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Sodium-Calcium-Magnesium Exchange Reactions on a Montmorillonitic Soil: I. Binary Exchange Reactions¹

ABSTRACT

The binary cation exchange processes, Na Ca, Na Mg, and Ca Mg, were investigated at 298 K for a montmorillonitic soil separate (principally clay and silt) suspended in a 50 mol m^–3 perchlorate background at pH 6, 6.6, or 7. The resulting exchange isotherms indicated some preference on the soil for Ca and Mg over Na, and for Ca over Mg. These preferences were attributed primarily to organic matter associated with the soil separate, since previous studies had shown that pure montmorillonite exhibits essentially no preference among the three cations under the same conditions as in the present experiments. The exchange isotherms at pH 6, 6.6, and 7 were congruent for all three binary exchange reactions, showing that, at these pH values, proton competition did not affect the exchange selectivity of the soil. The total adsorbed metal charge, Q₀, however, increased with the pH value, as expected from the presence of organic matter in the soil, and varied with the surface composition of the exchanger. This variation was rationalized as a net effect of the ionic potential and Lewis acid softness properties of the exchangeable metal cations. Greater ionic potential was considered to result in greater proton displacing power, whereas greater Lewis acid softness was considered to result in stronger complex formation between a metal cation and organic functional groups. Thus, increasing ionic potential could produce an increase in Q₀, but increasing Lewis acid softness could produce a decrease in exchangeability and consequent decrease in Q₀. Ionic potential was the dominant consideration in Na Mg exchange, whereas Lewis acid softness played the principal role in Na Ca and Mg Ca exchange.

¹ Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521.

² Postdoctoral Research Soil Scientist, Professor of Soil Science, and Research Associate III, respectively.

Received for publication January 23, 1984. Accepted for publication April 30, 1984.

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