Kinetics of Phenol and Aniline Adsorption and Desorption on an Organo—Clay

The use of organo-clays in wastewater treatment, as alternatives to activated C for sorbing pollutants, and in clay liners is of great interest to soil and environmental scientists. There is a lack of information, however, on the kinetics and mechanisms of organo-clay interactions with pollutants. Accordingly, in this study, the time-dependent adsorption and desorption of phenol and aniline on hexadecyltrimethylammonium-montmorillonite (HDTMA-Montmorillonite) from aqueous solutions were determined in a stirred-flow chamber. The experiments were conducted at two concentration levels, 0.001 and 0.01 mol L ' for aniline, phenol, and their mixed solutions. Carbon-labeled (C) solutes were used in the study. Adsorption of both phenol and aniline was completed within 40 min and desorption in 20 min. When the sorptives were mixed, the rates of adsorption-desorption were not significantly affected. The rate of phenol adsorption was influenced by sorptive concentration while the rate of aniline adsorption was independent of sorptive concentration. Various mechanisms appear to be involved in the adsorption processes. O POLLUTANT SORPTION on clay-organic complexes (organo-clays) has been of interest for the last two decades to soil and environmental scientists concerned with organic pollutant mobility in soils, natural groundwater, and surface water bodies, and to engineers seeking a method for removing harmful organic compounds in wastewater treatment processes. When organic metal cations are adsorbed on the cation exchange sites of clays, the surface properties of the clays may be significantly modified. Longchain alkylammonium cations adsorbed on clays, for example, change the nature of the surface from hydrophilic to more hydrophobic (Mortland et al., 1986). Whereas enzyme (protein) or organic pollutant adsorption on smectite is ordinarily pH dependent, enzymes and organics may be retained on clay-organic complexes such as HDTMA+-smectite by hydrophoP.-C. Zhang, Research Center, State Univ. of New York College at Oswego, Oswego, NY 13126; D.L. Sparks, Dep. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303. Contribution from the Delaware Agric. Exp. Stn. no. 1469 and the Dep. of Plant and Soil Sciences no. 310. Received 5 Dec. 1991. *Corresponding author. Published in Soil Sci. Soc. Am. J. 57:340-345 (1993) bic binding, which is independent of pH (Garwood et al., 1983; Boyd and Mortland, 1985a,b, 1986). Such an interaction involves hydrophobic portions of the enzyme interacting with the hydrophobic alkyl group on the mineral surface. The ability of smectites to adsorb aromatics, such as benzene and phenol, from aqueous solution is greatly increased by the replacement of inorganic exchangeable cations with TMA ions (McBride et al., 1977). For example, Cu-smectite removed only 4% benzene and 0% phenol from aqueous solutions. However, TMA-smectite removed 47% benzene and 27% phenol under the same experimental conditions. The polycation CPC was used to block cation-exchange sites on montmorillonite and modify its surface properties (Srinivasan and Fogler, 1990a,b). Electrokinetic measurements demonstrated that the sorbed polycations were essentially nonexchangeable. About 90% of the cationic surfactant (CPC) was apparently irreversibly bound to the surface. Flocculation and peptization were performed to establish that the adsorbed surfactant moiety was oriented with its hydrocarbon tail towards the surface. The modified montmorillonite bound organic pollutants as strongly as granulated activated carbon (Srinivasan and Fogler, 1990a). The adsorption-desorption kinetics of organic compounds on organo-clays has not been extensively studied. Most investigators have concentrated on equilibrium aspects, because they were interested in the modification of the clay and the ability of the modified surface to sorb organic pollutants. To better understand and predict how organo-clays affect the mobility and retention of organic pollutants with time, it is imperative that the kinetics of these reactions be understood. Such information is lacking in the literature and would assist in evaluating the usefulness of organo-clays in wastewater treatment, as alternatives to activated carbon for adsorbing organic pollutants, and in clay liners. Accordingly, in this study, we investigated the kinetics of phenol and aniline adsorpAbbreviations: HDTMA, hexadecyltrimethylammonium; TMA, tetramethylamrnonium; CPC, cetylpyridinium; DI, double deionized; CEC, cation-exchange capacity; **, significant at the 0.01 confidence level. ZHANG & SPARKS: PHENOL AND ANILINE ADSORPTION AND DESORPTION KINETICS 341 tion-desorption on an organo-clay, HDTMAmontmorillonite. MATERIALS AND METHODS Experimental Procedures Montmorillonite (SWY-1, Crook Co., Wyoming) used in the study was obtained from the Source Clays Repository (Clay Minerals Society, Columbia, MO). It was suspended in a HC1 solution of pH 3 to remove carbonates. Then, H2O2 was added to the suspension and the suspension was placed in a water bath at 315 to 318 K for 6 h to oxidize the naturally occurring organic materials. The clay suspension was then dialyzed in a membrane tube immersed in DI water and the water was changed daily until the conductivity of the suspension equaled that of the DI water. The clay suspension was fractioned into the <2H,m fraction by centrifugation, saturated with Ca using 0.5 mol L' CaCl2, and dialyzed in DI water to wash out excess electrolyte. The HDTMA-montmorillonite was prepared using the procedure of Boyd et al. (1988). Hexadecyltrimethylammonium bromide (Aldrich Chemical Co., Milwaukee, WI) was added to the montmorillonite suspension in an amount that equaled the CEC of the montmorillonite clay, approximately 90 cmoy kg. After 2 h of standing, the organo-clay was washed with DI water and then freeze-dried. A static study was conducted to obtain adsorption isotherms. The HDTMA-montmorillonite (0.500 g) was placed in a 30mL centrifuge tube and a C-labeled phenol or aniline solution was added. Ionic strength in the suspension was adjusted to 0.01 M with a 0.1 mol LCaCl? solution. After shaking the suspension for 6 h, the time at which equilibrium was attained, the suspension was centrifuged at 17, 210 x g for 20 min. A 1-mL aliquot of the supernatant solution was placed in a scintillation vial containing 10 mL of scintillation solution (ScintiVerse E, Fisher Scientific, Pittsburgh, PA). The activity of the radioactive isotope in the vial was determined with a Beckman LS 5000 TA scintillation counter (Beckman Instruments, Fullerton, CA). For each treatment, the same procedure was performed with a reference sample (phenol or aniline in CaCl? solution without HDTMA-montmorillonite). The adsorption of phenol or aniline was determined from the difference in activities of the treatment and its reference. We assumed that the C-labeled phenol or aniline molecules were adsorbed on the organo-clay like the unlabeled molecules. All experiments were conducted at a temperature of 297 ± 1 K and a measured pH of 6.4 ± 0.3. Kinetics of adsorption and desorption of phenol and aniline and a mixture of these two organics on HDTMA-montmorillonite were studied at concentrations of 0.001 and 0.01 mol L". The kinetics were studied using the stirred-flow reaction chamber first developed by Carski and Sparks (1985) and later modified by Toner et al. (1989). A magnetic stir star, 0.500 g of HDTMA-montmorillonite, and 5.0 mL of a 0.003 mol L' CaCl2 solution were placed in the chamber, complete with influent and effluent ports and a plunger base. A prefilter and filter were fitted just below the effluent port to retain the modified clay, a top was attached, and a known volume of CaCl2 solution was injected into the chamber. The plunger base was then used to displace the excess air from the chamber, thus enabling precise control of the reaction volume. An LKB 2132 Microperpex peristaltic pump (LKB-Produkt AB, Bromma, Sweden) was used to maintain a constant sorptive flow rate of 0.001 L min-'. The CaCl2 solution was passed through the chamber for 1 h to wash out any free HDTMA+ before the organic sorptive solution was introduced into the chamber. For adsorption studies, effluent was collected at 1-min intervals for the first 30 min and then at 2-min intervals. For the desorption experiments, effluent was collected every 1 min for the first 20 min and then at intervals of 2, 3, and 5 min. The adsorption-desorption procedures were followed without organo-clay to construct a dilution curve to distinguish the effects of dilution from adsorption and desorption effects on effluent concentration (Carski and Sparks, 1985; Sadusky and Sparks, 1991) and also to verify the equation derived to calculate the dilution factor, as discussed below. After running the CaCl2 solution through the chamber, Clabeled phenol or aniline solution was pumped through the chamber at a flow rate of 0.001 L min-. Effluent was collected in vials containing 10 mL of scintillation solution. The adsorption studies were continued until an equilibrium in adsorption was attained. Following adsorption, desorption was effected by passing 0.003 mol L' CaCl2 through the chamber until an equilibrium was reached. Analysis of the organics was determined as described for the static studies. Analysis of Kinetic Data The quantity of phenol and aniline adsorption-desorption on HDTMA-montmorillonite was calculated from the differences in concentration of influent and effluent solutions at the time of sampling and in the dilution factor, defined below. For adsorption, the following equation was used to calculate the amount of organic material adsorbed on HDTMA-montmorillonite as a function of time: (2a = = I [(C, Q/A/ (C,+A, C,)V]/m [1]