Sulfur-ferrihydrite-modified biochar for simultaneous immobilization of arsenic and cadmium in co-contaminated water/soil: performance and mechanisms

Environmental contamination by arsenic (As), and cadmium (Cd) presents a significant remediation challenge due to their distinct chemical speciation, adsorption affinities, and transport behaviors. In this work, a sulfur-ferrihydrite-functionalized biochar (SFB) was engineered as a versatile soil amendment capable of immobilizing both Cd(II), and As(III) simultaneously. Comprehensive characterization confirmed that sulfur species and ferrihydrite nanoparticles were homogeneously distributed across the biochar surface, resulting in a hierarchically porous composite enriched with reactive functional groups and enhanced redox-active interfaces. Adsorption experiments revealed that SFB achieved maximum uptake capacities of 76.69 mg g−1 for Cd, and 8.28 mg g−1 for As, substantially exceeding those of rice straw biochar (BC), ferrihydrite (FH), and ferrihydrite-modified biochar (FB). The adsorption behavior conformed well to the Langmuir isotherm, and pseudo-second-order kinetic models, suggesting that both physical adsorption and chemical interactions governed Cd(II)/As(III) sequestration. Spectroscopic and microscopic analyses indicated that Cd(II) was immobilized mainly through ion exchange, surface coordination, and the formation of CdS and FeS precipitates. In contrast, As retention proceeded via oxidation of As(III) to As(V), followed by strong inner-sphere complexation with Fe–O functional groups. These coupled redox and coordination processes created synergistic interfacial sites capable of effectively capturing both cationic and anionic contaminants. Soil incubation experiments further confirmed the effectiveness of SFB, showing maximum reductions in bioavailable Cd (32.59%–41.26%), and As (50.06%–64.06%), along with their transformation into more stable residual fractions. Overall, the results demonstrate that SFB represents a promising amendment for the simultaneous stabilization of Cd and As in co-contaminated agricultural soils.