Abstract: A series of modified fluid loss tests was carried out to investigate the hydraulic conductivities (k) of typical sodium treated Ca-bentonite (CaB) and natural Na-bentonite (NaB) contaminated by lead-cadmium (Pb-Cd) and chromium (Cr). The results showed that when the bentonite was exposed to Pb-Cd concentration lower than six mmol/L, its permeability coefficient exhibited a smaller increase by approximately 2 to 3 times, while it increased significantly when Pb-Cd concentration increased to 10 mmol/L. Under the action of potassium chromate, the k of Cr-contaminated sodium-treated CaB sample was slightly lower than that of uncontaminated samples, whereas the k of Cr-contaminated NaB sample was approximately 2 to 8 times higher than that of uncontaminated samples. The results can be attributed to the different forms of chromium existing in bentonite slurry under soil-liquid interaction in different pH-Eh conditions. Since chromium exists in the anionic complex as Cr(VI) in sodium treated CaB slurry, the negatively charged bentonite particles increase, which enhances the dispersed state of the bentonite. In contrast, Cr(III) cation is identified in NaB slurry, which leads to the aggregation of bentonite particles due to the diffuse double layer contraction. A comprehensive analysis was conducted on the variation of chemical compatibility of bentonites under ionic strength obtained from this study associated with those reported in the literature. It is found that a threshold of ionic strength exists, k increases more than an order of magnitude beyond the threshold.

Key words: bentonite, hydraulic conductivity, heavy metal, chemical species