Solvation Effect on Organic Compound Interactions in Soil Organic Matter MIKHAIL BORISOVER,* MINOLEN REDDY, AND ELLEN R. GRABER Institute of Soil, Water and Environmental Sciences, The Volcani Center ARO, P.O. Box 6, Bet Dagan, 50250 Israel 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. Introduction The hydration status of soil organic matter (SOM) is well- known to affect its structure. Multiple processes including hydrogen bonding between polar functional groups, con- formation rearrangements, and bridging via metal cations may occur in SOM humic substances when partially or completely dehydrated (1-4). These processes cause the macromolecular substances to shrink, creating a more condensed structure in the dehydrated state. It is expected that such a condensed structure may have an increased thermodynamic and/or kinetic potential to resist sorption of organic compounds due to diminished availability of sorption sites and reduced sorbate diffusivity. Shrinkage of the SOM structure may strongly affect desorption oforganic compounds, contributing to decreased rates of organic sorbate release under dry or wet conditions (4). Comparing sorption of selected organic compounds on dryand hydrated SOM-rich sorbentsisa meansofevaluating the effect ofhydration on sorption and can elucidate certain properties and structures ofcondensed SOM. An increase in vapor-phase sorption oftrichloroethylene on Aldrich humic acid salt at an intermediate water humidity(5)demonstrates that interactions between polar SOM moieties have an important role in limiting sorption in dry SOM. A strong increase in activity-normalized sorption of the specifically interacting compounds pyridine and phenol on SOM from water as compared with n -hexadecane was also reported (6).This hydration-assisted sorption was interpreted in terms ofdisruption ofSOM polar contacts in the presence ofwater. A weak reduction in sorption of benzene, carbon tetra- chloride, and trichloroethylene by SOM-rich sorbents from water as compared with vapor-phase sorption on dry SOM (7) shows that, for these compounds, complete hydration of the SOM sorbent did not result in exposure of new effective sorption sites. Complete hydration of SOM involves dissociation of carboxylic groups and strong hydration ofmanyother polar functionalities, thus disrupting SOM structure. We hypoth- esized that by measuring sorption of a selected organic compound on a SOM-rich sorbent from different organic media ranging from nonpolar to polar, different extents of solvent-disrupting effects could be tested. On the basis of such experiments,it would be possible to distinguish between cases where the solvent may assist in activity-normalized sorption ofthe probe compound (solvent-assisted sorption) and cases where the solvent would successfullycompete with the probe compound for sorption sites (solvent-competitive sorption). This comparison could be made more sophisti- cated by measuring sorption from n -hexadecane mixtures with chosen polar solvents,makingit possible to observe the gradualexposure ofnewsitesfor SOM-sorbate interactions. By comparing the hydration effect with the organic solvent effect,it could be possible to elaborate on the role ofhydration in sorption. Such a solvent-based approach should not be confused with studies of sorption on soil from aqueous solutions of miscible organic solvents (e.g., refs 8-10) or studies of sorption on soil from nonaqueous solvents (e.g., refs 11 and 12). In experiments with water-miscible solvents, the effect of the solvent on SOM structure can only be examined in relation to a stronglyhydrated SOM structure and thus cannot elucidate organic solvent or water effects on a dehydrated SOM structure. In sorption experiments from nonaqueous solvents on mineral-rich soils, the strong sorption potential of dry mineral surfaces interferes with evaluating SOM- organic compound interactions. Sorption data for organic compounds on SOM-rich sorbents from organic solvents are scarce and limited mainly to sorption isotherms measured from hydrocarbons (4, 6, 13, 14). Apparently, there is no activity-based comparison of organic compound sorption on SOM-rich sorbents from organic media with different solvation abilities. As such, we examine sorption of a selected specifically interacting compound, pyridine, by a model soil organic matter from different organicmedia including n -hexadecane, acetonitrile,acetone,and n- hexadecane mixtureswith either acetonitrile or acetone and compare it with sorption from water. By using an activity-based comparison of sorption data, we intend to distinguish between solvent-assisted and solvent-competitive sorption behavior.Pyridine wasselected according to its ability to undergo specific interactions with SOM (15). On the basis of gas-phase -SOM distribution coefficients (15), it may be calculated that pyridine interacts with hydrated SOM 2500 times stronger than its nonpolar analogue, benzene. Pyridine is capable of forming strong *Corresponding author telephone: 972-3-968-3314; fax: 972-3- 960-4017; e-mail: vwmichel@volcani.agri.gov.il. Environ. Sci. Technol. 2001, 35, 2518-2524 2518 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 12, 2001 10.1021/es001810d CCC: $20.00  2001 American Chemical Society Published on Web 05/11/2001