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Percent of Copper Diffusing in the Adsorbed and Electrolyte-Solution Phases in Kaolinite and Montmorillonite¹

ABSTRACT

The apparent diffusion coefficient, D_e, of a cation diffusing in soil was proposed to be D_e^m = [D_s^m(dC_s/dC_sa} + ¹D_a^m (dC_a/dC_sa)] / [1 + dC_p/dC_sa] where D_s^m and ¹D_a^m are the diffusion coefficients of the cation in the electrolyte-solution phase and in the adsorbed phase (exchangeable cations), respectively, and where the frame of reference in each case is with respect to the entire soil mass. The symbols C_s, C_a and C_p are concentrations of electrolyte-solution cations, adsorbed cations, and precipitated or immobilized cations, respectively; C_sa is defined to be the sum of C_s and C_a. The concentrations are expressed with respect to the total volumetric water content of the system. The quantities, dC_s/dC_sa, dC_a/dC_sa, and dC_p/dC_sa were evaluated in experiments independent of the measurement of D_e^m. The relative contributions of D_s^m and ¹D_a^m to D_e^m were evaluated over a large range of C_sa values for Cu²⁺ diffusing in water-saturated kaolinite and montmorillonite. In kaolinite clay with a cation exchange capacity (CEC) of 0.065 meq g^-1, the percent of total diffusion due to diffusion in the adsorbed phase was 100, 52, 34, 22, 18, 9, and 7 at C_sa concentrations of 0.015, 0.02, 0.03, 0.04, 0.05, 0.06, and 0.08 meq g^-1, respectively. In montmorillonite clay with a CEC of 0.70 meq g^-1, the percent of total diffusion due to diffusion in an adsorbed phase was 100, 57, 42, 34, 28, 20, and 13 at C_sa concentrations of 0.03, 0.05, 0.10, 0.15, 0.20, 0.40, and 0.70 meq g^-1, respectively.

¹ Contribution from the Department of Agronomy, Kentucky Agr. Exp. Sta., Lexington 40506. This paper (no. 71-330) is published with the approval of the Director of the Kentucky Agr. Exp. Sta.

² Professor, Associate Professor, and Assistant Professor, respectively, Department of Agronomy, University of Kentucky.

Received for publication March 22, 1971. Accepted for publication August 27, 1971.