Removal of high concentrations of zinc, cadmium, and nickel heavy metals by Bacillus and Comamonas through microbially induced carbonate precipitation

Heavy metal pollution in urban freshwater, driven by anthropogenic activities, poses significant risks to aquatic ecosystems and human health due to its toxicity and persistence. Recently, urease-producing bacteria have gained attention for their ability to remove heavy metals through microbial-induced carbonate precipitation (MICP). In this study, eight urease-producing bacteria were exposed to individual solutions of zinc (Zn²⁺), cadmium (Cd²⁺), and nickel (Ni²⁺) at concentrations ranging from 0 to 6 mM to assess their resistance. Three strains-Bacillus subtilis HMZC1 (B2), Bacillus sp. HMZCSW (B6), and Comamonas sp. HMZC (B11)-survived at 4 mM and 6 mM, while most others could not tolerate 4 mM. Their urea-degrading ability was tested at different pH levels, identifying an optimal pH of 7 for MICP. Heavy metal carbonate precipitation experiments at 4 mM and 6 mM revealed that all three strains achieved > 93% removal of Zn²⁺, Ni²⁺, and Cd²⁺ within 72 h. Comamonas sp. HMZC exhibited the highest efficiency, achieving > 95% removal of certain heavy metals at 6 mM. Statistical analysis using one-way ANOVA revealed significant differences (p < 0.05) in heavy metal removal efficiencies among the strains for certain treatment conditions (Cd²⁺ and Zn²⁺ at 4 mM), although not all comparisons reached statistical significance. Scanning Electron Microscopy and X-ray Diffraction confirmed the morphology and composition of the precipitated heavy metal carbonates. Our findings demonstrate that urease-producing bacteria can effectively immobilize multiple heavy metals, highlighting the MICP process as a practical and sustainable biological approach for ecological restoration and wastewater treatment.