We reacted the water extract of a milling waste, containing concentrations of Cu, Cd, and Zn in excess of USEPA (1974) drinking water limits in batch systems with two calcareous soils to determine the mechanisms and rates of Cu and Cd sorption. The pH of the extract was 4 to 4.5, and the predominant anion was SO4. The computer program GEOCHEM was used to calculate metal activities in the "equilibrated" suspensions.
Calculated activities of Cu and Cd indicated that the suspensions of soil containing 30% carbonate were undersaturated with respect to oxide and carbonate pure solid phases at the lower levels of metal addition. As additions of Cu and Cd increased, the activities approached the solubility of tenorite (CuO) and octavite (CdCO3), respectively. At low levels of addition adsorption on carbonate surfaces apparently controlled solution concentrations. Carbonates dissolved from the soil containing 0.2% carbonates, and desorption with 0.01 M CaCl2 suggested that cation exchange processes were involved in metal sorption. The sorption of Cu, Cd, and Zn required 14 d to reach time-invariant concentrations in suspensions of the soil containing 30% carbonates, but only about 1 d in the 0.2% carbonate soil suspension.
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