The optimization strategy for Cd and Pb immobilization in soil by meta-analysis combined with numerical modeling

Chemical immobilization is one of the most effective technologies to remediate sites with heavy metals, while the selection of proper immobilization material and the determination of its dose ratio remains a difficult problem that limits the remediation efficiency. This paper conducted a meta-analysis on 489 independent observations about the immobilization of heavy metals, in which immobilization materials were divided into biochar, phosphate, lime, metal oxides and clay minerals. By statistical analysis from these observations, the material dose ratio was found to be the most important parameter that controlled the immobilization efficiency, and the Freundlich adsorption model was successfully applied to calculate the bioavailable heavy metal (BHM) contents after immobilization. Based on the calculation results, phosphate was the most effective material to immobilize Pb. Lime was most effective when the initial bioavailable Cd content was 0.1~1 mg kg⁻¹, while the immobilization effects by lime and phosphate were close when the initial bioavailable Cd content was 10 mg kg⁻¹. Besides the material dose ratio, soil pH and organic matter content (OM) had negative correlations, while the fraction of initial BHMs (FB) and the immobilization time had positive correlations with the immobilization efficiency. A numerical model considering the material dose ratio, initial soil pH, OM, FB and immobilization time was established to calculate the residual BHM contents after immobilization. However, these factors only explained the immobilization effect by less than 45%, indicating that other factors such as the sub-kind of material and modification methods would also affect the immobilization effect. These results could help optimize the immobilization material type and its dose ratio in practical engineering.