Behaviour and mechanism of cadmium immobilization in contaminated soil by calcium carbide residue-enhanced MICP

A novel technique that couples microbially induced calcite precipitation (MICP) and calcium carbide residue (CCR) is proposed for immobilizing Cd²⁺ in contaminated soil. The properties and mechanism of CCR-enhanced MICP were investigated through a series of experimental analyses considering factors such as heavy metal concentration, curing time, and the effect of Ca²⁺. The unconfined compressive strength (UCS) increased with increasing curing time and reached a maximum value at 28 d, and the leaching concentration of Cd²⁺ decreased and tended to level off with increasing curing time. The addition of CCR enhanced the immobilization performance of Cd²⁺ through the MICP method, resulting in UCSs that were 3.8–4.2 times those of samples without CCR and leaching concentrations of Cd²⁺ that were 38.9–69.2% lower at a curing time of 28 d. The addition of Ca²⁺ to cementation solutions further improved the immobilization effectiveness, resulting in the UCSs of the samples increasing by 18.7–49.8% and the leaching concentrations of Cd²⁺ decreasing by 11–40% CaCO₃ and its hydration products can immobilize Cd²⁺ through coprecipitation, reducing its toxicity by converting weak acid-extractable cadmium into residual cadmium. Consequently, Sporosarcina pasteurii combined with CCR improved the UCS of the treated contaminated soil and greatly decreased cadmium migration.