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.
Gases within soil voids are known to increase pore fluid compressibility, hinder pore pressure development, and enhance cyclic resistance of soils. Soil microorganisms within void spaces produce CO2, extracellular polymeric substances (EPS), minerals such as CaCO3, and other metabolic products depending on specifics of metabolic processes. Bacteriogenic CO2 can therefore influence pore fluid compressibility, pore pressure development, and deformation response of soils during rapid loading. However, this issue remains relatively unexplored. Amounts of free and dissolved CO2 within soil voids are expected to depend on the types of soil microorganisms, their metabolic processes, and solubility in pore water. Influence of CO2 produced by two species of aerobic soil bacteria on pore pressure potential of sand samples was experimentally investigated in this study. Measurements were within about 10% of analytically derived relationships between CO2 content and pore water pressure. Pore pressure was also found to relate closely to volume of free CO2 and bacterial population size. Data obtained in this study further indicate that cyclic resistance of sand containing live bacterial populations is correlated with Skempton's pore pressure parameter, B, and amounts of EPS and CaCO3 through a relationship independent of bacterial species or metabolic pathway.
