Efficient removal of Hg2+ from wastewater by a novel Cu-modified attapulgite: Adsorption performance and mechanism

Efficient removal of Hg2+ from wastewater by a novel Cu-modified attapulgite: Adsorption performance and mechanism | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Efficient removal of Hg2+ from wastewater by a novel Cu-modified attapulgite: Adsorption performance and mechanism Chongming Chen, Dong Li, Jinxing Yu, Kai Che This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4755442/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 19 Dec, 2024 Read the published version in Korean Journal of Chemical Engineering → Version 1 posted 4 You are reading this latest preprint version Abstract The development of low-cost and highly efficient adsorbents is essentially needed for removing Hg 2+ species from desulfurization sludge leaching wastewater. In this study, a series of novel Cu-modified attapulgite (Cu-ATP) adsorbents were synthesized via a simple HNO 3 treatment combined with an improved impregnation method. The Hg 2+ removal efficiency of these Cu-ATP adsorbents was investigated in simulated leaching wastewater. The effects of HNO 3 concentration, Cu precursor, Cu loading content, and other adsorption conditions on Hg 2+ removal using Cu-ATP were investigated. The results demonstrated that Cu-ATP prepared with CuSO 4 as the precursor and treated with 3 mol/L HNO 3 showed excellent Hg 2+ removal performance. Moreover, with increasing adsorbent content and adsorption time, the Hg 2+ removal efficiency of Cu-ATP first increased and then stabilized. However, with an increase in pH value, the Hg 2+ removal efficiency first increased and then decreased, whereas the removal showed a decreasing trend with increasing initial Hg 2+ concentration. The adsorption kinetics results indicated that Hg 2+ adsorption on Cu-ATP was well described by the pseudo-second-order model. Furthermore, various characterization methods, including Brunauer − Emmett − Teller analysis (BET), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), were employed to analyze the physicochemical properties of the adsorbents. The analyses confirmed that the superior Hg 2+ removal efficiency of Cu-ATP was mainly due to the complexation of Hg 2+ with chemisorbed oxygen produced by Cu doping and S species generated from the Cu precursor (CuSO 4 ). These findings underscore the potential of Cu-ATP as a cost-effective adsorbent for removing Hg 2+ from wastewater. Wastewater Hg2+ removal Cu-modified attapulgite removal mechanism Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 1. Introduction Mercury is a highly polluting heavy metal known for its persistence, non-degradability, and biological toxicity. It has been listed as a priority pollutant for control by various environmental agencies worldwide [ 1 ]. Coal-fired power plants are considered one of the major sources of anthropogenic mercury emissions and have thus attracted a lot of attention in recent years [ 2 ]. During the production process of coal-fired power plants, mercury migrates and transforms with the flue gas. Elemental mercury (Hg 0 ) is discharged into the atmosphere with the flue gas owing to the volatility of Hg 0 and its insolubility in water. However, oxidized mercury species (Hg 2+ ) are easily soluble in water. They are captured by the wet desulfurization system and then enriched in the desulfurization products (such as sludge and wastewater from desulfurization) [ 3 , 4 ]. The high concentration of Hg 2+ in desulfurization sludge significantly restricts the safe disposal and resource utilization of the sludge. [ 5 ]. In recent years, researchers have used chemical leaching methods to remove Hg 2+ from desulfurization sludge, and have achieved satisfactory Hg 2+ removal efficiency. Unfortunately, the high content of Hg 2+ in the leaching wastewater can cause secondary pollution and pose serious risks to the ecological environment and human health [ 6 ]. Moreover, the enrichment of Hg 2+ species in desulfurization wastewater complicates the treatment of desulfurization wastewater in coal-fired power plants. Therefore, developing efficient Hg 2+ removal technology for wastewater is crucial to prevent secondary pollution. This advancement is significant for improving chemical leaching technology for mercury-contaminated desulfurization sludge and for treating wastewater polluted by Hg 2+ species. Currently, studies on the removal of Hg 2+ species from wastewater produced during the leaching process of desulfurization sludge are few. Most existing studies have focused on the efficient removal of Hg 2+ from industrial wastewater [ 7 , 8 ]. The main technologies for Hg 2+ removal from wastewater include chemical precipitation, biological treatment, ion exchange, membrane separation, electrochemical methods, and adsorption. Among these, the adsorption method is favored by researchers owing to its low investment cost, high removal efficiency, lack of secondary pollution, and low energy consumption [ 9 – 11 ]. The adsorption method utilizes the physical and chemical characteristics of adsorbents and pollutants to transfer trace pollutant molecules dissolved in the water phase to solid-phase adsorbents. This process involves various interactions, such as van der Waals forces, hydrophobic effects, electrostatic effects, and ion exchange [ 5 ]. The key to the adsorption method is designing and developing adsorbents with high performance and low cost. With continuous advancements in adsorbents, materials such as metal–organic frameworks (MOFs), clay, carbon nanotubes, layered double hydroxides, and zeolites have been used to remove Hg 2+ from wastewater [ 12 – 15 ]. Attapulgite (ATP) is a water-rich magnesium aluminosilicate clay mineral with a chain-layer structure. Its crystal structure contains cations such as Na + , Ca 2+ , Fe 3+ , and Al 3+ , which endows it with excellent cation-exchange properties [ 16 ]. Natural ATP features abundant microporous channels, providing numerous adsorption sites for pollutant removal. Additionally, the presence of -OH active groups and excess charges on the ATP surface enhances its ion-exchange capabilities, surface electrostatic adsorption, and microporous diffusion properties, all of which improve the adsorptive performance of ATP [ 17 – 19 ]. Considering these advantages, ATP is commonly used for the adsorption removal of heavy metal ions in wastewater. While natural ATP is inexpensive and readily available, it contains various impurities, and its heavy metal-adsorption performance is relatively low [ 20 ]. Researchers have enhanced the specific surface area and micropore structure of ATP through metal load modification, acid treatment, organic modification, and ultrasonic modification. These methods increase the number of surface adsorption sites, thereby significantly improving the performance of ATP in adsorbing heavy metals [ 21 – 25 ]. However, there are only a few studies on Hg 2+ removal from wastewater using modified ATP, and the mechanism by which modified ATP improves the adsorption performance of heavy metals is still unclear. Additionally, the cation composition of desulfurization sludge leaching wastewater is more complex than that of traditional Hg 2+ -containing wastewater, which may affect the adsorption performance of modified ATP. This study employed a method combining acid treatment with metal modification to modify natural ATP and synthesize a series of Cu-modified ATP (Cu-ATP) adsorbents. The Hg 2+ removal efficiency of Cu-ATP was investigated in simulated desulfurization sludge leaching wastewater, and the effects of various reaction conditions on the Hg 2+ adsorption performance of Cu-ATP were explored. The optimal reaction conditions for Hg 2+ adsorption by Cu-ATP were determined. Additionally, the reaction mechanism of Hg 2+ adsorption on the Cu-ATP surface was analyzed according to adsorption activity and characterization results. These findings provide a foundation for developing low-cost, high-efficiency, environmentally friendly adsorbents and are significant for advancing chemical leaching technology for Hg-contaminated desulfurized sludge and treating Hg 2+ -polluted wastewater. 2. Materials and Methods 2.1. Adsorbent preparation The specific method for treating ATP adsorption materials with HNO 3 was as follows: 4 g of ATP was weighed, and 20 mL of HNO 3 solution with varying concentrations (1, 3, 5, 7 mol/L) was added to the beaker. The mixture was then heated and boiled for 2 h. After the obtained solid was washed with deionized water, it was calcined for 2 h at 200°C and then ground to 60 mesh to extract HNO 3 -treated ATP. A series of Cu-ATP adsorbents were prepared through an improved impregnation method. First, a certain amount of ATP pretreated with HNO 3 was added to an aqueous solution of a Cu precursor. The mixture was then stirred for 2 h in a 50°C thermostatic water bath, dried at 100°C for 12 h, and calcined at 450°C for 3 h in an N 2 /O 2 atmosphere. During the preparation process, various Cu-ATP adsorbents were obtained through the alteration of the types of Cu precursors (CuNO 3 , CuCl 2 , and CuSO 4 ) and the Cu content (0, 2, 4, 6, and 8%). 2.2. Adsorbent performance test The specific method for testing the adsorption performance of the adsorbent was as follows: 2 g of Cu-ATP adsorbent was added to 1000 mL of simulated wastewater containing 100 mg/L of Hg 2+ and stirred at room temperature for 40 min. After suction filtration, the residual Hg 2+ concentration in the filtrate was determined using an AFS-933 atomic fluorescence photometer, and the corresponding Hg 2+ removal rate was calculated. The Hg 2+ removal rate was calculated as follows: $$R=\frac{{{C_0} - {C_e}}}{{{C_0}}} \times 100\%$$ 1 . Here, C 0 is the initial Hg 2+ concentration in the solution, and C e represents the remaining Hg 2+ concentration in the filtrate. 2.3. Adsorbent characterization Several characterization methods were used to analyze the physicochemical properties of the adsorbents. The Brunauer − Emmett − Teller (BET) surface area, pore volume, and average pore diameter of the different adsorbents were obtained using a N 2 adsorption isotherms analyzer (ASAP 2020). The standard BET equation was used to calculate the specific surface area. The X-ray diffraction (XRD) method was employed to determine the crystallinity and the dispersivity of Cu species on the adsorbents pretreated under different conditions. The XRD patterns of the samples were collected using a Rigaku X-ray diffractometer with Cu-Kα radiation. To investigate the valence states of surface elements of different adsorbents, X-ray photoelectron spectroscopy (XPS) analysis was performed using an X-ray photoelectron spectrometer with Al-Kα radiation. The C 1s peak at 284.6 eV was used to calibrate the observed spectra. 3. Results and Discussion 3.1. Hg 2+ removal over ATP pretreated under different conditions 3.1.1. Effect of HNO 3 concentration on Hg 2+ removal The effects of HNO 3 concentrations on the Hg 2+ removal properties of ATP pretreated by HNO 3 were investigated, and the results are shown in Fig. 1 . As the HNO 3 concentration increased, the Hg 2+ removal efficiency of ATP initially increased and then decreased (Fig. 1 ). The optimal Hg 2+ removal performance was achieved at an HNO 3 concentration of 3 mol/L. This indicated that activation of ATP using an appropriate amount of HNO 3 could enhance the adsorption performance of ATP for Hg 2+ , but excessive HNO 3 concentrations inhibited the adsorption performance of ATP for Hg 2+ . This phenomenon presumably occurred because activation of ATP using an appropriate amount of HNO 3 could remove some cations from the crystal structure of ATP, thereby increasing the specific surface area and pore volume of ATP and enhancing its Hg 2+ -adsorption capacity [ 26 ]. However, under excess HNO 3 concentrations, the cations in the ATP octahedral structure were nearly completely dissolved, causing the tetrahedral sheet to lose support and the structure to collapse. This resulted in a reduction of the internal pore channels and specific surface area, thereby weakening the adsorption capacity of ATP. BET characterization of ATP also confirmed this hypothesis. Considering these experimental results, 3 mol/L of HNO 3 was selected as the optimal treatment condition for ATP, and follow-up adsorption experiments were conducted. 3.1.2. Effect of different Cu precursors on Hg 2+ removal Figure 2 illustrates the effects of different Cu precursors on the adsorption performance of Cu-modified ATP for Hg 2+ removal. ATP modified with various Cu precursors showed varying Hg 2+ adsorption capabilities. Among these, CuSO 4 -modified ATP exhibited the most effective Hg 2+ adsorption performance. This occurrence is probably attributable to the presence of sulfur (S) species in the ATP pore structure modified by CuSO 4 , which readily bound with Hg 2+ to form HgS, resulting in enhanced Hg 2+ adsorption capacity of Cu-ATP. Subsequent XPS characterization results also confirmed this hypothesis. Additionally, owing to Cu doping, some cations in ATP were replaced and thus created vacancy-deficient sites on the ATP surface. This promoted the migration and transformation of lattice oxygen on the ATP surface and enhanced HgO formation on the surface, which facilitated Hg 2+ adsorption [ 27 ]. The preference for formation of HgO on Cu-ATP was also why Hg 2+ predominantly existed in the form of HgO on the ATP surface. Given the superior Hg 2+ adsorption performance of CuSO 4 -modified ATP, CuSO 4 was chosen as the precursor for Cu in subsequent experiments. 3.1.3. Effect of Cu content on Hg 2+ removal Figure 3 illustrates the effects of different Cu contents on the Hg 2+ adsorption performance of Cu-ATP. As the Cu loading increased, the adsorption capacity of Cu-ATP for Hg 2+ initially improved and then declined. This suggests that appropriate Cu doping can effectively enhance Hg 2+ adsorption performance. However, excessive Cu loading reduced the Hg 2+ adsorption capacity of ATP. Increased Cu loading can block the internal micropores of ATP, reducing the specific surface area of Cu-ATP and thereby weakening its Hg 2+ adsorption capacity. BET results for Cu-ATP with varying Cu loading also support this hypothesis. 3.2. Effect of adsorption conditions on Hg 2+ removal 3.2.1. Effect of adsorbent content The effects of ATP and Cu-ATP addition on Hg 2+ adsorption properties were investigated (Fig. 4 ). Both ATP and Cu-ATP showed similar trends in Hg 2+ removal efficiency. With increasing adsorbent content, Hg 2+ removal efficiency initially increased and then stabilized. At an adsorbent content of 2 g, the Hg 2+ adsorption efficiency reached 90.7%. A further increase in adsorbent content did not significantly enhance Hg 2+ removal efficiency, attributable to the increase in active adsorption sites for Hg 2+ as the adsorbent content increased. The increased adsorption sites resulted in higher total Hg 2+ adsorption, thereby improving Hg 2+ removal efficiency. 3.2.2. Effect of adsorbent time The adsorption time influenced the contact process between the adsorbent and wastewater, thereby affecting the Hg 2+ removal performance. Therefore, the effects of adsorption time on Hg 2+ removal efficiency were investigated, and the results are shown in Fig. 5 . Within the 0–20 min range, both ATP and Cu-ATP showed a rapid increase in Hg 2+ removal efficiency with increasing adsorption time. The Hg 2+ removal efficiency of ATP stabilized around 25 min and slightly decreased after 50 min. After adsorption equilibrium was reached, the partial desorption of adsorbed Hg 2+ species occurred over time, resulting in a slight decrease in Hg 2+ removal efficiency. This instability in the adsorption process indicated noticeable desorption behavior on the ATP surface. The Hg 2+ removal efficiency of Cu-ATP stabilized at around 35 min of adsorption time. Moreover, extending the adsorption time did not lead to a decrease in Hg 2+ removal efficiency, which indicated stable adsorption of Hg 2+ on Cu-ATP without significant desorption behavior during the process. This result suggests that Hg 2+ species adsorbed on Cu-ATP surfaces were more stable than those on ATP surfaces. 3.2.3. Effect of pH value Considering that the leaching wastewater from desulfurization sludge is typically acidic, the performance of the adsorbent in removing Hg 2+ species under different pH conditions was investigated, and the results are shown in Fig. 6 . The Hg 2+ removal efficiency of both ATP and Cu-ATP first increased and then decreased notably within the pH range of 3–7. Specifically, at wastewater pH values exceeding 5, the Hg 2+ removal efficiency of both adsorbents decreased significantly. This indicated that the adsorbents demonstrated better Hg 2+ removal performance at pH 3–5, possibly because the acidic environment aided in eliminating impurity ions from the pore channels of the adsorbent, thereby maintaining a higher specific surface area and a pore structure. This finding aligns with the principle that acid treatment enhances ATP adsorption performance [ 28 ]. 3.2.4. Effect of initial Hg 2+ concentration The variation in the Hg 2+ concentration of leaching wastewater under different leaching conditions may impact the Hg 2+ removal performance of the adsorbent. Therefore, the effect of the initial Hg 2+ concentration in wastewater on the Hg 2+ removal performance of the adsorbent was investigated, and the results are shown in Fig. 7 . As the initial Hg 2+ concentration increased, the Hg 2+ removal efficiency of both ATP and Cu-ATP gradually decreased. This indicated that higher concentrations of Hg 2+ led to lower Hg 2+ removal performance by the adsorbent. A higher Hg 2+ concentration led to more Hg 2+ species being adsorbed onto the micropore structure and active sites of the adsorbent. This gradual saturation of adsorption sites reduced the capacity of the adsorbent to remove Hg 2+ and hence lowered its removal efficiency. 3.3. Adsorption kinetics of Hg 2+ To better investigate the adsorption process, the pseudo-first-order model (Eq. 2 ) and pseudo-second-order model (Eq. 3 ) were employed to evaluate the adsorption kinetics of Hg 2+ on ATP and Cu-ATP, respectively. $$\ln \left( {{q_e} - {q_t}} \right)=\ln {q_e} - {k_1}t$$ 2 , $$\frac{t}{{{q_t}}}=\frac{1}{{{k_2}q_{e}^{2}}}+\frac{1}{{{q_e}}}t$$ 3 , where q e (mg·g − 1 ) and q t (mg·g − 1 ) represent the amounts of adsorbed Hg 2+ species at equilibrium time and time t (min). k 1 (min − 1 ) and k 2 (g·(mg·min) −1 ) represent the rate constants of the pseudo-first-order model and the pseudo-second-order model, respectively. The Hg 2+ adsorption profiles on ATP and Cu-ATP over time are shown in Fig. 8 and Fig. 9 , respectively. Table 1 presents the kinetic parameters for Hg 2+ adsorption on ATP and Cu-ATP fitted by the aforementioned models. The adsorption kinetics of Hg 2+ on ATP are well described by the pseudo-first-order model, as indicated by the high correlation coefficient (R 2 = 0.995). This indicated that the adsorption of Hg 2+ on ATP mainly involved a physical process. However, for Cu-ATP, both models showed a higher correlation coefficient, which indicates that Cu addition altered the adsorption process of Hg 2+ on the adsorbent. This suggests that both physical and chemical adsorption processes occurred simultaneously. The pseudo-second-order model exhibited a higher correlation coefficient than the pseudo-first-order model, which suggests that the Hg 2+ adsorption on Cu-ATP was mainly a chemical adsorption process, which involved electron exchange between the adsorbent and the adsorbate. This process is also an important reason that Cu-ATP exhibited high Hg 2+ adsorption capacity of Cu-ATP. Subsequent XPS characterization of the adsorbent also confirmed that the Hg 2+ adsorption process induced electron transfer on the adsorbent surface, which enhanced Hg 2+ adsorption activity. Table 1 Kinetic parameters for adsorption Kinetic model Parameters ATP Cu-ATP Pseudo-first-order model q e (mg·g − 1 ) 29.87 44.74 k 1 (min − 1 ) 0.161 0.182 R 2 0.995 0.993 Pseudo-second-order model q e (mg·g − 1 ) 32.37 48.30 k 2 (g·(mg·min) −1 ) 0.008 0.007 R 2 0.971 0.997 3.4. Adsorbent characterization 3.4.1. BET A large specific surface area and good pore volume are crucial for ensuring high adsorption capacity in adsorbents. The specific surface area, pore volume, and average pore diameter of ATP samples treated under different conditions were measured, and the results are listed in Table 2 . Untreated ATP exhibited a low specific surface area and pore volume, likely owing to the presence of impurity ions. In contrast, ATP samples treated with HNO 3 showed significantly improved specific surface area and pore volume. This suggests that the HNO 3 treatment effectively removed impurity ions from ATP micropores, resulting in a superior pore structure. However, at HNO 3 concentrations exceeding 3 mol/L, the specific surface area and pore volume of ATP significantly decreased, likely because excessively high concentrations of HNO 3 can lead to the collapse of ATP channels, reducing both the specific surface area and internal channels and thereby weakening the adsorption capacity of ATP [ 27 , 28 ]. The Hg 2+ adsorption capacity of ATP treated with different concentrations of HNO 3 also supports this speculation. Additionally, after ATP was doped with Cu, the specific surface area and pore volume of ATP slightly decreased. The slight decrease is attributable to the relatively low Cu loading, which resulted in Cu being highly dispersed on the surface of ATP without significantly reducing its specific surface area and pore volume. This observation aligns well with the XRD characterization results. Table 2 Physical characteristics of ATP pretreated under different conditions Samples S BET (m 2 /g) V pore (cm 3 /g) D pore (nm) Natural ATP 36.2 0.09 11.3 1 mol/L HNO 3 -ATP 127.6 0.15 7.3 3 mol/L HNO 3 -ATP 141.3 0.13 7.5 5 mol/L HNO 3 -ATP 121.2 0.15 6.9 7 mol/L HNO 3 -ATP 109.8 0.14 7.1 2% Cu-ATP 132.2 0.12 6.3 4% Cu-ATP 126.6 0.11 6.4 6% Cu-ATP 120.3 0.11 6.1 8% Cu-ATP 114.2 0.12 6.5 3.4.2. XRD XRD was employed to examine the surface phase structures of ATP samples under various treatment conditions, and the results are depicted in Fig. 10 . ATP samples exhibited characteristic diffraction peaks, with peaks at 19.7° and 26.6° corresponding to the (040) and (400) planes of ATP clay [ 29 , 30 ]. Moreover, different ATP samples did not exhibit significant differences in the intensity of the abovementioned characteristic peaks, which indicated that Cu doping did not significantly alter the fundamental structure. Additionally, the patterns of ATP samples doped with 4–8% Cu exhibited characteristic peaks of CuO, indicating the presence of Cu species in the form of CuO crystallites on ATP. However, the pattern of 2% Cu-ATP exhibited no distinct peak of Cu species. This observation is attributable to the low loading of Cu, which resulted in the high dispersion of Cu on the surface of ATP. 3.4.3. XPS The adsorption kinetics results suggest that Hg 2+ adsorption on Cu-ATP is a chemical adsorption process. To further investigate the Hg 2+ adsorption process on Cu-ATP, the chemical valence states of elements on the surface of ATP, 6% Cu-ATP, and used Cu-ATP were analyzed through the XPS method. Figure 11 shows the Cu 2p XPS spectra of ATP samples pretreated under different conditions. No significant characteristic peaks of Cu species were detected on the surface of untreated ATP, indicating the absence of Cu species on the ATP surface. However, Cu-ATP exhibited clear characteristic peaks of Cu species both before and after adsorption. The binding energies located at 933.1 eV and 953.2 eV corresponded to Cu⁺ species, while the peaks near 934 eV and 954.2 eV corresponded to Cu 2+ species [ 31 , 32 ]. This result indicated that both Cu 2+ and Cu + occurred on the Cu-ATP surface. The coexistence of Cu²⁺ and Cu⁺ species was due to electron transfer during the Cu doping process. This electron transfer process promoted the formation of surface oxygen vacancy sites, thereby increasing the amount of chemisorbed oxygen and facilitating the migration of oxygen species, which enhanced the chemisorption behavior of the adsorbent [ 30 ]. The O 1s XPS spectra also confirmed that Cu doping increased the content of chemisorbed oxygen on the adsorbent surface. In addition, the Cu + /Cu 2+ ratio on the used Cu-ATP surface was significantly higher than that on fresh Cu-ATP, indicating that an electron transfer process occurred during the adsorption process, converting part of the Cu 2+ species into Cu + species. This process increased the formation of chemisorbed oxygen, which then combined with Hg 2+ to form stable HgO, thereby enhancing the chemisorbed behavior of the adsorbent. Subsequent Hg 4f XPS characterization also confirmed that the adsorbed mercury species mainly existed in the form of HgO. The O 1s XPS spectra of ATP samples pretreated under different conditions are displayed in Fig. 12 . The O 1s characteristic peaks of all samples could be fitted into two peaks. The peaks around 530.3 eV corresponded to the lattice oxygen (O β ), while the peaks near 531.6 eV corresponded to the chemisorbed oxygen (O α ) [ 33 ]. The O α ratio on the ATP surface was only 24.9%. However, after the doping of Cu into ATP, the O α ratio on the adsorbent surface increased to 33.9%, indicating that Cu doping increased the O α content on the adsorbent surface. Cu doping on the ATP surface induced the formation of oxygen vacancies, which could promote the migration of oxygen species, thereby producing more O α [ 30 ]. This enhancement was favorable for improving the chemisorption behavior of the adsorbent. The O α ratio on the surface of the used Cu-ATP was reduced to 22.8%, suggesting that the Hg 2+ adsorption process consumed the O α on the adsorbent surface. This can be explained by the reaction between Hg 2+ species and O α to form HgO during the Hg 2+ adsorption process, which led to a decrease in O α content. The Hg 4f XPS characterization also confirmed that the adsorbed mercury species mainly existed in the form of HgO. To investigate the mercury species formed on the Cu-ATP surface during the Hg 2+ adsorption process, XPS analysis was conducted to examine the form of mercury species on the surface of Cu-ATP before and after Hg 2+ adsorption. The results are shown in Fig. 13 . The Hg 4f XPS spectra indicated that no characteristic peak of mercury species was detected on fresh Cu-ATP, suggesting the absence of mercury species. However, after Hg 2+ adsorption, a characteristic peak of HgO species was observed, indicating that the mercury species mainly existed in the form of HgO on the used Cu-ATP surface [ 34 ]. The changes in surface oxygen on the adsorbent suggest that Hg 2+ species in solution combined with the chemisorbed oxygen on the Cu-ATP surface to form stable HgO, thus achieving efficient removal of Hg 2+ . Additionally, the spectrum of the used Cu-ATP exhibited a small characteristic peak of HgS species, indicating that some Hg 2+ species combined with sulfur ions to form stable HgS during the adsorption process [ 35 – 37 ]. Considering that ATP did not contain sulfur species, HgS species were likely present because sulfur species from the CuSO 4 precursor were loaded onto the ATP surface during the Cu loading process, resulting in the formation of HgS species. The formation of the HgS species is probably the main reason why Cu-ATP prepared using CuSO 4 as the precursor exhibited the best Hg 2+ adsorption performance among the different Cu precursors. 3.5. Adsorption mechanism analysis Cu-ATP exhibited superior Hg 2+ removal performance in simulated wastewater containing Hg 2+ species. The adsorption kinetics results confirmed that Hg 2+ adsorption on Cu-ATP fitted well with the pseudo-second-order model, indicating a chemical adsorption process. XPS characterization demonstrated that Cu species on the Cu-modified ATP adsorbent mainly existed in the form of Cu 2+ and Cu + , which could induce an electron transfer process, resulting in the formation of unsaturated sites on the Cu-ATP surface. This process can produce a large amount of O α on the Cu-ATP surface, leading to an increase in the Hg 2+ adsorption active sites on the adsorbent surface. The O 1s XPS spectra also confirmed the increase of O α on the Cu-ATP surface after Cu loading. During the Cu modification process, the S species in the Cu precursor CuSO 4 would transfer to the ATP surface, providing more adsorption active sites for Hg 2+ removal. The Hg 4f XPS spectra confirmed that the adsorbed Hg 2+ species existed mainly in the form of HgO and HgS on the Cu-ATP surface. The process of removing Hg 2+ by Cu-ATP is depicted in Fig. 14 . Hg 2+ species in wastewater were captured by chemisorbed oxygen and sulfur active sites on the Cu-ATP surface; consequently, stable HgO and HgS were generated, which enabled the efficient removal of Hg 2+ species from wastewater. 4. Conclusions A series of Cu-ATP adsorbents were synthesized, and the Hg 2+ removal performance of Cu-ATP in simulated desulfurization sludge leaching wastewater was investigated. Cu-ATP prepared using CuSO 4 as the precursor of Cu and treated with 3 mol/L HNO 3 demonstrated excellent Hg 2+ removal performance. The optimal Hg 2+ removal efficiency reached 90.7%, with a Cu-to-ATP ratio of 0.06. Moreover, the Hg 2+ removal efficiency of Cu-ATP initially increased with higher adsorbent content and longer adsorption times before stabilizing. However, as the pH value increased, the Hg 2+ removal efficiency first increased and then decreased, whereas the removal efficiency decreased with increasing initial Hg 2+ concentration. The adsorption kinetics revealed that Hg 2+ adsorption on Cu-ATP conformed well to the pseudo-second-order model, indicating a chemical adsorption process. BET and XRD analyses demonstrated that HNO 3 treatment effectively enhanced the specific surface area and pore volume of ATP. Following Cu loading, the specific surface area of the adsorbent slightly decreased, yet the crystal structure of ATP remained unaffected. The XPS results indicated that Cu species predominantly existed in the forms of Cu 2+ and Cu + on the Cu-ATP surface, significantly increasing the O α content and thereby enhancing the Hg 2+ removal efficiency of Cu-ATP. Moreover, S species from the CuSO 4 precursor were transferred to the Cu-ATP surface and provided additional active sites for Hg 2+ adsorption. Analysis of mercury morphology confirmed that adsorbed Hg 2+ species existed as stable HgO and HgS on the Cu-ATP surface, enabling the effective removal of Hg 2+ from wastewater. The findings reveal the potential of Cu-ATP as a cost-effective adsorbent for Hg 2+ removal in wastewater treatment. This study investigated the Hg 2+ removal performance of Cu-ATP, and future follow-up research should further explore the impact of other heavy metals (such as Cr, Ni, and Cu) coexisting in wastewater on the Hg 2+ adsorption properties of Cu-ATP. Declarations Acknowledgments This work was supported by the Project of State Grid Hebei Energy Technology Service, grant number TSS2022-05. We thank LetPub (www.letpub.com.cn) for its linguistic assistance during the preparation of this manuscript. References A. Ishag, Y. Yue, J. Xiao, X. Huang and Y. Sun, J. Clean Prod., 367 , 133111 (2022). Y. Li, J. Yu, Y. Liu, R. Huang, Z. Wang and Y. Zhao, J. Hazard. Mater., 427 , 128132 (2022). G. Chen, K. Han, C. Liu and B. Yan, J. Hazard. Mater., 414 , 125561 (2021). D. Liang, R. Jun, T. Ling and L. Hua, Fresenius Environ. Bull., 26 , 2066 (2017). Y. Kobayashi, F. Ogata, C. Saenjum, T. 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Hu, C. Duan and P. Li, Chem. Eng. J. , 482 , 148831 (2024). T. Falayi and F. Ntuli, J. Ind. Eng. Chem. , 20 , 1285 (2014). X. Zhang, T. Lan, Q. Yi, Y. Wang, D. Cheng and D. Zhang, Environ. Sci.-Nano , 11 , 2457 (2024). Q. Zhu, J. Zhu, M. Su, L. Liu, C. Wang, D. Ji, W. Bai and W. Shen, Environ. Eng. Res. 28 , 220343 (2023). G. Zhang, H. Wang, S. Guo, J. Wang and J. Liu, Appl. Surf. Sci. , 362 , 257 (2016). C. Chen, Y. Cao, S. Liu, J. Chen and W. Jia, Appl. Surf. Sci. , 480 , 537 (2019). S. Jiang, L. Zhao, J. Zhang, Y. Huang and X. Wang, Fuel , 349 , 128648 (2023). H. Shu, Y. Liu, Y. Jia, C. Dou and F. Bai, Sep. Purif. Technol. , 327 , 124940 (2023). Y. Cao, F. Wang, Q. Peng, C. Chen, S. Liu, W. Jia and R. Hao, Fuel , 347 , 128497 (2023). B. Li, Y. Hou, Y. Li, K. Peng, Z. Huang, J. Wang, L. Chang and K. Xie, Fuel Process. Technol. , 240 , 107578 (2023). J. Wang, L. Dai, S. Hu, H. Yin, M. Yang, A. Reheman and G. Yan, Water Sci. Technol. , 83 , 2921 (2021). M. Hu, H. Tian and J. He, Chin. J. Inorg. Chem. , 36 , 695 (2020). M. Hu, H. Tian and J. He, ACS Appl. Mater. Interfaces , 11 , 19200 (2019). Cite Share Download PDF Status: Published Journal Publication published 19 Dec, 2024 Read the published version in Korean Journal of Chemical Engineering → Version 1 posted Reviewers agreed at journal 28 Aug, 2024 Reviewers invited by journal 24 Jul, 2024 Editor assigned by journal 22 Jul, 2024 First submitted to journal 17 Jul, 2024 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. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4755442","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":331139050,"identity":"92f9fb48-5f76-46b9-89b9-c192a81badf4","order_by":0,"name":"Chongming Chen","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAxklEQVRIiWNgGAWjYDCCA0DM2ABiMR848OEHaVrYEg/O7CFNC4/xYQ42InTw3cgxYPi5wyaPf9qZD4cZeBjk+cUO4NciCdTC2HsmrVjidu6GwwUWDIYzZyfg12IA1MLM2HY4cYM0UMsMHoYEg9vEafkP1JLz4DAPG/FaDoC0MBCnRfLMswLG3rbkxBm30wyAgSxB2C98x5M3MPxss0vsn538+MOHHzby/NIEtDAIZJgjR7kEAeUgwH/8ARGqRsEoGAWjYEQDAFMQSvvrvX5nAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0001-6452-7395","institution":"State Grid Hebei Electric Power Company","correspondingAuthor":true,"prefix":"","firstName":"Chongming","middleName":"","lastName":"Chen","suffix":""},{"id":331139051,"identity":"f185847e-9558-43b1-a71c-c93ddc630c53","order_by":1,"name":"Dong Li","email":"","orcid":"","institution":"State Grid Hebei Electric Power Company","correspondingAuthor":false,"prefix":"","firstName":"Dong","middleName":"","lastName":"Li","suffix":""},{"id":331139052,"identity":"490e798b-9473-48f1-9162-81ea443efc0d","order_by":2,"name":"Jinxing Yu","email":"","orcid":"","institution":"State Grid Hebei Electric Power Company","correspondingAuthor":false,"prefix":"","firstName":"Jinxing","middleName":"","lastName":"Yu","suffix":""},{"id":331139053,"identity":"2337a5d1-6bb2-4cf5-bb3c-1fc39d4c90cd","order_by":3,"name":"Kai Che","email":"","orcid":"","institution":"State Grid Hebei Electric Power Company","correspondingAuthor":false,"prefix":"","firstName":"Kai","middleName":"","lastName":"Che","suffix":""}],"badges":[],"createdAt":"2024-07-17 10:23:35","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4755442/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4755442/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11814-024-00360-6","type":"published","date":"2024-12-19T15:57:21+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":62884188,"identity":"9b105985-ddde-4c28-af38-ca5db26f3c67","added_by":"auto","created_at":"2024-08-20 15:30:09","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":32902,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of HNO\u003csub\u003e3\u003c/sub\u003e concentration on Hg\u003csup\u003e2+\u003c/sup\u003e removal\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/45b0697cd526f665939ec2bd.png"},{"id":62884800,"identity":"a632781a-a231-4311-973d-9d7851d2ff66","added_by":"auto","created_at":"2024-08-20 15:38:08","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":53232,"visible":true,"origin":"","legend":"\u003cp\u003eEffects of different Cu precursors on Hg\u003csup\u003e2+\u003c/sup\u003e removal\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/c2b24b7ae6a4dccc704ace84.png"},{"id":62884195,"identity":"0cf8a70c-ada8-4718-a149-1bb7a97bf858","added_by":"auto","created_at":"2024-08-20 15:30:09","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":32603,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of Cu content on Hg\u003csup\u003e2+\u003c/sup\u003e removal\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/4c641d23935bf73449d5803d.png"},{"id":62884803,"identity":"6363b433-9d5f-4063-ab2e-53b8a7972897","added_by":"auto","created_at":"2024-08-20 15:38:09","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":39033,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of adsorbent content on Hg\u003csup\u003e2+\u003c/sup\u003e removal\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/4ebceb46cdbbffd00ce8cdac.png"},{"id":62884190,"identity":"fd0bfa17-83d0-4b6c-b7f4-a0178253bf57","added_by":"auto","created_at":"2024-08-20 15:30:09","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":39742,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of adsorbent time on Hg\u003csup\u003e2+\u003c/sup\u003e removal\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/d538dd83f247b7042e553f74.png"},{"id":62884181,"identity":"9efaf105-12e1-4051-94d9-0e022370486c","added_by":"auto","created_at":"2024-08-20 15:30:08","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":37957,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of pH value on Hg\u003csup\u003e2+\u003c/sup\u003e removal\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/a4bef3a1ecd402d3c77918cd.png"},{"id":62885234,"identity":"525e9460-dd39-47fd-8073-9e340f23e5b2","added_by":"auto","created_at":"2024-08-20 15:46:09","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":39962,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of initial Hg\u003csup\u003e2+\u003c/sup\u003e concentration on Hg\u003csup\u003e2+\u003c/sup\u003e removal\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/b64f5c432ba49a3d500eb531.png"},{"id":62884183,"identity":"9570a170-beef-4ddd-8987-d223be19fb03","added_by":"auto","created_at":"2024-08-20 15:30:09","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":40145,"visible":true,"origin":"","legend":"\u003cp\u003eAdsorption kinetics of Hg\u003csup\u003e2+\u003c/sup\u003e on ATP\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/89bd4335e4229ea600a98bd7.png"},{"id":62884801,"identity":"993c56d9-e01b-4e03-9188-27feed891cd3","added_by":"auto","created_at":"2024-08-20 15:38:09","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":40090,"visible":true,"origin":"","legend":"\u003cp\u003eAdsorption kinetics of Hg\u003csup\u003e2+\u003c/sup\u003e on Cu-ATP\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/4387a0ec16572030aa4eae2f.png"},{"id":62884805,"identity":"85ecc3da-090a-473b-85d5-ff9ef378eeb6","added_by":"auto","created_at":"2024-08-20 15:38:09","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":57298,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of Cu-ATP with different Cu loadings\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/8f6cd4258af1ffd0a3cca989.png"},{"id":62884185,"identity":"eb5748ce-00f2-4f9e-8097-6c3ae7099e43","added_by":"auto","created_at":"2024-08-20 15:30:09","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":50728,"visible":true,"origin":"","legend":"\u003cp\u003eCu 2p XPS spectra of Cu-ATP pretreated under different conditions\u003c/p\u003e","description":"","filename":"floatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/e4ca6ef2914c18016bb0fe3a.png"},{"id":62884804,"identity":"dacddd62-347b-4190-a359-63ba6de69aa5","added_by":"auto","created_at":"2024-08-20 15:38:09","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":57243,"visible":true,"origin":"","legend":"\u003cp\u003eO 1s XPS spectra of Cu-ATP pretreated under different conditions\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/3330fe6ed171c0859cd42525.png"},{"id":62884806,"identity":"d6e93ff6-2917-4025-9c68-46949b4f7e5e","added_by":"auto","created_at":"2024-08-20 15:38:09","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":39624,"visible":true,"origin":"","legend":"\u003cp\u003eHg 4f XPS spectra of Cu-ATP pretreated under different conditions\u003c/p\u003e","description":"","filename":"floatimage13.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/2a9d67c8a5c79b58a3ecda09.png"},{"id":62884194,"identity":"97a86384-57f5-4ef4-90a9-da75fc67aacd","added_by":"auto","created_at":"2024-08-20 15:30:09","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":93264,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of Hg\u003csup\u003e2+\u003c/sup\u003e removal on Cu-ATP\u003c/p\u003e","description":"","filename":"floatimage14.png","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/bd51edf770f375b611e7b60d.png"},{"id":72201693,"identity":"be7da067-fd7a-4532-9893-787206a61018","added_by":"auto","created_at":"2024-12-23 16:09:54","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1205025,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4755442/v1/6a285e92-1175-4539-bb42-0223d80c30bb.pdf"}],"financialInterests":"","formattedTitle":"Efficient removal of Hg2+ from wastewater by a novel Cu-modified attapulgite: Adsorption performance and mechanism","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eMercury is a highly polluting heavy metal known for its persistence, non-degradability, and biological toxicity. It has been listed as a priority pollutant for control by various environmental agencies worldwide [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Coal-fired power plants are considered one of the major sources of anthropogenic mercury emissions and have thus attracted a lot of attention in recent years [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. During the production process of coal-fired power plants, mercury migrates and transforms with the flue gas. Elemental mercury (Hg\u003csup\u003e0\u003c/sup\u003e) is discharged into the atmosphere with the flue gas owing to the volatility of Hg\u003csup\u003e0\u003c/sup\u003e and its insolubility in water. However, oxidized mercury species (Hg\u003csup\u003e2+\u003c/sup\u003e) are easily soluble in water. They are captured by the wet desulfurization system and then enriched in the desulfurization products (such as sludge and wastewater from desulfurization) [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. The high concentration of Hg\u003csup\u003e2+\u003c/sup\u003e in desulfurization sludge significantly restricts the safe disposal and resource utilization of the sludge. [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. In recent years, researchers have used chemical leaching methods to remove Hg\u003csup\u003e2+\u003c/sup\u003e from desulfurization sludge, and have achieved satisfactory Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency. Unfortunately, the high content of Hg\u003csup\u003e2+\u003c/sup\u003e in the leaching wastewater can cause secondary pollution and pose serious risks to the ecological environment and human health [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Moreover, the enrichment of Hg\u003csup\u003e2+\u003c/sup\u003e species in desulfurization wastewater complicates the treatment of desulfurization wastewater in coal-fired power plants. Therefore, developing efficient Hg\u003csup\u003e2+\u003c/sup\u003e removal technology for wastewater is crucial to prevent secondary pollution. This advancement is significant for improving chemical leaching technology for mercury-contaminated desulfurization sludge and for treating wastewater polluted by Hg\u003csup\u003e2+\u003c/sup\u003e species.\u003c/p\u003e \u003cp\u003eCurrently, studies on the removal of Hg\u003csup\u003e2+\u003c/sup\u003e species from wastewater produced during the leaching process of desulfurization sludge are few. Most existing studies have focused on the efficient removal of Hg\u003csup\u003e2+\u003c/sup\u003e from industrial wastewater [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The main technologies for Hg\u003csup\u003e2+\u003c/sup\u003e removal from wastewater include chemical precipitation, biological treatment, ion exchange, membrane separation, electrochemical methods, and adsorption. Among these, the adsorption method is favored by researchers owing to its low investment cost, high removal efficiency, lack of secondary pollution, and low energy consumption [\u003cspan additionalcitationids=\"CR10\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. The adsorption method utilizes the physical and chemical characteristics of adsorbents and pollutants to transfer trace pollutant molecules dissolved in the water phase to solid-phase adsorbents. This process involves various interactions, such as van der Waals forces, hydrophobic effects, electrostatic effects, and ion exchange [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The key to the adsorption method is designing and developing adsorbents with high performance and low cost. With continuous advancements in adsorbents, materials such as metal\u0026ndash;organic frameworks (MOFs), clay, carbon nanotubes, layered double hydroxides, and zeolites have been used to remove Hg\u003csup\u003e2+\u003c/sup\u003e from wastewater [\u003cspan additionalcitationids=\"CR13 CR14\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Attapulgite (ATP) is a water-rich magnesium aluminosilicate clay mineral with a chain-layer structure. Its crystal structure contains cations such as Na\u003csup\u003e+\u003c/sup\u003e, Ca\u003csup\u003e2+\u003c/sup\u003e, Fe\u003csup\u003e3+\u003c/sup\u003e, and Al\u003csup\u003e3+\u003c/sup\u003e, which endows it with excellent cation-exchange properties [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Natural ATP features abundant microporous channels, providing numerous adsorption sites for pollutant removal. Additionally, the presence of -OH active groups and excess charges on the ATP surface enhances its ion-exchange capabilities, surface electrostatic adsorption, and microporous diffusion properties, all of which improve the adsorptive performance of ATP [\u003cspan additionalcitationids=\"CR18\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Considering these advantages, ATP is commonly used for the adsorption removal of heavy metal ions in wastewater. While natural ATP is inexpensive and readily available, it contains various impurities, and its heavy metal-adsorption performance is relatively low [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Researchers have enhanced the specific surface area and micropore structure of ATP through metal load modification, acid treatment, organic modification, and ultrasonic modification. These methods increase the number of surface adsorption sites, thereby significantly improving the performance of ATP in adsorbing heavy metals [\u003cspan additionalcitationids=\"CR22 CR23 CR24\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. However, there are only a few studies on Hg\u003csup\u003e2+\u003c/sup\u003e removal from wastewater using modified ATP, and the mechanism by which modified ATP improves the adsorption performance of heavy metals is still unclear. Additionally, the cation composition of desulfurization sludge leaching wastewater is more complex than that of traditional Hg\u003csup\u003e2+\u003c/sup\u003e-containing wastewater, which may affect the adsorption performance of modified ATP.\u003c/p\u003e \u003cp\u003eThis study employed a method combining acid treatment with metal modification to modify natural ATP and synthesize a series of Cu-modified ATP (Cu-ATP) adsorbents. The Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency of Cu-ATP was investigated in simulated desulfurization sludge leaching wastewater, and the effects of various reaction conditions on the Hg\u003csup\u003e2+\u003c/sup\u003e adsorption performance of Cu-ATP were explored. The optimal reaction conditions for Hg\u003csup\u003e2+\u003c/sup\u003e adsorption by Cu-ATP were determined. Additionally, the reaction mechanism of Hg\u003csup\u003e2+\u003c/sup\u003e adsorption on the Cu-ATP surface was analyzed according to adsorption activity and characterization results. These findings provide a foundation for developing low-cost, high-efficiency, environmentally friendly adsorbents and are significant for advancing chemical leaching technology for Hg-contaminated desulfurized sludge and treating Hg\u003csup\u003e2+\u003c/sup\u003e-polluted wastewater.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Adsorbent preparation\u003c/h2\u003e \u003cp\u003eThe specific method for treating ATP adsorption materials with HNO\u003csub\u003e3\u003c/sub\u003e was as follows: 4 g of ATP was weighed, and 20 mL of HNO\u003csub\u003e3\u003c/sub\u003e solution with varying concentrations (1, 3, 5, 7 mol/L) was added to the beaker. The mixture was then heated and boiled for 2 h. After the obtained solid was washed with deionized water, it was calcined for 2 h at 200\u0026deg;C and then ground to 60 mesh to extract HNO\u003csub\u003e3\u003c/sub\u003e-treated ATP.\u003c/p\u003e \u003cp\u003eA series of Cu-ATP adsorbents were prepared through an improved impregnation method. First, a certain amount of ATP pretreated with HNO\u003csub\u003e3\u003c/sub\u003e was added to an aqueous solution of a Cu precursor. The mixture was then stirred for 2 h in a 50\u0026deg;C thermostatic water bath, dried at 100\u0026deg;C for 12 h, and calcined at 450\u0026deg;C for 3 h in an N\u003csub\u003e2\u003c/sub\u003e/O\u003csub\u003e2\u003c/sub\u003e atmosphere. During the preparation process, various Cu-ATP adsorbents were obtained through the alteration of the types of Cu precursors (CuNO\u003csub\u003e3\u003c/sub\u003e, CuCl\u003csub\u003e2\u003c/sub\u003e, and CuSO\u003csub\u003e4\u003c/sub\u003e) and the Cu content (0, 2, 4, 6, and 8%).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Adsorbent performance test\u003c/h2\u003e \u003cp\u003eThe specific method for testing the adsorption performance of the adsorbent was as follows: 2 g of Cu-ATP adsorbent was added to 1000 mL of simulated wastewater containing 100 mg/L of Hg\u003csup\u003e2+\u003c/sup\u003e and stirred at room temperature for 40 min. After suction filtration, the residual Hg\u003csup\u003e2+\u003c/sup\u003e concentration in the filtrate was determined using an AFS-933 atomic fluorescence photometer, and the corresponding Hg\u003csup\u003e2+\u003c/sup\u003e removal rate was calculated. The Hg\u003csup\u003e2+\u003c/sup\u003e removal rate was calculated as follows:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$R=\\frac{{{C_0} - {C_e}}}{{{C_0}}} \\times 100\\%$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e.\u003c/p\u003e \u003cp\u003eHere, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e is the initial Hg\u003csup\u003e2+\u003c/sup\u003e concentration in the solution, and \u003cem\u003eC\u003c/em\u003e\u003csub\u003ee\u003c/sub\u003e represents the remaining Hg\u003csup\u003e2+\u003c/sup\u003e concentration in the filtrate.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Adsorbent characterization\u003c/h2\u003e \u003cp\u003eSeveral characterization methods were used to analyze the physicochemical properties of the adsorbents. The Brunauer\u0026thinsp;\u0026minus;\u0026thinsp;Emmett\u0026thinsp;\u0026minus;\u0026thinsp;Teller (BET) surface area, pore volume, and average pore diameter of the different adsorbents were obtained using a N\u003csub\u003e2\u003c/sub\u003e adsorption isotherms analyzer (ASAP 2020). The standard BET equation was used to calculate the specific surface area. The X-ray diffraction (XRD) method was employed to determine the crystallinity and the dispersivity of Cu species on the adsorbents pretreated under different conditions. The XRD patterns of the samples were collected using a Rigaku X-ray diffractometer with Cu-Kα radiation. To investigate the valence states of surface elements of different adsorbents, X-ray photoelectron spectroscopy (XPS) analysis was performed using an X-ray photoelectron spectrometer with Al-Kα radiation. The C 1s peak at 284.6 eV was used to calibrate the observed spectra.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Hg\u003csup\u003e2+\u003c/sup\u003e removal over ATP pretreated under different conditions\u003c/h2\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e3.1.1. Effect of HNO\u003csub\u003e3\u003c/sub\u003e concentration on Hg\u003csup\u003e2+\u003c/sup\u003e removal\u003c/h2\u003e \u003cp\u003eThe effects of HNO\u003csub\u003e3\u003c/sub\u003e concentrations on the Hg\u003csup\u003e2+\u003c/sup\u003e removal properties of ATP pretreated by HNO\u003csub\u003e3\u003c/sub\u003e were investigated, and the results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. As the HNO\u003csub\u003e3\u003c/sub\u003e concentration increased, the Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency of ATP initially increased and then decreased (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The optimal Hg\u003csup\u003e2+\u003c/sup\u003e removal performance was achieved at an HNO\u003csub\u003e3\u003c/sub\u003e concentration of 3 mol/L. This indicated that activation of ATP using an appropriate amount of HNO\u003csub\u003e3\u003c/sub\u003e could enhance the adsorption performance of ATP for Hg\u003csup\u003e2+\u003c/sup\u003e, but excessive HNO\u003csub\u003e3\u003c/sub\u003e concentrations inhibited the adsorption performance of ATP for Hg\u003csup\u003e2+\u003c/sup\u003e. This phenomenon presumably occurred because activation of ATP using an appropriate amount of HNO\u003csub\u003e3\u003c/sub\u003e could remove some cations from the crystal structure of ATP, thereby increasing the specific surface area and pore volume of ATP and enhancing its Hg\u003csup\u003e2+\u003c/sup\u003e-adsorption capacity [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. However, under excess HNO\u003csub\u003e3\u003c/sub\u003e concentrations, the cations in the ATP octahedral structure were nearly completely dissolved, causing the tetrahedral sheet to lose support and the structure to collapse. This resulted in a reduction of the internal pore channels and specific surface area, thereby weakening the adsorption capacity of ATP. BET characterization of ATP also confirmed this hypothesis. Considering these experimental results, 3 mol/L of HNO\u003csub\u003e3\u003c/sub\u003e was selected as the optimal treatment condition for ATP, and follow-up adsorption experiments were conducted.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e3.1.2. Effect of different Cu precursors on Hg\u003csup\u003e2+\u003c/sup\u003e removal\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e illustrates the effects of different Cu precursors on the adsorption performance of Cu-modified ATP for Hg\u003csup\u003e2+\u003c/sup\u003e removal. ATP modified with various Cu precursors showed varying Hg\u003csup\u003e2+\u003c/sup\u003e adsorption capabilities. Among these, CuSO\u003csub\u003e4\u003c/sub\u003e-modified ATP exhibited the most effective Hg\u003csup\u003e2+\u003c/sup\u003e adsorption performance. This occurrence is probably attributable to the presence of sulfur (S) species in the ATP pore structure modified by CuSO\u003csub\u003e4\u003c/sub\u003e, which readily bound with Hg\u003csup\u003e2+\u003c/sup\u003e to form HgS, resulting in enhanced Hg\u003csup\u003e2+\u003c/sup\u003e adsorption capacity of Cu-ATP. Subsequent XPS characterization results also confirmed this hypothesis. Additionally, owing to Cu doping, some cations in ATP were replaced and thus created vacancy-deficient sites on the ATP surface. This promoted the migration and transformation of lattice oxygen on the ATP surface and enhanced HgO formation on the surface, which facilitated Hg\u003csup\u003e2+\u003c/sup\u003e adsorption [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. The preference for formation of HgO on Cu-ATP was also why Hg\u003csup\u003e2+\u003c/sup\u003e predominantly existed in the form of HgO on the ATP surface. Given the superior Hg\u003csup\u003e2+\u003c/sup\u003e adsorption performance of CuSO\u003csub\u003e4\u003c/sub\u003e-modified ATP, CuSO\u003csub\u003e4\u003c/sub\u003e was chosen as the precursor for Cu in subsequent experiments.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e3.1.3. Effect of Cu content on Hg\u003csup\u003e2+\u003c/sup\u003e removal\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e illustrates the effects of different Cu contents on the Hg\u003csup\u003e2+\u003c/sup\u003e adsorption performance of Cu-ATP. As the Cu loading increased, the adsorption capacity of Cu-ATP for Hg\u003csup\u003e2+\u003c/sup\u003e initially improved and then declined. This suggests that appropriate Cu doping can effectively enhance Hg\u003csup\u003e2+\u003c/sup\u003e adsorption performance. However, excessive Cu loading reduced the Hg\u003csup\u003e2+\u003c/sup\u003e adsorption capacity of ATP. Increased Cu loading can block the internal micropores of ATP, reducing the specific surface area of Cu-ATP and thereby weakening its Hg\u003csup\u003e2+\u003c/sup\u003e adsorption capacity. BET results for Cu-ATP with varying Cu loading also support this hypothesis.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Effect of adsorption conditions on Hg\u003csup\u003e2+\u003c/sup\u003e removal\u003c/h2\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1. Effect of adsorbent content\u003c/h2\u003e \u003cp\u003eThe effects of ATP and Cu-ATP addition on Hg\u003csup\u003e2+\u003c/sup\u003e adsorption properties were investigated (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Both ATP and Cu-ATP showed similar trends in Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency. With increasing adsorbent content, Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency initially increased and then stabilized. At an adsorbent content of 2 g, the Hg\u003csup\u003e2+\u003c/sup\u003e adsorption efficiency reached 90.7%. A further increase in adsorbent content did not significantly enhance Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency, attributable to the increase in active adsorption sites for Hg\u003csup\u003e2+\u003c/sup\u003e as the adsorbent content increased. The increased adsorption sites resulted in higher total Hg\u003csup\u003e2+\u003c/sup\u003e adsorption, thereby improving Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.2.2. Effect of adsorbent time\u003c/h2\u003e \u003cp\u003eThe adsorption time influenced the contact process between the adsorbent and wastewater, thereby affecting the Hg\u003csup\u003e2+\u003c/sup\u003e removal performance. Therefore, the effects of adsorption time on Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency were investigated, and the results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. Within the 0\u0026ndash;20 min range, both ATP and Cu-ATP showed a rapid increase in Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency with increasing adsorption time. The Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency of ATP stabilized around 25 min and slightly decreased after 50 min. After adsorption equilibrium was reached, the partial desorption of adsorbed Hg\u003csup\u003e2+\u003c/sup\u003e species occurred over time, resulting in a slight decrease in Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency. This instability in the adsorption process indicated noticeable desorption behavior on the ATP surface. The Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency of Cu-ATP stabilized at around 35 min of adsorption time. Moreover, extending the adsorption time did not lead to a decrease in Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency, which indicated stable adsorption of Hg\u003csup\u003e2+\u003c/sup\u003e on Cu-ATP without significant desorption behavior during the process. This result suggests that Hg\u003csup\u003e2+\u003c/sup\u003e species adsorbed on Cu-ATP surfaces were more stable than those on ATP surfaces.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section3\"\u003e \u003ch2\u003e3.2.3. Effect of pH value\u003c/h2\u003e \u003cp\u003eConsidering that the leaching wastewater from desulfurization sludge is typically acidic, the performance of the adsorbent in removing Hg\u003csup\u003e2+\u003c/sup\u003e species under different pH conditions was investigated, and the results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. The Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency of both ATP and Cu-ATP first increased and then decreased notably within the pH range of 3\u0026ndash;7. Specifically, at wastewater pH values exceeding 5, the Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency of both adsorbents decreased significantly. This indicated that the adsorbents demonstrated better Hg\u003csup\u003e2+\u003c/sup\u003e removal performance at pH 3\u0026ndash;5, possibly because the acidic environment aided in eliminating impurity ions from the pore channels of the adsorbent, thereby maintaining a higher specific surface area and a pore structure. This finding aligns with the principle that acid treatment enhances ATP adsorption performance [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section3\"\u003e \u003ch2\u003e3.2.4. Effect of initial Hg\u003csup\u003e2+\u003c/sup\u003e concentration\u003c/h2\u003e \u003cp\u003eThe variation in the Hg\u003csup\u003e2+\u003c/sup\u003e concentration of leaching wastewater under different leaching conditions may impact the Hg\u003csup\u003e2+\u003c/sup\u003e removal performance of the adsorbent. Therefore, the effect of the initial Hg\u003csup\u003e2+\u003c/sup\u003e concentration in wastewater on the Hg\u003csup\u003e2+\u003c/sup\u003e removal performance of the adsorbent was investigated, and the results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. As the initial Hg\u003csup\u003e2+\u003c/sup\u003e concentration increased, the Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency of both ATP and Cu-ATP gradually decreased. This indicated that higher concentrations of Hg\u003csup\u003e2+\u003c/sup\u003e led to lower Hg\u003csup\u003e2+\u003c/sup\u003e removal performance by the adsorbent. A higher Hg\u003csup\u003e2+\u003c/sup\u003e concentration led to more Hg\u003csup\u003e2+\u003c/sup\u003e species being adsorbed onto the micropore structure and active sites of the adsorbent. This gradual saturation of adsorption sites reduced the capacity of the adsorbent to remove Hg\u003csup\u003e2+\u003c/sup\u003e and hence lowered its removal efficiency.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Adsorption kinetics of Hg\u003csup\u003e2+\u003c/sup\u003e\u003c/h2\u003e \u003cp\u003eTo better investigate the adsorption process, the pseudo-first-order model (Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) and pseudo-second-order model (Eq.\u0026nbsp;\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) were employed to evaluate the adsorption kinetics of Hg\u003csup\u003e2+\u003c/sup\u003e on ATP and Cu-ATP, respectively.\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\ln \\left( {{q_e} - {q_t}} \\right)=\\ln {q_e} - {k_1}t$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e,\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\frac{t}{{{q_t}}}=\\frac{1}{{{k_2}q_{e}^{2}}}+\\frac{1}{{{q_e}}}t$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eq\u003c/em\u003e\u003csub\u003e\u003cem\u003ee\u003c/em\u003e\u003c/sub\u003e (mg\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and \u003cem\u003eq\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e (mg\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) represent the amounts of adsorbed Hg\u003csup\u003e2+\u003c/sup\u003e species at equilibrium time and time \u003cem\u003et\u003c/em\u003e (min). \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e (min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e (g\u0026middot;(mg\u0026middot;min)\u003csup\u003e\u0026minus;1\u003c/sup\u003e) represent the rate constants of the pseudo-first-order model and the pseudo-second-order model, respectively.\u003c/p\u003e \u003cp\u003eThe Hg\u003csup\u003e2+\u003c/sup\u003e adsorption profiles on ATP and Cu-ATP over time are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, respectively. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e presents the kinetic parameters for Hg\u003csup\u003e2+\u003c/sup\u003e adsorption on ATP and Cu-ATP fitted by the aforementioned models. The adsorption kinetics of Hg\u003csup\u003e2+\u003c/sup\u003e on ATP are well described by the pseudo-first-order model, as indicated by the high correlation coefficient (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.995). This indicated that the adsorption of Hg\u003csup\u003e2+\u003c/sup\u003e on ATP mainly involved a physical process. However, for Cu-ATP, both models showed a higher correlation coefficient, which indicates that Cu addition altered the adsorption process of Hg\u003csup\u003e2+\u003c/sup\u003e on the adsorbent. This suggests that both physical and chemical adsorption processes occurred simultaneously. The pseudo-second-order model exhibited a higher correlation coefficient than the pseudo-first-order model, which suggests that the Hg\u003csup\u003e2+\u003c/sup\u003e adsorption on Cu-ATP was mainly a chemical adsorption process, which involved electron exchange between the adsorbent and the adsorbate. This process is also an important reason that Cu-ATP exhibited high Hg\u003csup\u003e2+\u003c/sup\u003e adsorption capacity of Cu-ATP. Subsequent XPS characterization of the adsorbent also confirmed that the Hg\u003csup\u003e2+\u003c/sup\u003e adsorption process induced electron transfer on the adsorbent surface, which enhanced Hg\u003csup\u003e2+\u003c/sup\u003e adsorption activity.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eKinetic parameters for adsorption\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKinetic model\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eParameters\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eATP\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCu-ATP\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003ePseudo-first-order model\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eq\u003csub\u003ee\u003c/sub\u003e (mg\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e29.87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e44.74\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ek\u003csub\u003e1\u003c/sub\u003e (min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.161\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.182\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.995\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.993\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003ePseudo-second-order model\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eq\u003csub\u003ee\u003c/sub\u003e (mg\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e32.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e48.30\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ek\u003csub\u003e2\u003c/sub\u003e (g\u0026middot;(mg\u0026middot;min)\u003csup\u003e\u0026minus;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.008\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.007\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.971\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.997\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e3.4. Adsorbent characterization\u003c/h2\u003e \u003cdiv id=\"Sec18\" class=\"Section3\"\u003e \u003ch2\u003e3.4.1. BET\u003c/h2\u003e \u003cp\u003eA large specific surface area and good pore volume are crucial for ensuring high adsorption capacity in adsorbents. The specific surface area, pore volume, and average pore diameter of ATP samples treated under different conditions were measured, and the results are listed in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Untreated ATP exhibited a low specific surface area and pore volume, likely owing to the presence of impurity ions. In contrast, ATP samples treated with HNO\u003csub\u003e3\u003c/sub\u003e showed significantly improved specific surface area and pore volume. This suggests that the HNO\u003csub\u003e3\u003c/sub\u003e treatment effectively removed impurity ions from ATP micropores, resulting in a superior pore structure. However, at HNO\u003csub\u003e3\u003c/sub\u003e concentrations exceeding 3 mol/L, the specific surface area and pore volume of ATP significantly decreased, likely because excessively high concentrations of HNO\u003csub\u003e3\u003c/sub\u003e can lead to the collapse of ATP channels, reducing both the specific surface area and internal channels and thereby weakening the adsorption capacity of ATP [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The Hg\u003csup\u003e2+\u003c/sup\u003e adsorption capacity of ATP treated with different concentrations of HNO\u003csub\u003e3\u003c/sub\u003e also supports this speculation. Additionally, after ATP was doped with Cu, the specific surface area and pore volume of ATP slightly decreased. The slight decrease is attributable to the relatively low Cu loading, which resulted in Cu being highly dispersed on the surface of ATP without significantly reducing its specific surface area and pore volume. This observation aligns well with the XRD characterization results.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePhysical characteristics of ATP pretreated under different conditions\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSamples\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eS\u003csub\u003eBET\u003c/sub\u003e (m\u003csup\u003e2\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eV\u003csub\u003epore\u003c/sub\u003e (cm\u003csup\u003e3\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eD\u003csub\u003epore\u003c/sub\u003e (nm)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNatural ATP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e36.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e11.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1 mol/L HNO\u003csub\u003e3\u003c/sub\u003e-ATP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e127.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3 mol/L HNO\u003csub\u003e3\u003c/sub\u003e-ATP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e141.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5 mol/L HNO\u003csub\u003e3\u003c/sub\u003e-ATP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e121.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e6.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7 mol/L HNO\u003csub\u003e3\u003c/sub\u003e-ATP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e109.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2% Cu-ATP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e132.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e6.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4% Cu-ATP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e126.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e6.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6% Cu-ATP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e120.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e6.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8% Cu-ATP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e114.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e6.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section3\"\u003e \u003ch2\u003e3.4.2. XRD\u003c/h2\u003e \u003cp\u003eXRD was employed to examine the surface phase structures of ATP samples under various treatment conditions, and the results are depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. ATP samples exhibited characteristic diffraction peaks, with peaks at 19.7\u0026deg; and 26.6\u0026deg; corresponding to the (040) and (400) planes of ATP clay [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Moreover, different ATP samples did not exhibit significant differences in the intensity of the abovementioned characteristic peaks, which indicated that Cu doping did not significantly alter the fundamental structure. Additionally, the patterns of ATP samples doped with 4\u0026ndash;8% Cu exhibited characteristic peaks of CuO, indicating the presence of Cu species in the form of CuO crystallites on ATP. However, the pattern of 2% Cu-ATP exhibited no distinct peak of Cu species. This observation is attributable to the low loading of Cu, which resulted in the high dispersion of Cu on the surface of ATP.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section3\"\u003e \u003ch2\u003e3.4.3. XPS\u003c/h2\u003e \u003cp\u003eThe adsorption kinetics results suggest that Hg\u003csup\u003e2+\u003c/sup\u003e adsorption on Cu-ATP is a chemical adsorption process. To further investigate the Hg\u003csup\u003e2+\u003c/sup\u003e adsorption process on Cu-ATP, the chemical valence states of elements on the surface of ATP, 6% Cu-ATP, and used Cu-ATP were analyzed through the XPS method.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e shows the Cu 2p XPS spectra of ATP samples pretreated under different conditions. No significant characteristic peaks of Cu species were detected on the surface of untreated ATP, indicating the absence of Cu species on the ATP surface. However, Cu-ATP exhibited clear characteristic peaks of Cu species both before and after adsorption. The binding energies located at 933.1 eV and 953.2 eV corresponded to Cu⁺ species, while the peaks near 934 eV and 954.2 eV corresponded to Cu\u003csup\u003e2+\u003c/sup\u003e species [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. This result indicated that both Cu\u003csup\u003e2+\u003c/sup\u003e and Cu\u003csup\u003e+\u003c/sup\u003e occurred on the Cu-ATP surface. The coexistence of Cu\u0026sup2;⁺ and Cu⁺ species was due to electron transfer during the Cu doping process. This electron transfer process promoted the formation of surface oxygen vacancy sites, thereby increasing the amount of chemisorbed oxygen and facilitating the migration of oxygen species, which enhanced the chemisorption behavior of the adsorbent [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. The O 1s XPS spectra also confirmed that Cu doping increased the content of chemisorbed oxygen on the adsorbent surface. In addition, the Cu\u003csup\u003e+\u003c/sup\u003e/Cu\u003csup\u003e2+\u003c/sup\u003e ratio on the used Cu-ATP surface was significantly higher than that on fresh Cu-ATP, indicating that an electron transfer process occurred during the adsorption process, converting part of the Cu\u003csup\u003e2+\u003c/sup\u003e species into Cu\u003csup\u003e+\u003c/sup\u003e species. This process increased the formation of chemisorbed oxygen, which then combined with Hg\u003csup\u003e2+\u003c/sup\u003e to form stable HgO, thereby enhancing the chemisorbed behavior of the adsorbent. Subsequent Hg 4f XPS characterization also confirmed that the adsorbed mercury species mainly existed in the form of HgO.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe O 1s XPS spectra of ATP samples pretreated under different conditions are displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e. The O 1s characteristic peaks of all samples could be fitted into two peaks. The peaks around 530.3 eV corresponded to the lattice oxygen (O\u003csub\u003eβ\u003c/sub\u003e), while the peaks near 531.6 eV corresponded to the chemisorbed oxygen (O\u003csub\u003eα\u003c/sub\u003e) [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. The O\u003csub\u003eα\u003c/sub\u003e ratio on the ATP surface was only 24.9%. However, after the doping of Cu into ATP, the O\u003csub\u003eα\u003c/sub\u003e ratio on the adsorbent surface increased to 33.9%, indicating that Cu doping increased the O\u003csub\u003eα\u003c/sub\u003e content on the adsorbent surface. Cu doping on the ATP surface induced the formation of oxygen vacancies, which could promote the migration of oxygen species, thereby producing more O\u003csub\u003eα\u003c/sub\u003e [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. This enhancement was favorable for improving the chemisorption behavior of the adsorbent. The O\u003csub\u003eα\u003c/sub\u003e ratio on the surface of the used Cu-ATP was reduced to 22.8%, suggesting that the Hg\u003csup\u003e2+\u003c/sup\u003e adsorption process consumed the O\u003csub\u003eα\u003c/sub\u003e on the adsorbent surface. This can be explained by the reaction between Hg\u003csup\u003e2+\u003c/sup\u003e species and O\u003csub\u003eα\u003c/sub\u003e to form HgO during the Hg\u003csup\u003e2+\u003c/sup\u003e adsorption process, which led to a decrease in O\u003csub\u003eα\u003c/sub\u003e content. The Hg 4f XPS characterization also confirmed that the adsorbed mercury species mainly existed in the form of HgO.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo investigate the mercury species formed on the Cu-ATP surface during the Hg\u003csup\u003e2+\u003c/sup\u003e adsorption process, XPS analysis was conducted to examine the form of mercury species on the surface of Cu-ATP before and after Hg\u003csup\u003e2+\u003c/sup\u003e adsorption. The results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e. The Hg 4f XPS spectra indicated that no characteristic peak of mercury species was detected on fresh Cu-ATP, suggesting the absence of mercury species. However, after Hg\u003csup\u003e2+\u003c/sup\u003e adsorption, a characteristic peak of HgO species was observed, indicating that the mercury species mainly existed in the form of HgO on the used Cu-ATP surface [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. The changes in surface oxygen on the adsorbent suggest that Hg\u003csup\u003e2+\u003c/sup\u003e species in solution combined with the chemisorbed oxygen on the Cu-ATP surface to form stable HgO, thus achieving efficient removal of Hg\u003csup\u003e2+\u003c/sup\u003e. Additionally, the spectrum of the used Cu-ATP exhibited a small characteristic peak of HgS species, indicating that some Hg\u003csup\u003e2+\u003c/sup\u003e species combined with sulfur ions to form stable HgS during the adsorption process [\u003cspan additionalcitationids=\"CR36\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. Considering that ATP did not contain sulfur species, HgS species were likely present because sulfur species from the CuSO\u003csub\u003e4\u003c/sub\u003e precursor were loaded onto the ATP surface during the Cu loading process, resulting in the formation of HgS species. The formation of the HgS species is probably the main reason why Cu-ATP prepared using CuSO\u003csub\u003e4\u003c/sub\u003e as the precursor exhibited the best Hg\u003csup\u003e2+\u003c/sup\u003e adsorption performance among the different Cu precursors.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec21\" class=\"Section2\"\u003e \u003ch2\u003e3.5. Adsorption mechanism analysis\u003c/h2\u003e \u003cp\u003eCu-ATP exhibited superior Hg\u003csup\u003e2+\u003c/sup\u003e removal performance in simulated wastewater containing Hg\u003csup\u003e2+\u003c/sup\u003e species. The adsorption kinetics results confirmed that Hg\u003csup\u003e2+\u003c/sup\u003e adsorption on Cu-ATP fitted well with the pseudo-second-order model, indicating a chemical adsorption process. XPS characterization demonstrated that Cu species on the Cu-modified ATP adsorbent mainly existed in the form of Cu\u003csup\u003e2+\u003c/sup\u003e and Cu\u003csup\u003e+\u003c/sup\u003e, which could induce an electron transfer process, resulting in the formation of unsaturated sites on the Cu-ATP surface. This process can produce a large amount of O\u003csub\u003eα\u003c/sub\u003e on the Cu-ATP surface, leading to an increase in the Hg\u003csup\u003e2+\u003c/sup\u003e adsorption active sites on the adsorbent surface. The O 1s XPS spectra also confirmed the increase of O\u003csub\u003eα\u003c/sub\u003e on the Cu-ATP surface after Cu loading. During the Cu modification process, the S species in the Cu precursor CuSO\u003csub\u003e4\u003c/sub\u003e would transfer to the ATP surface, providing more adsorption active sites for Hg\u003csup\u003e2+\u003c/sup\u003e removal. The Hg 4f XPS spectra confirmed that the adsorbed Hg\u003csup\u003e2+\u003c/sup\u003e species existed mainly in the form of HgO and HgS on the Cu-ATP surface. The process of removing Hg\u003csup\u003e2+\u003c/sup\u003e by Cu-ATP is depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e. Hg\u003csup\u003e2+\u003c/sup\u003e species in wastewater were captured by chemisorbed oxygen and sulfur active sites on the Cu-ATP surface; consequently, stable HgO and HgS were generated, which enabled the efficient removal of Hg\u003csup\u003e2+\u003c/sup\u003e species from wastewater.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eA series of Cu-ATP adsorbents were synthesized, and the Hg\u003csup\u003e2+\u003c/sup\u003e removal performance of Cu-ATP in simulated desulfurization sludge leaching wastewater was investigated. Cu-ATP prepared using CuSO\u003csub\u003e4\u003c/sub\u003e as the precursor of Cu and treated with 3 mol/L HNO\u003csub\u003e3\u003c/sub\u003e demonstrated excellent Hg\u003csup\u003e2+\u003c/sup\u003e removal performance. The optimal Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency reached 90.7%, with a Cu-to-ATP ratio of 0.06. Moreover, the Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency of Cu-ATP initially increased with higher adsorbent content and longer adsorption times before stabilizing. However, as the pH value increased, the Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency first increased and then decreased, whereas the removal efficiency decreased with increasing initial Hg\u003csup\u003e2+\u003c/sup\u003e concentration. The adsorption kinetics revealed that Hg\u003csup\u003e2+\u003c/sup\u003e adsorption on Cu-ATP conformed well to the pseudo-second-order model, indicating a chemical adsorption process. BET and XRD analyses demonstrated that HNO\u003csub\u003e3\u003c/sub\u003e treatment effectively enhanced the specific surface area and pore volume of ATP. Following Cu loading, the specific surface area of the adsorbent slightly decreased, yet the crystal structure of ATP remained unaffected. The XPS results indicated that Cu species predominantly existed in the forms of Cu\u003csup\u003e2+\u003c/sup\u003e and Cu\u003csup\u003e+\u003c/sup\u003e on the Cu-ATP surface, significantly increasing the O\u003csub\u003eα\u003c/sub\u003e content and thereby enhancing the Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency of Cu-ATP. Moreover, S species from the CuSO\u003csub\u003e4\u003c/sub\u003e precursor were transferred to the Cu-ATP surface and provided additional active sites for Hg\u003csup\u003e2+\u003c/sup\u003e adsorption. Analysis of mercury morphology confirmed that adsorbed Hg\u003csup\u003e2+\u003c/sup\u003e species existed as stable HgO and HgS on the Cu-ATP surface, enabling the effective removal of Hg\u003csup\u003e2+\u003c/sup\u003e from wastewater. The findings reveal the potential of Cu-ATP as a cost-effective adsorbent for Hg\u003csup\u003e2+\u003c/sup\u003e removal in wastewater treatment. This study investigated the Hg\u003csup\u003e2+\u003c/sup\u003e removal performance of Cu-ATP, and future follow-up research should further explore the impact of other heavy metals (such as Cr, Ni, and Cu) coexisting in wastewater on the Hg\u003csup\u003e2+\u003c/sup\u003e adsorption properties of Cu-ATP.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the Project of State Grid Hebei Energy Technology Service, grant number TSS2022-05. We thank LetPub (www.letpub.com.cn) for its linguistic assistance during the preparation of this manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eA. Ishag, Y. Yue, J. Xiao, X. Huang and Y. Sun, \u003cem\u003eJ. Clean Prod.,\u003c/em\u003e \u003cstrong\u003e367\u003c/strong\u003e, 133111 (2022).\u003c/li\u003e\n\u003cli\u003eY. Li, J. Yu, Y. Liu, R. Huang, Z. Wang and Y. Zhao, \u003cem\u003eJ. Hazard. Mater.,\u003c/em\u003e \u003cstrong\u003e427\u003c/strong\u003e, 128132 (2022).\u003c/li\u003e\n\u003cli\u003eG. Chen, K. Han, C. Liu and B. Yan, \u003cem\u003eJ. Hazard. Mater.,\u003c/em\u003e \u003cstrong\u003e414\u003c/strong\u003e, 125561 (2021).\u003c/li\u003e\n\u003cli\u003eD. Liang, R. Jun, T. Ling and L. Hua, \u003cem\u003eFresenius Environ. Bull.,\u003c/em\u003e \u003cstrong\u003e26\u003c/strong\u003e, 2066 (2017).\u003c/li\u003e\n\u003cli\u003eY. Kobayashi, F. Ogata, C. Saenjum, T. Nakamura and N. 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Interfaces\u003c/em\u003e\u003cem\u003e,\u003c/em\u003e \u003cstrong\u003e11\u003c/strong\u003e, 19200 (2019).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"korean-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"kjce","sideBox":"Learn more about [Korean Journal of Chemical Engineering](http://link.springer.com/journal/11814)","snPcode":"11814","submissionUrl":"https://www.editorialmanager.com/kjce/default2.aspx","title":"Korean Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Subscription","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Wastewater, Hg2+ removal, Cu-modified attapulgite, removal mechanism","lastPublishedDoi":"10.21203/rs.3.rs-4755442/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4755442/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe development of low-cost and highly efficient adsorbents is essentially needed for removing Hg\u003csup\u003e2+\u003c/sup\u003e species from desulfurization sludge leaching wastewater. In this study, a series of novel Cu-modified attapulgite (Cu-ATP) adsorbents were synthesized via a simple HNO\u003csub\u003e3\u003c/sub\u003e treatment combined with an improved impregnation method. The Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency of these Cu-ATP adsorbents was investigated in simulated leaching wastewater. The effects of HNO\u003csub\u003e3\u003c/sub\u003e concentration, Cu precursor, Cu loading content, and other adsorption conditions on Hg\u003csup\u003e2+\u003c/sup\u003e removal using Cu-ATP were investigated. The results demonstrated that Cu-ATP prepared with CuSO\u003csub\u003e4\u003c/sub\u003e as the precursor and treated with 3 mol/L HNO\u003csub\u003e3\u003c/sub\u003e showed excellent Hg\u003csup\u003e2+\u003c/sup\u003e removal performance. Moreover, with increasing adsorbent content and adsorption time, the Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency of Cu-ATP first increased and then stabilized. However, with an increase in pH value, the Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency first increased and then decreased, whereas the removal showed a decreasing trend with increasing initial Hg\u003csup\u003e2+\u003c/sup\u003e concentration. The adsorption kinetics results indicated that Hg\u003csup\u003e2+\u003c/sup\u003e adsorption on Cu-ATP was well described by the pseudo-second-order model. Furthermore, various characterization methods, including Brunauer\u0026thinsp;\u0026minus;\u0026thinsp;Emmett\u0026thinsp;\u0026minus;\u0026thinsp;Teller analysis (BET), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), were employed to analyze the physicochemical properties of the adsorbents. The analyses confirmed that the superior Hg\u003csup\u003e2+\u003c/sup\u003e removal efficiency of Cu-ATP was mainly due to the complexation of Hg\u003csup\u003e2+\u003c/sup\u003e with chemisorbed oxygen produced by Cu doping and S species generated from the Cu precursor (CuSO\u003csub\u003e4\u003c/sub\u003e). These findings underscore the potential of Cu-ATP as a cost-effective adsorbent for removing Hg\u003csup\u003e2+\u003c/sup\u003e from wastewater.\u003c/p\u003e","manuscriptTitle":"Efficient removal of Hg2+ from wastewater by a novel Cu-modified attapulgite: Adsorption performance and mechanism","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-08-20 15:30:03","doi":"10.21203/rs.3.rs-4755442/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2024-08-28T15:32:04+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-07-24T09:30:46+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-07-22T11:53:07+00:00","index":"","fulltext":""},{"type":"submitted","content":"Korean Journal of Chemical Engineering","date":"2024-07-17T05:12:07+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"korean-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"kjce","sideBox":"Learn more about [Korean Journal of Chemical Engineering](http://link.springer.com/journal/11814)","snPcode":"11814","submissionUrl":"https://www.editorialmanager.com/kjce/default2.aspx","title":"Korean Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Subscription","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"89b10ce1-f42d-4c81-875b-8f433757c6c3","owner":[],"postedDate":"August 20th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-12-23T16:01:04+00:00","versionOfRecord":{"articleIdentity":"rs-4755442","link":"https://doi.org/10.1007/s11814-024-00360-6","journal":{"identity":"korean-journal-of-chemical-engineering","isVorOnly":false,"title":"Korean Journal of Chemical Engineering"},"publishedOn":"2024-12-19 15:57:21","publishedOnDateReadable":"December 19th, 2024"},"versionCreatedAt":"2024-08-20 15:30:03","video":"","vorDoi":"10.1007/s11814-024-00360-6","vorDoiUrl":"https://doi.org/10.1007/s11814-024-00360-6","workflowStages":[]},"version":"v1","identity":"rs-4755442","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4755442","identity":"rs-4755442","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}