Environ. Sci. Technol. 2007, 41, 3172-3178
Environmental Factors Influencing Sorption of Heterocyclic Aromatic Compounds to Soil ERPING BI, TORSTEN C. SCHMIDT,* AND STEFAN B. HADERLEIN Chair of Environmental Mineralogy, Centre for Applied Geoscience, University of Tuebingen, Sigwartstrasse 10, D-72076 Tuebingen, Germany
Heterocyclic organic compounds containing nitrogen, sulfur, or oxygen (NSOs) are an important class of groundwater contaminants related to the production and use of manufactured gas, heavy oils, and coal tar. Surprisingly little is known about the processes that control sorption and transport of NSOs in the subsurface. In this study, the effects of various environmental factors including temperature, ionic strength, and dissolved/sorbed ion composition on the sorption of NSOs have been investigated by means of a soil column chromatography approach. For the investigated compounds, increased temperature normally decreases their sorption to soil. The enthalpy change of the sorption process corroborates earlier findings that van der Waals forces dominate the sorption of S- and O-heterocyclic compounds such as thiophene, benzothiophene, benzofuran, and 2-methylbenzofuran. Ionic strength and ion composition (Ca2+ vs K+ at given ionic strength) of the aqueous phase show no significant effects on the sorption of these compounds. Previous studies demonstrated that for N-heterocyclic compounds, cation exchange and surface complex formation rather than partitioning into soil organic matter control their overall sorption. In contrast to S- and O-heterocyclic compounds, increasing ionic strength reduced the sorption of ionizable N-heterocyclic compounds (pyridine, 2-methylpyridine, quinoline, 2-methylquinoline, and isoquinoline), due to increased electrostatic competition by cations. At given ionic strength, an increase of the K+/Ca2+ ratio in the mobile phase enhanced the sorption of N-heterocyclic compounds, consistent with cation exchange of the protonated organic species as the dominating sorption process. Among the investigated N-heterocyclic compounds sorption of benzotriazole showed a peculiar feature in that ternary surface complexation with Ca2+ appears to be the dominant sorption mechanism.
Introduction Heterocyclic aromatic compounds containing nitrogen, sulfur, or oxygen (NSOs) are often found in groundwater (1, 2), especially at sites contaminated by coal tar. Accurate prediction of NSOs’ leaching potential and contamination of groundwater requires a sound knowledge of the influence of various factors on sorption of these compounds (3-9). * Corresponding author phone. +49-203-379-3311/-3308; fax +49203-379-2108; e-mail:
[email protected]; present address: University Duisburg-Essen, Chair of Instrumental Analysis, Lotharstr. 1, MF147, D-47048 Duisburg. 3172
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Published sorption studies on NSOs have been carried out mainly on quinoline and pyridine (10-20). Enhanced sorption of pyridine and quinoline was found in acidic soils (12), and cation exchange was suggested as major sorption mechanism (13-17, 20). A comprehensive study on sorption of NSOs to a set of different soils indicated that sorption of non-ionizable S- and O-heterocyclic compounds is correlated with soil organic carbon content while sorption of ionizable N-heterocyclic compounds is dominated by cation exchange and/or surface complexation (21). The fraction of these compounds present in cationic form and thus the solution pH has been identified as a key parameter determining their sorption to various types of soils. However, the effect of temperature, ionic strength, and ion composition on the sorption of NSOs has not been systematically studied. The effect of temperature on sorption is determined by the enthalpy change (∆H) for this process. Sorption enthalpies for partitioning of nonpolar organic compounds into soil organic matter are rather small (absolute values typically below 8 kJ/mol), thus partitioning processes can be expected to be quite insensitive to temperature (22). For compounds that may undergo specific interactions with surfaces, sorption enthalpies can show higher absolute values. Thus, studies on temperature dependence of sorption may help to characterize the sorption process at given conditions. In some situations, sorption enthalpies may vary significantly with concentration due to changing sorption mechanisms at different concentration ranges. For example, the sorption enthalpy of diuron to soils was found to be -50 to -60 kJ/ mol at very low aqueous concentration, but changed to -20 to 0 kJ/mol for high concentrations where partitioning was dominant (23). Thus, it is important to determine and compare either isosteric heats of sorption (i.e., for a fixed amount of sorbed solute) or ∆H values that are representative for a certain concentration range. For partitioning of apolar and weakly polar compounds into particulate organic matter, the effects of pH and ionic strength are usually negligible (22). For polar compounds, there is no consistent picture regarding the influence of ionic strength on sorption. The sorption of some herbicides (bromacil, simazine, norflurazon, and diuron) to soil increased with increasing CaCl2 concentration from 0.01 to 1.0 M (4). In contrast, the ionic strength of the soil solution had little or no effect on the sorption/transport characteristics of some other pesticides (atrazine, isoproturon, paraquat, and 2,4-D) (24). NSOs’ adsorption to soil clays can be substantially influenced by the exchangeable cations present. Among the common exchangeable base cations in soils (Ca2+, Mg2+, K+, and Na+), K+-saturated clays frequently demonstrate the strongest affinity for pesticides (25). The specific sorption of nitroaromatic compounds (NACs) to siloxane sites at mineral surfaces, particularly in homoionic clays, is suppressed by the presence of strongly hydrated cations (e.g., Li+, Na+, Mg2+, Ca2+, A13+). For more weakly hydrated cations (e.g., NH4+, K+), the distribution coefficient, Kd, of a given NAC, increases with decreasing free energy of hydration of the cation (6, 7). It was found that divalent cations (Sr2+, Ca2+, and Mg2+) enhance sorption of xanthan (an anionic polysaccharide) to a greater degree than monovalent cations (K+, Na+, and Li+) at the same ionic strength, by means of electrostatic interaction (26). In this study, the effect of temperature (5-40 °C), ionic strength (0-0.05 M CaCl2), and ion type (CaCl2 and KCl at an ionic strength of 0.015 M) on the sorption of NSOs at, and their transport in, a silty soil was investigated by soil column 10.1021/es0623764 CCC: $37.00
2007 American Chemical Society Published on Web 03/23/2007
TABLE 1. Identities and Properties of the Investigated Organic Compoundsa
a Notes: 1: data from database SRC (http://esc.syrres.com), except logK ow values of DMBF and 5MBT were calculated with the SRC-KowWin (http://www.syrres.com/esc/kowdemo.htm); 2: data from ref (28); 3: data from ref (29); *: pKa is valid for corresponding cation.
chromatography and the experimental results are discussed in terms of potential sorption mechanisms, i.e., sorption by means of van der Waals forces, cation exchange of cationic organic compounds, and surface complexation of neutral organic compounds. The major goals were (i) to infer from the effects of temperature, ionic strength, and cation type information on the predominant sorption mechanism(s), and (ii) to check for crucial environmental factors other than pH that might control the sorption of NSOs in the subsurface. Thus, we want to foster our understanding of NSO sorption by analysis of a comprehensive data set allowing the direct comparison of compound-specific differences in sorption behavior.
Materials and Methods Chemicals and Packing Materials. The following chemicals were obtained from Aldrich: pyridine (PY, 99%), 2-methylpyridine (2MPY, 99%), 1H-benzotriazole (BTA, 99%), quinoline (QUI, 98%), 2-hydroxyquinoline (2QUI, 98%), benzofuran(BF,99%),2-methylbenzofuran(2MBF,96%),1-benzothiophene (BT, 99%), isoquinoline (IQUI, 97%), naphthalene (Naph, 99%), and benzene (Benz, g99%). The following were obtained from Fluka: thiourea (THS, 99%), 2,3-dimethylbenzofuran (DMBF, g95%), quinaldine (2MQUI, 97%); from Acros Organics: pyrrole (PR, 99%), indole (IND, 99%), thiophene (THIO, 99%), toluene (99+%); and from ABCR: 5-methylbenzo[b]thiophene (5MBT, 98%). Toluene, benzene, and naphthalene were taken as reference compounds that would not undergo any specific interactions. Stock solutions were prepared in methanol (g99%, Merck), and all solutions were diluted with MilliQ water and stored at 4 °C. Important compound properties are given in Table 1. Eurosoil 4 was used as sorbent in this research as it exhibits average properties with regard to pH, organic carbon content, clay content, etc., compared with other Eurosoils. Eurosoil 4 (IRMM, Belgium) has a pH of 6.5 in 0.01 M CaCl2 with composition of silt 75.7%, clay 20.3%, and sand 4.1%. The organic carbon content is 1.55%. Cation exchange capacity (CEC) is 175 mmol/kg. The main soil minerals are SiO2 68.63%, Al2O3 12.07%, and Fe2O3 2.71% (27). Quartz (from Merck) was used for soil dilution in order to avoid column clogging. Both soil and quartz were manually ground with pestle and mortar to make the packing material more homogeneous.
TABLE 2. Packed Columns with Reference Materials column
L × i.d. (mm)
packing materialb
OC (%)
E4Q-1 60 × 3 Eurosoil4/quartz 1.03 and E4Q-3 ) 1:0.5 E4ba 53 × 3 Eurosoil4 1.55 a
b
density porosityc (g/cm3)d 0.444
2.69
0.476
2.72 c
Packed by Bischoff. The ratio in the mixture is in weight. Porosity was determined by traveling time of thiourea. d Solid density was estimated from the material composition and density data (quartz 2.65, silt 2.70, and clay 2.80).
Soil Column Chromatography. An HPLC system (Bischoff Analysentechnik und -gera¨te GmbH, Germany) equipped with stainless steel columns (5.3-cm long, 0.3-cm i.d., Bischoff, Germany; 6.0-cm and 25-cm long, 0.3-cm i.d., CS-Chromatography Service GmbH, Germany) packed with Eurosoil 4 was used in this study. Eurosoil 4 was ground to a grain size less than 0.063 mm. In order to avoid clogging, the soil was mixed with ground quartz of similar size before placing into the column. Detailed information on packing and method validation are described elsewhere (30). Column information is shown in Table 2. Dirac input was done by an injectionloop valve (5 µL). Methanol contents in the injected solutions were kept below 2% (v/v) to avoid any cosolvent effects. The experimental designs for investigating different factors are as follows. (1) Temperature: column E4b, flowrate 100 µL/min, 0.005 M CaCl2, four concentrations injected at temperatures of 5, 25, and 40 °C. Concentrations ranged from 2.0 to 351 mg/L but were within 1 order of magnitude for individual compounds. (2) Ionic Strength: columns E4Q-3, and E4Q-1, flowrate 100 µL/min, 25 °C, CaCl2 0.0, 0.005, 0.05 M in mobile phase. Injected concentrations ranged from 130 to 351 mg/L, but for each compound, it was kept constant for the different ionic strengths. (3) Cation Type: column E4Q-1, flowrate 100 µL/min, 25 °C, 0.005 M CaCl2 and 0.015 M KCl in mobile phase, injected concentration for each compound was kept constant for different mobile phase compositions. The injected concentrations for compounds were in a range from 127 to 338 mg/L. VOL. 41, NO. 9, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. van’t Hoff plots for the sorption of target compounds to Eurosoil 4 (Column E4b; at 5, 25, and 40 °C; CaCl2 0.005 M in mobile phase; flowrate 100 µL/min; injection volume 5 µL; injected concentration (mg/L): BT 146, 2MBF 151, 2MPY 30.5, BF 153, PY 31.7, THIO 151, 2MQUI 34.2, IQUI 16.3, QUI 35.3, 2QUI 45.8, IND 9.77, BTA 39.4). Thiourea (THS) was used as the conservative tracer to determine the dead time, to. The half mass point, i.e., the point at which the peak area can be divided into two parts having the same area, was used as mean retention time (t) as discussed in a previous paper (21). The retardation factor (Rf) of a target compound was determined by eq 1. In a small concentration range, isotherms can be approximately considered linear (23). For the studied compounds, the results showed that the concentration range was in or very close to the linear part of the isotherm. Therefore, the distribution coefficient (Kd) can be calculated by eq 2
Rf )
t - t′ to - t′
Rf ) 1 +
Fb ‚K θ d
(1)
(2)
where t′ ) traveling time through injector and connecting capillary between injector and UV detector (in this study, the volume of this part is 15 µL), Fb ) bulk density, and θ ) porosity.
Results and Discussions Dependence of Sorption on Temperature. The influence of temperature on the sorption of NSOs is shown in Figure 1. Sorption coefficients decrease with increasing temperature for all NSOs studied. This indicates that the various sorption processes involved are all exothermic. However, there is very little effect of temperature on the sorption coefficients of PY and 2MPY. According to the van’t Hoff equation, the enthalpy change, ∆H, was calculated from the slope and is shown in Figure 2.
lnKd ) ∆S/R - ∆H/R‚1/T
(3)
where ∆S is entropy change [kJ/(mol‚K)], ∆H is enthalpy change [kJ/mol], R is gas constant [8.3145 J/(mol‚K)], and T is temperature [K]. Kow was used as a descriptor for nonspecific interactions to categorize the ∆H values of NSOs (Figure 2). The enthalpy change (∆H) of all compounds is higher than -25 kJ/mol (the absolute values are less than 25 kJ/mol). This is consistent with the absence of covalent bonding (i.e., chemisorption which generally gives ∆H in the range from -60 to -80 kJ/ mol (31)) and with our observation that sorption is fully 3174
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FIGURE 2. Sorption enthalpies determined with Eurosoil 4 (Column E4b, temperature range 5-40 °C, flowrate 100 µL/min; CaCl2 0.005 M in mobile phase). There were two ∆H values at two different injected concentrations for each compound. The lower -∆H value was obtained from the higher injected concentration. The lower concentration range is from 10 to 45 mg/L, and the higher concentration range is from 150 to 350 mg/L. Group A refers to Sand O- heterocyclic compounds for which van der Waals forces are the main sorption mechanism. Group B comprises N-heterocyclic compounds for which specific interactions play an important role in sorption. Group C comprises compounds for which temperature shows little effect on their sorption. reversible. Three clusters of compounds can be observed in Figure 2. The ∆H of compounds in group A is in the range from -6.0 to -11.0 kJ/mol. This indicates that van der Waals forces (normally corresponding to a ∆H from -4 to -8 kJ/ mol (31)) dominate the sorption of these S- and O-heterocyclic compounds, which implies that partitioning into soil organic matter is the dominant sorption mechanism (21). Two distinct data points for each compound in Figure 2 show that ∆H changes with the aqueous concentration of N-heterocyclic compounds. Due to the sorbent surface being energetically heterogeneous, the enthalpy of adsorption varies with the amount adsorbed. A lower aqueous concen-
FIGURE 3. Effect of ion type (K+ vs Ca2+) in mobile phase at ionic strength of 0.015 M (column E4Q-1, flowrate 100 µL/min; 25 °C; pH 6.7 to 6.9 for both cases). tration (corresponding to a lower loading of adsorbate) leads to a more negative ∆H since the energetically favored sites are occupied first. However, the concentration range investigated was rather small (within 1 order of magnitude) and thus the observed differences in ∆H were not pronounced (although significantly higher than measurement uncertainties) in comparison with data variability in literature. This is in accordance with the only slight nonlinearity of sorption (i.e., Freundlich coefficients between 0.8 and 1 with the exception of 2MQUI) found in our previous study (21). For the compounds of group B, ∆H ranged from -15 to -22 kJ/mol. This implies that besides van der Waals forces, other interactions also play a role in sorption. For the ionizable compounds 2MQUI, QUI, and IQUI, cation exchange (corresponding to an average ∆H of -8 kJ/ mol (31)) dominates the overall sorption. With respect to 2QUI (existing mainly as the keto form (oxo tautomer) (29)), IND, and BTA, additional interactions other than cation exchange predominate the overall sorption. For BTA, surface complexation might be a significant process (see below). Previous results suggest that specific interactions also dominate the overall sorption of PY and 2MPY (21). However, the ∆H of PY and 2MPY are relatively low (-0 to -2 kJ/mol, group C in Figure 2). Since soil properties should affect all nitrogen heterocycles in the same manner, this distinct behavior might be due to specific properties of PY and 2MPY. Both of them are fully water miscible, and the molecular sizes are smaller than those of the other compounds, resulting in quite different energies of cavity formation and solvation in aqueous solutions compared with the other N-heterocycles. In summary, at lower temperature stronger sorption of compounds was observed. The different extent of temperature dependency further supports our previous hypothesis that specific interactions play an important role in the sorption of nitrogen heterocyclic aromatic compounds. Effect of Solution Properties on Cation Exchange. As shown in Figure 3, when changing the Ca2+/K+ ratio in the aqueous phase at constant ionic strength, there is no significant effect on the sorption of THIO, Tol, BF, 2MBF, DMBF, IND, 2QUI, and BTA. The variation of Kd values for these compounds ranged from 4% to 20%, and the higher deviations were found for the compounds with low Kd, for example THIO, for which the relative uncertainty in Kd is highest. In contrast, for the other compounds (PY, 2MPY, QUI, IQUI, and 2MQUI), Kd values increased substantially by a factor from 1.5 to 10. Exchangeable cations influence the sorption of cationic organic compounds mainly by competing for negatively charged sorption sites on the solid surfaces. The significantly higher sorption of protonated N-heterocyclic compounds
FIGURE 4. Effect of CaCl2 concentrations in the mobile phase on the sorption of NSOs (in E4Q-1 column, pH values at 0.005 and 0.05 M CaCl2 are 6.7 to 6.9). with K+ in the mobile phase confirms cation exchange as major sorption mechanism for these compounds. According to the Hoffmeister series hydrated K+ has a lower surface charge density and thus is much less effective than hydrated Ca2+ in competing with cationic organic compounds for sorption sites of negative charge (32). Among the exchangeable base cations commonly present in soils (Ca2+, Mg2+, K+, and Na+), minerals saturated with weakly hydrated cations (e.g., K+) show a higher affinity for many polar, nonionic pesticides compared with clays saturated with more strongly hydrated cations (25). Replacing Ca2+ by K+ in solution, however, did not cause substantial increase of BTA sorption. One reason for this might be a counteracting process, i.e., the diminishing of a surface complex formation of BTA, which involves Ca2+ (see below). As shown in Figure 4, for certain compounds (THIO, Tol, BF, 2MBF, DMBF, BT, BTA, IND, and 2QUI), the concentration of CaCl2 has a negligible effect on sorption. However, for compounds for which cation exchange is the main sorption mechanism, changing the CaCl2 concentration resulted in a substantial influence on the sorption/transport of these compounds. Both pH and ionic strength have an effect on the sorption of ionizable organic compounds. The pH determines the charge density on the solid surface and the fraction of sorbate in ionized form. In our experiments, there was no significant pH variation (6.7-6.9) resulting from changes in the CaCl2 concentration; thus, the fractions of the organic cationic species at different CaCl2 concentrations are relatively constant. The reduction in sorption at high CaCl2 concentrations results, therefore, from increased competition between Ca2+ and protonated organic compounds for binding sites on the solid matrix (22). Accordingly, this effect is most pronounced for compounds with pKa values near the solution pH (2MQUI, IQUI, 2MPY, and PY) of which a considerable fraction is present as cations. This is further evidence that cation exchange is the main sorption process of ionizable N-heterocyclic compounds. The results indicate that pKa is not the only parameter necessary to describe the sorption of these compounds (17), as the ionic strength of the system also has to be considered. One alternative effect of increasing the CaCl2 concentration is the potential formation of a (neutral) ion pair complex VOL. 41, NO. 9, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Effect of preload of high CaCl2 on the sorption of (A) BTA and (B) 2MPY (25 °C, flowrate 100 µL/min) in column E4Q-3. mAU: milli-absorption unit in the UV detector. Under these flow conditions, one pore volume corresponds to 113 s. [BH - Cl]0 (where B is an organic base) as suggested by others for the sorption of TBT+ (33-35). This process is, however, not consistent with the effects of pH on the sorption of NSOs (30) and thus not relevant to our system. Ternary Surface Complexation. Our previous work (21) suggested that surface complexation of the neutral species of BTA contributes to its overall sorption. Although stability constants of metal-BTA complexes are not available (e.g., in the NIST Standard Reference Database 46 (36)), the formation of BTA complexes, especially with copper, is wellknown (37) from its use as a corrosion inhibitor. We suggest that ternary surface complexation (38) of the neutral BTA species also contributes to the overall sorption of the compound in our system:
tSO- + Mem+ + L h tSOMeL(m-1)
(4)
where tSO- refers to a negatively charged site at the mineralwater interface, Me is metal ion, m is the valence of the metal ion, and L is the neutral organic ligand. For BTA, the complex formation with Ca2+ is postulated to occur both in aqueous solution and at the mineral surface and can be described as follows:
Ca2+ + BTA h [Ca - BTA]2+ (in solution)
(5)
[tSO-‚‚‚Ca2+]+ BTAh [tSO‚‚‚Ca - BTA]+ (on solid surface) (6) According to this model, the contribution of surface complexation to overall sorption depends on the Ca2+ density on the solid surface and is also affected by complexation in solution. In our experiments (see Figure 5), when increasing the CaCl2 concentration from 0 to 0.05 M (t1 to t2), there was a substantial increase of Ca2+ density on the solid surface, due to ion exchange between Ca2+ and H+ on the surface, as is indicated by the drop in the pH of the mobile phase. This leads to an increased sorption of BTA due to surface complexation (Figure 5A and Figure 6). Removal of Ca2+ from the mobile phase at the time step t3, leads to excess Ca2+ on the solid surface. Because of less [Ca - BTA]2+ complexation in solution, higher sorption and stronger tailing occurred in the BTA breakthrough curve (Figure 5A, t3). When BTA complexation in solution and on the solid surface was at equilibrium, the concentration of Ca2+ (0.005 to 0.05 M) had little effect on the sorption of BTA (Figure 6). Apparently, in this Ca2+ concentration range the formation of BTA complexes with Ca2+ both in solution and on the solid surface occurred to an extent that led to no net change in BTA retardation. According to the HSAB rules, a borderline ligand such as BTA should not form strong complexes with a hard sphere 3176
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FIGURE 6. Effect of Ca2+ concentration on the sorption of toluene, 2-methylpyridine, and benzotriazole. metal cation such as Ca2+. Nevertheless, our experimental data suggest that it is sufficiently strong to influence substantially the retardation of BTA. Another potential mechanism for BTA/Ca2+ interactions is a complex formation of the anionic species of BTA with Ca2+ ([BTA - Ca]+; [BTA2 - Ca]0) (39). This would prevent electrostatic repulsion between BTA- and the negatively charged solid surface. However, in our system the pH was much lower than the pKa of BTA, thus the fraction of anionic BTA species was negligible. In comparison, sorption of 2MPY decreased with increasing Ca2+ concentration due to the superposition of the effects of pH dependent speciation of 2MPY and increased competition between 2MPY+ and Ca2+ at high ionic strength (Figures 4 and 6). As shown in Figure 5B, changing CaCl2 concentration back to 0.0 M resulted in a similar pH as before, which means that the same fraction of cationic species is present. However, the sorption of 2MPY was lower than before, because the sorption sites on the solid surface were still occupied by Ca2+ ions. Unfortunately, there are no stability constants published for complexes of 2MPY with Ca2+ or Mg2+. The complex stability constants of PY-Ca2+ and PY-Mg2+ are 10-0.4 to 10-0.48 and 10-0.29 to 10-0.37, respectively (36). If the constants of 2MPY are of a similar magnitude, surface complexation involving Ca2+ and Mg2+ would play a minor role in the overall sorption of 2MPY in our system. In summary, the effects of various environmental factors influencing the sorption of NSOs to reference soils have been successfully studied by soil column chromatography. The lack of significant effects of ionic strength, ion type, and temperature on the sorption of O- and S-heterocyclic
compounds shows that partitioning to soil organic matter is the dominant sorption mechanism for these compounds. In contrast, specific interactions are involved in the sorption of N-heterocyclic compounds. The concentration of CaCl2 in the mobile phase significantly affects the cation exchange of organic cationic species with negatively charged surfaces. Thus, at given ionic strength, substituting Ca2+ with K+ in the mobile phase enhances the sorption of organic cationic species. Furthermore, our data suggest that ternary surface complexation with Ca2+ is involved in the sorption of benzotriazole. Although the specific sorption processes have been addressed for NSOs in this study, further investigations of the role of surface complexation for N-heterocyclic compounds are needed to provide a better understanding of their sorption behavior. An important step toward a quanitification of surface complexation will be the determination of currently lacking complex stability constants.
Acknowledgments We thank Kai Goss, Satoshi Endo, Hans Peter Arp, and Guohui Wang for reviewing the manuscript and their valuable comments. We also thank the anonymous reviewers for helpful remarks.
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Received for review October 4, 2006. Revised manuscript received February 2, 2007. Accepted February 16, 2007. ES0623764
