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Predicting Cation Mobility in Kaolinitic Media Based on Exchange Selectivities of Kaolinite

Agronomy Dep., Sturgis Hall, Louisiana State Univ., Baton Rouge, LA 70803

*Corresponding author.

ABSTRACT

Predictive modeling of cation transport in soils requires knowledge of the distribution of exchangeable species between solution and adsorbed phases. Such distribution of exchangeable species may be described with an exchange selectivity coefficient. Typically, the selectivity coefficient for any cation-exchange pair varies among soils. If the exchange complex of a soil is dominated by one type of exchanger, however, then the selectivity coefficient for that soil may be similar in magnitude to that for the principal mineral (or organic) constituent. Therefore, reasonably accurate predictions of cation transport in that soil might be obtained using exchange data for the dominant exchange material. This study was undertaken to determine whether exchange data for kaolinite could be successfully used in predicting Ca, Mg, and Na retention during movement in a Mahan series soil (clayey, kaolinitic, thermic Typic Hapludult). Binary-exchange isotherms for Ca-Mg, Ca-Na, and Mg-Na were generated for Ga-1 kaolinite (well crystallized). Selectivity coefficients derived from these data were used to describe cation exchange during miscible displacement through columns of (i) kaolinite admixed with acid-washed sand and (ii) Mahan soil. Use of the selectivity coefficients for kaolinite provided good predictions of cation transport in the kaolinite/sand mixture. Predictions for the kaolinitic Mahan soil were not as good. The presence of small amounts of 2:1 minerals may have influenced the overall exchange behavior of the Mahan soil. When the uncertainty in values for selectivity coefficients and other exchange or transport parameters was considered, however, the exchange selectivity data for kaolinite could adequately describe cation transport in the Mahan soil.

Approved for publication by the director of the Louisiana Agric. Exp. Stn. as Manuscript no. 90-09-5273.

Received for publication October 26, 1990.