Alkaline modification on schwertmannite promoted the simultaneous immobilization of arsenite and cadmium

Arsenic (As) and cadmium (Cd) co-contamination is of most concern in the environment. Schwertmannite (Sch), a poorly crystalline Fe(III) oxyhydroxy-sulfate mineral, is extremely efficient for As immobilization owing to its unique structure, but ineffective against cationic Cd (Cd(II)) due to the acidic pH after Sch addition. In this study, an alkaline modification method was proposed to improve the treatment efficiency of schwertmannite for As(III)-Cd(II) co-contamination. Results showed that alkali soaking had no obvious effect on mineral phase, and schwertmannite was the main mineral component. Alkali-modified Sch (Sch-OH) consisted of a large number of nanoparticles superimposed on each other, with specific surface area drastically increasing from 69.7 m2/g (Sch) to 261.9 m2/g. The sulfate content in Sch-OH structure decreased by 71.5%, which contributed to the reduction of maximum adsorption capacity for As(III) (from 116.09 to 81.06 mg/g at pH 6.5). The co-existing As(III) enhanced Cd(II) adsorption onto Sch-OH by forming ternary Fe-As-Cd complexes, corresponding to an increase in adsorbed amount from 8.8 to 11.3 mg/g (in 20 mg/L Cd(II) solution). Sch-OH exhibited a higher mineral stability than Sch under flooded condition at 40°C, and its retaining ability for As(III) was stronger. In a Cd-As polluted soil, the content of KH2PO4-extractable As decreased from 6.3 to 4.8 mg/kg after 10 days-incubation with 5% Sch-OH, and the immobilization efficiency of Cd reached 49.1%. It is necessary to explore more modification methods of schwertmannite to broaden its application in metal(loid)s remediation.