Sorption and Desorption of Copper on Soil Clay Components

Abstract

AbstractToxic effects have been reported due to the accumulation of Cu in soils receiving long‐term applications of swine (Sus scrofa) manure, sewage, municipal composts, and mining waste. The objective of this study was to quantify sorption and desorption of Cu on various clay and clay‐organic matter fractions separated from a Zook soil (fine, smectitic, mesic Cumulic Vertic Endoaquolls) located in central Iowa. The clay was separated into four particle‐size fractions and portions of each fraction were treated with H2O2 and/or dithionite‐citrate‐bicarbonate (DCB) for removal of organic matter and free‐iron compounds, respectively. A batch‐equilibration technique was employed to evaluate sorption of Cu at pH 6.0 in the presence of 0.01 M CaCl2. The sorption isotherms show that Cu was preferentially sorbed on organic matter associated with the coarse clay fraction of the soil. After removal of organic matter, the fine clay (<0.02 µm) exhibited higher Cu retention than did the coarse (0.2–2.0 µm) and medium clay (0.02–0.2 µm). The DCB treatment created or exposed high‐affinity sites for Cu on the fine clay, but had little effect on Cu sorption for the coarse day. Copper appears to be specifically sorbed on the surfaces of silicate clays in excess of that which can be accounted for by variable charge sites. The results also suggest that Fe oxides may coat lateral surfaces of layer silicates, blocking access of Cu to potential sorption sites. Substantial sorption‐desorption hysteresis was observed for all samples, suggesting that Cu forms high‐energy bonds with both organic matter and layer‐silicate surfaces.