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United States Department of Agriculture

Agricultural Research Service

Title: Relationship Between Cation Exchange Selectivity and Crystalline Swelling in Expanding 2:1 Phyllosilicates

Authors
item Laird, David
item Shang, Chao - UNIV. OF SASKATCHEWAN

Submitted to: Clays and Clay Minerals
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 6, 1996
Publication Date: N/A

Interpretive Summary: In soils, the availability of nutrients needed for plant growth depends in part on the ability of nutrients held on soil particles to trade places with other nutrients in the water between soil particles. Although scientists have long known that soil particles like some nutrients better than others, the reasons for these preferences are not well understood. In this paper we proposed a new theoretical model that explains how and why nutrients trade places. Also, we ran an experiment in which barium in water was allowed to trade places with magnesium held on a clay and vice versa. The results showed that the clay preferred barium over magnesium. The results also showed that the extent to which barium is preferred depends on how swollen the clay is with water. These results were predicted by the new theoretical model. The new model will provide a theoretical basis to aid in the design of nutrient management systems which facilitate crop growth while minimizing the loss of nutrients to leaching. Producers will benefit from more efficient nutrient management systems because their fertilizer bills will be lower, and the general public will benefit from less leaching of nutrients because the quality of ground water and water in lakes and rivers will be better.

Technical Abstract: A theoretical model is presented for the relationship between crystalline swelling and cation exchange selectivity of expanding 2:1 phyllosilicates. The model is a unified framework for understanding numerous ion exchange phenomena, including the influence of clay properties on selectivity, non-ideal exchange behavior, demixing, fixation, and hysteresis. The model is based on the assumption that changes in basal spacing of a clay are phase changes, and that each phase of a clay has a different selectivity coefficient for a particular cation exchange reaction. The model was tested by investigating Ba-Mg exchange on a synthetic fluoro-hectorite. Both basal spacings and the measured Vanselow selectivity coefficients were nearly constant for the forward reaction (Ba in solution replacing Mg on the clay). However, a basal spacing shift from 16 A to 18.5 A was observed for the reverse reaction. Vanselow selectivity coefficients (Kv) for Ba were an order of magnitude larger for samples with 16 A basal spacings than for samples with 18 A basal spacings, and there was a strong correlation between basal spacing and Kv (r2=0.85; p<0.001). The results of the Ba-Mg exchange experiment strongly support the validity of the proposed model.

Last Modified: 8/22/2014
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