Adsorption of Cd(II) and Cu(II) by Na‐Montmorillonite at Low Surface Coverage

Abstract

AbstractMontmorillonite has a heterogeneous charge distribution, and it is important to determine whether constant potential edge sites or constant charge sites control metal adsorption. The pH dependence of metal adsorption provides insight into the adsorption mechanism as well as being an important variable that will determine the solubility of Cd or Cu in soil systems. Adsorption of Cu and Cd by Na‐montmorillonite (2 g L−1) in 0.01M NaClO4 was measured over a pH range from 4 to 8.5 under humidified N2 gas. With the exception of experiments for Cu adsorption > 10 µM CuT, total soluble metal values were undersaturated with respect to known metal hydroxide solid phases. Kinetics for the adsorption of 50 µM CdT and CuT were rapid, with equilibrium occurring within 30 min. Experimental data for initial metal concentrations ranging from 50 to 0.1 µM yielded linear adsorption isotherms over the pH value range studied. Initial metal concentrations represent from 6 to 0.01% of the total sites available for metal binding, assuming an average of 0.8 mmol charge g−1 of clay. The pH dependence associated with Cd and Cu adsorption was reversible and much lower than observed for oxide minerals or for precipitation reactions. Adsorption of 2 µM CdT by 2.0, 0.2, and 0.02 g of clay L−1 suspensions showed that the fractional coverage of Cd on montmorillonite increased when the solid concentration decreased. As the fractional coverage approached saturation on the calculated amount of constant potential edge sites, the pH dependence increased and approached expected values of pH dependent charge on constant potential sites. This suggests that constant potential edge sites may primarily be responsible for Cd and Cu adsorption at low adsorption densities.