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Effect of Precipitation of Alkaline Earth Carbonates and Magnesium Hydroxide on Na-Ca-Mg Exchange in Wyoming Bentonite¹

ABSTRACT

Wyoming bentonite saturated with Ca and Mg at five different ratios was equilibrated with NaCl solution. It was found that the preference of the bentonite for Ca over Mg was not affected by the presence of exchangeable Na. The separation factor, ^Na_Ca+Mg, of the bentonite equilibrated with NaCl was compared to that of bentonite in which alkaline earth carbonates and magnesium hydroxide precipitated. It was found that in the presence of alkaline earth precipitation only, ^Na_Ca+Mg was a function of ionic strength and equivalent fraction of soluble Na. However, precipitation of the carbonates caused a different distribution of the exchangeable ions in the bentonite crystallite compared to the distribution in the absence of precipitation. Where carbonates precipitated, Na was found between the interlayers and Ca and Mg occupied the outside surfaces of the clay. As the pH of the equilibrium solution increased and additional precipitation of alkaline earth carbonates and magnesium hydroxide occurred, ^Na_Ca+Mg increased with decrease in ionic strength. Apparently this increase in the preference for Na was due to the restriction on Na, occupying the internal surface of the bentonite, to diffuse into the equilibrium solution because of hydroxide coating of the crystallites.

¹ Contribution from the Dep. of Land, Air and Water Resources, Univ. of California, Davis. This research was supported by the Kearney Foundation of Soil Science.

² Soil Scientist, Professor of Water Science, and Professor of Soil Science, respectively. The Senior Author was on a sabbatical leave from the Institute of Soils and Water, Volcani Center, Bet Dagan, Israel.

Received for publication December 13, 1982. Accepted for publication May 2, 1983.