Abstract
AbstractThe Vanselow selectivity coefficient was calculated for Mg → Ca exchange on a montmorillonitic soil separate containing organic matter. Seven sets of cation exchange data were analyzed numerically, encompassing pH values ranging from 5 to 7 in a 50 mol m−3 perchlorate medium and exchangeable sodium percentages ranging from 0 to 25%. The selectivity coefficient for Mg → Ca exchange was observed to decrease from a value near 1.5 at 0% Ca saturation to a value near 1.0 at 100% Ca saturation with no effect of pH on this trend in the range examined. At low Ca saturation of the soil, the selectivity coefficient increased by about 15% as the ESP increased from 0 to 25%. This behavior was shown through a model calculation to be consistent with a mechanism in which Na adsorbs preferentially on the montmorillonitic component of the exchange complex in the soil, leaving the nonmontmorillonitic portion of the exchange complex to adsorb Ca selectively over Mg.
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