Beyond calcite: Crude-urease EICP reveals metal-specific crystallogenetic pathways

Heloisa Dickinson, John MacDonald, Jaime Toney

During carbonate biomineralization, metal identity governs phase selection and metal partitioning, directing contaminants into fundamentally different crystallogenetic pathways even where bulk removal efficiencies remain high. However, the mechanisms controlling this divergence remain incompletely understood. Here, we compare the mineralogical fate of Pb, Co and Cr under standardized urease-driven enzyme-induced carbonate precipitation (EICP) using a crude soybean-derived urease extract across a metal concentration range of 2–20 mM. Metal retention pathways were investigated using SEM–EDS, powder X-ray diffraction with Rietveld refinement, and synchrotron Nano-XRF mapping. Despite identical precipitation conditions, the three metals followed markedly different mineralogical trajectories. Pb and Co systems maintained high alkalinity (final pH ≈ 8.1–8.2) and high removal efficiencies (>99% for Pb and 97–98% for Co), yet their mineralogical evolution differed fundamentally. Pb exhibited a concentration-dependent evolution characterized by the progressive stabilization of discrete cerussite populations within otherwise calcite-dominated assemblages. Cerussite was absent at 2 mM, accounted for ~3 wt% at intermediate loadings, and reached ~6 wt% at 20 mM, coexisting with CaCO₃ domains exhibiting progressive chemical zonation and variable Pb incorporation. In contrast, Co remained diffusely associated with calcite-dominated precipitates across all concentrations, showing no evidence of chemical zonation or discrete cobalt carbonate phases. Cr exhibited consistently lower final pH (7.5–7.8) and the greatest departure from carbonate-controlled mineralization. Progressive inhibition of ureolysis with increasing Cr loading reduced carbonate precipitation and lowered removal efficiency from 97% at T1 to 87% at T3. At the highest loading, Cr-rich, Ca-poor domains developed alongside elongated prismatic crystals and Ca–S-rich precipitates with gypsum-like morphologies, collectively indicating a shift away from carbonate-dominated precipitation pathways. Together, these findings establish metal identity as a first-order control on carbonate biomineralization, governing phase selection and metal partitioning independently of bulk removal efficiency. Despite these divergent mineralogical outcomes, sunflower-like radial aggregates recurred across all systems, except Cr-T3, suggesting a shared, plausibly precursor-mediated crystallization framework. Removal efficiency is therefore a poor proxy for mineralogical fate, with important implications for predicting contaminant mobility and the long-term stability of biomineralization-based immobilization strategies.