Adsorption of phenol and m-chlorophenol onorganobentonites and repeated thermal regeneration

S.H. Lin*, M.J. Cheng

Department of Chemical Engineering, Yuan Ze University, Chungli 320, Taiwan, Peoples Republic of China

Received 24 November 2000; received in revised form 2 April 2001; accepted 2 April 2001

Abstract

Experimental investigations were conducted on the adsorption characteristics of phenol and m-chlorophenol by organobento-

nites. The organobentonites were prepared by modifying natural bentonite with various quaternary ammonium salts includingtetramethylammonium bromide, hexadecyltrimethylammonium bromide, benzyl-triethylammonium bromide, tetraethylammoniumbromide and cetylpyridinium bromide. The adsorption characteristics of phenol and chlorophenol by these organobentonites wereexamined in detail. The empirical Freundlich isotherm was found to describe well the equilibrium adsorption data. Thermal

regeneration of spent organobentonites was also investigated and operating conditions of 200�C and 2 h heating were found to yieldvery good results. # 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Organobentonite; Phenol; m-Chlorophenol; Adsorption; Regeneration

1. Introduction

There is growing public concern about the widespreadcontamination of surface and ground waters by variousorganic compounds due to the rapid development ofchemical and petrochemical industries over the pastseveral decades. In addition, during the manufacturingand processing stages of organic chemicals, largeamounts of wastewaters are often generated. Theorganic contents in these wastewaters usually exceed thelevel for safe discharge. Therefore, removal of organiccompounds from the wastewater has become an integralpart of wastewater treatment systems used in the che-mical and petrochemical industries.In the past several decades, extensive research has

been conducted to develop innovative and promisingadsorbent materials for dealing with the treatment pro-blem of contaminated industrial effluents. The ultimategoal of this endeavor is to identify an effective andinexpensive adsorbent for the volatile organic com-pound (VOC) removal from aqueous solutions. Orga-noclays are the adsorbent materials that have been

generally considered by many investigators to have thepotential of meeting this requirement [1,2].Among the various organoclays, organobentonites

are the most widely investigated by many researchers [3–19]. Organobentonites are produced by replacingexchangeable inorganic cations (e.g. Na+ Ca2+, H+) onthe internal and external mineral surfaces of bentonitewith quaternary alkylammonium cations. Naturallyoccurring bentonite is not efficient as an adsorbent for theuptake of hydrophobic organic pollutants from aqueoussolution due primarily to the electrically charged andhydrophilic characteristics of its surface. With thisexchange treatment, bentonite surfaces are drasticallyaltered. As a result, organobentonites have enhancedsorption capacities for nonionic organic pollutants [1,2].Due to their unique sorption capabilities, organo-

bentonites have been investigated for a wide variety ofenvironmental applications. Organobentonites werefound to be rather effective in removing various organicpollutants from the industrial wastewaters [3–19].However, the previous investigations focused primarilyon the various fundamental aspects of the adsorptionprocess, such as the adsorption isotherms, kinetics oradsorption efficiencies under different operating condi-tions. One important issue not considered by the pre-vious investigations is the regeneration of exhausted

0956-053X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0956-053X(01 )00029-0

Waste Management 22 (2002) 595–603

www.elsevier.nl/locate/wasman

* Corresponding auhtor. Tel.: +886-3-463-8910; fax: +886-3-455-

9373.

E-mail address: ceshlin@saturn.yzu.edu.tw (S.H. Lin).

organobentonites. Like other adsorbents, organobento-nites will eventually become saturated over time. Withoutregeneration, exhausted organobentonites would requiredisposal as solid wastes. In order tominimize this problemand save on adsorbent costs, regeneration is essential. Inthe present work, experimental tests were conducted toevaluate the thermal regeneration process for spentorganobentonites. Several quaternary ammonium salts,including tetramethylammonium bromide (TMAB),hexadecyltrimethylammonium bromide (HDTMAB),benzyltriethyl- ammonium bromide (BTEAB), tetra-ethylammonium bromide (TEAB) and cetylpyridiniumbromide (CPB), were used to prepare organobentonitesfor the experimental tests. The adsorption character-istics and equilibrium isotherms of the prepared orga-nobentonites were also examined in detail.

2. Experimental investigations

The natural bentonite was obtained from Fu ChangChemical Co. (Taoyuan, Taiwan). The bentonite (<3mm) was in the Na+ form and industrial grade. It wasproduced in the eastern part of Taiwan and used asreceived. The bentonite was found to have a cationexchange capacity (CEC) of 0.58 meq/g and a BETsurface area of 168 m2/g. All quaternary ammoniumsalts (TMAB, HDTMAB, BTEAB, TEAB and CPB)were obtained from Aldrich Chemical Co. (Milwaukee,WI, USA) and were reagent grade. Phenol and m-chlorophenol were obtained from E. Merck GmbH(Darmstadt, Germany) and were also of reagent grade.Stock solutions containing quaternary ammonium

salts of desired concentrations (between 2 and 8 m/m%)were prepared. One hundred millilitres of the qua-ternary ammonium salt solution was placed in a beakerand 20 g of pre-dried bentonite added. The mixture wasstirred at room temperature by a magnetic stirrer for 2h, followed by vacuum filtering and washing severaltimes with deionized water. The organobentonites weredried at 50�C in an oven and then activated for 1 h at110�C. The dry organobentonite was ground to lessthan 60 standard mesh (0.246 mm). Organobentoniteswere prepared using other quaternary ammonium saltsat different concentrations. The BET surface area andits micropore volume were measured using a Micro-meritics porosimeter (Model ASAP 200, MicromeriticsInstrument Corp., Norcross, GA, USA) with N2 ascarrier gas. The organic carbon content of organo-bentonites was determined by the standard method [20].A JOEL scanning electron micrograph (SEM; ModelJSM-5410, Joel Instrument, Inc., Tokyo, Japan) wasused to obtain the SEM images of the natural bentoniteand the exhausted and regenerated organobentonitesIn the equilibrium adsorption tests, 25 ml of prepared

solution containing phenol or chlorophenol were placed

in a flask and 0.5 g of organobentonite added. Theflasks were placed in a constant temperature bath whichwas set at a desired temperature and at 50 rpm speed. Atest run lasted over 12 h to ensure adsorption equili-brium. At the end, samples were taken and centrifugedfor phenol or chlorophenol concentration measure-ments using a Hewlett Parkard gas chromatograph(Model 5890 series II, Hewlett Parkard Instrument,Inc., CO, USA), equipped with a capillary column andFID detector. The equilibrium adsorption tests wereperformed for phenol or chlorophenol solution withinitial concentration between 50 and 300 mg/l.For regeneration, the spent organobentonite was air

dried at room temperature. The organobentonite was putin a quartz cylinder (25 cm long and 8 cm I.D.) whichwas externally heated by an electric heater. The quartzcylinder was purged with N2 gas for 10 min to ensure noair remained in the cylinder and both ends of the cylinderwere sealed before the electric heater was turned on.Heating under an N2 environment prevented oxidationof quaternary amine on the organobentonite, whichcould happen under atmospheric conditions at elevatedtemperature, as observed in the experimental tests, andrendered the regenerated organobentonite useless. Regen-eration temperatures between 100 and 350�C were testedusing regeneration runs of over 2 h. The regeneratedorganobentonites were then used for phenol or chlor-ophenol adsorption tests. The phenol or chlorophenolremoval using regenerated organobentonites served as abasis for evaluation of the regeneration efficiency.

3. Results and discussion

3.1. Preparation and sorption

The BET surface areas and the micropore volumes forvarious organobentonites prepared with 2 and 4 m/m%quaternary ammonium salt solution are listed in Table 1.After exchange treatment, quaternary ammonium salt

Table 1

The BET surface areas and micropore volumes of organobentonitesa

Quaternary amine concentration

2 wt.% 4 wt.%

Sa Vm Sa Vm

HDTMAB 66.9 1.14 37.4 0.86

TMAB 49.9 0.95 26.5 0.61

BTEAB 68.4 0.57 8.4 0.19

CPB 122.5 0.89 12.3 0.28

TEAB 68.4 0.56 8.4 0.19

a Sa, BET surface area, m2/g; Vm, micropore volume, ml/g; TMAB,

tetramethylammonium bromide; HDTMAB, hexadecyltrimethyl-

ammonium bromide; BTEAB, benzyltriethylammonium bromide;

TEAB, tetraethylammonium bromide; CPB, cetylpyridinium bromide.

596 S.H. Lin, M.J. Cheng /Waste Management 22 (2002) 595–603

apparently entered the interstitial spaces of bentoniteand was adsorbed on its mineral surfaces. Hence boththe BET surface area and the micropore volume of thetreated bentonites were significantly reduced in com-parison with these of original bentonite as the qua-ternary ammonium salt concentration increased.For adsorption of organic contaminants on organo-

bentonites, two mechanisms have been reported: (1)partition process; and (2) surface absorbent process [16].When large size organic molecule, for exampleHDTMAB, is interacted into bentonite, partition pro-cess plays an important role. For this mechanism, theadsorption of organic contaminants is functionally andconceptually similar to the dissolution of organic con-taminants in a bulk phase organic solvent such as octa-nol. This mechanism results from the surface propertychange from hydrophilic to hydrophobic because thelong tail of organic molecule forms hydrophobic phasein the interlayer of organobentonite. If this mechanism isa major function of organobentonite prepared, the moreintercalation of organic molecule, the higher adsorptioncapacity of organobentonite. On the other hand, whensmall size organic molecule, such as TMAB, is inter-calated into bentonite, surface adsorbent mechanism isimportant. For this mechanism, the surface of organo-bentonite can be viewed as containing isolate qua-ternary ammonium exchange cations that are separatedby free planar aluminosilicate mineral surfaces. Theincrease of phenol or chlorophenol with an increase ofHDTMAB and CPB concentrations, shown in Fig. 1(a)and (b), indicates the partition process mechanism isimportant. The relative constant adsorption of phenoland chlorophenol on other organobentonites is due tothe surface adsorbent mechanism.Also depicted in Fig. 1(a) and (b) is that the phenol

and chlorophenol removal by BTEAB-bentonite is verygood, being over 80% for BTEAB concentration as lowas 2 m/m%. The HDTMAB- and CPB-bentonites pre-pared using optimum 6 m/m% quaternary ammoniumsalt concentration achieve approximately the sameremoval efficiency for chlorophenol adsorption. However,the TMAB- and TEAB-bentonites had rather low phe-nol and chlorophenol removal for TMAB and TEABconcentrations. The exact reason for this is not known.It may be related to the relative low organic carboncontent of those organobentonites, as seen in Table 2which shows significantly lower organic carbon contentsof TMAB- and TEAB-bentonites than others. Certainly,high organic carbon content may not fully account forthe high adsorption capacity of BTEAB-bentoniteshown in Fig. 1. The BET surface area and the micro-pore volume or size could also play a crucial role.Another point worth looking into is the amount of

organobentonite relative to the amount of phenol orchlorophenol required to effect a good adsorption. Thisaspect is depicted in Fig. 2 for a fixed 10 ml of aqueous

Table 2

Organic carbon content of organobentonites at various surfactant

concentrations

Surfactanta Organic Carbon

0% 2% 4% 6% 8%

BTEAB 0.076 0.29 0.62 0.73 0.76

TMAB 0.076 0.083 0.094 0.122 0.125

CPB 0.076 0.16 0.98 1.52 1.91

HDTMAB 0.076 0.19 1.06 1.69 1.75

TEAB 0.076 0.082 0.087 0.093 0.094

a Surfactant concentration in m/m%. TMAB, tetra-

methylammonium bromide; HDTMAB, hexadecyltrimethyl-ammo-

nium bromide; BTEAB, benzyltriethylammonium bromide; TEAB,

tetraethylammonium bromide; CPB, cetylpyridinium bromide.

Fig. 1. Phenol and chlorophenol removal as function of surfactant

concentration for various quaternary ammonium salts with 0.5 g

organobentonite and 25 ml aqueous solution containing 100 mg/l

phenol or chlorophenol.

S.H. Lin, M.J. Cheng /Waste Management 22 (2002) 595–603 597

solution containing 100 mg/l initial phenol or chlor-ophenol. Apparent in both the top and bottom graphs isthat 8 g of organobentonite/g adsorbate would be areasonable amount for all adsorption cases. Moreorganobentonite does not appear to yield additionalbenefit. Also noted here is that the TMAB- and TEAB-bentonites consistently have low capacity for phenoland chlorophenol adsorption.The two general adsorption isotherms that can be

used to describe the equilibrium adsorption relation arethe well-known monolayer Langmuir and empiricalFreundlich model [21,22] which are represented by

qe ¼abCe

1þ bCeð1Þ

qe ¼ KC1=ne ð2Þ

where qe and Ce are the equilibrium adsorption capacityof organo-bentonite and the equilibrium phenol orchlorophenol concentration in the aqueous solution,respectively, and a, b, K and n are the constant isothermparameters. The results of present equilibrium adsorp-tion tests reveals that the Freundlich isotherm repre-sents the observed data considerably better theLangmuir isotherm. Fig. 3 displays the model fit to theobserved data (solid symbols). The isotherm parametersobtained from the model fit are listed in Table 3. Themodel fit is very good indeed due to large r2 close to 1.

3.2. Regeneration tests

In practical applications of organobentonite for phe-nol or chlorophenol adsorption, the organobentoniteswill eventually become exhausted. Disposal of the phe-nol- or chlorophenol-laden organobentonites thus is animportant issue that needs to be addressed otherwise theexhausted organobentonites would create a new roundof environmental problem. In the previous investiga-tions, this issue has received relatively little attention inthe past [23,24]. In the present study, experimental testswere conducted to thermally regenerate the spent orga-nobentonites in an electric oven under N2 environment.Results from preliminary tests in this study indicatedthat regeneration under atmospheric conditions at ele-vated temperature caused oxidation of quaternaryamines on the surface of organobentonites that led to aconsiderable loss of the adsorption capacity (over 80%)of the regenerated organobentonites. Such an oxidativedecomposition could be considerably minimized under anitrogen environment and the results of this regenera-tion process have been satisfactory. Fig. 4 compares theSEM images (5000�) of the natural bentonite (topimage), phenol-laden BTEAB- and CPB-bentonites (a),BTEAB- and CPB-bentonites regenerated at 200�C (b)and 300�C (c). It is seen that there are some slightchanges on the mineral surfaces among the naturalbentonite, phenol-laden and regenerated organobento-nites. Furthermore, the high regeneration temperaturehad a significant effect on the color of regenerated

Table 3

Constant parameters of the Freundlich isotherma

Adsorbenta Adsorbate K 1/n r2

BTEAB Phenol 0.606 0.531 0.978

Chlorophenol 0.667 0.634 0.997

HDTMAB Phenol 0.015 1.237 0.986

Chlorophenol 0.138 0.991 0.998

CPB Phenol 0.012 1.149 0.994

Chlorophenol 0.643 0.678 0.989

a HDTMAB, hexadecyltrimethyl-ammonium bromide; BTEAB,

benzyltriethylammonium bromide; CPB, cetylpyridinium bromide.

Fig. 2. Phenol and chlorophenol removal as function of the relative

amount of quaternary ammonium salts with 0.5 g organobentonite

and 25 ml aqueous solution containing 100 mg/l phenol or chlor-

ophenol.

598 S.H. Lin, M.J. Cheng /Waste Management 22 (2002) 595–603

organobentonites, as seen in Table 4. The color ofregenerated BTEAB-bentonite did not change for regen-eration temperature up to 300�C. But regenerated CPB-and TMAB-bentonites turned brown at 200 and 250oC,respectively. At 300oC, all regenerated organobentonites,

except for BTEAB-bentonite, all became dark in color.The reason for the color changes of organobentonitesregenerated at higher temperature is not exactly known.It could be due to the fact that at sufficiently highregeneration temperature, phenol or chlorophenol onthe mineral surfaces of some organobentonites waspartially or completely decomposed, resulting in gen-eration of colored organic molecules. The phenol orchlorophenol decomposition appeared to depend on thetype of quaternary ammonium salt used for organo-bentonite preparation. Such a color change of phenol orchlorophenol during high temperature decompositionwas also observed in wet air oxidation of those com-pounds in the aqueous solution [25,26]. The color changesof regenerated organobentonites did have some effecton their adsorption capability, as will be shown later.An important aspect of spent organobentonite regen-

eration is the time and temperature required to effect agood regeneration efficiency. Fig. 5(a) and (b) demon-strate the phenol and chlorophenol removal of regener-ated organobentonites as a function of regenerationtime at constant temperature 200�C. The phenolremoval is seen to increase with the regeneration timeand approaches a steady value after 1.5 to 2 h. Hence aregeneration time of 2 h was adopted for the presentstudy and can be recommended for regeneration purposeThe effect of regeneration temperature on the phenol

and chlorophenol removal of regenerated organobento-nites is demonstrated in Fig. 6. The phenol and chlor-ophenol removal increase with an increase in theregeneration temperature and reach a peak approxi-mately around 200 to 250�C. After that, the phenol andchlorophenol removal start to fall off. As mentionedearlier, some regenerated organobentonites started tochange color after regeneration at or beyond 200�C.The color change was due presumably to generation ofsmall colored organic molecules of phenol and chlor-ophenol upon their decompositions. Some of thosesmall organic molecules generated by phenol or chlor-ophenol decomposition could become adsorbed on theactive mineral surfaces of regenerated organobentonites,leading to a reduced adsorption capacity. Moreover, thequaternary ammonium salts originally adsorbed on themineral surfaces could either be partially removed ordecomposed during regeneration at high temperature.This can also partially account for the reduced phenoland chlorophenol removal of most of the regeneratedorganobentonites. From the results revealed in this fig-ure, it could be conclude that 200�C is a good tempera-ture for exhausted organobentonite regeneration.To further verify the regeneration efficiency of spent

organobentonites by the procedure adopted in the pre-sent study and at the recommended regeneration condi-tions of 200�C for 2 h, the phenol and chlorophenolremoval of regenerated organobentonites for up to fiveadsorption/regeneration cycles is demonstrated in Fig. 7.

Table 4

Colors of organobentonites regenerated at different temperaturesa

Temperature 100�C 150�C 200�C 250�C 300�C

BTEAB White White White White White

TMAB White White White Brown Dark

CPB White White Brown Dark Dark

HDTMAB White White White Dark Dark

a Natural bentonite was white. TMAB, tetramethylammonium

bromide; HDTMAB, hexadecyltrimethyl-ammonium bromide;

BTEAB, benzyltriethylammonium bromide; TEAB, tetra-

ethylammonium bromide; CPB, cetylpyridinium bromide.

Fig. 3. Fit of the Freundlich isotherm to the equilibrium adsorption

data of phenol (a) and chlorophenol (b) with different initial phenol or

chlorophenol concentrations between 50 and 300 mg/l.

S.H. Lin, M.J. Cheng /Waste Management 22 (2002) 595–603 599

Fig. 4. Comparison of the SEM images of natural bentonite, phenol-laden and regenerated organonbentonites.

600 S.H. Lin, M.J. Cheng /Waste Management 22 (2002) 595–603

For both phenol and chlorophenol adsorptions, theregeneration of BTEAB-, TMAB- and CPB-bentoniteshas been quite good with only rather slight decay in theremoval efficiency. But, there was quite significantdecrease in the regeneration efficiency of CPB-bentonitefor each cycle. Even with such an efficiency decrease,60% chlorophenol removal after five cycles for regener-ated CPB-bentonite is still acceptable in view of reduceddisposal cost of exhausted CPB-bentonite.

4. Conclusions

Phenol and m-chlorophenol adsorption by surfactant-modified bentonites was investigated in the presentstudy. Phenol or chlorophenol adsorption character-istics were examined in terms of the type of quaternaryamine used, amount of organobentonite required forefficient adsorption. Also considered are the regenera-tion of spent organobentonites and development of

operating conditions for such purpose. Based on the testresults, the following conclusions are drawn:

1. For the five quaternary ammonium salts testedhere, BTEAB-bentonite performs the best, with anefficiency of over 80% for phenol or chlorophenoladsorption. HDTMAB and CPB-bentonite with 6m/m% concentration also does well with chlor-ophenol adsorption but not phenol. TMAB- andTEAB-bentonites did not do well with an overadsorption efficiency of lower than 40%.

2. An optimum ratio of the amount of organobento-nite to that of phenol or chlorophenol wasobserved to exist at 8 g/g.

3. Empirical Freundlich isotherm was found todescribe well the organobentonite adsorption ofphenol and chlorophenol.

Fig. 6. Phenol and chlorophenol removal of regenerated organo-

bentonites as a function of regeneration temperature with 2 h regen-

eration time.

Fig. 5. Phenol and chlorophenol removal of regenerated organo-

bentonites as a function of regeneration time with 200�C regeneration

temperature.

S.H. Lin, M.J. Cheng /Waste Management 22 (2002) 595–603 601

4. Regeneration of spent organobentonites showsthat 200�C and 2 h were good operating condi-tions for this purpose. Little decrease in the phenoland chlorophenol removal for regeneratedBTEAB-, MTAB- and HDTMAB-bentonites wasobserved even after five adsorption/regenerationcycles. For CPB-bentonite, loss of phenol andchlorophenol removal efficiency was significant foreach cycle. Even so, reasonable chlorophenolremoval efficiency above 60% could still be main-tained in comparison with original 80%.

5. Color change of some regenerated organobento-nites was observed when the regeneration tem-perature was high above 200�C. The color changewas presumably caused by phenol and chlor-ophenol decomposition. The high regenerationtemperature could possibly also cause partialdetachment and/or decomposition of quaternaryammonium salt, resulting in a reduced adsorptioncapability of regenerated organobentonites.

Acknowledgements

The authors are grateful to the National ScienceCouncil, Taiwan, People’s Republic of China for thefinancial support (under the grant NSC89-2214-E155-003) of this research. The authors are also grateful tothe anonymous reviewers for the many good sugges-tions that have been incorporated in the final version ofthis paper.

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