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Cation Exchange Equilibria in Soils Expressed as Cation‐Ligand Complex Formation
Author(s) -
Shaviv A.,
Mattigod S. V.
Publication year - 1985
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/sssaj1985.03615995004900030007x
Subject(s) - chemistry , ligand (biochemistry) , equilibrium constant , ionic bonding , ion exchange , soil water , ionic strength , analytical chemistry (journal) , inorganic chemistry , thermodynamics , ion , aqueous solution , chromatography , physics , organic chemistry , soil science , biochemistry , receptor , environmental science
A model for representing cation exchange reactions as part of the equilibrium reactions in a soil‐water system is proposed. The exchange reaction in this model is considered to be a composite of two half complexation reactions. These complexation reactions are assumed to occur between the exchanging cations and the negatively charged surfaces considered as hypothetical free ligands. The model was added as a subroutine to the GEOCHEM program which calculates ionic equilibrium in soil water systems. Two formation constants, K′ Caf for the Ca‐ligand complex and K′ Naf for the Na‐ligand complex were estimated by assuming K′ 2 Naf /K′ Caf = K V Ca‐Na = 1 in the range 1 > E Ca > 0.7. A value of log K′ Caf = 2.5 was found to provide the best fit to the relationship between E S = ( E Na /(CEC‐ E Na )) vs. SAR given by the U.S. Salinity Laboratory Report (1954). The fitted value of K′ Caf was used for calculating the K‐ligand formation constant; K′ Kt and the Mg‐ligand formation constant, K′ Mgr , using the appropriate experimental Vanselow selectivity coefficient. The calculated formation constants corresponding to the cations in a given soil water system were used to predict the distribution of the cations in solution and on the solid phase by means of the GEO‐CHEM program. Good agreement was found for the K‐Ca and Ca‐Mg exchange in soils when comparing experimental data to the ones predicted by the proposed model.