Critical role of soil-applied molybdenum dioxide composite biochar material in enhancing Cr(VI) remediation process: The driver of Fe(III)/Fe(II) redox cycle.

Heavy metal contamination of agricultural land due to sewage irrigation, over-application of fertilizers and pesticides, and industrial activities. Biochar, due to its rich functional groups and excellent electrochemical performance, is used for the remediation of heavy metal-contaminated farmland. However, the remediation mechanism remains uncertain due to the influence of minerals and multi-element composite pollution on soil. Therefore, introducing transition metal oxide MoO₂ to prepare biochar composite remediation materials enhances the adsorption and reduction of soil Cr (Ⅵ). This study compared the differences in Cr (Ⅵ) improvement under different pollution systems and pH conditions and explored the potential mechanism of Fe (Ⅲ)/Fe (Ⅱ) redox cycling in Cr (Ⅵ) remediation. The results showed that both biochar MoO₂ ball-milling composite (BC + M) and biochar-loaded MoO₂ (BC/M) retained the original biochar (BC) remediation method for Cr (Ⅵ). Among them, the remediation of BC/M was the most stable, with the maximum remediation value ranging from approximately 6.52 to 58.58 mg/kg. In different pollution systems, Cd and Pb exhibited competitive adsorption toward Cr (Ⅵ), but they enhanced Cr (Ⅵ) remediation by promoting adsorption and self-complexation. In acidic conditions (pH = 4), BC/M showed the best remediation effect, with a reduction kinetic constant of 34.61 × 10^-3 S^-1 and a maximum adsorption capacity of 61.64 mg/g. Fe (Ⅲ)/Fe (Ⅱ) redox cycling accelerated the reduction of Cr (Ⅵ) (R² = 0.81), and MoO₂ promoted the Fe (Ⅲ)/Fe (Ⅱ) redox cycle. BC/M enhanced the Fe (Ⅱ) formation efficiency by 66.39% and 71.81% compared to BC + M and BC at pH = 4. The introduction of MoO₂ and biochar composite materials enhanced the reduction process of Cr (Ⅵ), with BC/M achieving the optimal remediation level. This study reveals the potential mechanisms of MoO₂ and biochar composite materials in soil Cr (Ⅵ) remediation, providing a reference and insight for the preparation of Cr (Ⅵ) remediation materials and the treatment of contaminated farmland.