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Self-Diffusion of Tritiated Water in Montmorillonite and Kaolinite Clay¹

ABSTRACT

The quick-freeze technique of measuring diffusion coefficients of ions in clays and soils was adapted to the measurement of self-diffusion coefficients of tritiated water in clays. The self-diffusion coefficients of tritiated water were approximately equal in the kaolinite and montmorillonite clays. The water contents, oven-dry weight basis, used for montmorillonite were 125.0, 118.5, 100.0, and 87.0%; the water contents used for kaolinite were 60.5, 55.0, 43.5, and 39.0%. The calculated average number of water layers on each mineral surface ranged from 3.5 to 5.5 for montmorillonite and from 17 to 26 for kaolinite. The self-diffusion coefficients in creased in a linear manner as the average number of water layers present on each mineral surface increased; a slightly different rate of increase was obtained for each of the two clays. The lack of difference of the diffusion coefficients in the two clays could not be explained on the basis of charge density of the clays nor was a difference in viscosity a plausible explanation. A longer path length of the diffusing water molecules in the kaolinite and a smaller relative mobility of the diffusing water molecules in the montmorillonite is given as a possible explanation.

¹ Contribution from the Department of Agronomy, Kentucky Agr. Exp. Sta., Lexington, and the Department of Agronomy, Arkansas Agr. Exp. Sta., Fayetteville. Presented before Div. S-1, Soil Sci. Soc. Amer., Nov. 6, 1967, at Washington, D. C.

² Associate Professor, University of Kentucky, and Professor, University of Arkansas, respectively.

Received for publication May 8, 1967. Accepted for publication January 17, 1968.