In the past few decades, solidification/stabilization (S/S) technology has been put forward for the purpose of improving soil strength and inhibiting contaminant migration in the remediation of heavy metal-contaminated sites. Cement, lime, and fly ash are among the most common and effective binders to treat contaminated soils. During S/S processing, the main interactions that are responsible for improving the soil’s behaviors can be summarized as gelification, self-hardening, and aggregation. Currently, precipitation, incorporation, and substitution have been commonly accepted as the predominant immobilization mechanisms for heavy metal ions and have been directly verified by some micro-testing techniques. While replacement of Ca²⁺/Si⁴⁺ in the cementitious products and physical encapsulation remain controversial, which is proposed dependent on the indirect results. Lead and zinc can retard both the initial and final setting times of cement hydration, while chromium can accelerate the initial cement hydration. Though cadmium can shorten the initial setting time, further cement hydration will be inhibited. While for mercury, the interference impact is closely associated with its adapted anion. It should be pointed out that obtaining a better understanding of the remediation mechanism involved in S/S processing will contribute to facilitating technical improvement, further extension, and application.

Microbial induced carbonate precipitation (MICP) is a natural bio-mediated process that has been investigated for soil stabilization and heavy metal immobilization in soil and groundwater. This study analyzed the effect and mechanism of MICP for the solidification/stabilization of tailings sand with multi-heavy metals. When the concentration of cementation solution (CS) is 1.0 mol/L and the optical density(OD₆₀₀) is 1.6, the unconfined compressive strength of tailings sand treated by MICP is the largest, and the solidification efficiency of heavy metals in tailings sand is also the highest. The macroscopic and microscopic observations reveal that the mechanism of MICP solidification of tailings is bacterial outer oxide, hydroxide, alkaline carbonate, and carbonate precipitation.

Microbial-induced carbonate precipitation (MICP) is an effective technology for repairing sites contaminated by heavy metals. In this work, Sporosarcina pasteurii was cultured and mixed with a cementing fluid as a binder to remediate Pb-contaminated silt. The effects of varying experimental parameters, including Pb concentration and dry density, were also tested and analyzed. The leaching and strength characteristics and the MICP improvement mechanism of the Pb-contaminated silt were studied. Samples with dry densities of 1.50 g/cm³ and 1.55 g/cm³ exhibited the highest unconfined compression strengths (UCS). Scanning electron microscopy showed that not all CaCO₃ crystals produced a cementation effect, with some filling pores in an invalid cementation form. The results showed that MICP remediation of low Pb²⁺ concentration-contaminated silt could meet the relevant Chinese environmental safety standards. Low Pb concentrations helped improve MICP-treated, Pb-contaminated silt strength, whereas high Pb concentrations significantly reduced this strength. Testing to determine the tolerance of an active microbe, Sporosarcina pasteurii, showed that trace amounts of Pb promoted its growth, thus improving the MICP effect, whereas excessive Pb had a toxic effect, which reduced MICP effectiveness. Mercury injection experiments showed that MICP produced CaCO₃; this mainly filled soil mesopores and macropores and, thus, improved the soil UCS. Scanning electron microscopy showed that not all CaCO₃ crystals produced a cementation effect, with some filling pores in an invalid cementation form. MICP was innovatively applied to silt sites with heavy metal pollutants while considering the soil compaction in actual construction, thus broadening the application scope of MICP, optimizing the construction process, and reducing the construction cost.