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Published online 1 May 2008
Published in J Environ Qual 37:817-823 (2008)
DOI: 10.2134/jeq2007.0217
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Organic Compounds in the Environment

Sorption of Aromatic Compounds to Clay Mineral and Model Humic Substance–Clay Complex: Effects of Solute Structure and Exchangeable Cation

Yingjie Zhang, Dongqiang Zhu* and Hongxia Yu

State Key Lab. of Pollution Control and Resource Reuse, and School of the Environment, Nanjing Univ., Jiangsu 210093, P.R. China

* Corresponding author (zhud{at}nju.edu.cn).

Received for publication May 2, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Clay minerals and humic substance (HS)–clay complexes are widely distributed in soil environments. Improved predictions on the uptake of organic pollutants by soil require a better understanding of fundamental mechanisms that control the relative contribution from organic and inorganic constituents. Five selected aromatic compounds varying in electronic structure, including nonpolar phenanthrene (PHEN), 1,2,4,5-tetrachlorobenzene (TeCB), polar 1,3-dinitrobenzene (DNB), 2,6-dichlorobenzonitrile (dichlobenil [DNL]), and 1-naphthalenyl methylcarbamate (carbaryl [CBL]), were sorbed separately from aqueous solution to Na+–, K+–, Cs+–, and Ca2+–saturated montmorillonites with and without the presence of dissolved HS at pH about 6. Upon normalizing for hydrophobic effects by solute aqueous solubility, the overall trend of sorptive affinity to HS-free K+–clay is DNB >> DNL, CBL > PHEN, TeCB, indicating preferential adsorption of the polar solutes. With the presence of HS, sorption of PHEN, TeCB, and CBL increases by several times compared with the pure clay, attributed to HS-facilitated hydrophobic partition (PHEN and TeCB) or H-bonding (CBL). The enhanced sorption of PHEN by HS is cation dependent, where Cs+ shows the strongest facilitative effect. Coadsorption of HS does not affect sorption of DNB and DNL to clays except that of DNB to Ca2+–clay because cation–dipole interactions between the polar group (NO2 or CN) of solute and weakly hydrated exchangeable cations dominate the overall sorption.

Abbreviations: CBL, carbaryl • DNB, 1,3-dinitrobenzene • DNL, 2,6-dichlorobenzonitrile • GC, gas chromatography • HOC, hydrophobic organic compounds • HS, humic substance • Kd, solid-to-solution distribution coefficient • KOC, organic carbon-based distribution coefficient • KOW, solute n-octanol–water partition coefficient • NAC, nitroaromatic compounds • OC, organic carbon • PAH, polycyclic aromatic hydrocarbon • PHEN, phenanthrene • SOM, soil organic matter • SW, water solubility • TeCB, 1,2,4,5-tetrachlorobenzene • XRD, X-ray diffraction


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
SORPTION to soil greatly affects the fate and transport of hydrophobic organic compounds (HOCs) in the environment. Clay minerals and soil organic matter (SOM) are two major soil constituents. Clay tends to bind SOM to form stable aggregates, and most SOM exists as SOM–clay complexes (Stevenson, 1982; Schnitzer, 1986). Improved predictions on the uptake of HOCs to soil require better knowledge of the relative contribution from organic and inorganic constituents to the sorption process. Thus, elucidation of fundamental mechanisms that determine sorption of organic pollutants to clay minerals and SOM–clay complexes is of great theoretical and practical importance.

Recently, HOC sorption to model humic substance (HS)–mineral complexes has been intensively studied for a better understanding of the similar process in soil (Murphy et al., 1990; Onken and Traina, 1997; Celis et al., 1998; Liu et al., 2000; Li et al., 2003; Wang and Xing, 2005; Feng et al., 2006; Hengpraprom et al., 2006). The presence of HS even at a loading less than 1% organic carbon (OC) could greatly increase sorption of nonpolar HOCs (e.g., polycyclic aromatic hydrocarbons [PAHs]) to clay minerals, implying that hydrophobic partition into organic constituents plays a primary role in the sorption process (Onken and Traina, 1997; Wang and Xing, 2005). It is well established that the uptake of apolar HOCs (PAHs, polychlorinated biphenyls, and chlorinated benzenes) by natural soil and sediment correlates well with the content of organic matter or OC of the geosorbent (Chiou et al., 1979; 1983; Karickhoff et al., 1979; Means et al., 1980; Celis et al., 2006). The predominance of hydrophobic effects in the sorption of apolar HOCs to soil is further verified by the good linear free energy relationship that is often found between OC-based distribution coefficient (KOC) and solute n-octanol–water partition coefficient (KOW) or water solubility (SW) (Chiou, 2002).

Unlike the apolar compounds, sorption of the polars (e.g., organic acids and nitroaromatic compounds [NACs]) to clay minerals seems not to be facilitated or to be somewhat inhibited by the presence of HS due to the overwhelming polar interactions of solute with mineral surface and/or blockage of adsorption sites by the bound HS (Celis et al., 1999; Liu et al., 2000; Li et al., 2003). It is well documented that smectite clays exchanged with weakly hydrated cations (e.g., K+ and Cs+) show strong sorptive affinities toward many polar HOCs, including NACs, dichlobenil, atrazine, and carbaryl (Laird et al., 1992; Haderlein et al., 1996; Boyd et al., 2001; Sheng et al., 2001; De Oliveira et al., 2005). For example, sorption of 1,3,5-trinitrobenzene and 6-methyl-2,4-dinitrophenol to K+–montmorillonite is much higher than expected (i.e., the solid-to-solution distribution coefficient [Kd] is as high as 105 L kg–1) (Boyd et al., 2001; Sheng et al., 2002), which is attributed to cation–dipole interactions between the nitro (NO2) group and K+. Alternatively, water molecules coordinated to strongly hydrated cations (e.g., Na+ and Ca2+) impair such polar interactions with exchangeable cations, and therefore sorption is suppressed.

Much progress has been made in characterizing chemistry and structure of HS–mineral complexes. One important finding is that mineral surfaces fractionate HS in sorption, which seems to be dependent on type and surface property of the mineral (Kaiser et al., 1997; Specht et al., 2000; Zhou et al., 2001; Kaiser, 2003; Hur and Schlautman, 2003 and 2004; Wang and Xing, 2005; Feng et al., 2006; Hengpraprom et al., 2006). Based on the solid 13C or 1H nuclear magnetic resonance technique, previous studies revealed that kaolinite and montmorillonite preferentially sorb the aliphatic fractions over the aromatic fractions of HS, likely due to greater hydrophobicity of the former (Wang and Xing, 2005; Feng et al., 2006). Moreover, Hengpraprom et al. (2006) found that the sorptive affinities of three humic acids from different sources are inversely related to their polarities (i.e., larger O/C ratios reflect higher polarities and generally lower hydrophobicity). In contrast, Fe and Al oxyhydroxides (e.g., goethite, ferrihydrite, and amorphous Al(OH)) seem to preferentially sorb HS fractions rich in aromatic factions (Kaiser et al., 1997; Zhou et al., 2001; Kaiser, 2003). This was attributed to strong complexation of acidic ligands, particularly those associated with aromatic structures, with the oxyhydroxide surface. Despite the potential importance, no adequate attention has been paid on the impact of exchangeable cations on adsorption of HS to clay minerals. For instance, like polar NACs, relatively strong cation–dipole interactions might exist between weakly hydrated exchangeable cations (e.g., K+ and Cs+) and HS components containing strongly polar groups (e.g., C=O), hence fractionating HS in adsorption.

Previous studies of HOC sorption to HS–clay complexes have been limited to individual classes of compounds (e.g., nonpolar PAHs or polar NACs). In this study, we systematically estimated the impact of solute structure and exchangeable cation in an interplayed matrix on HOC sorption to a domestic montmorillonite with and without the presence of a commercial humic acid at the dissolved state. X-ray diffraction (XRD) was explored to characterize changes in clay basal spacing on HS adsorption. The relative contribution from organic and inorganic constituents and the predominant sorption mechanism are discussed for each case sorption process.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Materials
To prepare Na+–clay, 20 g of domestic montmorillonite (CEC = 110 cmol kg–1) (Fenghong Inc., Zhejiang Province, China) was mixed with 1 L of 0.5 mol L–1 NaCl aqueous solution for 24 h, and the suspension containing clay fraction (<2 µm) was collected by repeated centrifugation and resuspension in 0.5 mol L–1 NaCl. Suspension of clay saturated with K+, Cs+, or Ca2+ was prepared by washing/centrifuging the Na+–clay suspension three times with a 0.1 mol L–1 appropriate chloride solution. The clay suspension was then dialyzed to remove excess salts using membrane tubes (3500 Daltons) (Union Carbide, Houston, TX) until a negative chloride test was performed using AgNO3. The resulting clay suspensions were freeze-dried, and the obtained homoionic clays of Na+, K+, Cs+, and Ca2+ were stored in a desiccator for later use.

Sorbates include two nonpolar compounds, phenanthrene (PHEN) (Fluka, Riedel-de Haën, Seelze, Germany) and 1,2,4,5-tetrachlorobenzene (TeCB) (Aldrich, St. Louis, MO), and three polar compounds, 1,3-dinitrobenzene (DNB) (Aldrich), 2,6-dichlorobenzonitrile (dichlobenil [DNL]) (Chem Service, West Chester, PA), and 1-naphthalenyl methylcarbamate (carbaryl [CBL]) (Chem Service). Chemical structures of the sorbates are given in Fig. 1 , and selected properties are listed in Table 1 . Humic substance (HS) of a commercial humic acid (Fluka) extracted from natural coal was used as received to prepare model HS–clay complexes. The elemental analysis (Elementar Vario MICRO, Heraeus, Germany) on freeze-dried HS gave C (50.0%), H (3.5%), O (39.9%), N (0.8%), and S (1.1%). The ash content was <5% based on the elemental analysis.


Figure 1
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  		Fig. 1. Chemical structures of sorbates.

 

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  		Table 1. Water solubility (SW) and n-octanol–water partition coefficient (KOW) for sorbates.

 
Batch Sorption
Sorption was performed in PTFE-lined screw cap glass vials of 22 mL capacity receiving 100 mg Na+–, K+–, Cs+–, or Ca2+–clay for sorption to clay and to HS–clay complex or vials of 40 mL capacity receiving 30 to 100 mg HS for sorption to pure HS. To prepare HS solution for sorption to HS–clay complex, 50 mg of HS was dissolved by 5 mL 0.1 mol L–1 NaOH and mixed with distilled water to 1 L to reach an apparent HS concentration of 50 mg L–1. The resulting HS solution was adjusted to pH 7.0 by 0.1 mol L–1 HCl, followed by filtration through a 0.45-µm membrane. The HS solution was then mixed with NaCl, KCl, or CsCl salt to prepare a background solution containing 0.01 mol L–1 electrolyte for sorption to the corresponding cation-exchanged clay with the presence of HS. To avoid precipitation formed between the dissolved HS and Ca2+ (which instantly developed when they were mixed), HS water solution was used for sorption to HS–Ca2+–clay. The salt-free HS solution has a TOC of 24 ± 1 mg L–1 (TOC 5000A; Shimadzu, Japan), with SD calculated from three replicates. The homoionic clay was mixed with sufficient volume of corresponding HS solution to eliminate headspace (the mass ratio of HS to clay was about 0.01). For sorption experiments of pure clay, the background solution was 0.01 mol L–1 appropriate chloride. For sorption experiments of pure HS, the background solution was 0.01 mol L–1 KCl. The pH of clay suspension and HS–clay suspension was adjusted to about 6.0 by 0.1 mol L–1 HCl and 0.1 mol L–1 NaOH (equilibrium pH = 6.0 ± 0.3, measured at the end of sorption experiments). The pH of the HS suspension was unadjusted (equilibrium pH = 6.0 ± 0.1). Organic solute was added in a carrier of methanol that was kept below 0.1% by volume to minimize co-solvent effects on solute. Sorption samples were shaken by an orbital shaker at room temperature for 3 d.

After centrifugation, the solute TeCB was extracted from an aliquot of the aqueous phase with hexanes and analyzed by gas chromatography (GC) with electron-capture detection using a 60 m x 0.25 mm DB-1 capillary column (J&W Scientific, Folsom, CA). The rest of the solutes were analyzed directly by high-performance liquid chromatography with a UV detector using a 4.6 x 150 mm HC-C18 column (Agilent, Santa Clara, CA). Isocratic elution was performed under the following conditions: 60% methanol:40% water (v/v) with a wavelength of 238 nm for DNB; 80% methanol:20% water (v/v) with a wavelength of 254 nm for PHEN; 75% methanol:25% water (v/v) with a wavelength of 210 nm for DNL; 70% methanol:30% water (v/v) with a wavelength of 280 nm for CBL. Calibration curves included at least seven standards over the test concentration range. When GC/electron-capture detection was used, calibration curves were fit to a power law expression to account for the detector response nonlinearity. Separate studies showed that the dissolved HS has no interference with the total aqueous concentration measurement of organic solute in high-performance liquid chromatography or GC analyses compared with the HS-free condition. Adsorbed mass was assumed to be equal to the difference between added mass and mass in the solution aqueous phase. To take account for solute loss from processes other than sorption to the sorbent (i.e., sorption to septum and glassware and volatilization), calibration curves were obtained separately from samples receiving the same treatment as the sorption samples but no sorbent.

XRD Analyses
Clays exchanged with different cations with and without coadsorption of HS were filtered through 0.45-µm membranes under vacuum. The obtained clay film was allowed to equilibrate with water vapor at 100% humidity for at least 24 h in a closed vial before the analysis of XRD (X'TRA, ARL, Switzerland). One portion of the clay was freeze-dried for quantifying the OC content by elemental analysis.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Sorption isotherms to HS-free and HS-sorbed clays saturated with different cations and to solid HS were fit to the Freundlich model (q = KFCe,n) by regression on the linear format (lnq = lnKF + n lnCe), where q (mmol kg–1) and Ce (mmol L–1) are the equilibrium sorbed and total aqueous concentrations, respectively; KF (mmol1–n Ln kg–1) is the Freundlich affinity coefficient; and n (unitless) is the Freundlich linearity index. The isotherms are shown from Fig. 2 through Fig. 5, and the parameters are given in Table 2 . In general, the Freundlich model provides good fits. The Kd for different sorbate–sorbent combinations was also calculated (values at 0.01 SW and 0.1 SW shown in Table 2).


Figure 2
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  		Fig. 2. Sorption isotherms for all compounds. (a) Original for K+–clay and humic substance (HS)–K+–clay. (b) Normalized for hydrophobic effect using solute aqueous solubility (SW) for K+–clay. CBL, carbaryl; DNB, 1,3-dinitrobenzene; DNL, dichlobenil; PHEN, phenanthrene; TeCB, 1,2,4,5-tetrachlorobenzene.

 

Figure 5
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  		Fig. 5. Sorption isotherms for all compounds to humic substances. (a) Original. (b) Normalized for hydrophobic effect using solute aqueous solubility (SW). CBL, carbaryl; DNB, 1,3-dinitrobenzene; DNL, dichlobenil; PHEN, phenanthrene; TeCB, 1,2,4,5-tetrachlorobenzene.

 

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  		Table 2. Freundlich model parameters KF and n ± SD for isotherms measured for organic adsorption to humic substance (HS), clay, and HS–clay complexes with different exchangeable cations, along with solid-to-solution distribution coefficient (Kd) calculated from the Freundlich model at specific aqueous concentrations of 0.01 SW and 0.1 SW.

 
Sorption to Clay
The Effect of Solute Structure
Sorption isotherms of different solutes to HS-free K+–montmorillonite are highly divergent (Fig. 2a), and direct comparison in sorptive affinity between different classes of solutes is difficult due to their distinct concentration scopes. The Freundlich linearity index (n) covers a wide range, from 0.59 (TeCB) to 1.34 (PHEN) (Table 2), indicating varied sorptive linearity (i.e., departures from n = 1 imply nonlinear sorption). Uncharged regions of the siloxane surface of smectites are considered somewhat hydrophobic and may serve as effective adsorption sites of HOCs via hydrophobic effects (Jaynes and Boyd, 1991; Laird and Fleming, 1999; Aggarwal et al. 2006a, 2006b). At a first-order approximation, hydrophobic effects can be normalized to the same level for different solutes by dividing the equilibrium solution concentration by solute aqueous solubility (Chiou, 2002). The normalized isotherms are shown in Fig. 2b. Upon normalizing for the hydrophobic effects, the overall trend of sorptive affinity to K+–clay is DNB >> DNL, CBL > PHEN, TeCB, correlating well with polarity of solute. It seems that the enhanced sorption of the polar solutes (DNB, DNL, and CBL) over the nonpolar solutes (PHEN and TeCB) was due to cation–dipole interactions between the polar solutes and exchangeable cations on the clay surface.

It is well established that smectites exchanged with weakly hydrated cations such as K+ manifest strong sorptive affinities toward NACs (Haderlein et al., 1996; Boyd et al., 2001; Sheng et al., 2002). Boyd et al. (2001) and Sheng et al. (2002) attributed the results mainly to cation–dipole interactions between the nitro (NO2) group and exchangeable cation. Moreover, cation–dipole interactions induced by polar groups of DNL (CN) and CBL (C = O) have also been proposed to explain their relatively high sorption to smectites (Sheng et al., 2001; De Oliveira et al., 2005). In this study, sorption of DNL and CBL is much lower than that of DNB to K+–clay, as shown by the original isotherms (Fig. 2a) and by the normalized isotherms (Fig. 2b). The reason is that, due to lower polarity, the CN and C = O groups interact less strongly with exchangeable cations than the NO2 group, and DNL and CBL contain less polar substituents than DNB.

The Effect of Exchangeable Cation
The type of exchangeable cation influences sorption of nonpolar PHEN (Fig. 3 ) and polar DNB (Fig. 4 ) to montmorillonite, with much stronger effects observed on DNB. The overall trend of cation-dependent sorption is Cs+ > K+, Ca2+ > Na+ for PHEN, and Cs+ >> K+ >> Na+ > Ca2+ for DNB, which roughly inversely correlates with the hydration energy of cation. The largest differences in cation effects, measured by Kd (Table 2), are about three times for PHEN (between Cs+ and Na+) and more than two factors for DNB (between Cs+ and Ca2+). The extremely high sorption of DNB by K+– and Cs+–clays can be attributed to cation–dipole interactions between the NO2 group and the exchangeable cation (Boyd et al., 2001; Sheng et al., 2002). Alternatively, Na+– and Ca2+–clays show much lower sorption of DNB because such interactions are impaired due to the strong competitive hydration reactions of these cations.


Figure 3
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  		Fig. 3. Sorption isotherms for phenanthrene to clays and humic substance (HS)–clay complexes saturated with different cations of Na+, K+, Cs+, and Ca2+.

 

Figure 4
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  		Fig. 4. Sorption isotherms for 1,3-dinitrobenzene to clays and humic substance (HS)–clay complexes saturated with different cations of Na+, K+, Cs+, and Ca2+.

 
Previous studies suggested that sorption of nonpolar PHEN to smectites is dominated by hydrophobic effects (Hundal et al., 2001). In smectites exchanged with less hydrated cations (e.g., K+ and Cs+), the smaller size of cation hydration shell results in less swelling of the clay and larger size of the adsorption domain of the siloxane surface, hence increasing hydrophobic effects in HOC sorption (Jaynes and Boyd, 1991; Laird and Fleming, 1999; Li et al., 2003; Aggarwal et al. 2006a, 2006b). Furthermore, relatively weak cation–{pi} interactions may exist between {pi}-donor compounds (e.g., PAHs) and weakly hydrated exchangeable cations (e.g., Cs+) on mineral surfaces (Zhu et al., 2004), helping to explain the observed cation effect on PHEN sorption.

For K+– and Cs+–clays, because cation–dipole interactions of DNB are strong enough to surpass the sum of all sorption driving forces (mainly hydrophobic effects) of PHEN, sorption of DNB is much stronger than PHEN. On the contrary, for Na+– and Ca2+–clays, sorption of DNB is about one order of magnitude lower than PHEN. This is because for these clays the cation–dipole interactions of DNB become much weaker and cannot compensate the shortage in free energy contributed from the hydrophobic effects to the sorption process (i.e., PHEN is much more hydrophobic than DNB, as justified by KOW and SW values in Table 1).

Sorption to Humic Substance
Sorption of different solutes to solid HS shows a decreasing order of PHEN, TeCB > DNL > DNB, CBL (Fig. 5a ). Upon normalizing for hydrophobic effects by solute aqueous solubility, the order becomes DNB > CBL > DNL > PHEN, TeCB (Fig. 5b), suggesting nonhydrophobic sorptive forces of the polar solutes. The same trend was observed for sorptive nonlinearity (see Freundlich n values in Table 2), indicating that the relative contribution from nonhydrophobic forces to the overall sorption gradually decreases along the sequence from DNB to PHEN and TeCB (i.e., ideal hydrophobic partition would lead to linear sorption). Corresponding to structures of the polar solutes, the nonhydrophobic forces may include polar (dipole–dipole, dipole–induced dipole) interactions, H-bonding, and {pi}–{pi} electron–donor–acceptor (EDA) interactions. Previous studies have proposed H-bonding and {pi}–{pi} EDA interactions between chloroacetanilide pesticides (e.g., alachlor and metolachlor) and soil humic acids (Senesi 1992; Senesi et al., 1994; Liu et al., 2000). The highest normalized sorption of DNB can be explained by strong polar interactions due to the high polarity of solute and possible {pi}–{pi} EDA interactions (i.e., NACs are strong {pi} acceptors; Zhu and Pignatello, 2005a). The higher sorption of CBL relative to DNL can be accounted for by the strong H-bonding ability of CBL; that is, the O-C(=O) (H acceptor) and NH (H acceptor or H-donor) structures of CBL form stronger H-bonds than the CN group of DNL.

Sorption to HS–Clay Complex
Characterization of HS–Clay Complexes
X-ray diffraction patterns of HS-sorbed clay and HS-free clay exchanged with the same cation are identical (data not shown), suggesting that HS is restricted to the external surface of clay tactoids. This is in agreement with earlier results that the basal spacing of a reference montmorillonite (SWy-2) exchanged with K+ is not affected by coadsorption of humic acids (Li et al., 2003). The OC contents of Na+–, K+–, Cs+–, and Ca2+–clays associated with HS are 0.16, 0.21, 0.26, and 0.31%, respectively. The OC contents of pure clays are below the detection limit (0.01%). The largest OC content of Ca2+–clay is likely due to the strong complexation of Ca2+ with the ionic structures (e.g., COO–) of HS. In previous studies, Feng et al. (2006) also showed that Ca2+ facilitates adsorption of humic acids to montmorillonite compared with Na+. The higher HS adsorption by Cs+–clay than Na+–and K+–clays can be explained by the enhanced hydrophobicity of clay surface due to the least cation hydration of Cs+. Moreover, relatively strong cation–dipole interactions may exist between Cs+ and HS polar groups (e.g., C=O and OH), hence facilitating adsorption of HS. Further studies are required to better understand the cation effect on HS adsorption to smectites.

The Effect of Solute Structure
For K+–saturated montmorillonite, coadsorption of HA results in diverse effects on sorptive linearity and affinity for different solutes (see Freundlich n and Kd values in Table 2). The presence of HA increases sorptive affinities of PHEN, TeCB, and CBL by 3 to 6 times, but it leaves that of DNB and DNL nearly unchanged. The enhanced sorption of PHEN and TeCB can be attributed to hydrophobic partition into the bound HS. Alternatively, because sorption of DNB and DNL is dominated by cation–dipole interactions with the clay surface, the relative contribution from HS to the overall sorption becomes negligible. However, the enhanced sorption of CBL by the presence of HS cannot be explained by HS-facilitated hydrophobic partition because otherwise a stronger trend would have been observed on DNL. This is because, compared with CBL, DNL is more hydrophobic (see SW and KOW values in Table 1) but invokes slightly weaker polar interactions with the clay surface (i.e., justified by the normalized isotherms on hydrophobic effects; Fig. 2b). Combining the results of sorption to pure clay and to pure HS, we propose that H-bonding with the bound HS is likely responsible for the enhanced sorption of CBL to the HS–clay complex.

The Effect of Exchangeable Cation
The enhanced sorption of PHEN to HS-complexed clays is dependent on exchangeable cations (Fig. 3). Furthermore, there is no consistent relationship between the enhanced sorption and the OC content of clay complex. Sorption to HS–Cs+–clay is about two times higher than HS–clays exchanged with other cations despite its moderate OC content (i.e., Ca2+ [0.31] > Cs+ [0.26] > K+ [0.21] > Na+ [0.16]). This is probably because clays exchanged with Cs+, the least hydrated cation, preferentially sorb the more hydrophobic moieties of HS that favor partition of organic solute. Alternatively, the presence of HS shows negligible impacts on the sorption of DNB to Na+–, K+–, and Cs+–clays and increases the sorption to Ca2+–clay by about two times (Fig. 4). For Na+–, K+–, and Cs+–clays, cation–dipole interactions between the NO2 group of DNB and exchangeable cations dominate the sorption process. However, for Ca2+–clay, such interactions become impaired due to the strong cation hydration, and thus sorption to HS plays a more important role.

Model Analyses
Assuming that the organic and inorganic constituents function individually as two sorbent phases, the relative contribution from the HS component can be estimated by subtracting the clay sorption from the HS–clay sorption. Thus, KOC of the bound HS is calculated by the following illustrative model:

Formula 1[1]
where q and Ce are the sorbed and solution concentrations, respectively, at sorption equilibrium to HS–K+–clay; KF and n are Freundlich parameters for sorption to K+–clay; and fOC is the OC content of the HS–clay complex. The calculated KOC is 46,000 ± 3000 L kg–1 for PHEN (average values), 22,000 ± 3000 for TeCB, and 2100 ± 500 for CBL. These values are remarkably higher than the measured KOC for sorption to pure HS (i.e., 3000–4800 L kg–1 for PHEN [lower and upper boundary values], 5000–11,000 L kg–1 for TeCB, and 240–420 L kg–1 for CBL).

Considering the complexity of the interactive system under study, it is not surprising to find that the simple composite model combining sorption to pure HS and to pure clay cannot predict the overall sorption to HS–clay complexes. Binding to the clay surface may result in compositional and conformational changes in the bound HS compared with the bulk HS. Wang and Xing (2005) reported in earlier studies that the KOC of PHEN to HS–clay complexes is several fold higher than that to the bulk HS and increases as the HS loading decreases. The investigators attributed the observations to the low polarity of the bound HS due to fractionation and the more condensed structure of the bound HS with lower loading. The KOC (46,000 ± 3000 L kg–1) of PHEN to the HS–clay complex in the present study greatly exceeds that to polyethylene (KOC = 14,000 L kg–1, converted from the reported Kd value [Zhu and Pignatello, 2005b]), a model sorbent dictated by the inert polymethylene unit that is capable of only hydrophobic partition. Therefore, the enhanced KOC to HS–clay complexes relative to the pure HS may not be fully ascribed to changes in polarity of the bound HS from fractionation. Due to the low sorbed concentration of HS (OC content ≤0.31%) and the high surface area of montmorillonite, the sorbed HS is expected to form individual patches on the external surface of clay. In previous studies, Mayer et al. (2004) proposed a configuration of the sorbed organic matter as patches on the clay surface centered around high-energy edge sites. This configuration of the bound HS enables adsorption of organic solutes on the HS-coated clay surface, which is more thermodynamically favored compared with absorption into the bulky HS or polyethylene due to the lack of extra cost of pushing apart HS/polyethylene structures to create a cavity for the sorbate.

Once covered and complexed by HS, the clay surface may not be fully exposed and accessible to organic solutes in adsorption. The estimated contribution from the clay surface to sorption would probably be higher than the true case, resulting in an underestimated KOC value by the illustrative model of Eq. [1]. The further complexity comes from the dissolved HS in aqueous solution, which could also bind organic solutes and affect their activity coefficients. Accordingly, the Kd value calculated from the total aqueous phase concentration in this study can only be considered "apparent" and is different from that calculated from the concentration of free solute.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
Sorption of HOCs to HS–clay complexes suspended in aqueous solution is a complex, cumulative process of multiple interacting pathways. The relative contribution from organic and inorganic constituents and the predominant sorption mechanism depend on the physical/chemical properties of the interactive sorbate–sorbent system. Regardless of the presence of HS, cation–dipole interactions between the polar group of solute and exchangeable cation dominate sorption of DNB and DNL. Weakly hydrated cations (K+ and Cs+) favor such sorptive interactions on the clay surface due to less competitive cation hydration. Compared with the HS-free clay, coadsorption of HS greatly enhances sorption of PHEN, TeCB, and CBL via facilitated hydrophobic partition (PHEN and TeCB) or H-bonding (CBL). Independent of exchangeable cations, the XRD-based basal spacing of clay is not modified by the presence of HS, suggesting that HS is restricted at the external surface of clay. However, the type of exchangeable cation influences the sorbed concentration and possibly the fractions of HS on the clay surface, which in turn determine the HS-enhanced sorption of PHEN. The results in this study highlight the importance of solute structure and exchangeable cation to sorption of HOCs to HS–clay complexes.


   ACKNOWLEDGMENTS
 
This work was supported by the China National Science Foundation (grant 20637030 and grant 20647002) and by the Jiangsu Province Science Foundation (BK2006128). We thank Mr. Wei Zhu (Center of Modern Analysis, Nanjing University) for assisting with some experiments.


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