Comprehensive Performance Assessment and Mechanistic Insights into Zero-Valent Iron/Ferrous Hydroxide Complex Composites for Optimized Se(IV) Sequestration in Aqueous Media

Comprehensive Performance Assessment and Mechanistic Insights into Zero-Valent Iron/Ferrous Hydroxide Complex Composites for Optimized Se(IV) Sequestration in Aqueous Media | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Comprehensive Performance Assessment and Mechanistic Insights into Zero-Valent Iron/Ferrous Hydroxide Complex Composites for Optimized Se(IV) Sequestration in Aqueous Media Yanjun Du, Qing Zhou, Jiankun Zhao, Hexi Wu, Xiaoyan Li, Yibao Liu, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7734481/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract 79 Se is one of the key fission products in spent fuel, with a half - life of 6.5×10 4 a. Owing to its exceptionally high mobility and extremely low solubility, this isotope can effortlessly permeate into groundwater, resulting in widespread contamination. This type of pollution poses an extremely grave threat to both ecosystems and human well-being. Therefore, the long-term safe fixation and disposal of 79 Se has become a challenging and cutting-edge topic in research. In this study, Ferrous Hydroxide Complex (FHC) was employed as a carrier to explore its effectiveness in removing selenium (IV). Zero-valent iron/Ferrous Hydroxide Complex (Fe 0 /FHC) was synthesized using simple and cost-effective methods, and their performance in selenium (IV) removal was evaluated. The findings demonstrated that, under specific conditions-pH 7.0, a 40-min adsorption period, 35% Fe 0 content, and a ratio of solid to liquid that was 0.20 g L − 1 , the Fe 0 /FHC composite achieved a Se(IV) adsorption capacity of 201.62 mg g − 1 . Kinetic analysis showed that adsorption behavior matches the pseudo-second-order model, suggesting that the chemical process was the dominant adsorption mechanism. Isothermal adsorption modeling further showed that the Langmuir model provided a better fit, suggesting that the adsorption predominantly occurred through monolayer coverage. Thermodynamic investigations indicated that the adsorption of Se(IV) by Fe 0 /FHC was an exothermic reaction. Further analysis via FT-IR and XPS showed that Fe 0 /FHC's removal of Se(IV) involves electrostatic adsorption, complexation, and reduction precipitation processes. Overall, the Fe 0 /FHC composite offered distinct benefits, including ease of synthesis, a large specific surface area, and exceptional adsorption capacity. These characteristics endowed Fe 0 /FHC with great potential in treating selenium-containing wastewater. Consequently, as a new adsorption material, it held broad application prospects in this field. Nascent FHC Iron Selenium reduction adsorption mechanism Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1 Introduction With nuclear energy developing rapidly, safe radioactive waste disposal is an urgent critical issue [ 1 – 3 ]. High-level radioactive waste repositories use a multi-barrier system for long-term isolation, yet ensuring their long-term stability under complex geological conditions is tough [ 4 , 5 ]. Selenium is redox-sensitive with five natural oxidation states: Se(-II), Se(-I), Se(0), Se(IV), and Se(VI). Se(IV) is the most toxic and bioavailable, ten times more hazardous than Se(VI) [ 6 ]. It has a long half-life (6.5×10⁴ a) and high mobility [ 7 – 9 ]. Corrosion or aging of storage tanks due to high temperature, pressure, radiation, or crustal stress can cause Se(IV) to dissolve into groundwater and spread, contaminating groundwater and threatening ecosystems and human health [ 10 , 11 ]. Se(IV) exists as negatively charged selenate (SeO 4 2− ), making it hard for near-field buffers, engineering barriers, or geological media to immobilize it [ 12 , 13 ]. Traditional adsorption methods are also ineffective in fixing Se(IV) [ 14 ]. Selenium removal methods include chemical reduction, ion exchange, reverse osmosis technology, biological treatment, adsorption, and chemical precipitation, among others. Both domestic and international research has explored various experimental approaches in this field [ 15 , 16 ]. Among these, adsorption methods have gained widespread popularity due to their cost-effectiveness, operational flexibility, broad applicability, and the recyclability of materials [ 17 – 19 ]. In recent years, with the increasingly severe problem of global water pollution, the research and development of efficient, convenient and green water selenium adsorbent has become a hot research topic in the intersection field of environmental science and material chemistry. Commonly employed adsorbents encompass natural materials [ 20 , 21 ], iron-based compounds [ 22 , 23 ], and carbon-based substances [ 24 , 25 ]. Iron oxides and hydroxides, known for their strong affinity for selenium ions, serve as significant adsorbents in sediment, functioning as an essential element in immobilizing selenium within the environment [ 26 ]. Various iron-based adsorbents demonstrate high adsorption capacity across a broad pH range and in the presence of competing ions [ 27 , 28 ]. Ferrous Hydroxide Complex (FHC) has potent reducing capabilities. Consequently, FHC can effectively reduce high-valence radionuclides such as Selenium and Uninium [ 29 , 30 ]. In natural geological environments rich in FHC, FHC can interact with ⁷⁹Se, which significantly reduces the migration capacity of 79 Se over a long geological time scale [ 31 , 32 ]. Nevertheless, in the absence of externally introduced strong reductants, FHC exhibits limited reduction capacity and a slow reaction rate, rendering it incapable of fully reducing selenite to FeSe 2 [ 33 ]. Consequently, the limitation of incomplete reduction constrains its wider utilization to a certain degree. Owing to its high reactivity, substantial specific surface area, cost-effectiveness, potent reducing ability, and facile modification potential, nZVI has found extensive application in treating wastewater contaminated with selenium [ 34 , 35 ]. Although nZVI can remove Se(IV) to a certain extent, its low mechanical strength, chemical instability, and aggregation behavior pose significant barriers to its widespread adoption [ 36 ]. In response to these constraints, researchers have adopted a strategy of anchoring nZVI onto supporting materials. This composite design enhances the nZVI's catalytic activity, promotes uniform dispersion, and ensures sustained stability under operational conditions [ 37 ]. Dong studied the removal effect of selenium (IV) using nZVI and columnar bentonite (Al-bent). Al-bent could significantly promote the migration of selenium from the liquid phase to the iron surface, accelerated the reduction process, and thereby improved the removal efficiency [ 38 ]. The objective of this research was to boost the capacity of FHC in eliminating Se(IV) from water-based solutions. By employing FHC as the base material and incorporating zero-valent iron as an additive, Fe 0 /FHC was successfully prepared. The research showed that Fe 0 /FHC could be used as components in chemical barriers, efficiently preventing the movement of high-valence soluble isotopes like 79 Se in repositories designed. 2 Experimental Detailed information regarding the characterization techniques and the approaches used for fitting experimental data can be found in the Supplemental Information (SI). 2.1 Materials Reagents such as FeCl 3 ·6H 2 O, FeSO 4 ·7H 2 O, C 2 H 5 OH, KBH 4 and Na 2 SeO 3 were sourced from MACKLIN in China. 2.2 Preparation of Fe 0 A precise amount of FeCl 3 ·6H 2 O was measured out and then placed into a beaker. A mixed solvent was then prepared by gradually adding ethanol and deionized water with stirring. Potassium borohydride was gradually added drop by drop to the reaction mixture until the reaction reached completion. The obtained product underwent several rounds of washing to remove impurities, followed by vacuum drying at 80°C for 10 h to obtain zero-valent iron (Fe 0 ) material. 4Fe+3BH+9HO→4Fe↓+3HBO+12H+6H↑ (1) 2.3 Preparation of FHC We accurately weighed ferrous sulfate heptahydrate (chemical formula: FeSO₄·7H₂O) using an analytical balance and transferred it to a 250 mL pre-washed beaker. Following the molar ratio specified in the experimental protocol, we precisely pipetted 1 mol L − 1 sodium hydroxide solution into the beaker and slowly added it dropwise while adjusting the reaction system's concentration with deionized water. Under continuous magnetic stirring, we ensured thorough mixing of reactants to achieve complete reaction. After completion, centrifugal separation was employed followed by multiple washes with deionized water until no impurity ions were detected in the eluates. Finally, the purified product was dried for 24 h at 60 ℃ in a vacuum drying oven, yielding high-purity ferrous hydroxide complex (abbreviated as FHC) [ 33 ]. Fe 2+ +OH − →FeOH + (2) Fe 2+ +2OH − →Fe(OH) 2 ↓ (3) Fe(OH) 2 +OH − →Fe(OH) 3 − ↓ (4) 2.4 Synthesis of Fe 0 /FHC Fe 0 and FHC were measured out in accordance with a predetermined mass ratio, and Fe 0 /FHC composites were subsequently synthesized. The detailed preparation steps are illustrated in Fig. 1 . 3 Results and discussion 3.1 Characterizations Scanning electron microscopy (SEM) was employed to analyze the microstructures of zero-valent iron (Fe 0 ), ferrous hydroxide complex (FHC), composite materials (Fe 0 /FHC), and selenium (Se)-adsorbed products (Fe 0 /FHC-Se). The outcomes are depicted in Fig. 2 (a-d). Figure 2 (a) reveals that Fe 0 possessed a rod-like structure but suffered from notable agglomeration [ 39 ]. In contrast, FHC exhibited a sheeted morphology as shown in Fig. 2 (b) [ 29 ]. Figure 2 (c) demonstrates that the composite material's surface was adorned with nanoscale FHC pine needles. This observation confirms the formation of the Fe 0 /FHC composite. Furthermore, the composite material exhibited excellent dispersibility, suggesting that the incorporation of FHC effectively hinders Fe 0 aggregation and significantly enhances the composite's specific surface area. The scanning electron microscopy (SEM) analysis of Fig. 2 (d) demonstrated that the successful adsorption of selenium on the material surface triggered significant morphological changes. The originally homogeneous material particles transformed into fragmented structures with varying sizes, while interparticle aggregation became markedly more pronounced compared to the conditions before selenium adsorption. These structural modifications-particularly the increased fragmentation and aggregation-likely result from selenium's chemical adsorption process on the material surface. Our hypothesis suggests that selenium's adsorption altered the material's surface chemistry, which in turn affected interparticle interactions and consequently induced this substantial microstructural transformation. Table 1 reveals that selenium (Se) accounts for up to 6.12% by weight in the adsorbed product. Figure 3 illustrates the presence of Fe, C, S, O, and Se. Table 1 Scanning results of element content in Fe 0 /FHC-Se Element Line type Weight percentage/% Atomic percentage/% C K linear system 5.60 13.72 O K linear system 28.43 52.32 S K linear system 0.31 0.29 Fe L linear system 59.54 31.39 Se L linear system 6.12 2.28 Total 100.00 100.00 The infrared spectroscopy results shown in Fig. 4 (a) clearly demonstrate the significant changes in the functional groups of the material. Detailed spectral analysis reveals a -OH peak around 3430 cm⁻¹ for both ferrous hydroxide complex (FHC) and Fe 0 /FHC composites. This characteristic peak likely results from the combined effects of stretching and bending vibrations of surface -OH functional groups [ 40 ], indicating that a specific quantity of water molecules have been adsorbed on the adsorbent surface. This phenomenon also provides a reasonable explanation for the C = C double bond peak observed at 1630 cm − 1 . A weak absorption peak at 1110 cm − 1 corresponds to the mode of C-O bonds in FHC surface -OH groups, as reported in the literature [ 41 ]. Notably, compared with pure Fe 0 samples, Fe 0 /FHC composites exhibit two distinct Fe-O bond stretching vibration peaks at 1400 cm − 1 and 562 cm − 1 , in the high-frequency and low-frequency regions, respectively [ 42 ]. These significant spectral shifts and characteristic peak positions in FT-IR spectra fully confirm the successful preparation of Fe 0 /FHC composites, supported by solid experimental data. As shown in Fig. 4 (b), Fe 0 /FHC exhibited weaker characteristic peaks compared to FHC. This phenomenon likely resulted from the interactions between Fe 0 and FHC, which enhanced the dispersion of Fe 0 and thereby decreased the crystallinity and stability. Importantly, all peak positions matched standard FHC card values [ 29 , 43 ]. Additionally, for Fe 0 and Fe 0 /FHC, at a 2θ value of 44.9°, characteristic peaks corresponding are clearly discernible. Furthermore, the XRD spectra of all samples did not reveal any notable impurity peaks, indicating that the synthesized samples possessed a high degree of purity. In essence, XRD analysis validated the successful preparation of Fe 0 /FHC. The X-ray photoelectron spectroscopy (XPS) analysis in Fig. 4 (c) reveals distinct characteristic peaks at specific energy positions of 285.08 eV, 531.08 eV, and 711.08 eV for the Fe 0 , FHC, and Fe 0 /FHC samples respectively. The C 1s peak at 285.08 eV originates from various carbon species adsorbed on the sample surface, including organic contaminants or surface carbon layers. The O 1s peak at 531.08 eV unequivocally indicates oxygen presence, which is consistent with both the hydroxyl functional groups in FHC materials and the iron oxide-hydroxyl coexistence in Fe 0 /FHC composites. The Fe 2p 3/2 peak at 711.08 eV directly confirms iron's oxidation state as + 3. Notably, the Fe 0 /FHC composite exhibits significantly stronger Fe 2p 3/2 peaks compared to pure FHC material, demonstrating effective iron content enhancement through Fe 0 loading. These XPS results not only accurately identify the chemical states of carbon, oxygen, and iron but also reveal unique elemental distribution patterns and surface chemistry characteristics of Fe 0 /FHC composites. From Fig. 4 (d), the nitrogen adsorption isotherms measured experimentally exhibit typical characteristics of Class IV curves, which closely align with the adsorption behavior of mesoporous materials as defined in the IUPAC classification standards. Specifically, the isotherms demonstrate single-molecule layer adsorption characteristics in the low-pressure region, followed by significant capillary condensation as pressure increases, and the formation of hysteresis loops in the medium-high pressure range. These features conclusively confirm the material's well - ordered mesoporous structure [ 39 ]. The loading of Fe 0 onto FHC resulted in a significant augmentation of its specific surface area, yielding a hierarchical pore structure with enhanced mass transfer properties. This modification was particularly advantageous for Se(IV) adsorption, as it promotes rapid diffusion and strong interaction with adsorbent sites. Figure 4 (e) illustrates that FHC underwent a notable mass reduction as the temperature rose. Particularly at 60 ℃ to 280 ℃, FHC experienced a 10.20% weight loss attributed to carbon disintegration within Fe 0 /FHC [ 44 ]. Conversely, Fe 0 /FHC only lost 4.53% of its weight from 0 ℃ to 800 ℃, demonstrating that the incorporation of zero-valent iron enhanced the thermal stability of the Fe 0 /FHC composites. This characteristic enabled Fe 0 /FHC composites to exhibit exceptional thermal stability, making them particularly promising for high-temperature catalytic or adsorption processes. 3.2 Removal performance studies 3.2.1 Effect of Fe 0 proportions A series of Fe 0 /FHC were synthesized with tailored Fe 0 loading amounts to achieve different compositional ratios. The relationship between composite ratio and Se(IV) removal efficiency was systematically evaluated, as shown in Fig. 5 (a). Statistical analysis confirmed that Fe 0 loading amount serves as a critical determinant of Se(IV) removal performance. As the mass fraction of elemental iron (Fe 0 ) in the composite increased from 15% to 55%, the removal efficiency of Se(IV) initially improved and subsequently declined. Experimental findings reveal that alterations in the amount of Fe 0 loaded have a profound impact on the Se(IV) removal capability of the Fe 0 /FHC system. When an excessive quantity of Fe 0 is loaded, there is an overabundance of active sites, which in turn decreases both the specific surface area of the material and its reaction activity. On the contrary, if the Fe 0 loading is insufficient, there won't be enough active sites available for the reactions to occur. In either case, the Se(IV) removal performance of the Fe 0 /FHC system is significantly compromised. Hence, it is crucial to keep the Fe 0 loading within the optimal range of 35% to achieve efficient Se(IV) removal in real-world applications. 3.2.2 Effect of pH Figure 5 (b) demonstrated that pH strongly influenced Se(IV) removal efficiency. While pure Fe 0 showed a consistent decline in Se(IV) removal with increasing pH, both FHC and Fe 0 /FHC composites exhibited an initial increase followed by a decrease in removal efficiency across the pH range tested. In general, the Fe 0 /FHC composite material demonstrated superior Se(IV) removal performance, reaching 94.28% (pH = 7.0). As a critical parameter for assessing solution environments, pH significantly influences material surface chemistry and fundamentally determines the existence forms and chemical behavior patterns of selenium in aqueous solutions. Detailed analysis reveals that under strongly acidic conditions (pH < 2.0), selenium primarily exists stably as selenite molecules (H 2 SeO 3 ). When pH rises to the neutral-to-weakly alkaline range of 2.64–8.36, over 95% of selenium converts into hydrogen selenite ions (HSeO 3 − ). As pH continues to rise beyond 8.0 into alkaline conditions, selenium predominantly transforms into selenate ions (SeO 3 2− ). Test data in Fig. 5 (c) demonstrate that the Fe 0 /FHC composite material has an isoelectric point of 6.9, which plays a decisive role in its surface charge characteristics. Under acidic conditions (pH < 6.9), the Fe 0 /FHC surface exhibits a positive charge. As pH increases from 3.0 to 6.9, the proportion of HSeO₃ − in solution progressively rises. The positive surface charge creates strong electrostatic attraction with negatively charged HSeO 3 − ions, leading to a significant enhancement in Fe 0 /FHC's removal efficiency for HSeO 3 − . This optimal removal effect is achieved at the isoelectric point pH 6.9. However, when the pH value continues to rise beyond 6.9 to 8.0, the surface charge properties of Fe 0 /FHC reverse from positive to negative, and a static repulsion is generated with the similarly negatively charged HSeO 3 − , resulting in a gradual decrease in the removal efficiency with the increase of pH value. 3.2.3 Solid-liquid ratio Figure 5 (d) reveals that at low solid-to-liquid ratios, Fe 0 /FHC exhibited higher Se(IV) adsorption capacity per unit mass, but the limited total adsorbent quantity resulted in relatively low overall Se(IV) removal efficiency. With a rise in the solid-to-liquid ratio, the number of accessible adsorption sites increased, thereby leading to an elevated removal efficiency. In experimental studies, when the solid-liquid ratio was optimized to 0.20 g L − 1 , the Fe 0 /FHC composite demonstrated exceptional selenium(IV) removal performance with an efficiency of 92.25% and an adsorption capacity of 70.73 mg g − 1 . This result indicates that the material achieves high pollutant removal efficiency at low dosage. Considering the need to balance treatment effectiveness with economic costs in practical applications while maintaining both high removal rates and good adsorption properties, comprehensive analysis confirmed 0.20 g L − 1 as the optimal solid-liquid ratio for this experiment. Therefore, in subsequent experimental series, we uniformly adopted this optimized ratio to ensure reliable and comparable experimental results. 3.2.4 Reaction time and kinetics As shown in Fig. 6 (a), experiments at different temperatures (293 K, 298 K, 303 K, and 308 K) demonstrated that elevated ambient temperature significantly affects the Se(IV) removal performance of Fe 0 /FHC composite materials. The data revealed a slight upward trend in Se(IV) removal efficiency as temperature increased from 293 K to 308 K. The removal rate followed consistent temporal patterns across all temperatures: rapid removal occurred during the initial 0–10 min, followed by a slower growth phase between 10–40 min, with adsorption equilibrium achieved around 40 min. Therefore, this study established 40 min as the standard reaction time for subsequent experiments. Analysis supported by formulas S3 and S4 in supplementary information led to the development of a kinetic model for Fe 0 /FHC's Se(IV) adsorption process. Fitted results are shown in Fig. 6 (b) and 6(c), with parameters summarized in Table 2 . Model evaluation confirmed excellent fitting performance of the quasi-second-order kinetic model, indicating that chemical adsorption mechanisms dominate the process. During the initial stage (0–5 min), chemical reaction rates determined removal efficiency, with interfacial interactions influencing adsorption dynamics [ 45 ]. Kinetic analysis further revealed an increase in the quasi-second-order kinetic model's rate constant k 2 when temperature rose from 293 K to 308 K, validating the temperature-promoting effect on adsorption kinetics. The results confirmed that higher temperatures promoted Se(IV) adsorption by Fe 0 /FHC, supporting the previously observed slight efficiency gain with temperature rise and validating temperature's role. Table 2 Kinetic model fitting parameters for Se(IV) removal by Fe 0 /FHC Temperature/K Quasi-first order dynamics Quasi-second order dynamics Q e /(mg g − 1 ) K 1 /(L min − 1 ) R 2 Q e /(mg g − 1 ) K 2 /(g mg − 1 min − 1 ) R 2 293 37.0831 0.0759 0.9358 76.2777 0.0026 0.9975 298 22.1849 0.0744 0.8876 73.9098 0.0045 0.9982 303 12.8736 0.0591 0.7200 72.4113 0.0066 0.9985 308 9.7961 0.0517 0.7226 71.2251 0.0100 0.9994 3.2.5 Initial selenium concentration and isotherms Figure 6 (d) reveals that across the tested temperature range, the Se(IV) adsorption capacity of Fe 0 /FHC composites exhibited a positive correlation with initial Se(IV) concentration, demonstrating consistent concentration-dependent behavior at all experimental temperatures. At initial Se(IV) concentrations above 100 mg L − 1 , the adsorption capacity attained 201.62 mg g − 1 . This study applied the formulas S5 and S6 from the system support information to model and analyze the isothermal adsorption behavior of Se(IV) on Fe 0 /FHC composite materials. As shown in Fig. 7 (a) and 7(b), the fitted curves closely matched the experimental data, confirming the applicability of the selected model. Table 3 lists the fitting parameters indicating that Se(IV) primarily adsorbed as a monolayer on the Fe 0 /FHC surface. Temperature-dependent adsorption studies revealed that the equilibrium adsorption capacity of Fe 0 /FHC for Se(IV) increased continuously from 293 K to 308 K, suggesting an endothermic reaction where higher temperatures enhance adsorption efficiency. Therefore, due to its simple synthesis process, low raw material costs, and excellent adsorption performance, Fe 0 /FHC shows significant application value and market potential as a novel. Table 3 Isothermal adsorption model parameters for Fe 0 /FHC removal of Se(IV) Temperature/K Langmuir isotherm adsorption model Freundlich adsorption isotherm model Q m /(mg g − 1 ) K L /(L mg − 1 ) R 2 1/n K F R 2 293 139.6648 -0.1027 0.9923 0.1986 59.4199 0.8331 298 191.9386 -0.1001 0.9908 0.2307 66.6943 0.9254 303 235.8491 -0.1076 0.9888 0.2653 66.9048 0.9636 308 277.7778 -0.0997 0.9791 0.2972 68.7001 0.9371 3.2.6 Effect of temperature and thermodynamics As depicted in Fig. 7 (c), the Se(IV) removal efficiency by the Fe 0 /FHC exhibited a slight upward trend with temperature elevation from 293 K to 308 K across all tested initial concentrations. Furthermore, the removal rate demonstrated a concentration-dependent pattern, initially increasing with higher initial Se(IV) levels before reaching an asymptotic plateau. This could be attributed to the ample availability of active sites at lower initial concentrations, allowing Se(IV) ions to be readily adsorbed. However, when the initial concentration of selenium(IV) in the solution gradually increases and reaches a specific threshold, the limited active sites on the adsorbent surface will be progressively occupied, eventually approaching saturation. At this saturated state, even if the concentration of selenium(IV) in the solution continues to increase, its improvement effect on the overall removal rate becomes negligible. To gain deeper insights into the essential characteristics of this adsorption process, we conducted systematic and precise fitting analysis of the thermodynamic curves of Fe 0 /FHC adsorbing Se(IV) based on the thermodynamic formulas S7, S8, and S9 provided in SI. From Fig. 7 (d), the experimental data showed good consistency with the theoretical model. The detailed fitting parameters listed in Table 4 reveal that the value of ΔH 0 is positive, clearly indicating that this adsorption process is an endothermic reaction, which is more favorable under higher temperatures. This is because the additional thermal energy from temperature increase effectively enhances the interaction between selenium ions and the active sites on the Fe 0 /FHC surface, significantly improving adsorption efficiency. The microscopic mechanism may be attributed to the arrangement at the solid - liquid interface during adsorption, where such structural changes paradoxically facilitate selenium ion adsorption. More crucially, all experimentally measured ΔG 0 values were negative, which thermodynamically confirms a spontaneous process. Notably, as temperature increases, ΔG 0 showed a significant upward trend, indicating that the spontaneous driving force of this adsorption process becomes further enhanced under higher temperature conditions [ 46 , 47 ]. In conclusion, the thermodynamic analysis produced results that consistently aligned with the predictions of the isotherm adsorption model, thereby validating the theoretical framework across all experimental conditions. Table 4 Thermodynamic fitting parameters for Se(IV) removal by Fe 0 /FHC C 0 (mg L − 1 ) Δ H 0 / (kJ mol − 1 ) Δ S 0 /(J mol − 1 K − 1 ) Δ G 0 / (kJ mol − 1 ) 293 K 298 K 303 K 308 K 5 89.6997 344.3657 -11.1706 -13.0536 -14.4621 -16.4425 10 77.4563 298.9793 -9.9691 -11.8826 -13.1946 -14.5012 15 66.3303 254.7722 -8.2823 -9.6459 -10.8679 -12.1187 20 50.6747 195.0741 -6.1167 -7.9069 -8.6754 -9.0809 25 45.7458 172.5910 -4.5853 -6.0082 -6.6472 -7.2301 30 41.9668 157.0044 -3.9800 -4.9441 -5.5279 -6.4000 40 38.3025 138.8454 -2.3112 -3.1762 -3.7737 -4.4212 50 34.2964 122.7466 -1.5689 -2.4091 -2.9522 -3.4254 60 31.9891 112.9006 -0.9937 -1.7748 -2.2840 -2.6974 80 89.6997 344.3657 -11.1706 -13.0536 -14.4621 -16.4425 100 77.4563 298.9793 -9.9691 -11.8826 -13.1946 -14.5012 3.2.7 Effect of coexisting cations We systematically investigated the mechanisms by which three common coexisting cations (K⁺, Na⁺, Ca²⁺) affect the performance of Fe 0 /FHC composites in removing Se(IV) from aqueous solutions. The results are visually presented in Fig. 8 (a) (adsorption kinetics curves) and 8(b) (removal efficiency). Analysis of Fig. 8 (a) clearly demonstrates that all three cations significantly enhance the Se(IV) removal efficiency of Fe 0 /FHC composites, though their effects show distinct differences. Specifically, Ca²⁺ (divalent) exhibits a promoting effect, while Na⁺ and K⁺ (monovalent) demonstrate weaker enhancing effects. Within the pH range of 3.0–7.0, the predominant form of Se(IV) is HSeO 3 − anion. The cations form electrostatic bridging interactions with HSeO 3 − on the Fe 0 /FHC surface through their positive charges. This charge neutralization effect not only strengthens surface adsorption but also facilitates subsequent reduction reactions. Notably, higher - valence cations (e.g., Ca²⁺ compared to Na⁺ and K⁺) generate stronger electrostatic attraction, significantly accelerating HSeO 3 − migration to the material surface. This optimized interfacial reaction kinetics enhances overall Se(IV) removal efficiency. Kinetic data in Fig. 8 (b) further indicate that cation addition dramatically shortens the adsorption equilibrium time from several hours to approximately 30 min. Of particular significance, the Ca²⁺ system demonstrates the fastest reaction rate and achieves the highest removal efficiency, which conclusively confirms the cationic promotion effect on Se(IV) removal. Table 5 provides detailed pH change patterns across all experimental groups, showing consistent pH increases with cation addition, reaching a maximum of 8.62. Table 5 pH of the solution before and after adding cations pH before adsorption pH after adsorption Blank K + Na + Ca 2+ 3.0 7.22 7.37 8.14 8.06 3.5 8.24 8.19 8.30 8.25 4.0 8.36 8.35 8.38 8.35 4.5 8.38 8.48 8.44 8.42 5.0 8.55 8.54 8.52 8.62 5.5 8.52 8.51 8.46 8.60 6.0 8.51 8.35 8.34 8.53 6.5 8.45 8.22 8.29 8.52 7.0 8.42 8.16 8.18 8.44 7.5 7.22 7.37 8.14 8.06 3.2.8 Effect of coexisting anions Prevalent coexisting anions such as F − , NO 3 − , CO 3 2− , PO 4 3− , and Humic Acid were chosen to investigate their impact. As illustrated in Fig. 8 (c), NO 3 − and PO 4 3− notably hindered the removal of Se(IV). In acidic environments, both NO 3 − and PO 4 3− displayed some oxidizing characteristics, thereby competing with the reduction of HSeO 3 − . Experimental results demonstrate that coexisting anions exhibit significant competitive effects on the reduction of HSeO 3 − . Specifically, certain anions engage in intense competitive adsorption with HSeO 3 − at reaction sites, which partially hinders the reduction process and significantly reduces the removal efficiency of tetravalent selenium (Se(IV)). Comparative experiments reveal that F − , CO₃² − , and humic acid show negligible competitive adsorption with HSeO 3 − , rendering their competitive influence negligible. The kinetic curves in Fig. 8 (d) clearly indicate that the presence of competing anions leads to reduced reaction rates and decreased removal efficiency of HSeO 3 − in Fe 0 /FHC composites, particularly a more pronounced decrease in reaction rate. This phenomenon strongly demonstrates that competitive anions inhibit the reduction of HSeO 3 − following the addition of anions like F − , NO 3 − , CO 3 2− , PO 4 3− , and Humic Acid. Notably, NO 3 − and PO 4 3− had particularly pronounced impacts on removal. Additionally, Table 6 reveals that after adsorption experiments involving these anions, the solution's pH could increase to 8.89. Table 6 pH of solution before and after adding anions pH before adsorption pH after adsorption Blank F − NO 3 − CO 3 2− PO 4 3− Humic Acid 3.0 5.58 5.49 4.35 5.64 4.51 5.35 3.5 6.15 5.76 4.64 5.95 4.94 6.13 4.0 6.34 6.54 4.93 6.21 5.36 6.41 4.5 6.81 7.22 5.36 6.64 5.52 7.03 5.0 7.29 7.36 5.50 6.76 5.69 7.13 5.5 7.30 7.41 5.61 7.24 6.21 7.31 6.0 7.42 7.80 5.70 7.53 6.60 7.43 6.5 7.87 8.27 6.17 7.90 6.96 7.69 7.0 8.31 8.34 6.78 8.18 6.92 8.12 7.5 8.83 8.59 7.16 8.37 7.17 8.89 3.3 Removal mechanism Through comparison with the original conditions without adsorption treatment, the results shown in Fig. 9 (a) clearly reveal that the functional group characteristic peaks on the surface of Fe 0 /FHC underwent significant changes after adsorption treatment. Specifically, the intensity of characteristic peaks representing hydroxyl groups (-OH), carbon-carbon double bonds (C = C), carbon-oxygen bonds (C-O), and iron-oxygen bonds (Fe-O) all increased markedly. These peak intensities demonstrate that during the adsorption process, hydroxyl groups, various oxygen-iron-containing functional groups actively participated in complexation reactions with selenium. Notably, the characteristic peak of Se-O bonds was detected for the first time at the 882 cm − 1 wavenumber, providing direct and conclusive experimental evidence for the successful adsorption of Se(IV) by Fe 0 /FHC [ 48 ]. Notably, the post-adsorption spectrum exhibited no discernible Se-Se bond vibration peak, providing additional evidence that stable chemical interactions-rather than mere physical adsorption-occurred. As shown in the XPS analysis results in Fig. 9 (b), a distinct Se 3d characteristic peak is clearly observed at the binding energy position of 56.08 eV, which holds significant characterization value. By comparing with standard spectral database, this feature corresponds to the 3d orbital electron binding energy of selenium. This experimental result fully demonstrates that the Fe 0 /FHC composite material successfully captured and immobilized selenium from the solution during adsorption experiments. This discovery directly validates the excellent adsorption performance of the Fe 0 /FHC composite material for selenium pollutants, providing reliable experimental evidence for subsequent research on its application in heavy metal pollution control [ 33 ]. To precisely elucidate the chemical state distributions and spectral characteristics of Se 3d and Fe 3p orbitals, this study employed advanced peak decomposition fitting techniques to carry out meticulous deconvolution of the XPS high-resolution spectrum (Fig. 9 (c)). The analysis revealed that the prominent peak at 55.2 eV originated from the superposition of Se(0) 3d 5/2 (55.3 eV) and Fe(II)-O 3p 3/2 (55.9 eV). Similarly, the shoulder peak at 58.3 eV was predominantly attributed to Fe(III)-O 3p 3/2 (58.0 eV) and Se(IV) 3d 5/2 (58.7 eV), where its broadened profile highlighted the heterogeneous distribution of surface oxidation states [ 47 , 49 ]. The X-ray photoelectron spectroscopy (XPS) analysis in Fig. 9 (d) reveals five characteristic peaks in the Fe 2p orbital before selenium adsorption begins. These peaks correspond to binding energies of 710.1 eV, 711.5 eV, 718.3 eV, 724.0 eV, and 729.0 eV. Following selenium adsorption, all peak positions shifted to 710.2 eV, 712.0 eV, 718.1 eV, 724.3 eV, and 730.7 eV respectively. Notably, the relative intensities of these peaks also changed significantly beyond their positional shifts. Detailed analysis shows that the peaks at 711.5 eV and 724.0 eV correspond to the spin-orbital splitting peaks of Fe 2p 3/2 and Fe 2p 1/2 electrons, which exhibited particularly pronounced variations during adsorption. The systematic shift in binding energy positions and marked intensity changes strongly indicate that iron plays a crucial role in selenium complexation and actively participates in surface chemical reactions [ 39 ]. These findings provide essential experimental evidence for further investigation into the iron-selenium interaction mechanism. Figure 9 (e) provides a detailed analysis of the high-resolution C 1s X-ray photoelectron spectroscopy (XPS) profiles on the sample surface. Before adsorption, the results of Gaussian-Lorentz peak fitting revealed four distinct C 1s peaks: the 284.3 eV peak from sp² hybridized C-C bonds, 285.0 eV from C-O single bonds, 288.2 eV from C = O double bonds, and 290.8 eV from O-C = O carboxyl groups. After selenium(IV) ion adsorption, these peaks shifted by varying amounts: C-C to 284.4 eV, C-O to 285.2 eV, C = O to 288.3 eV, and O-C = O to 290.5 eV. Notably, the C = O bond showed significant intensity reduction, strongly indicating its crucial role in the selenium adsorption process. In contrast, while C-C, C-O, and O-C = O bonds also shifted slightly, their effects were much less pronounced than those of the C = O bond. This comparison demonstrates that these three chemical bonds played relatively minor roles in selenium adsorption, with their contributions being far less significant than that of the C = O bond. As shown in Fig. 9 (f), X-ray photoelectron spectroscopy (XPS) was employed to characterize the material surface before initiating the adsorption reaction. The results revealed two distinct O 1s spectral peaks at binding energies of 529.5 eV and 530.8 eV. Specifically, the 529.5 eV peak corresponds to the chemical state of Fe-O bonds, while the 530.8 eV peak reflects the vibrational mode of H-O bonds. Following the adsorption process, both peaks shifted slightly: the Fe-O peak moved to 529.4 eV, and the H-O peak shifted to 530.5 eV. Notably, the Fe-O peak not only shifted in position but also showed a significant reduction in intensity and broadening in shape. This phenomenon strongly indicates that iron played a crucial role in removing selenium ions-with some iron atoms being oxidized into iron oxides or iron hydroxides. In contrast, the H-O peak exhibited a completely different trend, showing enhanced intensity and sharpened shape. These changes clearly demonstrate that during adsorption, water molecules interacted significantly with selenium ions, causing partial detachment and altering the chemical environment. Based on FT-IR and XPS analyses, Se(IV) removal by Fe 0 /FHC occurred through two main mechanisms: (1) electrostatic attraction facilitating Se(IV) ion adsorption onto the material surface, and (2) complexation with surface functional groups including C = O, H-O and Fe-O bonds, which formed stable chemical structures. Figure 10 shows the potential complexing mechanism. 4 Conclusions This study triumphantly synthesized Fe 0 /FHC via a combined reduction-ball milling approach and systematically evaluated their Se(IV) adsorption performance and the underlying interaction mechanisms. Characterization analysis indicated that the synthesized Fe 0 /FHC displayed a sheeted morphology. Experimental data strongly confirm that under optimized conditions-precisely regulating the solution pH to neutral (7.0), setting the adsorption reaction duration at 40 min, maintaining zero-valent iron (Fe 0 ) content at 35%, and fixing the solid-to-liquid ratio at 0.20 g L − 1 -the Fe 0 /FHC composite adsorbent exhibits outstanding adsorption performance for tetravalent selenium [Se(IV)], with a maximum adsorption capacity reaching 201.62 mg g − 1 . In-depth kinetic analysis reveals that the adsorption process's dynamic characteristics closely align with the pseudo-second-order reaction model (correlation coefficient R 2 > 0.99), strongly supporting the dominant role of chemical adsorption mechanisms in Se(IV) removal. Further isothermal adsorption modeling analysis demonstrates that experimental data shows significantly higher agreement with the Langmuir isothermal model compared to other models, indicating that Se(IV) adsorption on Fe 0 /FHC surfaces primarily occurs through uniform single-molecular-layer adsorption. Thermodynamic parameter measurements reveal a negative enthalpy change (ΔH 0 < 0) throughout the adsorption process, conclusively demonstrating that Fe 0 /FHC's Se(IV) adsorption constitutes an exothermic spontaneous process. It is worth noting that in the investigation of the influence of interfering substances, it was found that the presence of common anions (such as F − , CO 3 2− , etc.) and natural organic matter (such as humic acid) had little effect on the adsorption efficiency of Se(IV), and the reduction rate of removal did not exceed 5%, which fully demonstrated the good applicability of this adsorbent in the actual complex water environment. The Fe 0 /FHC composite prepared in this study demonstrated superior Se(IV) removal efficiency when compared with conventional adsorbents. The material exhibited unique advantages: low production cost, facile synthesis, strong reductive capability, high adsorption capacity, and rapid removal kinetics. Evaluated as a promising reactive barrier material, it showed potential for blocking soluble multivalent radionuclide ( 79 Se) migration. Declarations CRediT authorship contribution statement Yanjun Du : Writing-Reviewing and Editing. Qing Zhou : Methodology. Jiankun Zhao : Investigation. Hexi Wu : Validation. Xiaoyan Li : Resources. Yibao Liu : Conceptualization. Zhanggao Le : Visualization. Data availability Data will be provided upon request. Acknowledgement This work was financially supported by National Natural Science Foundation of China (12565020), Doctoral Scientific Research Start-up Fund Project of East China University of Technology (DHBK2024017) and Special topic of national science and technology major project in deep earth (2025ZD1007303-03). References Dong D, Wang Z, Guan J (2025) Research on safe disposal technology and progress of radioactive nuclear waste [J]. Nucl Eng Des 435:113934 Clayton R, Kirk J, Banford A (2025) A review of radioactive waste processing and disposal from a life cycle environmental perspective [J]. Clean Technol Envir 27(2):665–682 Kuhlman K, Bartol J, Carter A (2024) Scenario development for safety assessment in deep geologic disposal of high-level radioactive waste and spent nuclear fuel: A review [J]. Risk Anal 44(8):1850–1864 Li N, Duan X, Peng G (2025) Engineering barriers in deep geological disposal: Implications for radioactive nuclide migration and long-term safety [J]. J Environ Radioactiv 285:107670 Xiang L, Liu X, Liu P (2025) Research insights of argillaceous-based site for high-level radioactive waste disposal in China [J]. J Radioanal Nucl Ch 334(1):795–805 Liang L, Yang W, Guan X (2013) Kinetics and mechanisms of pH-dependent selenite removal by zero valent iron [J]. Water Res 47(15):5846–5855 He H, Liu J, Dong Y (2019) Sorption of selenite on Tamusu clay in simulated groundwater with high salinity under aerobic/anaerobic conditions [J]. J Environ Radioactiv 203:210–219 Yang J, Shi K, Sun X (2019) An approach for the efficient immobilization of 79 Se using Fe-OOH modified GMZ bentonite [J]. Radiochim Acta 108(2):113–126 Ma Z, Shan C, Liang J (2018) Efficient adsorption of Selenium(IV) from water by hematite modified magnetic nanoparticles [J]. Chemosphere 193:134–141 Lee J, Kim K, Kim I (2023) High-efficiency deep geological repository system for spent nuclear fuel in Korea with optimized decay heat in a disposal canister and increased thermal limit of bentonite [J]. Nucl Eng Technol 55(4):1540–1554 Turner J, Berry T, Bowman M (2023) Role of the geosphere in deep nuclear waste disposal-An England and Wales perspective [J]. Earth-Sci Rev 242:104445 Chikkamath S, Patel M, Kar A (2023) Diffusion of radioisotopes in Fe-montmorillonite relevant to geological disposal of HLW [J]. Phys Chem Earth 131:103429 Ruiz-Fresneda M, Martinez-Moreno M, Povedano-Priego (2023) Impact of microbial processes on the safety of deep geological repositories for radioactive waste [J]. Front Microbiol 14:3389–3409 Tyupina E, Kozlov P, Krupskaya V (2023) Application of Cement-Based Materials as a Component of an Engineered Barrier System at Geological Disposal Facilities for Radioactive Waste-A Review [J]. Energies 16(2):605–640 He Y, Xiang Y, Zhou Y (2018) Selenium contamination, consequences and remediation techniques in water and soils: A review [J]. Environ Res 164:288–301 Tran A, Wang P, Zhang S, Jovanovic M, Siewert B, Moser S, Lindsey J (2024) Synthesis of a model phyllobilin bearing an optical marker [J]. New J Chem 48:2855–2865 Suazo-Hernández J, Manquián-Cerda K, Mora M (2021) Efficient and selective removal of Se(VI) and As(V) mixed contaminants from aqueous media by montmorillonite-nanoscale zero valent iron nanocomposite [J]. J Hazard Mater 403:123639 Benis K, Damuchali A, Soltan J (2020) Treatment of aqueous arsenic-A review of biochar modification methods [J]. Sci Total Environ 739:139750 Wang S, Xiao K, Mo Y (2020) Selenium(VI) and copper(II) adsorption using polyethyleneimine-based resins: Effect of glutaraldehyde crosslinking and storage condition [J]. J Hazard Mater 386:121637 Hanada K, Watanabe S, Inagawa A (2021) Sulfated Steelmaking Slags as Se(IV) Adsorbents: Effects of Preparation Conditions on Adsorption Performance [J]. Isij Int 61(1):506–512 Li D, Yan W, Guo X (2020) Removal of selenium from caustic solution by adsorption with Ca-Al layered double hydroxides [J]. Hydrometallurgy 191:105231 Okonji S, Dominic J, Pernitsky D (2020) Removal and recovery of selenium species from wastewater: Adsorption kinetics and co-precipitation mechanisms [J]. J Water Process Eng 38:101666 Xu W, Zhu J, Johnson T (2020) Selenium isotope fractionation during adsorption by Fe, Mn and Al oxides [J]. Geochim Cosmochim Ac 272:121–136 Benis K, Damuchali A, Mcphedran K (2020) Treatment of aqueous arsenic - A review of biosorbent preparation methods [J]. J Environ Manage 273:111126 Zhang L, Jiang S, Guan Y (2021) Efficient removal of selenate in water by cationic poly(allyltrimethylammonium) grafted chitosan and biochar composite [J]. Environ Res 194:110667 Wang J, Xie L, Li S (2021) Probing the In Situ Redox Behavior of Selenium on a Pyrite Surface by Scanning Electrochemical Microscopy [J]. J Phys Chem C 125(5):3018–3026 Kalaitzidou K, Zouboulis A, Mitrakas M (2020) Cost evaluation for Se(IV) removal, by applying common drinking water treatment processes: Coagulation/precipitation or adsorption [J]. J Environ Chem Eng 8(5):104209 Yue P, Chen N, Peak D (2020) Oxygen atom release during selenium oxyanion adsorption on goethite and hematite [J]. Appl Geochem 117:104605 Zhang Y, Fu M, Wu D, Zhang Y (2017) Immobilization of selenite from aqueous solution by structural ferrous hydroxide complexes [J]. Rsc Adv 7:13398–13405 Ma B, Charlet L, Fernandez-Martinez A, Kang M, Made B (2019) A review of the retention mechanisms of redox-sensitive radionuclides in multi-barrier systems [J]. Appl Geochem 100:414–431 Wen Z, Zhang Y, Zhou X, Chen R (2017) Effective As(III) and As(V) immobilization from aqueous solution by nascent ferrous hydroxide colloids (FHC) [J]. Sep Purif Technol 176:395–401 Tian Q, Guo B, Chuaicham C, Sasaki K (2020) Mechanism analysis of selenium (VI) immobilization using alkaline-earth metal oxides and ferrous salt [J]. Chemsphere 248:126123 Du Y, Zhou Q, Zhao J (2023) Adsorptive Removal of Selenium(IV) from Aqueous Solution by Ferrous Hydroxide Complex-Zero Nickel Composites [J]. Solvent Extr Ion Exc 41(5):680–696 Wang Z, Lü C, Wang Y (2024) Zero-valent iron (ZVI) facilitated in-situ selenium (Se) immobilization and its recovery by magnetic separation: Mechanisms and implications for microbial ecology [J]. J Hazard Mater 473:134591 Fan Z, You Y, Ding K (2024) In-situ constructing an innovative and versatile nanotraps for incorporation of zero-valent iron nanoparticles into zeolite imidazole framework-8 towards high efficient detoxification and imprisonment of U(VI) and Se(IV) from water via a combination study of capture performance and underlying mechanism aspects [J]. Chem Eng J 494:153032 Chalastara K, Demopoulos G (2022) Mechanism of galvanic reduction of selenate oxyanions and surface immobilization by nano zero-valent iron aggregates under anaerobic conditions: towards high electron efficiency [J]. Environ Sci-Wat Res 8(9):1910–1922 Wu M, Teng X, Liang X (2023) Supporting nanoscale zero-valent iron onto shrimp shell-derived N-doped biochar to boost its reactivity and electron utilization for selenite sequestration [J]. Chemosphere 319:137979 Dong H, Chen Y, Sheng G (2016) The roles of a pillared bentonite on enhancing Se(VI) removal by ZVI and the influence of co-existing solutes in groundwater - ScienceDirect [J]. J Hazard Mater 304(3):306–312 Du Y, Fu X, Zhou Q (2023) Preparation of zero-valent iron-nickle bimetallic composite for Se(IV) adsorption from aqueous solution [J]. J Radioanal Nucl Ch 386:123944 Yu W, Guo H, Hou C (2022) Removal of Arsenate from Aqueous Solution by Synthetic Siderite-Modified Biochar: Characteristics and Mechanisms [J]. Water Air Soil Poll 233(7):278–292 Wang H, Tsang Y, Wang Y (2018) Adsorption capacities of poorly crystalline Fe minerals for antimonate and arsenate removal from water: adsorption properties and effects of environmental and chemical conditions [J]. Clean Technol Envir 20(10):2169–2179 Nguyen T, Tran H, Nguyen T (2022) Single-step removal of arsenite ions from water through oxidation-coupled adsorption using Mn/Mg/Fe layered double hydroxide as catalyst and adsorbent [J]. Chemosphere 295:133370 Qiu D (2020) Fabrication of dynamic zero-valent iron/MnO 2 nanowire membrane for efficient and recyclable selenium separation [J]. Sep Purif Technol 230:115847 Lee S, Kim Y, Chang B (2020) Enhanced Arsenic (III and V) Removal in Anoxic Environments by Hierarchically Structured Citrate/FeCO 3 Nanocomposites [J]. Nanomaterials-Basel 10(9):1773–1795 Bakather O, Fard A, Ihsanullah (2017) Enhanced Adsorption of Selenium Ions from Aqueous Solution Using Iron Oxide Impregnated Carbon Nanotubes [J]. Bioinorg Chem Appl 2017:4323619 Wang Z, Zhang H, Tang R (2022) An investigation of Re(VII) and Se(IV) adsorption by Tamusu clay: effect of time, pH, ionic strength, temperature and organic acids [J]. J Radioanal Nucl Ch 331(9):3461–3473 Wu J, Guo B, Kang M (2022) Comparative study on the reductive immobilization of Se(IV) by Beishan granite and Tamusu claystone [J]. Appl Geochem 146:105447 Zhijian L, Qingqing W, fujuan D (2022) Reduction of selenite to selenium nanospheres by Se(IV)-resistant Lactobacillus paralimentarius JZ07 [J]. Food Chem 393:133385 Ma B, Nie Z, Liu C, Kang M, Bardelli F, Chen F, Charlet L (2014) Kinetics of FeSe 2 oxidation by ferric iron and its reactivity compared with FeS 2 [J]. Sci China Chem 57:1300–1309 Additional Declarations No competing interests reported. Supplementary Files SupportingInformation.docx GraphicalAbstractforreview.docx Highlights.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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16:47:30","extension":"html","order_by":29,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":160709,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/dba728a8cd4dbe87012d167f.html"},{"id":93615478,"identity":"62ef7921-cc03-4d92-9ac2-25788ac64035","added_by":"auto","created_at":"2025-10-15 16:47:29","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":763633,"visible":true,"origin":"","legend":"\u003cp\u003eSynthesis diagram of Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite material.\u003c/p\u003e","description":"","filename":"image1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/fc188979d69c82e0dc157cf5.jpeg"},{"id":93615479,"identity":"8b86bcc3-17ee-44cc-8183-9d58b9f82693","added_by":"auto","created_at":"2025-10-15 16:47:29","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":552482,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of (a) Fe\u003csup\u003e0\u003c/sup\u003e, (b) FHC, (c) Fe\u003csup\u003e0\u003c/sup\u003e/FHC, (d) Fe\u003csup\u003e0\u003c/sup\u003e/FHC-Se.\u003c/p\u003e","description":"","filename":"image2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/5655d5e72e624004d477c293.jpeg"},{"id":93615484,"identity":"ec07b28a-d9b1-40b9-ac85-d7ef4b175b21","added_by":"auto","created_at":"2025-10-15 16:47:29","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":268711,"visible":true,"origin":"","legend":"\u003cp\u003e(a-e) EDS mapping and the corresponding elemental distribution mapping of iron, sulfur, carbon, oxygen, selenium, (f) EDS spectrum of selenium loaded on Fe\u003csup\u003e0\u003c/sup\u003e/FHC.\u003c/p\u003e","description":"","filename":"image3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/8609356f0cee3e86b1b36c4a.jpeg"},{"id":93616168,"identity":"493073f7-804a-407c-bbc1-fdca892112f1","added_by":"auto","created_at":"2025-10-15 16:55:29","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":391213,"visible":true,"origin":"","legend":"\u003cp\u003e(a) FT-IR spectra, (b) XRD spectra, (c) XPS full spectra, (d) N\u003csub\u003e2\u003c/sub\u003e Adsorption-desorption isotherms, (e) Thermogravimetric curves, (f) Appearance of Se(IV) in aqueous solution at different pH values.\u003c/p\u003e","description":"","filename":"image4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/2ce9c51af366a54a242c75c6.jpeg"},{"id":93616171,"identity":"56723845-7169-4948-9423-d02f5732c75b","added_by":"auto","created_at":"2025-10-15 16:55:29","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":188795,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Effect of Fe\u003csup\u003e0\u003c/sup\u003e ratio on Se(IV) removal by Fe\u003csup\u003e0\u003c/sup\u003e/FHC (pH = 5.0, \u003cem\u003eM\u003c/em\u003e =7.5 mg, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e = 10 mg L\u003csup\u003e-1\u003c/sup\u003e, \u003cem\u003eV \u003c/em\u003e= 25 mL, \u003cem\u003et\u003c/em\u003e = 120 min, \u003cem\u003eT\u003c/em\u003e = 298 K) , (b) Effect of pH on Se(IV) removal by Fe\u003csup\u003e0\u003c/sup\u003e/FHC (\u003cem\u003eM\u003c/em\u003e = 7.5 mg, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e = 10 mg L\u003csup\u003e-1\u003c/sup\u003e, \u003cem\u003eV \u003c/em\u003e= 25 mL, \u003cem\u003et\u003c/em\u003e = 120 min, \u003cem\u003eT\u003c/em\u003e = 298 K), (c) Zeta potential values of Fe\u003csup\u003e0\u003c/sup\u003e/FHC with variation of pH, (d) Removal capacity and removal percentage of Se(IV) removal by Fe\u003csup\u003e0\u003c/sup\u003e/FHC (pH = 7.0, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e = 10 mg L\u003csup\u003e-1\u003c/sup\u003e, \u003cem\u003eV\u003c/em\u003e = 25 mL, \u003cem\u003et\u003c/em\u003e = 120 min, \u003cem\u003eT \u003c/em\u003e= 298 K).\u003c/p\u003e","description":"","filename":"image5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/b2d51a2a653243bf4dbffe9d.jpeg"},{"id":93616170,"identity":"1d7f44c7-49a1-4e5d-b5cb-456162d0c327","added_by":"auto","created_at":"2025-10-15 16:55:29","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":187200,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Effect of contact time on Se(IV) removal by Fe\u003csup\u003e0\u003c/sup\u003e/FHC (pH = 7.0, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e = 10 mg L\u003csup\u003e-1\u003c/sup\u003e, \u003cem\u003eM\u003c/em\u003e = 10 mg, \u003cem\u003eV\u003c/em\u003e = 50 mL), (b) Pseudo-first-order kinetic plots for Se(IV) adsorption on Fe\u003csup\u003e0\u003c/sup\u003e/FHC, (c) Pseudo-second-order kinetic plots for Se(IV) adsorption on Fe\u003csup\u003e0\u003c/sup\u003e/FHC, (d) Effect of initial selenium concentration on Se(IV) removal by Fe\u003csup\u003e0\u003c/sup\u003e/FHC (pH = 7.0, t= 40 min, \u003cem\u003eM\u003c/em\u003e = 10 mg, \u003cem\u003eV\u003c/em\u003e = 50 mL).\u003c/p\u003e","description":"","filename":"image6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/fe44fd5f2b404153930ce65e.jpeg"},{"id":93615485,"identity":"342cce56-9c71-48c0-a8fb-1d47b6438f2d","added_by":"auto","created_at":"2025-10-15 16:47:29","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":243670,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Linear relationship between ln(C\u003csub\u003ee\u003c/sub\u003e/Q\u003csub\u003ee\u003c/sub\u003e) and C\u003csub\u003ee\u003c/sub\u003e for Se(IV) removal on Fe\u003csup\u003e0\u003c/sup\u003e/FHC, (b) Linear relationship between lnQ\u003csub\u003ee\u003c/sub\u003e and lnC\u003csub\u003ee\u003c/sub\u003e for Se(IV) removal on Fe\u003csup\u003e0\u003c/sup\u003e/FHC, (c) Effect of temperature on Se(IV) removal by Fe\u003csup\u003e0\u003c/sup\u003e/FHC (pH = 7.0, \u003cem\u003eM\u003c/em\u003e = 10 mg, \u003cem\u003eV\u003c/em\u003e = 50 mL, t= 40 min), (d) Linear relationship between lnK\u003csub\u003ed\u003c/sub\u003e and 1/T for Se(IV) removal on Fe\u003csup\u003e0\u003c/sup\u003e/FHC.\u003c/p\u003e","description":"","filename":"image7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/f31e3b4bcf6826428ccd20ee.jpeg"},{"id":93615489,"identity":"62172f57-88a0-404a-ad13-997d90aa14c0","added_by":"auto","created_at":"2025-10-15 16:47:29","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":209127,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Effect of cations on the removal of Se(IV) by Fe\u003csup\u003e0\u003c/sup\u003e/FHC at different pH values (t= 40 min, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e= 10 mg L\u003csup\u003e-1\u003c/sup\u003e, \u003cem\u003eM\u003c/em\u003e= 10 mg, \u003cem\u003eV\u003c/em\u003e= 50 mL, T= 298 K), (b) Effect of cations on the removal of Se(IV) by Fe\u003csup\u003e0\u003c/sup\u003e/FHC at different times (pH= 7.0, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e= 10 mg L\u003csup\u003e-1\u003c/sup\u003e, \u003cem\u003eM\u003c/em\u003e= 10 mg, \u003cem\u003eV\u003c/em\u003e= 50 mL, T= 298 K), (c) Effect of anions on the removal of Se(IV) by Fe\u003csup\u003e0\u003c/sup\u003e/FHC at different pH values (t= 40 min, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e= 10 mg L\u003csup\u003e-1\u003c/sup\u003e, \u003cem\u003eM\u003c/em\u003e= 10 mg, \u003cem\u003eV\u003c/em\u003e= 50 mL, T= 298 K), (d) Effects of anions on the removal of Se(IV) by Fe\u003csup\u003e0\u003c/sup\u003e/FHC under different time conditions (pH= 7.0, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e= 10 mg L\u003csup\u003e-1\u003c/sup\u003e, \u003cem\u003eM\u003c/em\u003e= 10 mg, \u003cem\u003eV\u003c/em\u003e= 50 mL, T= 298 K).\u003c/p\u003e","description":"","filename":"image8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/7553ea4a2abba6a226e4233c.jpeg"},{"id":93615494,"identity":"732750b1-f6b4-4d5c-aaee-14fe19a7f3ff","added_by":"auto","created_at":"2025-10-15 16:47:29","extension":"jpeg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":442777,"visible":true,"origin":"","legend":"\u003cp\u003e(a) FT-IR scan of Fe\u003csup\u003e0\u003c/sup\u003e/FHC and Fe\u003csup\u003e0\u003c/sup\u003e/FHC-Se(IV), (b) XPS full scan images of Fe\u003csup\u003e0\u003c/sup\u003e/FHC and Fe\u003csup\u003e0\u003c/sup\u003e/FHC-Se(IV), (c) Se(3d)/Fe(3p) high-resolution spectrum after adsorption, (d) High-resolution spectra of Fe 2p before and after adsorption, (e) High-resolution spectra of C 1s before and after adsorption, (e) High-resolution spectra of O 1s before and after adsorption.\u003c/p\u003e","description":"","filename":"image9.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/cedb5c364d766ee9814d7852.jpeg"},{"id":93616987,"identity":"b95f061b-27cb-4725-b0de-a518480cb20b","added_by":"auto","created_at":"2025-10-15 17:03:29","extension":"jpeg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":736969,"visible":true,"origin":"","legend":"\u003cp\u003eMechanism diagram of Fe\u003csup\u003e0\u003c/sup\u003e/FHC adsorption of Se(IV).\u003c/p\u003e","description":"","filename":"image10.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/2767c56a1a19fb4ac5670643.jpeg"},{"id":102910198,"identity":"deef3dd4-6b9e-4aa3-a149-3a88b924f73e","added_by":"auto","created_at":"2026-02-18 09:57:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5299951,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/4832fa8e-2d9c-4c38-9a33-54106f70dd4a.pdf"},{"id":93616986,"identity":"cd747dd6-5d79-42b2-9cd4-57d1c201a9f9","added_by":"auto","created_at":"2025-10-15 17:03:29","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":23122,"visible":true,"origin":"","legend":"","description":"","filename":"SupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/43d5c5979a237f834b56eb76.docx"},{"id":93615492,"identity":"1ba51b78-6cbb-413e-a063-a5e620187934","added_by":"auto","created_at":"2025-10-15 16:47:29","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":1820476,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstractforreview.docx","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/bb81a49c6b43e2378d7a19c0.docx"},{"id":93615483,"identity":"f939188e-ae6e-4640-aed7-c66099c3f85b","added_by":"auto","created_at":"2025-10-15 16:47:29","extension":"docx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":13388,"visible":true,"origin":"","legend":"","description":"","filename":"Highlights.docx","url":"https://assets-eu.researchsquare.com/files/rs-7734481/v1/8d7f5ab07eebd50c35c93fe6.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Comprehensive Performance Assessment and Mechanistic Insights into Zero-Valent Iron/Ferrous Hydroxide Complex Composites for Optimized Se(IV) Sequestration in Aqueous Media","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eWith nuclear energy developing rapidly, safe radioactive waste disposal is an urgent critical issue [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. High-level radioactive waste repositories use a multi-barrier system for long-term isolation, yet ensuring their long-term stability under complex geological conditions is tough [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Selenium is redox-sensitive with five natural oxidation states: Se(-II), Se(-I), Se(0), Se(IV), and Se(VI). Se(IV) is the most toxic and bioavailable, ten times more hazardous than Se(VI) [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. It has a long half-life (6.5\u0026times;10⁴ a) and high mobility [\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Corrosion or aging of storage tanks due to high temperature, pressure, radiation, or crustal stress can cause Se(IV) to dissolve into groundwater and spread, contaminating groundwater and threatening ecosystems and human health [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Se(IV) exists as negatively charged selenate (SeO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e), making it hard for near-field buffers, engineering barriers, or geological media to immobilize it [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Traditional adsorption methods are also ineffective in fixing Se(IV) [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eSelenium removal methods include chemical reduction, ion exchange, reverse osmosis technology, biological treatment, adsorption, and chemical precipitation, among others. Both domestic and international research has explored various experimental approaches in this field [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Among these, adsorption methods have gained widespread popularity due to their cost-effectiveness, operational flexibility, broad applicability, and the recyclability of materials [\u003cspan additionalcitationids=\"CR18\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. In recent years, with the increasingly severe problem of global water pollution, the research and development of efficient, convenient and green water selenium adsorbent has become a hot research topic in the intersection field of environmental science and material chemistry. Commonly employed adsorbents encompass natural materials [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], iron-based compounds [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], and carbon-based substances [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIron oxides and hydroxides, known for their strong affinity for selenium ions, serve as significant adsorbents in sediment, functioning as an essential element in immobilizing selenium within the environment [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Various iron-based adsorbents demonstrate high adsorption capacity across a broad pH range and in the presence of competing ions [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eFerrous Hydroxide Complex (FHC) has potent reducing capabilities. Consequently, FHC can effectively reduce high-valence radionuclides such as Selenium and Uninium [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. In natural geological environments rich in FHC, FHC can interact with ⁷⁹Se, which significantly reduces the migration capacity of \u003csup\u003e79\u003c/sup\u003eSe over a long geological time scale [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. Nevertheless, in the absence of externally introduced strong reductants, FHC exhibits limited reduction capacity and a slow reaction rate, rendering it incapable of fully reducing selenite to FeSe\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Consequently, the limitation of incomplete reduction constrains its wider utilization to a certain degree.\u003c/p\u003e\u003cp\u003eOwing to its high reactivity, substantial specific surface area, cost-effectiveness, potent reducing ability, and facile modification potential, nZVI has found extensive application in treating wastewater contaminated with selenium [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. Although nZVI can remove Se(IV) to a certain extent, its low mechanical strength, chemical instability, and aggregation behavior pose significant barriers to its widespread adoption [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn response to these constraints, researchers have adopted a strategy of anchoring nZVI onto supporting materials. This composite design enhances the nZVI's catalytic activity, promotes uniform dispersion, and ensures sustained stability under operational conditions [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. Dong studied the removal effect of selenium (IV) using nZVI and columnar bentonite (Al-bent). Al-bent could significantly promote the migration of selenium from the liquid phase to the iron surface, accelerated the reduction process, and thereby improved the removal efficiency [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe objective of this research was to boost the capacity of FHC in eliminating Se(IV) from water-based solutions. By employing FHC as the base material and incorporating zero-valent iron as an additive, Fe\u003csup\u003e0\u003c/sup\u003e/FHC was successfully prepared. The research showed that Fe\u003csup\u003e0\u003c/sup\u003e/FHC could be used as components in chemical barriers, efficiently preventing the movement of high-valence soluble isotopes like \u003csup\u003e79\u003c/sup\u003eSe in repositories designed.\u003c/p\u003e"},{"header":"2 Experimental","content":"\u003cp\u003eDetailed information regarding the characterization techniques and the approaches used for fitting experimental data can be found in the Supplemental Information (SI).\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Materials\u003c/h2\u003e\u003cp\u003eReagents such as FeCl\u003csub\u003e3\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO, FeSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;7H\u003csub\u003e2\u003c/sub\u003eO, C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e5\u003c/sub\u003eOH, KBH\u003csub\u003e4\u003c/sub\u003e and Na\u003csub\u003e2\u003c/sub\u003eSeO\u003csub\u003e3\u003c/sub\u003e were sourced from MACKLIN in China.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Preparation of Fe\u003csup\u003e0\u003c/sup\u003e\u003c/h2\u003e\u003cp\u003eA precise amount of FeCl\u003csub\u003e3\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO was measured out and then placed into a beaker. A mixed solvent was then prepared by gradually adding ethanol and deionized water with stirring. Potassium borohydride was gradually added drop by drop to the reaction mixture until the reaction reached completion. The obtained product underwent several rounds of washing to remove impurities, followed by vacuum drying at 80\u0026deg;C for 10 h to obtain zero-valent iron (Fe\u003csup\u003e0\u003c/sup\u003e) material.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003e4Fe+3BH+9HO→4Fe↓+3HBO+12H+6H↑ (1)\u003c/h3\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Preparation of FHC\u003c/h2\u003e\u003cp\u003eWe accurately weighed ferrous sulfate heptahydrate (chemical formula: FeSO₄\u0026middot;7H₂O) using an analytical balance and transferred it to a 250 mL pre-washed beaker. Following the molar ratio specified in the experimental protocol, we precisely pipetted 1 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e sodium hydroxide solution into the beaker and slowly added it dropwise while adjusting the reaction system's concentration with deionized water. Under continuous magnetic stirring, we ensured thorough mixing of reactants to achieve complete reaction. After completion, centrifugal separation was employed followed by multiple washes with deionized water until no impurity ions were detected in the eluates. Finally, the purified product was dried for 24 h at 60 ℃ in a vacuum drying oven, yielding high-purity ferrous hydroxide complex (abbreviated as FHC) [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eFe\u003csup\u003e2+\u003c/sup\u003e+OH\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026rarr;FeOH\u003csup\u003e+\u003c/sup\u003e (2)\u003c/p\u003e\u003cp\u003eFe\u003csup\u003e2+\u003c/sup\u003e+2OH\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026rarr;Fe(OH)\u003csub\u003e2\u003c/sub\u003e\u0026darr; (3)\u003c/p\u003e\u003cp\u003eFe(OH)\u003csub\u003e2\u003c/sub\u003e +OH\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026rarr;Fe(OH)\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026darr; (4)\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Synthesis of Fe\u003csup\u003e0\u003c/sup\u003e/FHC\u003c/h2\u003e\u003cp\u003eFe\u003csup\u003e0\u003c/sup\u003e and FHC were measured out in accordance with a predetermined mass ratio, and Fe\u003csup\u003e0\u003c/sup\u003e/FHC composites were subsequently synthesized. The detailed preparation steps are illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Results and discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Characterizations\u003c/h2\u003e\u003cp\u003eScanning electron microscopy (SEM) was employed to analyze the microstructures of zero-valent iron (Fe\u003csup\u003e0\u003c/sup\u003e), ferrous hydroxide complex (FHC), composite materials (Fe\u003csup\u003e0\u003c/sup\u003e/FHC), and selenium (Se)-adsorbed products (Fe\u003csup\u003e0\u003c/sup\u003e/FHC-Se). The outcomes are depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a-d). Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a) reveals that Fe\u003csup\u003e0\u003c/sup\u003e possessed a rod-like structure but suffered from notable agglomeration [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. In contrast, FHC exhibited a sheeted morphology as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b) [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(c) demonstrates that the composite material's surface was adorned with nanoscale FHC pine needles. This observation confirms the formation of the Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite. Furthermore, the composite material exhibited excellent dispersibility, suggesting that the incorporation of FHC effectively hinders Fe\u003csup\u003e0\u003c/sup\u003e aggregation and significantly enhances the composite's specific surface area. The scanning electron microscopy (SEM) analysis of Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d) demonstrated that the successful adsorption of selenium on the material surface triggered significant morphological changes. The originally homogeneous material particles transformed into fragmented structures with varying sizes, while interparticle aggregation became markedly more pronounced compared to the conditions before selenium adsorption. These structural modifications-particularly the increased fragmentation and aggregation-likely result from selenium's chemical adsorption process on the material surface. Our hypothesis suggests that selenium's adsorption altered the material's surface chemistry, which in turn affected interparticle interactions and consequently induced this substantial microstructural transformation. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e reveals that selenium (Se) accounts for up to 6.12% by weight in the adsorbed product. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e illustrates the presence of Fe, C, S, O, and Se.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eScanning results of element content in Fe\u003csup\u003e0\u003c/sup\u003e/FHC-Se\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eElement\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eLine type\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eWeight percentage/%\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eAtomic percentage/%\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eK linear system\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e5.60\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e13.72\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eO\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eK linear system\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e28.43\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e52.32\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eS\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eK linear system\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.29\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFe\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eL linear system\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e59.54\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e31.39\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSe\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eL linear system\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e6.12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2.28\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTotal\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e100.00\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e100.00\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe infrared spectroscopy results shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a) clearly demonstrate the significant changes in the functional groups of the material. Detailed spectral analysis reveals a -OH peak around 3430 cm⁻\u0026sup1; for both ferrous hydroxide complex (FHC) and Fe\u003csup\u003e0\u003c/sup\u003e/FHC composites. This characteristic peak likely results from the combined effects of stretching and bending vibrations of surface -OH functional groups [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e], indicating that a specific quantity of water molecules have been adsorbed on the adsorbent surface. This phenomenon also provides a reasonable explanation for the C\u0026thinsp;=\u0026thinsp;C double bond peak observed at 1630 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. A weak absorption peak at 1110 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to the mode of C-O bonds in FHC surface -OH groups, as reported in the literature [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Notably, compared with pure Fe\u003csup\u003e0\u003c/sup\u003e samples, Fe\u003csup\u003e0\u003c/sup\u003e/FHC composites exhibit two distinct Fe-O bond stretching vibration peaks at 1400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 562 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, in the high-frequency and low-frequency regions, respectively [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. These significant spectral shifts and characteristic peak positions in FT-IR spectra fully confirm the successful preparation of Fe\u003csup\u003e0\u003c/sup\u003e/FHC composites, supported by solid experimental data.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b), Fe\u003csup\u003e0\u003c/sup\u003e/FHC exhibited weaker characteristic peaks compared to FHC. This phenomenon likely resulted from the interactions between Fe\u003csup\u003e0\u003c/sup\u003e and FHC, which enhanced the dispersion of Fe\u003csup\u003e0\u003c/sup\u003e and thereby decreased the crystallinity and stability. Importantly, all peak positions matched standard FHC card values [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. Additionally, for Fe\u003csup\u003e0\u003c/sup\u003e and Fe\u003csup\u003e0\u003c/sup\u003e/FHC, at a 2θ value of 44.9\u0026deg;, characteristic peaks corresponding are clearly discernible. Furthermore, the XRD spectra of all samples did not reveal any notable impurity peaks, indicating that the synthesized samples possessed a high degree of purity. In essence, XRD analysis validated the successful preparation of Fe\u003csup\u003e0\u003c/sup\u003e/FHC.\u003c/p\u003e\u003cp\u003eThe X-ray photoelectron spectroscopy (XPS) analysis in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c) reveals distinct characteristic peaks at specific energy positions of 285.08 eV, 531.08 eV, and 711.08 eV for the Fe\u003csup\u003e0\u003c/sup\u003e, FHC, and Fe\u003csup\u003e0\u003c/sup\u003e/FHC samples respectively. The C 1s peak at 285.08 eV originates from various carbon species adsorbed on the sample surface, including organic contaminants or surface carbon layers. The O 1s peak at 531.08 eV unequivocally indicates oxygen presence, which is consistent with both the hydroxyl functional groups in FHC materials and the iron oxide-hydroxyl coexistence in Fe\u003csup\u003e0\u003c/sup\u003e/FHC composites. The Fe 2p\u003csub\u003e3/2\u003c/sub\u003e peak at 711.08 eV directly confirms iron's oxidation state as +\u0026thinsp;3. Notably, the Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite exhibits significantly stronger Fe 2p\u003csub\u003e3/2\u003c/sub\u003e peaks compared to pure FHC material, demonstrating effective iron content enhancement through Fe\u003csup\u003e0\u003c/sup\u003e loading. These XPS results not only accurately identify the chemical states of carbon, oxygen, and iron but also reveal unique elemental distribution patterns and surface chemistry characteristics of Fe\u003csup\u003e0\u003c/sup\u003e/FHC composites.\u003c/p\u003e\u003cp\u003eFrom Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(d), the nitrogen adsorption isotherms measured experimentally exhibit typical characteristics of Class IV curves, which closely align with the adsorption behavior of mesoporous materials as defined in the IUPAC classification standards. Specifically, the isotherms demonstrate single-molecule layer adsorption characteristics in the low-pressure region, followed by significant capillary condensation as pressure increases, and the formation of hysteresis loops in the medium-high pressure range. These features conclusively confirm the material's well - ordered mesoporous structure [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. The loading of Fe\u003csup\u003e0\u003c/sup\u003e onto FHC resulted in a significant augmentation of its specific surface area, yielding a hierarchical pore structure with enhanced mass transfer properties. This modification was particularly advantageous for Se(IV) adsorption, as it promotes rapid diffusion and strong interaction with adsorbent sites.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(e) illustrates that FHC underwent a notable mass reduction as the temperature rose. Particularly at 60 ℃ to 280 ℃, FHC experienced a 10.20% weight loss attributed to carbon disintegration within Fe\u003csup\u003e0\u003c/sup\u003e/FHC [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. Conversely, Fe\u003csup\u003e0\u003c/sup\u003e/FHC only lost 4.53% of its weight from 0 ℃ to 800 ℃, demonstrating that the incorporation of zero-valent iron enhanced the thermal stability of the Fe\u003csup\u003e0\u003c/sup\u003e/FHC composites. This characteristic enabled Fe\u003csup\u003e0\u003c/sup\u003e/FHC composites to exhibit exceptional thermal stability, making them particularly promising for high-temperature catalytic or adsorption processes.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Removal performance studies\u003c/h2\u003e\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\u003ch2\u003e3.2.1 Effect of Fe\u003csup\u003e0\u003c/sup\u003e proportions\u003c/h2\u003e\u003cp\u003eA series of Fe\u003csup\u003e0\u003c/sup\u003e/FHC were synthesized with tailored Fe\u003csup\u003e0\u003c/sup\u003e loading amounts to achieve different compositional ratios. The relationship between composite ratio and Se(IV) removal efficiency was systematically evaluated, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a). Statistical analysis confirmed that Fe\u003csup\u003e0\u003c/sup\u003e loading amount serves as a critical determinant of Se(IV) removal performance. As the mass fraction of elemental iron (Fe\u003csup\u003e0\u003c/sup\u003e) in the composite increased from 15% to 55%, the removal efficiency of Se(IV) initially improved and subsequently declined. Experimental findings reveal that alterations in the amount of Fe\u003csup\u003e0\u003c/sup\u003e loaded have a profound impact on the Se(IV) removal capability of the Fe\u003csup\u003e0\u003c/sup\u003e/FHC system. When an excessive quantity of Fe\u003csup\u003e0\u003c/sup\u003e is loaded, there is an overabundance of active sites, which in turn decreases both the specific surface area of the material and its reaction activity. On the contrary, if the Fe\u003csup\u003e0\u003c/sup\u003e loading is insufficient, there won't be enough active sites available for the reactions to occur. In either case, the Se(IV) removal performance of the Fe\u003csup\u003e0\u003c/sup\u003e/FHC system is significantly compromised. Hence, it is crucial to keep the Fe\u003csup\u003e0\u003c/sup\u003e loading within the optimal range of 35% to achieve efficient Se(IV) removal in real-world applications.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\u003ch2\u003e3.2.2 Effect of pH\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b) demonstrated that pH strongly influenced Se(IV) removal efficiency. While pure Fe\u003csup\u003e0\u003c/sup\u003e showed a consistent decline in Se(IV) removal with increasing pH, both FHC and Fe\u003csup\u003e0\u003c/sup\u003e/FHC composites exhibited an initial increase followed by a decrease in removal efficiency across the pH range tested. In general, the Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite material demonstrated superior Se(IV) removal performance, reaching 94.28% (pH\u0026thinsp;=\u0026thinsp;7.0). As a critical parameter for assessing solution environments, pH significantly influences material surface chemistry and fundamentally determines the existence forms and chemical behavior patterns of selenium in aqueous solutions. Detailed analysis reveals that under strongly acidic conditions (pH\u0026thinsp;\u0026lt;\u0026thinsp;2.0), selenium primarily exists stably as selenite molecules (H\u003csub\u003e2\u003c/sub\u003eSeO\u003csub\u003e3\u003c/sub\u003e). When pH rises to the neutral-to-weakly alkaline range of 2.64\u0026ndash;8.36, over 95% of selenium converts into hydrogen selenite ions (HSeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e). As pH continues to rise beyond 8.0 into alkaline conditions, selenium predominantly transforms into selenate ions (SeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e). Test data in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c) demonstrate that the Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite material has an isoelectric point of 6.9, which plays a decisive role in its surface charge characteristics. Under acidic conditions (pH\u0026thinsp;\u0026lt;\u0026thinsp;6.9), the Fe\u003csup\u003e0\u003c/sup\u003e/FHC surface exhibits a positive charge. As pH increases from 3.0 to 6.9, the proportion of HSeO₃\u003csup\u003e\u0026minus;\u003c/sup\u003e in solution progressively rises. The positive surface charge creates strong electrostatic attraction with negatively charged HSeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e ions, leading to a significant enhancement in Fe\u003csup\u003e0\u003c/sup\u003e/FHC's removal efficiency for HSeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. This optimal removal effect is achieved at the isoelectric point pH 6.9. However, when the pH value continues to rise beyond 6.9 to 8.0, the surface charge properties of Fe\u003csup\u003e0\u003c/sup\u003e/FHC reverse from positive to negative, and a static repulsion is generated with the similarly negatively charged HSeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, resulting in a gradual decrease in the removal efficiency with the increase of pH value.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section3\"\u003e\u003ch2\u003e3.2.3 Solid-liquid ratio\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(d) reveals that at low solid-to-liquid ratios, Fe\u003csup\u003e0\u003c/sup\u003e/FHC exhibited higher Se(IV) adsorption capacity per unit mass, but the limited total adsorbent quantity resulted in relatively low overall Se(IV) removal efficiency. With a rise in the solid-to-liquid ratio, the number of accessible adsorption sites increased, thereby leading to an elevated removal efficiency. In experimental studies, when the solid-liquid ratio was optimized to 0.20 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite demonstrated exceptional selenium(IV) removal performance with an efficiency of 92.25% and an adsorption capacity of 70.73 mg g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. This result indicates that the material achieves high pollutant removal efficiency at low dosage. Considering the need to balance treatment effectiveness with economic costs in practical applications while maintaining both high removal rates and good adsorption properties, comprehensive analysis confirmed 0.20 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e as the optimal solid-liquid ratio for this experiment. Therefore, in subsequent experimental series, we uniformly adopted this optimized ratio to ensure reliable and comparable experimental results.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section3\"\u003e\u003ch2\u003e3.2.4 Reaction time and kinetics\u003c/h2\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a), experiments at different temperatures (293 K, 298 K, 303 K, and 308 K) demonstrated that elevated ambient temperature significantly affects the Se(IV) removal performance of Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite materials. The data revealed a slight upward trend in Se(IV) removal efficiency as temperature increased from 293 K to 308 K. The removal rate followed consistent temporal patterns across all temperatures: rapid removal occurred during the initial 0\u0026ndash;10 min, followed by a slower growth phase between 10\u0026ndash;40 min, with adsorption equilibrium achieved around 40 min. Therefore, this study established 40 min as the standard reaction time for subsequent experiments. Analysis supported by formulas S3 and S4 in supplementary information led to the development of a kinetic model for Fe\u003csup\u003e0\u003c/sup\u003e/FHC's Se(IV) adsorption process. Fitted results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b) and 6(c), with parameters summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Model evaluation confirmed excellent fitting performance of the quasi-second-order kinetic model, indicating that chemical adsorption mechanisms dominate the process. During the initial stage (0\u0026ndash;5 min), chemical reaction rates determined removal efficiency, with interfacial interactions influencing adsorption dynamics [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. Kinetic analysis further revealed an increase in the quasi-second-order kinetic model's rate constant k\u003csub\u003e2\u003c/sub\u003e when temperature rose from 293 K to 308 K, validating the temperature-promoting effect on adsorption kinetics. The results confirmed that higher temperatures promoted Se(IV) adsorption by Fe\u003csup\u003e0\u003c/sup\u003e/FHC, supporting the previously observed slight efficiency gain with temperature rise and validating temperature's role.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eKinetic model fitting parameters for Se(IV) removal by Fe\u003csup\u003e0\u003c/sup\u003e/FHC\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eTemperature/K\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u003cp\u003eQuasi-first order dynamics\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e\u003cp\u003eQuasi-second order dynamics\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eQ\u003csub\u003ee\u003c/sub\u003e/(mg g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cem\u003eK\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e/(L min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eQ\u003csub\u003ee\u003c/sub\u003e/(mg g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u003cem\u003eK\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e/(g mg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e293\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e37.0831\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.0759\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.9358\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e76.2777\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.0026\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.9975\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e298\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e22.1849\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.0744\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.8876\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e73.9098\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.0045\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.9982\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e303\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e12.8736\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.0591\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.7200\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e72.4113\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.0066\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.9985\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e308\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e9.7961\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.0517\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.7226\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e71.2251\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e0.0100\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.9994\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section3\"\u003e\u003ch2\u003e3.2.5 Initial selenium concentration and isotherms\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(d) reveals that across the tested temperature range, the Se(IV) adsorption capacity of Fe\u003csup\u003e0\u003c/sup\u003e/FHC composites exhibited a positive correlation with initial Se(IV) concentration, demonstrating consistent concentration-dependent behavior at all experimental temperatures. At initial Se(IV) concentrations above 100 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the adsorption capacity attained 201.62 mg g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. This study applied the formulas S5 and S6 from the system support information to model and analyze the isothermal adsorption behavior of Se(IV) on Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite materials. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a) and 7(b), the fitted curves closely matched the experimental data, confirming the applicability of the selected model. Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e lists the fitting parameters indicating that Se(IV) primarily adsorbed as a monolayer on the Fe\u003csup\u003e0\u003c/sup\u003e/FHC surface. Temperature-dependent adsorption studies revealed that the equilibrium adsorption capacity of Fe\u003csup\u003e0\u003c/sup\u003e/FHC for Se(IV) increased continuously from 293 K to 308 K, suggesting an endothermic reaction where higher temperatures enhance adsorption efficiency. Therefore, due to its simple synthesis process, low raw material costs, and excellent adsorption performance, Fe\u003csup\u003e0\u003c/sup\u003e/FHC shows significant application value and market potential as a novel.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eIsothermal adsorption model parameters for Fe\u003csup\u003e0\u003c/sup\u003e/FHC removal of Se(IV)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eTemperature/K\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u003cp\u003eLangmuir isotherm adsorption model\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e\u003cp\u003eFreundlich adsorption isotherm model\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eQ\u003csub\u003em\u003c/sub\u003e/(mg g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eK\u003csub\u003eL\u003c/sub\u003e/(L mg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1/n\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eK\u003csub\u003eF\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e293\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e139.6648\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.1027\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.9923\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0.1986\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e59.4199\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.8331\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e298\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e191.9386\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.1001\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.9908\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0.2307\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e66.6943\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.9254\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e303\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e235.8491\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.1076\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.9888\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0.2653\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e66.9048\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.9636\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e308\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e277.7778\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-0.0997\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.9791\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0.2972\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e68.7001\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.9371\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section3\"\u003e\u003ch2\u003e3.2.6 Effect of temperature and thermodynamics\u003c/h2\u003e\u003cp\u003eAs depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(c), the Se(IV) removal efficiency by the Fe\u003csup\u003e0\u003c/sup\u003e/FHC exhibited a slight upward trend with temperature elevation from 293 K to 308 K across all tested initial concentrations. Furthermore, the removal rate demonstrated a concentration-dependent pattern, initially increasing with higher initial Se(IV) levels before reaching an asymptotic plateau. This could be attributed to the ample availability of active sites at lower initial concentrations, allowing Se(IV) ions to be readily adsorbed. However, when the initial concentration of selenium(IV) in the solution gradually increases and reaches a specific threshold, the limited active sites on the adsorbent surface will be progressively occupied, eventually approaching saturation. At this saturated state, even if the concentration of selenium(IV) in the solution continues to increase, its improvement effect on the overall removal rate becomes negligible. To gain deeper insights into the essential characteristics of this adsorption process, we conducted systematic and precise fitting analysis of the thermodynamic curves of Fe\u003csup\u003e0\u003c/sup\u003e/FHC adsorbing Se(IV) based on the thermodynamic formulas S7, S8, and S9 provided in SI. From Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(d), the experimental data showed good consistency with the theoretical model. The detailed fitting parameters listed in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e reveal that the value of ΔH\u003csup\u003e0\u003c/sup\u003e is positive, clearly indicating that this adsorption process is an endothermic reaction, which is more favorable under higher temperatures. This is because the additional thermal energy from temperature increase effectively enhances the interaction between selenium ions and the active sites on the Fe\u003csup\u003e0\u003c/sup\u003e/FHC surface, significantly improving adsorption efficiency. The microscopic mechanism may be attributed to the arrangement at the solid - liquid interface during adsorption, where such structural changes paradoxically facilitate selenium ion adsorption. More crucially, all experimentally measured ΔG\u003csup\u003e0\u003c/sup\u003e values were negative, which thermodynamically confirms a spontaneous process. Notably, as temperature increases, ΔG\u003csup\u003e0\u003c/sup\u003e showed a significant upward trend, indicating that the spontaneous driving force of this adsorption process becomes further enhanced under higher temperature conditions [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. In conclusion, the thermodynamic analysis produced results that consistently aligned with the predictions of the isotherm adsorption model, thereby validating the theoretical framework across all experimental conditions.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eThermodynamic fitting parameters for Se(IV) removal by Fe\u003csup\u003e0\u003c/sup\u003e/FHC\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eC\u003csub\u003e0\u003c/sub\u003e (mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eΔ\u003cem\u003eH\u003c/em\u003e\u003csup\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sup\u003e/ (kJ mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eΔ\u003cem\u003eS\u003c/em\u003e\u003csup\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sup\u003e/(J mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"4\" nameend=\"c7\" namest=\"c4\"\u003e\u003cp\u003eΔ\u003cem\u003eG\u003c/em\u003e\u003csup\u003e0\u003c/sup\u003e/ (kJ mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003e293 K\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003e298 K\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003e303 K\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003e308 K\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e89.6997\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e344.3657\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e-11.1706\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e-13.0536\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e-14.4621\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e-16.4425\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e77.4563\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e298.9793\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e-9.9691\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e-11.8826\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e-13.1946\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e-14.5012\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e66.3303\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e254.7722\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e-8.2823\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e-9.6459\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e-10.8679\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e-12.1187\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e50.6747\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e195.0741\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e-6.1167\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e-7.9069\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e-8.6754\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e-9.0809\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e45.7458\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e172.5910\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e-4.5853\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e-6.0082\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e-6.6472\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e-7.2301\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e41.9668\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e157.0044\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e-3.9800\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e-4.9441\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e-5.5279\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e-6.4000\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e40\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e38.3025\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e138.8454\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e-2.3112\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e-3.1762\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e-3.7737\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e-4.4212\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e34.2964\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e122.7466\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e-1.5689\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e-2.4091\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e-2.9522\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e-3.4254\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e60\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e31.9891\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e112.9006\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e-0.9937\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e-1.7748\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e-2.2840\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e-2.6974\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e80\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e89.6997\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e344.3657\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e-11.1706\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e-13.0536\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e-14.4621\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e-16.4425\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e100\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e77.4563\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e298.9793\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e-9.9691\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e-11.8826\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e-13.1946\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e-14.5012\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section3\"\u003e\u003ch2\u003e3.2.7 Effect of coexisting cations\u003c/h2\u003e\u003cp\u003eWe systematically investigated the mechanisms by which three common coexisting cations (K⁺, Na⁺, Ca\u0026sup2;⁺) affect the performance of Fe\u003csup\u003e0\u003c/sup\u003e/FHC composites in removing Se(IV) from aqueous solutions. The results are visually presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a) (adsorption kinetics curves) and 8(b) (removal efficiency). Analysis of Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a) clearly demonstrates that all three cations significantly enhance the Se(IV) removal efficiency of Fe\u003csup\u003e0\u003c/sup\u003e/FHC composites, though their effects show distinct differences. Specifically, Ca\u0026sup2;⁺ (divalent) exhibits a promoting effect, while Na⁺ and K⁺ (monovalent) demonstrate weaker enhancing effects. Within the pH range of 3.0\u0026ndash;7.0, the predominant form of Se(IV) is HSeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e anion. The cations form electrostatic bridging interactions with HSeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e on the Fe\u003csup\u003e0\u003c/sup\u003e/FHC surface through their positive charges. This charge neutralization effect not only strengthens surface adsorption but also facilitates subsequent reduction reactions. Notably, higher - valence cations (e.g., Ca\u0026sup2;⁺ compared to Na⁺ and K⁺) generate stronger electrostatic attraction, significantly accelerating HSeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e migration to the material surface. This optimized interfacial reaction kinetics enhances overall Se(IV) removal efficiency. Kinetic data in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(b) further indicate that cation addition dramatically shortens the adsorption equilibrium time from several hours to approximately 30 min. Of particular significance, the Ca\u0026sup2;⁺ system demonstrates the fastest reaction rate and achieves the highest removal efficiency, which conclusively confirms the cationic promotion effect on Se(IV) removal. Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e provides detailed pH change patterns across all experimental groups, showing consistent pH increases with cation addition, reaching a maximum of 8.62.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003epH of the solution before and after adding cations\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003epH before adsorption\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e\u003cp\u003epH after adsorption\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBlank\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eK\u003csup\u003e+\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eNa\u003csup\u003e+\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eCa\u003csup\u003e2+\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e7.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e7.37\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e8.14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.06\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e8.24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e8.19\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e8.30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.25\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e4.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e8.36\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e8.35\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e8.38\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.35\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e4.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e8.38\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e8.48\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e8.44\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.42\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e5.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e8.55\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e8.54\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e8.52\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.62\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e5.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e8.52\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e8.51\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e8.46\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.60\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e6.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e8.51\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e8.35\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e8.34\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.53\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e6.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e8.45\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e8.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e8.29\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.52\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e7.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e8.42\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e8.16\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e8.18\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.44\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e7.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e7.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e7.37\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e8.14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.06\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec18\" class=\"Section3\"\u003e\u003ch2\u003e3.2.8 Effect of coexisting anions\u003c/h2\u003e\u003cp\u003ePrevalent coexisting anions such as F\u003csup\u003e\u0026minus;\u003c/sup\u003e, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e, and Humic Acid were chosen to investigate their impact. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(c), NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e notably hindered the removal of Se(IV). In acidic environments, both NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e displayed some oxidizing characteristics, thereby competing with the reduction of HSeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. Experimental results demonstrate that coexisting anions exhibit significant competitive effects on the reduction of HSeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. Specifically, certain anions engage in intense competitive adsorption with HSeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e at reaction sites, which partially hinders the reduction process and significantly reduces the removal efficiency of tetravalent selenium (Se(IV)). Comparative experiments reveal that F\u003csup\u003e\u0026minus;\u003c/sup\u003e, CO₃\u0026sup2;\u003csup\u003e\u0026minus;\u003c/sup\u003e, and humic acid show negligible competitive adsorption with HSeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, rendering their competitive influence negligible. The kinetic curves in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(d) clearly indicate that the presence of competing anions leads to reduced reaction rates and decreased removal efficiency of HSeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e in Fe\u003csup\u003e0\u003c/sup\u003e/FHC composites, particularly a more pronounced decrease in reaction rate. This phenomenon strongly demonstrates that competitive anions inhibit the reduction of HSeO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e following the addition of anions like F\u003csup\u003e\u0026minus;\u003c/sup\u003e, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e, and Humic Acid. Notably, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e had particularly pronounced impacts on removal. Additionally, Table\u0026nbsp;\u003cspan refid=\"Tab6\" class=\"InternalRef\"\u003e6\u003c/span\u003e reveals that after adsorption experiments involving these anions, the solution's pH could increase to 8.89.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab6\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 6\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003epH of solution before and after adding anions\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003epH before adsorption\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"6\" nameend=\"c7\" namest=\"c2\"\u003e\u003cp\u003epH after adsorption\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBlank\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eF\u003csup\u003e\u0026minus;\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003ePO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eHumic Acid\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e5.58\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e5.49\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e4.35\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e5.64\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e4.51\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e5.35\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e6.15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e5.76\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e4.64\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e5.95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e4.94\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e6.13\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e4.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e6.34\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e6.54\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e4.93\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e6.21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e5.36\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e6.41\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e4.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e6.81\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e7.22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e5.36\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e6.64\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e5.52\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e7.03\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e5.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e7.29\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e7.36\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e5.50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e6.76\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e5.69\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e7.13\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e5.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e7.30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e7.41\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e5.61\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e7.24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e6.21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e7.31\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e6.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e7.42\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e7.80\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e5.70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e7.53\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e6.60\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e7.43\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e6.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e7.87\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e8.27\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e6.17\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e7.90\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e6.96\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e7.69\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e7.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e8.31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e8.34\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e6.78\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.18\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e6.92\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e8.12\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e7.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e8.83\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e8.59\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e7.16\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e8.37\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e7.17\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e8.89\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec19\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Removal mechanism\u003c/h2\u003e\u003cp\u003eThrough comparison with the original conditions without adsorption treatment, the results shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(a) clearly reveal that the functional group characteristic peaks on the surface of Fe\u003csup\u003e0\u003c/sup\u003e/FHC underwent significant changes after adsorption treatment. Specifically, the intensity of characteristic peaks representing hydroxyl groups (-OH), carbon-carbon double bonds (C\u0026thinsp;=\u0026thinsp;C), carbon-oxygen bonds (C-O), and iron-oxygen bonds (Fe-O) all increased markedly. These peak intensities demonstrate that during the adsorption process, hydroxyl groups, various oxygen-iron-containing functional groups actively participated in complexation reactions with selenium. Notably, the characteristic peak of Se-O bonds was detected for the first time at the 882 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e wavenumber, providing direct and conclusive experimental evidence for the successful adsorption of Se(IV) by Fe\u003csup\u003e0\u003c/sup\u003e/FHC [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. Notably, the post-adsorption spectrum exhibited no discernible Se-Se bond vibration peak, providing additional evidence that stable chemical interactions-rather than mere physical adsorption-occurred.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs shown in the XPS analysis results in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(b), a distinct Se 3d characteristic peak is clearly observed at the binding energy position of 56.08 eV, which holds significant characterization value. By comparing with standard spectral database, this feature corresponds to the 3d orbital electron binding energy of selenium. This experimental result fully demonstrates that the Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite material successfully captured and immobilized selenium from the solution during adsorption experiments. This discovery directly validates the excellent adsorption performance of the Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite material for selenium pollutants, providing reliable experimental evidence for subsequent research on its application in heavy metal pollution control [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eTo precisely elucidate the chemical state distributions and spectral characteristics of Se 3d and Fe 3p orbitals, this study employed advanced peak decomposition fitting techniques to carry out meticulous deconvolution of the XPS high-resolution spectrum (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(c)). The analysis revealed that the prominent peak at 55.2 eV originated from the superposition of Se(0) 3d\u003csub\u003e5/2\u003c/sub\u003e (55.3 eV) and Fe(II)-O 3p\u003csub\u003e3/2\u003c/sub\u003e (55.9 eV). Similarly, the shoulder peak at 58.3 eV was predominantly attributed to Fe(III)-O 3p\u003csub\u003e3/2\u003c/sub\u003e (58.0 eV) and Se(IV) 3d\u003csub\u003e5/2\u003c/sub\u003e (58.7 eV), where its broadened profile highlighted the heterogeneous distribution of surface oxidation states [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe X-ray photoelectron spectroscopy (XPS) analysis in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(d) reveals five characteristic peaks in the Fe 2p orbital before selenium adsorption begins. These peaks correspond to binding energies of 710.1 eV, 711.5 eV, 718.3 eV, 724.0 eV, and 729.0 eV. Following selenium adsorption, all peak positions shifted to 710.2 eV, 712.0 eV, 718.1 eV, 724.3 eV, and 730.7 eV respectively. Notably, the relative intensities of these peaks also changed significantly beyond their positional shifts. Detailed analysis shows that the peaks at 711.5 eV and 724.0 eV correspond to the spin-orbital splitting peaks of Fe 2p\u003csub\u003e3/2\u003c/sub\u003e and Fe 2p\u003csub\u003e1/2\u003c/sub\u003e electrons, which exhibited particularly pronounced variations during adsorption. The systematic shift in binding energy positions and marked intensity changes strongly indicate that iron plays a crucial role in selenium complexation and actively participates in surface chemical reactions [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. These findings provide essential experimental evidence for further investigation into the iron-selenium interaction mechanism.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(e) provides a detailed analysis of the high-resolution C 1s X-ray photoelectron spectroscopy (XPS) profiles on the sample surface. Before adsorption, the results of Gaussian-Lorentz peak fitting revealed four distinct C 1s peaks: the 284.3 eV peak from sp\u0026sup2; hybridized C-C bonds, 285.0 eV from C-O single bonds, 288.2 eV from C\u0026thinsp;=\u0026thinsp;O double bonds, and 290.8 eV from O-C\u0026thinsp;=\u0026thinsp;O carboxyl groups. After selenium(IV) ion adsorption, these peaks shifted by varying amounts: C-C to 284.4 eV, C-O to 285.2 eV, C\u0026thinsp;=\u0026thinsp;O to 288.3 eV, and O-C\u0026thinsp;=\u0026thinsp;O to 290.5 eV. Notably, the C\u0026thinsp;=\u0026thinsp;O bond showed significant intensity reduction, strongly indicating its crucial role in the selenium adsorption process. In contrast, while C-C, C-O, and O-C\u0026thinsp;=\u0026thinsp;O bonds also shifted slightly, their effects were much less pronounced than those of the C\u0026thinsp;=\u0026thinsp;O bond. This comparison demonstrates that these three chemical bonds played relatively minor roles in selenium adsorption, with their contributions being far less significant than that of the C\u0026thinsp;=\u0026thinsp;O bond.\u003c/p\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(f), X-ray photoelectron spectroscopy (XPS) was employed to characterize the material surface before initiating the adsorption reaction. The results revealed two distinct O 1s spectral peaks at binding energies of 529.5 eV and 530.8 eV. Specifically, the 529.5 eV peak corresponds to the chemical state of Fe-O bonds, while the 530.8 eV peak reflects the vibrational mode of H-O bonds. Following the adsorption process, both peaks shifted slightly: the Fe-O peak moved to 529.4 eV, and the H-O peak shifted to 530.5 eV. Notably, the Fe-O peak not only shifted in position but also showed a significant reduction in intensity and broadening in shape. This phenomenon strongly indicates that iron played a crucial role in removing selenium ions-with some iron atoms being oxidized into iron oxides or iron hydroxides. In contrast, the H-O peak exhibited a completely different trend, showing enhanced intensity and sharpened shape. These changes clearly demonstrate that during adsorption, water molecules interacted significantly with selenium ions, causing partial detachment and altering the chemical environment.\u003c/p\u003e\u003cp\u003eBased on FT-IR and XPS analyses, Se(IV) removal by Fe\u003csup\u003e0\u003c/sup\u003e/FHC occurred through two main mechanisms: (1) electrostatic attraction facilitating Se(IV) ion adsorption onto the material surface, and (2) complexation with surface functional groups including C\u0026thinsp;=\u0026thinsp;O, H-O and Fe-O bonds, which formed stable chemical structures. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e shows the potential complexing mechanism.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4 Conclusions","content":"\u003cp\u003eThis study triumphantly synthesized Fe\u003csup\u003e0\u003c/sup\u003e/FHC via a combined reduction-ball milling approach and systematically evaluated their Se(IV) adsorption performance and the underlying interaction mechanisms. Characterization analysis indicated that the synthesized Fe\u003csup\u003e0\u003c/sup\u003e/FHC displayed a sheeted morphology. Experimental data strongly confirm that under optimized conditions-precisely regulating the solution pH to neutral (7.0), setting the adsorption reaction duration at 40 min, maintaining zero-valent iron (Fe\u003csup\u003e0\u003c/sup\u003e) content at 35%, and fixing the solid-to-liquid ratio at 0.20 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e-the Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite adsorbent exhibits outstanding adsorption performance for tetravalent selenium [Se(IV)], with a maximum adsorption capacity reaching 201.62 mg g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. In-depth kinetic analysis reveals that the adsorption process's dynamic characteristics closely align with the pseudo-second-order reaction model (correlation coefficient R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.99), strongly supporting the dominant role of chemical adsorption mechanisms in Se(IV) removal. Further isothermal adsorption modeling analysis demonstrates that experimental data shows significantly higher agreement with the Langmuir isothermal model compared to other models, indicating that Se(IV) adsorption on Fe\u003csup\u003e0\u003c/sup\u003e/FHC surfaces primarily occurs through uniform single-molecular-layer adsorption. Thermodynamic parameter measurements reveal a negative enthalpy change (ΔH\u003csup\u003e0\u003c/sup\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0) throughout the adsorption process, conclusively demonstrating that Fe\u003csup\u003e0\u003c/sup\u003e/FHC's Se(IV) adsorption constitutes an exothermic spontaneous process. It is worth noting that in the investigation of the influence of interfering substances, it was found that the presence of common anions (such as F\u003csup\u003e\u0026minus;\u003c/sup\u003e, CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, etc.) and natural organic matter (such as humic acid) had little effect on the adsorption efficiency of Se(IV), and the reduction rate of removal did not exceed 5%, which fully demonstrated the good applicability of this adsorbent in the actual complex water environment. The Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite prepared in this study demonstrated superior Se(IV) removal efficiency when compared with conventional adsorbents. The material exhibited unique advantages: low production cost, facile synthesis, strong reductive capability, high adsorption capacity, and rapid removal kinetics. Evaluated as a promising reactive barrier material, it showed potential for blocking soluble multivalent radionuclide (\u003csup\u003e79\u003c/sup\u003eSe) migration.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCRediT authorship contribution statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eYanjun Du\u003c/strong\u003e\u003cstrong\u003e:\u003c/strong\u003e Writing-Reviewing and Editing. \u003cstrong\u003eQing Zhou\u003c/strong\u003e\u003cstrong\u003e:\u0026nbsp;\u003c/strong\u003eMethodology. \u003cstrong\u003eJiankun Zhao\u003c/strong\u003e\u003cstrong\u003e:\u003c/strong\u003e Investigation. \u003cstrong\u003eHexi Wu\u003c/strong\u003e\u003cstrong\u003e:\u003c/strong\u003e Validation. \u003cstrong\u003eXiaoyan Li\u003c/strong\u003e: Resources. \u003cstrong\u003eYibao Liu\u003c/strong\u003e\u003cstrong\u003e:\u003c/strong\u003e Conceptualization. \u003cstrong\u003eZhanggao Le\u003c/strong\u003e\u003cstrong\u003e:\u003c/strong\u003e Visualization.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData will be provided upon request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was financially supported by National Natural Science Foundation of China (12565020), Doctoral Scientific Research Start-up Fund Project of East China University of Technology (DHBK2024017) and Special topic of national science and technology major project in deep earth (2025ZD1007303-03).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eDong D, Wang Z, Guan J (2025) Research on safe disposal technology and progress of radioactive nuclear waste [J]. Nucl Eng Des 435:113934\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eClayton R, Kirk J, Banford A (2025) A review of radioactive waste processing and disposal from a life cycle environmental perspective [J]. Clean Technol Envir 27(2):665\u0026ndash;682\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKuhlman K, Bartol J, Carter A (2024) Scenario development for safety assessment in deep geologic disposal of high-level radioactive waste and spent nuclear fuel: A review [J]. Risk Anal 44(8):1850\u0026ndash;1864\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLi N, Duan X, Peng G (2025) Engineering barriers in deep geological disposal: Implications for radioactive nuclide migration and long-term safety [J]. J Environ Radioactiv 285:107670\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eXiang L, Liu X, Liu P (2025) Research insights of argillaceous-based site for high-level radioactive waste disposal in China [J]. J Radioanal Nucl Ch 334(1):795\u0026ndash;805\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLiang L, Yang W, Guan X (2013) Kinetics and mechanisms of pH-dependent selenite removal by zero valent iron [J]. Water Res 47(15):5846\u0026ndash;5855\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHe H, Liu J, Dong Y (2019) Sorption of selenite on Tamusu clay in simulated groundwater with high salinity under aerobic/anaerobic conditions [J]. J Environ Radioactiv 203:210\u0026ndash;219\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYang J, Shi K, Sun X (2019) An approach for the efficient immobilization of \u003csup\u003e79\u003c/sup\u003eSe using Fe-OOH modified GMZ bentonite [J]. Radiochim Acta 108(2):113\u0026ndash;126\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMa Z, Shan C, Liang J (2018) Efficient adsorption of Selenium(IV) from water by hematite modified magnetic nanoparticles [J]. Chemosphere 193:134\u0026ndash;141\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLee J, Kim K, Kim I (2023) High-efficiency deep geological repository system for spent nuclear fuel in Korea with optimized decay heat in a disposal canister and increased thermal limit of bentonite [J]. Nucl Eng Technol 55(4):1540\u0026ndash;1554\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTurner J, Berry T, Bowman M (2023) Role of the geosphere in deep nuclear waste disposal-An England and Wales perspective [J]. Earth-Sci Rev 242:104445\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eChikkamath S, Patel M, Kar A (2023) Diffusion of radioisotopes in Fe-montmorillonite relevant to geological disposal of HLW [J]. Phys Chem Earth 131:103429\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eRuiz-Fresneda M, Martinez-Moreno M, Povedano-Priego (2023) Impact of microbial processes on the safety of deep geological repositories for radioactive waste [J]. Front Microbiol 14:3389\u0026ndash;3409\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTyupina E, Kozlov P, Krupskaya V (2023) Application of Cement-Based Materials as a Component of an Engineered Barrier System at Geological Disposal Facilities for Radioactive Waste-A Review [J]. Energies 16(2):605\u0026ndash;640\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHe Y, Xiang Y, Zhou Y (2018) Selenium contamination, consequences and remediation techniques in water and soils: A review [J]. Environ Res 164:288\u0026ndash;301\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTran A, Wang P, Zhang S, Jovanovic M, Siewert B, Moser S, Lindsey J (2024) Synthesis of a model phyllobilin bearing an optical marker [J]. New J Chem 48:2855\u0026ndash;2865\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSuazo-Hern\u0026aacute;ndez J, Manqui\u0026aacute;n-Cerda K, Mora M (2021) Efficient and selective removal of Se(VI) and As(V) mixed contaminants from aqueous media by montmorillonite-nanoscale zero valent iron nanocomposite [J]. J Hazard Mater 403:123639\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBenis K, Damuchali A, Soltan J (2020) Treatment of aqueous arsenic-A review of biochar modification methods [J]. Sci Total Environ 739:139750\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang S, Xiao K, Mo Y (2020) Selenium(VI) and copper(II) adsorption using polyethyleneimine-based resins: Effect of glutaraldehyde crosslinking and storage condition [J]. J Hazard Mater 386:121637\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHanada K, Watanabe S, Inagawa A (2021) Sulfated Steelmaking Slags as Se(IV) Adsorbents: Effects of Preparation Conditions on Adsorption Performance [J]. Isij Int 61(1):506\u0026ndash;512\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLi D, Yan W, Guo X (2020) Removal of selenium from caustic solution by adsorption with Ca-Al layered double hydroxides [J]. Hydrometallurgy 191:105231\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eOkonji S, Dominic J, Pernitsky D (2020) Removal and recovery of selenium species from wastewater: Adsorption kinetics and co-precipitation mechanisms [J]. J Water Process Eng 38:101666\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eXu W, Zhu J, Johnson T (2020) Selenium isotope fractionation during adsorption by Fe, Mn and Al oxides [J]. Geochim Cosmochim Ac 272:121\u0026ndash;136\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBenis K, Damuchali A, Mcphedran K (2020) Treatment of aqueous arsenic - A review of biosorbent preparation methods [J]. J Environ Manage 273:111126\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang L, Jiang S, Guan Y (2021) Efficient removal of selenate in water by cationic poly(allyltrimethylammonium) grafted chitosan and biochar composite [J]. Environ Res 194:110667\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang J, Xie L, Li S (2021) Probing the In Situ Redox Behavior of Selenium on a Pyrite Surface by Scanning Electrochemical Microscopy [J]. J Phys Chem C 125(5):3018\u0026ndash;3026\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKalaitzidou K, Zouboulis A, Mitrakas M (2020) Cost evaluation for Se(IV) removal, by applying common drinking water treatment processes: Coagulation/precipitation or adsorption [J]. J Environ Chem Eng 8(5):104209\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYue P, Chen N, Peak D (2020) Oxygen atom release during selenium oxyanion adsorption on goethite and hematite [J]. Appl Geochem 117:104605\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang Y, Fu M, Wu D, Zhang Y (2017) Immobilization of selenite from aqueous solution by structural ferrous hydroxide complexes [J]. Rsc Adv 7:13398\u0026ndash;13405\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMa B, Charlet L, Fernandez-Martinez A, Kang M, Made B (2019) A review of the retention mechanisms of redox-sensitive radionuclides in multi-barrier systems [J]. Appl Geochem 100:414\u0026ndash;431\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWen Z, Zhang Y, Zhou X, Chen R (2017) Effective As(III) and As(V) immobilization from aqueous solution by nascent ferrous hydroxide colloids (FHC) [J]. Sep Purif Technol 176:395\u0026ndash;401\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTian Q, Guo B, Chuaicham C, Sasaki K (2020) Mechanism analysis of selenium (VI) immobilization using alkaline-earth metal oxides and ferrous salt [J]. Chemsphere 248:126123\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDu Y, Zhou Q, Zhao J (2023) Adsorptive Removal of Selenium(IV) from Aqueous Solution by Ferrous Hydroxide Complex-Zero Nickel Composites [J]. Solvent Extr Ion Exc 41(5):680\u0026ndash;696\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang Z, L\u0026uuml; C, Wang Y (2024) Zero-valent iron (ZVI) facilitated in-situ selenium (Se) immobilization and its recovery by magnetic separation: Mechanisms and implications for microbial ecology [J]. J Hazard Mater 473:134591\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eFan Z, You Y, Ding K (2024) In-situ constructing an innovative and versatile nanotraps for incorporation of zero-valent iron nanoparticles into zeolite imidazole framework-8 towards high efficient detoxification and imprisonment of U(VI) and Se(IV) from water via a combination study of capture performance and underlying mechanism aspects [J]. Chem Eng J 494:153032\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eChalastara K, Demopoulos G (2022) Mechanism of galvanic reduction of selenate oxyanions and surface immobilization by nano zero-valent iron aggregates under anaerobic conditions: towards high electron efficiency [J]. Environ Sci-Wat Res 8(9):1910\u0026ndash;1922\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWu M, Teng X, Liang X (2023) Supporting nanoscale zero-valent iron onto shrimp shell-derived N-doped biochar to boost its reactivity and electron utilization for selenite sequestration [J]. Chemosphere 319:137979\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDong H, Chen Y, Sheng G (2016) The roles of a pillared bentonite on enhancing Se(VI) removal by ZVI and the influence of co-existing solutes in groundwater - ScienceDirect [J]. J Hazard Mater 304(3):306\u0026ndash;312\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDu Y, Fu X, Zhou Q (2023) Preparation of zero-valent iron-nickle bimetallic composite for Se(IV) adsorption from aqueous solution [J]. J Radioanal Nucl Ch 386:123944\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYu W, Guo H, Hou C (2022) Removal of Arsenate from Aqueous Solution by Synthetic Siderite-Modified Biochar: Characteristics and Mechanisms [J]. Water Air Soil Poll 233(7):278\u0026ndash;292\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang H, Tsang Y, Wang Y (2018) Adsorption capacities of poorly crystalline Fe minerals for antimonate and arsenate removal from water: adsorption properties and effects of environmental and chemical conditions [J]. Clean Technol Envir 20(10):2169\u0026ndash;2179\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eNguyen T, Tran H, Nguyen T (2022) Single-step removal of arsenite ions from water through oxidation-coupled adsorption using Mn/Mg/Fe layered double hydroxide as catalyst and adsorbent [J]. Chemosphere 295:133370\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eQiu D (2020) Fabrication of dynamic zero-valent iron/MnO\u003csub\u003e2\u003c/sub\u003e nanowire membrane for efficient and recyclable selenium separation [J]. Sep Purif Technol 230:115847\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLee S, Kim Y, Chang B (2020) Enhanced Arsenic (III and V) Removal in Anoxic Environments by Hierarchically Structured Citrate/FeCO\u003csub\u003e3\u003c/sub\u003e Nanocomposites [J]. Nanomaterials-Basel 10(9):1773\u0026ndash;1795\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBakather O, Fard A, Ihsanullah (2017) Enhanced Adsorption of Selenium Ions from Aqueous Solution Using Iron Oxide Impregnated Carbon Nanotubes [J]. Bioinorg Chem Appl 2017:4323619\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang Z, Zhang H, Tang R (2022) An investigation of Re(VII) and Se(IV) adsorption by Tamusu clay: effect of time, pH, ionic strength, temperature and organic acids [J]. J Radioanal Nucl Ch 331(9):3461\u0026ndash;3473\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWu J, Guo B, Kang M (2022) Comparative study on the reductive immobilization of Se(IV) by Beishan granite and Tamusu claystone [J]. Appl Geochem 146:105447\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhijian L, Qingqing W, fujuan D (2022) Reduction of selenite to selenium nanospheres by Se(IV)-resistant Lactobacillus paralimentarius JZ07 [J]. Food Chem 393:133385\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMa B, Nie Z, Liu C, Kang M, Bardelli F, Chen F, Charlet L (2014) Kinetics of FeSe\u003csub\u003e2\u003c/sub\u003e oxidation by ferric iron and its reactivity compared with FeS\u003csub\u003e2\u003c/sub\u003e [J]. Sci China Chem 57:1300\u0026ndash;1309\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Nascent FHC, Iron, Selenium, reduction, adsorption, mechanism","lastPublishedDoi":"10.21203/rs.3.rs-7734481/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7734481/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003csup\u003e79\u003c/sup\u003eSe is one of the key fission products in spent fuel, with a half - life of 6.5\u0026times;10\u003csup\u003e4\u003c/sup\u003e a. Owing to its exceptionally high mobility and extremely low solubility, this isotope can effortlessly permeate into groundwater, resulting in widespread contamination. This type of pollution poses an extremely grave threat to both ecosystems and human well-being. Therefore, the long-term safe fixation and disposal of \u003csup\u003e79\u003c/sup\u003eSe has become a challenging and cutting-edge topic in research. In this study, Ferrous Hydroxide Complex (FHC) was employed as a carrier to explore its effectiveness in removing selenium (IV). Zero-valent iron/Ferrous Hydroxide Complex (Fe\u003csup\u003e0\u003c/sup\u003e/FHC) was synthesized using simple and cost-effective methods, and their performance in selenium (IV) removal was evaluated. The findings demonstrated that, under specific conditions-pH 7.0, a 40-min adsorption period, 35% Fe\u003csup\u003e0\u003c/sup\u003e content, and a ratio of solid to liquid that was 0.20 g L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite achieved a Se(IV) adsorption capacity of 201.62 mg g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Kinetic analysis showed that adsorption behavior matches the pseudo-second-order model, suggesting that the chemical process was the dominant adsorption mechanism. Isothermal adsorption modeling further showed that the Langmuir model provided a better fit, suggesting that the adsorption predominantly occurred through monolayer coverage. Thermodynamic investigations indicated that the adsorption of Se(IV) by Fe\u003csup\u003e0\u003c/sup\u003e/FHC was an exothermic reaction. Further analysis via FT-IR and XPS showed that Fe\u003csup\u003e0\u003c/sup\u003e/FHC's removal of Se(IV) involves electrostatic adsorption, complexation, and reduction precipitation processes. Overall, the Fe\u003csup\u003e0\u003c/sup\u003e/FHC composite offered distinct benefits, including ease of synthesis, a large specific surface area, and exceptional adsorption capacity. These characteristics endowed Fe\u003csup\u003e0\u003c/sup\u003e/FHC with great potential in treating selenium-containing wastewater. Consequently, as a new adsorption material, it held broad application prospects in this field.\u003c/p\u003e","manuscriptTitle":"Comprehensive Performance Assessment and Mechanistic Insights into Zero-Valent Iron/Ferrous Hydroxide Complex Composites for Optimized Se(IV) Sequestration in Aqueous Media","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-15 16:47:24","doi":"10.21203/rs.3.rs-7734481/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"bb232fcb-0e67-47f1-906a-4817fa4b5159","owner":[],"postedDate":"October 15th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-02-18T09:56:03+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-15 16:47:24","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7734481","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7734481","identity":"rs-7734481","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}