MICP versus MIPP for Cd immobilization: kinetics, products, leaching stability and conversion feasibility

Microbially induced carbonate/phosphate precipitation (MICP/MIPP) are prevalent techniques for heavy metal (HM) remediation. However, systematic comparative studies on their functional strains, mineralization efficiency, precipitate stability, and integrated application potential are still scarce. Here, the MICP-functional strain Sporosarcina sp. DF-C31 and MIPP-functional strain Burkholderia orbicola PSB4 were employed to investigate their cadmium (Cd 2+ ) mineralization performance. The MICP system achieved 98.98% Cd 2+ removal efficiency within 36 h and generated stable calcite precipitate within 72 h, while MIPP immobilized 98.39% of Cd 2+ within 168 h and yielded highly crystalline hydroxyapatite (HAP) within 120 h. The MICP precipitates consisted of a Ca 1-x Cd x CO 3 solid solution, and the MIPP precipitates were composed of Ca 10-x Cd x (PO 4 ) 6 (OH) 2 . Multiple characterization techniques collectively demonstrated that Cd immobilization proceeded via Ca-site substitution and lattice incorporation in both systems. Specifically, MIPP adopted a pathway of surface adsorption followed by lattice migration, while MICP operates via coprecipitation-mediated entrapment. In two-month acid leaching tests, Cd 2+ release from MICP precipitates increased significantly under aggressive low-pH conditions. For MICP precipitates, Cd 2+ release rates reached 23.86% and 1.70% at pH 2 and pH 4, respectively. In contrast, MIPP precipitates showed only a moderate increase to 10.18% and 0.55%. The sequential MICP-to-MIPP phase conversion process was further evaluated. At an initial Cd concentration of 50 mg/L, HAP formation was negligible at 30 °C and only partial (35.98% conversion) at 50 °C. The corresponding Cd removal efficiencies remained unchanged and increased by 1.1%, respectively. Overall, this integrated strategy provides a condition-dependent remediation approach, where the stability enhancement is critically dependent on the extent of HAP conversion.