Preliminary evaluation of Bacillus licheniformis-mediated carbonate precipitation for lead and cadmium immobilization in landfill leachate

Microbially induced carbonate precipitation (MICP) is a promising bioremediation strategy for heavy-metal immobilization; however, its performance in chemically complex landfill leachate remains insufficiently understood. This study provides a preliminary evaluation of Bacillus licheniformis-mediated MICP for simultaneous Pb²⁺ and Cd²⁺ immobilization in synthetic and real landfill leachate. The initial dissolved metal concentrations measured in the prepared leachate systems were 0.23–0.32 mg/L for Pb²⁺ and 0.062–0.080 mg/L for Cd²⁺. During treatment, pH increased to 7.3–8.0, indicating ureolysis-driven alkalinization favorable for carbonate precipitation. ORP decreased to 38–51 mV, reflecting a shift from more oxidative initial conditions toward a less oxidative environment, rather than truly reductive conditions. COD removal reached 31–42%, suggesting partial organic load reduction alongside biomineralization. Based on dissolved metal reductions before and after treatment, Cd²⁺ removal exceeded 80% and reached > 98%, whereas Pb²⁺ removal ranged from 32% to 62%. SEM–EDX, FTIR, and XRD analyses confirmed carbonate-mediated immobilization. The precipitates were dominated by aragonite-type CaCO₃, which provided a carbonate mineral matrix for Pb and Cd association, while the detection of cerussite (PbCO₃) and otavite (CdCO₃) indicated discrete metal carbonate formation as an additional immobilization pathway. These findings suggest that B. licheniformis-driven MICP can improve leachate chemistry and promote Pb²⁺ and Cd²⁺ immobilization under chemically complex leachate conditions. However, the results should be interpreted as preliminary evidence of short-term immobilization rather than definitive proof of long-term field-scale stability.