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Sodium-Copper Exchange on Wyoming Montmorillonite in Chloride, Perchlorate, Nitrate, and Sulfate Solutions

National Exposure Res. Lab., U.S. EPA, 960 College Station Rd., Athens, GA 30605-2700

D. L. Sparks

Delaware Agric. Exp. Stn., Dep. of Plant and Soil Sciences, College of Agricultural Sciences, Univ. of Delaware, Newark, DE 19717-1303

*Corresponding author (zhang.zhenzhong{at}epamail.epa.gov).

ABSTRACT

The effect of anions on the Na-Cu exchange reactions on montmorillonites is not well understood and discrepancies exist in the literature. This study was conducted to examine the role of anions on the Na-Cu exchange reaction on Wyoming montmorillonite. The copper exchange capacity (CuEC) and Na-Cu exchange reactions on Wyoming montmorillonite were studied in Cl^–, ClO^–₄, NO^–₃, and SO^2–₄ solution media at a constant total metal charge concentration of 0.0200 mol_c L^–1. We found that the values of CuEC were similar in Cl^–, ClO^–₄, NO^–₃, and SO^2–₄ electrolyte media, and the quantity of Cu extracted by Na₂-EDTA solution was only 1.0 to 1.2% of the CuEC and was not affected by anions. The apparent adsorbed metal charge, Q, and Vanselow selectivity coefficients, K_v, were determined across a wide range of exchanger phase composition, with Cu²⁺ occupying 30 to 95% of the charge sites. We found that the values of Q increased with Cu saturation in each of the anionic media, and the increase was not affected by anions. The increase in Q was attributed to decrease in negative adsorption of anions. We observed that K_v was independent of exchanger composition in each of the four anionic media. The values of K_v in the SO^–₄ medium were lower than those in other anionic media because of the complexation reaction between Cu²⁺ and SO^2–₄. The data showed that Cu²⁺ ions were not specifically adsorbed under our experimental conditions and that the exchanger phase behaved as an ideal mixture. It was concluded that no detectable monovalent complexes such as CuCl⁺ and CuNO⁺₃ were adsorbed on Wyoming montmorillonite in the Na-Cu exchange reaction.

Published as Miscellaneous Paper no. 1601 of the Delaware Agric. Exp. Stn.

Received for publication May 18, 1995.

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