Cement-soda residue (CSR) has been proven to be an effective binder for treating heavy metal-contaminated soils, and the durability is its most important characteristic. In this study, the effects of acid rain (AR) on the leaching behavior of CSR-solidified/stabilized, zinc-contaminated soils were investigated using flexible-wall soil column leaching tests. After leaching, some parameters were determined such as the unconfined compressive strength (UCS) and permeability coefficient of the samples, the concentrations of Zn²⁺ and Ca²⁺ in the filtrate. The test results showed that after AR leaching, the UCS of the solidified soil samples decreased and the permeability coefficient increased, while the zinc concentration in the filtrate always met the third grade of the applicable standard, the Chinese National Environmental Quality Standards (<1 mg⋅L^-1). To reveal the binding mechanism, scanning electron microscopy (SEM) and mercury intrusion testing (MIP) were used to observe the microscopic characteristics of the soil samples. At the micro scale, the MIP and SEM results confirmed that the hydration products in the soil samples-hydrated calcium silicate, calcium hydroxide, and calcium zincate hydrate-partially dissolved during AR leaching, resulting in the loss of their internal structure. Consequently, the high alkalinity of the soda residue contributed to H⁺ neutralization in the AR leaching agent, indicating that soda residue can not only solidify heavy metal zinc ions effectively but can also buffer the erosive effect of AR on soil.

Tailing sand contains a large number of heavy metals and sulfides that are prone to forming acid mine drainage (AMD), which pollutes the surrounding surface environment and groundwater resources and damages the ecological environment. Microbially induced calcium carbonate precipitation (MICP) technology can biocement heavy metals and sulfides in tailing sand and prevent pollution via source control. In this study, through an unconfined compressive strength test, permeability test, and toxic leaching test (TCLP), the curing effect of MICP was investigated in the laboratory and the effect of grouting rounds on curing was also analyzed. In addition, the curing mechanism of MICP was studied by means of Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray diffraction spectroscopy (XRD), and scanning electron microscopy (SEM). The experimental results showed that MICP could induce calcium carbonate precipitation through relatively complex biochemical and physicochemical reactions to achieve the immobilization of heavy metals and sulfides and significantly reduce the impact of tailing sand on the surrounding environment.

Red-bed argillaceous siltstone is one of main types of rocks composing unstable slopes on both banks of water diversion project from the Yangtze to Huaihe River and is prone to deterioration after subjected to drying-wetting cycle conditions during water diversion project operation. The study of the damage degradation and the constitutive relationship of rocks subjected to drying-wetting cycle process is therefore of critical scientific importance. In this paper, laboratory tests were conducted to investigate the effect of drying-wetting cycles on the mechanical properties and microstructural characteristics. The experimental results show that the microstructure of the sample surface is no longer dense and uniform and the porosity of tested samples significantly increases with drying-wetting cycles increasing. After subjected to cycles of drying-wetting, the compaction stage of the stress-strain curve becomes longer with more pronounced nonlinear features, while uniaxial compression strength (UCS) and elastic modulus drop obviously. Combined with the evolution characteristics of microstructure found as the porosity grew, UCS and elastic modulus gradually decline. Additionally, from the perspective of energy analysis, the input energy density and dissipated energy density gradually extended with the increasing strain, and the elastic energy density increased first and then declined. As the number of drying-wetting cycles increases, the energy density of samples all present linear fell. Based on the evolution of dissipated energy, a new damage model of red-bed argillaceous siltstone subjected to drying-wetting cycles was established by introducing compaction coefficient. The coefficients of determination (⁠ $R2$⁠) of the constitutive model are always greater than 0.85, indicating that the proposed model is generally in good agreement with the experimental data under different drying-wetting cycles. This paper has specific research and reference value to understand the damage evolution characteristics of red-bed argillaceous siltstone under cyclic drying-wetting.

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.