Synergistic effects of Mg and Fe-modified bimetallic biochar for immobilization of As and Cd in agricultural soils

Arsenic (As) and cadmium (Cd) are persistent heavy metals commonly found in agricultural environments, where they pose a serious risk due to their high potential for bioaccumulation in the food chain. Therefore, effective management through advanced agricultural practices is crucial to minimize their presence and immobilize these contaminants in soil. In pursuit of this goal, we conducted a study to evaluate the potential of biochar (BC) and modified BC enriched with magnesium (Mg) and iron (Fe) for the adsorption of As and Cd in contaminated agricultural soils. We synthesized rice straw-derived BC, Mg-modified BC (Mg-BC), Fe-modified BC (Fe-BC), and a bimetallic BC (Bi-BC), incorporating both Fe and Mg. Following synthesis, these BC were applied to contaminated soils to assess changes in soil physicochemical properties, the immobilization of As and Cd, and the uptake of these metals by Ipomoea aquatica (commonly known as water spinach). The BC were characterized using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD), which revealed notable structural and compositional changes. Characterization confirmed the successful incorporation of Mg and Fe into the Mg-BC, Fe-BC, and Bi-BC samples. Application of the modified BC significantly improved several soil parameters, including pH (5.7–7.8), cation exchange properties (7.4–11 cmol + .kg −1 ), organic matter (2.1–4.8 %), water holding capacity (18.9–24.0 %), redox potential (204.6–273.3 mV). Enzyme activities, including dehydrogenase, alkaline phosphatase, β-glucosidase, and urease activity increased by 41.1 %, 61.2 %, 58.5 %, and 97.4 %, respectively, in Bi-BC treated soils compared to CK. Furthermore, the application of Bi-BC reduced soil Cd and As(III) concentrations by 59.0 % and 60.0 %, respectively, and decreased their accumulation in the edible parts of water spinach by 71.31 % and 82.5 %. This not only enhanced plant growth but also significantly reduced the bioavailability of these toxic metals. These findings indicate that Bi-BC is more effective than BC, Mg-BC, and Fe-BC in immobilizing Cd and As, leading to improved soil quality. The synergistic effects of Mg and Fe modifications present a sustainable and efficient strategy for remediating soils co-contaminated with As and Cd.