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TECHNICAL REPORTS

Organic Compounds in the Environment

Using Surfactant-Modified Clays to Determine Sorption Mechanisms for a Representative Organic Base, Quinoline

Soil and Water Science Department, University of Florida, Gainesville, Florida 32611

^* Corresponding author (Kizza{at}ufl.edu).

Received for publication December 15, 2006.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Sorption of a representative ionizable nitrogen heterocycle, quinoline (pK_a = 4.92), was investigated to determine the relative contributions of the neutral and protonated species to the overall process. Batch sorption experiments were conducted on surfactant-modified clays that were synthesized from the exchange of hexadecyltrimethylammonium cations for resident sodium cations on a specimen smectite (Swy-2) at 0, 60, 80, and 100% of the clay's cation exchange capacity (CEC). Hexadecyltrimethylammonium exchange creates highly effective organic partitioning domains within the clay interlayers in proportion to their coverage on the exchange complex. The fractionally exchanged clays, therefore, provided discrete exchange and organic partitioning domains for the protonated and neutral species of quinoline. Data were described by a combined Langmuir-linear isotherm that permitted independent characterization of both sorption components. Results indicated that cationic sorption dominated but that the neutral species can contribute substantially given sufficient organic carbon content relative to the CEC and at pH above the pK_a of quinoline. The data obtained in this study for quinoline demonstrated that the combined isotherm (including cation exchange and hydrophobic partitioning terms) describes sorption data and compares favorably with the purely empirical Freundlish isotherm.

Abbreviations: CEC, cation exchange capacity • f_oc, fraction of organic carbon • HDTMA, hexadecyltrimethylammonium • NHC, nitrogen heterocycle • OC, organic carbon

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
QUINOLINE represents an important class of contaminants, nitrogen heterocyclic compounds (NHCs), whose fate in the environment deserves attention. They are most commonly found in energy-derived wastes such as shale oil retort waste waters, coal tar derivatives, and coal tar processing wastes derived from the conversion of fossil to synthetic fuels (Leenheer et al., 1982; Periera et al., 1983). Coal tar is produced from the distillation of bituminous coal and is diverse in composition. Thus, a variety of NHCs have been associated with surface and subsurface waters near coal processing facilities, including high concentrations of quinoline, acridine, and pyridine derivatives (Periera et al., 1983; Sims and O'Loughlin, 1989). A similar array of compounds has been identified in shale oil retort waters (Leenheer et al., 1982). Therefore, environmental loadings of these compounds can be substantial, necessitating a greater understanding of the mechanisms governing their proliferation in the environment.

Quinoline is a representative NHC whose environmental fate is dictated by its ability to speciate in solution subject to pH. Quinoline and other NHCs are organic bases containing nitrogen moieties capable of accepting protons and becoming positively charged (Eq. [1]).

[1]

Q⁰ is neutral quinoline, and QH⁺ is the protonated form. Overall sorption on soils and sediments can stem from a number of mechanisms, including exchange of the cationic species, solvophobic partitioning of the neutral species into organic carbon (OC), cooperative sorption from interactions with previously sorbed species, and chemisorption at colloid surfaces. A comprehensive treatment of total sorption, therefore, might require discrimination among the full complement of potential individual contributions, which may prove unwieldy for many applications. However, the two mechanisms that apparently dominate sorption are those that control the chemical analogs of the two individual organic base species: exchange of the cation and partitioning of the neutral species into OC (Banwart et al., 1982; Hassett et al., 1983; Ainsworth et al., 1987; Nichols and Evans, 1991; Fabrega et al., 2001).

Exchange of organic base cations is expected to be fundamentally similar to exchange of permanent organic cations. The protonated forms of NHCs like quinoline feature a single charge paired to a comparatively large hydrophobic substituent (Fig. 1 ). Thus, they are bound at mineral surfaces by the same forces that promote highly selective exchange of permanent organic cations (Ainsworth et al., 1987; Zachara et al., 1990). In essence, the electrostatic component to cation exchange is augmented by the interaction of the hydrocarbonaceous moieties of the organic cation with neutral domains on mineral surfaces. Therefore, the strength of adsorption has been correlated with the size of the hydrocarbonaceous substituents of the protonated base. Other pertinent observations include a decline in sorption with increased pH or ionic strength, the correlation of sorption with the ionized species fraction in solution and the cation exchange capacity (CEC), a vulnerability to competition from other cations, and a body of spectroscopic evidence (Ainsworth et al., 1987; Zachara et al., 1986; Traina and Onken, 1991; Zierath et al., 1980; Doehler and Young, 1961).

View larger version (6K): [in this window] [in a new window]   Fig. 1. Representative nitrogen heterocyclic compounds.

Thus, depending on system pH and the base character of the sorbate, the potential exists for describing organic base sorption using model analogs for permanent cations and neutral organic molecules. The present goal is to determine if evaluation of these two primary components can be used to approximate the potentially more complex overall process. The ability to observe uptake of either species, however, is related to the strength of organic partitioning relative to the strength of cation exchange. In general, selectivity for organic cations is demonstrably high in relation to the uptake of polar and nonpolar neutral molecules. Consequently, abundant direct indications of the contribution to overall sorption of the neutral species of organic bases relative to the cation are essentially absent because when the two species coexist in the same system, the impact of cation adsorption tends to obscure contributions from the neutral molecule. However, conditions commonly exist for which neutral species sorption can be substantial: high pH, a high proportion of OC relative to CEC, and high solution concentrations of the sorbate. Controlling these variables to delineate individual sorption mechanisms in natural sorbents can be difficult. Soils possess a suite of properties that can be difficult to reliably manipulate or characterize. Therefore, we used fractionally exchanged organoclays as comparatively stable sorbent materials that could be more simply manipulated and defined based on experimental demands with respect to the relative abundance of exchanger and organic sorbent domains.

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Organoclays as Model Sorbents
Organoclays are synthesized from the exchange of resident metallic cations on clay surfaces by organic alkylammonium cations. Specifically, organophillic clays are produced from exchange with relatively long-chain alkylammonium cations (>12 carbons). This replacement strongly enhances the hydrophobicity of the clay interlayers, making them excellent sorbents for neutral hydrophobic compounds (Jaynes and Boyd, 1991; Nzengung et al., 1996; Boyd et al., 1988). Thus, organoclays have been proposed as components in a variety of waste containment barriers to inhibit the migration of toxic organic contaminants.

Exchange with alkylammonium cations ordinarily is taken to completion, resulting in full coverage of the exchange complex to facilitate maximum sorption of hydrophobic organic compounds. However, the amount of exchange can be tailored to specific objectives. For instance, it has been shown that clays fractionally exchanged with hexadecyltrimethylammonium (HDTMA) cations can maintain an organophilic character in proportion with their fractional coverage of the exchange complex, within certain limits (Boyd et al., 1988; Bonczek et al., 2002). Depending on the level of exchange, HDTMA exists as monolayer, bilayer, or transitional arrangements within the interlayer space and can potentially coexist with sodium within the same interlayer (Bonczek et al., 2002). In essence, below 60% coverage of the exchange complex, HDTMA and sodium coexist in discrete domains within the same interlayer and interlayer spacing as measured by X-ray diffraction is governed largely by the alkylammonium cations. At coverages >70%, HDTMA bilayers form in conjunction with some residual interstratified monolayer phases. Regardless of the configuration, it has been shown that the organophilic domains remained accessible to nonpolar, uncharged solution constituents and did not impede the movement of water to exchange domains populated by sodium (Bonczek et al., 2002). In other words, for long-chain cations like HDTMA, the respective sorption domains remain discrete regardless of surface coverage, unlike the case for much smaller organic cations, which potentially can distribute randomly within the interlayer (Sheng and Boyd, 1998). Therefore, the relative proportions of exchange and organic domains may be manipulated to serve a variety of experimental objectives through fractional coverage of the exchange complex. The sorbent CEC for the present group of experiments was controlled through coverage of a Na-smectite by HDTMA cations at levels of 0, 60, 80, and 100% of the CEC. The measured CECs of the resulting organoclays decreased in proportion to an increase in the fraction of OC (f_oc), providing different relative amounts of each sorbent domain that could be used to explore their impacts on the overall sorption process.

Preparation of Organoclays
Na-montmorillonite (Swy-2) was obtained from the Source Clay Repository of the Clay Minerals Society (Colombia, MO). Reagent-grade hexadecyltri-methylammonium chloride was obtained from Fisher Scientific (Pittsburgh, PA). Organoclays were prepared from the wet-sedimented fraction obtained from the specimen clay as described by Nzengung et al. (1996). The <2-µm fraction was washed three times with a 0.1 M NaCl solution, rinsed twice with a 0.05 M NaCl solution, and rinsed with deionized water. The CEC of the resultant Na-montmorillonite was determined by ⁴⁵Ca isotopic exchange at pH 6 and was equal to 819 (mmol) kg^–1. Na-clay suspensions containing 20 g of clay were treated with solutions containing HDTMA equal to the desired coverage of the CEC (60, 80, and 100%) and allowed to equilibrate overnight while being shaken continuously. The resulting organoclays were centrifuged, washed, and freeze-dried. Cation exchange capacity values for the fractionally exchanged clays were determined by ⁴⁵Ca isotopic exchange, and OC contents were determined from the amount of HDTMA adsorbed at each level. Total C of the organoclays was determined from combustion, with subsequent measurement of CO₂ evolved. The clays were stored in desiccators over P₂O₅ until used.

Quinoline Sorption Isotherms
Reagent-grade quinoline was obtained from Aldrich Chemical. Quinoline has a pKa of 4.92 (Chorover et al., 1999) and an octanol-water partitioning coefficient of 102 (Zachara et al., 1986). Equilibrium sorption of quinoline from aqueous solution on clays and organoclays was examined using a stirred batch method (Nkedi-Kizza et al., 1985) with a solution/clay ratio of 300:1. Initial quinoline solution concentrations ranged from 0.25 to 4.40 mmol L^–1 in 0.02 M NaCl. The pH was adjusted through addition of HCl or NaOH. Clay was weighed into 30-mL polypropylene centrifuge tubes, and the appropriate aqueous quinoline solutions were added. Duplicates were used for each initial concentration. The tubes were agitated on a reciprocal shaker at ambient temperature (25°C) for 24 h (Nzengung et al., 1996; Nkedi-Kizza et al., 1985). Equilibrium was reached within 8 h. Samples of the equilibrium solutions were obtained from the supernatant after centrifugation. Quinoline concentrations were determined by high performance liquid chromatography using ultraviolet detection ( = 280 nm), a C-18 column, and a 0.50 M ammonium acetate/methanol mobile phase in a ratio of 30:70. Sorbed quinoline was determined as the difference between the amounts initially reacted and that remaining in the equilibrium solution.

Describing Quinoline Sorption: Combined Isotherm
To determine the relative magnitudes of sorption for each quinoline species, a combined Langmuir-linear isotherm was used. The combined isotherm is essentially a combination of the two isotherm equations that describe the dominant mechanisms apparently responsible for overall sorption of organic bases.

[2]

The first term in Eq. [2] defines adsorption of the cation in terms of an H-type isotherm (Langmuir), and the second term describes linear partitioning of the neutral species between the solution and OC (Freundlich-linear). S_T is total sorption (mmol kg^–1), QH⁺ (mmol L^–1) is the protonated form of quinoline, and Q⁰ (mmol L^–1) is the neutral form. S_max (mmol kg^–1) is the sorption maximum for the cation, K_oc (L kg^–1) is the OC partitioning coefficient for neutral quinoline, f_oc is the fraction of OC of the sorbent, and k (L mmol^–1) is a constant. For the neutral form, the linear Freundlich isotherm is given by Eq. [3].

[3]

Where S is the amount adsorbed (mmol kg^–1), K_D = K_oc f_oc (L kg^–1), and C is the solution concentration (mmol L^–1). The nonlinear Freundlich isotherm is given by Eq. [4].

[4]

Where k_f is the Freundlich sorption coefficient (mmol kg^–1)/(mmol L^–1)^N, and N is the Freundlich constant.

Based on the acid dissociation constant (K_a) of quinoline, the concentration of the neutral and protonated forms can be expressed relative to total quinoline (Q_T) as

[5]

Thus, at a pH approximately 2 units below or above the solute's pKa value, Eq. [2] can produce Langmuir or linear isotherms, respectively. At intermediate pH values, where both components of quinoline are active, the combined isotherm is L-type and resembles the Freundlich isotherm. However, Eq. [2] possesses an advantage over the Freundlich equation in that the combined isotherm incorporates a rudimentary theoretical framework to produce the frequently observed L-type isotherm common to many reported data, and the model coefficients are related to recognizable sorbent–sorbate parameters, specifically, the CEC and OC content of the sorbent and the OC partitioning coefficient of the sorbate.

	Results and Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Application of the Linear and Langmuir Terms of the Combined Isotherm
Initial results for quinoline sorption on the Na-clay and the fully exchanged HDTMA clay at pH 3 and pH 8, respectively, indicated that each term in the combined isotherm equation was closely descriptive of the experimental data (Fig. 2 ). In other words, the fundamental assumptions relative to the application of the Langmuir and linear terms seemed to be appropriate within the present context. Here, C (mmol L^–1) is the equilibrium solution concentration of both species, and S (mmol kg^–1) is the amount adsorbed of both species.

View larger version (12K): [in this window] [in a new window]   Fig. 2. Sorption of quinoline on Na-clay and 100% hexadecyltrimethylammonium (HDTMA) organoclay at pH 3 and pH 8.

Data in Fig. 2 suggest the relative strengths of sorption for the neutral and protonated species of quinoline. At low solution concentrations, sorption of the cation is far stronger than absorption of the neutral species owing to high affinity for the protonated base at low clay surface loadings of quinolinium. At higher solution concentrations, this dominance declines as adsorption of the cation approaches S_max. In mixed sorbents containing CEC and OC, potential adsorption dominance depends on two factors in addition to pH: (i) the solution concentration of quinoline and (ii) the magnitude of the OC content relative to the exchange capacity. This latter relation is integral to sorption of NHCs. Exchange affinity is characteristically higher than the strength of molecular adsorption, frequently leading to neglect of the neutral species as a significant component to overall sorption. However, given sufficient OC in relation to the exchange capacity, the natural dominance of cationic adsorption may be overcome, and the influence of the neutral species on the overall process may be made more evident.

Sorption on Fractionally Exchanged Organoclays at pH 3 and pH 8
The previous experiments on Na-clay and fully exchanged HDTMA clays demonstrated the applicability of the Langmuir and linear terms in the combined isotherm to independent sorption of each species. Additional experiments on fractionally exchanged clays at pH 3 and 8 were conducted to test the models on mixed sorbents and to determine the parameters S_max, k, and K_oc for subsequent use in the combined isotherm. These parameters then were used to describe sorption data on fractionally exchanged clays at intermediate levels of pH where both species of quinoline participate in sorption.

Sorption of the cation at pH 3 on the 60 and 80% organoclays was well described by the Langmuir term of the combined isotherm and indicated that the magnitudes of the coefficients k and S_max were unique to each sorbent (Table 1 ). Data in Fig. 3 and Table 1 indicate that both coefficients declined with increasing HDTMA coverage up to 80% of the CEC. However, sorption maxima for both partially exchanged organoclays were lower than the measured CECs. Quinolinium adsorption has been shown to be vulnerable to competition from metallic cations, including Na⁺, which was used here as a background electrolyte (Ainsworth et al., 1987; Zachara et al., 1986). Additionally, HDTMA and sodium ions can occupy the same interlayers, particularly for the 60% coverage (Bonczek et al., 2002). The distribution of Na⁺ and HDTMA domains within the interlayer is not necessarily uniform or homogeneous (Bonczek et al., 2002; Sheng and Boyd, 1998). This property could constrain interlayer access of quinolinium cations to particular "avenues" dictated by the proximity and arrangement of Na⁺ and HDTMA domains within the interlayer space and impose diffusive constraints on exchange. This tends to result in sorption maxima that are lower than the measured exchange capacity.

View larger version (14K): [in this window] [in a new window]   Fig. 3. Sorption of quinoline on Na-clay and fractionally exchanged clays at pH 3.

Table 1 summarizes the values determined above for the constants K_oc, S_max, and k, which were used in the combined isotherm to predict subsequent data collected at intermediate pH values on the fractionally exchanged organoclays. Thus, these parameters were held constant for each sorbent, with further model functionality dictated only by compound speciation. Table 1 also indicates sorbent OC content and CEC. Organic carbon contents were between 106 and 174 (g kg^–1), which is high relative to most mineral soils. However, this assessment must be balanced by recognition of the accompanying exchange capacities, which were also unusually high. Thus, it is the relative magnitudes of the OC content and the CEC, rather than the absolute abundance of either, that is central to the sorption of ionizable organic compounds like quinoline.

Sorption of neutral quinoline at pH 8 was linear and well correlated with the OC content of the three organoclays (Fig. 4 ), supporting the hypothesis that the second term in the combined isotherm is governed by an aqueous-organic partitioning mechanism. K_oc values for the organoclays were essentially constant (average K_oc = 375), although they were significantly greater than the K_oc value estimated from the octanol–water partitioning coefficient for quinoline. This is generally consistent with reported sorption of neutral aromatic compounds on organoclays (Nzengung et al., 1996; Lee et al., 1989; Boyd et al., 1988).

View larger version (10K): [in this window] [in a new window]   Fig. 4. Sorption of quinoline on three organoclays at pH 8.

View larger version (15K): [in this window] [in a new window]   Fig. 5. Combined isotherm simulation on the 80% organoclay (A) at pH 3.9 and (B) at pH 5 illustrating the contributions of the neutral and protonated species of quinoline to total sorption.

View larger version (13K): [in this window] [in a new window]   Fig. 6. Combined isotherm simulation on the 60% organoclay (A) at pH 7.3 and 80% organoclay (B) at pH 7.2 illustrating the contributions of the neutral and protonated species of quinoline to total sorption. For (B), neutral and combined/model contributions are equal.

A logical step to reconcile these two approaches was to compare them on a more equivalent basis by direct fitting of the combined isotherm. Direct fitting could involve optimization of three parameters (S_max, k, and K_oc). However, S_max and K_oc were based on sorbent or sorbate properties that are difficult to ignore in favor of a multiparameter optimization. The parameter k was the most tenuous in terms of its relation to definable properties and, therefore, was chosen.

Empirically, k sets the slope of the Langmuir equation at low solute concentrations and is frequently regarded as an "affinity coefficient" that traditionally has been deemed proportional to the energy of adsorption (Harter and Baker, 1977; Cavallaro and McBride, 1978; Sposito, 1979). It is sometimes believed, therefore, that the relative affinity of two solutes for a given sorbent can be expressed through k_, but several authors warn against liberal interpretations of the parameter (Harter and Baker, 1977; Griffin and Au, 1977; Sposito, 1979). Predicting k in a meaningful way also is not straightforward. Comparing even true affinity parameters for exchange from different experiments on different sorbents may not be warranted unless the equilibrium solutions are identical for each point on the isotherm because the parameter is also proportional to the competing ion concentration. Values and predictors of k are neither widely agreed upon nor documented, which strengthens its candidacy for optimization.

Data in Fig. 7A and 7B compare the optimized combined isotherm and the Freundlich isotherms near pH 5 for the 60 and 80% organoclays. At these pH values, both quinoline species are almost equally represented. The combined isotherm and the Freundlich isotherm predict the sorption data fairly well. The optimized k values were 4.71 and 4.32 for the 60 and 80% organoclays, respectively. However, these k values were not drastically different from those listed in Table 1. For the Freundlich isotherms, N values are <0.6 (Fig. 7A and 7B). These values are much smaller than what is reported in the literature for sorption of neutral organic chemicals in soils and sediments. For example, Karickhoff et al. (1979) and Nkedi-Kizza et al. (1985) have reported N values to be in the range 0.7 < N < 1. This likely is due to the strong sorption of the ionic species that follows the Langmuir isotherm at pH values close to the pK_a of quinoline.

View larger version (15K): [in this window] [in a new window]   Fig. 7. Comparison of the Freundlich isotherm and the optimized combined isotherm for quinoline on 60% (A) and 80% (B) exchanged organoclays at pH 5.2 and pH 5, respectively.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
For partially exchanged organoclays, it was evident that the combined isotherm described the shape and magnitude of the data well, despite using independently determined isotherm coefficients. The implication is that the magnitude of sorption on organoclays can be approximated given reasonable estimates of S_max, k, and K_oc. Thus, the combined isotherm applied to a controlled sorbent system offered a mechanistically referenced approach to quinoline sorption that is absent in the broadly applied Freundlich or L-type isotherm models. Further, at least two of the model coefficients, S_max and K_oc, bear a practical relation to widely easily determined sorbent and sorbate properties. L-type isotherms are strictly empirical and must be directly fit to a given data set to provide the appropriate coefficients, which subsequently are correlated with sorbent or sorbate properties without the benefit of a direct theoretical basis.

Separation of the sorption of the neutral and ionized species provided a compelling basis for a multimechanistic approach to overall organic base sorption. Factoring all or even most of the sorptive contributions for a more complex sorbent may be more difficult, but simple separation of the dominant processes in clays and organoclays lent credence to the view that organic base sorption, although frequently related to exchange parameters on the sorbent, embraces important processes beyond cation exchange.

The Langmuir isotherms were obtained for low pH values and, despite concerns relative to the applicability of this isotherm for cation exchange (Shukla and Mittal, 1979; Sposito, 1979; Harter and Baker, 1977), it performed well as an empirical component in the combined isotherm. Further, the Langmuir isotherm can be derived from basic ion exchange considerations (Schwartzenbach et al., 1993), and, even among its most energetic detractors, the Langmuir equation is a good empirical isotherm for describing cation adsorption by many clay or soil systems (Harter and Baker, 1977).

The neutral-species component of the model was exaggerated through the use of organoclays, which, in addition to their high OC contents, exhibit a marked affinity for neutral organic compounds. However, the magnitude of the cation exchange capacity in relation to the OC content of the partially exchanged organoclays fell within the range of many soils and sediments. Nonetheless, the objective in the present context was not necessarily to promote an extrapolation of these results to other more complex media (nor to rule it out) but was to demonstrate the utility of a mechanistically based approach that accords with the majority of the existing experimental data. Although exchange/adsorption component is a powerful determinant of sorption, the combined isotherm provides insight to, and an observable measure of the magnitude of, the neutral species contribution to overall sorption of organic bases like quinoline.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
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