Potassium-Calcium Exchange in a Multireactive Soil System: II. Thermodynamics
- P. M. Jardine and
- D. L. Sparks
Thermodynamics of K exchange were investigated in Ca-saturated samples from the Ap horizon of an Evesboro soil from Delaware. At 283 and 298 K the selectivity curves (lnkv vs. X̄K) showed preference for K at low values of NK (mole fraction of K in solution) and for Ca at higher values. This selectivity reversal may be attributed to exchange sites of varying reactivity for K and Ca ions and supports the hypothesis of the multireactive nature of the soil. Although K was selectively bound at low NK, the soil exhibited an overall Ca preference as noted by the positive standard free energy values (ΔG°). The standard enthalpy of exchange (ΔH°) was negative, which indicated very strong binding of K ions with some sites of the soil. This may be associated with the presence of vermiculitic clay minerals that predominated in the <2-µm clay fraction. A thermodynamic investigation was also initiated on the various size fractions of the soil (i.e., sand, silt, and clay) and on soil that was treated with cetyltrimethylammonium bromide (CTAB) or NaOCl-DCB. These treatments explained the differences in ionic selectivity observed in the Evesboro soil.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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