Optimization strategies for Cd and Pb immobilization in soil using meta-analysis combined with numerical modeling

Chemical immobilization is one of the most effective technologies for remediating sites with heavy metals, but the selection of proper immobilization material and determination of its dose ratio is a challenge that limits the remediation efficiency. In this study, we conducted a meta-analysis of 489 independent observations on the immobilization of heavy metals, in which the immobilization materials were divided into biochar, phosphate, lime, metal oxides, and clay minerals. The statistical analysis of these observations revealed that the material dose ratio was the most important parameter that controlled immobilization efficiency, and the Freundlich adsorption model was successfully applied to calculate the bioavailable heavy metal (BHM) concentration after immobilization. Based on the calculation results, phosphate was the most effective material for Pb immobilization. Lime was the most effective when the initial bioavailable Cd content was 0.1–1 mg kg^–1, whereas the immobilization effects of lime and phosphate were similar when the initial bioavailable Cd content was 10 mg kg^–1 .In addition to the material dose ratio, initial soil pH (pH) and organic matter (OM) content were negatively correlated, whereas the fraction of initial BHMs before immobilization (FB) and immobilization time were positively correlated with immobilization efficiency. A numerical model that considered the material dose ratio, pH, OM, FB, and immobilization time was established to calculate the residual BHM concentration after immobilization. However, these factors only explained less than 45% of the immobilization effect, indicating that other factors, such as the sub-type of the material and modification methods, also affect the immobilization effect. These results could help to optimize the type of immobilization material of heavy metals and its dose ratio in practical engineering applications.