Immobilization of Cd²⁺ in an aqueous environment using a two-step microbial-induced carbonate precipitation method

Previous researches indicate that the potent toxicity of cadmium hinders the efficacy of the microbial-induced carbonate precipitation (MICP) process for bioremediation of Cd²⁺ in aqueous environment. Increasing urea and calcium resource doses, introducing synergists, and utilizing urease-producing consortia can improve bio-immobilization performance of MICP. However, such measures may incur cost increases and/or secondary contamination. This study first verifies the substantial biotoxicity of Cd²⁺ for urease activity and then analyzes the practical limitation of traditional MICP using Bacillus pasteurii for bioremediation of Cd²⁺ in an aqueous environment containing 1-40 mM Cd²⁺ by a series tube tests and numerical simulation. Subsequently, a two-step MICP method, which separates urea hydrolysis and heavy metal precipitation, is introduced in this study to eliminate the inhibitory effect of heavy metal on urease activity. The concentrations of ammonium, Cd²⁺, and pH were monitored over time. The results indicate that the urease expression in B. pasteurii can be significantly inhibited by Cd²⁺ particularly at the concentration ranging from 10 to 40 mM, leading to pretty low efficacy of traditional MICP for bioremediation of Cd²⁺ (Cd²⁺ removal rate as low as 21.55-38.47% when the initial Cd²⁺ concentration = 40 mM). In contrast, when the two-step MICP method is applied, the Cd²⁺ can be almost completely immobilized, even though the concentration ratio of urea to Cd²⁺ is as low as 1.5:1.0, which is close to the theory minimum concentration ratio for the complete precipitation of carbonate to cadmium ions(1.0:1.0). Therefore, the cost-effective, environmentally sustainable, and straightforward two-step MICP method holds great potential for application in the bioremediation of Cd²⁺-contaminated solutions in high concentration.