Quasi‐thermodynamic basis of competitive‐adsorption coefficients for anionic mixtures in soils

Abstract

SUMMARY Equilibrium competitive‐adsorption coefficients derived from the application of the multi‐component Freundlich‐type Sheindorf‐Rebhun‐Sheintuch (SRS) equation appeared to have a soil‐specific, quasi‐thermodynamic basis. Using three soils, single‐solute adsorption isotherms at 288, 298, and 308 K were constructed with arsenate, phosphate, and molybdate. A Clausius‐Clapeyron‐type equation was used to estimate apparent isosteric heats of adsorption values ( q iso ). The adsorption of the three anions was characterized by different q iso values, and was generally in the order of H 2 PO 4 ‐>H 2 AsO 4 ‐>MoO 4 2‐ . The magnitudes of the SRS competitive coefficients derived from binary‐solute mixtures were in agreement with the distributions of the heats of adsorption of the individual anion‐soil systems. Higher competitive coefficients, indicating the preferential adsorption of solute i in a binary‐solute mixture containing solutes i and j, tended to associate with higher q iso values for solute i relative to solute j. However, the q iso values for the adsorption of molybdate by one soil underestimated the extent of competitive interactions between arsenate and molybdate.