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Comparative study on the thermal reactivation of spent adsorbents
Fuel Processing Technology 116 (2013) 358–365
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Comparative study on the thermal reactivation of spent adsorbents S. Román a,⁎, B. Ledesma b, A. Álvarez-Murillo a, J.F. González a a b
Department of Applied Physics, University of Extremadura, Avda. Elvas, s/n, CP 06006 Badajoz, Spain Department of Mechanical, Energetics and Materials Engineering, University of Extremadura, Avda. Elvas, s/n, CP 06006 Badajoz, Spain
a r t i c l e
i n f o
Article history: Received 14 May 2013 Received in revised form 17 July 2013 Accepted 21 July 2013 Available online 28 August 2013 Keywords: Regeneration Activated carbons Waste treatment Physical activation
a b s t r a c t Activated carbons previously used for p-Nitrophenol (PNP) adsorption were subjected to thermal reactivation in order to recover their initial porosity characteristics. Three activating agents were comparatively analysed (air, carbon dioxide and water steam). Regeneration results improved in the sequence air b CO2 b steam; steam activation almost removed all the adsorbate adsorbed on the carbon, achieving regeneration efficiency values up to 94% for N2 adsorption, and above 100% for PNP adsorption. The activation process did not cause a significant modification of the pore size distribution of the adsorbents, which remained microporous irrespective of the activating agent. The analysis of gases evolved was consistent with the chemical processes involved in the respective activations. There was a significant difference in the pattern followed by H2 in steam activations compared with CO2 and air. The prominence of water gas and water gas shift reactions were associated to this effect, which was also evident from the increase in CO and CO2 concentration. © 2013 Elsevier B.V. All rights reserved.
1. Introduction As the activated carbon market grows increasingly, the search for new technologies or precursors, which can improve the production processes and lower the associated costs is very important. Also, the economics of adsorption technology greatly depends upon the reactivation and reuse of the spent AC. By regeneration processes, the spent adsorbent is subjected to those conditions which enhance the desorption of the adsorbent. Some different regeneration methods have been proposed, such as thermal, solvent regeneration, microwave treatment, photo-phenton processes, biological methods, wet oxygen regeneration, and others [1–3]. However, due to its simplicity and low cost, thermal regeneration is the most often applied technique [4]. Thermal regeneration involves the inert heating of the exhausted adsorbent. This causes the desorption of a fraction of the adsorbate molecules from the carbon matrix, generally the most volatile ones, or those adsorbed by weak dispersion forces (i.e., physisorbed). In general, variable porosity regain values can be achieved by the pyrolysis of spent adsorbents. A review on the state of art found in the bibliography provides a wide variety of results, which are a function of a given adsorption system. Cazetta et al. [3] studied the thermal regeneration of Methylene Blue spent ACs and achieved a porosity regain near 63%. Carratalá et al. [5] subjected to thermal treatment ACs saturated with bencene and toluene and found regeneration efficiency values close to 100% ⁎ Corresponding author. Tel.: +34 924289600; fax: +34 924289601. E-mail address: [email protected] (S. Román). 0378-3820/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2013.07.019
for both adsorbates. Our research group, in a previous work [6], has studied the effect of thermal treatment of spent adsorbents exhausted with p-Nitrophenol (PNP). Phenols are produced in large quantities as a consequence of many anthropogenic activities, such as chemical, petrol, leather, steel, tinctoral or pharmaceutical industries, as well as the use of pesticides [7]. These compounds penetrate into ecosystems as the result of drainage off the municipal or industrial sewage and cause severe damage to living species; moreover, these compounds are very persistent and accumulate in food chains. Only in Europe, indirect emissions of phenolic compounds are above 2000 annual tonnes [8]. Among the various phenol-derived compounds, PNP removal is imperative, according to the High Volume Production Chemicals database [9]. Repetitive exposures to PNP can damage blood cells in the central nervous system and have mutagenic effects; likewise, prolonged exposures can harm both the liver and kidney [10]. Previous studies on the thermal regeneration of PNP spent ACs have confirmed the feasibility of restoring a significant fraction of the adsorbent initial porosity after the pyrolytic treatment. Regeneration efficiency values up to 70% were achieved. In general, all works agree in the fact that the pyrolysis process is not enough to restore the whole carbon adsorption capacity, because a fraction of adsorbate decomposition products are known to be retained on the carbon surface. The desorption is influenced by both the adsorbent, adsorbate and system characteristics. For example, for a given adsorbate, not only the pore volumes but also the pore size distribution characteristics affect the results of thermal regeneration [5]. Also, the surface chemistry of the adsorbent can influence its subsequent regeneration [11]. Regarding the adsorbate, features such as its molecular
S. Román et al. / Fuel Processing Technology 116 (2013) 358–365
structure or boiling point have to be considered; for example, it has been found that aromatics are easier to desorb due to their tendency to be adsorbed via non-specific interactions [5,12]. Moreover, the system characteristics such as temperature, solvation and pH [13], also may play a significant role on the regeneration process. In general, all the abovementioned factors affect the strength of the adsorbent–adsorbate interaction, and are in consequence decisive on the regeneration efficiency of the process. In order to improve porosity regain results, inert thermal treatment is often followed by the gasification of the carbon by an oxidizing gas, which is usually steam. This results in an improvement of regeneration results, via the selective removal of the residual organics remaining on the carbon surface. This treatment usually deblocks a fraction of the non-accessible porosity and also leads to a modification of the adsorbent surface chemistry, which can affect its adsorption properties. There is a compromise situation about the effects caused during carbon gasification; on the one hand, if the activation conditions are not good enough to selectively oxidize the adsorbed species, the AC adsorption capacity will not be fully restored. On the other hand, if the regeneration conditions are too harsh the original structure and porosity of the adsorbent are likely to be damaged. Besides, the burn-off involved in a regeneration process is a very important parameter to be taken into account, since it conditions the economical feasibility of the process; previous studies state that make up carbon used to replace the lost during regeneration can represent 20–40% of the total costs associated with GAC regeneration [14]. In general, for a given spent adsorbent, the potential overburning depends, as in the case of physical activation of chars, on the activation conditions (gasifying agent, temperature and time) [15]. Despite its potential to improve AC regeneration efficiency, the research carried out on the effect of different activating agents on a given spent adsorbent has not been investigated before, to the best of the authors known. The works made on reactivation of adsorbents have focussed on the effect of a single agent, usually steam [12], and less frequently, carbon dioxide [2] or air [5]. As a further step to complete previous research on the pyrolysis of spent adsorbents, this work aims to improve the abovementioned results, by means of oxidizing the carbonized matter blocking the adsorbent porosity. The activation of the carbon previously regenerated (RT/700) was comparatively investigated using different gasifying agents, namely air, carbon dioxide and steam. After reactivation, the ACs were characterized in terms of their porosity by means of N2 adsorption at −196 °C, Hg porosimetry, SEM imaging and FT-IR spectroscopy. Besides, the activation process was monitored by gas chromatography, in order to assess the composition of the gases released (H2, CO, CO2 and CH4), and relate it to the desorption process. 2. Experimental 2.1. Materials A commercial activated carbon (Carbsorb, CB) was selected. Previous to adsorption, CB was grounded and sieved. The adsorbate used was p-Nitrophenol (PNP), a phenolic compound supplied by Sigma-Aldrich. First, the fresh activated carbon was saturated with PNP. For this task, 2.4 g of CB carbon was added to 1 L of PNP solution (3 g L−1). The system was then left to equilibrium under a thermostated temperature of 298 K for 72 h, as previous works showed that these conditions guarantee the complete saturation of the carbon and the adsorption equilibrium, respectively [16]. In second place, the spent carbon was subjected to thermal regeneration (700 °C, 40 min, N2 flow of 100 mL min−1) was followed by physical activation, under the following experimental conditions:
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a) Air activation (100 cm3 min−1) at 633 K, during periods of time in the range 90–180 min. b) Carbon dioxide activation (40 cm3 min−1) at 1123 K, during activation times of 30–300 min. c) Water steam activation (0.4 g min− 1), at 1123 K, studying the influence of activation time in the range 30–90 min. The abovementioned experimental conditions were defined according to previous works which showed that they were suitable to favour the development of microporosity, enhancing the oxidation of the organic compounds remaining in the carbon surface after charring process [17]. The nomenclature used for the regenerated ACs was defined according to the pattern ActX/T/t, where X stands for the activating agent, T for the temperature used and t represents the activation time. For example, the sample ActA/360/90 refers to a carbon which was regenerated via air activation at 360 °C, during a period of time of 90 min. 2.2. Characterization techniques Nitrogen adsorption isotherms at −196 °C were made using a semiautomatic adsorption unit (AUTOSORB-1, Quantachrome). Previous to analyses, the samples were outgassed at 100 °C during 12 h. Experimental data were analysed by suitable methods [18] to calculate: (a) the value of the BET specific surface (SBET), (b) the external surface (SEXT) by the αs-method, using the reference non-porous solid proposed by Carrott et al. [19], (c) the percentage of internal surface (Sint), by the difference between SBET and SEXT, (d) the volume of micropores through the Dubinin–Radushkevich equation (VmiDR), and (e) the volume of mesopores (Vme), as the difference between the pore volume at p/p0 = 0.95 and p/p0 = 0.10. Moreover the external surface of the adsorbents was studied by Hg porosimetry (AUTOPORE 4900, Micromeritics), which allowed the determination of meso and macropore volume. Also, the surface morphology of the reactivated samples was analysed by Scanning Electron Micrograph (SEM, Hitachi, S-3600N) observation, and their surface chemistry was evaluated by FT-IR analyses (Perkin–Elmer 1720). The Regeneration Efficiency (RE, %) was determined according to Eq. (1), with two different adsorbates: gaseous N2 at −196 °C and PNP in aqueous solution at 25 °C. In the former case, N2 adsorption capacity was taken as the one corresponding at the relative pressure of 0.95. In the case of PNP, adsorption measurements were made based on previous works [12], by adding 0.05 g of adsorbent to 10 mL of 3 g L−1 PNP solution, for a period of time of 120 h, under stirring conditions (600 rpm). The concentration of the supernatant solutions was measured spectrophotometrically at 225 nm (Heλios-α Spectrophotometer). The amount of PNP adsorbed was calculated from the difference between initial and final concentration.
REð% Þ ¼
adsorption capacity of regenerated AC 100 Adsorption capacity of fresh AC
ð1Þ
3. Results and discussion 3.1. Characterization of activated carbons Figs. 1, 2 and 3 show the N2 adsorption isotherms at −196 °C of the carbons reactivated with air, carbon dioxide and steam, respectively. In these plots, the adsorption data corresponding to the fresh (CB) and spent (CBspent) carbons have been included. From the adsorption isotherm, characteristical textural parameters were determined, according to the models described in Section 2.1,
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Fig. 1. N2 adsorption isotherms at −196 °C for carbons activated with air.
Fig. 3. N2 adsorption isotherms at −196 °C for carbons activated with water steam.
and have been collected in Table 1. In this table, the values of burn-off (%) and regeneration efficiencies according to Eq. (1) are also displayed. Firstly, from the comparison of fresh and spent carbons, it is obvious that after its use, the adsorbent porosity becomes filled or blocked, finding a very marked decrease in the micropore volume. Thereafter, the pyrolysis treatment involves the removal of an important fraction of the adsorbate, and more than 60% of the microporosity is restored. Regarding the N2 access to the porous network up to a value of relative pressure of 0.95, as it is defined for the N2 regeneration efficiency parameter, one can see that a 68% of porosity was restored. The addition of the activation step has a clear effect on the final characteristics of the carbon, although the improvement depends markedly on the treatment atmosphere [20]. The results concerning each activating agent are discussed below. Firstly, from the values of burn-off it is obvious that each activating agent has a different reactivity, increasing in the sequence air b carbon dioxide b water steam, as it has been also found in previous pieces of research, regarding activation processes [17,21]. On the other hand, it is interesting to notice that the increase in the burn-off value as activation time is enlarged, does not involve in all cases a greater porosity development. For example for Steam series, there is a cut-point value at 60 min (sample ActS/850/60) from which this tendency is switched. For activation times greater than 60 min, a prolongated activation causes a decrease in the micro and mesopores values, with an increasing contribution of external surface, which might be indicative of external burning. The use of air gave rise to a slight recovery of porosity, in reference to the pyrolyzed sample (RT/700). In the case of sample ActA/360/90, an increase of 12% in the micropore volume was found, while a longer
treatment did not result advantageous. It is interesting to notice that all the carbons belonging to this series present N2 isotherms which can be classified as type I, according to BDDT classification, typical of microporous materials [22]. Meng et al. [23] have reported the total effectiveness of air during the regeneration of mesoporous MCM-41 based adsorbents, used for desulfurization processes. Previous works on the activation of chars by air [24], have shown that longer dwell times can help the diffusion of the oxygen towards the inner of the blocked pores and thus improve the oxidation; moreover, there is a cutpoint in which a more prolongated process results in external burning, giving rise to wider pore size distribution. From the results obtained in this work, none of these features is found; activating during longer activation times did not improve the porosity regain nor caused a widening of pore size distribution. Although in this study air activation did not provide a marked regain of porosity, it has to be highlighted the low cost of air activation processes, as compared to steam or carbon dioxide ones, not only because this gas is cheaper but also because of the lower thermal exigency. Also, it is worth noticing that this agent caused the lowest values of burn-off. This is very important, considering that the loss of AC by attrition, handling, washing-out or burn-off stands as one of the most important factors affecting regeneration costs [1]. In consequence, the use of this agent might be interesting for regeneration processes in which a high porosity efficiency is not required. Regarding CO2 regeneration processes, the results obtained revealed an improvement of results, as compared to air activation. The micropore volume was increased up to a 28% in reference to the thermally regenerated sample, which means more than 90% of the spent adsorbent. These results are quite better than those published by other authors which report regeneration efficiency values lower than 55% during carbon dioxide regeneration of ACs exhausted with organics [2]. Again, it is interesting to highlight that the carbon dioxide treatment did not increase the fraction of mesoporosity, not even for the samples treated during longer periods of time. The works on the use of CO2 in activation processes with lignocellulosic precursors [25,26] often report that this agent is prone to produce microporous adsorbents, and increasing activation times gives rise to a slow and gradual porosity widening. As it can be inferred from the corresponding adsorption isotherms (Fig. 2), all the carbons activated by CO2 show a rapid N2 uptake at low values of relative pressures followed by a well defined plateau (type I isotherms), in accordance with their microporous nature. No effect of the activation time on the mesoporosity distribution was found, even for the longest experiences. Also, the low reactivity of this agent is remarkable: the enlargement of the activation process from 90 to 300 min caused a very fair improvement of results. Finally, the results corresponding to steam activation will be analysed. In general, the results found in the bibliography often report
Fig. 2. N2 adsorption isotherms at −196 °C for carbons activated with carbon dioxide.
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Table 1 Burn off and Textural characteristics of CB, CBspent, RT/700. Regeneration efficiency values from N2 and PNP adsorption data.
CB RT/700 ActA/360/90 ActA/360/180 ActC/850/30 ActC/850/60 ActC/850/300 ActS/850/30 ActS/850/45 ActS/850/60 ActS/850/75 ActS/850/90 CBsat
Burn-off (%)
SBET, m2 g−1
Vmi(DR), cm3 g−1
Vme, cm3 g−1
SEXT, m2g−1
SINT, %
E.R.N2, %
E.R. PNF, %
– – 2.8 7.5 3.2 4.9 9.0 7.6 9.8 14.2 18.9 24.8 –
930 611 678 651 735 718 756 786 850 875 793 805 64
0.490 0.312 0.356 0.343 0.387 0.378 0.398 0.413 0.448 0.461 0.416 0.422 0.039
0.063 0.049 0.058 0.043 0.045 0.055 0.049 0.051 0.058 0.060 0.051 0.054 0.014
78 67 74 58 62 75 64 65 77 80 58 75 15
92 89 89 91 92 90 92 92 91 91 93 91 77
100 68 75 70 78 78 81 84 92 94 85 86 10
– 69 75 70 76 80 88 89 94 102 107 108 –
that this agent is the most advantageous, due to the fact that it can diffuse quickly on the porous network and activate the microporosity fast and also because it is cheaper than carbon dioxide. Actually, water steam activation is the most used treatment for activated carbon manufacturing processes. From our results, it can be observed that water steam stands out as the most interesting agent in order to develop the blocked porosity for
Fig. 4. Hg intrusion curves (a) and pore size distribution (b) for CB, CBspent and selected activated carbons.
PNP exhausted adsorbents. It succeeded in restoring more than 90% of the initial N2 carbon adsorption capacity (samples ActS/850/45 and ActS/850/60). The great values obtained for PNP regeneration efficiency have to be pointed out. Values above 100% were obtained for this parameter in the case of the samples made during longer activation times. These results suggest that the steam treatment of the adsorbent might be oxidizing the adsorbate located in the blocked porosity, but also reactivating the pristine carbon, and creating new features in the adsorbent which favour the PNP adsorption. Hg intrusion analyses were made on selected samples (ActA/360/90, ActC/850/30 and ActS/850/60) in order to get additional information on the meso and macroporosity of the adsorbents. In Fig. 4 (a and b) the intrusion curves and derived pore size distribution plots are shown, respectively. In order to favour the analysis, the values of meso and macropores (VmeP and VmaP, in cm3 g−1) calculated from porosimetry intrusion data, as well as the micropore volume (Vmi(DR), Table 1) from N2 adsorption, have been plotted for each adsorbent in Fig. 5. From Fig. 5a, several points can be raised up: on the one hand, it is obvious that the fresh sample is the one with the greatest intrusion volume (VmeP and VmaP equal to 0.301 and 0.121 cm3 g−1). Saturation of CB involves a marked decrease on Hg volume, both the mesopore and macropore volumes are lowered, in such a way that the pore size distribution is not apparently affected (see that the intrusion curves of CB and CBspent are rather parallel). It is worth noticing that after saturation, the reduction of VmeP and VmaP (0.195 and 0.095 cm3 g−1), which corresponds to a drop of 65 and 44% regarding the fresh one, respectively, is much lower than that corresponding to micropore volume as determined from N2 adsorption data (92%). Again, the permanence of PNP in the smallest pores of the carbon is verified. The activation of the spent sample by the three activating agents allowed the restoration of a fraction of the intrusion volume, especially in the case of the carbon regenerated with water steam. In the case of the carbon activated with air, the porosity restorage is lower, in
Fig. 5. Values of micro, meso and macropore volume for CB, CBsat and selected samples.
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agreement with N2 adsorption analyses at −196 °C. At this point it is worth noticing that although the activated samples do not overcome the intrusion volume of the fresh sample (see Figs. 4a and 5), they do present a greater growth of pore volume (dlogV/dlogD) in the macropore region (Fig. 4b). This indicates that activated samples have a wider variety of pore sizes, as compared to fresh and spent samples. The surface morphology of the adsorbents was examined by Scanning Electron Microscopy (SEM). Micrographies of each reactivated sample were taken at different magnifications, and are shown in Fig. 6(a–f). Likewise, the SEM image of the spent sample can be observed for comparison purposes from the Supplementary material (Fig. S1). From Fig. 6, it can be inferred that the reactivation of the spent adsorbent has a very different effect on the topographical features of the carbons, as it is described below. Besides, it is worth noticing the existence of white particles heterogeneously distributed on the carbon surface, on both the spent sample (Fig. S1) and reactivated carbons (Fig. 6). These particles are usually associated to the throng of inorganic material of the precursor, as a consequence of activation process [27]. Regarding the effect of each activating agent on the morphological characteristics of the carbons, it is obvious that air seems to be the least relevant, with a very scarce modification (Fig. 6a and d) as compared to the spent carbon (Fig. S1). A very dissimilar behaviour is found in the case of carbon dioxide and water steam. Firstly, in the case of the former agent (Fig. 6b and e), great cavities and large interconnected channels have been created, so that the whole carbon structure seems to be hollower than the spent carbon. In the case of water steam reactivated sample, the observation of the carbon particle at low magnification (Fig. 6c) shows a splintered surface, which could be related to the original cellular structure of the precursor. The analysis of the surface with a greater magnification reveals the presence of cylindrical macropores. It is very interesting to highlight that although both carbon dioxide and steam have caused
the development of large pores, their appearance is rather different. This behaviour has been also observed in a previous work about the preparation of ACs from walnut shells [21]; the fact that in both cases the tendency resembles so much can suggest that the activating pattern is quite similar, regardless the activation is made on a char or on a spent adsorbent. Finally, the surface chemistry of the adsorbents was analysed by FTIR spectroscopy. Fig. 7 displays the corresponding spectra, which were analysed according to suitable bibliography [28]. From Fig. 7, it can be inferred that the activating agent does not exert a significant influence on the organic chemical structure of the raw material. The bands found in the three cases are very similar, although their intensity can vary depending on the sample. For the three samples, the broad band centred at 3400 cm−1, is mostly attributed to hydrogen bonds, participating in adsorbed water molecules. Also, the adsorption band near 1100 cm−1, can suggest the presence of secondary hydroxyl groups. This latter band has also been related to ν(C\O) vibrations of peroxides and heterocyclic rings. The peaks overlapped around 2400 cm−1, present in all the adsorbents, are generally ascribed to symmetric or asymmetric stretching of aliphatic bonds in \CH, CH2 or CH3. On the other hand, the region of overlapping bands between 1650 and 1400 cm−1 can be attributed to the presence of coupled carbonyl groups as well as aromatic ring stretching.
3.2. Gas analysis In Fig. 8(a, b, c and d) the evolution of the concentration of gases (H2, CO, CO2 and CH4, respectively, in mol L− 1) with time has been plotted. In this figure, the left hand ordinate axis represents the gas
Fig. 6. SEM micrographs of activated carbons.
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Fig. 7. FT-IR spectra of activated carbons.
concentration corresponding to carbon dioxide and air activation, while the right hand axes refer to water steam activation results. From Fig. 8, several points can be highlighted. In the first place, it is found that the concentration profiles show very different features depending on the type of process. On the one hand, the concentration of the gases released in the case of air treatment shows very scant
values, in agreement with the slight effect found on the porosity regain of the carbons during the activation processes. In the case of CO2 activation, hydrogen exhibits a marked concentration peak at the beginning of the process, which decreases gradually with time, completely disappearing around 100 min. Ferro et al. have related the presence of hydrogen in the gas evolved from
Fig. 8. Evolution of gas concentration with time, during activation processes with air, H2Ov and CO2.
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regeneration processes to the decomposition of surface oxygen groups [11]. Regarding the production of CO and CO2, the analysis of the former one can be very useful to study the oxidation of the carbon via the participation of Boudouard equilibrium (Eq. (2)). CO2 þ C→2CO:
ð2Þ
From Fig. 8, it is can be inferred that its concentration increases at the beginning of the process up to a point, at 75 min approximately, in which it reaches a plateau, and then shows a second decrease around 125 min, which seems to indicate the existence of two different stages. It has to be taken into account that CO2 is fed to the reactor continuously; in consequence, a proper analysis might be made by subtracting the amount of gas measured, from the amount of gas fed to the reactor. This latter quantity can be calculated from the CO2 feeding flow rate (0.04 L min−1). This issue has been considered, and the accumulated amount of CO2 (moles) during the process has been determined for the activation process, and compared to the accumulated moles fed to the reactor (see Supplementary material Fig. S2). From this figure, it is observed that in fact, a fraction of the fed CO2 is consumed as the reaction proceeds; moreover, the difference is slightly more marked at the final stages, in coherence with the increase in CO shown in the concentration profiles (Fig. 8). Finally, both CO and CO2 are also prone to be evolved from the decomposition of oxygen surface groups [11]. Finally, in the case of water steam activation processes, H2, CO and CO2 show a marked increase along the whole process. Particularly, it is noticeable the drastic increase on the production of hydrogen (more than 50 fold), as compared to the results obtained with the other two activating agents. The continuous release of H2 along the process has been observed during the steam activation of olive stone chars [29]. Taking into account the increasing trends found for the former gases (CO and CO2) during steam activation, the increase in H2 might be associated to the prominence of equilibria water gas (Eq. (3)) and water gas shift (Eq. (4)). C þ H2 O→H2 þ CO
ð3Þ
CO þ H2 O→H2 þ CO2 :
ð4Þ
Moreover, while for air and carbon dioxide activations no emission of CH4 was found, this gas is released for water steam runs; with an increasing concentration up to a value of 0.92 mol L−1 after 100 min, approximately (Fig. 8d). Among the different equilibria participating in the production of this gas, methanation reaction (Eq. (5)) can be suggested. CO þ 3H2 →CH4 þ H2 O:
ð5Þ
4. Conclusions In this work, the thermal regeneration of activated carbons exhausted with p-Nitrophenol following their pyrolysis has been investigated, comparing the effect of air, carbon dioxide and water steam. The analysis of the experimental results allowed obtaining the following conclusions: – Simple inert thermal regeneration at 700 °C is not enough to restore the adsorbent initial porosity; a regeneration efficiency of 68%, as defined from N2 adsorption data at 77, was consistent with the permanence in the carbon matrix of a fraction of the products resulting from PNP adsorption. – The additional of an oxidation step was in all cases positive, improving the porosity regain, although the effect was very dependent on the type of activation process.
– Air activation provided a slight improvement of the carbon porosity (75%), so that it was estimated that the use of this agent would only be interesting for modest applications. The use of CO2 and especially water steam were very interesting, achieving in the latter case a regeneration efficiency of 94%, regarding N2 adsorption, and a total recuperation of the porosity available to PNP in aqueous media. Moreover, none of the treatments caused a significant modification on the pore size distribution. – The gas monitorization during activation allowed the identification of H2, CO, CH4 and CO2, which were related to the breaking up of chemisorbed products, surface functional groups, or other compounds that may have been created in the carbon surface as a result of thermal treatment. In general, steam activation processes stood out due to their greater production of hydrogen and methane. 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Wróblewska, Desorption of chloro-organic compounds from a bed of activated carbon, Journal of Colloid and Interface Science 285 (2) (2005) 518–524. [13] L.M. Pastrana-Martínez, M.V. López Ramón, C. Moreno-Castilla, Adsorption and thermal desorption of the herbicide fluoroxypyr on activated carbon fibers and cloth at different carbon fibers and cloth at different pH values, Journal of Colloid and Interface Science 331 (1) (2009) 2–7. [14] S.D. Lambert, G. San Miguel, N.J.D. Graham, Deleterious effects of inorganic compounds during thermal regeneration of GAC: a review, Journal American Water Works Association 94 (12) (2002) 109–119. [15] S. Yan, M. Fang, Z. Wang, Z. Luo, Regeneration performance of CO2-rich solvents by using membrane vacuum regeneration technology: relationships between absorbent structure and regeneration efficiency, Applied Energy 98 (2012) 357–367. [16] B. Ledesma, C.M. González-García, S. Román, Arrangements of PNP molecule onto activated carbons of different acidity, Influence of pH. Proceedings of the XXXVII Iberian Adsorption Meeting, Seville, 2012. [17] S. Román, J.F. González, C.M. González-García, F. Zamora, Control of pore development during CO2 and steam activation of olive stones, Fuel Processing Technology 89 (8) (2008) 715–720. [18] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, T. Siemieinewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity, Pure and Applied Chemistry 57 (4) (1985) 613–619. [19] P.J.M. Carrott, R.A. Roberts, K.S.W. Sing, Standard nitrogen adsorption data for nonporous carbons, Carbon 25 (1987) 769–774.
S. Román et al. / Fuel Processing Technology 116 (2013) 358–365 [20] J. Ribera Utrilla, M.A. Ferro-García, C. Moreno-Castilla, J.P. Joly, Thermal desorption of chlorophenols from activated carbons. Influence of treatment atmosphere, Carbon 32 (4) (1994) 743–746. [21] J.F. González, S. Román, C.M. González-García, J.M.V. Nabais, A.L. Ortiz, Porosity development in activated carbons prepared from walnut shells by carbon dioxide or steam activation, Industrial and Engineering Chemistry Research 48 (2009) 7474–7481. [22] S. Brunauer, L.S. Deming, W.E. Deming, E. Teller, On a theory of the Van der Waals adsorption of gases, Journal of the American Chemical Society 62 (1940) 1723–1729. [23] X. Meng, G. Qiu, G. Wang, Q. Cai, Y. Wang, Durable and regenerable mesoporous adsorbent for deep desulfurization of model jet fuel, Fuel Processing Technology 111 (2013) 78–85. [24] S. Román, J.F. González, J. Gañán, E. Sabio, C.M. González-García, A. Ramiro, V. Mangut, Preparation and characterization of carbons for the retention of halogens
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Comparative study on the thermal reactivation of spent adsorbents S. Román a,⁎, B. Ledesma b, A. Álvarez-Murillo a, J.F. González a a b
Department of Applied Physics, University of Extremadura, Avda. Elvas, s/n, CP 06006 Badajoz, Spain Department of Mechanical, Energetics and Materials Engineering, University of Extremadura, Avda. Elvas, s/n, CP 06006 Badajoz, Spain
a r t i c l e
i n f o
Article history: Received 14 May 2013 Received in revised form 17 July 2013 Accepted 21 July 2013 Available online 28 August 2013 Keywords: Regeneration Activated carbons Waste treatment Physical activation
a b s t r a c t Activated carbons previously used for p-Nitrophenol (PNP) adsorption were subjected to thermal reactivation in order to recover their initial porosity characteristics. Three activating agents were comparatively analysed (air, carbon dioxide and water steam). Regeneration results improved in the sequence air b CO2 b steam; steam activation almost removed all the adsorbate adsorbed on the carbon, achieving regeneration efficiency values up to 94% for N2 adsorption, and above 100% for PNP adsorption. The activation process did not cause a significant modification of the pore size distribution of the adsorbents, which remained microporous irrespective of the activating agent. The analysis of gases evolved was consistent with the chemical processes involved in the respective activations. There was a significant difference in the pattern followed by H2 in steam activations compared with CO2 and air. The prominence of water gas and water gas shift reactions were associated to this effect, which was also evident from the increase in CO and CO2 concentration. © 2013 Elsevier B.V. All rights reserved.
1. Introduction As the activated carbon market grows increasingly, the search for new technologies or precursors, which can improve the production processes and lower the associated costs is very important. Also, the economics of adsorption technology greatly depends upon the reactivation and reuse of the spent AC. By regeneration processes, the spent adsorbent is subjected to those conditions which enhance the desorption of the adsorbent. Some different regeneration methods have been proposed, such as thermal, solvent regeneration, microwave treatment, photo-phenton processes, biological methods, wet oxygen regeneration, and others [1–3]. However, due to its simplicity and low cost, thermal regeneration is the most often applied technique [4]. Thermal regeneration involves the inert heating of the exhausted adsorbent. This causes the desorption of a fraction of the adsorbate molecules from the carbon matrix, generally the most volatile ones, or those adsorbed by weak dispersion forces (i.e., physisorbed). In general, variable porosity regain values can be achieved by the pyrolysis of spent adsorbents. A review on the state of art found in the bibliography provides a wide variety of results, which are a function of a given adsorption system. Cazetta et al. [3] studied the thermal regeneration of Methylene Blue spent ACs and achieved a porosity regain near 63%. Carratalá et al. [5] subjected to thermal treatment ACs saturated with bencene and toluene and found regeneration efficiency values close to 100% ⁎ Corresponding author. Tel.: +34 924289600; fax: +34 924289601. E-mail address: [email protected] (S. Román). 0378-3820/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2013.07.019
for both adsorbates. Our research group, in a previous work [6], has studied the effect of thermal treatment of spent adsorbents exhausted with p-Nitrophenol (PNP). Phenols are produced in large quantities as a consequence of many anthropogenic activities, such as chemical, petrol, leather, steel, tinctoral or pharmaceutical industries, as well as the use of pesticides [7]. These compounds penetrate into ecosystems as the result of drainage off the municipal or industrial sewage and cause severe damage to living species; moreover, these compounds are very persistent and accumulate in food chains. Only in Europe, indirect emissions of phenolic compounds are above 2000 annual tonnes [8]. Among the various phenol-derived compounds, PNP removal is imperative, according to the High Volume Production Chemicals database [9]. Repetitive exposures to PNP can damage blood cells in the central nervous system and have mutagenic effects; likewise, prolonged exposures can harm both the liver and kidney [10]. Previous studies on the thermal regeneration of PNP spent ACs have confirmed the feasibility of restoring a significant fraction of the adsorbent initial porosity after the pyrolytic treatment. Regeneration efficiency values up to 70% were achieved. In general, all works agree in the fact that the pyrolysis process is not enough to restore the whole carbon adsorption capacity, because a fraction of adsorbate decomposition products are known to be retained on the carbon surface. The desorption is influenced by both the adsorbent, adsorbate and system characteristics. For example, for a given adsorbate, not only the pore volumes but also the pore size distribution characteristics affect the results of thermal regeneration [5]. Also, the surface chemistry of the adsorbent can influence its subsequent regeneration [11]. Regarding the adsorbate, features such as its molecular
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structure or boiling point have to be considered; for example, it has been found that aromatics are easier to desorb due to their tendency to be adsorbed via non-specific interactions [5,12]. Moreover, the system characteristics such as temperature, solvation and pH [13], also may play a significant role on the regeneration process. In general, all the abovementioned factors affect the strength of the adsorbent–adsorbate interaction, and are in consequence decisive on the regeneration efficiency of the process. In order to improve porosity regain results, inert thermal treatment is often followed by the gasification of the carbon by an oxidizing gas, which is usually steam. This results in an improvement of regeneration results, via the selective removal of the residual organics remaining on the carbon surface. This treatment usually deblocks a fraction of the non-accessible porosity and also leads to a modification of the adsorbent surface chemistry, which can affect its adsorption properties. There is a compromise situation about the effects caused during carbon gasification; on the one hand, if the activation conditions are not good enough to selectively oxidize the adsorbed species, the AC adsorption capacity will not be fully restored. On the other hand, if the regeneration conditions are too harsh the original structure and porosity of the adsorbent are likely to be damaged. Besides, the burn-off involved in a regeneration process is a very important parameter to be taken into account, since it conditions the economical feasibility of the process; previous studies state that make up carbon used to replace the lost during regeneration can represent 20–40% of the total costs associated with GAC regeneration [14]. In general, for a given spent adsorbent, the potential overburning depends, as in the case of physical activation of chars, on the activation conditions (gasifying agent, temperature and time) [15]. Despite its potential to improve AC regeneration efficiency, the research carried out on the effect of different activating agents on a given spent adsorbent has not been investigated before, to the best of the authors known. The works made on reactivation of adsorbents have focussed on the effect of a single agent, usually steam [12], and less frequently, carbon dioxide [2] or air [5]. As a further step to complete previous research on the pyrolysis of spent adsorbents, this work aims to improve the abovementioned results, by means of oxidizing the carbonized matter blocking the adsorbent porosity. The activation of the carbon previously regenerated (RT/700) was comparatively investigated using different gasifying agents, namely air, carbon dioxide and steam. After reactivation, the ACs were characterized in terms of their porosity by means of N2 adsorption at −196 °C, Hg porosimetry, SEM imaging and FT-IR spectroscopy. Besides, the activation process was monitored by gas chromatography, in order to assess the composition of the gases released (H2, CO, CO2 and CH4), and relate it to the desorption process. 2. Experimental 2.1. Materials A commercial activated carbon (Carbsorb, CB) was selected. Previous to adsorption, CB was grounded and sieved. The adsorbate used was p-Nitrophenol (PNP), a phenolic compound supplied by Sigma-Aldrich. First, the fresh activated carbon was saturated with PNP. For this task, 2.4 g of CB carbon was added to 1 L of PNP solution (3 g L−1). The system was then left to equilibrium under a thermostated temperature of 298 K for 72 h, as previous works showed that these conditions guarantee the complete saturation of the carbon and the adsorption equilibrium, respectively [16]. In second place, the spent carbon was subjected to thermal regeneration (700 °C, 40 min, N2 flow of 100 mL min−1) was followed by physical activation, under the following experimental conditions:
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a) Air activation (100 cm3 min−1) at 633 K, during periods of time in the range 90–180 min. b) Carbon dioxide activation (40 cm3 min−1) at 1123 K, during activation times of 30–300 min. c) Water steam activation (0.4 g min− 1), at 1123 K, studying the influence of activation time in the range 30–90 min. The abovementioned experimental conditions were defined according to previous works which showed that they were suitable to favour the development of microporosity, enhancing the oxidation of the organic compounds remaining in the carbon surface after charring process [17]. The nomenclature used for the regenerated ACs was defined according to the pattern ActX/T/t, where X stands for the activating agent, T for the temperature used and t represents the activation time. For example, the sample ActA/360/90 refers to a carbon which was regenerated via air activation at 360 °C, during a period of time of 90 min. 2.2. Characterization techniques Nitrogen adsorption isotherms at −196 °C were made using a semiautomatic adsorption unit (AUTOSORB-1, Quantachrome). Previous to analyses, the samples were outgassed at 100 °C during 12 h. Experimental data were analysed by suitable methods [18] to calculate: (a) the value of the BET specific surface (SBET), (b) the external surface (SEXT) by the αs-method, using the reference non-porous solid proposed by Carrott et al. [19], (c) the percentage of internal surface (Sint), by the difference between SBET and SEXT, (d) the volume of micropores through the Dubinin–Radushkevich equation (VmiDR), and (e) the volume of mesopores (Vme), as the difference between the pore volume at p/p0 = 0.95 and p/p0 = 0.10. Moreover the external surface of the adsorbents was studied by Hg porosimetry (AUTOPORE 4900, Micromeritics), which allowed the determination of meso and macropore volume. Also, the surface morphology of the reactivated samples was analysed by Scanning Electron Micrograph (SEM, Hitachi, S-3600N) observation, and their surface chemistry was evaluated by FT-IR analyses (Perkin–Elmer 1720). The Regeneration Efficiency (RE, %) was determined according to Eq. (1), with two different adsorbates: gaseous N2 at −196 °C and PNP in aqueous solution at 25 °C. In the former case, N2 adsorption capacity was taken as the one corresponding at the relative pressure of 0.95. In the case of PNP, adsorption measurements were made based on previous works [12], by adding 0.05 g of adsorbent to 10 mL of 3 g L−1 PNP solution, for a period of time of 120 h, under stirring conditions (600 rpm). The concentration of the supernatant solutions was measured spectrophotometrically at 225 nm (Heλios-α Spectrophotometer). The amount of PNP adsorbed was calculated from the difference between initial and final concentration.
REð% Þ ¼
adsorption capacity of regenerated AC 100 Adsorption capacity of fresh AC
ð1Þ
3. Results and discussion 3.1. Characterization of activated carbons Figs. 1, 2 and 3 show the N2 adsorption isotherms at −196 °C of the carbons reactivated with air, carbon dioxide and steam, respectively. In these plots, the adsorption data corresponding to the fresh (CB) and spent (CBspent) carbons have been included. From the adsorption isotherm, characteristical textural parameters were determined, according to the models described in Section 2.1,
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Fig. 1. N2 adsorption isotherms at −196 °C for carbons activated with air.
Fig. 3. N2 adsorption isotherms at −196 °C for carbons activated with water steam.
and have been collected in Table 1. In this table, the values of burn-off (%) and regeneration efficiencies according to Eq. (1) are also displayed. Firstly, from the comparison of fresh and spent carbons, it is obvious that after its use, the adsorbent porosity becomes filled or blocked, finding a very marked decrease in the micropore volume. Thereafter, the pyrolysis treatment involves the removal of an important fraction of the adsorbate, and more than 60% of the microporosity is restored. Regarding the N2 access to the porous network up to a value of relative pressure of 0.95, as it is defined for the N2 regeneration efficiency parameter, one can see that a 68% of porosity was restored. The addition of the activation step has a clear effect on the final characteristics of the carbon, although the improvement depends markedly on the treatment atmosphere [20]. The results concerning each activating agent are discussed below. Firstly, from the values of burn-off it is obvious that each activating agent has a different reactivity, increasing in the sequence air b carbon dioxide b water steam, as it has been also found in previous pieces of research, regarding activation processes [17,21]. On the other hand, it is interesting to notice that the increase in the burn-off value as activation time is enlarged, does not involve in all cases a greater porosity development. For example for Steam series, there is a cut-point value at 60 min (sample ActS/850/60) from which this tendency is switched. For activation times greater than 60 min, a prolongated activation causes a decrease in the micro and mesopores values, with an increasing contribution of external surface, which might be indicative of external burning. The use of air gave rise to a slight recovery of porosity, in reference to the pyrolyzed sample (RT/700). In the case of sample ActA/360/90, an increase of 12% in the micropore volume was found, while a longer
treatment did not result advantageous. It is interesting to notice that all the carbons belonging to this series present N2 isotherms which can be classified as type I, according to BDDT classification, typical of microporous materials [22]. Meng et al. [23] have reported the total effectiveness of air during the regeneration of mesoporous MCM-41 based adsorbents, used for desulfurization processes. Previous works on the activation of chars by air [24], have shown that longer dwell times can help the diffusion of the oxygen towards the inner of the blocked pores and thus improve the oxidation; moreover, there is a cutpoint in which a more prolongated process results in external burning, giving rise to wider pore size distribution. From the results obtained in this work, none of these features is found; activating during longer activation times did not improve the porosity regain nor caused a widening of pore size distribution. Although in this study air activation did not provide a marked regain of porosity, it has to be highlighted the low cost of air activation processes, as compared to steam or carbon dioxide ones, not only because this gas is cheaper but also because of the lower thermal exigency. Also, it is worth noticing that this agent caused the lowest values of burn-off. This is very important, considering that the loss of AC by attrition, handling, washing-out or burn-off stands as one of the most important factors affecting regeneration costs [1]. In consequence, the use of this agent might be interesting for regeneration processes in which a high porosity efficiency is not required. Regarding CO2 regeneration processes, the results obtained revealed an improvement of results, as compared to air activation. The micropore volume was increased up to a 28% in reference to the thermally regenerated sample, which means more than 90% of the spent adsorbent. These results are quite better than those published by other authors which report regeneration efficiency values lower than 55% during carbon dioxide regeneration of ACs exhausted with organics [2]. Again, it is interesting to highlight that the carbon dioxide treatment did not increase the fraction of mesoporosity, not even for the samples treated during longer periods of time. The works on the use of CO2 in activation processes with lignocellulosic precursors [25,26] often report that this agent is prone to produce microporous adsorbents, and increasing activation times gives rise to a slow and gradual porosity widening. As it can be inferred from the corresponding adsorption isotherms (Fig. 2), all the carbons activated by CO2 show a rapid N2 uptake at low values of relative pressures followed by a well defined plateau (type I isotherms), in accordance with their microporous nature. No effect of the activation time on the mesoporosity distribution was found, even for the longest experiences. Also, the low reactivity of this agent is remarkable: the enlargement of the activation process from 90 to 300 min caused a very fair improvement of results. Finally, the results corresponding to steam activation will be analysed. In general, the results found in the bibliography often report
Fig. 2. N2 adsorption isotherms at −196 °C for carbons activated with carbon dioxide.
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Table 1 Burn off and Textural characteristics of CB, CBspent, RT/700. Regeneration efficiency values from N2 and PNP adsorption data.
CB RT/700 ActA/360/90 ActA/360/180 ActC/850/30 ActC/850/60 ActC/850/300 ActS/850/30 ActS/850/45 ActS/850/60 ActS/850/75 ActS/850/90 CBsat
Burn-off (%)
SBET, m2 g−1
Vmi(DR), cm3 g−1
Vme, cm3 g−1
SEXT, m2g−1
SINT, %
E.R.N2, %
E.R. PNF, %
– – 2.8 7.5 3.2 4.9 9.0 7.6 9.8 14.2 18.9 24.8 –
930 611 678 651 735 718 756 786 850 875 793 805 64
0.490 0.312 0.356 0.343 0.387 0.378 0.398 0.413 0.448 0.461 0.416 0.422 0.039
0.063 0.049 0.058 0.043 0.045 0.055 0.049 0.051 0.058 0.060 0.051 0.054 0.014
78 67 74 58 62 75 64 65 77 80 58 75 15
92 89 89 91 92 90 92 92 91 91 93 91 77
100 68 75 70 78 78 81 84 92 94 85 86 10
– 69 75 70 76 80 88 89 94 102 107 108 –
that this agent is the most advantageous, due to the fact that it can diffuse quickly on the porous network and activate the microporosity fast and also because it is cheaper than carbon dioxide. Actually, water steam activation is the most used treatment for activated carbon manufacturing processes. From our results, it can be observed that water steam stands out as the most interesting agent in order to develop the blocked porosity for
Fig. 4. Hg intrusion curves (a) and pore size distribution (b) for CB, CBspent and selected activated carbons.
PNP exhausted adsorbents. It succeeded in restoring more than 90% of the initial N2 carbon adsorption capacity (samples ActS/850/45 and ActS/850/60). The great values obtained for PNP regeneration efficiency have to be pointed out. Values above 100% were obtained for this parameter in the case of the samples made during longer activation times. These results suggest that the steam treatment of the adsorbent might be oxidizing the adsorbate located in the blocked porosity, but also reactivating the pristine carbon, and creating new features in the adsorbent which favour the PNP adsorption. Hg intrusion analyses were made on selected samples (ActA/360/90, ActC/850/30 and ActS/850/60) in order to get additional information on the meso and macroporosity of the adsorbents. In Fig. 4 (a and b) the intrusion curves and derived pore size distribution plots are shown, respectively. In order to favour the analysis, the values of meso and macropores (VmeP and VmaP, in cm3 g−1) calculated from porosimetry intrusion data, as well as the micropore volume (Vmi(DR), Table 1) from N2 adsorption, have been plotted for each adsorbent in Fig. 5. From Fig. 5a, several points can be raised up: on the one hand, it is obvious that the fresh sample is the one with the greatest intrusion volume (VmeP and VmaP equal to 0.301 and 0.121 cm3 g−1). Saturation of CB involves a marked decrease on Hg volume, both the mesopore and macropore volumes are lowered, in such a way that the pore size distribution is not apparently affected (see that the intrusion curves of CB and CBspent are rather parallel). It is worth noticing that after saturation, the reduction of VmeP and VmaP (0.195 and 0.095 cm3 g−1), which corresponds to a drop of 65 and 44% regarding the fresh one, respectively, is much lower than that corresponding to micropore volume as determined from N2 adsorption data (92%). Again, the permanence of PNP in the smallest pores of the carbon is verified. The activation of the spent sample by the three activating agents allowed the restoration of a fraction of the intrusion volume, especially in the case of the carbon regenerated with water steam. In the case of the carbon activated with air, the porosity restorage is lower, in
Fig. 5. Values of micro, meso and macropore volume for CB, CBsat and selected samples.
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agreement with N2 adsorption analyses at −196 °C. At this point it is worth noticing that although the activated samples do not overcome the intrusion volume of the fresh sample (see Figs. 4a and 5), they do present a greater growth of pore volume (dlogV/dlogD) in the macropore region (Fig. 4b). This indicates that activated samples have a wider variety of pore sizes, as compared to fresh and spent samples. The surface morphology of the adsorbents was examined by Scanning Electron Microscopy (SEM). Micrographies of each reactivated sample were taken at different magnifications, and are shown in Fig. 6(a–f). Likewise, the SEM image of the spent sample can be observed for comparison purposes from the Supplementary material (Fig. S1). From Fig. 6, it can be inferred that the reactivation of the spent adsorbent has a very different effect on the topographical features of the carbons, as it is described below. Besides, it is worth noticing the existence of white particles heterogeneously distributed on the carbon surface, on both the spent sample (Fig. S1) and reactivated carbons (Fig. 6). These particles are usually associated to the throng of inorganic material of the precursor, as a consequence of activation process [27]. Regarding the effect of each activating agent on the morphological characteristics of the carbons, it is obvious that air seems to be the least relevant, with a very scarce modification (Fig. 6a and d) as compared to the spent carbon (Fig. S1). A very dissimilar behaviour is found in the case of carbon dioxide and water steam. Firstly, in the case of the former agent (Fig. 6b and e), great cavities and large interconnected channels have been created, so that the whole carbon structure seems to be hollower than the spent carbon. In the case of water steam reactivated sample, the observation of the carbon particle at low magnification (Fig. 6c) shows a splintered surface, which could be related to the original cellular structure of the precursor. The analysis of the surface with a greater magnification reveals the presence of cylindrical macropores. It is very interesting to highlight that although both carbon dioxide and steam have caused
the development of large pores, their appearance is rather different. This behaviour has been also observed in a previous work about the preparation of ACs from walnut shells [21]; the fact that in both cases the tendency resembles so much can suggest that the activating pattern is quite similar, regardless the activation is made on a char or on a spent adsorbent. Finally, the surface chemistry of the adsorbents was analysed by FTIR spectroscopy. Fig. 7 displays the corresponding spectra, which were analysed according to suitable bibliography [28]. From Fig. 7, it can be inferred that the activating agent does not exert a significant influence on the organic chemical structure of the raw material. The bands found in the three cases are very similar, although their intensity can vary depending on the sample. For the three samples, the broad band centred at 3400 cm−1, is mostly attributed to hydrogen bonds, participating in adsorbed water molecules. Also, the adsorption band near 1100 cm−1, can suggest the presence of secondary hydroxyl groups. This latter band has also been related to ν(C\O) vibrations of peroxides and heterocyclic rings. The peaks overlapped around 2400 cm−1, present in all the adsorbents, are generally ascribed to symmetric or asymmetric stretching of aliphatic bonds in \CH, CH2 or CH3. On the other hand, the region of overlapping bands between 1650 and 1400 cm−1 can be attributed to the presence of coupled carbonyl groups as well as aromatic ring stretching.
3.2. Gas analysis In Fig. 8(a, b, c and d) the evolution of the concentration of gases (H2, CO, CO2 and CH4, respectively, in mol L− 1) with time has been plotted. In this figure, the left hand ordinate axis represents the gas
Fig. 6. SEM micrographs of activated carbons.
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Fig. 7. FT-IR spectra of activated carbons.
concentration corresponding to carbon dioxide and air activation, while the right hand axes refer to water steam activation results. From Fig. 8, several points can be highlighted. In the first place, it is found that the concentration profiles show very different features depending on the type of process. On the one hand, the concentration of the gases released in the case of air treatment shows very scant
values, in agreement with the slight effect found on the porosity regain of the carbons during the activation processes. In the case of CO2 activation, hydrogen exhibits a marked concentration peak at the beginning of the process, which decreases gradually with time, completely disappearing around 100 min. Ferro et al. have related the presence of hydrogen in the gas evolved from
Fig. 8. Evolution of gas concentration with time, during activation processes with air, H2Ov and CO2.
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regeneration processes to the decomposition of surface oxygen groups [11]. Regarding the production of CO and CO2, the analysis of the former one can be very useful to study the oxidation of the carbon via the participation of Boudouard equilibrium (Eq. (2)). CO2 þ C→2CO:
ð2Þ
From Fig. 8, it is can be inferred that its concentration increases at the beginning of the process up to a point, at 75 min approximately, in which it reaches a plateau, and then shows a second decrease around 125 min, which seems to indicate the existence of two different stages. It has to be taken into account that CO2 is fed to the reactor continuously; in consequence, a proper analysis might be made by subtracting the amount of gas measured, from the amount of gas fed to the reactor. This latter quantity can be calculated from the CO2 feeding flow rate (0.04 L min−1). This issue has been considered, and the accumulated amount of CO2 (moles) during the process has been determined for the activation process, and compared to the accumulated moles fed to the reactor (see Supplementary material Fig. S2). From this figure, it is observed that in fact, a fraction of the fed CO2 is consumed as the reaction proceeds; moreover, the difference is slightly more marked at the final stages, in coherence with the increase in CO shown in the concentration profiles (Fig. 8). Finally, both CO and CO2 are also prone to be evolved from the decomposition of oxygen surface groups [11]. Finally, in the case of water steam activation processes, H2, CO and CO2 show a marked increase along the whole process. Particularly, it is noticeable the drastic increase on the production of hydrogen (more than 50 fold), as compared to the results obtained with the other two activating agents. The continuous release of H2 along the process has been observed during the steam activation of olive stone chars [29]. Taking into account the increasing trends found for the former gases (CO and CO2) during steam activation, the increase in H2 might be associated to the prominence of equilibria water gas (Eq. (3)) and water gas shift (Eq. (4)). C þ H2 O→H2 þ CO
ð3Þ
CO þ H2 O→H2 þ CO2 :
ð4Þ
Moreover, while for air and carbon dioxide activations no emission of CH4 was found, this gas is released for water steam runs; with an increasing concentration up to a value of 0.92 mol L−1 after 100 min, approximately (Fig. 8d). Among the different equilibria participating in the production of this gas, methanation reaction (Eq. (5)) can be suggested. CO þ 3H2 →CH4 þ H2 O:
ð5Þ
4. Conclusions In this work, the thermal regeneration of activated carbons exhausted with p-Nitrophenol following their pyrolysis has been investigated, comparing the effect of air, carbon dioxide and water steam. The analysis of the experimental results allowed obtaining the following conclusions: – Simple inert thermal regeneration at 700 °C is not enough to restore the adsorbent initial porosity; a regeneration efficiency of 68%, as defined from N2 adsorption data at 77, was consistent with the permanence in the carbon matrix of a fraction of the products resulting from PNP adsorption. – The additional of an oxidation step was in all cases positive, improving the porosity regain, although the effect was very dependent on the type of activation process.
– Air activation provided a slight improvement of the carbon porosity (75%), so that it was estimated that the use of this agent would only be interesting for modest applications. The use of CO2 and especially water steam were very interesting, achieving in the latter case a regeneration efficiency of 94%, regarding N2 adsorption, and a total recuperation of the porosity available to PNP in aqueous media. Moreover, none of the treatments caused a significant modification on the pore size distribution. – The gas monitorization during activation allowed the identification of H2, CO, CH4 and CO2, which were related to the breaking up of chemisorbed products, surface functional groups, or other compounds that may have been created in the carbon surface as a result of thermal treatment. In general, steam activation processes stood out due to their greater production of hydrogen and methane. 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