The ultra-large-capacity silica-supported ferric oxyhydroxide material: Preparation and mechanism on arsenic removal from water.

To address the challenges associated with conventional iron-based adsorbents, such as high cost and low arsenic capacity, this study reports the silica-supported ferric oxyhydroxide composite adsorbent (SiFeOH) for arsenic removal from water. A silica support was first prepared from sodium silicate and activated by methanesulfonic acid. Then, ferric oxyhydroxide was introduced onto the support through in-situ oxidative deposition. Structural characterization revealed that the material possesses an amorphous and non-porous structure, providing a fully accessible and effective external surface. Under the optimized Si/Fe molar ratio of 1:12(SiFeOH-1:12), the material exhibited maximum adsorption capacities for As(III) and As(V) as high as 77.74 and 115.96 mg/g, respectively, significantly outperforming conventional iron-based adsorbents (typically <50 mg/g). Thermodynamic analysis indicated that As(III) adsorption is an enthalpy-driven, exothermic process (ΔH = -29.03 kJ/mol), while As(V) adsorption is an entropy-driven, endothermic process (ΔH = +21.57 kJ/mol). Furthermore, XPS analysis confirmed the underlying mechanism: the removal of As(V) follows a non-redox, direct complexation pathway, whereas As(III) removal involves a more intricate redox-coupled complexation process. Both arsenic species are ultimately immobilized through ligand exchange with surface hydroxyl groups (Fe-OH) to form stable inner-sphere complexes. In treating an actual high-arsenic water sample, a dosage of only 30 mg of the material effectively reduced the arsenic concentration from 143.0 ppb to 3.0 ppb, well below the drinking water standard, while also demonstrating effective arsenic immobilization stability. This research not only develops a high-performance adsorbent but also elucidates the interfacial interaction mechanisms with different arsenic species from the perspectives of surface chemistry and electron transfer, thereby providing new materials and theoretical support for arsenic pollution remediation.