Heavy metal retention properties of kaolin and bentonite in a wide range of concentration and different pH conditions

• Comprehensive characterization of heavy metal retention on two reference clays
• Emphasizing on the extreme concentration and different pH conditions
• Acidification can result in a significant decrease in heavy metal retention.
• Different heavy metal retention isotherms for the bentonite and the kaolinite

Accumulation of heavy metals can occur on clay barriers or slurry-trench cutoff wall, resulting in their high concentrations at which the retention reaction takes place. Or high concentration conditions are often adopted to accelerate the reaction process at a laboratory scale for investigating their impacts on engineering behaviors (hydraulic or mechanical) of clays. So far, there is lack of data regarding heavy metal retention behavior on clays under those extreme concentration conditions. This paper aims to mitigate this gap with emphasis on pH effects. To do so, several series of sorption batch tests were conducted to characterize the retention capacity of three ubiquitous heavy metals (Pb, Zn and Cu) on two reference clays (Speswhite kaolin and Wyoming bentonite). A wide range of individual metal concentration varying from 0.1 to 100 mM and different pH conditions were tested. Results suggest that the acidifying the metal–clay system can result in a significant decrease in the heavy metal retention, which can be attributed to an increase in competition of H+ with metal ions to the surface sites and a decrease in metal ion precipitation as hydroxides at high pH levels. In addition, the heavy metal retention isotherm over a wide range of concentrations for the bentonite follows Temkin and Langmuir adsorption models while Freundlich and Langmuir models for the kaolin. Interestingly, at a high concentration of 10 mM, the Pb precipitation unlikely occurs in the presence of both clays tested over a wide range pH level (< 10), whereas it likely takes place in case of Cu and Zn.