Positively and negatively charged additives modulate microbial-induced carbonate precipitation (MICP) for lead-contaminated loess remediation

Abstract

The remediation of heavy metal-contaminated loess remains a critical environmental and geotechnical challenge. In this study, microbial-induced carbonate precipitation (MICP) was applied to Pb-contaminated loess with three representative additives: graphene oxide (GO), calcium lignosulfonate (Ca-Ls) and chitosan (CS). Mechanistic evaluation combined zeta potential analysis, scanning electron microscopy (SEM), unconfined compressive strength (UCS) tests and Pb²⁺ leaching experiments under freeze–thaw cycles. Results show that GO enhanced surface charge density, with zeta potential reaching about minus sixteen millivolts, contracted the diffuse double layer, and produced dense carbonate bridges. This treatment yielded the highest UCS, reaching about four hundred and sixty kilopascals initially and maintaining about three hundred and fifty kilopascals after nine freeze–thaw cycles. Pb²⁺ leaching in the GO group remained low, between fifty-five and sixty-eight milligrams per liter, corresponding to a reduction of about sixty-five percent compared with untreated loess. Ca-Ls achieved moderate improvements, retaining UCS at about three hundred and thirty kilopascals and restricting Pb²⁺ leaching to seventy to eighty milligrams per liter after cycling, consistent with uniform carbonate precipitation observed in SEM. In contrast, CS induced more negative potentials, about minus sixteen point two millivolts, but suppressed microbial activity, leading to patchy precipitation and higher leaching levels of ninety to ninety-five milligrams per liter. Collectively, the findings demonstrate that additive regulation of diffuse double layer characteristics and precipitation pathways governs both mechanical durability and heavy metal stabilization. GO provided the most favorable balance between strength and Pb immobilization, followed by Ca-Ls, while CS showed limited benefits. This study provides new insights into additive-assisted MICP as a practical and sustainable strategy for improving the environmental safety and engineering reliability of Pb-contaminated loess under freeze–thaw conditions.