Remediation of Cr(VI)-contaminated soil by ball milling modified zero-valent iron biochar composites: Insights into long-term stability and microbial community

Hexavalent chromium (Cr(VI)) in soil has received extensive attention because of its harmful effects on the environment. In this study, ball-milled zero-valent iron biochar composites (BM-ZVI/BC) were prepared and used for the remediation of artificial Cr(VI)-contaminated soil. The Cr(VI) in the soil could be reduced from 170.4 mg kg-1 to less than 3.0 mg kg-1 after 5 days and remained stable in the next 105 days. The immobilization efficiency for Cr(VI) and Cr was close to 100%. In addition, the bioavailability of Cr(VI) and Cr was reduced to 27.9% and 4.6%, respectively. BM-ZVI/BC not only played a role in the transformation of Cr into a more stable chemical form to reduce its toxicity and mobility in the soil, but also affected the soil physicochemical properties. Compared to zero-valent iron biochar composites (ZVI/BC), BM-ZVI/BC had a positive effect on recovering microbial diversity and richness, along with the enhanced growth of low-abundance phyla. The relative abundance of Arthrobacter, Bacillus and Exiguobacterium related to Cr-reduction was also increased in the BM-ZVI/BC remediated soil. BM-ZVI/BC has shown excellent resistance and stability under extreme environmental conditions of acid rain leaching, dry-wet and freeze-thaw cycles. This study demonstrates the potential of ball milling modification in preparing zero-valent iron biochar composites for the remediation of Cr(VI)-contaminated soil.