Enzyme-mediated multiphase precipitation: An innovative strategy for ecotoxic metal immobilization in aqueous systems

Ecotoxic metal contamination in wastewater and soil poses a critical environmental challenge due to its persistence, toxicity, and bioaccumulation potential. While conventional biogeotechnical methods like Enzyme-Induced Carbonate Precipitation (EICP) and Microbial Induced Carbonate Precipitation (MICP) have shown promise for metal immobilisation, their application is limited by excessive ammonium byproduct generation. In this paper, we present Enzyme-Mediated Multiphase Precipitation (EMMP), a novel approach that significantly reduces ammonium production while utilising waste-derived materials (i.e., soybean crude urease extract, bone meal and urea) to achieve simultaneous precipitation of metal-magnesium ammonium phosphates, carbonates, and calcium phosphates. We evaluated EMMP's effectiveness in removing nine metals (As, Cd, Co, Cr, Cu, Li, Ni, Pb, Zn) at three concentrations (2 mM, 5 mM, 20 mM). The process achieved removal efficiencies exceeding 95% for Pb, Cd, and Zn and 80% for Co, Ni, and Li through combined mechanisms of direct precipitation, co-precipitation, surface adsorption, and biomolecule-mediated interactions. Crystallographic analysis revealed calcite and struvite as primary mineral phases with distinct metal-dependent morphological variations. Relative to control conditions (defined as 100% ammonium production), EMMP reduced ammonium generation to below 5% for Cr, Cu, Pb, and As, to 20-45% for Ni, Co, and Cd, and maintained levels at 78-82% for Li and Zn. By integrating waste-derived materials and minimising ammonium generation, EMMP demonstrates an efficient, sustainable approach for metal remediation in aqueous environments, aligning with circular economy principles.