ENVIRONMENTAL ENGINEERING SCIENCEVolume 22, Number 5, 2005© Mary Ann Liebert, Inc.

Adsorption Removal of Phenol in Water and SimultaneousRegeneration by Catalytic Oxidation

Y.H. Wang,1 J.C. Zhang,2,* L.F. Song,2 J.Y. Hu,2 S.L. Ong,2 and W.J. Ng2

1The Key Laboratory of Science and Technology of Controllable Chemical ReactionsMinistry of Education

Beijing University of Chemical TechnologyBeijing 100029, China

2Department of Civil EngineeringNational University of Singapore

Singapore 119260

ABSTRACT

A series of sorbents were prepared, and their performance on removal of phenol from wastewater was in-vestigated in this study. It indicated that the suitable compositions for the prepared sorbent mainly con-sisted of 0.5 wt.% PtO, 4.5 wt.% MnO, 1.99 wt.% Fe2O3, and 2.4 wt.% CoO, respectively, and supportedon Al2O3, shortened as an APMFC1.2 sorbent, A, P, M, F, C indicated Al2O3, PtO, MnO, Fe2O3, CoO,respectively, and number 1.2 as the molar ratio of Co. Studies showed that the spent sorbent could be ef-fectively regenerated at 240°C by gas mixtures of 6.0 mol.% steam balanced with air. Multiadsorp-tion–regeneration cycles conducted on APMFC1.2 sorbent indicated that it had good regenerability at theexperimental conditions, and characterized results further confirmed that the APMFC1.2 sorbent had sta-ble structure, which implied APMFC1.2 sorbent could be a promising sorbent/catalyst to be used in theprocess of removal of organic compounds in wastewater by adsorption combined with catalytic oxidationmethods.

Key words: organic compounds; wastewater; adsorption; catalytic oxidation; sorbent and catalyst

*Corresponding author: Institute on Membrane Technology, CNR-ITM, c/o University of Calabria, Via P. Bucci 17/C, 87030 Rende (CS), Italy. Phone: 39-0984-49-2050; Fax: 39-0984-40-2103; E-mail: zhangjc@mail.buct.edu.cn; Zhangjc2005@yahoo.com.cn

INTRODUCTION

ORGANIC COMPOUNDS have been involved in the man-ufacturing of a wide variety of commercial chemi-

cal products. Utilization of these organic compounds ina manufacturing process invariably results in generationof different types of wastewater that contain significant

amounts of waste organic compounds, which is especiallytrue in many chemical and petrochemical industries. Dis-charge of these wastewaters without treatment into a nat-ural water body is undesirable, which can definitely up-set the water quality of the receiving water body. Inaddition to the potential toxicity of the organic com-pounds, very often the dissolved oxygen concentration of

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the receiving water body polluted by the untreated chem-ical wastewater can fall below the level deemed neces-sary for normal aquatic life (Lin and Ho, 1996; Guzzellaet al., 2002; Gogate and Pandit, 2004). Hence, increas-ingly stringent restrictions have been imposed by gov-ernment on the concentration of these organic compoundsin the wastewater for safe discharge. Treatment of thesewastewaters has thus become an integral part of the op-erations of chemical and petrochemical plants.

Traditionally, activated sludge treatment is believed asthe most useful method to deal with the chemical waste-waters, which contain various kinds of organic com-pounds because of its simplicity and relatively low cost.However, the micro-organisms in an activated sludge sys-tem, even well acclimatized, can only handle chemicalwastewaters containing relatively low concentrations oforganic compounds primarily due to low biodegradabil-ity and inhibitory effects of those organic compounds(Del Pozo and Diez, 2003; Gogate and Pandit, 2004; Jongand Parry, 2003). The use of wet air oxidation is a pop-ular method to treat various industrial toxic and refrac-tory wastes. Wet oxidation is a process of subcritical ox-idation of organic matter in an aqueous phase withoxygen (either in pure or as air) at elevated temperaturesand pressures (Mishra, 1993; 1995; Mantzavinos et al.,1997). The slow rate of oxidation coupled with refrac-tory nature of some complex materials, for example,polychlorinated biphenyls and polyaromatics, is a majorlimitation of the wet air oxidation process (Yang et al.,2002; Sun et al., 2003).

To solve mentioned problems, the adsorption tech-nique using activated carbon is commonly used to treatthe contaminated water (Hand et al., 1994; Crittenden etal., 1997). However, these processes are nondestructivein nature because contaminants are only transferred fromone phase to the other (Suri et al., 1999; Zhu et al., 2002).Moreover, the spent carbon may have to be handled ashazardous waste. Another process, heterogeneous photo-catalytic oxidation, which employs photoactive catalystsilluminated with UV light to generate highly reactive rad-icals, can mineralize organic compounds to nontoxicforms. This process is effective in laboratory studies un-der controlled conditions (Allemane et al., 1993; Nishi-jima et al., 2003). During field operations, however, itusually suffers from problems of photocatalyst fouling(Christopher et al., 2003; Fuchs et al., 2003; Gwon et al.,2003; Liikanen et al., 2003), and scavenging of the re-active radicals by inorganic species and natural organicmatters present in water.

A potential treatment strategy to remove and destroythe organic contaminants can be a combination of an ad-sorption–regeneration and catalytic oxidation in site(ARCO) (Guzzella et al., 2002). This treatment strategy

would be designed as two operational steps: (1) organiccompounds are removed from the fluid using adsorptionmethods; (2) the exhausted adsorbent is regenerated online using the mixtures of containing steam and oxygenso that the sorbent can be reused. So the problems asso-ciated with photocatalysis, that is, catalyst fouling, canbe eliminated. During practical application, two reactorscan be used: one is used to treat the wastewater, whilethe other is being regenerated on-line. This would pro-vide a continuous supply of treated water, while no wasteis generated. However, there was a relative lack of in-formation in the literature about the removal of organicpollutants from wastewater by ARCO.

The overall objective of this study was to evaluate thetechnical feasibility of the ARCO treatment strategy forthe removal and destruction of the selective organic com-pounds. Phenol was selected as the target of organic com-pounds dissolved into water in this study. This is becausephenol and its derivates represent an important class ofenvironmental water pollutants, which have been foundthat most phenolic are present in waste wasters frommany industries (petrochemical, pharmaceutical, plastic,and pesticidal chemical industries) (Vindic et al., 1993;Alejandre et al., 1998; Yoon et al., 2003). As these com-pounds have high toxicity and a carcinogenic character,they have caused considerable damage to the ecosystemin water bodies and human health. An effective and eco-nomic treatment process, which will completely miner-alize all the toxic species present in the water stream with-out leaving behind any hazardous residues, is in greatdemand.

The ARCO strategy was appraised by multicycles ofadsorption–regeneration, in which phenol was regardedas reagent of organic compounds. An effort of this studywas to evaluate the possibility of using catalytic oxida-tion method to simultaneously oxidize organic com-pounds during the regeneration. For this purpose, Al2O3

support was impregnated with catalytic components(mainly noble metals). The effects of sorbent composi-tions on the adsorption and regeneration performancewere investigated in the fixed-bed reactor. Finally, thefresh, reacted regenerated sorbents were characterized byusing BET, ICP-MS, and SEM. This paper mainly re-ported our experimental results for the prepared sorbentto be used in the process of removal of phenol fromwastewater by ARCO.

EXPERIMENTAL METHODS

Experimental setup

A schematic diagram of adsorption and regenerationwas shown in Fig. 1. This system consisted of three parts:

a feed blending station for preparing the adsorption or re-generation reactant mixtures with different compositions,an assembly of microreactor–electric oven with a multi-channel temperature controller, and an off-line HPLCequipped with a UV-VIS detector and C-18 column (Var-ian-Chrompack, Walnut Creek, CA). During the regen-eration process, the stainless steel tube of transferring thegenerated steam was heated to prevent steam condensa-tion before it was introduced into the regenerator. Thefeed blending station consisted of two metering pumpsfor driving the water containing of certain concentrationof phenol for adsorption and the treated water for regen-eration use, respectively. An air tube was used to providethe oxidant to oxidize the phenol contaminant, which wasadsorbed on the catalyst surface and micropores duringthe adsorption process, into water and carbon dioxide.The metering pumps were calibrated in advance. Themain parts of microreactor–electric oven assembly area fixed-bed reactor of 16 mm i.d., 400 mm long, and anelectric oven of 1200 W. The temperature during theadsorption was conducted on the ambient temperature.A wide range of temperatures at regeneration were in-vestigated to optimize the regeneration temperature,which would make the sorbent/catalyst with good re-

generability as well as higher activity. The temperatureof fixed-bed section was controlled with a temperatureprogrammable controller and measured with a mi-crothermal couple inserted into its centre through asmall jacket tube. The fixed-bed reactor was chargedwith 5 g sorbent, and some amount of quartz chips wasloaded in both sides of the adsorbent section.

Sorbent/catalyst preparation

The sorbents used in the experiment were first obtainedby impregnation of alumina, which was pretreated in ad-vance, with the solution containing 17.5–26.0 wt.% and5.9–13.1 wt.% of manganese nitrate and ferrite nitrate,respectively, followed by drying in an oven at 110°Covernight under vacuum condition and then impregnatedwith the solution of containing 5.8–12.8 wt.% of cobaltnitrate, followed by drying in an oven at 110°C lasted for24 h. The solution temperature during impregnation waskept constant at 55°C by using a thermostatic bath. Thematerial was then calcined in air in a muffle at 500°C for6 h. The sorbent of containing of platinum was similarlyprepared according to the above method except aluminasupport has been impregnated by the mentioned metals

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Figure 1. Schematic diagram of fixed-bed experimental setup. 1,2—pump; 3—mass flow controller; 4—mixer; 5—sorbent bed;6—temperature controller; 7—thermocouples; 8—adjusting valve.

Table 1. Operational parameters of adsorption and regeneration/oxidation.

Items Adsorption Regeneration/oxidation

Sorbent load (g) 5.0 5.0Temperature (°C) �25 180–300Pressure Ambient pressure Ambient pressureSpace velocity (h�1) 150 (liquid) 3,000 (gas)

compositions �60 mg/L phenol solution 2.0–8.0 mol. % steam � air

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in advance. All the prepared sorbents were kept in a glassutensil to be used for further investigation.

Fixed-bed adsorption tests

Vertical fixed-bed reactor with 16 mm i.d. was usedin this study, which was made of stainless steel to pre-vent corrosive and less adsorption during experiments.Before tests were investigated, some preliminary exper-iments were done to make the tests have good repro-ducibility. The adsorption experiments were first initiatedby introduction of the wastewater containing 60 mg/Lphenol, which was conducted on the ambient tempera-ture. When the complete breakthrough was finished, thenthe feed was switched to the mixtures of steam and air.The effects of compositions of the mixed regeneratedgases and temperatures on the sorbent performance wereinvestigated in this experiment. The experimental condi-tions were summarized in Table 1.

Sorbents characterization

Fresh, typical reacted and regenerated sorbents/cata-lysts were characterized. SEM characterization was ana-lyzed by using an S-3200N HITACH apparatus. The sur-face area of fresh, typical reacted and regeneratedsorbents/catalysts was investigated by using a Mi-cromeritics ASAP-2000 apparatus. The sorbent compo-sitions were measured by an Inductively Coupled

Plasma-Mass Spectrometry (ICP-MS, Agilent 7500, US)instrument.

RESULTS AND DISCUSSION

Characterization of fresh sorbents

Fresh sorbents were characterized. Table 2 was the re-sult of sorbent compositions characterized by ICP-MS.To investigate the effects of active components on thesorbent performance, the solution concentrations of man-ganese nitrate and ferrite nitrate were varied during thesorbent preparation. The total content of MnO and Fe2O3

(wt.%) as well as platinum was not varied during im-pregnation. To the Pt–Mn–Fe–O/Al2O3 sorbent, it wasshortened as APMFx, in which A, P, M, F, and x indi-cated the alumina, oxides of platinum, manganese, fer-rite, and the molar ratio of manganese to ferrite, respec-tively, as shown in Table 2. The cobalt was added intothe APMFx sorbent to further improve its performance,which was shortened as APMFCy. The meaning of A, P,M, and F were mentioned above; C and y indicated thecobalt oxide and molar ratio of cobalt in the sorbent, re-spectively, similarly shown in Table 2. The surface areaand main crystalline phase for the typical sorbent ofAPMFC1.2 was characterized by using a BET and XRDapparatus, respectively, as listed in Table 3. From Table3 results it showed that the characterization peaks of plat-

Table 2. Sorbent/catalyst compositions supported on AI2O3 (wt.%).

Mn�Fe Mn�Fe�CoSorbents MnO Fe2O3 (molar ratio) CoO (molar ratio) PtO

APMF1.0 3.08 3.41 1.0 — — 0.5APMF1.5 3.74 2.75 1.5 — — 0.5APMF2.0 4.18 2.31 2.0 — — 0.5APMF2.5 4.50 1.99 2.5 — — 0.5APMF4.0 5.09 1.41 4.0 — — 0.5APMFC0.4 4.50 1.99 2.5 1.2 2.5�1�0.4 0.5APMFC0.8 4.50 1.99 2.5 1.8 2.5�1�0.8 0.5APMFC1.2 4.50 1.99 2.5 2.4 2.5�1�1.2 0.5APMFC2.2 4.50 1.99 2.5 3.0 2.5�1�2.2 0.5

Table 3. Characterization of fresh and regenerated APMFC1.2 sorbents.

Sorbents BET surface area (m2/g) Main crystalline phasec

APMFC1.2a 158 Fe2O3, Mn2O3, CoO, Al2O3

APMFC1.2b 156 Fe2O3, Mn2O3, CoO, Al2O3

aFresh sorbent; bregenerated sorbent; ccould not detect PtO crystalline phase.

inum could not be obviously detected in this investiga-tion. The probably reason might be the less amounts ofplatinum contained in the sorbent, and was uniformlyloaded on the support.

Effect of molar ratio of Mn:Fe on sorbent performance

Figure 2 showed the breakthrough profiles of phenolconcentration in the effluent at different molar ratios ofmanganese to ferrite for the prepared sorbents. At the ad-sorption conditions listed in Table 1, a typical run for thesorbent to be achieved complete breakthrough was near4 days. The column run time data were converted to wa-ter treated per gram sorbent used in plotting Fig. 2. It in-dicated that the sorbent performance was definitely im-proved with increasing the molar ratio of manganese toferrite under the experimental conditions when its molarratio was below 2.5. According to a previous study(Zhang et al., 1999), when the sorbent had similar per-formance of removal of sulfur compounds from the coalgas at high temperature, the sorbent stability with highercontent of MnO would be better than that with lower con-tent of MnO. Hence, the suitable compositions for the in-vestigated sorbent probably consisted of 4.50 wt.% MnOand 1.99 wt.% Fe2O3, respectively, shortened to APMF2.5

sorbent. Further modification to improve the sorbent per-formance was conducted on this sorbent.

Investigation of cobalt on sorbent performance

Figure 3 showed breakthrough curves for sorbents con-taining different contents of cobalt oxide and sorbentwithout impregnated cobalt component, APMF2.5, re-spectively. It indicated that the sorbent performance has

been slightly improved by adding little amounts of cobaltoxide on the sorbent. The investigation also clearly indi-cated that the sorbent performance could not be furtherimproved when the content of cobalt oxide was over 2.4wt.% in this experiment. Thus, the suitable compositionsfor the investigated sorbent probably consisted of 4.50wt.% MnO, 1.99 wt.% Fe2O3, and 2.4 wt.% CoO, re-spectively, shortened to APMFC1.2 sorbent. The follow-ing investigation was carried out on this sorbent.

Effect of gas compositions on regenerated sorbent performance

Figure 4 showed breakthrough curves of phenol con-centration for the fresh and regenerated sorbents. The re-generated experiments were carried out using differentconcentrations of steam and air at 240°C regenerationtemperature. From the experimental results, it showedthat the suitable gas compositions to regenerate the spentsorbent should be carried out by 6.0 mol.% steam bal-anced with air.

Effect of temperature on regenerated sorbent performance

Figure 5 showed phenol breakthrough curves for thefresh and regenerated sorbents. Regenerated experimentswere carried out at different temperature with gas com-positions of 6.0 mol% steam and balanced with air. Theinvestigated results showed that the spent sorbent couldbe effectively regenerated at three different regeneratedtemperatures. However, trace amounts of phenol were de-tected from the effluent of the regenerated mixtures at180°C regeneration temperature. Thus, the suitable re-generation temperature combined with the comparatively

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Figure 2. Effects of molar ratios of Mn to Fe on breakthroughcurves of sorbent adsorption.

Figure 3. Effects of cobalt contents on breakthrough curvesof sorbent adsorption.

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lower energy consumption should be carried out at240°C.

Regenerability investigation of APMFC1.2 sorbent

The effects of multicycles of adsorption–regenerationon APMFC1.2 sorbent performance were shown in Fig.6. According to experimental results shown in Fig. 6, itindicated that APMFC1.2 sorbent had good regenerabil-ity under experimental conditions. The regeneratedAPMFC1.2 sorbent breakthrough curves almost becamethe same as the fresh sorbent when the adsorption–re-generation has undergone multicycles (Fig. 6 only

showed results of fresh, first, third, and fifth data), whichproofed that APMFC1.2 sorbent could be an ideal sorbentof the removal of trace organic compounds from waste-water by adsorption combined with catalytic oxidation.SEM patterns for the fresh and regenerated sorbents havebeen characterized (not shown in the text). Results indi-cated that there was not much difference between thefresh and regenerated sample patterns. BET and XRDcharacterized results were listed in Table 3, which fur-ther confirmed that the APMFC1.2 sorbent had a goodstable structure. This also implied this sorbent would havegood stability during the multiadsorption–regenerationcycles.

CONCLUSION

The experiment carried out in a fixed-bed adsorp-tion–regeneration in the site by using catalytic oxidationshowed that the prepared sorbent, APMFC1.2, had goodperformance of removal of the selected target organiccompounds, phenol, from wastewater. Under experi-mental conditions, when the fixed-bed reactor was loadedon 5 g APMFC1.2 sorbent, it could treat 25 liters of waste-water without phenol pollutant in the effluent. The spentsorbent could be effectively regenerated at 240°C withgas mixtures of 6.0 mol.% steam balanced with air. Mul-tiadsorption–regeneration results showed APMFC1.2 sor-bent had good performance of removal of phenol fromthe wastewater, which indicated that APMFC1.2 sorbenthad good stability. Characterized results for the fresh andused samples further confirmed that APMFC1.2 sorbenthad stable structure. From this primary study, it indicated

Figure 4. Effects of regeneration gas compositions on sor-bent breakthrough curves at 240°C regeneration temperature.

Figure 5. Effects of regeneration temperatures on sorbentbreakthrough curves regenerated with 6.0% steam balancedwith air.

Figure 6. Sorbent performance of multiadsorption–regener-ation cycles regenerated at 240°C and with 6.0% steam bal-anced with air.

that this hybrid method had good potential application ofremoval of trace phenol from the wastewater.

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