H-SCH was synthesized via H₂O₂ oxidation method as described by Regenspurg et al. (Regenspurg et al., 2004). Briefly, 5.56 g of FeSO₄.7H₂O was dissolved in 248 mL ultrapure water. Then, 2 mL of 30% H₂O₂ was added slowly and uniformly while stirring for 2 min (min). The resulting precipitate was incubated for 24 h (h) in a constant temperature shaker at 180 rpm and 25°C. The synthesized H-SCH was then filtered through a 0.45 μm filter and rinsed with deionized water more than five times until the pH value of the solution used to rinse SCH stabilized. Subsequently, the H-SCH was filtered again using a 0.45 μm filter and dried at 60 °C for 12 h. The dried SCH was then ground and passed through a 150-mesh sieve.

M-SCH was synthesized using the KMnO₄ oxidation method, optimized from the H₂O₂ oxidation method, as described by Cao et al. (Cao et al., 2021). In this method, 0.3 g of KMnO₄ replaced H₂O₂, with all other steps remaining the same as for H-SCH.

Y-SCH was synthesized using an ethanol modification method, optimized from the H₂O₂ oxidation method, as described by Zhang et al. (Zhang D. et al., 2022). Here, 248 mL of 20% ethanol solution replaced ultrapure water, with all other steps remaining the same as for H-SCH. The rate of adding the oxidant in the preparation of H-SCH, M-SCH, and Y-SCH was consistent.

To investigate the adsorption capacity and mechanism of SCH for As(III), batch experiments of adsorption kinetics and isothermal adsorption were conducted in the 50 mL centrifuge tubes containing 40 mL 0.001M NaCl electrolyte (pH 7) with 1.5 g/L SCHs. In the adsorption kinetics experiment, the time intervals were 5, 15, 30, 60, 120, 240, 360, 480, 720, 1,440 and 2,880 min, respectively, with an initial As(III) concentration of 100 mg/L at 20 °C. According to the results of adsorption kinetics experiment, the equilibrium time was set at 1,440 min for the isothermal adsorption experiments. To obtain the thermodynamic parameters and saturation adsorption capacity of SCH for As(III), isothermal adsorption experiments were conducted at temperatures of 20, 30, and 40°C, with initial As(III) concentration from 10 to 350 mg/L. All experiments were carried out in a thermostatic water bath oscillator at 180 rpm and triplicated. At the end of each experiment, supernatants and solids were obtained by centrifugation, respectively. The concentrations of As(III) and total Fe in the liquid samples were analyzed after passing through a 0.22 μm filter. To obtain a completely dry SCH sample, the collected SCHs were dried at 35°C for 48 h, and then fully ground for subsequent characterizations (Zhang D. et al., 2022; Wang X. M. et al., 2023). Before use, all laboratory glassware was soaked in 5% (v/v) HNO₃ solutions for at least 24 h, followed by repeated rinsing with deionized water (Milli-Q). All reagents used were analytical grade in this study.

Shewanella oneidensis strain MR-1 (MR-1) and Desulfovibrio desulfuricans subsp.desulfuricans (referred to as SRB hereafter) were selected in this study. The detailed bacteria culture conditions for MR-1 and SRB can be found in the Supplementary Text S1. All experimental vessels were sterilized by autoclaving and UV sterilization, and all experimental procedures were conducted in an anaerobic sterile glove box. The impact experiments of MR-1 and SRB were divided into two distinct groups: experimental (B and C groups) and control group (A group). The experimental group involved the addition of either MR-1 or SRB to investigate their effect on the adsorption process, while the control group was without the addition of MR-1 or SRB. Each group was conducted in the 50 mL centrifuge tubes, and As(III) solution prepared with sterilized deoxygenated ultra-pure water and SCH was added in a 50 mL centrifuge tube to reach a 40 mL system with 50 mg/L A(III) and 1.5 g/L SCH. The bacterial concentration of experimental group was 2×10⁸ cells/mL, and DL-sodium lactate (50 mM) was introduced as the carbon source. Each group was performed in triplicates, and the incubation period lasted for 24 h at 20 °C. At the end of each experiment, supernatants and solids were obtained by centrifugation. The concentrations of As(III), total Fe and Fe(II) in the liquid samples were analyzed after passing through a 0.22 μm filter. The collected SCHs were dried at 35°C for 48 h, and then fully ground for subsequent characterizations (Zhang D. et al., 2022; Wang X. M. et al., 2023).

The concentrations of total Fe and Fe(II) were analyzed by the 1,10-phenanthroline method (Gao et al., 2022) (UV-2550, Shimadzu, Japan). The concentration of As(III) was analyzed by atomic fluorescence spectroscopy (HG-AFS 3100, Beijing Created High-Guarantee Instrumentation, China), detailed testing methods are provided in Supplementary Text S3. The X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer (Germany) using Cu Kα radiation (40 kV, 30 mA, 0.1 s/step), and the data of the crystallite phase were taken in 2-theta from 15° to 80°. Fourier transform infrared (FTIR) spectroscopy was conducted with an IRTRACER-100 (Shimadzu, Japan) with the following settings: attenuated total reflection (ATR) mode, scanned in the wavenumber range of 4,000 cm⁻¹–400 cm⁻¹, with a resolution of 4 cm⁻¹. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALABXI + spectrometer (Thermo Fisher, USA) with monochromatized Al Kα radiation and the binding energy positions were calibrated against C1s at 284.8 eV. Scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) was conducted on an S-4800 scanning electron microscope (HITACHI, Japan). The specific surface area and pore volume were calculated based on the Brunauer-Emmett-Teller method (ASAP2020, Micromeritics, US).

The infrared peak area (IPA) of SCHs was calculated by curve integral in Origin 2021 (Li et al., 2018; Li et al., 2021). XPS data was drawn through Avantage 5.9931. Other pictures and data analysis were completed through Origin 2021 and Excel 2021.

The XRD pattern of H-SCH, M-SCH and Y-SCH (SCHs) exhibited eight characteristic peaks that matches the SCH standard PDF pattern (PDF: 47–1775) (Figure 1A). The XRD spectra of SCH in the existing literature are similar to the amorphous peaks observed in this study, which is due to SCH being a poorly crystalline iron hydroxyl sulfate mineral (Chen et al., 2020; Wang et al., 2020; Cao et al., 2021; Jia et al., 2024). The FTIR results (Figure 1B) indicated that the functional groups of the studied SCHs (H-SCH, M-SCH, and Y-SCH) were consistent with spectra in the literature, including surface -OH groups (3,206 cm^-1), outer-sphere SO₄ ^2- (1,087 and 977 cm^-1), structural SO₄ ^2- (605 cm^-1), as well as Fe-O (694 cm^-1) groups (Peak et al., 1999; Paikaray et al., 2012; Chen et al., 2022; Tian et al., 2023). These alignments confirmed the successful synthesis of SCHs using various methods (Vithana et al., 2014; Cao et al., 2021; Zhang D. et al., 2022; Naren et al., 2023). However, notable distinctions were observed in terms of elemental composition, functional group content, and surface morphology among the three types of SCHs. Differences in infrared peak area (IPA), representing different function group content, were observed among the synthesized SCHs (Supplementary Table S1) (Brülls et al., 2007; Li et al., 2018; Wieser et al., 2022). Both M-SCH and Y-SCH exhibited higher content of SO₄ ^2- and -OH. Additionally, EDS results (Supplementary Figure S1) and elemental composition analysis (Supplementary Table S2) confirmed that M-SCH and Y-SCH contained more sulfur, consistent with the higher content of SO₄ ^2- in these samples. The surface of H-SCH was relatively smooth (Supplementary Figure S1), while M-SCH and Y-SCH had uneven wrinkles, with Y-SCH displaying a wool ball-like structure. This uneven surface morphology contributed to an increased specific surface area and average pore volume of M-SCH and Y-SCH (Supplementary Table S3). These results indicated that compared to the H₂O₂ oxidation method, the KMnO₄ oxidation (M-SCH) and ethanol modification (Y-SCH) methods enhanced the structural properties of SCH.

Previous studies have shown that the adsorption of As(III) by SCH is often accompanied by the oxidation of As(III), with the oxidation rate related to the As/Fe ratio on the SCH surface (Paikaray et al., 2012). If As(III) oxidation occurs, competitive adsorption between As(III) and As(V) on SCH is expected. Theoretically, As(III) is relatively stable in homogeneous solutions with low oxidation rate by O₂ and Fe(III) in water environments (Gulledge and O'Connor, 1973; Oscarson et al., 1980; Zhao Z. et al., 2011). Within the 24 h reaction period in this study, continuous XPS characterization results revealed no peaks corresponding to As(V) and Fe(II) were observed (Supplementary Figure S2; Supplementary Figure S3) (Gan et al., 2015; Fan et al., 2023). This result indicated that O₂ and Fe(III) in system did not oxidize As(III) to As(V) in this study. The high concentration of As(III) (100 mg/L) in this system led to a high As/Fe ratio on the SCH surface, inhibiting As(III) oxidation (Burton et al., 2009; Paikaray et al., 2011; 2012).

In this study, all SCH samples reached adsorption equilibrium within 24 h (1,440 min, Supplementary Figure S4). The pseudo-second-order kinetic model (Supplementary, Section S2) provided a better fit for the changes in As(III) concentration, as shown in Supplementary Table S4. Furthermore, the adsorption of As(III) by the investigated SCHs was a spontaneous endothermic reaction (∆G < 0, ∆S > 0, ∆H > 0) (Albadarin et al., 2012), with a slight enhancement in adsorption capacity observed with increasing temperature (Supplementary Figures S5A-F). The adsorption capacity of SCHs for As(III) followed the order Y-SCH > M-SCH > H-SCH at temperatures of 20, 30, and 40 °C. Notably, at room temperature (20 °C), the SCH samples exhibited excellent adsorption capacities for As(III), with Y-SCH at 62.69 mg/g, M-SCH at 58.62 mg/g, and H-SCH at 52.16 mg/g (Supplementary Figures S5A-C). Considering the adsorption capacity for As(III), along with the larger specific surface area and average pore volume of M-SCH and Y-SCH (Supplementary Table S3), Y-SCH and M-SCH exhibit superior performance compared to H-SCH. Therefore, Y-SCH and M-SCH were selected as the focus for further investigation into the adsorption dynamics and the impact of microorganisms.

Typically, the pseudo-second-order kinetic model is used to describe chemical adsorption processes (Ho and McKay, 1999). However, in this work, the calculated ∆H values for M-SCH and Y-SCH were 10.65 and 30.78 kJ/mol, respectively (Supplementary Table S5; Supplementary Figures S5D-F, see Text S2 for calculation method). The ΔH for adsorption governed by chemical adsorption ranges between 40 and 120 kJ/mol (Alkan et al., 2004; Zhao D. et al., 2011). Since all ΔH values for the studied SCHs were less than 40 kJ/mol, this implies that physical adsorption predominantly governs the adsorption of As(III) by SCHs. Therefore, the specific adsorption mechanism between SCHs and As(III) needs further analysis using infrared peak area (IPA) analysis. Figure 2 showed a reduction in the IPA of -OH (3,206 cm^-1), outer-sphere SO₄ ^2- (1,087 and 977 cm^-1), and structural SO₄ ^2- (605 cm^-1) groups. This indicated the involvement of both -OH complexation and SO₄ ^2- exchange (including outer-sphere and structural SO₄ ^2-) during the adsorption of As(III) on M-SCH and Y-SCH (Burton et al., 2009; Carrero et al., 2017). However, the reduction in the content of -OH and SO₄ ^2- in M-SCH and Y-SCH was slight (Figures 3A–C). This suggests that -OH complexation and SO₄ ^2- exchange participate in the adsorption process, but they do not play a dominant role (Brülls et al., 2007; Li et al., 2018; Wieser et al., 2022). Additionally, considering the enhanced specific surface area and porosity of M-SCH and Y-SCH (Supplementary Table S3) favor the physical adsorption of As(III), physical adsorption should play a leading role in the adsorption of As(III) by these materials (Li and Wei, 2022). Compared to M-SCH, Y-SCH has a larger specific surface area and porosity, resulting in a stronger adsorption capacity (Supplementary Figure S4; Supplementary Figures S5A-C). These results indicated that the two optimized chemical oxidation methods primarily enhanced the physical adsorption capacity of SCH for As(III).

In the investigation of M-SCH and Y-SCH, the changes in mineral phase during the dynamic adsorption process were sequentially monitored (Figures 4A, B). The XRD spectra of both M-SCH and Y-SCH exhibited new peaks at 2-theta = 30.7° and 41.4° in the initial stage of the process (60 min), indicating the formation of new minerals within the system. Subsequently, the intensity of these peaks gradually diminished over time until they completely disappeared by the end of the process (1,440 min), reverting the system to a mineral composition primarily dominated by SCH. This phenomenon can be explained by the amorphous nature of SCH, which exhibits some solubility in an aqueous environment (K_sp = 18.0 ± 2.5) (Burton et al., 2008). Consequently, the dissolution of SCH in water released Fe(III), SO₄ ^2-, and H⁺ ions (Schoepfer and Burton, 2021) (Figures 5A–C). In the presence of As(III), the hydrolysis of Fe(III) led to the formation of As(III)-bearing ferrihydrite through a coprecipitation mechanism (Figure 5D) (Johnston et al., 2011; Yang et al., 2021).

Distinct peaks at 2-theta = 30.7° and 41.4° in Figure 4 correspond to the characteristic peaks of As(III)-bearing ferrihydrite. The deviation of these main XRD peaks from the standard ferrihydrite can be attributed to poor crystallinity induced by the coprecipitation or adsorption of As(III). This causes a shift in the main XRD peaks of standard ferrihydrite from 2-theta = 35.89°, 40.80°, and 46.28° (Figure 4, PDF 29–0712) approximately 5° to the left. Previous studies have also observed that the presence of As(III) leads to a leftward shift in the XRD peaks of ferrihydrite (Takaya et al., 2021). The newly formed As(III)-bearing ferrihydrite exhibited reduced crystallinity and diminished stability, making it susceptible to transformation into alternative mineral phases. Simultaneously, the pH of the aqueous solution continued to decrease due to the release of H⁺ from the dissolution of SCH (Figure 5B). Additionally, the concentration of released SO₄ ^2- from SCH dissolution and SO₄ ^2- exchange steadily increased. This created an environment of low pH and high SO₄ ^2- concentration conducive to the transformation of As(III)-bearing ferrihydrite into SCH (Ying et al., 2020; Schoepfer and Burton, 2021) (Figures 5A, B). Considering these factors collectively, the newly formed As(III)-bearing ferrihydrite exhibited poor stability and gradually transformed into SCH.

The transformation of As(III)-bearing ferrihydrite to SCH required the utilization of SO₄ ^2- and H⁺ in the solution (Johnston et al., 2011). However, SCH released SO₄ ^2- during the adsorption of As(III) via SO₄ ^2- exchange, resulting in an unexpected increase in SO₄ ^2- concentration over time (Figure 5A) (Burton et al., 2009; Antelo et al., 2012). Although the change of SO₄ ^2- levels was initially masked, a definitive indication was observed from the pH change (Figure 5B). The pH of M-SCH and Y-SCH systems gradually rose starting at 480 min. This change indirectly confirmed the XRD spectra (Figure 4), indicating the transformation of As(III)-bearing ferrihydrite to SCH after 480 min. This finding differs significantly from the lengthy transformation processes observed in long-term experiments, which can span days, months, or even years. These long-term studies often neglect to monitor SCH during the initial stages of the reaction, particularly within the first 24 h. In such cases, SCH is found to transform directly into goethite over an extended timeframe (Xie et al., 2017; Wang et al., 2020). This study represents the first instance where the generation of As(III)-bearing ferrihydrite was observed, followed by its subsequent transformation into SCH during the short-term (within 24 h) adsorption process between SCH and As(III).

Figure 6A showed a marked increase in the quantities of Fe(II) in groups supplemented with MR-1 and SRB (B and C groups), compared to groups without these supplements (A group). This increase highlighted the metabolic influence of MR-1 and SRB under the experimental conditions (As(III) concentration = 50 mg/L) (Zhang Z. et al., 2022; Ke et al., 2023). Additionally, Figure 6B demonstrated the ability of MR-1 and SRB to enhance the adsorption capacity of SCH for As(III). It is important to note that this conclusion accounts for the adsorption contributed by the MR-1 and SRB bacteria (Supplementary Figure S6). Compared to MR-1, SRB significantly increased the adsorption of As(III) by SCH. Extracellular polymeric substances (EPS) produced by MR-1 or SRB coated the surface of SCH, aiding in the complexation of As(III) (Ma et al., 2021; Zhang P. et al., 2021; Chen et al., 2023). Furthermore, MR-1 and SRB consumed significant amounts of H⁺ during metabolism (Bose et al., 2009; Zhang Z. et al., 2022), thereby increasing the system’s pH (Figure 6C). The higher pH promotes -OH complexation with As(III) on the surface of SCH, facilitating the adsorption of As(III) (Wang X. et al., 2023). The presence of EPS on the surface of SCH was confirmed by FTIR results (Figure 7), which showed a series of distinctive new peaks in the infrared spectrum at 1,250–1707 cm^-1 after the reaction, corresponding to the characteristic peaks of EPS (Eboigbodin and Biggs, 2008).

The mineral composition of M-SCH and Y-SCH in the MR-1 system showed negligible changes compared to the pre-reaction state (Figure 8A). Previous studies indicated that Fe(II) can promote the reductive dissolution and transformation of SCH (Fan et al., 2019; Fan et al., 2023). Remarkably, despite the considerable Fe(II) content in the MR-1 system (Figure 6A), no discernible mineral transformation of SCH was observed. This can be attributed to the adsorption of EPS produced by MR-1 onto the SCH surface. The presence of EPS not only enhanced the adsorption of As(III) but also occupied active sites, impeding electron transfer between Fe(II) and Fe(III) on the SCH surface (Yan et al., 2021; Wang Q. et al., 2023). This decelerated the mineral transformation process. Additionally, although the metabolic activity of MR-1 elevated the system’s pH (Figure 6C), the pH of the MR-1 system remained below the pH_-pcz of synthetic SCH, which is approximately 7 (Jönsson et al., 2005). Consequently, the SCH surface was positively charged, hindering the adsorption of Fe(II) due to electrostatic repulsion. As a result, electron transfer from Fe(II) to SCH was weakened, further retarding the mineral transformation process. These two factors collectively explain the absence of noticeable mineral transformation of SCH in the MR-1 system.

Different from the MR-1 system, a peak at approximately 2-theta = 30.5° was observed in the SRB system after the reaction (Figure 8B). This newly observed peak is recognized as the characteristic peak of As(III)-bearing ferrihydrite generated within the system (the specific identification process of As(III)-bearing ferrihydrite is discussed in details in section 3.3). Similar phenomena have also been observed in the reductive dissolution process of As-loaded SCH in the presence of S^2- (Zhang et al., 2016). The transformation of SCH to As(III)-bearing ferrihydrite occurred only in the SRB system due to the considerably higher concentration of Fe(III) compared to the MR-1 system (Figure 6A). S^2- generated by SRB exhibited a more pronounced reductive dissolution ability towards SCH compared to the Fe(II) produced by MR-1, resulting in a higher Fe(III) concentration in the SRB system (Poulton et al., 2004). However, the reduction ability of SRB towards Fe(III) is weaker than that of MR-1, leading to a higher proportion of Fe(III) in the SRB system (Figure 6A) (Zhang Y. et al., 2021). Under SRB mediation, the Fe(III) concentrations in the M-SCH and Y-SCH systems reached 11.57 mg/L and 9.39 mg/L, respectively, with pH values of 5.95 and 5.84 (Figures 6A–C). This concentration of Fe(III) and pH environment is highly favorable for the formation of ferrihydrite (Stumm and Morgan, 2013; Weatherill et al., 2016). However, the characteristic peaks of ferrihydrite on the XRD are not prominent (Figure 8B), which may be due to the inhibition of ferrihydrite formation in the SRB-mediated reducing environment (Cornell and Schwertmann, 2003; Stumm and Morgan, 2013).

Although no significant transformation of SCH occurred during the microbial influence experiment, both M-SCH and Y-SCH exhibited reductive dissolution in the presence of MR-1 and SRB. Additionally, the formation of As(III)-bearing ferrihydrite was observed in the SRB systems. The low crystallinity and poor stability of As(III)-bearing ferrihydrite, combined with the potential mineral transformation of SCH due to prolonged exposure to MR-1 and SRB, raise concerns regarding the release of As(III). Therefore, further investigations into the long-term stability and risks associated with the secondary release of SCH and As(III)-bearing ferrihydrite under the influence of MR-1 and SRB are warranted.