Dual remediation of cadmium and arsenic in paddy soils using iron-modified biochars from different raw materials: performance and mechanisms

Abstract Agricultural soils in China’s rice-producing regions face significant contamination from both cadmium (Cd) and arsenic (As), posing serious ecological and health risks. This study aimed to address the challenge of immobilizing both contaminants simultaneously using iron-modified biochars derived from rice straw (SIBC), pig manure (MIBC), and pine wood (PIBC). Characterization showed successful iron impregnation, with Fe content in SIBC, MIBC, and PIBC reaching 35.19%, 35.26%, and 35.79%, respectively. The modification also increased the specific surface area and porosity of the biochars, with PIBC showing a 10.46-fold increase. Incubation experiments demonstrated that iron-modified biochars were effective in reducing bioavailable Cd and As. PIBC reduced available Cd by 70.1%, while SIBC reduced available As by 29.2%. Sequential extraction analyses revealed that iron-modified biochars promoted the transformation of Cd into more stable, residual forms, and As into less available oxidizable and residual phases. Additionally, pore water chemistry showed a shift towards low-toxicity As(V), with SIBC and PIBC increasing As(V) content by 84.8% and 91.1%, respectively. Microbial analyses indicated that biochar amendments improved soil ecological health, increasing microbial α-diversity and the abundance of beneficial bacterial phyla like Proteobacteria, Acidobacteriota, and Gemmatimonadota. Among the biochars, SIBC was the most cost-effective and efficient amendment for the remediation of co-contaminated paddy soils, showing significant potential for reducing both Cd and As contamination.