Effect and mechanism of biochar as a support on immobilization of different heavy metals by iron oxides in a multi-contaminated soil

The effect and mechanism of biochar as a support on immobilization of different heavy metals (HMs) by iron oxides (FexOy) were still poorly understood. Herein, biochar-supported zero-valent iron (ZVI/BC) was fabricated via the simple one-step thermal reduction approach and applied to a soil multi-contaminated with cadmium (Cd), zinc (Zn), and lead (Pb) for 60-d. Results showed that individual pyrogenic FexOy effectively immobilized Cd but facilitated the release of Pb and Zn in soils. Biochar as a support compromised the immobilization of Cd by FexOy, possibly ascribing to cationic competition effect. ZVI/BC can synergistically immobilize Zn and Pb with the immobilization effectiveness of 33.7 % and 11.9 %, respectively owing to the dispersion effect of BC on ZVI. ZVI/BC addition increased soil pH, but soil pH increase can be not the key factor for the immobilization of HMs in soils. Moreover, biochar increased the diversity of soil bacteria community, but pyrogenic FexOy significantly lowered it. Further, ZVI/BC supplementation significantly stimulated the relative abundances of Fe(Ⅲ)-reducing bacteria (e.g., Geobacter and Acinetobacter) and sulfate-reducing bacteria (e.g., Desulfosporosinus and Desulfurispora) in soils, boosting the microbial reduction of Fe(Ⅲ) minerals and increasing the HCl-extractable Fe(Ⅱ) concentration. The adsorption/co-precipitation of secondary Fe minerals and formation of metal sulfide precipitates could play an important role in immobilization of HMs in soils. Overall, the present study highlighted the coupling of biotic and abiotic processes for stabilization remediation of HMs multi-contaminated soils by ZVI/BC.