Enhanced Cadmium and Arsenic Stabilization in Co-Contaminated Soils Using Ferromanganese-Modified Biochar: Evaluating Molar Ratio Effects

Cadmium (Cd) and arsenic (As) accumulate in soils due to their persistence, posing a global remediation challenge. This study investigated the effectiveness of ferromanganese-modified biochar (Fe–Mn–BC) for stabilizing Cd and As and explored the underlying mechanisms. Soil incubation experiments were conducted using TCLP, CaCl₂-extractable Cd, NaH₂PO₄-extractable As, and BCR sequential extraction. After 60 days of Fe–Mn–BC application, extractable Cd and As decreased from 0.65 to 0.29 mg kg−1 and from 11.42 to 6.99 mg kg−1, respectively, indicating enhanced stabilization. The amendment promoted transformation of mobile fractions (weak-acid Cd and reducible As) into more stable residual forms, thereby limiting metal mobility. Soil pH remained stable (7.67–7.96), supporting the stabilization process. Overall, Fe–Mn–BC reduced the bioavailability and ecotoxicity of Cd and As. Among treatments, Fe₀.₁–Mn₀.₁ BC showed the highest effectiveness, followed by Fe₀.₁–Mn₀.₀₅ BC, Fe₀.₀₅–Mn₀.₁ BC, and BC.