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Microporous and Mesoporous Materials 85 (2005) 7–15

Effect of microwave and conventional regeneration onthe microporous and mesoporous network and on the

adsorptive capacity of activated carbons

C.O. Ania, J.B. Parra *, J.A. Menendez, J.J. Pis

Instituto Nacional del Carbon, CSIC, Apartado 73, 33080 Oviedo, Spain

Received 11 December 2004; received in revised form 10 March 2005; accepted 13 June 2005Available online 27 July 2005

Abstract

The effect of different heating mechanisms on the porous network of activated carbons (AC) previously exhausted with phenolhas been studied. To this end, thermal treatment of the exhausted AC was performed using two experimental devices: a single modemicrowave device operating at 2450 MHz and a conventional electric furnace. By employing microwave energy, the regenerationtime was considerably shortened compared to conventional thermal heating. Moreover, microwave heating preserved the porousstructure of the regenerated AC more efficiently than treatment in a conventional device. In both cases successive regeneration cyclesreduced the microporosity of the activated carbons. However, conventional heating shifted the micropore size distribution to poresof narrower sizes. The apparent BET surface areas were also reduced significantly over the regeneration cycles. A loss of the adsorp-tive capacity of the carbon material was observed after six adsorption–desorption cycles in both systems. The phenol adsorptioncapacities decreased to a greater extent in the samples regenerated in the electric furnace.� 2005 Elsevier Inc. All rights reserved.

Keywords: Activated carbon; Thermal regeneration; Microwave energy; Phenol adsorption; Carbon deposits

1. Introduction

Over the last few decades, activated carbons (AC)have been employed in a wide number of applicationson an industrial scale, including technologies for thepurification of gases [1], and the removal of organic pol-lutants from water (i.e., purification of drinking water,and wastewater) [2–4]. Depending on their adsorptioncapacity, they become saturated after sustained use.Formerly, when an AC reached its saturation limit, itwas usually taken to a landfill and dumped. However,the enclosing and burying of hazardous waste is becom-ing increasingly unacceptable due to growing concern

1387-1811/$ - see front matter � 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.micromeso.2005.06.013

* Corresponding author. Tel.: +34 985 11 89 73; fax: +34 985 29 7662.

E-mail address: jbparra@incar.csic.es (J.B. Parra).

about the effect of pollutants on the environment andmore restrictive environmental regulations.

Efficient regeneration systems are required to permita wider application of carbon adsorption processesand to ensure their economic feasibility [5]. This hasmotivated companies to develop methods for regenerat-ing and reusing saturated activated carbon. Over theyears, a wide variety of regeneration techniques havebeen suggested and applied. These are based either ondesorption—induced by increasing the temperature orby displacement with solvents, or on decomposition in-duced by thermal, chemical, catalytic or microbiologicalprocesses [6,7]. By far the most extensively used tech-nique is thermal regeneration under steam or an inertatmosphere. Unfortunately, the regeneration process istime consuming and after successive heating and coolingcycles, the carbon becomes damaged.

8 C.O. Ania et al. / Microporous and Mesoporous Materials 85 (2005) 7–15

The application of microwave-heating technology forregenerating industrial waste activated carbon has beeninvestigated [8–10], with very promising results. Themain difference between microwave devices and conven-tional heating systems is in the way the heat is generated.Thermal regeneration is conventionally performed in ro-tary kilns or vertical furnaces, where the carbon bed isindirectly heated by conduction and convection. In themicrowave device, the microwaves supply energy di-rectly to the carbon bed. Energy transfer is not by con-duction or convection as in conventional heating, but isreadily transformed into heat inside the particles by di-pole rotation and ionic conduction [11]. When high fre-quency voltages are applied to a material, the responseof the molecules with a permanent dipole moment or in-duced dipole to the applied potential field is to changetheir orientation in the direction opposite to that ofthe applied field. The synchronized agitation of mole-cules then generates heat.

Microwave regeneration offers possible advantagesover conventional treatment. These include the rapidand precise control of the carbon bed temperature, amore compact furnace, and energy savings. Someauthors have also found that microwave regenera-tion gives rise to a better performance of the activatedcarbons in terms of ulterior adsorption capacity andrate of adsorption compared to conventional heating[10,12–14]. Consequently, processes based on micro-wave energy are gaining importance in several fieldsof application on an industrial scale, and microwavetechnology is now considered a promising availabletechnology for the regeneration of carbon materials[15,16].

The aim of this work was to acquire a deeper insightinto the thermal regeneration of activated carbons. Tothis end a comparative study of the regeneration of acti-vated carbons by conventional thermal methods andsingle mode microwave energy was performed. Changesin the porous texture of the regenerated carbons result-ing from the thermal treatment in both devices werestudied in relation to the changes in the load capacityof the adsorbents.

2. Experimental

2.1. Materials

According to the manufacturer, the selected activatedcarbons were obtained from the physical activation ofcoal (Q) and chemical activation of wood (CV) usingphosphoric acid. Sample Q was ground and a particlesize fraction of 0.212–0.075 mm was used in all theexperiments. The second carbon sample, CV, was sup-plied in powdered form. The as-received samples werewashed several times with deionised boiling water, dried

for 2 days at 110 �C and stored in a desiccator untilused.

2.2. Regeneration in an electric furnace and a microwave

device

The samples were saturated with phenol using astainless steel column of length 10 cm and internal diam-eter 1.5 cm. All the experiments were conducted at aninitial concentration of 2 g l�1, and a flow rate of4 ml min�1. The outlet concentration was continuouslymonitored by means of a UV–VIS spectrophotometerat the corresponding wavelength. The adsorptive capac-ities were obtained by integrating over the entire break-through curve. When the saturation point was reached,the exhausted AC was washed with distilled water for2 h to eliminate any excess of pollutant, and dried over-night in an oven at 110 �C to reduce the moisture con-tent. Afterwards, the sub-samples of the exhausted ACwere regenerated.

Regeneration of the exhausted AC was carried out inan electric furnace (EF) and in a single mode microwavedevice (MW). The resultant samples were labelled sam-ple-EF or sample-MW, respectively. The exhaustedsamples were regenerated at 850 �C under an inert(N2) atmosphere. The spent AC was placed in a verticalquartz reactor and purged with inert gas. The samplewas then heated using a nitrogen flow rate of10 ml min�1. The inert atmosphere was maintained dur-ing the heating up and cooling-down intervals.

The experimental equipment for the microwaveregeneration of AC has been described elsewhere [17].The temperature of the regeneration process wasmeasured with an optical pyrometer. Samples will bedenoted as the carbon samples followed by CiRi—Cibeing the cycles of saturation and Ri the number ofregenerations, plus a reference to the experimental de-vice. For instance, sample QC2R2 MW corresponds toQ activated carbon after two subsequent adsorption/regeneration cycles in the microwave device. The effectof the heating mechanism in the absence of the adsor-bate was also evaluated; the blanks will be denoted asthe carbon samples followed by EF or MW.

2.3. Textural and chemical characterisation

Textural characterisation was carried out by measur-ing the N2 and CO2 adsorption isotherms at �196 and0 �C, respectively. Narrow microporosity (pore diame-ter < 0.7 nm), medium-sized microporosity (0.7 < porediameter < 2 nm) and mesoporosity were evaluated bymeans of the DFT model [18] applied to the nitrogenadsorption isotherms. This subdivision of microporosityprovided by the DFT method, although not in strictaccordance with that established by IUPAC, is widelyaccepted [19].

C.O. Ania et al. / Microporous and Mesoporous Materials 85 (2005) 7–15 9

The micropore size distribution was assessed fromCO2 adsorption at 0 �C [20]. For this purpose, theDRS equation was also applied to the CO2 adsorptionisotherms, the b parameter being 0.36 [21] and the pack-ing area for the CO2 molecule 0.187 nm2 [22].

The materials were further characterized by ultimateanalysis and the determination of the point of zerocharge. The PZC values were determined by mass-titra-tion as described elsewhere [23].

1 2 3 4 5 6Number of regenerations

200

400

600

800

Inpu

t pow

er (W

) QCV

Fig. 1. Variation in the input power applied during the regenerationassays in the microwave device.

3. Results and discussion

3.1. Regeneration in the electric furnace and microwavedevice

The exposure time of the activated carbon at the tem-perature of regeneration was set as the reaction timenecessary to attain a 3% degree of burn-off under aCO2 atmosphere, in order to compare the results withthose obtained under oxidizing conditions, as discussedin previous works [17]. This parameter was measured inboth experimental devices. The time was found to be 9times shorter when the samples were regenerated bymeans of microwave energy (4 min) compared to theconventional EF (37 min). The time needed to rise fromroom temperature to 850 �C was also lower when micro-wave energy was applied. The AC reached the experi-mental temperature in 5–6 min when heated bymicrowave energy, whereas 10–13 min were necessaryfor the samples treated in the EF.

The influence of thermal treatment on the porousstructure of non-saturated AC has been reported previ-ously. In general, microwave heating of non-saturatedactivated carbon preserves the porous structure of theparent sample to a greater extent, than treatment carriedout in the EF. Specific surface areas and micropore vol-umes in the samples treated in the MW did not differnoticeably from those of the parent AC. Whereas,changes were greater after treatment of the non-satu-rated AC in the EF, giving rise to some blocking ofthe porous structure [17].

3.1.1. Temperature uniformity in the microwave device

Water has a high dielectric loss factor, so relativelysmall differences in moisture content between sampleswill result in different temperatures. To eliminate thechanges in the strength of the electric fields in the mate-rial and the power dissipated as a consequence of varia-tions in moisture content, the saturated samples weredried at 110 �C.

During microwave heating, the energy is applied di-rectly into the sample. The temperature reached willnot only depend on the fraction of the supplied energyabsorbed by the sample but also on the nature of theAC (dielectric properties, degree of order of the basal

planes, chemical composition, etc.) [24,25], its particlesize, and the amount of microwave power applied tothe sample [11]. For a given AC, the temperature canbe modified by adjusting the input power. In the presentwork, the temperature was fitted by using a variable in-put power, taking into account the requirements of eachsample. The saturated AC reached the experimentaltemperature (850 �C) after 5–6 min, this temperatureremaining practically stable during the treatment. Incontrast about 14 min was needed when regenerationwas conducted in the conventional electric equipment.The amount of carbon, moisture content and particlesize were found to be approximately the same for allthe assays.

When the exhausted carbons were exposed to themicrowaves, the input power used to reach the regener-ation temperature (850 �C) had to be increased witheach regeneration cycle from 300 to 800 W (Fig. 1).These differences are probably due to the changes in-duced in the microstructures of the carbon, and the sat-uration degree. Nevertheless, it was difficult to establishany correlation between the saturation cycles and thefraction of supplied energy that was adsorbed. After aseries of adsorption/desorption cycles the changes in-duced in the chemical composition and/or porous struc-ture of the regenerated AC may alter the dielectricproperties of the adsorbents considerably. As a resultthe fraction of absorbed energy needed to reach the finaltemperature is also modified.

In most cases, heating the samples in the microwavedevice was quite fast, less than 6 min being needed toreach 850 �C, except for sample CV in the first regener-ation cycle. In general, a rapid initial heating up to140 �C was followed by a period during which volatileswere removed from the sample. There was then a rapidincrease in temperature up to 850 �C. Subsequent treat-ments of the exhausted activated carbons (i.e., a second

25 150 275 400 525

Temperature (ºC)

0.80

0.85

0.90

0.95

1.00

Wei

ght l

oss

(%)

QCV

Fig. 2. TG curves of the as-received activated carbons.

10 C.O. Ania et al. / Microporous and Mesoporous Materials 85 (2005) 7–15

and successive series of regeneration cycles) led to a stea-dy increase in temperature. Desorption of the adsorbateretained on the activated carbon was observed after200 �C (change in the colour of the evolved gases).

The slow heating rate of the CV sample during thefirst regeneration cycle was also observed for the heatingof the as-received sample (non-saturated). In this case,the period of gas removal at low temperatures was muchlonger, approximately 12 min being needed to reach850 �C. During this step, a large amount of gases wasobserved to evolve from the carbon bed, and the frac-tion of power absorbed by the activated carbon de-creased suddenly. The microwaves applied to thecarbon bed were not absorbed, and a considerable per-centage of the energy was transmitted by the sample.This might be explained in terms of the different chemi-cal composition of the two activated carbons, if it is as-sumed that some changes occur on the surface chemistryof the AC during microwave heating. Indeed, thisassumption was proved correct by the chemical analysesof the regenerated samples and also when the AC wasused again to adsorb phenol, a point which will be dis-cussed later.

Sample CV presented high oxygen (9.7%) and volatilematter (22%) contents, compared to the Q sample (1.9%and 2.5%, respectively). Volatiles and oxygen-containingfunctional groups were evolved during heating, theamount of gases released being much larger for sampleCV than for sample Q, as might be expected. The gasesevolved from the sample appeared to hinder microwaveabsorption, thereby impeding the heating of the AC.This fact was corroborated by: (i) the large amount oftransmitted power detected at the end of the microwave

Table 1Chemical and textural properties of the parent and regenerated samples, from

BET method DFT method

SBET

(m2 g�1)CBET Vnarrowmicropores

(cm3 g�1)Vm

(cm

Q 1149 262 0.126 0.2Q EF 1022 353 0.136 0.1QC1R1 EF 822 302 0.119 0.1QC3R3 EF 340 130 0.024 0.0QC6R6 EF 162 88 0.002 0.0Q MW 1083 309 0.131 0.1QC1R1 MW 932 289 0.111 0.1QC3R3 MW 595 145 0.052 0.0QC6R6 MW 257 105 0.021 0.0

CV 1414 157 0.112 0.2CV EF 961 287 0.142 0.1CVC1R1 EF 908 273 0.137 0.0CVC3R3 EF 525 248 0.066 0.0CVC6R6 EF 374 159 0.038 0.0CV MW 1062 297 0.142 0.1CVC1R1 MW 961 268 0.132 0.1CVC3R3 MW 547 265 0.023 0.0CVC6R6 MW 386 175 0.018 0.0

cavity, and (ii) the maintenance of a constant tempera-ture in the carbon bed during the process, even whenthe input power applied was increased. Once the volumeof gases evolved had diminished (all the volatiles havingalready evolved) the sample was heated rapidly, whilethe amount of transmitted power decreased abruptly.During subsequent regeneration cycles, the CV sampledid not show the same behaviour, as the volatiles andmost of the oxygen content had already been desorbedin the first step (cf. Table 1). Consequently, the perfor-mance observed after the first few cycles was more sim-ilar to that of the Q sample.

These results were corroborated by the thermalanalysis of the as-received activated carbons (Fig. 2).The mass loss profile of the CV sample at low tempera-

N2 adsorption isotherms at �196 �C, using the BET and DFT methods

Oxygen (%) pH PZC

edium-micropores3 g�1)

Vmesopores

(cm3 g�1)

11 0.134 1.9 9.279 0.065 1.1 9.829 0.071 1.2 10.363 0.051 1.0 9.524 0.037 1.0 9.392 0.101 1.4 10.866 0.087 0.9 10.285 0.068 0.9 9.537 0.051 0.9 9.3

43 0.530 9.7 2.205 0.347 6.8 3.599 0.330 6.8 3.652 0.275 4.2 7.335 0.249 2.4 8.763 0.462 6.1 4.216 0.352 5.7 4.556 0.286 4.0 7.143 0.262 1.9 9.2

0.0 0.2 0.4 0.6 0.8 1.0

0

100

200

300

400QQ N850 EFQ N850 MW

500

600

700

800

900CVCV N850 EFCV N850 MW

Volu

me

adso

rbed

(cm

3 g-1, S

TP)

(cm

3 g-1, S

TP)

p/po

C.O. Ania et al. / Microporous and Mesoporous Materials 85 (2005) 7–15 11

ture was much more noticeable than that of the Q sam-ple, indicating that the volatile matter of the CV samplehad evolved from low temperatures. Furthermore, theproximate analysis of the CV sample showed a large vol-atile matter content.

3.1.2. Characterization of the regenerated activated

carbons

The characteristics of the pore structure of the parentand regenerated activated carbons are presented inTable 1. The N2 adsorption isotherms at �196 �C areshown in Figs. 3 and 4. All the adsorption isothermsare type I on the BDDT classification [26], indicatingthat they are mainly microporous materials. However,the desorption branch of sample CV presents a hystere-sis loop at high relative pressures, pointing to a consid-erable development of mesoporosity.

After the first cycle, the shape of the nitrogen iso-therms moved steadily downwards, this effect beingmore evident for the samples regenerated in the conven-tional furnace. Successive cycles caused a sharp decrease

0.0 0.2 0.4 0.6 0.8 1.0

0

100

200

300

400

500

600QQ C1R1 EFQ C3R3 EFQ C6R6 EFQ C1R1 MWQ C3R3 MWQ C6R6 MW

0.0 0.2 0.4 0.6 0.8 1.0

0

100

200

300

400

500

600

700

800

900CVCV C1R1 EFCV C3R3 EFCV C6R6 EFCV C1R1 MWCV C3R3 MWCV C6R6 MW

p/po

p/po

Volu

me

adso

rbed

(cm

3 g-1, S

TP)

Volu

me

adso

rbed

(cm

3 g-1, S

TP)

Fig. 3. N2 adsorption isotherms at �196� corresponding to the as-received and the regenerated samples after 1, 3 and 6 regenerationcycles.

0.0 0.2 0.4 0.6 0.8 1.0

0

100

200

300

400

Volu

me

adso

rbed

p/po

Fig. 4. N2 adsorption isotherms at �196 �C corresponding to thesamples treated in the absence of adsorbate.

in the amount of adsorbed nitrogen. After 3 and 6regeneration cycles the porous network of the adsorbentwas found to be extremely damaged. Nevertheless,dielectric heating preserved the texture to a greater ex-tent than the conventional method.

The abrupt knee in the nitrogen adsorption isothermsof the regenerated samples indicates that the micropo-rosity of these samples is mainly composed of pores ofa small diameter. As regards mesoporosity, the hystere-sis loop of the CV sample at high relative pressures ap-peared in all its counterparts. After 6 regenerationcycles, despite the significant fall in the uptake of nitro-gen, mesoporosity seemed to be noticeably wellpreserved.

As can be seen in Table 1, there is an abrupt decreasein the surface areas of the regenerated samples. BET val-ues (cf. Table 1) decreased considerably over the succes-sive regeneration cycles, values being 50% lower thanthose of the parent samples after 3 cycles. This effectwas more evident when thermal treatment was con-ducted in the electric furnace. For instance, the apparentBET surface area of sample Q fell by 86% after 6 cycles,

12 C.O. Ania et al. / Microporous and Mesoporous Materials 85 (2005) 7–15

compared to 76% in the microwave. These changes wereeven more remarkable for the Q sample than for the CVsample (except in the first cycle). The greater decrease inthe BET area observed in the CV sample after the firstfew cycles might be associated with internal reorganiza-tions in this activated carbon as a consequence of thelarge amount of gases evolved during pyrolysis. TheCBET values of the regenerated samples confirmed thatmicroporosity was constricted due to the thermal treat-ment of the exhausted samples, as suggested by theabrupt knee in the nitrogen adsorption isotherms. Theseresults also prove that the adsorbate was only partiallydesorbed during pyrolysis. As a consequence of this par-tial compound removal, the porous structure of the sam-ple remained blocked and hence the apparent BETsurface area was reduced.

Thermal heating at high temperature of the non-sat-urated materials induced a partial collapse of the porousstructure (structural annealing). This effect, which hasbeen reported in the literature by numerous authors[27], was more significant for the CV samples.

The textural properties of the samples heated up in theabsence of the adsorbate were better than those observedafter the first regeneration, underlining the important roleof the phenol molecules and their desorption process inthe changes produced in the porous structure of the AC.

The evolution of medium-micro and mesopores wasevaluated by the DFT model applied to the nitrogenadsorption isotherms (cf. Table 1). The mesopore vol-umes decreased substantially after the first cycle, this de-crease being somewhat smaller for the following cycles.For instance, the volume of mesopores in the Q seriesafter 6 regeneration cycles fell by 72% when the regener-ation was conducted in the EF, against 62% in themicrowave device. In contrast, the decrease amountedto 51% and 53% for regeneration in the EF and themicrowave device, respectively, for the CV series. Themedium-sized micropore volume decreased sharply asregeneration proceeded. For instance, regeneration ofQ in the electric furnace gave rise to a decrease of 90%in the micropore volume compared to 82% when regen-eration was conducted using microwave energy.

This suggests that the porosity of the samples wasconstricted as a consequence of the regeneration pro-cess. Gradual blocking of the porous structure seemedto occur because a fraction of the adsorbed moleculesdid not evolve from the carbon surface during regenera-tion. These molecules may have remained inside thepore network of the AC, and decomposed due to thehigh temperature of the process, giving rise to a carbo-naceous residue [28–30]. The deposited carbon may haveappeared as coke deposits at the entrance of the pores.The process is very similar to chemical vapour deposi-tion, and has been widely reported in the literature [30].

The blockage, however, was less evident for the CVseries, probably due to the highly developed mesoporos-

ity of the parent sample (0.134 vs 0.530 cm3 g�1 for Qand CV, respectively). Mesoporosity may permit a bet-ter connection between the microporous pattern of theactivated sample (where adsorption mainly takes place),and the exterior of the carbon, permitting the moleculesretained to evolve more rapidly from the carbon. In thecase of the Q series, the lack of a well developed meso-porosity delayed the migration of the desorbed mole-cules. Consequently, the formation of coke depositsand damage to porosity was enhanced.

The micropore size distribution calculated by apply-ing the DRS equation to the CO2 adsorption isothermsat 0 �C is shown in Fig. 5. The results show a gradualdecrease in the height of the maximum pore volume withthe number of regeneration cycles. When regenerationwas performed by conventional techniques, the decreasein micropore volume was accompanied by a downwardshift to pores of narrower sizes. This suggests that regen-eration treatment introduced some molecular sieve ef-fects into the porous structure of the regeneratedactivated carbons, most likely due to blocking effectsat the entrance of the pores of a larger size. In contrast,in the case of regeneration in the MW, a slow andgradual fall was observed and the pore width remainedalmost unaltered, indicating that obstruction of thepores of a larger size occurred to a lesser extent In thecase of the CV series, a shift to pores of a narrow sizeafter the first cycle and in the absence of an adsorbatewas also observed, regardless of the heating mechanismemployed. This can be attributed to the damage to theporous structure of the sample as a consequence of thetemperature of regeneration (volatiles evolved duringpyrolysis).

These results are in good agreement with those in-ferred from nitrogen adsorption data. If phenol mole-cules are not completely removed from the inner poresof the activated carbon, they decompose inside the por-ous structure, creating coke deposits. Thus, the micro-porous network of the AC is blocked and the volumeof medium-sized micropores and mesopores accessibleto the nitrogen probes decreases. At the same time, car-bon deposition from phenol decomposition may occurat the entrance to the pores, giving rise to narrower poresize distributions, as evidenced by applying the DRSmethod to the CO2 adsorption data.

The changes induced in the porous structure ofthe regenerated samples are due to a twofold effect; theannealing effect in the carbonaceous skeleton when thesamples are exposed to high temperatures, and the crea-tion of coke deposits within the pore structure [28–30],as a consequence of which the surface area decreasesand the micropore distribution shifts towards narrowerpores, regardless of the experimental device employed.

Textural properties are preserved to a greater extentin the case of microwave energy. This might be due tothe different heating mechanisms in both devices, along

0.0 0.5 1.0 1.5 2.0L (nm)

0.0

0.5

1.0

1.5

QQ EFQC1 EFQC3 EFQC6 EF

0.0

0.5

1.0

1.5

0.0 0.5 1.0 1.5 2.0L (nm)

QQ MWQC1 MWQC3 MWQC6 MW

0.0 0.5 1.0 1.5 2.0L (nm)

0.0

0.5

1.0

1.5

CVCV EFCVC1 EFCVC3 EFCVC6 EF

0.0

0.5

1.0

1.5

0.0 0.5 1.0 1.5 2.0L (nm)

CVCV MWCVC1 MWCVC3 MWCVC6 MW

Por

e Vo

lum

e (c

m3 g-1

, STP

) P

ore

Volu

me

(cm

3 g-1, S

TP)

Fig. 5. Micropore size distribution obtained from the DRS method applied to the CO2 adsorption isotherms at 0 �C.

C.O. Ania et al. / Microporous and Mesoporous Materials 85 (2005) 7–15 13

with the shorter exposure time at the temperature ofregeneration in the microwave system. In the micro-wave, due to the shorter exposure time, the AC were lessprone to suffer structural porous blockage.

Whereas heating by conventional means occurs byconvection, microwaves heat the samples from theinside, creating a thermal temperature gradient thatdecreases towards the surface of the material [31]. Dueto this temperature gradient, there is a heat flow fromthe inside of the particle (hot area) to the outside (coolarea), unlike the case of conventional heating. Conse-quently, the desorbed molecules produced in the coreof the carbon bed diffuse towards the lower temperatureregion more quickly.

Since desorption occurs at the surface of the carbonbed, the phenol molecules inside the AC have to migrate

to the surface. As the diffusion towards the surface is therate-determining step in the process, the desorption pro-cess is favored during microwave irradiation.

While the desorbed molecules are being transferredthrough the carbon bed towards the surface, unwantedparallel reactions such as the decomposition of theevolved molecules may occur. These reactions wereunfavored in the microwave device, due to the shortexposure time and the temperature gradient whichenhanced the desorption process. Therefore, the innerporous structure of the samples, which would otherwisehave been destroyed, was better preserved.

In the conventional method, however, the AC isheated by convective heat transfer from a high-tempera-ture gas and by conduction from the surface to the cen-tre, with a temperature gradient that is in the inverse

14 C.O. Ania et al. / Microporous and Mesoporous Materials 85 (2005) 7–15

direction to that of microwave heating; i.e., a lower tem-perature in the centre and a higher one in the outerregion.

There is a point in conventional heating where themolecules have substantially evolved, but there remainsan exhausted core. Transmission of heat to this innercore is slower by conventional techniques, so thatdesorption from this central core is more difficult. Asregeneration develops from the surface to the centre,the desorbed molecules produced near the centre haveto traverse a high temperature surface region. For thisreason, it is difficult to prevent the decomposition ofthe desorbed molecules inside the porous network ofthe activated carbon. This causes the physical ruptureof the pore walls and a greater obstruction of the pores,compared to the samples regenerated with microwaveenergy (cf. Table 1).

It is well known that thermal treatment of activatedcarbons decreases their oxygen content. Most of theoxygen is originally present in the form of functionalgroups located around the edges of the carbon layerplanes. As a consequence of the regeneration tempera-ture, the oxygenated functional groups of the CV sampleare evolved as CO and CO2 during the process. Thelarge amount of gases evolved from the CV sample,compared to the Q sample, is due to its high oxygenand volatile contents. Furthermore, the PZC values(Table 1) showed that the regeneration treatment gaverise to highly basic materials, regardless of the heatingmechanism. This effect is much more noticeable for theCV sample, which initially showed an acidic nature.Most of the functional groups of the CV sample wereeliminated during the first and second regeneration cy-cles, as the PZC values after the third regeneration cyclewere of a basic nature.

The results of the loading capacity of phenol on theparent and regenerated activated carbons are presentedin Table 2. The adsorptive capacities of the regeneratedsamples were greater when dielectric heating was em-ployed in the regeneration process. There are two rea-sons for this behaviour: firstly, the more developedtextural properties of the samples regenerated with

Table 2Adsorption capacities of the as-received and the thermally regeneratedsamples

Q 226 CV 177

EF MW EF MW

Adsorptive capacities (mg g�1)

QC1R1 161 425 CVC1R1 210 238QC2R2 113 307 CVC2R2 167 265QC3R3 71 115 CVC3R3 147 157QC4R4 55 82 CVC4R4 91 104QC5R5 40 70 CVC5R5 88 96QC6R6 32 59 CVC6R6 65 83

microwave energy, and secondly, the changes inducedin the surface chemistry after regeneration.

Using conventional heating (EF) the adsorptioncapacity of the AC was observed to decrease graduallywith subsequent regeneration cycles, this effect beingmore evident for the CV series. This decrease is in agree-ment with the partial desorption of phenol molecules andthe blockage of the porous structure. As the porositydiminished, the adsorption capacity also decreased, de-spite the increasing basicity of the samples (cf. Table 1).

In contrast, it is worth noting that after one regener-ation cycle, despite the slight deterioration of the porousstructure of the parent activated carbons—compared tothe EF furnace, adsorptive capacities after MW regener-ation were unexpectedly high. The loading capacity ofphenol exceeded its initial value, particularly for theCV sample.

Adsorptive capacities after the second and even thethird regeneration cycles in the MW device were also sig-nificantly high. Afterwards, the adsorptive capacitiesslowly decreased. For instance, after the first cycle thephenol adsorption capacity in the Q sample was425 mg g�1in the MW, compared to 161 mg g�1 in theelectric furnace. After 6 cycles, the decrease in adsorp-tion capacity compared to the initial sample was 70%in the MW versus 82% in the EF. Thus it can be inferredthat microwave energy contributes to an increase in theefficiency of the regeneration of saturated AC, and in thelife span of the activated carbon.

This behaviour can be explained in terms of texturalchanges and surface chemistry. Since thermal treatmentof the activated carbons eliminated the oxygenatedsurface functional groups, the p-electron density of thegraphene layers increased, as did the dispersive interac-tions between the phenol and the p-electron density ofthe carbon [32–34]. Accordingly, there was an increasein the chemisorption contribution to the overall loadingcapacity of phenol, this capacity increasing extensivelyafter the first regeneration cycle. In the case of the CVseries, the increase in the contribution of chemisorptionmight be expected to be more pronounced, due to theprimary acidic nature of this AC. In fact, deteriorationof the textural properties due to the high amount ofgases evolved during heating at high temperaturerestricted the increase in the overall uptake of phenol.

4. Conclusion

Dielectric heating of activated carbons was found tobe more rapid and efficient than conventional methods,although the input power of the microwaves needed tobe increased when the samples were saturated. Afterthe first regeneration cycle, any further increase in theregeneration/adsorption step meant an increase ofdouble the amount of power applied. Nevertheless, the

C.O. Ania et al. / Microporous and Mesoporous Materials 85 (2005) 7–15 15

main advantage of using microwave energy is that thetreatment can be performed in a relatively short periodof time, which implies a lower consumption of gas andenergy and a process that is still economically attractiveand feasible.

The changes induced in the porous structure of theregenerated samples are due to a twofold effect; on theone hand the partial collapse of porosity due to the ther-mal heating itself, and secondly the formation of cokedeposits within the pore network as a consequence ofthe decomposition of phenol molecules that did notevolve.

When microwave heating was applied a temperaturegradient was created in the samples, which decreasedfrom the core towards the surface of the material. Thus,any desorbed molecules migrated rapidly to the surface,the latter having a lower temperature than the core ofthe sample. This reduced the formation of coke depositsinside the porous structure of the AC, resulting in a bet-ter preservation of the textural characteristics of theactivated carbon.

In the conventional heating, the desorbed moleculesproduced near the centre decomposed inside the porousnetwork of the activated carbon, before they could becompletely evolved from the carbon surface. Thus, cokeresidues blocked the pore structure of the adsorbent,leading to a greater decrease in the apparent BET sur-face areas and micropore volumes.

Adsorptive capacities were found to be much higherwhen regeneration was carried out in the microwave de-vice, even after various cycles of regeneration, comparedto those of the conventional furnace. This is attributedmainly to the changes induced in the surface chemistryand textural properties of the regenerated samples.

Acknowledgements

Work carried out with a financial grant from theEuropean Coal and Steel Community (Project 7220-PR-139). The authors wish to thank Dr. Martın-Gullonfor his considerate and helpful assistance with the soft-ware tool used for the DRS calculation.

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