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 Cd 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 Ca. The unconfined compressive strength (UCS) increased with increasing curing time and reached a maximum value at 28 d, and the leaching concentration of Cd decreased and tended to level off with increasing curing time. The addition of CCR enhanced the immobilization performance of Cd through the MICP method, resulting in UCSs that were 3.8-4.2 times those of samples without CCR and leaching concentrations of Cd that were 38.9-69.2% lower at a curing time of 28 d. The addition of Ca 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 Cd decreasing by 11-40% CaCO and its hydration products can immobilize Cd 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.