An Inquiry into the Oxidative-Reductive Actions of 79Se4+

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He This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7580301/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 The long-term disposal risks associated with the key radioactive isotope 79 Se 4+ remain a subject of debate, with the specific mechanism of containment yet to be determined. This study, reveals a novel observation in the field, showing that 79 Se 4+ can be efficiently reduced and sequestered by precipitating as Se 0 . Our findings indicate that the reduction of Se 4+ is controlled by the "H + inhibition" mechanism. The rate at pH ~ 8.0 (𝑘 = 4.22×10 −3 mol·L −1 ·h −1 ) is 1131 times faster than the rate at pH 6.3 (𝑘 = 3.73×10 −6 mol·L −1 ·h −1 ). Granite's influence is more nuanced than pH, establishing a distinctive reaction environment at the solid-liquid interface distinct from the bulk solution, leading to reactions occurring above a pH of 5.7 (pHpzc). Additionally, we examined the long-term retention mechanism of Se 4+ in the solid phase, confirming that adsorbed Se 4+ undergoes continuous reduction instead of dissociating into the solution. This validates the effectiveness of the engineering design in the prolonged immobilization of 79 Se 4+ . The data and conclusions presented in this study offer crucial support for the safety assessment of geological disposal repositories and relevant environmental research. 79Se redox reaction mobility granite Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Deep geological disposal is considered the most viable method for handling high-level radioactive waste[ 1 , 2 ]. It is essential to comprehend the diverse geochemical properties of enduring fission nuclides such as 79 Se, 129 I, and 99 Tc[ 3 , 4 ], which have half-lives extending over hundreds of thousands of years. This comprehension is crucial for guiding the engineering design, construction, safety assessment, and ongoing maintenance of geological disposal facilities. Among these nuclides, selenium-79 is notable for its ability to exist in various valence states and environmental forms[ 5 , 6 ]. In its reduced state, 79 Se demonstrates valences of 0, -1, and − 2, typically existing in a solid form with low solubility and mobility[ 7 , 8 ]. Conversely, in its oxidized state, 79 Se exhibits valences of + 4 and + 6, presenting as oxygen-containing anions with high solubility and strong mobility[ 6 ]. Given that the high-level radioactive waste disposal facility is situated in a low-oxygen water environment[ 9 – 11 ], selenium will mainly be present as Se 4+ [ 12 ]。Therefore, comprehending the redox behavior of Se 4+ under disposal site conditions is essential for a thorough evaluation of the migration risk of 79 Se in the disposal repository. While Se 4+ can be adsorbed by a multi-barrier system consisting of bentonite and granite [ 9 , 13 – 15 ], potential changes in the speciation of adsorbed 79 Se 4+ are also linked to its redox behavior. Research in this area is currently fragmented[ 16 – 18 ]. In groundwater environments, Fe 2+ is the predominant reductive agent among groundwater constituents[ 19 – 21 ]. The low oxygen levels near disposal sites are mainly due to the presence of Fe 2+ in groundwater[ 22 , 23 ]. Although Se and Fe have similar standard electrode potentials[ 18 , 24 ], which limits Fe 2+ 's thermodynamic ability to reduce Se 4+ , the occurrence and rate of oxidation-reduction processes are influenced by factors such as pH and the reaction medium (presence of solid phase). The interaction of Se 4+ and Fe 2+ at the granite-groundwater interface affects the redox process. Prolonged contact between high-level waste containers and groundwater leads to corrosion of the iron matrix, releasing Fe 2+ into groundwater and increasing its concentration[ 25 ]. In the mildly alkaline conditions of deep geological repositories, there is speculation that 79 Se 4+ could undergo reduction. Nevertheless, the precise process, extent, influencing factors, and mechanisms of this reaction remain unclear. Hence, a thorough and systematic investigation of the redox interaction between Se 4+ and Fe 2+ is essential. This study investigates the redox reaction between Se 4+ and Fe 2+ , with a specific focus on the impact of pH and granite. Detailed analysis of the underlying mechanisms was carried out to elucidate the long-term evolution of 79 Se 4+ in granite. These findings are fundamental for understanding the geochemical behavior of Se 4+ and play a significant role in the design and safety assessment of geological repositories for high-level radioactive waste. 2. Experimental section 2.1 Experimental materials, characterization techniques, and experimental conditions The granite utilized in the study, along with its characterization results (see Figure S1 ), is described in a previous publication for reference[ 18 ]. The stable isotope carrier, Na 2 SeO 3 [ 26 ] (purity > 99%), was procured from AMRESCO in Solon, Ohio, USA for use in this research. Deionized water (18.2 MΩ·cm) was purified by boiling, cooling, and bubbling with high-purity nitrogen inside a glove box before application[ 27 ]. The grey Se(0) samples[ 28 ] were sourced from Sigma Aldrich and Alfa Aesar. Additionally, all other chemicals used were of analytical grade. To maintain anoxic conditions, all chemicals and solutions were equilibrated in a glove box for a minimum of 3 days. The experiments were carried out in a low-oxygen glove box[ 29 ], with the temperature set at 23 ± 2 degrees Celsius, unless otherwise specified in the study. The article utilizes various characterization techniques[ 25 ], including Brunauer Emmett Teller (BET), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) spectroscopy, X-ray Fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) spectroscopy. Se 4+ concentrations in the filtrate were quantified using inductively coupled plasma optical emission spectrometer (ICP-OES), while Fe 2+ concentrations were determined through the 1,10-phenanthroline method[ 30 , 31 ]. For more detailed information, please refer to Text S1. 2.2 Experiments 2.2.1 Preparation of the buffer solutions The natural groundwater environment functions as a crucial buffer system, maintaining a stable pH level. This highlights the necessity of precise pH control for research accuracy. Buffer solutions[ 27 , 32 – 34 ] were utilized in all reactions to counteract sudden or localized pH changes effectively. These prepared buffers have sufficient capacity to regulate pH fluctuations during reactions. Continuous pH monitoring was conducted to confirm the efficacy of the buffer system throughout the experiments. Further details on the composition and establishment of the buffer systems can be found in Text S2. 2.2.2 Oxidation-reduction reactions Groundwater typically maintains a slightly alkaline pH of around 8. To advance our understanding in this field and provide insights for applications such as Se 4+ wastewater treatment, we deliberately investigated various pH levels - specifically pH 4.8, 6.5, 7.5, and 8.0. In addition to pH, our study will also examine the influence of granite. Accordingly, we conducted two sets of experiments at each pH level. The first set focused on homogeneous reactions involving Se 4+ and Fe 2+ exclusively, while the second set, termed heterogeneous reactions, introduced a specified quantity of granite into the system. 2.3 Spectroscopic analyses and thermodynamic calculations The PHREEQC code[ 35 ] was utilized to compute the speciation and coupling pH-Eh diagrams of selenium and iron in aqueous solution. The Llnl.dat database was updated with the most recent NEA thermodynamic data for selenium[ 36 ] and the ANDRA thermodynamic data for iron compounds[ 37 ]. The Gibbs free energy calculations were based on the authoritative works: "Chemical Thermodynamics of Selenium"[ 36 ] and "Thermodynamics des produits de corrosion"[ 37 ]. 3 Results and Discussion 3.1 The impact of H + The data presented in Fig. 1 illustrates a significant correlation between pH levels and the decrease in Se 4+ and Fe 2+ concentrations. pH is a key determinant of the reaction outcomes. Particularly, in alkaline conditions (pH approximately 7.5 to 8.0), the decrease rates of Se 4+ and Fe 2+ concentrations are notably higher compared to acidic environments (pH around 4.8 to 6.3). Particularly, at a pH of around 8.0, the Se 4+ and Fe 2+ concentrations in the solution can rapidly decrease to about 10% of their initial levels within 120 hours. To analyze the rate of Se 4+ concentration decline under varying conditions, we calculated the average rate, \(\:\stackrel{-}{k}\) , over a reaction time t using the formula: \(\:\stackrel{-}{k}=\frac{{{\Delta\:}}_{{\text{S}\text{e}}^{4+}}}{{t}_{total}}\) , measured in mol·L −1 ·h −1 , where t represents the reaction time in hours, and \(\:{{\Delta\:}}_{{\text{S}\text{e}}^{4+}}\) is the decrease in Se 4+ concentration during the reaction period in mol·L −1 . The detailed calculations are provided in Fig. 1 D. It is noteworthy that the average rate at pH ~ 8 (4.22×10 −3 mol·L − 1 ·h −1 ) exceeds the rate at pH ~ 6.3 (3.73×10 −6 mol·L −1 ·h −1 ) by a factor of 1131. A graphical representation illustrating the relationship between Fe 2+ concentration (y-axis) and variations in Se 4+ concentration (x-axis) was created. Linear regression analysis of the results presented in Fig. 1 C revealed a slope of 3.8 at pH levels of 7.5 and 8.0, aligning with a 4:1 stoichiometric ratio for the Fe 2+ and Se 4+ reaction described by the equation: Se 4+ + 4Fe 2+ → Se(0) + 4Fe 3+ (1) The observation indicates that Se 4+ can be reduced by Fe 2+ , with the results showing a more favorable reaction in alkaline conditions. Upon analyzing the specific reaction equation, it is noted that the interaction between Se 4+ and Fe 2+ leads to a continuous release of H + ions, as described in Text S3, suggesting that H + ions act as inhibitors in the reaction. Higher pH levels result in lower concentrations of H + ions, thereby promoting the reaction. In alkaline conditions, the presence of OH − ions can neutralize the generated H + ions, thereby facilitating the reaction. Conversely, the presence of H + in acidic conditions hinders the reaction. Consequently, no significant variation in Se 4+ and Fe 2+ levels is observed at pH ~ 4.8, with only a slight decrease in Se 4+ and Fe 2+ concentrations at pH ~ 6.3 (Fig. 1 ). To confirm the "H + inhibition" mechanism, three homogeneous experiments were conducted at pH levels of approximately 6.75, 7.00, and 7.25, as depicted in Fig. 2 . As shown in Fig. 2 , despite a slight increase in pH from around 6.75 to 7.25, there was a significant discrepancy in the concentration variations of Se 4+ and Fe 2+ : the reaction was markedly hindered at pH levels of about 6.75 and 7.00. Conversely, the reaction rate experienced a substantial increase as the pH approached alkaline conditions (around 7.25). The data presented in Table 1 illustrates that the average rate constant, \(\:\stackrel{-}{k}\) , is 6.93×10 − 5 at pH ~ 7.25, significantly exceeding the value at pH ~ 6.5 (8.26×10 − 6 ) by a factor of 8.4. The observed positive correlation between reaction rate and pH, as depicted in Figure S2, provides evidence for the hypothesis that the interplay between Se 4+ and Fe 2+ is influenced by the concentration of H + ions. Table 1 A summary of the average rates and associated data for homogeneous reactions at pH levels of 6.75, 7.00, and 7.25. pH Se 4+ mol·L − 1 Fe 2+ mol·L − 1 \(\:{\varDelta\:}_{{Se}^{4+}}\) , mol·L − 1 \(\:{t}_{total}\) , hours \(\:\stackrel{-}{k}\) , mol·L − 1 ·h − 1 6.75 1×10 − 4 4×10 − 4 0.0119 1440 8.26×10 − 6 7.00 1×10 − 4 4×10 − 4 0.03888 1440 2.70×10 − 5 7.25 1×10 − 4 4×10 − 4 0.09985 1440 6.93×10 − 5 3.2 The impact of granite In Figs. 3 A and 3 B, it is evident that the effect of granite is not significant under alkaline conditions. In line with the homogeneous reaction (Fig. 1 ), both Se 4+ and Fe 2+ concentrations exhibit a rapid decrease over time, especially at pH ~ 8.0, resulting in the total exhaustion of both species within a 48 -hours period. The slope of 3.98 shown in Fig. 3 C at pH ~ 8.0 aligns closely with the stoichiometric ratio of 4:1 for the Fe 2+ and Se 4+ reaction, as depicted in Eq. 1. These results strongly suggest that in the groundwater environment of disposal sites, 79 Se 4+ will undergo reduction. Furthermore, even in the event of leakage from glass-solidified bodies, the migration of 79 Se 4+ with groundwater (pH ~ 8) is improbable, as it promptly converts into a solid phase. However, under acidic conditions, the influence of granite is significant. When pH ~ 4.8, there was a noticeable decrease in Se 4+ concentration (Fig. 3 A), followed by a stabilization trend, which is couldn’t be observed in the homogeneous reaction (Fig. 1 A). This is only the results of adsorption [ 18 ] but redox, as illustrated by the unchanging Fe 2+ concentration (Fig. 3 B). At a pH of 6.3, a substantial decrease of Se 4+ about 55% (from 0.1 mM to 0.45 mM) was noted. The data presented in Fig. 3 D indicates that the average reaction rate is 1.43×10 − 5 mol·L − 1 ·h − 1 , which is approximately 3.83 times higher than under homogeneous conditions (3.73×10 − 5 mol·L − 1 ·h − 1 ). The decrease in selenium concentration cannot be solely attributed to Se 4+ adsorption by granite, as evidenced by two key observations. Firstly, granite shows limited Se 4+ adsorption at pH levels above 6, significantly lower than the adsorption observed at pH ~ 4.8 (approximately 20%). Additionally, the adsorption of Fe 2+ in granite is less than 25% at pH ~ 6.4 (refer to Figure S3), whereas in the reaction system, it decreases by 62.5% (Fig. 3 B). These findings strongly suggest the involvement of non-adsorption reactions in the solution, such as redox reactions. What is the reason behind the absence of the reaction under homogeneous conditions at a pH of about 6.3 (Fig. 1 ), while it does occur in the presence of granite? It is known that a lower concentration of H + in the reaction pathway increases the likelihood of the reaction. Therefore, the presence of granite is believed to counteract the H + ions in the system. This interaction is associated with the surface charge of the mineral, specifically referred to as the point of zero charge (pH pzc ). Experimental data reveals a pH pzc value[ 38 ] of around 5.7 for granite (Text S4). Consequently, when the pH surpasses 5.7, the granite surface becomes negatively charged[ 38 ], neutralizing the adverse effects of H + ions and promoting redox reactions at pH 6.3. In contrast, at approximately pH 4.8, the granite surface becomes positively charged, allowing the inhibitory effects of H + ions to persist. As a result, over an extended period (approximately 6240 hours), only the adsorption of Se 4+ onto the granite occurs in the heterogeneous reaction (Fig. 3 A). 3.3 Impact of solid-liquid ratio As mentioned earlier, the influence of 10 g/L granite on the alkaline reaction (pH ~ 7.5, ~ 8.0) is minimal. However, the substantial presence of granite at the disposal site necessitated an investigation of the solid-liquid ratio as a critical variable. The results are illustrated in Fig. 4 . The concentration variations of Se 4+ in the solution are notably affected by the solid-liquid ratio. The changes in Se 4+ concentration exhibit two distinct stages. Initially, an increase in the solid-liquid ratio speeds up the decline in Se 4+ concentration. Subsequently, in the second stage, as the reaction progresses, the increase in the solid-liquid ratio slows down the decrease in Se 4+ . T These alterations are prominently evident in the enlarged segment of Fig. 4 . The concentration variation of Se 4+ and Fe 2+ (Fig. 5 ) reveals that, apart from the slope observed at a solid-to-liquid ratio of 10 g/L (3.87), which closely matches the stoichiometric ratio of the redox reaction of 4:1, the concentrations of Se 4+ and Fe 2+ exhibit a curved relationship at other solid-to-liquid ratios. Additionally, with an increase in the solid-to-liquid ratio, the curvature of the curves intensifies. This suggests that the decline in Se 4+ and Fe 2+ concentrations at higher solid-liquid ratios may not be solely due to redox reactions but could involve other factors such as adsorption. The decrease in Se 4+ concentration in the solution is a result of two main factors: adsorption onto granite or reduction by Fe 2+ . Granite has a limited capacity to adsorb Se 4+ , with a higher solid-to-liquid ratio enhancing Se 4+ adsorption and consequently leading to a faster decrease in Se 4+ concentration in the liquid phase. Meanwhile, granite has a greater affinity for adsorbing Fe 2+ over Se 4+ (Figure S3), resulting in decreased Fe 2+ levels in the liquid phase at higher solid-to-liquid ratios. This situation is detrimental to Se 4+ reduction. As a result, the presence of granite impedes the decrease of Se 4+ in the liquid phase. These two conflicting mechanisms significantly impact the overall reaction kinetics. At the onset of the reaction, the adsorption of Se 4+ by granite is the primary reason for the decrease in Se 4+ concentration in the liquid phase. However, this adsorption diminishes rapidly with increasing pH. At around pH 7.5, during the initial 50 hours of the reaction, the decline in Se 4+ concentration is solely attributed to adsorption. Conversely, at pH 8, the adsorption capacity of granite is notably weak, with a slight advantage observed only within the first 4 hours of the reaction, as depicted in the magnified section of Fig. 4 C. As the adsorption process concludes, Se 4+ in the liquid phase reacts with Fe 2+ , further reducing Se 4+ concentration. Nonetheless, due to granite's adsorption of Fe 2+ , the concentration of Fe 2+ in the liquid phase remains low (Fig. 4 B), resulting in a slower redox rate (Table 2). This accounts for the slower decrease in Se 4+ concentration under high solid-liquid ratios. In summary, the presence of granite favors the reduction of Se 4+ under acidic conditions, while it hinders Se 4+ reduction under alkaline conditions. Table 2 . The average rates and relevant data for each reaction are outlined at solid-liquid ratios of 10, 30, 50, 70, and 100 g/L, with corresponding pH values of 7.5 and 8.0. pH Se 4+ , mol·L − 1 Fe 2+ , mol·L − 1 S/L, g·L − 1 \(\:{\varDelta\:}_{{Se}^{4+}}\) , mol·L − 1 \(\:{t}_{total}\) , hours \(\:\stackrel{-}{{\text{k}}^{{\prime\:}}}\) , mol·L − 1 ·h − 1 7.5 1×10 − 4 4×10 − 4 10 0.0967 408 2.37×10 − 4 1×10 − 4 4×10 − 4 30 0.0992 408 2.43×10 − 4 1×10 − 4 4×10 − 4 50 0.0988 408 2.42×10 − 4 1×10 − 4 4×10 − 4 70 0.0982 408 2.41×10 − 4 1×10 − 4 4×10 − 4 100 0.101 504 2.00×10 − 4 8.0 1×10 − 4 4×10 − 4 10 0.0940 80 1.18×10 − 3 1×10 − 4 4×10 − 4 30 0.0930 120 7.75×10 − 4 1×10 − 4 4×10 − 4 50 0.0894 240 3.73×10 − 4 1×10 − 4 4×10 − 4 70 0.0892 168 5.31×10 − 4 1×10 − 4 4×10 − 4 100 0.0874 240 3.64×10 − 4 3.4 The calculation of Gibbs free energy and electromotive force The Gibbs free energy (ΔrG) is a critical parameter in assessing the reversibility of chemical reactions[ 39 ]. Through analysis of the pH-Eh diagram[ 38 ] (Figure S4) and associated thermodynamic data (Table S1 ), Se 0 and Fe 2 O 3 have been identified as the thermodynamically stable products in our system. The Δ r G for the reaction was determined (refer to Figure S5, Text S3), with all calculated Δ r G values (Table 3 ) found to be negative, indicating thermodynamic feasibility. Furthermore, Δ r G decreases with increasing pH. Experimental findings indicate that the REDOX reaction occurs only when Δ r G reaches − 160.12 kJ·mol − 1 , corresponding to a pH > 6.4. Table 3 Gibbs free energy and potential across different reactions. ID pH Se 4+ , mol·L − 1 Fe 2+ , mol·L − 1 Δ r G, kJ·mol − 1 The electromotive force, E θ , mV 1 4.80 1e-4 4e-4 -96.20 479.50 2 6.40 1e-4 4e-4 -160.12 928.54 3 6.75 1e-4 4e-4 -174.10 1026.77 4 7.00 1e-4 4e-4 -184.09 1096.93 5 7.25 1e-4 4e-4 -194.08 1167.10 6 7.50 1e-4 4e-4 -204.06 1237.26 7 8.00 1e-4 4e-4 -224.04 1377.58 Monitoring the redox potential is essential for understanding the driving force behind redox reactions. We investigated the potentials of Se 4+ and Fe 2+ solutions, along with the reaction process (see Table 3 ) (refer to Text S5). Our findings revealed that the reaction potentials (E θ , depicted by pentagonal dots in Fig. 5 ) approached 0 mV, falling below the oxidation potential of Se 4+ at the specified pH (indicated by red circle points in Fig. 5 ) yet surpassing the reduction potential of Fe 2+ (depicted by gray circle points in Fig. 5 ), signifying the occurrence of the redox reaction. The lower ΔrG values and higher reaction potential indicated favorable conditions for the redox reaction. Notably, at pH ~ 8.0, with ΔrG = -224.04 kJ·mol-1 and E = 1377.58 mV, the REDOX reaction proceeded smoothly. 3.5 Species analysis-XPS As previously mentioned, the evolution of Se 4+ following adsorption is a significant issue. It is crucial to investigate whether the adsorbed Se 4+ can undergo reduction, as this determines whether the long-term retardation of 79 Se 4+ is effective. XPS data analysis was conducted to monitor the evolution of solid-phase species over varying reaction durations in a solution of 70 g/L at pH 7.5 and 8.0. Detailed peak positions and areas are presented in Table S2 and visually depicted in Figs. 6 . As shown in Fig. 6 , the ratio of Fe 2+ /Fe 3+ [ 40 ] peak areas decreased gradually over time, as outlined in Table S2. This observation suggests that the adsorbed Fe 2+ gradually reacted with Se 4+ , leading to a progressive decrease in Fe 2+ concentration in the solid phase. Furthermore, the absence of a Se 4+ peak in all reactions indicates that Se 0 is the sole Se species present in the solid phase[ 41 ]. This suggests that although Se 4+ may initially be immobilized in its adsorbed state, it eventually undergoes reduction to Se 0 . Therefore, unless other relevant factors affecting the safe disposal of Se 0 are considered, our study effectively illustrates the minimal long-term disposal risks associated with 79 Se 4+ . 4. Conclusions and perspectives In this study, we investigated the redox behavior of Se 4+ with a specific focus on the impact of pH and granite as crucial variables. We elucidated the underlying mechanisms and clarified the prolonged inhibition mechanism of 79 Se 4+ . Our findings reveal that the reaction kinetics are significantly influenced by the concentration of H + , with the reaction rate consistently increasing at higher pH levels. The redox reaction encounters challenges or may be impeded at pH levels below 7, while it proceeds readily at pH levels above 7. Notably, at a pH of approximately 8, the rate constant (k) can reach 4.22×10 − 3 mol·L − 1 ·h − 1 in a homogeneous system, representing a more than 1000-fold increase compared to the rate at pH 6.3 (3.73×10 − 6 mol·L − 1 ·h − 1 ). However, in the presence of granite in the reaction system, the reaction can occur under acidic conditions (pH > 5.7). The unique solid-liquid interface created by granite is distinct from the bulk solution. The OH − ions at its interface can neutralize the H + ions produced by the reaction, thereby promoting the reaction at a pH close to the point of zero charge (pzc) of the granite (~ 5.7). For instance, at pH ~ 6.3, approximately 55% of Se 4+ was observed to react. In our study, we also examined the thermodynamics of the reaction by analyzing the changes in Gibbs free energy (ΔG) and reaction potential (ΔE). A smaller ΔG corresponds to a larger ΔE, indicating a more favorable reaction. At pH ~ 8.0, the values were ΔG=-224.04 kJ·mol − 1 and ΔE = 1377.58 mV, signifying a highly facile progression of the redox reaction. Through the pH-Eh diagram and XPS analysis, we have identified Se 0 as the predominant species on the solid-phase surface. It has been observed that the adsorbed Se 4+ undergoes continuous reduction process over time instead of dissociation into the solution. This finding implies that under repository conditions (pH ~ 8), 79 Se 4+ will be immobilized in the repository in reduced form, thereby presenting a low migration risk. The data and conclusions presented herein offer crucial theoretical support for the construction of geological repositories and safety assessments, while also guiding research on other redox-sensitive radionuclides. Declarations Author Contribution Xiang Li: Investigation, Data curation. Jingju Li: Writing –original draft, Software, Data curation. Shirong Wang: Supervision, Formal analysis. Yun Zhou: Data curation.Jiangang He: Writing – review & editing, Supervision, Methodology. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 22106057), Fundamental Research Funds for the Central Universities of China (lzujbky-2023-41, lzujbky-2024-jdzx14), The Youth Fund Project of Hainan Provincial Natural Science Foundation (423 QN 323). 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Mattanovich, The potential of CO2-based production cycles in biotechnology to fight the climate crisis. Nature communications, 2023. 14 (1): p. 6978. Tang, S., et al., General synthesis of high-entropy single-atom nanocages for electrosynthesis of ammonia from nitrate. Nature communications, 2024. 15 (1): p. 6932. Li, L., et al., Hydrodynamics- and hydrochemistry-affected microbial selenate reduction in aquifer: Performance and mechanisms. The Science of the total environment, 2021. 768 : p. 145331. Additional Declarations No competing interests reported. Supplementary Files Supplementarydata.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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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1","display":"","copyAsset":false,"role":"figure","size":412960,"visible":true,"origin":"","legend":"\u003cp\u003e(A) and (B) illustrates the variations in Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e concentrations in a homogeneous system. The pH levels were adjusted to 4.8, 6.3, 7.5, and 8.0; (C) displays the correlation between the concentrations of Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e; (D) present the average rate.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7580301/v1/2c0ae226728c457c71a2a1d6.png"},{"id":92590310,"identity":"d25e8f3b-3759-4e24-a101-13207db43725","added_by":"auto","created_at":"2025-10-01 11:41:03","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":411550,"visible":true,"origin":"","legend":"\u003cp\u003e(A) and (B) demonstrate the variations in Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e concentrations, respectively, under homogeneous conditions at pH levels around 6.75, 7.00, and 7.25.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7580301/v1/ac35a3ac0c6282bfae759288.png"},{"id":92590311,"identity":"ef0f7d8d-e558-4501-a852-20de280dacb7","added_by":"auto","created_at":"2025-10-01 11:41:03","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":420633,"visible":true,"origin":"","legend":"\u003cp\u003e(A) and (B) illustrate the alterations in Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e concentrations within a heterogeneous system, respectively; (C) presents the fitted results of Se\u003csup\u003e4+\u003c/sup\u003e concentration as a function of Fe\u003csup\u003e2+\u003c/sup\u003e concentration; (D) provides the average rates and relevant data for each reaction.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7580301/v1/c81c9fdd1f24f06d9a265dac.png"},{"id":92591331,"identity":"dfe1f6d0-040e-4d8e-b561-caf8775765be","added_by":"auto","created_at":"2025-10-01 11:57:03","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":531598,"visible":true,"origin":"","legend":"\u003cp\u003eA, B for the concentrations of Se\u003csup\u003e4+\u003c/sup\u003e vary under different solid-liquid ratios at pH 7.5 and 8.0 respectively; C, D for the concentrations of Fe\u003csup\u003e2+\u003c/sup\u003e vary under different solid-liquid ratios at pH 7.5 and 8.0 respectively.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7580301/v1/55d7a9acc9868099e1eaf5e3.png"},{"id":92590549,"identity":"62f63a87-8f55-49cb-af9e-c3f87a5810cd","added_by":"auto","created_at":"2025-10-01 11:49:03","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":381067,"visible":true,"origin":"","legend":"\u003cp\u003eThe changes in Fe\u003csup\u003e2+\u003c/sup\u003e concentration relative to alterations in Se\u003csup\u003e4+\u003c/sup\u003e concentration at two pH levels: (A) 7.5, and (B) 8.0.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7580301/v1/62341f3d1a66a68d3c3ef4ab.png"},{"id":92590312,"identity":"78bc97a1-cac5-4d8c-9196-9c5c72a9d2b7","added_by":"auto","created_at":"2025-10-01 11:41:03","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":136938,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 5. \u003c/strong\u003eThe changes in electric potential in relation to pH levels. The red and black solid lines represent the theoretical electric potentials of Se\u003csup\u003e4+\u003c/sup\u003e/Se\u003csup\u003e0\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e/Fe\u003csup\u003e2+ \u003c/sup\u003epair, respectively. The black pentagram signifies the measured electromotive force for the actual chemical reaction.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7580301/v1/f28b8aa65d28ed1a1304c249.png"},{"id":92591697,"identity":"1c293450-c8bb-4215-b980-f62926b148bb","added_by":"auto","created_at":"2025-10-01 12:05:03","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":457770,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 6\u003c/strong\u003e. The XPS analysis results of Se and Fe in the solid phase.A and A' for 1 day and 15 days at pH 7.5, respectively. B and B' for 1 day and 15 days at pH 8.0, respectively. The solid-liquid ratio is 70 g/L.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7580301/v1/cf39b8da08d951fa666e1bb0.png"},{"id":98501285,"identity":"9f235b34-1fe7-4085-a546-3586bdd1b860","added_by":"auto","created_at":"2025-12-18 09:41:25","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3295203,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7580301/v1/937e7f16-7019-4b80-8a20-1e8af4e07e1e.pdf"},{"id":92590313,"identity":"224fa942-d193-41b7-b6f9-d3746fdd435d","added_by":"auto","created_at":"2025-10-01 11:41:03","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":2945145,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementarydata.docx","url":"https://assets-eu.researchsquare.com/files/rs-7580301/v1/9f462c6b3d4155dc31d7ee00.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eAn Inquiry into the Oxidative-Reductive Actions of \u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e79\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003eSe\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e4+\u003c/strong\u003e\u003c/sup\u003e\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eDeep geological disposal is considered the most viable method for handling high-level radioactive waste[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. It is essential to comprehend the diverse geochemical properties of enduring fission nuclides such as \u003csup\u003e79\u003c/sup\u003eSe, \u003csup\u003e129\u003c/sup\u003eI, and \u003csup\u003e99\u003c/sup\u003eTc[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], which have half-lives extending over hundreds of thousands of years. This comprehension is crucial for guiding the engineering design, construction, safety assessment, and ongoing maintenance of geological disposal facilities. Among these nuclides, selenium-79 is notable for its ability to exist in various valence states and environmental forms[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. In its reduced state, \u003csup\u003e79\u003c/sup\u003eSe demonstrates valences of 0, -1, and \u0026minus;\u0026thinsp;2, typically existing in a solid form with low solubility and mobility[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Conversely, in its oxidized state, \u003csup\u003e79\u003c/sup\u003eSe exhibits valences of +\u0026thinsp;4 and +\u0026thinsp;6, presenting as oxygen-containing anions with high solubility and strong mobility[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Given that the high-level radioactive waste disposal facility is situated in a low-oxygen water environment[\u003cspan additionalcitationids=\"CR10\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], selenium will mainly be present as Se\u003csup\u003e4+\u003c/sup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]。Therefore, comprehending the redox behavior of Se\u003csup\u003e4+\u003c/sup\u003e under disposal site conditions is essential for a thorough evaluation of the migration risk of \u003csup\u003e79\u003c/sup\u003eSe in the disposal repository. While Se\u003csup\u003e4+\u003c/sup\u003e can be adsorbed by a multi-barrier system consisting of bentonite and granite [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], potential changes in the speciation of adsorbed \u003csup\u003e79\u003c/sup\u003eSe\u003csup\u003e4+\u003c/sup\u003e are also linked to its redox behavior. Research in this area is currently fragmented[\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn groundwater environments, Fe\u003csup\u003e2+\u003c/sup\u003e is the predominant reductive agent among groundwater constituents[\u003cspan additionalcitationids=\"CR20\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. The low oxygen levels near disposal sites are mainly due to the presence of Fe\u003csup\u003e2+\u003c/sup\u003e in groundwater[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Although Se and Fe have similar standard electrode potentials[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e], which limits Fe\u003csup\u003e2+\u003c/sup\u003e's thermodynamic ability to reduce Se\u003csup\u003e4+\u003c/sup\u003e, the occurrence and rate of oxidation-reduction processes are influenced by factors such as pH and the reaction medium (presence of solid phase). The interaction of Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e at the granite-groundwater interface affects the redox process. Prolonged contact between high-level waste containers and groundwater leads to corrosion of the iron matrix, releasing Fe\u003csup\u003e2+\u003c/sup\u003e into groundwater and increasing its concentration[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. In the mildly alkaline conditions of deep geological repositories, there is speculation that \u003csup\u003e79\u003c/sup\u003eSe\u003csup\u003e4+\u003c/sup\u003e could undergo reduction. Nevertheless, the precise process, extent, influencing factors, and mechanisms of this reaction remain unclear. Hence, a thorough and systematic investigation of the redox interaction between Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e is essential.\u003c/p\u003e\u003cp\u003eThis study investigates the redox reaction between Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e, with a specific focus on the impact of pH and granite. Detailed analysis of the underlying mechanisms was carried out to elucidate the long-term evolution of \u003csup\u003e79\u003c/sup\u003eSe\u003csup\u003e4+\u003c/sup\u003e in granite. These findings are fundamental for understanding the geochemical behavior of Se\u003csup\u003e4+\u003c/sup\u003e and play a significant role in the design and safety assessment of geological repositories for high-level radioactive waste.\u003c/p\u003e"},{"header":"2. Experimental section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Experimental materials, characterization techniques, and experimental conditions\u003c/h2\u003e\u003cp\u003eThe granite utilized in the study, along with its characterization results (see Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e), is described in a previous publication for reference[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The stable isotope carrier, Na\u003csub\u003e2\u003c/sub\u003eSeO\u003csub\u003e3\u003c/sub\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] (purity\u0026thinsp;\u0026gt;\u0026thinsp;99%), was procured from AMRESCO in Solon, Ohio, USA for use in this research. Deionized water (18.2 MΩ\u0026middot;cm) was purified by boiling, cooling, and bubbling with high-purity nitrogen inside a glove box before application[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. The grey Se(0) samples[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e] were sourced from Sigma Aldrich and Alfa Aesar. Additionally, all other chemicals used were of analytical grade. To maintain anoxic conditions, all chemicals and solutions were equilibrated in a glove box for a minimum of 3 days. The experiments were carried out in a low-oxygen glove box[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e], with the temperature set at 23\u0026thinsp;\u0026plusmn;\u0026thinsp;2 degrees Celsius, unless otherwise specified in the study.\u003c/p\u003e\u003cp\u003eThe article utilizes various characterization techniques[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e], including Brunauer Emmett Teller (BET), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) spectroscopy, X-ray Fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) spectroscopy. Se\u003csup\u003e4+\u003c/sup\u003e concentrations in the filtrate were quantified using inductively coupled plasma optical emission spectrometer (ICP-OES), while Fe\u003csup\u003e2+\u003c/sup\u003e concentrations were determined through the 1,10-phenanthroline method[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. For more detailed information, please refer to Text S1.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Experiments\u003c/h2\u003e\u003cdiv id=\"Sec5\" class=\"Section3\"\u003e\u003ch2\u003e2.2.1 Preparation of the buffer solutions\u003c/h2\u003e\u003cp\u003eThe natural groundwater environment functions as a crucial buffer system, maintaining a stable pH level. This highlights the necessity of precise pH control for research accuracy. Buffer solutions[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan additionalcitationids=\"CR33\" citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e] were utilized in all reactions to counteract sudden or localized pH changes effectively. These prepared buffers have sufficient capacity to regulate pH fluctuations during reactions. Continuous pH monitoring was conducted to confirm the efficacy of the buffer system throughout the experiments. Further details on the composition and establishment of the buffer systems can be found in Text S2.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section3\"\u003e\u003ch2\u003e2.2.2 Oxidation-reduction reactions\u003c/h2\u003e\u003cp\u003eGroundwater typically maintains a slightly alkaline pH of around 8. To advance our understanding in this field and provide insights for applications such as Se\u003csup\u003e4+\u003c/sup\u003e wastewater treatment, we deliberately investigated various pH levels - specifically pH 4.8, 6.5, 7.5, and 8.0. In addition to pH, our study will also examine the influence of granite. Accordingly, we conducted two sets of experiments at each pH level. The first set focused on homogeneous reactions involving Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e exclusively, while the second set, termed heterogeneous reactions, introduced a specified quantity of granite into the system.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Spectroscopic analyses and thermodynamic calculations\u003c/h2\u003e\u003cp\u003eThe PHREEQC code[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] was utilized to compute the speciation and coupling pH-Eh diagrams of selenium and iron in aqueous solution. The Llnl.dat database was updated with the most recent NEA thermodynamic data for selenium[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e] and the ANDRA thermodynamic data for iron compounds[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. The Gibbs free energy calculations were based on the authoritative works: \"Chemical Thermodynamics of Selenium\"[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e] and \"Thermodynamics des produits de corrosion\"[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Results and Discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1 The impact of H\u003csup\u003e+\u003c/sup\u003e\u003c/h2\u003e\n \u003cp\u003eThe data presented in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e illustrates a significant correlation between pH levels and the decrease in Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e concentrations. pH is a key determinant of the reaction outcomes. Particularly, in alkaline conditions (pH approximately 7.5 to 8.0), the decrease rates of Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e concentrations are notably higher compared to acidic environments (pH around 4.8 to 6.3). Particularly, at a pH of around 8.0, the Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e concentrations in the solution can rapidly decrease to about 10% of their initial levels within 120 hours. To analyze the rate of Se\u003csup\u003e4+\u003c/sup\u003e concentration decline under varying conditions, we calculated the average rate, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{-}{k}\\)\u003c/span\u003e\u003c/span\u003e, over a reaction time t using the formula: \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{-}{k}=\\frac{{{\\Delta\\:}}_{{\\text{S}\\text{e}}^{4+}}}{{t}_{total}}\\)\u003c/span\u003e\u003c/span\u003e, measured in mol\u0026middot;L\u003csup\u003e\u0026minus;1\u003c/sup\u003e\u0026middot;h\u003csup\u003e\u0026minus;1\u003c/sup\u003e, where t represents the reaction time in hours, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{{\\Delta\\:}}_{{\\text{S}\\text{e}}^{4+}}\\)\u003c/span\u003e\u003c/span\u003e is the decrease in Se\u003csup\u003e4+\u003c/sup\u003e concentration during the reaction period in mol\u0026middot;L\u003csup\u003e\u0026minus;1\u003c/sup\u003e. The detailed calculations are provided in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eD. It is noteworthy that the average rate at pH ~\u0026thinsp;8 (4.22\u0026times;10\u003csup\u003e\u0026minus;3\u003c/sup\u003e mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;h\u003csup\u003e\u0026minus;1\u003c/sup\u003e) exceeds the rate at pH ~\u0026thinsp;6.3 (3.73\u0026times;10\u003csup\u003e\u0026minus;6\u003c/sup\u003e mol\u0026middot;L\u003csup\u003e\u0026minus;1\u003c/sup\u003e\u0026middot;h\u003csup\u003e\u0026minus;1\u003c/sup\u003e) by a factor of 1131.\u003c/p\u003e\n \u003cp\u003eA graphical representation illustrating the relationship between Fe\u003csup\u003e2+\u003c/sup\u003e concentration (y-axis) and variations in Se\u003csup\u003e4+\u003c/sup\u003e concentration (x-axis) was created. Linear regression analysis of the results presented in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eC revealed a slope of 3.8 at pH levels of 7.5 and 8.0, aligning with a 4:1 stoichiometric ratio for the Fe\u003csup\u003e2+\u003c/sup\u003e and Se\u003csup\u003e4+\u003c/sup\u003e reaction described by the equation:\u003c/p\u003e\n \u003cp\u003eSe\u003csup\u003e4+\u003c/sup\u003e + 4Fe\u003csup\u003e2+\u003c/sup\u003e \u0026rarr; Se(0)\u0026thinsp;+\u0026thinsp;4Fe\u003csup\u003e3+\u003c/sup\u003e (1)\u003c/p\u003e\n \u003cp\u003eThe observation indicates that Se\u003csup\u003e4+\u003c/sup\u003e can be reduced by Fe\u003csup\u003e2+\u003c/sup\u003e, with the results showing a more favorable reaction in alkaline conditions. Upon analyzing the specific reaction equation, it is noted that the interaction between Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e leads to a continuous release of H\u003csup\u003e+\u003c/sup\u003e ions, as described in Text S3, suggesting that H\u003csup\u003e+\u003c/sup\u003e ions act as inhibitors in the reaction. Higher pH levels result in lower concentrations of H\u003csup\u003e+\u003c/sup\u003e ions, thereby promoting the reaction. In alkaline conditions, the presence of OH\u003csup\u003e\u0026minus;\u003c/sup\u003e ions can neutralize the generated H\u003csup\u003e+\u003c/sup\u003e ions, thereby facilitating the reaction. Conversely, the presence of H\u003csup\u003e+\u003c/sup\u003e in acidic conditions hinders the reaction. Consequently, no significant variation in Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e levels is observed at pH\u0026thinsp;~\u0026thinsp;4.8, with only a slight decrease in Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e concentrations at pH\u0026thinsp;~\u0026thinsp;6.3 (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). To confirm the \u0026quot;H\u003csup\u003e+\u003c/sup\u003e inhibition\u0026quot; mechanism, three homogeneous experiments were conducted at pH levels of approximately 6.75, 7.00, and 7.25, as depicted in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eAs shown in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, despite a slight increase in pH from around 6.75 to 7.25, there was a significant discrepancy in the concentration variations of Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e: the reaction was markedly hindered at pH levels of about 6.75 and 7.00. Conversely, the reaction rate experienced a substantial increase as the pH approached alkaline conditions (around 7.25). The data presented in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e illustrates that the average rate constant, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{-}{k}\\)\u003c/span\u003e\u003c/span\u003e, is 6.93\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e at pH\u0026thinsp;~\u0026thinsp;7.25, significantly exceeding the value at pH\u0026thinsp;~\u0026thinsp;6.5 (8.26\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e) by a factor of 8.4. The observed positive correlation between reaction rate and pH, as depicted in Figure S2, provides evidence for the hypothesis that the interplay between Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e is influenced by the concentration of H\u003csup\u003e+\u003c/sup\u003e ions.\u003c/p\u003e\n \u003cp\u003e\u003c/p\u003e\n \u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eA summary of the average rates and associated data for homogeneous reactions at pH levels of 6.75, 7.00, and 7.25.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003epH\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSe\u003csup\u003e4+\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003emol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFe\u003csup\u003e2+\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003emol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varDelta\\:}_{{Se}^{4+}}\\)\u003c/span\u003e\u003c/span\u003e,\u003c/p\u003e\n \u003cp\u003emol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{t}_{total}\\)\u003c/span\u003e\u003c/span\u003e,\u003c/p\u003e\n \u003cp\u003ehours\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{-}{k}\\)\u003c/span\u003e\u003c/span\u003e,\u003c/p\u003e\n \u003cp\u003emol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6.75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.0119\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1440\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8.26\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7.00\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.03888\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1440\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.70\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7.25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.09985\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1440\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6.93\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 The impact of granite\u003c/h2\u003e\n \u003cp\u003eIn Figs. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eA and \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eB, it is evident that the effect of granite is not significant under alkaline conditions. In line with the homogeneous reaction (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e), both Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e concentrations exhibit a rapid decrease over time, especially at pH\u0026thinsp;~\u0026thinsp;8.0, resulting in the total exhaustion of both species within a 48 -hours period. The slope of 3.98 shown in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eC at pH\u0026thinsp;~\u0026thinsp;8.0 aligns closely with the stoichiometric ratio of 4:1 for the Fe\u003csup\u003e2+\u003c/sup\u003e and Se\u003csup\u003e4+\u003c/sup\u003e reaction, as depicted in Eq. 1. These results strongly suggest that in the groundwater environment of disposal sites, \u003csup\u003e79\u003c/sup\u003eSe\u003csup\u003e4+\u003c/sup\u003e will undergo reduction. Furthermore, even in the event of leakage from glass-solidified bodies, the migration of \u003csup\u003e79\u003c/sup\u003eSe\u003csup\u003e4+\u003c/sup\u003e with groundwater (pH\u0026thinsp;~\u0026thinsp;8) is improbable, as it promptly converts into a solid phase.\u003c/p\u003e\n \u003cp\u003eHowever, under acidic conditions, the influence of granite is significant. When pH\u0026thinsp;~\u0026thinsp;4.8, there was a noticeable decrease in Se\u003csup\u003e4+\u003c/sup\u003e concentration (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eA), followed by a stabilization trend, which is couldn\u0026rsquo;t be observed in the homogeneous reaction (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA). This is only the results of adsorption [\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e] but redox, as illustrated by the unchanging Fe\u003csup\u003e2+\u003c/sup\u003e concentration (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eB). At a pH of 6.3, a substantial decrease of Se\u003csup\u003e4+\u003c/sup\u003e about 55% (from 0.1 mM to 0.45 mM) was noted. The data presented in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eD indicates that the average reaction rate is 1.43\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which is approximately 3.83 times higher than under homogeneous conditions (3.73\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). The decrease in selenium concentration cannot be solely attributed to Se\u003csup\u003e4+\u003c/sup\u003e adsorption by granite, as evidenced by two key observations. Firstly, granite shows limited Se\u003csup\u003e4+\u003c/sup\u003e adsorption at pH levels above 6, significantly lower than the adsorption observed at pH\u0026thinsp;~\u0026thinsp;4.8 (approximately 20%). Additionally, the adsorption of Fe\u003csup\u003e2+\u003c/sup\u003e in granite is less than 25% at pH\u0026thinsp;~\u0026thinsp;6.4 (refer to Figure S3), whereas in the reaction system, it decreases by 62.5% (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eB). These findings strongly suggest the involvement of non-adsorption reactions in the solution, such as redox reactions.\u003c/p\u003e\n \u003cp\u003eWhat is the reason behind the absence of the reaction under homogeneous conditions at a pH of about 6.3 (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e), while it does occur in the presence of granite? It is known that a lower concentration of H\u003csup\u003e+\u003c/sup\u003e in the reaction pathway increases the likelihood of the reaction. Therefore, the presence of granite is believed to counteract the H\u003csup\u003e+\u003c/sup\u003e ions in the system. This interaction is associated with the surface charge of the mineral, specifically referred to as the point of zero charge (pH\u003csub\u003epzc\u003c/sub\u003e). Experimental data reveals a pH\u003csub\u003epzc\u003c/sub\u003e value[\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e] of around 5.7 for granite (Text S4). Consequently, when the pH surpasses 5.7, the granite surface becomes negatively charged[\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e], neutralizing the adverse effects of H\u003csup\u003e+\u003c/sup\u003e ions and promoting redox reactions at pH 6.3. In contrast, at approximately pH 4.8, the granite surface becomes positively charged, allowing the inhibitory effects of H\u003csup\u003e+\u003c/sup\u003e ions to persist. As a result, over an extended period (approximately 6240 hours), only the adsorption of Se\u003csup\u003e4+\u003c/sup\u003e onto the granite occurs in the heterogeneous reaction (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eA).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3 Impact of solid-liquid ratio\u003c/h2\u003e\n \u003cp\u003eAs mentioned earlier, the influence of 10 g/L granite on the alkaline reaction (pH\u0026thinsp;~\u0026thinsp;7.5, ~\u0026thinsp;8.0) is minimal. However, the substantial presence of granite at the disposal site necessitated an investigation of the solid-liquid ratio as a critical variable. The results are illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. The concentration variations of Se\u003csup\u003e4+\u003c/sup\u003e in the solution are notably affected by the solid-liquid ratio. The changes in Se\u003csup\u003e4+\u003c/sup\u003e concentration exhibit two distinct stages. Initially, an increase in the solid-liquid ratio speeds up the decline in Se\u003csup\u003e4+\u003c/sup\u003e concentration. Subsequently, in the second stage, as the reaction progresses, the increase in the solid-liquid ratio slows down the decrease in Se\u003csup\u003e4+\u003c/sup\u003e. T These alterations are prominently evident in the enlarged segment of Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eThe concentration variation of Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e) reveals that, apart from the slope observed at a solid-to-liquid ratio of 10 g/L (3.87), which closely matches the stoichiometric ratio of the redox reaction of 4:1, the concentrations of Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e exhibit a curved relationship at other solid-to-liquid ratios. Additionally, with an increase in the solid-to-liquid ratio, the curvature of the curves intensifies. This suggests that the decline in Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e concentrations at higher solid-liquid ratios may not be solely due to redox reactions but could involve other factors such as adsorption.\u003c/p\u003e\n \u003cp\u003eThe decrease in Se\u003csup\u003e4+\u003c/sup\u003e concentration in the solution is a result of two main factors: adsorption onto granite or reduction by Fe\u003csup\u003e2+\u003c/sup\u003e. Granite has a limited capacity to adsorb Se\u003csup\u003e4+\u003c/sup\u003e, with a higher solid-to-liquid ratio enhancing Se\u003csup\u003e4+\u003c/sup\u003e adsorption and consequently leading to a faster decrease in Se\u003csup\u003e4+\u003c/sup\u003e concentration in the liquid phase. Meanwhile, granite has a greater affinity for adsorbing Fe\u003csup\u003e2+\u003c/sup\u003e over Se\u003csup\u003e4+\u003c/sup\u003e (Figure S3), resulting in decreased Fe\u003csup\u003e2+\u003c/sup\u003e levels in the liquid phase at higher solid-to-liquid ratios. This situation is detrimental to Se\u003csup\u003e4+\u003c/sup\u003e reduction. As a result, the presence of granite impedes the decrease of Se\u003csup\u003e4+\u003c/sup\u003e in the liquid phase. These two conflicting mechanisms significantly impact the overall reaction kinetics. At the onset of the reaction, the adsorption of Se\u003csup\u003e4+\u003c/sup\u003e by granite is the primary reason for the decrease in Se\u003csup\u003e4+\u003c/sup\u003e concentration in the liquid phase. However, this adsorption diminishes rapidly with increasing pH. At around pH 7.5, during the initial 50 hours of the reaction, the decline in Se\u003csup\u003e4+\u003c/sup\u003e concentration is solely attributed to adsorption. Conversely, at pH 8, the adsorption capacity of granite is notably weak, with a slight advantage observed only within the first 4 hours of the reaction, as depicted in the magnified section of Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eC. As the adsorption process concludes, Se\u003csup\u003e4+\u003c/sup\u003e in the liquid phase reacts with Fe\u003csup\u003e2+\u003c/sup\u003e, further reducing Se\u003csup\u003e4+\u003c/sup\u003e concentration. Nonetheless, due to granite\u0026apos;s adsorption of Fe\u003csup\u003e2+\u003c/sup\u003e, the concentration of Fe\u003csup\u003e2+\u003c/sup\u003e in the liquid phase remains low (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eB), resulting in a slower redox rate (Table\u0026nbsp;2). This accounts for the slower decrease in Se\u003csup\u003e4+\u003c/sup\u003e concentration under high solid-liquid ratios. In summary, the presence of granite favors the reduction of Se\u003csup\u003e4+\u003c/sup\u003e under acidic conditions, while it hinders Se\u003csup\u003e4+\u003c/sup\u003e reduction under alkaline conditions.\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eTable 2\u003c/strong\u003e. The average rates and relevant data for each reaction are outlined at solid-liquid ratios of 10, 30, 50, 70, and 100 g/L, with corresponding pH values of 7.5 and 8.0.\u003c/p\u003e\n \u003cdiv align=\"char\" class=\"colspec\"\u003e\u003cbr\u003e\u003c/div\u003e\n \u003ctable id=\"Taba\" border=\"1\"\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003epH\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSe\u003csup\u003e4+\u003c/sup\u003e,\u003c/p\u003e\n \u003cp\u003emol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFe\u003csup\u003e2+\u003c/sup\u003e,\u003c/p\u003e\n \u003cp\u003emol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eS/L,\u003c/p\u003e\n \u003cp\u003eg\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varDelta\\:}_{{Se}^{4+}}\\)\u003c/span\u003e\u003c/span\u003e,\u003c/p\u003e\n \u003cp\u003emol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{t}_{total}\\)\u003c/span\u003e\u003c/span\u003e,\u003c/p\u003e\n \u003cp\u003ehours\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{-}{{\\text{k}}^{{\\prime\\:}}}\\)\u003c/span\u003e\u003c/span\u003e,\u003c/p\u003e\n \u003cp\u003emol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"5\"\u003e\n \u003cp\u003e7.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.0967\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e408\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.37\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.0992\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e408\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.43\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.0988\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e408\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.42\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e70\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.0982\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e408\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.41\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e100\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.101\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e504\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.00\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"5\"\u003e\n \u003cp\u003e8.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.0940\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e80\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.18\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.0930\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e120\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e7.75\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.0894\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e240\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.73\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e70\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.0892\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e168\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.31\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e100\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.0874\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e240\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.64\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003e3.4 The calculation of Gibbs free energy and electromotive force\u003c/h2\u003e\n \u003cp\u003eThe Gibbs free energy (\u0026Delta;rG) is a critical parameter in assessing the reversibility of chemical reactions[\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e]. Through analysis of the pH-Eh diagram[\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e] (Figure S4) and associated thermodynamic data (Table \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e), Se\u003csup\u003e0\u003c/sup\u003e and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e have been identified as the thermodynamically stable products in our system. The \u0026Delta;\u003csub\u003er\u003c/sub\u003eG for the reaction was determined (refer to Figure S5, Text S3), with all calculated \u0026Delta;\u003csub\u003er\u003c/sub\u003eG values (Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e) found to be negative, indicating thermodynamic feasibility. Furthermore, \u0026Delta;\u003csub\u003er\u003c/sub\u003eG decreases with increasing pH. Experimental findings indicate that the REDOX reaction occurs only when \u0026Delta;\u003csub\u003er\u003c/sub\u003eG reaches \u0026minus;\u0026thinsp;160.12 kJ\u0026middot;mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to a pH\u0026thinsp;\u0026gt;\u0026thinsp;6.4.\u003c/p\u003e\n \u003cp\u003e\u003c/p\u003e\n \u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eGibbs free energy and potential across different reactions.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eID\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003epH\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSe\u003csup\u003e4+\u003c/sup\u003e, mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFe\u003csup\u003e2+\u003c/sup\u003e, mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u0026Delta;\u003csub\u003er\u003c/sub\u003eG, kJ\u0026middot;mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eThe electromotive force, E\u003csup\u003e\u0026theta;\u003c/sup\u003e, mV\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.80\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-96.20\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e479.50\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6.40\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-160.12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e928.54\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6.75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-174.10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1026.77\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e7.00\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-184.09\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1096.93\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e7.25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-194.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1167.10\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e7.50\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-204.06\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1237.26\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8.00\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4e-4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-224.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1377.58\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003c/p\u003e\n \u003cp\u003eMonitoring the redox potential is essential for understanding the driving force behind redox reactions. We investigated the potentials of Se\u003csup\u003e4+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e solutions, along with the reaction process (see Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e) (refer to Text S5). Our findings revealed that the reaction potentials (E\u003csup\u003e\u0026theta;\u003c/sup\u003e, depicted by pentagonal dots in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e) approached 0 mV, falling below the oxidation potential of Se\u003csup\u003e4+\u003c/sup\u003e at the specified pH (indicated by red circle points in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e) yet surpassing the reduction potential of Fe\u003csup\u003e2+\u003c/sup\u003e (depicted by gray circle points in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e), signifying the occurrence of the redox reaction. The lower \u0026Delta;rG values and higher reaction potential indicated favorable conditions for the redox reaction. Notably, at pH\u0026thinsp;~\u0026thinsp;8.0, with \u0026Delta;rG = -224.04 kJ\u0026middot;mol-1 and E\u0026thinsp;=\u0026thinsp;1377.58 mV, the REDOX reaction proceeded smoothly.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n \u003ch2\u003e3.5 Species analysis-XPS\u003c/h2\u003e\n \u003cp\u003eAs previously mentioned, the evolution of Se\u003csup\u003e4+\u003c/sup\u003e following adsorption is a significant issue. It is crucial to investigate whether the adsorbed Se\u003csup\u003e4+\u003c/sup\u003e can undergo reduction, as this determines whether the long-term retardation of \u003csup\u003e79\u003c/sup\u003eSe\u003csup\u003e4+\u003c/sup\u003e is effective. XPS data analysis was conducted to monitor the evolution of solid-phase species over varying reaction durations in a solution of 70 g/L at pH 7.5 and 8.0. Detailed peak positions and areas are presented in Table S2 and visually depicted in Figs. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eAs shown in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e, the ratio of Fe\u003csup\u003e2+\u003c/sup\u003e/Fe\u003csup\u003e3+\u003c/sup\u003e[\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e] peak areas decreased gradually over time, as outlined in Table S2. This observation suggests that the adsorbed Fe\u003csup\u003e2+\u003c/sup\u003e gradually reacted with Se\u003csup\u003e4+\u003c/sup\u003e, leading to a progressive decrease in Fe\u003csup\u003e2+\u003c/sup\u003e concentration in the solid phase. Furthermore, the absence of a Se\u003csup\u003e4+\u003c/sup\u003e peak in all reactions indicates that Se\u003csup\u003e0\u003c/sup\u003e is the sole Se species present in the solid phase[\u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e]. This suggests that although Se\u003csup\u003e4+\u003c/sup\u003e may initially be immobilized in its adsorbed state, it eventually undergoes reduction to Se\u003csup\u003e0\u003c/sup\u003e. Therefore, unless other relevant factors affecting the safe disposal of Se\u003csup\u003e0\u003c/sup\u003e are considered, our study effectively illustrates the minimal long-term disposal risks associated with \u003csup\u003e79\u003c/sup\u003eSe\u003csup\u003e4+\u003c/sup\u003e.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Conclusions and perspectives","content":"\u003cp\u003eIn this study, we investigated the redox behavior of Se\u003csup\u003e4+\u003c/sup\u003e with a specific focus on the impact of pH and granite as crucial variables. We elucidated the underlying mechanisms and clarified the prolonged inhibition mechanism of \u003csup\u003e79\u003c/sup\u003eSe\u003csup\u003e4+\u003c/sup\u003e. Our findings reveal that the reaction kinetics are significantly influenced by the concentration of H\u003csup\u003e+\u003c/sup\u003e, with the reaction rate consistently increasing at higher pH levels. The redox reaction encounters challenges or may be impeded at pH levels below 7, while it proceeds readily at pH levels above 7. Notably, at a pH of approximately 8, the rate constant (k) can reach 4.22\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in a homogeneous system, representing a more than 1000-fold increase compared to the rate at pH 6.3 (3.73\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). However, in the presence of granite in the reaction system, the reaction can occur under acidic conditions (pH\u0026thinsp;\u0026gt;\u0026thinsp;5.7). The unique solid-liquid interface created by granite is distinct from the bulk solution. The OH\u003csup\u003e\u0026minus;\u003c/sup\u003e ions at its interface can neutralize the H\u003csup\u003e+\u003c/sup\u003e ions produced by the reaction, thereby promoting the reaction at a pH close to the point of zero charge (pzc) of the granite (~\u0026thinsp;5.7). For instance, at pH\u0026thinsp;~\u0026thinsp;6.3, approximately 55% of Se\u003csup\u003e4+\u003c/sup\u003e was observed to react. In our study, we also examined the thermodynamics of the reaction by analyzing the changes in Gibbs free energy (ΔG) and reaction potential (ΔE). A smaller ΔG corresponds to a larger ΔE, indicating a more favorable reaction. At pH\u0026thinsp;~\u0026thinsp;8.0, the values were ΔG=-224.04 kJ\u0026middot;mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and ΔE\u0026thinsp;=\u0026thinsp;1377.58 mV, signifying a highly facile progression of the redox reaction. Through the pH-Eh diagram and XPS analysis, we have identified Se\u003csup\u003e0\u003c/sup\u003e as the predominant species on the solid-phase surface. It has been observed that the adsorbed Se\u003csup\u003e4+\u003c/sup\u003e undergoes continuous reduction process over time instead of dissociation into the solution. This finding implies that under repository conditions (pH\u0026thinsp;~\u0026thinsp;8), \u003csup\u003e79\u003c/sup\u003eSe\u003csup\u003e4+\u003c/sup\u003e will be immobilized in the repository in reduced form, thereby presenting a low migration risk. The data and conclusions presented herein offer crucial theoretical support for the construction of geological repositories and safety assessments, while also guiding research on other redox-sensitive radionuclides.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eXiang Li: Investigation, Data curation. Jingju Li: Writing \u0026ndash;original draft, Software, Data curation. Shirong Wang: Supervision, Formal analysis. Yun Zhou: Data curation.Jiangang He: Writing \u0026ndash; review \u0026amp; editing, Supervision, Methodology.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eThis work was supported by National Natural Science Foundation of China (No. 22106057), Fundamental Research Funds for the Central Universities of China (lzujbky-2023-41, lzujbky-2024-jdzx14), The Youth Fund Project of Hainan Provincial Natural Science Foundation (423 QN 323).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eMartinez-Moreno, M.F., et al., \u003cem\u003eImpact of compacted bentonite microbial community on the clay mineralogy and copper canister corrosion: a multidisciplinary approach in view of a safe Deep Geological Repository of nuclear wastes.\u003c/em\u003e Journal of hazardous materials, 2023. \u003cstrong\u003e458\u003c/strong\u003e: p. 131940.\u003c/li\u003e\n\u003cli\u003eShukla, A., et al., \u003cem\u003eCharacterization of novel thorium tolerant Ochrobactrum intermedium AM7 in consort with assessing its EPS-Thorium binding.\u003c/em\u003e Journal of hazardous materials, 2020. \u003cstrong\u003e388\u003c/strong\u003e: p. 122047.\u003c/li\u003e\n\u003cli\u003ePearce, C.I., et al., \u003cem\u003eEvaluation of materials for iodine and technetium immobilization through sorption and redox-driven processes.\u003c/em\u003e The Science of the total environment, 2020. \u003cstrong\u003e716\u003c/strong\u003e: p. 136167.\u003c/li\u003e\n\u003cli\u003eWakabayashi, T., \u003cem\u003eConcept of a fast breeder reactor to transmute MAs and LLFPs.\u003c/em\u003e Scientific reports, 2021. \u003cstrong\u003e11\u003c/strong\u003e(1): p. 22443.\u003c/li\u003e\n\u003cli\u003eHo, M.S., et al., \u003cem\u003eRetention of immobile Se(0) in flow-through aquifer column systems during bioreduction and oxic-remobilization.\u003c/em\u003e The Science of the total environment, 2022. \u003cstrong\u003e834\u003c/strong\u003e: p. 155332.\u003c/li\u003e\n\u003cli\u003ePoulain, A., et al., \u003cem\u003eSelenium Nanowire Formation by Reacting Selenate with Magnetite.\u003c/em\u003e Environmental science \u0026amp; 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This study, reveals a novel observation in the field, showing that \u003csup\u003e79\u003c/sup\u003eSe\u003csup\u003e4+\u003c/sup\u003e can be efficiently reduced and sequestered by precipitating as Se\u003csup\u003e0\u003c/sup\u003e. Our findings indicate that the reduction of Se\u003csup\u003e4+\u003c/sup\u003e is controlled by the \"H\u003csup\u003e+\u003c/sup\u003e inhibition\" mechanism. The rate at pH\u0026thinsp;~\u0026thinsp;8.0 (\u0026#119896; = 4.22\u0026times;10\u003csup\u003e\u0026minus;3\u003c/sup\u003e mol\u0026middot;L\u003csup\u003e\u0026minus;1\u003c/sup\u003e\u0026middot;h\u003csup\u003e\u0026minus;1\u003c/sup\u003e) is 1131 times faster than the rate at pH 6.3 (\u0026#119896; = 3.73\u0026times;10\u003csup\u003e\u0026minus;6\u003c/sup\u003e mol\u0026middot;L\u003csup\u003e\u0026minus;1\u003c/sup\u003e\u0026middot;h\u003csup\u003e\u0026minus;1\u003c/sup\u003e). Granite's influence is more nuanced than pH, establishing a distinctive reaction environment at the solid-liquid interface distinct from the bulk solution, leading to reactions occurring above a pH of 5.7 (pHpzc). Additionally, we examined the long-term retention mechanism of Se\u003csup\u003e4+\u003c/sup\u003e in the solid phase, confirming that adsorbed Se\u003csup\u003e4+\u003c/sup\u003e undergoes continuous reduction instead of dissociating into the solution. This validates the effectiveness of the engineering design in the prolonged immobilization of \u003csup\u003e79\u003c/sup\u003eSe\u003csup\u003e4+\u003c/sup\u003e. The data and conclusions presented in this study offer crucial support for the safety assessment of geological disposal repositories and relevant environmental research.\u003c/p\u003e","manuscriptTitle":"An Inquiry into the Oxidative-Reductive Actions of 79Se4+","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-01 11:40:58","doi":"10.21203/rs.3.rs-7580301/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":"a60b5aad-de68-468b-bd86-ac3823c57550","owner":[],"postedDate":"October 1st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-12-18T09:39:45+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-01 11:40:58","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7580301","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7580301","identity":"rs-7580301","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}