Abstract:

[Objective] In practical remediation projects, leaching heavy metal-contaminated soils with high clay content is highly challenging. This is not only due to the low permeability of high-clay-content soils, but also because of the unclear influencing patterns and mechanisms of clay content on soil leaching. Current studies on the effects of clay content primarily focus on mixtures of sand and fine-grained soils, lacking systematic investigations into clay soils. Moreover, previous studies investigate the physicomechanical properties of soils with different clay contents, without comparing their effects on heavy metal adsorption and desorption. Studies on the correlation between heavy metal and clay content rely on field sampling methods. However, due to varying soil samples and contamination types, along with complex influencing factors, the conclusions are inconsistent. Therefore, this study intends to artificially prepare soil samples with different clay contents to eliminate complex influencing factors, investigate differences in their leaching performance and geotechnical properties, and analyze their correlations. In doing so, the single-factor influencing pattern of clay content can be obtained. [Methods] Based on the principle that soil particles of different sizes had different settling speeds in solution, soil samples with clay contents of 20%, 30%, 40%, and 50% were prepared via sedimentation method. First, the basic physical properties of soil samples were analyzed, followed by consolidation tests to investigate the porosity ratios, compression characteristics, permeability, and consolidation behaviors of soils with different clay contents, as well as their microscopic characteristics under scanning electron microscope (SEM). Through batch oscillation-centrifugation experiments, the adsorption of heavy metals (Cu and Zn) by the soil samples and their desorption characteristics during heavy metal removal using citric acid were studied, with analysis incorporating pH values and particle size variations. [Results] Soils with different clay contents exhibited significant differences in physicomechanical properties. Soils with a high clay content had larger specific surface areas, lower relative densities, more muscovite and chlorite minerals, and fewer quartz and albite minerals. Additionally, these soils had higher liquid-plastic limits and greater compressibility, while showing lower permeability and porosity ratios, with more soil particle aggregates observed microscopically. The increase in clay content significantly enhanced the soil’s adsorption capacity for heavy metal ions, while deteriorating the desorption performance of citric acid during leaching. As the clay content increased from 20% to 50%, the maximum heavy metal adsorption increased by up to 50%, whereas the maximum desorption rate decreased by up to 20%. Notably, pronounced differences were observed between 20% and 30% clay contents, mainly attributed to the formation of numerous aggregates during this stage, which enhanced the adsorption performance of the soil for heavy metals. In addition, the correlations of clay content with Zn and Cu differed. Zn was more difficult to remove than Cu in high-clay-content contaminated soils. [Conclusions] The experimental findings demonstrate that the differences in various physicomechanical and adsorption-desorption characteristics caused by different clay contents significantly influence the selection of technologies and parameters for soil leaching. Therefore, practical remediation projects must integrate relevant research and experimental analyses to fully consider the effects of clay content, so as to better achieve the goals of contaminated soil remediation.

Key words: heavy metal contamination, soil leaching, clay content, sedimentation method, consolidation test, adsorption-desorption