Chemosphere 74 (2009) 1062–1068 0045-6535/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.10.054 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere 1. Introduction Sorption of organic pollutants to the soil matrix is the key process that affects their ecotoxicological impact, environmen- tal mobility and rate of degradation. It has long been established that organic matter is the most important sorbent phase for non- ionic compounds. The sorption of non-ionic compounds to organic matter is most commonly described as a partitioning interaction, which can be quantified in terms of K OC , the C-normalized parti- tion coefficient (Chiou, 1989). However, K OC values for a given com- pound often vary considerably between soils (Wauchope et al., 2002), indicating that sorption of nonionic compounds cannot be explained entirely as a simple partitioning between soil solution and a homogeneous organic matter phase. Other non-partition- ing sorption behaviour, such as decreasing affinity with increas- ing sorbate concentration, desorption hysteresis and competitive sorption, provides further evidence against simple partitioning (Allen-King et al., 2002; Wauchope et al., 2002). A number of more sophisticated models have been developed in an attempt to better describe and predict sorption behaviour (Allen-King et al., 2002). Most models include multiple organic matter components with different sorption properties. In such models, variability in K OC between soils is explained by differences in the relative proportions of different organic matter components. The models mostly differ in the proposed composition of the highly sorptive and usually non-partitioning component, which has var- iously been attributed to black carbon (Chiou, 1995; Koelmans et al., 2006), “carbonaceous geosorbents”, including coal and ker- ogen as well as black carbon (Cornelissen et al., 2005), aliphatic domains (Mao et al., 2002; Salloum et al., 2002) and “condensed” (Weber and Huang, 1996) or “glassy” (Xing and Pignatello, 1997) domains. An alternative cause of K OC variability may be that not all organic matter is accessible to sorbate molecules. We recently showed that HF-treatment can increase soil K OC for diuron and phenanthrene by a factor of 2–3, and attributed this to organic matter-mineral inter- actions that block organic matter sorption sites in the whole soils (Ahangar et al., 2008b). A number of other studies are consistent with this interpretation (Celis et al., 2006; Bonin and Simpson, 2007). It is not only interactions with minerals that may block sorption sites on organic matter. Kohl and Rice (1999) reported that removal of soil lipids increased the sorption affinity of three soils for aro- matic sorbates. Lipid extraction was also found to increase sorption non-linearity. A number of similar findings have been reported recently. Chilom et al (2005) found that lipid extraction increased the sorption affinity of PAHs for soils and sediments. Tremblay et al. (2005) reported lipid extraction increased K OC for phenanthrene by up to an order of magnitude for a sediment and its humin frac- tion, and also increased K OC for its humic acid fraction. Drori et al. The effect of lipids on the sorption of diuron and phenanthrene in soils Ahmad Gholamalizadeh Ahangar a,b, * , Ronald J. Smernik a , Rai S. Kookana a,c , David J. Chittleborough a a Soil and Land Systems, School of Earth and Environmental Sciences, The University of Adelaide, Waite Campus, Urrbrae, SA 5064, Australia b Department of Soil Science, College of Agriculture, University of Zabol, Zabol, Iran c CSIRO, Land and Water, PMB 2, Glen Osmond, SA 5064 Australia article info abstract Article history: Received 11 August 2008 Received in revised form 28 October 2008 Accepted 28 October 2008 Available online 6 December 2008 The influence of lipids on the sorption of diuron and phenanthrene to soils was investigated. Accelerated solvent extraction (ASE) was used to extract lipids from twelve soil horizons. Extractable lipids accounted for 3–13% of organic C. The organic carbon-normalized sorption coefficients (K OC ) for diuron and phen- anthrene were consistently higher for the lipid-extracted soils than for the whole soils (average of 31% for diuron and 29% for phenanthrene), indicating that lipids compete for or block sorption sites on the organic matter. Sorption experiments on one pair of HF-treated soils indicated that the blocking effects of minerals and lipids are independent, since lipid extraction and HF-treatment combined increased K OC by more than either treatment alone. Lipids extracted from whole and HF-treated soils were very similar in composition, consisting predominantly of long-chain polymethylene structures. K OC of the lipid itself was lower than for any of the whole soils and soil fractions (lipid extracted and HF-treated) for diuron, but higher for phenanthrene. Solid-state 13 C NMR spectra of the HF-treated soils before and after lipid extrac- tion indicated that 15–20% of alkyl C was removed by ASE and that no other structures were affected. © 2008 Elsevier Ltd. All rights reserved. Keywords: Sorption Diuron Lipid Phenanthrene * Corresponding author. Address: Soil and Land Systems, School of Earth and Environmental Sciences, The University of Adelaide, Waite Campus, Urrbrae, South Australia, 5064, Australia. Tel.: +61 8 8303 6850; fax: +61 8 8303 6511. E-mail address: ahmad.gholamalizadehahangar@student.adelaide.edu.au (A.G. Ahangar).