Feasibility of Microbially Induced Carbonate Precipitation to Enhance the Internal Stability of Loess under Zn-Contaminated Seepage Conditions

Abstract

Loess is widely distributed in Northwestern China and serves as the preferred engineering construction material for anti-fouling barriers. Heavy metal contamination in soil presents significant challenges to the engineering safety of vulnerable loess structures. Hence, there is an urgent need to investigate the impact of heavy metal ions on their percolation performance. In order to investigate the effectiveness of microbially induced carbonate precipitation (MICP) using Sporosarcina pasturii (CGMCC1.3687) bacteria in reducing internal seepage erosion, a saturated permeability test was conducted on reshaped loess under constant water head saturation conditions. The response of loess to deionized water (DW) and ZnCl2 solution seepages was analyzed by monitoring changes in cation concentration over time, measuring Zeta potential, and using scanning electron microscopy (SEM). The results indicate that the hydrolysis of Zn2+ creates an acidic environment, leading to the dissolution of carbonate minerals in the loess, which enhances its permeability. The adsorption of Zn2+ ions and the resulting diffusion double-layer (DDL) effect reduce the thickness of the diffusion layer and increase the number of free water channels. Additionally, the permeability of loess exposed to ZnCl2 solution seepage significantly increased by 554.5% compared to loess exposed to deionized water (DW) seepage. Following the seepage of ZnCl2 solutions, changes in micropore area ratio were observed, decreasing by 48.80%, while mesopore areas increased by 23.9%. MICP treatment helps reduce erosion and volume shrinkage in contaminated loess. Carbonate precipitation enhances the erosion resistance of contaminated loess by absorbing or coating fine particles and creating bridging connections with coarse particles. These research results offer new perspectives on enhancing the seepage properties of saturated loess in the presence of heavy metal erosion and the geochemical mechanisms involved.

Feasibility Study of Applying Enzyme-Induced Carbonate Precipitation (EICP) without Calcium Source for Remediation of Lead-Contaminated Loess

“…The Guanzhong region, situated in the middle reaches of the Yellow River, is recognized as the largest loess area globally, covering approximately 6.4 × 10 5 km 2 . Soil samples utilized in this research were extracted from the loess in Tongchuan City, Shaanxi Province [20]. Disturbed loess samples were gathered from depths of around 0.5-1 m below the surface and transported to the laboratory for analysis.…”

“…Soil particles suspended in aqueous solutions can attract ions with opposite charges, forming strong connections near the particles, which is known as the Stern layer [20,32]. Ions further away create weaker connections, forming the diffuse layer.…”

“…To this end, a Zeiss Gemini sigma 300 scanning electron microscope (Oberkochen, Germany) was applied to characterize particle morphology and inter-particle connection between soil particles. The methodology for characterizing soil microstructures in our research was adapted from previous studies by Xu et al [21], He et al [20], and Hou et al [23].…”

“…Therefore, when leaching lead-contaminated loess, it is crucial to consider cation exchange and adsorption processes. The intrusion of lead ions disrupts the natural charge equilibrium of clay minerals, leading to the exchange of Na + , K + , Ca 2+ , and Mg 2+ ions adsorbed on clay minerals, resulting in a higher ion concentration in leachate [20]. Heavy metal ions in the exchange phase can also reduce the thickness of the double layer (DDL), induce clay particle flocculation, and increase pore space [34].…”