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Potassium‐Calcium Exchange in a Multireactive Soil System: II. Thermodynamics
Author(s) -
Jardine P. M.,
Sparks D. L.
Publication year - 1984
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1984.03615995004800010008x
Subject(s) - chemistry , silt , selectivity , cation exchange capacity , potassium , enthalpy , ion exchange , bromide , inorganic chemistry , clay minerals , ionic strength , soil water , mineralogy , ion , thermodynamics , soil science , geology , organic chemistry , paleontology , physics , catalysis , aqueous solution
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 (ln k v vs. X̄ K ) showed preference for K at low values of N K (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 N K , the soil exhibited an overall Ca preference as noted by the positive standard free energy values (Δ G °). The standard enthalpy of exchange (Δ H °) 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.