Environ. Sci. Technol. 2002, 36, 4689-4697
Solubility-Normalized Combined Adsorption-Partitioning Sorption Isotherms for Organic Pollutants SYBILLE KLEINEIDAM, CHRISTOPH SCHU ¨ TH, AND PETER GRATHWOHL* Center for Applied Geoscience, Applied Geology Group, University of Tu ¨ bingen, Sigwartstrasse 10, D-72076 Tu ¨ bingen, Germany
Equilibrium sorption isotherms were measured for five different low-polarity organic compounds (benzene, trichloroethene, 1,2- and 1,4-dichlorobenzene, and phenanthrene) over a wide concentration range. The investigated sorbents can be grouped into the following three classes: (1) humic soil organic matter, which shows linear sorption isotherms (solely partitioning, as observed in the peat sample); (2) carbon materials, which were thermally altered (due to their natural history or industrial production) and thus contain a high specific surface area and exhibit nonlinear isotherms, and (3) pure engineered microporous materials (e.g., zeolites and activated carbon), where adsorption is solely due to a pore-filling process. Sorption of all compounds was fitted very well by the Polanyi-Dubinin-Manes (PDM) model, which for sorbents containing humic organic matter (e.g., peat) was combined with linear partitioning. Both the partitioning and the Polanyi-Dubinin-Manes model predict unique sorption isotherms of similar compounds if the solubilitynormalized aqueous concentration is used. In addition, an inverse linear relationship between the distribution coefficient (Kd) and water solubility, which was very well confirmed by the data, is obtained. This also leads to unitequivalent Freundlich sorption isotherms and explains the often observed apparent correlation between sorption capacity at a given concentration (e.g., Freundlich coefficient) and sorption nonlinearity (Freundlich exponent).
Introduction Sorption of hydrophobic organic contaminants to organic matter (OM) has received much attention in recent years (1). Most studies agree that OM is the most important parameter governing the sorption/desorption, and thus fate and transport, of these compounds in soils and sediments. In contrast to the well-established partitioning model (2-4), often nonlinear sorption isotherms and extremely high sorption capacities are observed. Two basic concepts have been developed to explain such sorption phenomena as described in detail elsewhere (5). Whereas one concept favors the analogy between OM and polymers (summarized in 6-8), others emphasize the explanation in the heterogeneity of the OM and the presence of small amounts of high-surfacearea carbonaceous material with high sorption capacities (e.g., 9-11). * Corresponding author phone: 49-7071-2975429; fax: 49-70715059; e-mail:
[email protected]. 10.1021/es010293b CCC: $22.00 Published on Web 10/04/2002
2002 American Chemical Society
In the OM-polymer analogy, the sorption isotherms are described by a dual-mode model where sorption occurs in soft (rubbery - partitioning) and hard (glassy - nonlinear behavior) parts of OM (e.g. (12, 13)). Evidence for soft and hard regions in humic substances was found in spectroscopic measurements (8, 14), and the existence of a glass transition temperature, where a change from soft to hard components occurs, was reported (15). Organic matter in a glassy state, which is assumed to contain isolated holes (16), displays desorption hysteresis, but below the glass transition temperature no desorption hysteresis was observed (15, 17). Soil organic matter is a very heterogeneous mixture of different materials such as residues from microbial degradation of higher plants, and it may contain black carbon and coal particles (5). Thermally altered carbonaceous particles, e.g., from incomplete combustion of plant materials, or condensates from the gas phase (soot), are often termed black carbon, and this form of carbon is known to play a major role in the global carbon cycle (18). Recently, Kleineidam et al. (11), Karapanagioti et al. (19), and Ghosh et al. (20) showed that coals, charcoal, and soot or coke play an important role in the sorptive uptake of organic pollutants such as phenanthrene in sedimentary rocks, fluvial sands, soils and sediments, respectively. Ru ¨ gner et al. (21) showed that strong and nonlinear sorption in these carbonaceous particles is likely due to the presence of micropores (pore diameter
