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Ionic Transport in Soil with Ion-exchange Reaction¹

ABSTRACT

A mathematical model describing the transport of two ions in an exchange material such as soil, was described. Concentration profiles of displacing ion, A, and an ion being displaced, B, were obtained using the numerical solution of the two simultaneous transport equations of A and B. The variations in the concentration distributions were investigated by varying the selectivity coefficient of ion exchange reactions, valences of A and B, the magnitude of incoming concentration, and cation exchange capacity under constant flow rate of soil solution and dispersion coefficient. The results of the analyses indicated that the concentration distributions of displacing and displaced ions within a soil column can be related to the selectivity coefficient of the ion exchange reaction, exchange capacity, and concentration of the displacing ion. As the ion exchange selectivity coefficient became large, the front of the displacing ion became sharper and the maximum concentration of the displaced ion became greater. The depth of penetration of the displacing ion was not great since the ion effectively replaced the native ion that resided on the exchanger phase. For divalent-monovalent exchange, the cation exchange capacity, selectivity coefficient, and concentration of the displacing solution played a role in determining the shape of the distribution curves of displacing and displaced ions.

¹ Contribution from the Dep. of Soil Science, Univ. of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2.

² Professor, Dep. of Soil Science, Univ. of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2.

Received for publication December 14, 1984. Accepted for publication July 11, 1985.