Mechanisms of cadmium retention on enzyme-induced carbonate precipitation (EICP) of Ca/Mg: Nucleation, chemisorption, and co-precipitation

As a new engineering technique, enzyme induced calcite precipitation (EICP) has demonstrated significant ecological benefits for bio-cementation and bioremediation, with urease serving as a key contributor to carbonate mineralization. This study investigated the enormous potential of urease-aided carbonate precipitation on the retention of free cadmium ions, also dig into its underlying mechanisms. Changes in pH and electrical conductivity (EC), as well as urease activity, were measured over six days. The carbonate precipitates generated during EICP process (with and without cadmium added) were analyzed by ATR-FTIR and XRD tests to determine the molecular composition and mineral types under different Ca and Mg ratios. SEM-EDS investigated the differences in carbonate morphology and internal mineral formation. The results showed that urease existed in the solution stably, however its activity decreased slightly with time. It was found that carbonate types and morphology mostly depend on calcium and magnesium ratios within pure EICP system, but with rare correlation in the presence of cadmium added solutions. The Cd2+ ions were sufficiently immobilized as carbonate precipitates and identified as pure otavite (CdCO3), calcite co-precipitation (CaCO3-Cd), vaterite/aragonite chemisorption (CaMg(CO3)2). This suggested the role of urease-based EICP for a better means for metal bio-recovery and sequestration from the polluted environment.