Remediation potential of biochar for As and Cd by modifying soil physicochemical properties: a conceptual model elucidating stabilization mechanism based on conditional probability theory

Abstract Biochar has increasingly attracted the attention of researchers owing to its excellent stabilization effect on heavy metals and resource recycling properties. To better describe the stabilization process of heavy metals under the action of biochar, this study combined literature data integration and soil culture experiments, constructed a conceptual model of As and Cd stabilization under different biochar additions through the conditional probability theory, investigated the effects of biochar on soil properties and key components, identified the factors influencing the model parameters, and analyzed As and Cd stabilization by biochar and their mechanisms. Biochar addition improved soil particle size distribution and pore structure and increased soil pH and organic matter content but decreased cation exchange and total reducing substances. When the amount of biochar was 5%, tyrosinoid substance in soil decreased by 33% and aromatic protein substance increased by 32%. Biochar promoted the conversion of aromatic proteins to fulvic acids and reduced soil FeS 2 content. The leaching concentrations of As and Cd were reduced to 197.97 and 101.24 μg L −1 , respectively, when the addition of biochar was increased to 5%. As and Cd were transformed to more stable reducible, residual, and oxidizable states. Model 1 was more suitable for describing the stabilizing effect of biochar on As and Cd in the present study, and the stabilizing parameters decreased with an increase in biochar dosage. Partial least square-path modeling analyses showed that the soil cation exchange capacity (β = 0.358, p < 0.005) and pH (β = 0.358, p < 0.005) had a significant positive effect on the stabilizing parameters, and amorphous iron (β = −0.410, p < 0.005) had a significant negative effect on the stabilization parameters. Through the synergistic effect, the effective concentrations of both As and Cd were reduced, and Cd remediation exhibited some competitive advantages. In addition to soil adsorption and chelation, the main stabilization mechanisms of biochar for As included redox and those for Cd included ion exchange and co-precipitation. The study revealed that the conceptual model of stabilization based on conditional probabilistic theory can effectively describe the stabilization of As and Cd by biochar through changing soil physicochemical properties. The method could facilitate quantitative identification of the stabilization effect of biochar in heavy metal-contaminated soils. Graphical abstract