Study on the Mechanism of Cd-Contaminated Soil Remediation Using NaOH–KMnO4–FeCl3 Composite-Modified Biochar for Sustainable Utilization

Heavy metal pollution is characterized by long-term accumulation and recalcitrance to degradation, which poses a serious threat to soil ecosystems and groundwater environments. To improve the remediation efficiency of biochar for cadmium (Cd)-contaminated soil, this study took unmodified biochar (BC) as the control and systematically explored the remediation potential of NaOH–KMnO4–FeCl3 composite-modified biochar (GBC). Combined with a Brassica napus L. pot experiment, the effects of modified biochar on soil Cd passivation, soil physicochemical properties, and B. napus biomass were analyzed. After composite modification, GBC had its surface ash removed and exhibited a more regular pore structure, with successful loading of iron–manganese oxides. Although partial changes in the microporous structure caused a decrease in CO2 adsorption, the number of surface-active sites increased. Both biochars significantly increased soil carbon content, nitrogen and phosphorus nutrient levels, and electrical conductivity, while promoting B. napus biomass accumulation and reducing its Cd enrichment. Among them, the GBC-1.5 treatment group exhibited the most significant increase in B. napus biomass, which was 33.66% higher than that of the control group (CK). However, soil pH increased with the increase in BC but decreased with the increase in GBC application rate. In terms of Cd passivation effect, both biochars showed excellent remediation performance. When the application rate was 3%, the Cd passivation rate of the GBC-3 treatment group reached 35.87%, which was 5.29% higher than that of the BC-3 treatment group. The loading of iron–manganese oxides further enhanced the effectiveness and stability of chemical adsorption. This study provides an important reference for achieving sustainable utilization of soil heavy metal remediation.