The mechanism of chromate sorption by three variable charge soils

Adsorption of chromate and desorption of the pre-adsorbed chromate were studied using three representative variable charge soils from the south of China. The mechanisms of the adsorption were discussed based on the hydroxyl release and the change of ζ potential during the chromate adsorption. The adsorption and desorption of chromate followed the same order: the Hyper-Rhodic Ferralsol > the Rhodic Ferralsol > the Haplic Acrisol. The adsorption and the desorption both increased with elevation of the equilibrium chromate concentration and decreased with increasing of the soil solution pH. The percentage of the specific adsorption of chromate was 54.0–59.4%, 54.3–60.3%, and 43.9–46.2% for the Hyper-Rhodic Ferralsol, the Rhodic Ferralsol, and the Haplic Acrisol, respectively; the percentage of the electrostatic adsorption was 40.0–46.6%, 39.7–45.8%, and 50.8–56.5% for the three soils, respectively. These findings suggest that both the specific adsorption and the electrostatic adsorption contributed to the chromate adsorption by the variable charge soils. The hydroxyl release during the chromate adsorption shared the same trend with the adsorption envelopes, and decreased with increasing of pH. This is attributed to the exchange of chromate with the hydroxyl on the soil particle surfaces and the formation of a chemical bond between chromate and the surface. Our results indicate that the adsorption of chromate resulted in a shift of ζ potential-pH curves of the soil colloids to negative values, which suggests that the adsorption increased the negative surface charge and decreased the surface potential of the soil colloids.