Application of core-shell structured materials based on biologically active calcium carbonate for heavy metal...

Previous studies have demonstrated that immobilized cadmium carbonate (CdCO₃) materials were prone to decomposition in acidic environments (pH < 5.6), potentially causing secondary pollution. To address this limitation, a novel bio-CaCO₃ material was developed using microbially induced carbonate precipitation (MICP) technology. This material effectively adsorbed cadmium (Cd) onto its surface during the remineralization to form bio-CaCO₃-Cd. Subsequently, the composite material (bio-CaCO₃-CdCO₃) was synthesized by leveraging its unique urease activity. This core-shell structured material not only exhibits high Cd²⁺ capture capability but also maintains stability under acidic conditions, achieving 56.2 %-96.3 % Cd²⁺ immobilization at pH levels ranging from 3.5 to 5.6. Urease activity analyses revealed that bio-CaCO₃-0.1M, synthesized with 0.1 M CaCl₂ and urea, exhibited 23-fold higher urease activity compared to samples prepared under 1.2M conditions. Cd²⁺ tolerance tests further confirmed that bio-CaCO₃-0.1M maintained a removal efficiency of ∼90 % across Cd²⁺ concentrations ranging from 10 to 100 mg/L. Additionally, X-ray diffraction (XRD) and Scanning electron microscope (SEM) analyses validated that bio-CaCO₃-0.1M initially adsorbs Cd²⁺ to form CdCO₃, then utilized urease activity to drive re-biomineralization. Energy dispersive spectroscopy (EDS) tests indicated a reduction of Cd²⁺ contents in the crystal surface from 9.75 % to 0.05 %, following the remineralization. Notably, a comprehensive evaluation through cost-benefit analysis revealed that the bio-CaCO₃ system can generate enormous economic and social benefits, which proved the reasonableness of assumptions and the feasibility of large scale application. This unique core-shell structure significantly enhanced practical applications, and provided a green and sustainable strategy to mitigate secondary pollution associated with the re-leaching of mineralized heavy metals in extreme environments.