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

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 (Ca1-xMgxCO3) 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.