Remediation of As(III)-contaminated soil by biochar-loaded Fe-Mn oxides: Remediation efficiency and mechanism.

Iron-manganese oxide modified biochar (FMO-BC) was prepared for remediation of contaminated soil with As content of 1300 mg kg-1. The remediation scheme mainly includes oxidizing As(III) into less mobile As(V) by manganese oxide, stabilizing As(V) by iron oxide, and simultaneously reducing the aggregation of iron-manganese oxide (FMO) by BC support. The remediation effect and detailed mechanism were investigated through leaching tests, freeze-thaw experiment, sequential extraction procedure, and a series of spectroscopic/microscopic analyses. The results showed that the optimal FMO loading rate on BC was 25%. Owning to neutralization of alkaline BC and acidic hydrolysate of ferric iron, the addition of FMO-BC caused only a little change in soil pH, thereby mitigating the risk of As leaching under high pH condition. The FMO-BC treatment is more effective than FMO. After freeze-thaw cycles, the further dispersion of FMO and the presence of more functional groups enhanced the treatment effect. At 7 wt% of FMO-BC, the leaching concentration of As was reduced to 0.456 mg L-1, and the stabilization efficiency reached 98.6%. The FMO-BC treatment reduced fractions of exchangeable and carbonate-bound forms of As from 41.1% to 1.84%, substantially reducing the environmental risk. FMO-BC oxidized most As(III) in the soil into As(V), facilitated the formation of Fe-O-As and Mn-O-As complexes. The stabilization mechanism included oxidation, precipitation, and complexation reactions. By loading on BC, the aggregation of FMO was reduced and FMO dosage can be lowered. The proposed technique showed high efficiency, environmental friendliness, and great application prospect.