• Distinct remediation mechanisms between fermentation broths and bacterial suspensions were clarified. • A “dual-enzyme pulse–precipitation feedback” mechanism was revealed. • Urease and carbonic anhydrase exhibit synergy by regulating CO₂ and HCO₃⁻. • Microbes remediate heavy metals via carbonate skeleton-Mn composites pathway. Microbial remediation for heavy metal contamination of soil is a sustainable remediation technology. This study elucidates the microbial regulatory mechanisms of Bacillus thuringiensis and/or Acinetobacter calcoaceticus in Cd-Pb-contaminated soil. The 45-day incubation study demonstrated that co-treatment of Bacillus thuringiensis and Acinetobacter calcoaceticus achieved rapid Cd immobilization, with the high-dose Bacillus thuringiensis group immobilizing >80% of bioavailable Cd by day 30. Elevated soil pH showed a significant negative correlation with reduced metal bioavailability. Spearman correlation coefficients between Cd/Pb bioavailability and soil pH ranged from -0.505 to -0.605 (p < 0.01), highlighting the key role of alkalinization. CO₂ fluxes peaked early in the incubation period and closely tracked temporal fluctuations in urease activity. Short-term mechanistic experiments confirmed that urease and carbonic anhydrase synergistically drove carbonate precipitation, inducing heavy metal binding to form stable mineral phases. XRD and SEM-EDS analyses revealed the in situ formation of crystalline phases and spatial co-localization of Ca, Pb, and Cd within precipitation zones. Based on these findings, we propose a three-stage synergistic mechanism including enzymatic pH elevation, carbonate matrix induction and formation of heavy metal-manganese composite phases, which provides theoretical support and a practical basis for microbially driven in situ heavy metal passivation.
Wan et al. (Sun,) studied this question.