Performance and Mechanism of Zn-Contaminated Soil through Microbe-Induced Calcium Carbonate Precipitation

Abstract

Zn is a toxic heavy metal that seriously endangers human health and ecological stability. For a long time, traditional remediation techniques have been used to remediate Zn-contaminated soil prone to other problems such as secondary contamination. In recent years, due to the great danger posed by Zn pollution, there has been an increasing interest in applying eco-friendly and sustainable methods to remediate Zn-contaminated soil. Therefore, in this study, microbially induced calcium carbonate precipitation (MICP) technology was used to bioremediate zinc ions by transforming ionic heavy metals into insoluble solid-phase minerals. Through the unconfined compressive strength (UCS) test, direct shear (DS) test, and penetration test (PT), the results showed that the unconfined compressive strength of the treated specimens increased by 187.2~550.5%, the cohesion increased significantly compared with the internal friction angle of specimens, and the permeability coefficient can be reduced by at least one order of magnitude. During the treatment of Zn pollutants, the mobility of heavy metal zinc ions was significantly reduced, the percentage of exchangeable state Zn content was significantly reduced, and the leaching concentration of zinc ions in Zn-contaminated soil was reduced to about 20 mg/L, which was significantly lower than the limit in the standard (100 mg/L). These results were further confirmed by scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses, which indicated coprecipitation of calcium carbonate (CaCO3) and ZnCO3. The microbial solidification/stabilization of Zn-contaminated soil was most effective when the curing age of 28 d, the cementation solution concentration of 1 mol/L, and the cementation solution ratio of 1:2. Therefore, the bio-immobilization of zinc ions by MICP has the potential for application as a low-cost and eco-friendly method for heavy metal remediation.