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

Heloisa Dickinson, John MacDonald, Jaime Toney

Abstract

Enzyme-induced carbonate precipitation (EICP) has emerged as a versatile approach for soil improvement and contaminant immobilisation, yet performance is commonly evaluated from bulk metal removal efficiency while the mineralogical fate of retained metals remains poorly understood. Here, Pb, Co and Cr are compared under identical urease-driven EICP conditions using a crude soybean-derived urease extract, with metal retention pathways resolved through SEM–EDS, powder X-ray diffraction with Rietveld refinement, FTIR spectroscopy and chemical-field imaging/μXRF mapping. Across all treatments (2–20 mM), Pb and Co amended systems maintained high alkalinity (final pH ≈ 8.1–8.2) and high removal efficiencies (>99% for Pb; 97–99% for Co) but exhibited contrasting mineralogical outcomes. Pb showed a concentration-dependent transition from calcite-dominated assemblages to progressive stabilisation of a discrete Pb-carbonate population: cerussite was absent at low Pb, emerged at intermediate loading (~3 wt%) and increased at high loading (~6 wt%), coexisting with CaCO₃ domains bearing low Pb association and intra-particle chemical zoning. In contrast, Co remained associated with calcite-dominated products across all concentrations, with no evidence for a discrete cobalt carbonate phase; Co occurs at low wt% levels distributed diffusely within CaCO₃-rich domains. Chromium displayed the strongest divergence from carbonate-dominated behaviour. Increasing Cr loading reduced ureolysis, precipitate mass (to ~0.5 g at 20 mM) and removal efficiency (to ~87%), and was accompanied by an abrupt transition to abundant elongated prismatic morphologies. SEM–EDS identifies chemically discrete Cr-rich, Ca-poor domains together with Ca–S-rich precipitates exhibiting gypsum-like habits, indicating a shift from carbonate-controlled mineralisation towards sulphate-associated retention pathways at high loading. Together, these results demonstrate that under otherwise identical crude-enzyme EICP conditions, metal identity redirects carbonate biomineralization into distinct crystallogenetic pathways, with direct implications for predicting immobilisation mechanisms and remediation design.