Abstract The sorption characteristics of soils, which are fundamental to contaminant attenuation, depend on many soil-related and contaminant-related factors. Chief among the soil-related factors is soil composition. This study investigates smectite–calcite-contaminant and kaolinite–calcite-contaminant interactions for different heavy metals (HM) concentrations in order to address the impact of carbonate and heavy metals on micro-structural variations of clayey soils. To achieve the stated objectives, several batch equilibrium, consolidation and XRD (X-ray diffraction) analyses were performed. These experiments were conducted on a bentonite (smectite) sample, a kaolinite sample and several mixtures of kaolinite and carbonate. Furthermore, this study investigates the effect of HM cations in a smectite clay, with and without the benefit of sodium carbonate and sodium nitrate, with respect to the mixture's consolidation behaviour and transmission characteristics. Characterization experiments conducted in the study included determination of changes in the buffering capacity and microstructure characteristics of the clay (smectite and kaolinite) relative to the presence of varying concentrations and types of pore fluids and physical experiments focused on the consolidation behaviour of the smectite samples. The role of buffering capacity and macrostructure and microstructure changes on the transient (evolutionary) characteristics and properties of the clays can be seen from the results obtained. Double-layer swelling forces and buffering capacity are seen to be dominant factors in the performance of the clays — apparently through their influence on formation and restructuring of microstructures in the overall clay–carbonate mixtures. Exposure of the buffered smectite to Cu²⁺ ions in the midst of consolidation results in the added feature of chemico-osmotic consolidation — depending on the concentration of Cu²⁺ ions available, thus raising the question of chemical path dependency or chemical history on consolidation performance. Furthermore, the process of interaction of the kaolinite–carbonate and HM ions was monitored through XRD analyses of the soil samples at various stages of the tests. The results reported in this paper show that the consequence of a tighter aggregation of clay particles after soil–contaminant interaction causes the development or restructuring of microstructures into tighter micro-structural units. This produces a macrostructure with more macro-pores than the original natural clay sample which causes a reduction in XRD peak intensity of laboratory contaminated soil samples. It can be seen that once the quantity of laboratory added carbonate is increased, the addition of HM to soil sample has a minimal impact on the micro-structural change of clayey soils.

Research Highlights ► This study investigates clay-calcite-heavy metals (HM) interactions. ► It addresses the impact of carbonate and HM on micro-structural variations of clayey soils. ► The consequence of stiffer consolidation characteristics and a tighter aggregation of clay particles after soil-contaminant interaction causes the development or restructuring of microstructures into tighter micro-structural units. ► The enhanced buffering capability of the clay samples due to the laboratory addition of carbonate allowed them to resist settlement and sedimentation when exposed to low concentrations of HM.