Study on the Synergistic Passivation Effect of Cd and As in Soil by Iron Modified Biochar from Different Sources

Cadmium (Cd) and arsenic (As) are highly toxic heavy metals that pose serious risks to human health. This study investigated the synergistic passivation effects of iron-modified biochar derived from different feedstocks (rice straw, pig manure, and leaf litter) on Cd and As in contaminated soil. The physicochemical properties of the modified biochars were characterized using X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), and Brunauer-Emmett-Teller (BET) analysis. The results confirm the successful loading of Fe³⁺ onto the biochar surfaces and show significant increases in specific surface area, micropore surface area, and total pore volume compared with unmodified biochars. A soil incubation experiment demonstrated that all iron-modified biochars effectively reduced the bioavailability of Cd and As. Leaf litter-derived biochar (PIBC) achieved the highest reduction in Cd, decreasing its available content by 70.0% compared with the control (CK). In contrast, rice straw-derived biochar (SIBC) showed the greatest effect on As, reducing its concentration to 0.08 mg·kg^-1 by day 20. Correlation analysis reveal that soil pH, soil organic matter (SOM), and cation exchange capacity (CEC) were significantly associated with Cd and As availability, indicating that iron-modified biochar enhances passivation through alterations in soil physicochemical properties. Overall, this study highlights a sustainable strategy for the utilization of agricultural waste and the remediation of Cd- and As-contaminated soils.