Nitrogen and Calcium Addition Promoted Cadmium Immobilization in Soil by Urease-Producing Bacteria

Objective Microbial induced carbonate precipitation (MICP) is a bioremediation technology that utilizes the metabolic activities of urease-producing bacteria to immobilize metal ions, showing strong potential for application in the remediation of heavy metal-contaminated soil. Urea and calcium are key substrates influencing the metal ion immobilization efficiency in this process. However, the impact of their dosage on the MICP process remains unclear.

Method This study conducted solution and soil incubation experiments to explore Cd immobilization efficiency under different urea (0 ~ 3.072%, w/w) and calcium chloride (CaCl₂, 0 ~ 3.072%, w/w) dosages, using control (CK) and bacteria-only addition (J) as controls.

Results Overall, the addition of urea and calcium significantly improved the removal rate of Cd, which was less affected by the dosage of urea, but more affected by the dosage of calcium. Adding 0.048% ~ 0.192% CaCl₂ had the highest removal rate. When only urea was added, otavite (CdCO₃, PDF#99-000-2764) formed as the primary phase for immobilizing Cd²⁺. In contrast, adding calcium stimulated urease-producing bacterial activity, enhancing the MICP process. This resulted in the precipitation of both vaterite (CaCO₃, PDF#99-000-4172) and otavite (CdCO₃, PDF#99-000-2764) to fix Cd²⁺. Exogenous addition of urea and calcium chloride both affect nitrogen transformation and reduce the availability of soil phosphorus.

Conclusion The results demonstrate that applying nitrogen and calcium can effectively regulate urease-producing bacterial activity, enhancing Cd immobilization and stimulating the soil nitrogen cycle. These findings offer novel approaches for the effective immobilization of Cd in agricultural soils.