Mg/Ca ratio-regulated phase differentiation of microbially derived CaCO₃ and its effects on Cu²⁺, Cd²⁺, and Pb²⁺...

Efficient stabilization of heavy-metal co-contaminated wastewater remains limited by poor material selectivity, secondary sludge, and unclear immobilization mechanisms. In this study, Bacillus licheniformis ys isolated from groundwater was used to construct mineralization systems with Mg/Ca molar ratios of 1, 3, 6, and 12, producing microbially derived CaCO₃ with distinct polymorphs for Cu²⁺, Cd²⁺, and Pb²⁺ immobilization. With increasing Mg/Ca ratio, the mineral phase shifted from magnesian calcite (Ca_1-xMg_xCO₃) to monohydrocalcite (CaCO₃·H₂O) and then to an aragonite (CaCO₃)-monohydrocalcite mixed phase. In single-metal systems, the mixed phase favored Cu²⁺ removal, while monohydrocalcite showed relatively stable high-capacity immobilization of Cd²⁺ and Pb²⁺. In ternary systems, the priority was Pb²⁺>Cu²⁺>Cd²⁺. Monohydrocalcite showed the highest Pb²⁺ removal, whereas the aragonite-monohydrocalcite mixed phase was more favorable for competitive Cu²⁺ immobilization; in contrast, Cd²⁺ immobilization was strongly hindered by ionic competition and mainly occurred as surface-bound, poorly crystalline, or X-ray amorphous species rather than distinct crystalline Cd-bearing phases. Overall, Pb²⁺ showed the highest removal efficiency and fastest reaction rate, whereas Cu²⁺ and Cd²⁺ were more polymorph-sensitive. Carbonate polymorphs controlled both capacity and immobilization pathways, including surface precipitation, localized new-phase formation, poorly crystalline sequestration, and phase transformation. The kinetic data were best described by the pseudo-second-order model, suggesting that surface chemical reactions were important during immobilization. This study clarifies how Mg/Ca-regulated carbonate polymorphism differentially controls heavy-metal immobilization, providing a basis for designing microbially derived CaCO₃ materials for multi-metal remediation and understanding microbial mineralization functions under different Mg/Ca conditions.