Environ. Sci. Technol. 2008, 42, 7201–7206
Organic Sorbate-Organoclay Interactions in Aqueous and Hydrophobic Environments: Sorbate-Water Competition M I K H A I L B O R I S O V E R , * ,† Z E V G E R S T L , † FAINA BURSHTEIN,‡ SHMUEL YARIV,‡ AND URI MINGELGRIN† Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization, Bet Dagan, 50250, Israel, Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
Received April 23, 2008. Revised manuscript received July 8, 2008. Accepted July 17, 2008.
Sorption of nitrobenzene, phenol, and m-nitrophenol from water and n-hexadecane was measured on Na-montmorillonite and organoclays in which 41 and 90% of the exchange capacity of the Na-clay was occupied by hexadecyltrimethylammonium. The strength of sorbate-sorbent interactions in n-hexadecane for all three sorbents was in the following order: nitrobenzene < phenol < m-nitrophenol. The magnitude of the distribution coefficients suggests that the contribution to solute uptake of partitioning between n-hexadecane and the organic pseudophase of the dried organoclays is minor, whereas the major contribution is from adsorptive sorbate-sorbent interactions. Sorption isotherms obtained in different solvents were compared using a sorbate activity scale. In the organoclays, the stronger the tendency of a sorbate to interact with sorption sites, the less pronounced is the reduction in the activitybased sorption due to competition with water. The order of this reduction for the different sorbates is nitrobenzene > phenol > m-nitrophenol. The weakening of sorbate-sorbent interactions resulting from water-sorbate competition might be mitigated by interaction between the organic sorbate and sorbed water molecules. Since the more strongly interacting organic compounds are less susceptible to suppression of sorption in the presence of water, hydrating organoclays may result in an increased differentiation between “weakly” and “strongly” interacting (“nonpolar” and “polar”) compounds in the organoclay phase.
Introduction Clay minerals, in which exchangeable inorganic cations are replaced by organic cations (i.e., organoclays) have engendered considerable, long-term interest (1) due to their potential to interact with organic compounds in aqueous systems. In organoclays prepared using quaternary ammonium cations (QAC), both the siloxane surfaces and the QAC ions can contribute to the overall sorption of organic compounds, with their relative contributions dependent on * Corresponding author: phone 972-3-968-3314; fax: 972-3-9604017; e-mail:
[email protected]. † Agricultural Research Organization. ‡ The Hebrew University of Jerusalem. 10.1021/es801116b CCC: $40.75
Published on Web 09/04/2008
2008 American Chemical Society
the QAC size, packing configurations, interionic distances, and the basal spacings of the organoclays (2-4). Interactions of organic compounds with an organoclay may be a combination of partitioning into the organic pseudophase (“dissolution” domain) and adsorptive interactions. With flexible long-chain QACs, significant solute partitioning into the organic pseudophase of the organoclays is expected. Organoclays prepared with primary alkyl-ammonium cations exhibited hydrogen bonding between the NH of the ammonium moiety and some organic sorbates (5). One important role assigned to the sorbed organic cations is the weakening of the water network associated with clay surfaces and thus reducing the amount of complexed water at the clay surface (2, 4, 6). Considering the role of organoclay-water interactions in sorption of organic compounds from aqueous systems, it is useful to compare the sorption of organic compounds on hydrated and on dried (low-hydrated) organoclays. In comparison to the enormous amount of published data on the sorption of organic compounds by organoclays from water, studies comparing the sorption of organic compounds by hydrated and dried organoclays are scarce. A marked suppressive effect was demonstrated by comparing sorption of benzene and of trichloroethylene onto n-hexadecyltrimethylammonium (HDTMA)-exchanged smectite from the gas phase to their sorption from water (7). Distinct sorbate-water competition was indicated for the sorption of benzene, toluene, and xylene onto tetramethylammonium (TMA)exchanged smectite (8, 9) and for toluene, xylene, and tetrachloromethane on TMA- and tetramethylphosphonium exchanged smectites (10). The above studies were performed with nonpolar organic sorbates (aromatic hydrocarbons and chloro-hydrocarbons). Many environmentally important pollutants that can be removed with the aid of organoclays (e.g., nitro-substituted compounds, phenols, amines) are capable of (specific) interactions that are essentially different from those of nonpolar organic sorbates. For these types of sorbateorganoclay interactions, the effect of the sorbent hydration is unknown. These specific interactions may be associated also with essentially different sorption mechanisms (e.g., water bridging and sorbate interactions with complexed water) that cannot be envisioned from earlier studies of hydration effect on interactions of nonpolar organic compounds with organoclays. Hence, sorption of selected organic compounds capable of strong, specific interactions was examined on two organoclays from water and from a saturated hydrocarbon solvent (n-hexadecane). n-Hexadecane is a nonpolar solvent that is typically not capable of specific interactions with organic compounds (e.g., ref 11). Comparison between sorption on dried organoclays from n-hexadecane and from water, using the sorbate’s activity scale, may provide an insight into the role of water in sorbate-organoclay interactions. By examining sorption of organic compounds that display different strengths of interaction with organoclays, it is possible to estimate the capability of a sorbate to successfully compete with water. The environmental significance of the investigated topic is manifold: understanding the role of water activity (or moisture content) is of critical importance for a proper design of filters and liners constructed using organoclays (especially for air filters, e.g., ref 12) and for our ability to predict the behavior of liners and filters in the presence of effluents rich in cosolvents or of nonaqueous pollution streams (such as those present in fuel storage areas). Moisture content will VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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strongly affect the efficacy of organoclays when used as carriers for controlled release of bioactive agents (e.g. ref 13), and hence, sorbate-organoclay interactions in different media at various water activities may be of importance. Our conceptual understanding of the distribution of pollutants in liquid and solid media can be improved if the role of water is given its due weight using a sorbate activity-based analysis. Finally, soils are a mixture of mineral matrices complexed with organic components and are exposed to fluctuations in moisture content. By studying the well defined organoclay systems under various moisture regimes and comparing them to real-world soil systems, considerable light can be shed on the latter, thereby improving our ability to estimate risks and to design cleanup procedures. Thus, the overall aim of this study was to examine the relationship between the strength of the interactions of a sorbate with the organoclay (as measured in n-hexadecane) and the extent of the inhibitory effect of sorbate-water competition on the sorption. Our hypothesis is that strongly (and in particular specifically) interacting organic compounds may exhibit comparable interactions in both the hydrated and dried (e.g., n-hexadecane-saturated) organoclay phases.
Experimental Section Materials. Wyoming bentonite (Na-montmorillonite) with a CEC of 90.7 meq/100 g was obtained from Fisher Scientific (U.S.). The organic matter content of the clay was about 0.2% w/w (14). The moisture content of the freeze-dried bentonite used in subsequent sorption experiments did not exceed 2% w/w. Phenol (Phe; 99%), nitrobenzene (Nb; 99%), mnitrophenol (NPhe; 99%) (used as probe organic sorbates), and n-HDTMA bromide were purchased from Aldrich-Sigma. n-Hexadecane (>99%, Fluka) and water (Millipore-filtered) were used as solvents. Relevant physicochemical properties of the selected probe compounds are listed in Table S1 (Supporting Information). The HDTMA-exchanged clays were prepared as described in ref 15, washed, and freeze-dried. The organoclays had C contents of 8.5 and 16.7% w/w (measured by ThermoFinnigan C/N analyzer) and moisture contents of 2.8 and 0.7% w/w (determined by oven-drying at 105 °C), respectively. These C contents correspond to 41 and 90% exchange of the inorganic cation by HDTMA, respectively. The original clay and the HDTMA-exchanged clays are designated here as Naclay (no exchange), OC-41, and OC-90, respectively. Sorption Experiments. Details of the batch sorption protocol and analytical procedures are provided in the Supporting Information, together with Table S2 that summarizes the particulars of the sorption runs (range of solute concentrations, sorbent to solution ratios, fraction of the total amount of sorbate added that was sorbed, details of the sorption kinetics measurements, and innate pH of the aqueous systems). Calculation of Activities. Compound activities referred to the pure (supercooled) liquid state were calculated using equilibrium concentrations in solutions and the thermodynamic data on the compound’s vapor-solution distribution (as in ref 16; detailed in the Supporting Information). m-Nitrophenol is partially ionized in aqueous solutions (34% in the OC-90 system, 45% in the OC-41 system, and 50% in the Na-clay system; based on its pK and aqueous solution pHs reported in Supporting Information Tables S1 and S2, respectively). Hence, the activities of nonionized m-nitrophenol in aqueous systems were calculated using the concentration of nonionized m-nitrophenol instead of the total equilibrium solution concentration.
Results and Discussion Sorption from n-Hexadecane. Figure 1 summarizes the sorption data obtained for the three sorbates from n7202
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FIGURE 1. Sorption of the organic compounds on dried Na-clay, OC-41 and OC-90 from n-hexadecane: sorbed concentrations are plotted against solution concentration (a, c, e), and distribution coefficients, Kd,i are plotted against sorbed concentration (b, d, f). Error bars in all figures correspond to the standard deviation, and if not seen, do not exceed the symbol size. hexadecane. The order of sorption on different sorbents from n-hexadecane was nitrobenzene < phenol < m-nitrophenol. This order is more evident in comparing the inert solvent distribution coefficients Kd,i (the ratio of sorbed concentration to solution concentration) for any given sorbed concentration (Figure 1b, d, and f). All three sorbates exhibited rather large Kd,i values on the different sorbents, indicating that the effect of n-hexadecane uptake on the Kd,i values and on the compounds’ sorbed concentrations is negligible (ref 17; more details in the Supporting Information). The large Kd,i values imply also that sorption from n-hexadecane by organoclays (Figure 1d and f) cannot be considered as a simple partitioning between a hydrocarbon solvent and the organic fraction of the sorbent described as a hydrocarbon-like pseudophase. Such partitioning should be characterized by a distribution coefficient of a magnitude close to 1 (or even less than 1, based on the organic matter content of the organoclays) rather than by the large coefficients measured. The magnitude of the Kd,i values indicates that the role of the sorbate’s partitioning between n-hexadecane and the organic pseudophase of dried organoclays is minor, whereas the major contribution is by adsorptive sorbate-sorbent interactions. Electron donor-acceptor interactions with the siloxane groups of clay sorbents may contribute to the Kd,i values of organic sorbates containing a nitro-group (1, 18). Sorption on nonexchanged or partially HDTMA-exchanged clays (Figure 1a-d), may involve interactions of the nitrogroup with water associated with sodium cations as well as directly with less hydrated sodium ions (1, 19, 20). Water not removed during freeze-drying can be only partially detected by heating at 105 °C. A basal spacing of 1.12 nm was obtained for the freeze-dried Na-clay at 25 °C
as compared with 0.97 nm for the Na-clay after heating at 360 °C for three hours suggesting that a certain amount of water may still be found in the interlayer space of the freezedried Na-clay (15). The presence of a hydroxyl group in a molecule can result in additional hydrogen-bonding contributions to the overall sorbate-sorbent interactions. This powerful additional OH-related potential is clearly seen when comparing the sorption of nitrobenzene to that of mnitrophenol (Figure 1), and it could be associated with interactions between the sorbate and sorbent-complexed water and/or siloxane surfaces (1, 19, 21). Probably, due to the presence of an OH-group, phenol and m-nitrophenol demonstrate the saturation of certain sorption sites in the OC-41 sorbent at relatively low solute concentrations where sorbed concentration does not change significantly with the concentration in solution (Figure 1c) and the Kd,i values show a steep drop (Figure 1d). In the OC-41 and Na-clay, water associated with the inorganic cation could partially block siloxane surfaces, as can the bulky HDTMA cations (7), but on the other hand, the cation-associated water can serve as an H-bond partner. Different openings of the clay structure and the interplay between different types of sorption sites determine the Kd,i values for nitrobenzene and m-nitrophenol when comparing sorbents with a different extent of HDTMA-exchange. The relative strength of phenol interactions is similar to that of nitrobenzene on Na-clay (Figure 1a and b) and approaches that of m-nitrophenol on OC-90 (Figure 1e and f). This shift correlates with the extent of sodium replacement by HDTMA and could be related to the effect of water associated with the exchangeable sodium cations. Compared to m-nitrophenol, phenol may be less capable of interacting with water associated with the inorganic cation or siloxane surfaces partially blocked by water in the less exchanged clays. One reason for the reduced interaction of phenol is phenol’s lower H-bonding ability due to the absence of an electron-withdrawing nitro-group. On the other hand, there is an additional H-bonding potential of m-nitrophenol due to the interaction between the nitro-group serving as an H-acceptor and inorganic cation-associated water as Hdonor. Thus, the nitrobenzene < phenol Phe > NPhe. The stronger the tendency of a sorbate to interact with the sorption sites, the less pronounced is the reduction in sorption due to water-induced competition. When comparing sorption of the different sorbates from n-hexadecane (Figure 1a,c, and e), the stronger sorbatesorbent interactions seem to result in saturation of sorption sites at relatively low solute concentrations. For such a strongly interacting sorbate, water will have a lesser effect on the sorbate-sorbent interactions. It can also be seen from the data presented in Figures 2–4, that the inhibitory effect of water increases as the fraction of the surface not covered by the organic cation increases (Na-clay > OC-41 > OC-90). This is in agreement with the
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expected role of adsorptive sites in uptake of organic compounds by dried organoclays (7) and supports our initial hypothesis that strongly interacting (e.g., specifically interacting, polar) organic compounds may undergo comparable interactions in the hydrated and dried (n-hexadecanesaturated) organoclay phases. The order of reduction in sorption (plotted vs activity) on the Na-clay due to the presence of water (Figure 4b, d, and f) is not the same as on the organoclays; the reduction for NPhe seems to be even greater than for nitrobenzene. Although the reason for the difference in behavior of Na-clay as compared with that of the organoclays is not completely clear, it may be related to the difference in the opening of the Na-clay structure in the presence of water and nhexadecane and in the ability of the sorbates to intercalate into the dried Na-clay. Based on the thermal XRD analysis of the complexes formed by dried Na-bentonite with nitrobenzene and m-nitrophenol, it was suggested that both organic compounds are sorbed from n-hexadecane on the external clay surfaces (15). Yet, m-nitrophenol, being a strongly interacting sorbate, might have partially penetrated into the interlayer space of Na-clay from n-hexadecane. This is less likely to have occurred with nitrobenzene. In the presence of water, where swelling of the Na-clay occurs, both former compounds intercalate into the clay structure (15). Hence, the competitive effect of water on sorption could be partially compensated by increase in the available sorbent surface. This increase of the available surface of the hydrated clay as compared with the dried sorbent can be greater for nitrobenzene than for m-nitrophenol. Therefore, overall absolute sorption reduction is expected to be greater for m-nitrophenol as compared with nitrobenzene. This waterrelated expansion effect would not be significant for sorption onto organoclays due to the already expanded d-spacing caused by the presence of the HDTMA cation. A decrease in the Kd,a value (ratio between sorbed concentration and activity) with increasing sorption implies a gradual increase in the degree of saturation of sorption sites. In accordance with the competition model, such a decrease in Kd,a will be mitigated by the presence of a competing sorbate, for example, water. The decrease in Kd,a should be less evident or disappear altogether for sorbate/ sorbent pairs demonstrating a strong hydration-induced suppression of interactions, due to saturation of the sorption sites with the competing water molecules. This is distinctly seen for sorption of nitrobenzene and phenol onto OC-90, phenol onto OC-41 and nitrobenzene onto the Na-clay (see Supporting Information Figures S3-S5). The attenuation of sorbate-sorbent interactions as a result of water-sorbate competition for sorption sites can be mitigated, in part, by interactions between those organic sorbates capable of strong specific interactions with water molecules in the sorbent phase. Complexed water is obviously present in the hydrated Na-clay and the partially exchanged clay (OC-41) and, to a lesser extent, in the almost fully exchanged clay (OC-90). Water vapor sorption by a 100% HDTMA-exchanged smectite (Wyoming bentonite) was studied such that water “solubility” in the organoclay phase may be estimated to be approximately 50 mg/g (7). Water molecules that occupy sorption sites in the organoclay phase may themselves be alternative sorption sites and serve as water-bridges (1, 15). Such a sorption-enhancing waterbridge effect could be responsible for the greater mnitrophenol interactions with hydrated OC-41 as compared with dried OC-41 at the higher sorbate activities (Figure 3f). It is possible that not only the strong competitive potential of m-nitrophenol but also its interaction with complexed water molecules help to nullify the hydration effect on m-nitrophenol-sorbent interactions on OC-90 (Figure 2f).
One implication of these results is that since the more strongly interacting organic compounds are less susceptible to suppression of sorption in the presence of water, hydrating organoclays may result in an increased differentiation between “weakly” and “strongly” interacting (“nonpolar” and “polar”) compounds in the organoclay phase. This enhanced differentiation may be of importance for organoclay-based sorptive systems designed to function at variable moisture contents as it may result in greater sorbent selectivity at higher water activities. Another important consequence of these results is our understanding of the mechanisms of interaction of organic compounds with soil organic matter (SOM). The assemblages of SOM and mineral components present in soils are considerably more heterogeneous than are the investigated organoclays, and hence, the latter cannot directly serve as models for the complex systems that are soils. Yet, comparison between the behavior of soils and of organoclays vis-a-vis organic sorbates may shed light on the nature of the interaction of organic sorbates with SOM. For example, a different effect of sorbent hydration on the interactions of organic compounds with organoclays and with SOM was observed. While sorbent hydration may significantly enhance the sorbate’s interactions with SOM (16), it strengthened interactions to a much lesser extent or even weakened them in the case of the organoclays. This difference may be associated with the large number of functional groups present in SOM and the noncovalent, intra- as well as intermolecular links they form. Such links block sorbate access but are broken in the presence of water. This effect is not present in the investigated organoclays which lack functional groups in the organic layer. This is in agreement with the previously proposed idea that the enhancement of sorbate interactions by the hydration of SOM is related to the selective hydration of SOM fragments which are exposed due to the rupture of noncovalent SOM linkages (16, 17).
Acknowledgments This work was funded by the Italian government and carried out in the framework of the Italian-Israeli Cooperation on Environmental Technologies. We thank Nadezhda Bukhanovskaya (Institute of Soil, Water, and Environmental Sciences, The Volcani Center, Israel) for help in carrying out the experimental work on sorption. The comments of the reviewers did a lot to improve the clarity of the presentation and are greatly appreciated.
Supporting Information Available Two tables; five figures; details of the sorption and analytical protocols; activity calculations; estimation of the effect of n-hexadecane uptake on the determination of the sorbate distribution coefficients; demonstration of the effect of partial ionization of phenols on the comparison between sorption from water and from n-hexadecane. This material is available free of charge via the Internet at http://pubs.acs.org.
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ES801116B
