Using Eggshell and Seashell as Calcium Sources in Biogenic Calcite Precipitation for the Removal and Immobilization of Zinc

This study investigates the potential of various calcium (Ca) sources in ureolysis-based MICP technology for the remediation of Zn-contaminated soils. Eggshell (ES) and seashell (SS) waste were used as Ca sources to evaluate Zn removal in aqueous solutions (0, 1, 2, and 3 mM Zn), and Zn immobilization in contaminated soil (75, 150, 200, and 250 mg Zn kg−1). Zn concentrations in water, and its fractions in soil (soluble-exchangeable [Zns-ex] and carbonate [Zncar]) were measured. The Ca sources significantly influenced Zn removal in solution, with the highest removal rate (84.4%) observed in the SS treatment at 2 mM Zn. The morphology and elemental composition of the precipitates varied significantly with the Ca sources. XRD analysis indicated zinc carbonate, calcium zinc carbonate, and calcium zinc carbonate hydrate as the predominant minerals in SS, ES, and CaCl2 treatments, respectively. Additionally, the SS treatment demonstrated the most substantial reduction in Zns-ex concentration (67.3%) and the highest increase in the soil Zncar fraction (2.6 times) at a Zn concentration of 200 mg kg−1. In conclusion, calcium derived from seashell waste was found to be effective as an alternative to CaCl2 and could be a cost-effective calcium source in the MICP process for zinc immobilization.