Solvation effect on organic compound interactions in soil organic matter

We examine sorption of pyridine by soil organic matter (SOM) from different organic media including n-hexadecane, acetonitrile, acetone, and n-hexadecane mixtures with either acetonitrile or acetone and compare it with sorption from water. By using an activity-based comparison, we distinguish between solvent-assisted and solvent-competitive sorption behavior. Pyridine was selected because it forms strong complexes with phenolic and carboxylic groups, such that site interactions should dominate interactions in SOM. It is anticipated that pyridine sorption will be illustrative of the importance of disrupting strong interactions in a condensed, shrunken SOM phase for many organic compounds. It was generally found that activity-normalized pyridine uptake was assisted by polar solvent molecules rather than suppressed due to competition. An explanation is tendered on the basis of our earlier hypothesis of water-assisted disruption of polar SOM contacts. Certain polar moieties of dry SOM are unavailable for compound sorption due to strong interactions between them. By penetrating SOM structure, solvent molecules (and water) solvate (hydrate) polar moieties creating new sorption sites. Solvent molecules must solvate both moieties of the polar contact, such that the driving force for solvent-assisted sorption is solvation of the partner of the disrupted contact that does not directly interact with the sorbate.