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Ingeniería ambiental
  Water Research 36 (2002) 1107–1114 Removal of phenol from water by adsorption–flocculationusing organobentonite Yun-Hwei Shen* Department of Resources En g ineerin g , National Chen g  Kun g  Uni  v ersity, Tainan, Taiwan 70101, ROC  Received 9 January 2001; received in revised form 30 May 2001; accepted 25 June 2001 Abstract Bentonite modified with short chain cationic surfactant might be the basis of a new approach to removing organicpollutants from water. The treatment process involves dispersing bentonite to the contaminated water and then addinga small cationic surfactant so as to result in flocs which are agglomerates of organobentonite and bound organicpollutants. The flocs are then removed from the solution by sedimentation. Experimental results indicate that BTMA-bentonite displays a high affinity for phenol, possibly because phenol molecules interact favorably with the benzene ringin BTMA ion through increased  p 2 p  type interactions. Under appropriate operating conditions, 90% phenol removaland nearly 100% bentonite recovery could be achieved by the adsorption–flocculation process using BTMA-bentonite.Additionally, the insensitivity of the process to the changing ionic strength of the solution and rapid adsorption kineticsmade adsorption–flocculation with BTMA-bentonite attractive for continuous treatment of large volumes of industrialwastewater. The bentonite may function as a recyclable surfactant support for the adsorption and subsequentcombustion of organic pollutants. r 2002 Elsevier Science Ltd. All rights reserved. Keywords:  Organobentonite; Adsorption; Flocculation; Phenol 1. Introduction A montmorillonitic clay, bentonite, is characterizedmainly by an Al octahedral sheet placed between two Sitetrahedral sheets. The isomorphous substitution of Al 3+ for Si 4+ in the tetrahedral layer and Mg 2+ forAl 3+ in the octahedral layer results in a net negativesurface charge on the clay. This charge imbalance isoffset by exchangeable cations (typically Na + andCa 2+ ) at the clay surface. The layered structure of theclay facilitates expansion after wetting. Na + and Ca 2+ are strongly hydrated in the presence of water, resultingin a hydrophilic environment at the clay surface.Consequently, natural bentonite is an ineffective sorbentfor nonionic organic compounds (NOC) in water,although it has a high surface area. As is well known,simple ion-exchange reactions can significantly modifythe surface properties of natural bentonite. Whenorganic cations (cationic surfactants) of the form(CH 3 ) 3 NR + , where R is an alkyl hydrocarbon,occupy the exchange sites of bentonite clay, thesurface properties transform from hydrophilic tohydrophobic. Recent studies have shown that themolecular structure of cationic surfactants used tomodify clay affects the mechanism of NOC sorption[1–4]. Smith et al. [1] demonstrated that small organiccations create a relatively rigid, nonpolar surfaceamenable to NOC uptake by adsorption, a surfacecharacterized by a relatively high NOC uptake, isothermnonlinearity, and competitive sorption. In contrast,large organic cations create an organic partitionmedium for NOC uptake through the conglomera-tion of their flexible alkyl chains, a medium character-ized by a relatively low NOC uptake, isotherm linearity,and noncompetitive sorption. Lee et al. [5] confirmedthe effectiveness of smectite intercalated with smalltetramethylammonium (TMA) ion in removing *Tel.: +886-6-2757575-62820; fax: +886-6-238-0421. E-mail address: (Y.-H. Shen).0043-1354/02/$-see front matter r 2002 Elsevier Science Ltd. All rights reserved.PII: S0043-1354(01)00324-4  benzene from water, and its potential for purifyingbenzene-contaminated water.Activated carbon is widely applied for removingorganic pollutants from water. The porous nature of this adsorbent material and its high internal surface areaare favorable properties for adsorption. The adsorptionof organic compounds by activated carbon is partlycontrolled by physical interactions, including size exclu-sion and microporosity effects [6,7]. As the molecularsize of the organic compound increases, the rate of diffusion into the pores of the granular activated carbon(GAC) decreases. Further increases in the molecular sizedecrease sorption due to limited access to the interiorpore structure of the carbon. Powdered activated carbon(PAC) with rapid adsorption kinetics caused by a smallparticle size may be used to increase carbon utilizationand reduce the volume of the adsorber relative to a GACadsorber. However, recovering small PAC particlesfrom treated water may be awkward.Dispersed in solution, bentonite exhibits a largesurface area comparable to PAC because of swellingand layer delamination. Additionally, as noted pre-viously, bentonite modified by the addition of the smallcationic surfactant exhibits a high sorption capacity forNOC. Binding of cationic surfactant onto the negativelycharged surface of bentonite also causes charge neu-tralization of the bentonite and subsequent coagulation.The floc is formed from all the bentonite in solution andcan easily be removed from the solution by sedimenta-tion. This quality combined with its ability to hold NOCmakes bentonite flocculation with small cationic surfac-tant a possible basis for a new approach to wastewaterand chemical separation techniques.This study examines the feasibility of using bentonitemodified by the addition of the small cationic surfactantto remove NOC from water. The treatment processinvolves dispersing bentonite to the contaminated waterand subsequently adding a small cationic surfactant tocreate flocs, which are an agglomerate of organobento-nite and bound NOC molecules. The flocs are thenremoved from the solution by sedimentation. Fig. 1presents a flow diagram of the proposed process. Thisstudy evaluates the effectiveness of the proposed processin removing NOC from water and examining the majoroperating parameters that may control the process.Phenol is chosen to test the feasibility of the proposedprocess because it is classified by the US EPA as apriority pollutant and represents one of the morechallenging classes of pollutants requiring removal fromwaste streams. 2. Materials and methods 2.1. Materials Bentonite having cation-exchange capacity (CEC) of 97meq100g  1 (determined by the BaCl 2  –Triethanol-amine method [8]) obtained from Aldrich Chemical Co.was used. It contained 3.6% sand, 7.3% silt, and 89.1%clay with an average particle size of 5 m m as reported bythe supplier. Quaternary ammonium compounds wereobtained from Aldrich Chemical Co. and used asreceived. Table 1 lists the molecular formulae, chemicalpurities, and abbreviations of these compounds. TheNOC tested herein, phenol, was of analytical grade,purchased from Riedel-de Haen and used withoutfurther purification. 2.2. Phenol batch sorption experiments Each batch test sample in the sorption experimentswas prepared in a 22ml glass tube, with 0.2g of bentonite together with a pre-calculated volume of  Fig. 1. Flow diagram of the proposed adsorption–flocculation process. Y.-H. Shen / Water Research 36 (2002) 1107–1114 1108  cationic surfactant solution and phenol stock (inmethanol) and sufficient deionized water to bring thetotal fluid volume to 22ml. The organobentonites wereprepared at different saturated levels corresponding tothe different amounts of CEC occupied by cationicsurfactant. For example, the 50%, 75%, and 100% CECsaturated organobentonites were prepared by adding0.097, 0.1455, and 0.194mmol of cationic surfactant,respectively, into the above mentioned 22ml glass tube.The pH of each sample was controlled at about 7.0 byHCl and NaOH solutions. The glass tubes were sealedwith Teflon-lined septa and secured with open-portscrew caps. Next, the samples were equilibrated for 8hby rotating on a tube rotator at room temperature.Preliminary kinetic investigations revealed that sorptionequilibrium was reached in less than 1h. The aqueousphase was separated by centrifugation at 5000rpm for1h. Aliquots of the supernatant were withdrawn fromeach sample for phenol analysis. The difference in theamount before and after sorption reveals the amount of phenol sorbed. All samples were run in duplicates, andthe recovery of phenol ranged from 96% to 101%.Results of control experiment indicate the negligiblesorption of phenol on clean bentonite surface. Phenolconcentration was determined by using a high-pressureliquid chromatography instrument (HPLC, Jasco, Ja-pan), equipped with a reverse-phase column (C-18,Phase Sep, UK) and an ultraviolet (UV) detector(270nm, Model 975, Jasco, Japan) with isocraticoperation (1ml min  1 ) of mobile phase (40/60vol%acetonitrile/aqueous solution). 2.3. Flocculation experiments For each test, a 1.2g sample of bentonite was stirredin 190ml of distilled water in a 250ml beaker, fitted withfour 0.25in wide baffle plates and a 1in diameterpropeller, for 10min. The suspension was then adjustedto pH 7.0 and further conditioned for 2min. Afterconditioning, different amounts of organic cation (in 10-ml solution) corresponding to the different levels of saturation of the benotonite’s CEC were then added tothe suspension while the propeller was rotated at800revmin  1 . After 3min of rapid mixing, the samplewas stirred for a further 10min at 200revmin  1 , thenleft to settle for 1h. The concentration of bentonite inthe suspension was estimated by absorbance measure-ments taken with a Shimazdu UV-160A spectrophot-ometer using 10-mm cuvattes at a wavelength of 650nm.A calibration curve of the absorbance vs. bentoniteconcentration was obtained. Samples were taken beforeand after flocculation, and the absorbance was deter-mined. The percentage of bentonite removal wascalculated as usual. 2.4. Adsorption–flocculation experiments For each test, a pre-weighed amount of bentonite wasadded to 190ml of phenol containing solution in a250ml beaker fitted with four 0.25in wide baffle platesand a 1in diameter propeller to stir the sample. Thesuspension was then adjusted to pH 7.0 and conditionedfor 3min. Different amounts of organic cation (in 10-mlsolution), corresponding to the different levels of bentonite’s CEC occupied were then added to thesuspension, while the propeller was rotating at 800revmin  1 . After 3min of rapid mixing, the sample wasstirred for a further 10min at 200revmin  1 , and thenleft to settle for 1h. The concentration of phenol andbentonite in the supernatant were determined asdescribed above. 3. Results and discussion 3.1. Phenol batch sorption experiments Fig. 2 presents the sorption isotherms of phenol fromwater, onto the TMA-, BTMA-, TEA-, BTEA-, andTMH-bentonites, respectively, at 100% saturation of the benotonite’s CEC. The adsorption of phenol on theBTMA- and BTEA-bentonite corresponds to the L-typeisotherm in Giles’ classification [9], reflecting a relativelyhigh affinity between the adsorbate and adsorbent. Theshape of the adsorption isotherms of TMA-, TEA-, andTMH-bentonites are S-type isotherms in Giles’ classifi-cation, suggesting that the adsorbate–adsorbate inter-action is stronger than the adsorbate–adsorbent inter-action. The relatively high affinity between phenol andBTMA- and BTEA-bentonite is probably the result of  Table 1Molecular formulas, abbreviations, and percent chemical purities of five quaternary ammonium compoundsSurfactant Formula Abbreviation Purity (%)Trimethylamine hydrochloride N(CH 3 ) 3 H + Cl  TMH 98Tetramethylammonium bromide N(CH 3 ) 4+ Br  TMA 98Benzyltrimethylammonium bromide N(CH 3 ) 3 (CH 2 C 6 H 5 ) + Br  BTMA 97Tetraethylammonium bromide N(C 2 H 5 ) 4+ Br  TEA 99Benzyltriethylammonium bromide N(C 2 H 5 ) 3 (CH 2 C 6 H 5 ) + Br  BTEA 99 Y.-H. Shen / Water Research 36 (2002) 1107–1114  1109  the phenol molecules interacting favorably with thebenzene ring in BTMA and BTEA ions throughincreased  p 2 p  type interactions [10,11]. Meanwhile,the saturated uptake of phenol by BTMA-bentonite(34mgg  1 ) is considerably higher than that by BTEA-bentonite (21mgg  1 ), presumably due to the smallersize of the BTMA ion. Compared to the larger BTEAions, the intercalated smaller BTMA ions can accumu-late more tightly with only a slight steric interference onthe interlamellar surfaces of the bentonite, to form arelatively rigid, nonpolar organic carbon surface forhigher phenol uptake. Fig. 2 demonstrated that bento-nite modified by the BTMA ion exhibits a high phenoluptake capacity under a low concentration of phenol inthe solution, and may be an ideal organobentonite foradsorption–flocculation.Fig. 3 shows the sorption isotherms of phenol onBTMA-bentonites saturated at different levels of bento-nite’s CEC. The adsorption capacity of phenol onBTMA-bentonite saturated at 50–150% levels did notdiffer very significantly. Restated, the uptake of phenolon BTMA-bentonite does not increase with the numberof intercalated BTMA ions from 50% to 100%,implying that not all the intercalated BTMA ions areavailable for phenol uptake. Presumably, only thoseBTMA ions intercalated at local high charge densityregions of the interlamellar surfaces of bentonite canaggregate effectively to form a relatively rigid, nonpolarsurface for phenol uptake. As expected, the sites withinthe local high charge density areas in bentonite arepreferentially occupied when bentonite exchanges withBTMA ions. Fig. 3 shows that BTMA-bentonitessaturated from 50% to 150% levels of bentonite’sCEC are equally effective in removing phenol fromwastewater. 3.2. Flocculation experiments Fig. 4 presents the flocculation of bentonite suspen-sion at about pH 7.0, expressed in terms of percentage of solid removal, as a function of surfactant dosage(expressed in terms of %CEC of bentonite saturated).Evidently, BTMA, TEA, and BTEA ions cause effectivesolid removal (>99%) at surfactant dosages above 60%saturation of bentonite’s CEC. Clearly, the observedflocculation behavior can be attributed to the adsorptionof cationic surfactant, resulting in the charge neutraliza-tion of bentonite. The restabilization of bentoniteparticles was not observed even at a surfactant dosageof 200% saturation of bentonite’s CEC, presumably dueto the very large surface area of bentonite in suspension.Based on the results from phenol batch sorption testsand flocculation tests, BTMA-bentonite with 75%saturation of bentonite’s CEC was selected in thefollowing adsorption–flocculation experiments basedon the consideration of both treatment efficiency andcost. Fig. 2. Sorption isotherms of phenol on TMH-, TMA-, BTMA-, TEA-, and BTEA-bentonite. Y.-H. Shen / Water Research 36 (2002) 1107–1114 1110
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