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Journal of Environmental Quality 31:1972-1979 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Organic Compounds in the Environment

Retention Equations of Nonionic Organic Chemicals in Soil Column Chromatography with Methanol–Water Eluents

Feng Xu, Xinmiao Liang* and Bingcheng Lin

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 161 Zhongshan Road, Dalian 116011, China

* Corresponding author (liangxm{at}mail.dlptt.ln.cn)

Received for publication November 14, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Research efforts dealing with chemical transportation in soils are needed to prevent damage to ground water. Methanol-containing solvents can increase the translocation of nonionic organic chemicals (NOCs). In this study, a general log-linear retention equation, log k' = log k'w - S{phi} (Eq. [1]), was developed to describe the mobilities of NOCs in soil column chromatography (SCC). The term {phi} denotes the volume fraction of methanol in eluent, k' is the capacity factor of a solute at a certain {phi} value, and log k'w and -S are the intercept and slope of the log k' vs. {phi} plot. Two reference soils (GSE 17204 and GSE 17205) were used as packing materials, and were eluted by isocratic methanol–water mixtures. A model of linear solvation energy relationships (LSER) was applied to analyze the k' from molecular interactions. The most important factor determining the transportation was found to be the solute hydrophobic partition in soils, and the second-most important factor was the solute hydrogen-bond basicity (hydrogen-bond accepting ability), while the less important factor was the solute dipolarity–polarizability. The solute hydrogen-bond acidity (hydrogen-bond donating ability) was statistically unimportant and deletable. From the LSER model, one could also obtain Eq. [1]. The experimental k' data of 121 NOCs can be accurately explained by Eq. [1]. The equation is promising to estimate the solute mobility in pure water by extrapolating from lower-capacity factors obtained in methanol–water mixed eluents.

Abbreviations: LSER, linear solvation energy relationships • NOC, nonionic organic chemical • RPLC, reversed-phase liquid chromatography • SCC, soil column chromatography


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
GROUND WATER is a major drinking source for many citizens, and its contamination by nonionic organic chemicals (NOCs) has become a worldwide concern since some chemicals were discovered in ground water (Jury et al., 1987; Teso et al., 1996). Research about factors affecting the transportation of NOCs in soils is needed to prevent further contamination of ground water (Line et al., 1998; Fava et al., 2000). However, it is costly and tedious to use a conventional lysimeter facility (Suzuki et al., 1998) to measure the leaching potential of a chemical in a soil–water system. In addition, the results are often dependent on the complicated experimental conditions, difficult to compare with each other (Byers et al., 1995; Gawlik et al., 2000). Another common technique is the column method (Farran and Chentouf, 2000), in which extraneous material (e.g., quartz) is needed to mix with soil (as a packing material), so as to increase the permeability of the column. The aqueous solution of the solute under investigation continuously flows through the column. The eluent should be collected, off-line concentrated, and determined by a chromatographic system. Such a complicated procedure restricts the speed of determination. Moreover, for many NOCs, the time of retardation with water as an eluent is long. An alternative is to use methanol–water mixed solvents. Rao et al. (1985) found that the soil sorption coefficients (Kd) of NOCs decreased exponentially with increasing the volume fractions of methanol ({phi}) in the bulk solution. Using the methanol–water mixtures, the Kd measuring procedure could be considerably speeded up due to the increase of chemical solubility (Rao et al., 1985; Nkedi-Kizza et al., 1985). Nkedi-Kizza et al. (1985) found that the methanol–water solvents did not alter the amount of soil organic matter. Brusseau et al. (1991) reported that methanol did not influence the soil sorptive property even when the soil was immersed in a {phi} = 1 methanol–water mixture. Although their studies were limited to only several solutes and soil and methanol–water combinations, it was promising that methanol hardly affected the sorptive property of soil. These basic facts stimulated us to investigate in detail the influence of methanol on the retention behavior of soil columns, as described below.

Xu et al. (1999) developed a soil column chromatographic (SCC) method using naturally occurring soils as packing materials in liquid chromatographic columns and methanol–water mixtures (or water) as eluents, to study the sorption–leaching process of NOCs in soils. In this method, chromatograms were online recorded by UV detection. The capacity factor (k') was used, instead of the retention time, to characterize the mobility of the chemical, because the k' remained constant when experimental conditions, such as column length and flow rate, were changed. By adjusting the methanol volume fraction ({phi}) in the eluent, the migration time of a chemical was reduced to a value much smaller than the one in water eluent. As a consequence, the lab-scale screening of leaching ability of a group of herbicides was feasible (Xu et al., 1999). Furthermore, based on the existence of good correlations between the soil organic partition coefficient (Koc) and k' (measured in SCC) at different {phi} values, the Koc values of solutes could be easily obtained from k' values (Xu et al., 2001). The best Koc vs. k' correlation occurs in pure water eluent ({phi} = 0), due to similar soil–water media for Koc and k' determinations (Xu et al., 2001). Unfortunately, it is still not easy to measure the capacity factor (k'w) in pure water, especially for highly hydrophobic compounds that exhibit long migration time. In this case, the common method is to determine the k' in mixed eluents first, and then extrapolate to k'w (in water). Therefore, it is important to find an equation capable of describing the relationship between solute retention and methanol fraction.

In conventional reversed-phase liquid chromatography (RPLC), Eq. [1] is a powerful equation for describing the retention of an NOC in for example a C18 column and methanol–water systems, and the hydrophobic partition mechanism dominates the retention (Jinno and Kawasaki, 1984; Chen et al., 1995):

[1]
where -S is the slope of the linear log k' vs. {phi} plot, decided by the applied stationary phase, eluent, solute, and temperature, and log k'w is the intercept of the plot, that is, the log k' at {phi} = 0.

The linear solvation energy relationships (LSER) were widely used to examine the retention properties in RPLC from the point of view of molecular interactions (Sadek et al., 1985; Carr et al., 1986). In the model equation (Eq. [2]), P0 denotes the intercept, and the coefficients (m, s, b, and a) denote contributions of the solute properties of intrinsic van der Waals volume (VI/100), dipolarity–polarizability ({pi}*), hydrogen-bond basicity (ßm), and hydrogen-bond acidity ({alpha}m), respectively. Therefore, the cavity term (or hydrophobic partition term, mVI/100) measures the process of squeezing the solvent molecules to provide a suitable enclosure for the solute, the dipolarity–polarizability term (s{pi}*) measures the interactions of solute–solvent dipole–dipole and dipole-induced dipole, and hydrogen bonding terms measure the effects involving the solute as hydrogen-bond acceptor (HBA base) (bßm) and the solvent as hydrogen-bond donor (HBD acid), and the solute as HBD acid (a{alpha}m) and the solvent as HBA base:

[2]

In conventional RPLC, the leading and master terms affecting k' are the solute size (VI/100) and the ability of solute to accept a hydrogen bond (ßm). The contributions of the dipolarity–polarizability term ({pi}*) and the ability of solute to donate a hydrogen bond ({alpha}m) are small and negligible (Sadek et al., 1985).

The objectives of the present study are to (i) investigate in detail the influence of methanol–water mixed eluents on the retention properties over a soil column, (ii) employ the LSER model to analyze the retention in SCC with different methanol fractions, and (iii) evaluate the usefulness of Eq. [1] for describing NOC retention in SCC.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Chemicals
Carbamates and phenylureas were synthesized in our institute (Yang and Lu, 1999). Other NOCs were obtained from Aldrich (Milwaukee, WI), Bayer AG (Bayer Landwirschaftszentrum, Monheim, Germany), Fluka (Buchs, Switzerland), and Sigma (St. Louis, MO). They were of the highest purity available, and were checked by RPLC on C18 to find no impurity peaks existing. Methanol (Shangdong Yuwang Chemical Factory, Jinan, China) was chromatographic grade. Water was Milli-Q pure (Millipore, Bedford, MA).

Two reference soils were obtained from Bayer Landwirtschaftszentrum with following principal properties: GSE 17204 soil consisting of 2.82% organic carbon, pH 6.6 (in 0.01 M CaCl2), 0.09 mol kg-1 cation exchange capacity, 9.5% clay, 12.4% silt, and 78.1% sand; and GSE 17205 soil consisting of 0.86% organic carbon, pH 5.8 (in 0.01 M CaCl2), 0.03 mol kg-1 cation exchange capacity, 1.4% clay, 9.1% silt, and 89.5% sand.

Equipment
Retention measurement was performed at a Waters high performance liquid chromatographic system (Waters Associates, Milford, MA), consisting of two 515 pumps, a Rheodyne (Berkeley, CA) 7725i injection valve with a 200-µL loop, a UV 2487 detector, an AT-130 column heater (Tianjin Autoscience Co., Tianjin, China), three homemade stainless steel SCC columns, and data acquisition software (DL 800; Dalian Institute of Chemical Physics, Dalian, China). The tube connecting injection valve and detector was so short that the extra-column volume was negligible.

Preparation of Soil Columns
Each of the two soils (approximately 14.2 g) was divided into 11 portions, incrementally and tightly packed into the SCC column (100-mm length x 10-mm internal diameter) at the same packing height (9 mm) with a homemade pressurizing device. Both column ends contained a piece of nylon membrane (Millipore) of 0.45-µm pore size and a stainless steel flow-rate allocation disc, respectively. The column bulk density reached 1.8 g cm-3. After preconditioning by 80 mL of ({phi} = 0.90) methanol–water mixture at a flow rate of 1 mL min-1, the column was used in the following SCC experiment or stored for future use with ({phi} = 0.90) methanol–water mixture filled in and both ends sealed tightly.

Soil Column Chromatography Experiment
Isocratic methanol–water mixtures with the methanol volume fraction ({phi}) ranging from 0.90 to 0 in decrements of 0.05 were used as eluents. The flow rate was 1.0 mL min-1, and the column pressure was generally less than 300 kg cm-2. The column temperature was controlled at 30 ± 0.1°C. At a certain {phi} value, the hold-up time of 1{per thousand} w/v NaNO2 solution (detected at 220 nm) and the retention time of each chemical (detected at its maximum wavelength) were determined one by one at the apex of respective chromatographic peaks. After all determinations at the eluent of that {phi} value were fulfilled, the column was reconditioned by {phi} = 0.90 methanol–water eluent for 30 min, and then flushed with about 40 mL of eluent at the next {phi} value prior to the subsequent determination. The retention time (tR) and the hold-up time (t0) were originated from the mean value of duplicate determinations. The capacity factor (k') of each solute was calculated by the equation k' = (tR - t0)/t0. Generally, the k' values of replicated measurements differed by less than 3%. The directly determined k' ranged from 0 to 180. Much larger k' values were obtained by using Eq. [1]. The maximum concentration of solutes was 0.1 g mL-1 in {phi} = 0.90 methanol–water solution. To avoid overloading of the column, two solutions of a solute at the concentration ratio of 1:10 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 repeated until constant retention time was obtained.

Experiment with a Column Exposed to a Methanol–Water Eluent
A newly packed GSE 17204 soil column, which was never preconditioned by methanol-containing eluent, was applied to measure the capacity factors [denoted as k'w] of NOCs in water eluent first. Then, the column was conditioned by 80 mL of ({phi} = 0.90) methanol–water eluent at 1 mL min-1. Successively, the capacity factors at {phi} = 0.90 to 0.10 were measured so as to obtain the capacity factor (k'w) at {phi} = 0 by using Eq. [1]. A subset (n = 84) of solutes with log k'w < 2.5 were used, because direct measurement of k'w was feasible. Other solutes with log k'w > 2.5 were not chosen, because the retention was too long to be determined directly. Finally, the k'w data were compared with k'w data.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Influence of Methanol on Column Behavior
Figure 1 is a comparison of the log k'w values extrapolated from Eq. [1] with the log k'w values directly measured in pure water on an unpreconditioned GSE 17204 soil column. The equation, log k'w = (1.23 ± 0.01) x log k'w, with a correlation coefficient (r) of 0.989, was obtained for 84 solutes whose log k'w values were <2.5. The result that log k'w is 23% larger than log k'w is a little different from the results of Brusseau et al. (1991) and Bouchard (1998), who reported that methanol did not affect the sorptive property of soils. The organic matter in soil is a three-dimensional polymer chain perforated into "voids" (Chiou et al., 1983; Nkedi-Kizza et al., 1989). While the polymer exists in a condensed state in water, it expands in methanol–water mixtures (Freeman and Cheung, 1981). It is speculated that soil organic matter may sorb methanol to render more organic matter available for sorption, resulting in k'w values a little larger than k'w. Nevertheless, the influence of methanol is relatively small, and it might be quantified by the above k'w vs. k'w correlation. Thus, we used the preconditioned column in subsequent experiments. The preconditioned column could endure the subsequent change of methanol fraction ({phi}) from 0.90 to 0, and the system pressure and the solute capacity factor remained nearly constant when the eluent returned to its previous {phi} value.



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  		Fig. 1. Relationship between the k'w values extrapolated from the log-linear equation (Eq. [1]) and the k'w(direct) values directly measured in a water eluent on an unpreconditioned GSE 17204 soil column at 30°C.

 
Linear Solvation Energy Relationship Analyses of Capacity Factors in Various Methanol–Water Mixtures
The capacity factors (k') of 58 NOCs with wide hydrophobicity were adopted on the GSE 17204–filled column in the eluents of {phi} = 0.15 to 0.60. The eluents at {phi} > 0.60 and {phi} < 0.15 were not chosen, because the k' values (at {phi} > 0.60) for some solutes were too small to be determined accurately, or too large (log k' > 2.5 at {phi} < 0.15) to be determined quickly. The solvatochromic parameters of the NOCs were collected from the literature (Abraham et al., 1982; Kamlet et al., 1987), and were assembled in Table 1, together with a group of k' values obtained at {phi} = 0.50. These solutes were chosen because their solvatochromic parameters have been well tested in RPLC in the literature. Other solutes, whose solvatochromic parameters might be derived from empirical equations but needed for further verification, were not adopted in the LSER analyses.


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  		Table 1. Solvatochromic parameters and capacity factors obtained at the {phi} = 0.50 methanol–water mixture over GSE 17204 soil.{dagger}

 
Application of the LSER model to k' was performed by multifactor least-squares regressions to Eq. [2]. The results are summarized in Table 2, and the following conclusions could be drawn.


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  		Table 2. Coefficients of the equation, log k' = XYZ0 + mVI/100 + s{pi}* + bßm + a{alpha}m, on the GSE 17204 soil column at various methanol fractions ({phi}).

 
First, all regressions are satisfactory with r ranging from 0.980 to 0.990, and the standard deviations (SD) of log k' ranging from 0.11 to 0.24. Hence it is promising to predict k' at a certain {phi} via the corresponding correlation and the solvatochromic parameters of NOCs. The comparison of the predicted k' values with the directly measured k' values at {phi} = 0.50 shows satisfactory results (see Fig. 2 and Eq. [3]):

[3]



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  		Fig. 2. Relationship between the predicted k' values by the linear solvation energy relationships (LSER) model and the experimentally measured k' values. Column: GSE 17204 soil; eluent: {phi} = 0.50 methanol–water mixture; flow rate: 1 mL min-1; temperature: 30°C.

 
Second, regarding the results in Table 2, variations in VI and ßm from solute to solute cause much more contribution to k' than variations in {alpha}m and {pi}*. The coefficient s is statistically significant at the 95% confidence level by a Student's t test, but it causes only a small change in k'. The coefficient a closes to zero, and is not statistically significant at the 95% confidence level. If the contribution of the term VI/100 is set as the basis, the relative contributions of the second-most important term ßm at {phi} = 0.15 to 0.60 are -b/m = 0.63 to 0.74, while the relative contributions of the least important coefficient {pi}* are only s/m = 0.13 to 0.24. Hence, the two most important factors of a solute are those that measure the cavity formation (mVI/100) and the hydrogen-bond accepting ability (bßm), while the effect of solute dipolarity–polarizability (s{pi}*) is small.

Third, the sign of the coefficient m shows that increasing the solute size—everything else being held constant—will extend its residing over the soil phase. The sign of the coefficient b indicates that increasing the solute hydrogen-bond accepting ability will reduce its retention on the soil. The sign of the coefficient s manifests that raising {pi}* may increase the interaction between the solute and the polar organic matter in soil and increase the solute retention a little, but the influence is relatively small. These results give a clear overview of the retention mechanism in SCC that solute hydrophobicity is the dominant factor in determining the retention, and the solute hydrogen-bond accepting ability is the next-most important factor.

Fourth, in all instances, the coefficients m and -b linearly decrease as {phi} increases:

[4a]

[4b]

It is known that the coefficients m and b are defined as m = M({delta}21 - {delta}2s) and b = B({alpha}s - {alpha}1), where M and B are constants, {delta} is the solubility parameter, {alpha} is the hydrogen-bond acidity, and subscripts 1 and s denote eluent and stationary phase, respectively (Kamlet et al., 1988). Among the solvents commonly used in liquid chromatography, water has the highest cohesive energy ({delta}1 = 23.4) and HBD acidity ({alpha}1 = 1.17), while methanol has moderate cohesive energy ({delta}1 = 14.3) and HBD acidity ({alpha}1 = 0.93) (Abraham et al., 1982; Carr, 1993). With increasing {phi} in the eluent, the cohesiveness of the eluent decreases, while the cohesive energy of the soil phase increases, so m value will decrease. At the same time, with increasing {phi}, the HBD acidity in the soil phase increases, while the HBD acidity in the eluent slightly alters, so the -b value will decrease. The considerations show that the trends of Eq. [4a] and [4b] are reasonable.

As the HBD acidity is not statistically important and can be disregarded from Eq. [2], and the dipolarity–polarizability term is less important and changes slightly while changing {phi}, substituting Eq. [4a] and [4b] in Eq. [2] and combining the s{pi}* term with the intercept (P0) yields Eq. [5]:

[5]

The equation depicts that the capacity factor log-linearly decreases with increasing methanol fraction in the eluent. The result is consistent with the retention equation (Eq. [1]) in conventional RPLC.

Retention Equation in Soil Column Chromatography
The capacity factors of all 121 solutes with a wide range of hydrophobicity were measured over GSE 17204 and GSE 17205 soils at {phi} varying from 0.90 to 0. Some representative results are shown in Fig. 3 . A linear relationship between log k' and {phi} can be seen on both columns. All experimental data were nicely fitted to Eq. [1], and the parameters (log k'w and S) are listed in Table 3, yielding r > 0.99 for all the NOCs studied. The results demonstrate the validity of Eq. [1] in SCC.



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  		Fig. 3. Fits of the capacity factors (k') vs. methanol volume fractions ({phi}) according to Eq. [1] in 30°C methanol–water mixed eluents over (A) GSE 17204 soil and (B) GSE 17205 soil.

 

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  		Table 3. Fitting parameters of the retention equation, log k' = log k'w - S{phi}, for nonionic organic chemicals over the GSE 17204 and GSE 17205 soil columns eluted by methanol–water mixtures (methanol volume fraction {phi} from 0.90 to 0) at 30°C.{dagger}

 
As seen from Fig. 3 and Table 3, larger intercepts (log k'w) are observed in GSE 17204 soil, which has a higher amount of organic carbon (2.82%), while smaller intercepts (log k'w) are observed in GSE 170205 soil, which has a lower amount of organic carbon (0.86%). These results are reasonable, because the soil organic carbon amount is known to be the dominant factor in soil affecting the sorption and transportation of lipophilic compounds (Chiou et al., 1983).

In general, the mobility sequence of solutes can be discerned by the k' difference on one soil (Xu et al., 1999). The k' can also be adjusted by changing the methanol fraction in the eluents. For example, the retention of log k'w > 2.5 is too large to be measured directly, but when {phi} increases to 0.60, almost all log k' values could be reduced to a degree less than 2.5. Hence, the mobility sequence of chemicals could be compared from either k' values in an eluent with elevated {phi} value, or the extrapolated k'w values. Furthermore, the coefficients of Eq. [1] can be simply decided even with the retention data at two {phi} points. Subsequently, the log k' data at any other {phi} can be estimated via Eq. [1]. As a chromatographic platform, the SCC technique may benefit from other RPLC theories due to the existence of similar retention rules between SCC and RPLC.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The use of methanol–water mixed eluents reduced the solute retention considerably. Methanol itself might increase the retention a little, but the increment can be calculated by an empirical correlation to the retention on a column not preconditioned. From the LSER analyses we found that solute variations in size and hydrogen-bond basicity cause higher contributions for the solute retention on soil than the variations in dipolarity–polarizability and hydrogen-bond acidity. The solute retention in SCC is mainly dominated by hydrophobic partition, and second by solute hydrogen-bond basicity. Increasing solute size enlarges its affinity for the soil phase; decreasing solute hydrogen-bond basicity also increases its residing on the soil phase. Based on these findings, a log-linear retention equation was obtained. The equation was verified by the retention of 121 NOCs over two soil columns eluted by a wide range of methanol–water mixtures ({phi} = 0 to 0.90) with high regression coefficients (r > 0.99). The method is potentially applicable for estimating the leaching potential of an NOC in soil–water systems, and also helpful for fast screening of a large number of NOCs in their environmental applications.


   ACKNOWLEDGMENTS
 
We thank Fan Su, Miansheng Bao, and Qing Zhang for invaluable experiment assistance, and Ying Yang for kind donation of some synthesized chemicals.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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