Influence of the presence of heavy metals on the behaviour of bentonites

Due to its high swelling capacity, contaminant adsorption capacity, and low value of hydraulic conductivity, compacted bentonite is used as a liner material at the waste disposal site. However, the presence of various heavy metal pollutants in the leachate could affect the swelling and sorption capacity of bentonite and in turn reduce its usefulness as a liner material. Hence, to design a clay liner it is quite essential to study the behaviour of bentonites in the presence of the different chemicals present in the leachate. In this study, the effects of heavy metals of varying concentrations were experimentally investigated on the behaviour of two bentonites with different mineralogical compositions. The bentonites were evaluated for their free swelling, Atterberg limits, swelling potential, swelling pressure, and hydraulic conductivity in the presence of different concentrations of Zn²⁺, Pb²⁺, and Cu²⁺ metal ions. The results showed that with the increase in the heavy metal concentration the liquid limit, free swelling, swelling pressure, and swelling potential of the bentonite decreased and the hydraulic conductivity increased. These reductions are attributed to the adsorption of heavy metals to the bentonite and decrease in the thickness of the double layer. The effects due to metal ion concentration were found to be more pronounced for the bentonite of higher quality which is marked by a higher swelling capacity, liquid limit, cation exchange capacity, exchangeable sodium percentage, and specific surface area.