RELATION OF WATER AND NEUTRAL ORGANIC COMPOUNDS IN THE INTERLAYERS OF MIXED C a / T R I

-Organoclays were prepared by exchanging Ca 2+ in a Ca2*-saturated smectite partially or fully with trimethylphenylammonium (TMPA) cations. The mechanistic function of these organoclays as adsorbents for neutral organic compounds in aqueous solution was examined. TMPA cations were found to take a random distribution on the surfaces of mixed Ca/TMPA-smectites. The presence of TMPA, and its random distribution, resulted in water associated with the clay surfaces being held more weakly. Apparently, the interspersing of TMPA and Ca 2§ ions prohibits the formation of a stable network of water molecules around Ca 2§ Water molecules associated with the siloxane surface in mixed Ca/TMPA-clays are removed during the adsorption of neutral organic compounds from bulk water, leaving only --11 strongly held water molecules around each Ca 2§ as opposed to --58 water molecules in homoionic Ca 2tsmectite. These results demonstrate that the amount of water associated with the clay surfaces and interlayers depends on the nature of the exchange cation(s), and not on the amount of available siloxane surface area by itself. We conclude that in TMPA-smectites the TMPA cations function as nonhydrated pillars, and sorption of organic solutes occurs predominantly on the adjacent siloxane surfaces, which are hydrophobic in nature. The water molecules around Ca 2+ in mixed Ca/TMPA-smectites obscures some of the siloxane surfaces. This diminishes sorption capacity, in an amount roughly equivalent to the fraction of the CEC occupied by Ca 2' , because organic solutes cannot displace the waters of hydration of Ca 2+. Key Words--Adsorption, Organoclay, Smectite, TMPA. I N T R O D U C T I O N Due to the i somo r phous subs t i tu t ion in the a luminosi l ica te layers, na tura l c lay minera l s usual ly have a ne t nega t ive cha rge w h i c h is ba l anced by a lka l i -me ta l and a l k a l i n e e a r t h m e t a l ca t ions such as Na § and Ca 2§ The s t rong hydra t ion o f these inorganic cat ions renders the mine ra l surfaces hydrophi l ic . Ear ly studies (Stark 1948; H a n s o n and Nex 1953; Call 1957; Spencer et al. 1969; Spencer and Cl ia th 1970) on vapor phase sorpt ion by soils and clays demons t r a t ed that pes t ic ides and o ther re la t ively nonpo la r organic compounds are not s t rongly adsorbed on such hydra ted minera l surfaces. The surface properties of clay minerals can be dramatically changed by simple ion exchange of inorganic cations with a variety of organic cations such as quaternary a m m o n i u m cations of the form [(CH3)3NR] + or [ (CH3)2NRR'] +. Previous studies have shown (Mort land et al. 1986; Boyd, Lee et al. 1988; Boyd, Mort land et al. 1988; Boyd, Sun et al. 1988; Boyd et al. 1991) that the organoclays are effective sorbents for removing organic compounds f rom water, and may therefore be useful in treating contaminated waters, and as components of clay barriers in waste disposal reservoirs. Soil clays can be conver ted to organoclays in this fashion, and this substantially enhances their sorptive capabilities for organic contaminants (Boyd, Lee et al. 1988; Lee et al. 1989). The formation of subsurface organoclay sorptive zones via in-situ injections of cationic surfactant solutions has been suggested (Boyd et al. 1991; Bun'is and Antwor th 1992). f f properly placed, such zones could intercept and immobi l ize subsurface chemical plumes, perhaps followed by degradation of the immobi l ized contaminants (Nye et al. 1994). The sorpt ive proper t ies of the o rganoc lays fo rmed wi th qua te rna ry a m m o n i u m cat ions depend largely on the size of Rgroup (Boyd et al. 1991; Mor t l and et al. 1986; Boyd, Sun et al. 1988). W h e n R is a large alkyl g roup such as hexadecyl , the o rganoc lays fo rmed f rom smect i te were ef fec t ive sorbents for pheno l and ch lo ropheno l s compared to unmodi f i ed smecti te . A recent s tudy s h o w e d that c lays modi f ied wi th hexadec y l t r i m e t h y l a m m o n i u m ( H D T M A ) e f f ec t i ve ly so rb neutra l organic con taminan t s v ia e i ther s ingular or mul t ip le m e c h a n i s m ( s ) (Sheng et al. 1996a). For aliphat ic c o m p o u n d s such as t r i ch loroe thy lene and carbon te t rachlor ide, par t i t ioning into the organic phase der ived f rom the cong lomera t i on of H D T M A is the s ingular sorp t ion m e c h a n i s m by H D T M A c l a y s , s imi lar to solute par t i t ion ing in to soil organic mat te r (Chiou et al. 1979, 1983). However , the i so the rms descr ib ing sorpt ion of a l iphat ics by H D T M A c l a y s are genera l ly type III (Gregg and Sing 1982) compared to the l inear i so the rms c o m m o n l y o b s e r v e d for sorp t ion by soil organic matter. This is due to the m u c h h igher degree of sorp t ion into the H D T M A phase, as compared to soil organic matter, w h i c h substant ia l ly changes the compos i t i on of H D T M A phase and causes the so lvency of H D T M A phase for solutes to increase. Mul t ip le m e c h a n i s m s are i nvo lved in the sorpt ion of Copyright 9 1998, The Clay Minerals Society 10 Vol. 46, No. 1, 1998 Water and organic compounds in TMPA-smectites 11 Table 1. Properties of TMPA-smectites. Clay Sample name OC% % of CEC occupied by TMPA d(001) (A) 1 SAC-TMPA. 17 1.66 17 14.42, 18.89 2 SAC-TMPA.38 3.57 38 14.12, 18.97 3 SAC-TMPA.65 5.88 65 14.16 4 SAC-TMPA.75 6.75 75 14.95 5 SAC-TMPA 1.0 8.84 100 14.38 aromatic compounds by HDTMA-clays. Aromatic molecules solvate the cationic ammonium centers and alkyl chains of HDTMA, and are concomitantly adsorbed on the vacated mineral surfaces, yielding a sigmoid isotherm. The combination of solvation (a sigmoid isotherm) and partitioning (a type III isotherm) produces a double-sigmoid isotherm for aromatic solute sorption by HDTMA-clays. In contrast to the solvation and partitioning behavior of organoclays exchanged with large organic cations such as HDTMA, organoclays formed with small organic cations display adsorptive properties. Due to their small size, organic cations such as tetramethylammonium (TMA) or TMPA do not form a continuous organic phase on the clay surfaces and in the interlayers. Rather, they are physically isolated, leaving unobscured (free) siloxane mineral surfaces between the ammonium pillars. Lee et al. (1989, 1990) studied the adsorption of benzene and substituted benzenes by TMA-smectite. Sorption of organic compounds as vapors by the dry clay and as solutes from water was examined. Sorption by the dry clay manifested type II isotherms (Gregg and Sing 1982) and was not strongly dependent on sorbate size. However, shape-selective adsorption of aromatic molecules from water by TMAsmectite was evidenced by high uptake of benzene and progressively lower uptake of larger aromatic molecules. This phenomenon was ascribed to the shrinkage of interlamellar cavities by water. Jaynes and Boyd (1990) reported that TMPA-smectite was also an effective adsorbent of water soluble aromatic hydrocarbons including benzene, toluene, ethylbenzene, p-xylene, butylbenzene and naphthalene, and did not show the strong shape-selective adsorption characteristics of TMA-smectite. It was suggested that the lower degree of hydration of TMPA as compared to TMA accounted for the different adsorption characteristics of the corresponding organoclays. In a subsequent study (Jaynes and Boyd 1991), the charge density of smectite was chemically reduced, resulting in different TMPA contents in the organoclays. Adsorption was found to increase as layer charge and TMPA content decreased. Jaynes and Boyd (1991) concluded that the siloxane surface of smectite is hydrophobic, and aromatic molecules were mainly adsorbed on the siloxane surfaces between TMPA cations. TMPA cations had little direct effect on adsorption, and functioned as nonhydrated "pillars" to keep the smectite interlayers open for adsorption. Considering the important effects of charge reduction and TMPA density on the adsorption of aromatic hydrocarbons by TMPA-smectites, it was of interest to evaluate the adsorptive properties of mixed Ca/TMPAsmectites. Differences would be expected between reduced-charge TMPA smectite and mixed CatTMPAsmectite because charge reduction leaves the hydrophobic siloxane surfaces available for the adsorption of organic compounds while Ca 2§ in mixed Ca/TMPAsmectite will occupy the surfaces and may be hydrated. The overall objective of the current work is to evaluate the function of TMPA and Ca 2+ cations in the adsorption of organic compounds on mixed Ca/TMPAsmectites. MATERIALS AND METHODS Smectite from a Wyoming bentonite (designated SAC), which has Na as the primary exchangeable cation and a cation-exchange capacity (CEC) of --90 cmolc kg -1 (centimoles of charge kg -1) (Jaynes and Boyd 1990) was obtained from the American Colloid Company (Chicago, Illinois). The <2 Ixm clay fractions were obtained by the wet sedimentation method, and subsequently Ca-saturated. TMPA bromide (Aldrich Chemical Co., Milwaukee, Wisconsin) was used to prepare organoclays. To obtain mixed Ca/TMPA clay, aqueous TMPA solutions were added to 2 L of smectite suspensions containing 25 g of Ca-saturated smectite in amounts less than the CEC of the clay. To fully saturate the smectite with TMPA, the total amount of TMPA added was equal to 3 times the CEC of the smectite. The mixtures of the smectite suspensions and TMPA solution were stirred overnight at room temperature (--23 ~ The clay suspensions were then washed with distilled water repeatedly until free of bromide ions (no white AgBr precipitate from addition of AgNO3), and subsequently quick-frozen and freeze-dried. Organic carbon (OC) contents were determined using Dohrmann DC-190 high temperature TOC analyzer (Rosemount Analytical Inc., Santa Clara, California), and used to calculate the percent of CEC occupied by TMPA cations (Table 1). The batch equilibration technique was used to quantify TMPA adsorption and Ca 2§ release. Ca2*-saturated smectite (0.1 g) was weighed into a series of 25-mL Corex glass centrifuge tubes containing different volumes of aqueous TMPA bromide solutions (3.6 cmol L-~), corresponding to 0.16, 0.32, 0.4, 0.6, 0.8, 0.96, 1.2, 1.6, 2.4, 4.0, 6.0, 8.0 times CEC of the smectite, followed by adding distilled water to bring the total volume to 20 mL. The tubes were shaken on a reciprocating shaker at room temperature for 4 d. After centrifugation at 8000 rpm (relative centrifugal force, RCF = 4302 g) for 10 rain, a portion of each supernatant was taken and diluted to a concentration of be1 2 S h e n g a n d B o y d Clays and Clay Minerals Figure 1. (right). 200 ........ , ........ , ........ , I . . . . I . . . . , . . . . i ' i ~ TMPA adS~ tiOn t ~'~ 160 --o-Ca rel 200