Desorption of Polycyclic Aromatic Hydrocarbons from Soil in the Presence of Dissolved Organic Matter: Effect of Solution Composition and Aging Ingrid Kogel-Knabner,* Kai U. Totsche, and Bernd Raber ABSTRACT The effect of dissolved organic matter (DOM) on the desorption of polycyclic aromatic hydrocarbons (PAHs) from soil was studied with 14 C-benzo[a]pyrene and 14 C-pyrene in different soil-aqueous so- lution systems and after prolonged periods of aging (110 and 216 d). The desorption of 14 C-benzo[a]pyrene and 14 C-pyrene was strongly affected by the composition and properties of the aqueous phase. Desorption linearly increased with increasing DOM concentrations. Distribution coefficients (log A',,,-') for the desorption of "C-labeled PAHs were about 3.5 (pyrene) and 25 (benzol aJpyrene) times lower in the presence of DOM. The enhancement of PAH desorption between various types of DOM is controlled by the molecular weight distribu- tion of DOM. Dissolved organic matter with a high proportion of high molecular weight components (>14 000 dalton) has a high affinity for PAHs and thus is more effective in desorbing PAHs from soil. In addition, the desorption of PAHs from soil is controlled by the time elapsed since contamination. In the first 36 d following contami- nation with 14 C-benzo[a]pyrene, a decrease of the PAH concentration that could be desorbed in the solution phase was observed. The desorb- able fraction did not show any further changes for up to 183 d after contamination, suggesting a fraction of PAH that is not or very slowly desorbing from soil. The major proportion (>98%) of the desorbable PAH-fraction is apparently bound at sites that show a rate-limited release of PAHs. This is corroborated by the model discrimination conducted with three different models, which favors a two-site equilib- rium-nonequilibrium model. T HE fate of organic contaminants in soils is often estimated by considering the contaminant-soil in- teraction as the primary process (Brusseau and Rao, 1989). More recently, attention has been turned toward the effects of surfactants, cosolvents, and natural organic Ingrid Kogel-Knabner, Lehrstuhl ftir Bodenkunde, Technische Uni- versitat Miinchen, D-85350 Freising-Weihenstephan, Germany; Kai U. Totsche, Abt. Bodenphysik, Universitat Bayreuth, D-95440 Bay- reuth, Germany; and Bernd Raber, Geologisches Landesamt NRW, Postfach 1080, D-47710 Krefeld, Germany. Received 29 Mar. 1999. *Corresponding author (koegel@weihenstephan.de). Published in J. Environ. Qual. 29:906-916 (2000). colloids on the fate and transport of hydrophobic or- ganic chemicals (HOCs) in soils (McCarthy and Za- chara, 1989; Magee et al., 1991; Dunnivant et al., 1992; Totsche et al., 1997). Natural organic matter comprises the most important component in soil solution capable of facilitating the transport of HOCs. Organic molecules have been shown to form microscale hydrophobic envi- ronments in the aqueous phase and to act as a mobile sorbent phase for HOCs (Chiou et al., 1986; McCarthy et al., 1989). Several studies focused on factors controlling the in- teraction between DOM and HOCs in aqueous systems. The extent of HOC-DOM association was found to be enhanced with increasing DOM concentration and the hydrophobicity of the sorbate (Chiou et al., 1986; Mer- kelbach et al., 1993; Maxin and Kogel-Knabner, 1995; Raber and Kogel-Knabner, 1997). The relative amount of hydrophobic sorption sites of dissolved macromole- cules, expressed as average molecular weight composi- tion of DOM, seems to have a decisive effect on the extent of HOC binding as well (Jota and Hassett, 1991; Herbert et al., 1993; Raber and Kogel-Knabner, 1997). Further properties of DOM solutions, such as pH or ionic strength, seem to be of minor importance for the binding of a PAH to humic acids or DOM from soils (Chiou et al., 1986; Jota and Hassett, 1991; Raber et al., 1998). Most of the studies characterizing the interaction between HOCs and DOM have been performed with dissolved organic materials of various origins, such as alkaline-extracted organic acids or commercial humic and fulvic acids (Kogel-Knabner and Totsche, 1998). Previous studies have demonstrated that these com- pounds are not representative for natural DOM in soils. Aqueous-extracted DOM from soil or organic material Abbreviations: DOM, dissolved organic matter; DOC, dissolved or- ganic carbon; PAH, polycyclic aromatic hydrocarbon; HOC, hy- drophobic organic chemical; OC, total organic carbon concentration. Published May, 2000