Sorption of Pyrene to Dissolved Humic Substances and Related Model Polymers. 1. Structure -Property Correlation FRANK-DIETER KOPINKE,* ANETT GEORGI, AND KATRIN MACKENZIE Departm ent of Rem ediation Research, UFZsCentre for Environm ental Research Leipzig-Halle, Perm oserstrasse 15, D-04318, Leipzig, Germ any Solid-phase microextraction (SPME) was employed to determine sorption coefficients for pyrene on dissolved humic substances (DHS) of various origins and also on model polymers to gain a better understanding of the relationships between sorption potential and sorbent structures. The sorption potential of DHS from very different sources is described by an empirical two-parameter correlation involving the polarity (O/H atomic ratio) and the aromaticity (ǫ 280nm ) as descriptors. On the other hand, sorption experiments with well-defined model polymers, poly(acrylic acid) esters, led to the conclusion that aliphatic chains may be more effective than aromatic moieties in binding PAHs. The molecular weight of the model sorbents was found to have a significant influence on the sorption potential. Introduction Humic substances (HS) have been the subject of numerous scientific studies due to their environmental importance. The fate of hydrophobic organic compounds (HOCs), i.e., their transport and bioavailability in the environment, is strongly affected by their interactions with HS. Sorption experiments with dissolved organic matter (DOM) often reveal drastic differences in the sorption properties of HS of various origins (1-6). On one hand, this variability raises questions about the real mechanisms of hydrophobic sorp- tion. On the other hand, modeling of sorption-related processes in the environment is made much more difficult. Furthermore, the mechanistic basis is not yet sufficiently clarified to allow the development of a model for the prediction ofsorption coefficientson DHS.Proposalsalready made in the literature range from a partition of the sorbates between water and micelle-like structures of DHS (7) to specific interactions of the sorbates with defined sorption sites of the DHS (8). By correlating sorption properties with structuralparameters ofDHS, it is possible to better identify the structures and building blocks of DHS that are decisive for their sorption behavior. One approach toward under- standing sorption properties of DHS is to correlate sorption coefficients with those parameters of DHS that can be experimentally determined. By comparing the sorption behavior of various sorbents, generally a correlation between sorption potential and the content of functional groups, i.e., varying polarity of the sorbents,wasfound.In severalinvestigations,the conclusion has been drawn that with increasingpolarityofthe sorbents, the sorption capabilitywith respect to hydrophobic sorbates decreases. The atomic ratios (O+N)/ C (9, 10), H / O (11),and O/ C (12) commonly serve as measures of polarity. As wellas this negative influence ofthe polarity,a positive influence ofincreasing molecular weight and aromaticity of the organic matter (OM)upon the sorption ofPAHs has been observed (6, 13). The aromaticity is usually given as the proportion of aromatic C or H atoms measured by means of 13 C or 1 H NMR spectroscopy, respectively, or as specific absorptivities at 250-280 nm. It is commonly accepted that aromatic moieties in humic substances are the main sites for hydrophobic sorption of PAHs. Although such a correla- tion appearsplausible from the chemicalpoint ofview,there is as yet no clear mechanistic basis upon which to explain the role of aromatic units in OM for specific interactions with PAHs. Gauthier et al. (3) explained the sorption- enhancing effect of aromatic structures in OM as being due to a higher polarizability and more favorable van der Waals interaction with the PAH. Specific interactions are also discussed,such as charge-transfer complexes where the PAH acts as electron donor and the aromatic units in the OM as electron acceptors (14). Such specific intermolecular inter- actions cannot be satisfactorily explained with a simple partition model. Chiou et al. (15)attribute the enhanced partition ofPAHs as compared with other nonpolar solutes to a better compatibilitybetween the cohesive energydensitiesofPAHs and the aromatic components in SOM. However, the difference between the KOC values of 5 for soils and 7 for sediments (a factor of 2) is related to the difference in the contents of polar groups rather than to different aromatic carbon fractions. Uhle et al. (16) suggest that PCBs capable of free rotation of the phenyl rings around the 1,1′ carbon bond are able to interact more favorably than their ortho-substituted con- geners with DHS. This observation was attributed to more favorable steric arrangements between aromatic rings of sorbent and sorbates. However, taking recommended K OW values (from the same source that was used by Uhle et al.), there is a strong KOC-KOW correlation with no discrimination of the ortho-substituted PCBs. Apparently, the substitution pattern ofPCBsisalreadyreflected in theirpartition behavior regardlessofthe rigidityofpartition media,humicsubstances, or 1-octanol. Therefore, it does not seem to be conclusive to derive specific steric requirements for interactions between aromaticcompoundsand humicsubstancesfrom thesedata. DHSpropertiessuch asaromaticityand molecular weight have been shown to be significantly intercorrelated (6). Therefore,empiricalcorrelationsbetween sorption potential and specific structural parameters (e.g., between KDOC and aromaticity of DHS) must be interpreted with caution. In other words,even ifthe sorption behavior for PAHscorrelates well with the number of aromatic sites in DHS, the specific interactions with aromatic centers do not necessarily have to be the determining sorption mechanisms. Perminova et al. (17) investigated relationships between structure and bindingaffinitiesofdissolved humicsubstances for PAHs on an exceptionally large data basis (3 PAHs, 26 humic and fulvic acids).Theytested the relevance ofvarious structural properties of the HS as molecular descriptors for estimation of binding affinities. The KDOC values, measured by the fluorescence quenching technique (FQT), correlated with several descriptors of aromaticity of the HS including atomicH/Cratio,absorptivityat280nm,and some 13 C NMR *Corresponding author e-mail: kopinke@san.ufz.de; phone: ++49 (0)341 235 3264; fax: ++49 (0)341 235 2492. Environ. Sci. Technol. 2001, 35, 2536-2542 2536 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 12, 2001 10.1021/es000233q CCC: $20.00  2001 American Chemical Society Published on Web 05/16/2001