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Contribution of Adsorbed Cations to Diffusion in Clay-Water Systems¹

ABSTRACT

Self-diffusion of sodium and calcium ions was measured in bentonites containing various amounts of water and the respective chlorides. The contribution of mobile exchangeable cations to diffusion was calculated from the data by subtracting measured rates of free electrolyte diffusion from the total rate of self-diffusion measured in these systems. Mobile exchangeable cations accounted for from 3 to over 500 times as much diffusive movement as was accounted for by free electrolyte cations in these systems. The fractions of the exchangeable cations that were in the diffuse layer were estimated from the contribution of exchangeable ions to diffusion, the fluidity of water near clay particles and the calculated concentration distributions of the adsorbed cations. Approximately 70% of exchangeable sodium ions were mobile. Approximately 25% of exchangeable calcium ions were mobile.

¹ Contribution from Northern Plains Branch, Soil and Water Conservation Research Division, ARS, USDA, and the Colorado Agr. Exp. Sta., Ft. Collins. Scientific Journal Series 1060. Portions of this work were supported by the National Science Foundation and Regional Research Project W-68.

² Formerly graduate research assistant, Colorado State University, now Research Officer, Canada Agr. Res. Sta., Lethbridge, Alberta, Canada; Research Soil Scientist, USDA, and Agronomist (Soils), Colorado State University; and Research Soil Scientist, USDA, Fort Collins, Colo.

Received for publication March 23, 1965. Accepted for publication October 7, 1965.