thermal regeneration of an activated carbon.pdf

7/27/2019 THERMAL REGENERATION OF AN ACTIVATED CARBON.pdf

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Pergamon

000%6223(95)00090-9

Carbon Vol.33, No. 10, pp. 1417-1423,1995Copyright 0 1995 ElsevierScienceLtdPrinted in Great Britain. All rights reservedOLW8-6223/959.50 + 0.00

THERMAL REGENERATION OF AN ACTIVATED CARBONEXHAUSTED WITH DIFFERENT SUBSTITUTED PHENOLSC. MORENO-CASTILLA, .* J RIVERA-UTRILLA, J . P. J OLY,~ M. V. LOPEZ-RAM~N,

M. A. FERRO-GARCIA and F. CARRASCO-MAR~NDepartamento de Quimica Inorginica, Facultad de Ciencias, Universidad de Granada, 18071 Granada,SpainLaboratoire dApplication de la Chimie g lEnvironnement, CNRS, Universitk Lyon 1, ESCIL, 69622Villeurbanne cedex, France(Recei ved 22 Januar y 1995; accept ed in revised orm 1 M ay 1995)

Abstract-The thermal desorption process of phenol, m-aminophenol, p-cresol and p-nitrophenol froman activated carbon has been studied. For this purpose, the activated carbon-phenolic compound systemwas heated to 1100 K in a He flow following determination of weight loss and the evolution of lightgases by a thermobalance and by mass spectrometry, respectively. Results showed that during the heattreatment, part of the phenol evolved from the activated carbon and was deposited at the outlet of thereactor and part underwent degradation reactions to light gases and to a residue that remained on thesurface of the activated carbon. Finally, the variation of the adsorption capacity of the activated carbonafter several adsorption-regeneration cycles has also been studied.Key Words-Regeneration, activated carbons, substituted phenols.

1. INTRODUCTIONActivated carbons are widely used to remove organicpollutants from water but, depending on their adsorp-tion capacity, they become saturated after some time.If these activated carbons need to be reused asadsorbents, they therefore have to be regenerated.One of the cheapest and most versatile methods ofregenerating activated carbons is by thermal treat-ment in a given atmosphere[ 1,2].

In previous papers, activated carbons from aSpanish bituminous coal were found to have bothhigh surface area and appropriate porosity[3] toextensively adsorb phenolic compounds such as:phenol, p-cresol, m-aminophenol, chlorophenols andp-nitrophenol[4].The thermal regeneration process of an activatedcarbon from almond shells saturated with both ortho-chlorophenol and metachlorophenol has been studiedpreviously [ 5,6]. The physisorbed fraction was foundto be eliminated as chlorophenol molecules and/orheavy products of their degradation. The chemi-sorbed fraction, however, underwent degradationreactions giving light products such as H,, H,O, COand CO,, with the oxygen surface groups of theactivated carbon playing an important role in theproduction of these gases, and also yielded a residueor polymer that remained on the surface of theactivated carbon.

The aim of this paper is to gain more insight intothe thermal desorption processes of phenols fromactivated carbons. Thus, in the present study fourphenolic compounds (phenol, p-cresol, m-amino-phenol and p-nitrophenol) have been chosen, which

*To whom correspondence should be addressed.

together with the previously published work carriedout on chlorophenols[ 5-71, will enable us to developa more general picture on the thermal desorptionprocesses of the phenolic compounds, which are ofgreat importance in the regeneration processes ofexhausted activated carbons.

2. EXPERIMENTALThe activated carbon used in this study (APlO)

was obtained by pyrolysis in N2 and steam activationof a Spanish bituminous coal. Details of the methodof preparation and characterization of this activatedcarbon and the values of its more important texturaland chemical parameters have been reported in previ-ous papers[3,8,9]. Some of these are as follows:surface area=828 mz g- (determined from the N,adsorption isotherm by applying the BET method);pore volume accessible to water =0.52 cm3 g-l; andpH = 10.0.

The activated carbon was independently saturatedwith phenol (P), p-cresol (PC), m-aminophenol(MAP) and p-nitrophenol (PNP) as follows: 0.5 g ofactivated carbon were added to 100 cm3 of differentphenolic solutions (1 g 1-l). The suspensions werethermostatized at 298 K and mechanically shaken for3 days. They were then filtered out and the spentactivated carbon was dried in an oven at 393 K for2 hours, after which the cooled activated carbon wascarefully weighed. Table 1 summarizes the amountsof phenolic compounds adsorbed on the activatedcarbon and determined by spectrophotometry (X,),the amounts remaining on the activated carbon afterdrying in the oven (X) and the percentages of phenolic

CM 33-10-c 1417


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1418 C. MORENO-CASTILLA et al.Table 1. Adsorption capacity, X,, of the activated carbon APlO. Amount of phenoliccompound left on the carbon after drying, X. Amounts of oxygen contained in theadsorbed phenol, X,, and contained in the light gases, X,, detected by TPD-MS

X, X X0 X0Sample mg phenol g- C %W mg 0 g- C A(X,-X,)AP-10 9AP- lo-phenol 167 136 18.6 23 35 12AP-lo-m-aminophenol 178 157 11.8 23 44 21AP-lo-p-cresol 209 181 13.6 27 42 15AP-lo-p-nitrophenol 196 180 8.2 62 70 8

compounds released from the activated carbon duringthe oven drying process (%W).

Thermal regeneration of the spent activatedcarbon samples was followed by both TGA andTPD-MS techniques. TGA experiments were carriedout with a CI Electronics thermobalance (modelMK3). About 10 mg of sample was heated in anultrapure, dry He flow (200 cm3 mini) at a heatingrate of 20 K min- up to 1100 K.

The TPD-MS experiments were carried out byplacing about 200 mg of sample in a dynamic quartzreactor equipped with a short response time capillarythermocouple in its center. The sample was heatedin a He flow (60cm3 min-) at a heating rate of20 K min- up to 1100 K. On-line gas analysis wasfollowed with a quadrupole mass spectrometer (MS)with a capillary introduction system heated at 373 K.The MS apparatus was able to detect only light gases.The apparatus and data processing have beendescribed in detail in previous publications [ 10-121.

3. RESULTSThe data in Table 1 show that during the oven

drying process of the different APlO-phenolic com-pound systems, a fraction of the adsorbed phenolwas released from the activated carbon, caused bythe drag of the adsorbate molecules by the waterevaporation. The weight loss percentage (%W) islower for p-nitrophenol because of its higher inter-action with the surface of the activated carbon[4].

The TPD-MS spectrum of activated carbon APlOis depicted in Fig. 1, and those corresponding to thefour APlO-phenolic compound systems in Figs 2-5.Table 2 summarizes the total amounts of light gases

Qci>2 0.25t .k.

0300 500 700 900 1100T(K)

Fig. 1. TPD-MS pattern of activated carbon APlO in a Hehow. Heating rate: 20K min-. n H,; 0 H,O; l CO;0 co,.

300 500 700 900 1100TIKI

Fig. 2. TPD-MS pattern of the APlO-phenol system in aHe flow. Heating rate: 20 K mini. W H,; 0 H,O; + CO;0 co,.

300 400 500 600 700 800 900 1000 1100T(K)

Fig. 3. TPD-MS pattern of the APlO-m-aminophenolsystem in a He flow. Heating rate: 20 K min-. n H,;0 H,O; + CO; 0 CO,; 0 N,; 0 NH,.

1 0,sSLc 0.4

0300 400 500 600 700 800 900 1000 1100

T (K)Fig. 4. TPD-MS pattern of the APlO-p-cresol system in aHe flow. Heating rate: 20 K min-. H H,; 0 H,O; l CO;0 COz; 0 N,; 0 CH,.

detected by MS in the TPD experiments whosedesorption profiles are shown in the above figures.

Table 1 also includes the amounts of oxygen corre-sponding to both the adsorbed phenol (X6) deter-mined from X and the degradation compoundsdetected by TPD-MS and given in Table 2 (X,).

In order to evaluate the total amount of thedifferent phenols eliminated during the thermal regen-


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S

4

$3s=2%

I

0

Thermal regeneration of an activated carbonTable2. Amounts (pmol g-) of light compounds evolved from APlO andAPlO-phenolic compound systems heated up to 1100 K in a He flowSample H2 H,O CO CO* CH, NH, NO N,AP-10 14 410 94 39 - - - -AP-lo-phenol 28.5 710 1188 154 - - - -AP-lo-m-aminophenol 488 1098 1002 330 - 201 - 299AP-lo-p-cresol 658 1003 1137 243 223 - - -AP-lo-p-nitrophenol 253 1563 1397 723 - - 110 455

1419

no-0

3 ac 0

300 400 SO0 600 700 800 900 I OW 1100T(K)

Fig. 5. TPD-MS pattern of the APlO-p-nitrophenol systemin a He flow. Heating rate: 20K min-. n H,; 0 H,O;+ CO; 0 CO,; 0 N,; A NO.eration of the samples, these regeneration processeswere followed in a thermobalance determining thetotal weight loss as a function of temperature. Figures6-9 show both the DTG thermograms and the overallmass calculated from MS analysis for eachAPlO-phenolic compound system. Whereas DTGdata inform about the total weight loss per gram ofsample, the TPD-MS data correspond only to theweight of the light compounds detected by MS; thus,comparison of both thermograms for the same sample

300 4m so0 600 700 800 900 1000 1100T(K)

Fig. 6. DTG thermogram (--- ) and overall mass of lightgases calculated from TPD-MS (-) for the APlO-phenolsystem.

300 400 SO0 600 700 BOO 900 1000 1100T(K)

Fig. 7. DTG thermogram (--- ) and overall mass of lightgases calculated from TPD-MS (-) for the APlO-m-aminophenol system.

0.3

r^ ,.* 0. 2a555 0. 1

0

(p&&q_,300 SO0 700 900 1100

T(K)Fig. 8. DTG thermogram (--- ) and overall mass of lightgases calculated from TPD-MS (-) for the APlO-p-cresolsystem.

300 400 500 600 700 800 900 1000 1100T(K)

Fig. 9. DTG thermogram (---) and overall mass of lightgases calculated from TPD-MS (-) for the APlO-p-nitrophenol system.

is beneficial because it simultaneously informs usabout the total amount desorbed and the fractionconverted to light products.

4. DISCUSSIONThe TPD-MS results for activated carbon APlO,

(Fig. 1 and Table 2) are typical of carbonous materialswith a low oxygen content. The amount of COdesorbed is more than the double that correspondingto COz. According to the basic nature of thiscarbon[4], this CO might come from surface groupssuch as pyrone, quinone and carbony1[3,13]. Theamount of oxygen in the light products detected byTPD-MS, up to 1100 K for APlO (9 mg g- of C),is much lower than its oxygen content (8%), deter-mined by elemental analysis.4.1 API O -phenol system

During thermal treatment of the APlO-phenolsystem up to 1100 K the following are observed(Fig. 6):


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1420 C. MOKENO-CASTILL.A t 01.

(i)

(ii)

(iii)

Between 300 and 425 K, 6% of the phenol (P)adsorbed is released and light gases are notdetected by MS. This P will come from thephysisorbed fraction and is deposited at theoutlet of the reactor either as P or as a heavycompound[5].Between 425 and 925 K, the profiles of bothDTG and MS curves are similar in shape withtwo maximum peaks at about 550 and 675 K.respectively, although the amplitude of the DTGcurve is greater than that of MS. These resultsindicate that the P desorbed in this region corres-ponds to that chemisorbed[ 51 and is evolved aslight gases detected by MS and as heavy productsdetected only by TG and deposited at the outletof the reactor. The similarity of both the DTGand MS profiles suggests that light gases andheavy products proceed from parallel processes.Above 925 K, both DTG and MS profiles arecoincident. This shows that only the light gasesdetected by MS are released in this region; thesegases will also come from the chemisorbedfraction.

The total weight loss of this system detected byTG by heating to 1100 K was 71% of the adsorbedphenol (X). This means that there is a residue on thesurface of the activated carbon that proceeds fromthe chemisorbed fraction and that should be rich incarbon[ 141. The amount of light gases detected byMS was only 48%. The difference in weight lossbetween TG and MS (33%) is due to P and heavycompounds, coming from both the physisorbed andchemisorbed fractions evolved from the activatedcarbon and deposited at the outlet of the reactor.Figure 2 and Table 2 show the TPD profiles of thelight gases and their amounts, respectively. Asexpected from thermal degradation of P, the maindegradation compounds are CO and H,O. Theoxygen surface groups of the activated carbon shouldparticipate in some of the phenol degradation pro-cesses since the total amount of oxygen correspondingto the degradation compounds (35 mg g- of carbon)is greater than that corresponding to the adsorbedP(23mg g-r of C) (Table 1). Participation of theoxygen surface groups of the activated carbons in thethermal regeneration process is expected becausethe adsorption of phenolic compounds on the surfaceof the activated carbons implies a donor-acceptorcomplex mechanism involving carbonyl oxygens ofthe carbon surface acting as electron donors and thearomatic ring of the adsorbate acting as the accep-tor [ 151. Therefore, as reported previously [ 51, theseoxygen surface groups play an important role in thethermal regeneration of the activated carbons bycracking the adsorbate molecules,

4.2 API&m-aminophenolDuring the thermal treatment of the APlO-m-

aminophenol system up to 1100 K the following areobserved (Fig. 7):

(i)

(ii)

Between 300 and 450 K, the weight loss detectedby DTG is 31% of the initial amount adsorbed(X) and it comes from both the physisorbedfraction of MAP and essentially physisorbedwater, although the contribution of the latter isnegligible, (see Fig. 3). This MAP is not detectedby MS, as in the above system, and is depositedat the outlet of the reactor.Above 475 K, both the DTG and MS curves aremore or less coincident, implying that all MAPreleased is degraded to light gases detected byMS (Hz, H,O, CO, CO,. NH, and N, (see Fig. 3and Table 2). These light gases come from thechemisorbed MAP fraction. and, as in the abovesystem. the oxygen surface groups of the acti-vated carbon participate in the degradation reac-tions (see the value of A (X0-X,) in Table 1).At the end of the experiments, the total masseliminated with regard to the initial amount ofMAP adsorbed (X) was 83% (detected by TG),which indicates that a carbonaceous residue isalso left on the surface of the activated carbonproceeding from the chemisorbed fraction. Thetotal mass of light gases detected by MS was48% of the initial MAP adsorbed.

In this case the difference between the weight lossobtained by TG and MS. 350/o, should essentiallycorrespond to the physisorbed fraction evolved fromthe activated carbon between 300 and 450 K anddeposited at the outlet of the reactor, because asshown before no heavy products were released above450 K. Therefore, in this system, in contrast to theabove, the chemisorbed fraction evolved only aslight gases.4.3 API O-p-crrsol

Regarding the APlO-p-cresol system, the followingpoints can be deduced from Fig. 8:(i)

(ii)

Below 380 K. only PC is released (whichamounts to 1.5% of the initial PC adsorbed, X)without any defined peak in the DTG curve.This behaviour is different to that found in theabove systems, and implies either that there is asmall amount of PC physisorbed or that duringthe heat treatment a large amount of physisorbedPC is transformed to chemisorbed PC. Thislatter transformation has been observed beforein other activated carbonphenohc compoundsystemsC2.71.

The amounts of both PC and MAP phy-sisorbed by the activated carbon should besimilar because, as shown recently[4], the inter-action of both molecules with the surface of thecarbon is also similar due to the fact that bothsubstituents of the phenolic compound (methyland amino) are electron-donating to the aro-matic ring. Therefore, from the above twoassumptions, the transformation from physi-sorbed to chemisorbed PC should hold.Between 380 and 925 K, the DTG and MS


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Thermal regeneration of an activated carbon 1421profiles are similar in shape, but the amplitude hydrogens, which are themselves relatively mobile.of the DTG signal is greater than that of the As re-adsorption of PC is possible, a greater pressureMS. This means that in this range of temperature of p-cresol in the gas phase results in a more intensethe chemisorbed fraction of PC evolved as light re-adsorption and consequently favours the forma-gases and heavy compounds, which were tion of the chemisorbed form from the physisorbeddetected and not detected by MS, respectively. one. Reaction of some of the benzylic radicals withThis behaviour is also analogous to that of the oxygen surface groups might be the reason for theAPlO-phenol system. low temperature CO evolution.

(iii) Surprisingly, the MS signal is higher in amplitudethan that of DTG above 925 K. The explanationof this apparent anomaly is to be found in thedifferent experimental conditions in both tech-niques. Thus, the mass of the sample is about 20times greater and the inert gas flow -about 3times lower in MS than in TG. The pressure ofPC is therefore about 60 times higher in MSthan in TG experiments. Re-adsorption ofphysisorbed PC should thus be favoured in thecase of MS experiments. It is noteworthy thatre-adsorption of physisorbed chlorophenols hasbeen reported recently [ 71.

Another interesting feature in Fig. 4 is the evolu-tion of methane between 750 and 1100 K. This meth-ane should originate from a part of PC that did notundergo the benzylic abstraction described above andis probably due to its thermal demethylation.Desorbed methane amounts to 223 pmol g- ofcarbon, a very small quantity compared to theamount of methyl in the adsorbed PC (X), i.e.1680 pmol g-. This result agrees with the assumptionthat a part of PC undergoes abstraction of benzylichydrogen at the beginning of the heat treatment.

In turn, re-adsorption of physisorbed PC favoursits transformation into a chemisorbed form, whichfurther transforms and decomposes during heat treat-ment. In the case of MS experiments, a greateramount of chemisorbed PC is formed around 380 Kand consequently more degradation products areexpected. In the present case, most of these productsshould be light gases evolved above 925 K. Thisapparent anomaly thus corroborates the assumptionthat physisorbed PC is transformed into the chemi-sorbed species.

The total weight loss up to 1100 K of the APlO-p-cresol system determined by TG was 46% of thetotal amount of PC adsorbed, indicating that theamount of residue left on the surface of the activatedcarbon is the greatest of all the systems studied. Thisresult can be also explained by the mechanism pro-posed in the scheme above [ 11. Thus, the benzylicradicals formed could undergo reactions such as theformation of polymers on the surface of the activatedcarbon and/or its stabilization through the reactionwith dangling carbon atoms of the activated carbonmicrostructure, giving rise to stable C-C bonds.

More information about this transformation canbe gained from the results of TPD-MS (Fig. 4). Acharacteristic of the MS spectrum of PC is theevolvement of CO at low temperatures, which canonly proceed from chemisorbed species. Thus, CO isevolved from PC at 150 K lower than from MAP(Fig. 3). The beginning of CO desorption is alsohigher for P (Fig. 2). Besides, the amount of COdesorbed up to 700 K, is around 1.4 times greater forPC than for MAP. This CO desorbs into two peaksat 420 and 560 K, which are not found in the MAPspectrum. This suggests that low temperature COdesorption is due to the methyl group of PC.

The proposed mechanism for the transformationof physisorbed PC into chemisorbed PC in the firststep of the heat treatment is as follows:

The total weight loss due to light gases was 29%;17% (46- 29) of the total weight loss detected byTG, therefore, corresponds to heavy compounds thatare evolved from the activated carbon and depositedat the outlet of the reactor.

4.4 API 0-p-nitrophenolDuring the heat treatment of the APlO-p-

nitrophenol system up to 1100 K the following areobserved (Fig. 9):(i)

Gas phase Ph-CH,11 - H' (ii)Sorbed phase Ph-CH, --+ Ph-CH,-

physisorbed chemisarbed

+reaction with oxygen surface groupsreaction with dangling C atoms (1)polymerization

Physisorbed PC desorbs and at the same time reactson the surface of the carbon to produce a chemisorbedspecies through the abstraction of one of its benzylic

Below 450 K, the physisorbed p-nitrophenol isdesorbed and there is no appreciable evolutionof light gases from the system. As in othersystems, the PNP desorbed is deposited at theoutlet of the reactor either unchanged or as aheavy compound. The amount desorbed up tothis temperature is 10% of the total amountadsorbed.Between 450 and 750 K, the DTG and MSprofiles have the same shape, with two peaksappearing at around 550 and 700 K. The peakat 550 K obtained from the DTG experimentpresents a higher intensity than that obtainedfrom the MS experiment, indicating that bothlight gases and heavy compounds are releasedfrom the system at this temperature. These pro-ducts will stem from the chemisorbed fraction ofPNP. The amplitude of the peak at 550 K is thehighest found of all the systems studied.


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1422 C. MORENO-CASTILLA etal.On the contrary, the peak at 700 K is smaller in

amplitude and that obtained from DTG is fairlycoincident to that from MS. The profiles of thelight gases evolved in this temperature range areshown in Fig. 5. The parallelism of the profilesof all the gases, i.e. H20, CO, COa, NO and N,is worth noting, as it is implied that they alloriginate from the same groups of reactions. Thepresence of N2 and NO among the gases evolvedindicates that the nitro group of PNP wasreduced at the interface of the system, which inturn would give additional amounts of CO, CO,and H,O (Table 2). These redox processes areexothermic, and heat evolution from the systemwas studied by differential scanning calorimetry(DSC) up to 1100 K in the same experimentalconditions as the TPD-MS experiments. Theequipment used was a Perkin-Elmer, Model3600. The DSC curve obtained is depicted inFig. 10. At the temperatures corresponding tothe above peaks (550 and 700 K) two exothermicpeaks are observed whose values are -59 and- 63 kJ/mol PNP chemisorbed, respectively,which corroborate the occurrence of the abovereactions.

From the amounts of NO and Nz given inTable 2 and the amount of PNP chemisorbed(the difference between X and the PNP phy-sisorbed that evolved up to 450 K), all the nitrogroups present in the PNP chemisorbed partici-pate in the redox reactions that take place at550 and 700 K.

(iii) Above 750 K, the DTG profile shows a con-tinuous increase while the MS profile remainsalmost parallel to the abscissa, indicating that alarge amount of heavy compounds are released.This can be explained by increased volatility ofthe heavy compounds formed on the surface ofthe activated carbon compared to other systems,resulting in the evolution of greater amounts inthis temperature range, or it could be a conse-quence of the above redox reactions causingsome degree of depolymerization of the carbonsurface. With respect to the light gases (Fig. 5)

20001 ASW

W-lOOO-

-2500. .:a

-40001273 473 6'3 T W) 873 1073

Fig. 10. DSC of the APlO-p-nitrophenol system.

evolved in this region, CO is the main product,with negligible amounts of water evolved.

The weight loss of this system determined by TGup to 1100 K was 285 mg g- C, comparing thisvalue with the weight of adsorbed p-nitrophenol(180 mg/g C) it is clear that beside p-nitrophenol,part of the carbonous matrix is eliminated. Theseresults also confirm that the activated carbon isattacked by the nitro groups during its thermalregeneration.

The results obtained in this work, together withthose previously published[2,5-71 on the thermaldesorption of different phenolic compounds fromactivated carbons enable us to draw the followingconclusions. During the adsorption process of thephenolic compound on activated carbons from aque-ous solutions this is physisorbed and chemisorbed.When this activated carbon-phenolic compoundsystem is heat-treated up to 1100 K in an inert gasflow, the physisorbed phenol evolves at a low temper-ature from the surface of the carbon and is depositedat the outlet of the reactor either as itself or as aheavy compound. At the same time some of thephysisorbed phenol can be transformed to chemi-sorbed phenol. This transformation is favoured by:the experimental conditions (amount of sample, gasflow, heating rate), readsorption of the phenol com-pound and the formation of radical species.

The fraction of phenolic compound chemisorbedis converted to: light gases and heavy products (insome cases) that evolve from the surface of theactivated carbon, and also to a carbonaceous residueor polymer that remains on the surface of the carbon.

In the production of light gases, the oxygen surfacecomplexes of the activated carbon play an importantrole acting as cracking centers for the phenolic com-pound molecules. In all cases, except in that of PNP,the total amount of light gases evolved is quitesimilar, with a higher oxygen content in these gasesthan in the chemisorbed phenolic compound. Thebehaviour of PNP is different due to the attack ofthe nitro group on the surface of the carbon.

Pyrolysis of chlorophenols in an inert atmospherehas been shown to give a high yield of graphiteAake[ 141, the formation of a carbonaceous residueor polymer on the activated carbon surface in allcases, except in the PNP thermal desorption, couldtherefore be related with the condensation of largearomatic radicals into an extended sp structure. Ifthis is so, the formation of this residue would berelated to the H, evolution. Thus, in the case of theactivated carbon-PC system the amount of H,evolved is the greatest (see Table 2), which alsocorresponds to the highest amount of residue left.

4.5 Adsorption-regeneration cyclesThe adsorption capacity of the activated carbon

APlO for each phenolic compound after its regenera-tion has also been studied. For this purpose, the spentactivated carbon was heat-treated in dry N, flow


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Thermal regeneration of an activated carbon 1423

0 I 2 3 4 5 6 7Number of cycles

Fig. 11. Variation of the adsorption capacity of the activatedcarbon APlO as a function of the adsorption-regenerationcycle. Phenol, A; m-aminophenol, 0; p-cresol, 0;p-nitrophenol, 0.(100cm3 min-) up to 973 K (heating rate20 k min-) and this temperature was maintainedfor 1 hour. The regenerated samples were again sat-urated with the corresponding phenolic compound,determining their new adsorption capacity. Theseadsorption-regeneration cycles were determined upto 7 times. The results obtained as a function of thenumber of cycles are depicted in Fig. 11. The valueof cycle 0 corresponds to the adsorption capacity ofthe original activated carbon APlO.The capacity of APlO to adsorb phenolic com-pounds decreases as the number of adsorption-regen-eration cycle increases. The extent of this decreasedepends on the phenol type. Thus, the most pro-nounced decrease occurs with the PNP system whoseadsorption capacity after the three first cyclesundergoes a marked decrease from 196 to 39 mg g-C, showing a negligible value after the sixth cycle.

The lowest decrease in adsorption capacity withthe adsorption-regeneration cycles is detected for the

APlO-p-cresol system, since after the fourth cycle thiswas reduced to 50%, remaining almost constant forfurther cycles in spite of the fact that the residue lefton the surface in this system was the highest. Finally,the reduction of the adsorption capacity of APlOafter seven cycles for both phenol and p-nitrophenolis around 80%.Acknowledgements-Financial support from CICYT(Project No. AMB92-1032) and the Spanish-FrenchJoint Research Programme (No. 223B) is gratefullyacknowledged.

1.2.3.4.5.

6.7.

8.9.

10.11.12.13.14.15.

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