Crystalline swelling of organo-modified clays in ethanol–water solutions
Introduction
Permeable reactive barriers offer an effective method for the control of hydrocarbon-contaminated groundwater, as they are capable of reacting physically or chemically with contaminants, thereby minimizing contaminant transport without significant reductions in the permeability of the barrier to the treated groundwater (Guerin et al., 2002). Often, the pollutants are organic solvents or organic species with limited solubilities in water but having high affinity to organo-clays as determined from sorption isotherms Jaynes and Boyd, 1991, Koh and Dixon, 2001, Bonczek et al., 2002. Alther, 1995, Alther, 2002 showed that organo-clays are cost effective as pretreatments when used ahead of activated carbon sorbents in pump-and-treat technologies. Recently, Slade and Gates (2004) showed that, when exposed to neat toluene, hexadecyltrimethylammonium (HDTMA) smectites swell to ∼33–39 Å from interstratified mono- and bimolecular interlayer arrangements (initially at ∼15–17 Å) and to 42–45 Å from bimolecular (17.7 Å) and paraffinlike (>21 Å) interlayer arrangements. Xu and Boyd (1995) and Bonczek et al. (2002) found that these materials sorb increasing amounts of organics with increasing organic carbon content. Slade and Gates (2004) showed that the organic carbon in these organo-clay intercalates consisted of both charge balancing HDTMA cations and intercalated organic salt molecules.
Organically modified clays are commonly used as emulsifiers (thinners) of water–oil systems (e.g., drilling muds) but can also be dispersed in oils (thickeners) to form lubricating greases with lower susceptibilities to heating Jordan, 1963, Van Olphen, 1963. The degree of dispersion is related to the ability of the organo-clay to sorb organic moieties and to swell to form long-range particle–particle interactions Lagaly et al., 1983, Moraru, 2001. Jordan (1963) reported that the gel volumes of bentonites saturated with various alkylammonium cations increased by as much as 27 times when dispersed in n-alkanols of varying carbon tail lengths. Weiss (1963) observed that 2:1 layer silicates, intercalated with n-alkylammonium cations and solvated with n-alkanols or n-alkylamines of like carbon tail length, resulted in layer spacing increases consistent with the formation of bimolecular layers of n-alkyl moieties within the interlayers. Lagaly (1976) further observed that various n-alkanols were capable of expanding alkylammonium 2:1 layer silicate intercalates to layer spacings as great as 70 Å and postulated that the thickness of the bimolecular layer was dependent on the most frequent end-to-end arrangement of alkanol–alkylammonium pairs. Lagaly and Witter (1982) and Lagaly et al. (1983) found that alkylammonium-exchanged clays swelled in various binary (organic-water) mixtures, that the ratio of organic–inorganic solvent sorbed by the organo-clay was constant and independent of the bathing solution mixture at intermediate composition. Furthermore, the presence of inorganic salts, the alkyl tail length (number of carbons) and the layer charge density all influenced the swelling of organo-modified clays. Moraru (2001) found that organic cation coverage up to the cation exchange capacity (CEC) of the clay enhanced gel formation in organic solvents (e.g., toluene) and observed that Bingham yield stress values of these dispersions were maximal at organic cation coverage equivalent to the CEC. Slade and Gates (2004) showed that the incorporation of HDTMA-Br above that amount required to satisfy the CEC increased the ability of HDTMA clays to absorb and disperse in toluene. The initiation of organo-clay dispersion in solvents is related to the ability of certain anions (e.g., Cl−, ClO4−, I−) to impede the formation of solvent clusters within the interlayer pore-space Lagaly and Witter, 1982, Lagaly, 1984, Lagaly, 1987. Enhancing the expansive capacity of organo-modified clays in the presence of polar organic solvents forms the basis of various polymer–clay nanocomposite synthesis methods (e.g., Gilman et al., 2000, Moraru, 2001).
As part of an overall goal to develop clay-based reactive barriers for the remediation of hydrocarbon-contaminated groundwater, this paper aims to determine factors affecting swelling of two commercially available bentonites and three reference clay minerals, modified with the organic cation benzyloctadecyldimethylammonium (BODMA), in solutions of ethanol and water. BODMA was selected because steric effects were expected to yield greater interlayer porosities of BODMA intercalates than from di-alkyl-substituted quaternary ammonium cations. Ethanol–water solutions were used in the first instance because of the ease by which solvation reactions could be controlled. Inorganic salt (NaCl) was added to the solutions to mimic saline groundwater conditions.
Section snippets
Purification and characterization of the smectites
The smectites studied were purified by sedimentation/centrifugation methods to remove the nonswelling components, and XRD and IR were used to check their purity. The montmorillonitic fractions of two bentonites were studied. The Miles bentonite (United Minerals, Miles, Queensland) is composed of about 71% montmorillonite with the remaining materials being quartz, opal CT, feldspar and zeolite (Gates et al., 2002). Arumpo bentonite (Arumpo, Muldoora, New South Wales) is composed of randomly
BODMA smectite properties
The structural formulae and layer charge characteristics are presented in Table 1. The octahedral cation occupancies provide a range in layer charge and, in general, the tetrahedral charge increases with increasing total layer charge. Upton montmorillonite has the lowest tetrahedral charge. Of the two beidellite samples, the SBCa-1, has a very low octahedral charge compared to the De Lamar beidellite.
The %C and %N in the BODMA smectite complexes (Table 2) indicate that, while some benzyl carbon
Conclusions
Equilibration of BODMA smectites with increasing amounts of ethanol in ethanol–water solutions resulted in expansion of the interlayer spaces consistent with changes in interlayer configuration from bimolecular layers of BODMA cations, with their aliphatic carbon tails parallel to the layer plane, to bimolecular layers of BODMA cations whose aliphatic tails reside at a low angle (∼30°) with respect to the layer plane. Organo-modified clays have the capacity to swell more in organic solvents
Acknowledgements
The author acknowledges the many insightful discussions with and advice from P. Slade, G. Churchman and K. Norrish, assistance with organo-clay preparation by J. Anderson and for sample analyses by M. Fritz, G. Riley, M. Raven and S. McClure. J. Post (Vector Engineering, California) kindly provided the De Lamar beidellite.
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