Enhancing Coimmobilization Capacity of Schwertmannite for Arsenic and Cadmium through pH Elevation after Chemical Oxidation

Schwertmannite (Sch) is a promising environmental functional material for remediation of arsenic pollution, particularly arsenite [As(III)], due to its unique adsorption mechanism. However, its application in metal–metalloid cocontamination is greatly limited by its ineffective immobilization of cationic metals. In this study, a preparation method of chemical oxidation–pH elevation was proposed to optimize the immobilization capacity of schwertmannite for As and cadmium [Cd(II)], which referred to promoting solution pH to neutral conditions after FeSO₄–H₂O₂ chemical oxidation. The resultant schwertmannite mixture (Sch-M) had a slightly increased Fe content (by 14.2–17.7%) and a reduced sulfate content (by 24.5–42.1%) compared to Sch. Sch-M displayed stronger mineral phase stability under anaerobic Fe(II)-catalyzed condition. The adsorption capacity of Sch-M for As(III) was basically consistent with that of Sch (172.9–211.3 vs 196.5 mg/g) and reached 19.7–22.4 mg/g for Cd(II) at pH 6.5. Sch-M performed better stabilization effects for metals and metalloids in a contaminated soil than zerovalent iron. In addition, its passivation ability was resistant to acidic conditions (pH 2.6). The practical application of the Sch-based environmental functional material in the remediation of metal(loid)-contaminated soils is worthy of in-depth study.