Study on the effect of pH on MICP curing of cadmium contaminated tailings

The Microbial Induced Calcium Carbonate Precipitation (MICP) method stands as an environmentally sustainable approach for the solidification and remediation of contaminated soils. However, limited research has been dedicated to understanding the impact of pH on MICP treatment of heavy metals. In this study, we conducted MICP remediation experiments involving cadmium (Cd) solution and Cd-contaminated tailings using Sporosarcina pasteurii ATCC 11859 at varying pH levels of 5, 7, 9, and 11. Our findings indicate that in the aqueous solution tests, effective immobilization of Cd2+ occurred across the pH range of 5 to 11. The optimal immobilization of Cd2+ was observed at a pH of 9. In contrast, during the tailings consolidation and remediation tests, the introduction of acidic drenching solutions expedited the leaching of Cd2+ from the tailings. In this context, MICP primarily transformed the acid-soluble fraction of Cd within the tailings into reducible and residual states. Importantly, the effectiveness of this passivation process exhibited a positive correlation with the rate of carbonate generation. The analytical characterization through FTIR, XRD, XPS, and SEM-EDS of biomineralization precipitation offers insights into the mechanisms underlying the fixation of heavy metals by MICP and the solidification of tailings. MICP accomplishes the immobilization of heavy metal ions through a combination of biosorption and calcium carbonate adsorption, ion exchange, and co-precipitation processes. Furthermore, MICP enhances the friction angle of soil particles and promotes cohesion among them through coating, bonding, and bridging effects.