Reduction and immobilization of Cd(II) and As(III) using sulfur-ferrihydrite-biochar as an amendment in water and soil: Investigation of the Mechanism of Remediation

The transformation behaviors of arsenic (As) and cadmium (Cd) in contaminated soils are generally complex process due to their distinct chemical and physical characteristics, which poses challenge for remediation. This study proposes an efficient strategy for the simultaneous immobilization of Cd and As using sulfur-ferrihydrite-modified biochar (SFB) as an organic amendment. A series of experiments, including batch and pot experiments, was conducted under controlled conditions. The results showed that the maximum sorption capacities of Cd and As by SFB were 76.69 mg kg ^-1 and 8.28 mg kg ^-1 , respectively, which were significantly higher than those of biochar (BC), ferrihydrite (FH) and ferrihydrite-biochar (FB). This higher sorption capacity is attributed to synergistic interactions between biochar and ferrihydrite. The sorption process of Cd and As by SFB follows the Langmuir isothermal sorption model and the pseudo-second-order kinetic model, indicating a combination of physical sorption and chemisorption mechanisms. The removal mechanisms for As primarily involve precipitation, oxidation and complexation, while those for Cd mainly include ion exchange, complexation, precipitation, and electrostatic sorption. Application of SFB reduced the bioavailable forms of Cd and As in the soil, shifting their chemical forms toward more stable residual states and enhancing immobilization. Overall, the SFB is a novel and effective adsorbent by immobilizing Cd and As in agricultural soils, promoting safer crops production in contaminated field.