Journal of Environmental Quality 30:1618-1623 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Organic Compounds in the Environment
Prediction of Soil Organic Partition Coefficients by a Soil Leaching Column Chromatographic Method
Feng Xua,
Xinmiao Liang*,a,
Bingcheng Lina,
Fan Sua,
Karl-Werner Schrammb and
Antonius Kettrupb
a Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 161 Zhongshan Road, Dalian 116011, P.R. China
b GSF-National Research Center for Environment and Health, Institute of Ecological Chemistry, Ingolstädter Landstrasse 1, D-85764 Neuherberg, Germany
* Corresponding author (liangxm{at}mail.dlptt.ln.cn)
Received for publication November 22, 2000.
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ABSTRACT
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The soil organic partition coefficient (Koc) is one of the most important parameters to depict the transfer and fate of a chemical in the soil–water system. Predicting Koc by using a chromatographic technique has been developing into a convenient and low-cost method. In this paper, a soil leaching column chromatograpy (SLCC) method employing the soil column packed with reference soil GSE 17201 (obtained from Bayer Landwirtschaftszentrum, Monheim, Germany) and methanol–water eluents was developed to predict the Koc of hydrophobic organic chemicals (HOCs), over a log Koc range of 4.8 orders of magnitude, from their capacity factors. The capacity factor with water as an eluent 
could be obtained by linearly extrapolating capacity factors in methanol–water eluents (k') with various volume fractions of methanol (
). The important effects of solute activity coefficients in water on k'w and Koc were illustrated. Hence, the correlation between log Koc and log k'w (and log k') exists in the soil. The correlation coefficient (r) of the log Koc vs. log k'w correlation for 58 apolar and polar compounds could reach 0.987, while the correlation coefficients of the log Koc–log k' correlations were no less than 0.968, with ranging from 0 to 0.50. The smaller the , the higher the r. Therefore, it is recommended that the eluent of smaller , such as water, be used for accurately estimating Koc. Correspondingly, the r value of the log Koc–log k'w correlation on a reversed-phase Hypersil ODS (Thermo Hypersil, Kleinostheim, Germany) column was less than 0.940 for the same solutes. The SLCC method could provide a more reliable route to predict Koc indirectly from a correlation with k'w than the reversed-phase liquid chromatographic (RPLC) one.
Abbreviations: HOC, hydrophobic organic chemical • RPLC, reversed-phase liquid chromatography • SLCC, soil leaching column chromatography
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INTRODUCTION
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THE soil organic partition coefficient in soil–water systems (Koc) plays a significant role in modeling the sorption behavior of a pollutant in soil and eventually leaching to ground water (OECD, 1996). The term Koc is defined as the concentration ratio of a solute sorbed onto the soil phase and in the aqueous phase at equilibrium state. The batch equilibrium method was a conventional method for Koc measurement, but it would generally take 4 to 96 h or longer to reach equilibrium. So the method is not suitable for rapid screening of lots of compounds at one time. As an alternative, many indirect RPLC methods have been proposed in which liquid chromatographic packing materials, such as octadecyl-, octyl-, cyanopropyl-, and phenyl-silica (Kördel et al., 1995; Kaune et al., 1998) were adopted to determine the capacity factor (k') through a chromatogram and then to estimate Koc from log Koc–log k' correlations. These methods are based on the fact that the compound with a higher Koc value elutes later on some stationary phases than the compound with a lower Koc value. Because of the large discrepancy existing between RPLC packing phases and soil (in the batch method), the Koc estimation is not completely satisfactory, especially for compounds of different classes. Therefore, other packing materials such as immobilized humic acid, which to some degree imitate soil, were proposed (Pussemier et al., 1994).
A column packed with a naturally occurring soil should be the most appropriate material in the log Koc–log k' correlations due to the similar soil–solvent conditions between k' and Koc measurements. A miscible displacement technique through a breakthrough curve has been proposed (Rao et al., 1985; Nkedi-Kizza et al., 1989; Brusseau et al., 1991) for Koc measurement on soil columns with a 25-mm internal diameter, but the technique is still tedious. We developed a soil leaching column chromatographic (SLCC) method (Xu et al., 1999a,b,c), in which a reference soil was dryly packed into a smaller internal diameter column (10 mm), and methanol–water mixtures or pure water were used as eluents. The diluted methanol solution of a solute was injected through an injection valve, and the chromatographic peaks were monitored by an online UV detector. Through measuring the capacity factors of HOCs, we found that the eluents containing methanol could considerably accelerate the eluting or leaching process and thus established a log k'– linear retention equation, by which it is possible to compare the mobilities of HOCs at any methanol fraction.
The purpose of this investigation is to estimate Koc values of HOCs by establishing better log Koc–log k' (and log k'w) correlations in SLCC, to evaluate the influence of on the correlations, and to validate the soil columns as better than the reversed-phase Hypersil ODS columns and the batch method for obtaining Koc.
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THEORETICAL
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As the injection amount of a solute is very small, its concentration in an eluent is always infinitely diluent. Multilayer formation of solute molecules on the soil surface is thus precluded. Based on a displacement adsorption model in which the solute is distributed between a homogeneous interfacial layer and the bulk aqueous phase (Hammers et al., 1982), the capacity factor of a solute on the soil can be described as:
 | [1] |
where subscript 1 denotes solute, and 2 denotes water. The terms n1,sorbed (or n2,sorbed) and n1,water (or n2,water) are the mole numbers of adsorbed solute (or water) and of solute (or water) in the eluent, respectively. The terms x1,sorbed and x1,water are the molar fractions of the solute in the interfacial layer and in the eluent; at a sufficiently small solute amount they are inversely proportional to the corresponding infinite dilution activity coefficients, 
1,interface and 1,water, respectively. Eq. [1] also holds when methanol–water mixtures are used as eluents (Henuion et al., 1981; Locke, 1974).
The soil organic partition coefficient of a solute can be described as:
 | [2] |
where c1,soil (or c1,water) is the solute concentration in the soil (or aqueous) phase, and vsoil (or vwater) refers to the molar volumes of the soil (or aqueous) phase. The activity coefficient in the soil phase is denoted by 1,soil. Combining Eq. [1] and [2] gives:
 | [3] |
As the 1,soil and 1,interface values are usually unknown, Eq. [3] may be presented as a linear equation:
 | [4] |
or
 | [5] |
in which A1, A2, B1, and B2 are regression parameters whose magnitude depends on the temperature, the methanol content in the eluent, and the class of solutes. Log k' is a linear function of the methanol fraction in the eluent as in Eq. [6] (Xu et al., 1999a,b):
 | [6] |
where -S is the slope of the log k' vs. linear relationship.
Hence, log Koc should have better correlations with log k' at different (see Eq. [7]):
 | [7] |
in which A and B are regression parameters. Comparisons of Eq. [3], [4], and [7] show that Eq. [4] holds only when log
is constant or correlates with log k'w.
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MATERIALS AND METHODS
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All solutes were obtained from Aldrich (Milwaukee, WI), Fluka (Buchs, Switzerland), and Sigma (St. Louis, MO) and had the highest available purity. High performance liquid chromatography–grade methanol and Milli-Q water (Millipore, Bedford, MA) were used in all experiments. A reference soil, GSE 17201 (from Bayer Landwirtschaftszentrum, Monheim, Germany), had the following properties: organic carbon, 2.48%; pH, 6.3; cation exchange capacity, 10.0 cmol kg-1; clay, 7.2%; silt, 12.3%; and sand, 80.5%. A stainless steel column (10 mm i.d. x 100 mm) was packed with the above soil with a bulk density of 1.8 g cm-3. The soil column packing technique is described in detail in Xu et al. (1999c). Another column (4.6 mm i.d. x 150 mm) was packed with 3-µm Hypersil ODS. The SLCC experiment was carried out on a conventional high performance liquid chromatographic (HPLC) system (Waters Associates, Milford, MA) consisting of two Waters 510 pumps, a Rheodyne (Berkeley, CA) 7725i injection valve, an AT-130 column heater (Tianjin Autoscience Co., Tianjin, China) thermostated at 25.0 ± 0.1°C, a Waters UV 486 detector set at the maximum wavelength of each compound, and a DL 800 Workstation (Dalian Institute of Chemical Physics, Dalian, China). Various isocratic methanol–water mixtures with methanol volume fractions () from 0.90 to 0 in decrements of 0.10 were used as eluents at a flow rate of 1.0 mL min-1. The capacity factor was calculated according to k' = (tR - t0)/t0, where tR and t0 are the retention time (means of duplicate injections) of the solute investigated and of a nonretained solute (NaNO2), respectively. In general, the capacity factors of replicated measurements of a solute differed by less than 2%. The determined k' values in this study covered the range of 0 to 180. The even larger ones were obtained by using Eq. [6]. For changing the methanol composition, the soil column was first rinsed with 0.90 methanol–water eluent for ca. 30 min; after that approximately 40 mL experimental eluent was delivered. In order to avoid overloading of the column (particularly in the case of strongly retained solutes requiring larger injections), two solutions of a solute at different methanol concentrations (in the ratio 1:10, generally) were injected and the corresponding retention times were compared. When they were different, a new solution was prepared from the less concentrated one (10-time dilution) and the experiment was repeated until constant retention times were obtained.
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RESULTS AND DISCUSSION
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Effects of Activity Coefficients on k' from the Soil Column Method and on Koc
Xu et al. have found that capacity factors of non-electrolytes on a soil column at various methanol fractions () can be described by Eq. [6] with correlation coefficients (r) larger than 0.99, meaning that the log k' linearly decreases with increasing (Xu et al., 1999a). The intercepts (log k'w) and slopes (-S) were obtained by fitting experimental log k' data with (0 
0.90). The log k'w values of 64 solutes investigated are shown in Table 1 together with the log 1,water and log Koc data (by the batch equilibrium method) from the literature.
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Table 1. Comparison of the calculated values of log Koc from soil leaching column chromatography (SLCC) with the values obtained from reversed-phase liquid chromatography (RPLC) on a Hypersil ODS column and the literature values obtained by the batch equilibrium method.
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Table 2. Values of A and B in Eq. [7], log Koc = A + B log k', in GSE 17201 reference soil (or in a Hypersil ODS column)–methanol–water systems. For identification of solute classes, see Table 1.
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The log k'w on the soil column, and the literature log Koc data are both linearly related to log 1,water (see Fig. 1A and 1B), which can be depicted by Eq. [8] and [9]:
 | [8] |
 | [9] |
The above two equations clearly show that log k'w on the soil column and log Koc are affected by the solute activity coefficient at infinite dilution in water, which could be expected by Eq. [1] and [2]. Accordingly, log Koc should also have a close connection with log k'w.
Log Koc versus Log k' Correlation on the Soil Leaching Column Chromatographic Method
The tested solutes include apolar (Classes I–III, see Table 1), low-polar (Classes IV–V), polar (Class VI), and other unrelated polar (Class VII) compounds. Their capacity factors on the soil and on the ODS column at different values were measured. Some results of their correlations with the literature log Koc data were given in Table 2. The k' data at > 0.50 were not listed herein, because a higher methanol amount in the eluent will decrease the log Koc vs. log k' correlations, which will be discussed below.
Inspection of Table 2 shows that the best log Koc–log k' correlation on the soil column is found in apolar and low-polar solutes (Classes I–V) using water as an eluent, with correlation coefficients (r) up to 0.992 and standard deviations (SD) down to 0.128. In polar classes (VI–VII), even though the r is relatively low (0.940), probably due to more complicated interactions for polar solutes in soil–water systems than for apolar solutes, there is still a satisfactory correlation for all solutes in Classes I through VII, with r = 0.987 and SD = 0.149. In Fig. 2, log Koc is plotted against log k'w on the soil column for Solute Classes I through V and I through VII. Although strong polar solutes such as phenols and nitrogen-containing compounds were still included in the whole solute classes (I–VII), the coefficients (A = 1.397 and B = 0.799) of the log Koc vs. log k'w relationship in Classes I through VII are almost the same as those (A = 1.361 and B = 0.805) in Classes I through V.
Because different surrounding conditions exist between conventional Koc (in soil and water) and k'w (in ODS and water) measurements, the log Koc–log k'w correlation on ODS is worse than that on soil. Table 2 shows that the highest r value for the log Koc–log k'w correlation on ODS is only 0.939 for Solute Classes I through V and 0.903 for Solute Classes I through VII, which means that the correlation on ODS is not high enough to be used to predict Koc values of HOCs through their k' values, even for apolar and weakly polar solutes.
Eluent Effects on Log Koc–Log k' Correlations
Inspection of the data presented in Table 2 also suggests a general trend wherein the correlation decreases with increasing . This is due to the influence of methanol in eluents. When water was used as an eluent in SLCC, the equation, log Koc = 0.799 log k'w + 1.397, was adopted to calculate Koc and the results (log Koc(cal)) are listed in Table 1. Only two outliers (|log Koc - log Koc(cal)| 
0.30) are observed (i.e., acetophenone and anisole). The predicted Koc values of other solutes are satisfactorily in accord with the corresponding values from the literature. On ODS, the equation from Table 2, log Koc = 0.800 log k'w + 0.188, was adopted and the predicted values (log Koc(cal)) are also listed in Table 1. However, as many as 24 outliers were found, owing to different solute–solvent–stationary phase interactions between k' and Koc measurements. These can also be seen from the average value 
and the standard deviation (SD) of 
log Koc values (= log Koc - log Koc(cal)) for all solutes in Table 1. For the 58 solutes on the soil, 
= -0.00017 and SD = 0.14; whereas on ODS, = -0.0027 and SD = 0.39. Hence, SLCC approach gives better agreement with literature Koc values than the RPLC one on ODS.
If Koc is estimated from k' values measured at > 0, the number of outliers appears to increase gradually, which could be seen from the slow decrease of the r value with increasing . The reason is elucidated as follows. Solutes from different classes can be classified well by sets of p and q parameters in the relationship between their S and log k'w values (Xu et al., 1999a):
 | [10] |
The fitting p and q values are given in Table 3. Combination of Eq. [5], [6], and [10] yields:
 | [11] |
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Table 3. Fitting values, p and q, in Eq. [10], S = p log k'w + q, for various solute classes in GSE 17201 reference soil–methanol–water systems at 25.0°C.
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The scattering of the data points in log k' vs. log Koc plots is largely due to the last two terms on the right-hand side of Eq. [11], and the different p, q data sets for the various solute classes as shown in Table 3. Therefore, in order to screen the Koc values of many solutes quickly, the methanol–water eluents at higher could be used. In fact, the r values at ranging from 0 to 0.50 on the soil are not less than 0.968. However, if reliable Koc estimation is required for the solutes from different classes, it is recommended that the k' values with smaller (such as water) be used as the eluent.
Soil Leaching Column Chromatographic and Batch Equilibrium Methods
The SLCC method outweighs the batch equilibrium method for Koc determination in three aspects. First, the former could save the expense, obviously. A soil column packed with ca. 14 g of reference soil could last for several months to several years, and the solute amount for each injection onto the column is in the nanogram to microgram level. Second, the SLCC approach is more rapid in Koc determination and especially suitable for the estimation of Koc values for many compounds. In the batch method, multiple variables (i.e., water to soil ratio, equilibrium time, bulk concentration range, extraction and determination of solute sorbed on soil, etc.) should be predetermined through tedious conditional tests. Very few and simple variables in the SLCC experiment will be encountered, such as the methanol volume fraction in the eluent () and UV detection wavelength. The Koc value of a solute might be determined from its easily measured capacity factor by using the log Koc–log k'w (or log k') correlation, which was obtained from the known or published Koc data of model compounds. By using the SLCC method, the typical Koc determination time for a solute is only several to tens of minutes. Because the capacity factors of solutes could be quickly determined on one soil column, it is possible to compare and screen the Koc values of a large number of samples on the same soil. Third, the SLCC approach could be used to determine the Koc values for a wide variety of HOCs. As a chromatographic method, the SLCC is also eligible for Koc determinations for degradable, photosensitive, and strongly hydrophobic chemicals, which are difficult to test by the batch method. Whether the SLCC approach could give reliable Koc values for ionizable compounds still deserves further investigation.
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CONCLUSIONS
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The important effect of solute activity coefficients at infinite dilution solution on Koc and on k'w in SLCC lays the foundation for a general and better log Koc–log k'w correlation for HOCs on various soils (experimental verification of the correlations on two other soil columns will be reported in a forthcoming paper). The extrapolated log k'w values, holding water as an eluent, were obtained through a log k'– linear equation. The correlation coefficient of the log Koc–log k' relationship increases with a decrease of due to the similarity of soil–solvent surroundings between Koc and k' measurements. If one needs to obtain accurate Koc data, the log Koc–log k'w correlation at = 0 is recommended. For indirect determination of Koc values of HOCs from different classes, the SLCC method is expected to be more realistic than methods using RPLC on ODS columns. The log Koc range reaches 4.8 orders of magnitude and may be extended even more broadly.
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ACKNOWLEDGMENTS
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Our special thanks go to Dr. Wenzhong Wu (Institute of Hydrobiology, Chinese Academy of Sciences) and Dr. A. Yediler (Institute of Ecological Chemistry, GSF, Germany) for their generous offers of some chemicals and the soil, and to Mr. Miansheng Bao and Dr. Qing Zhang for their assistance to the experiment.
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ABSTRACT
INTRODUCTION
