Effect of Acidic Conditions on Surface Properties and Metal Binding Capacity of Clay Minerals

Clays are one of the most abundant minerals on the Earth's surface, and they play a significant role in global trace element cycling because of their ability to adsorb and incorporate a range of metal cations. However, to what extent clay surface reactivities are affected by changes in pH of the surrounding environment remains poorly understood. To address this uncertainty, we investigated three common clay minerals (kaolinite, illite, and montmorillonite) and compared their surface properties and metal binding capacity prior to and after acidic treatment (pH ranges from 0 to 7). We report that (1) the morphologies and crystal structures of the three studied clay minerals do not differ markedly after exposure to an acidic solution, (2) all three clays display increases in surface area after the pH 0 acidic treatment, and (3) cadmium adsorption to all three clays decreased after the pH 0 acidic treatment. Our results suggest that the variation in metal adsorption capacity is related to acidic modification of clay surface properties. Importantly, this work shows the relevance of acidic environments on surface-controlled reactions of trace metals to clay minerals.