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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Chemosphere 86 (2012) 634–640 Contents lists available at SciVerse ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere Linear and non-linear relationships between soil sorption and hydrophobicity: Model, validation and influencing factors ⇑ Yang Wen, Li M. Su, Wei C. Qin, Ling Fu, Jia He, Yuan H. Zhao Key Laboratory for Wetland Ecology and Vegetation Restoration of National Environmental Protection, Department of Environmental Sciences, Northeast Normal University, Changchun, Jilin 130024, PR China article info abstract Article history: The hydrophobic parameter represented by the octanol/water partition coefficient (logP) is commonly Received 5 July 2011 used to predict the soil sorption coefficient (Koc). However, a simple non-linear relationship between log- Received in revised form 1 November 2011 Koc and logP has not been reported in the literature. In the present paper, soil sorption data for 701 com- Accepted 1 November 2011 pounds was investigated. The results show that logK is linearly related to logP for compounds with logP Available online 9 December 2011 oc in the range of 0.5–7.5 and non-linearly related to logP for the compounds in a wide range of logP. A non- linear model has been developed between logKoc and logP for a wide range of compounds in the training Keywords: set. This model was validated in terms of average error (AE), average absolute error (AAE) and root-mean Sorption squared error (RMSE) by using an external test set with 107 compounds. Nearly the same predictive Partition coefficient Validation capacity was observed in comparison with existing models. However, this non-linear model is simple, Soil/sediment and uses only one parameter. The best model developed in this paper is a non-linear model with six cor- Hydrophobicity rection factors for six specific classes of compounds. This model can well predict logKoc for 701 diverse compounds with AAE = 0.37. The reasons for systemic deviations in these groups may be attributed to the difference of sorption mechanism for hydrophilic/polar compounds, low solubility for highly hydrophobic compounds, hydrolysis of esters in solution, volatilization for volatile compounds and highly experimental errors for compounds with extremely high or low sorption coefficients. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction which is constant for each chemical, when foc > 0.001. In situations where sorbents have low organic carbon, high clay contents, and Sorption processes play a major role in determining the envi- for chemicals that have highly polar or ionisable functional groups ronmental transport and fate of organic chemicals. This is because that may significantly interact with polar or charged sites on sor- the transport and fate of a chemical can be dependent on how the bent surfaces, the Koc approach may not be suitable (Bintein and chemical distributes between the solid and aqueous phases. The Devillers, 1994; Doucette, 2003). Selection of appropriate soil to most frequently used parameter to indicate the soil mobility of a solution ratios for sorption studies depends on the distribution chemical is the soil sorption distribution coefficient, Kd, defined coefficient Kd and the relative degree of sorption desired. For max- as the concentration ratio of solute in aqueous and solid phases imum precision in adsorption experiments, it is preferable to ad- at equilibrium. The Kd is usually normalized to the organic carbon just the sorbent concentration so that the percent removed is in content foc yielding Koc = Kd/foc. It reflects the fact that soil organic the 20–80% range, preferably around 50%, at equilibrium (Delle carbon is the major sorption domain for neutral organic com- Site, 2001). Outside this range, relative measurement errors can pounds (Schüürmann et al., 2006). become a dominant factor. To obtain the maximum precision, The sorption of an organic chemical on a natural solid is a very 1:1–1:100 soil/solution ratios were recommended for the com- complicated process, which involves both solute and sorbent prop- pounds with low and high sorption coefficients (OECD Guideline, erties. Organic carbon content foc is an important factor affecting 2001). However, these ranges of sorbent concentration can give sorption process. Low organic carbon content in the sorbent can re- solids effect. A variety of studies have suggested that solid concen- sult in hydrophilic sorption, rather than hydrophobic sorption for tration (or soil/solution ratio) can significantly affect compound hydrophobic compounds, especially for highly polar compounds sorption coefficients (Zhao and Lang, 1996). This effect appears to (Karickhoff, 1984). The soil/water sorption coefficient (Kd) can be result from the presence of suspended particles and dissolved or- normalized to give an organic-carbon partition coefficient (Koc), ganic matter contributed by the solids, which were not removed from suspension in the separation procedure. It increases the ⇑ Corresponding author. Tel.: +86 431 85099550; fax: +86 431 85955338. amount of solute in the aqueous phase and makes observed sorp- E-mail addresses: [email protected], [email protected] (Y.H. Zhao). tion coefficients decrease, with the effect being greater for more 0045-6535/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2011.11.001 Author's personal copy Y. Wen et al. / Chemosphere 86 (2012) 634–640 635 hydrophobic compounds. In general, soil pH changes have minor between logKoc and logP has not been reported in the literature. effects on the sorption of non-ionic molecules. On the other hand, In the present paper, a soil sorption data base for 701 compounds for ionic compounds, the sorption coefficient is quite sensitive to previously reported by Schüürmann et al. (2006) and Gramatica the pH of the sorbing soil because of different sorption contribu- et al. (2007) was investigated. The aim of the work is to develop tions from ionic and non-ionic species (Franco et al., 2009). The col- a general global model and class-specific models between logKoc loidal surfaces of most natural soils are negatively charged and and logP for a wide range of compounds. The robustness of the therefore have an affinity for positively charged molecules, but developed models will be tested by external validation and model not much affinity for negatively charged molecules (Bintein and quality of the fit will be compared with the MCI and logP based Devillers, 1994). Anionic species (from acids) have quite low sorp- models with correction factors from the KOCWIN program (Epi tion coefficients because they are repelled by the negative net Suite package, version 4.0) (http://www.epa.gov/oppt/exposure/ charge of soil surface, while cationic species (from bases) are quite pubs/episuitedl.htm). At the same time, the factors that influencing strongly sorbed (Wauchope et al., 2002). Not only the organic car- Koc, such as sorption mechanism, solubility, equilibrium time, ion- bon content, soil to solution ratio, sorbent concentration and pH ization, transformation and soil/solution ratio will be discussed described above can affect the sorption coefficient, but tempera- basing on the residuals (observed – predicted) for the well classi- ture, ionic strength, degradation, and presence of surfactants, co- fied compounds in this paper. solutes and co-solvents can also make a significant impact. More detailed reviews of factors influencing sorption have been presented by many investigators (Pignatello and Xing, 1996; Cousins 2. Materials and methods et al., 1999; Seth et al., 1999; Delle Site, 2001; Wauchope et al., 2002; Doucette, 2003). 2.1. Measured Koc data Many of the reported quantitative structure–activity relation- The experimental data of soil sorption coefficients (K ), nor- ships (QSARs) for predicting Koc are based on the relationship be- oc malized on organic carbon content, of 701 organic compounds tween Koc and determined or calculated molecular parameters. The hydrophobic parameters, octanol/water partition coefficient were taken from the literature (Schüürmann et al., 2006; Gramat- (logP) or solubility (logS), are commonly used to predict the soil ica et al., 2007) and compiled into a single database. Schüürmann sorption coefficient. These two physico-chemical properties are et al. (2006) presented 571 organic chemicals and Gramatica highly inter-correlated and consequently these two QSAR models et al. (2007) presented 643 organic chemicals. The total number can be considered as identical or parallel (Sabljic´ et al., 1995). A re- of the compounds reported in the two data sets is 701. There are view of the existing literature revealed numerous examples of log- 513 overlapping compounds in the two data sets. Some of the overlapping compounds were taken from the same source

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