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Organic Compound Sorption Enthalpy and Sorption Mechanisms in Soil Organic Matter

Institute of Soils and Water, Agricultural Research Organization, The Volcani Center, P.O.B. 6, Bet Dagan, 50250, Israel.

^* Corresponding author (vwgraber{at}volcani.agri.gov.il).

ABSTRACT

Small exothermic or even endothermic heats of sorption for organic compounds in soil organic matter (SOM) have been traditionally considered evidence for a partitioning mechanism into the bulk of SOM. This derives from considerations of solute transfer enthalpy (H_tr) from aqueous solution to organic solvent and partial molar enthalpies of solution in organic solvent (H^s_sol) and aqueous (–H^w_sol) phases, where H_tr = H^s_sol – H^w_sol. The dissolution of many solutes in organic solvents is accompanied by positive solution enthalpies and conforms to the condition H_tr > –H^w_sol. It was suggested by analogy of SOM to organic solvents, that solute sorption enthalpies (H_i) in SOM that conform to the analogous condition (i.e., H_i > H^w_sol) are consistent with partitioning into SOM. Systems that do not conform to this condition (i.e., H_i < H^w_sol) were considered to be consistent with adsorption. The purpose of this analysis is to point out the fallacies associated with this reasoning and to demonstrate that sorption enthalpy cannot be used in this way as evidence for a particular sorption mechanism. It is shown that for partitioning of numerous organic solutes into different organic solvents, H^s_sol < 0, such that H_tr < –H^w_sol. If SOM is considered the solvent phase, it is clear that H_i < –H^w_sol cannot be taken to indicate surface adsorption. This is particularly true for systems that provide an opportunity for strong force specific interactions inside of SOM. Similarly, it is shown that systems involving adsorption of nonpolar organic compounds at mineral surfaces often conform to the condition suggested as evidence for partitioning.