Adsorption of p-nitrophenol on an activated carbon with different oxidations

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1 Carbon 41 (003) Adsorption of p-nitrophenol on an activated carbon with different oxidations a a b b a, S. Haydar, M.A. Ferro-Garcıa, J. Rivera-Utrilla, J.P. Joly * Laboratoire d Application de la Chimie a` l Environnement, UMR 5634 CNRS-Universite Claude Bernard Lyon 1, 43 Bd. du Onze Novembre 1918, 696 Villeurbanne cedex, France b Departamento de Quımica Inorganica, Facultad de Ciencias, Universidad de Granada, Granada, Spain Received 31 May 00; accepted August 00 Abstract An activated carbon prepared from olive stones has been modified through oxidation by nitric acid or sodium hypochlorite. These treatments introduced large amounts of oxygen groups, which were characterized by mass-spectrometry, temperature-programmed desorption (DTP MS). Both CO - and CO-evolving groups were created by these oxidation treatments. A part of these oxidized samples was then outgassed under vacuum up to 83 K in order to remove most of the CO -evolving groups from their surface. Oxidized samples have a smaller surface area than the original sample. The subsequent partial outgassing increases the surface area which, however, does not reach the value it had before oxidation. p-nitrophenol (PNP) adsorption isotherms from aqueous solutions were determined at 98 K for the original, oxidized, and partly outgassed samples. The results confirm the presence of an intermediate plateau at low equilibrium PNP concentration (at about 10 mg/ l). The relative effects of textural versus surface chemistry on PNP uptakes are then discussed. The presence of CO-evolving groups showed no influence on PNP uptakes. The conclusion is that models in which carbonylic groups are basic adsorption sites for substituted phenols can be ruled out for the entire isotherm of PNP obtained with the original carbon. These models are also unlikely for PNP adsorption on oxidized and partly outgassed samples. 00 Elsevier Science Ltd. All rights reserved. Keywords: A. Activated carbon; B. Oxidation; C. Adsorption 1. Introduction to the adsorbate, so that a relationship between surface area and adsorption capacity is generally not observed. Con- The adsorption of phenol and substituted phenols from versely, some narrow pores, if accessible, may favor a aqueous solution on activated carbons has been intensively strong physical interaction between the adsorbate moleinvestigated for decades. However, this subject remains cules and the surface of carbon. The influence of textural highly controversial as described in a recent review by properties of carbons in substituted phenols adsorption, Radovic et al. [1]. There is a general agreement that and in particular the favorable role of small micropores, textural characteristics and surface chemistry of activated has been recognized by numerous authors up to recently carbons play a fundamental role in substituted phenol [ 9]. adsorption but discrepancies appear when an in-depth Among many other subjects discussed in the literature, description of this phenomenon is attempted. the nature of the adsorption sites for substituted phenols is A general concept in adsorption is that the larger the a major source of discrepancies as largely reported in Ref. specific surface area of the solid, the larger is the maxi- [1]. Many authors consider electron-rich regions located in mum adsorption capacity. Nevertheless, with microporous graphene layers interacting with the p electrons of the activated carbons, a part of the surface can be inaccessible aromatic ring of phenols (p p argument [1]), while numerous other authors arrived at or supported the conclusion *Corresponding author. Tel.: ; fax: 133- that some basic carbonyl surface groups form a bond with the aromatic ring of phenol (donor acceptor argu- address: jean-pierre.joly@univ-lyon1.fr (J.P. Joly). ment [1]). These two assumptions developed independent /0/$ see front matter 00 Elsevier Science Ltd. All rights reserved. PII: S (0)

2 388 S. Haydar et al. / Carbon 41 (003) ly since they were, respectively, introduced by Coughlin rite ions in excess in the solution were destroyed by and Ezra [10] on the one hand and by Mattson et al. [11] 3 chlorhydric acid (0.1 M; 70 cm ). The carbon was then on the other hand. Apart from these hypotheses, superficial washed, separated by filtration and dried by the same carboxylic groups, which are able to form a hydrogen bond procedures as described above. with the OH function of phenols, are other possible A part of the carbons AC-N and AC-Cl underwent an adsorption sites [1]. additional treatment in order to remove a part of the Indeed, the experimental discrimination between these initially present oxygen groups. This treatment consisted of adsorption models is not easy because the introduction of heating the carbon under vacuum up to 83 K by TPD. oxygen groups in an activated carbon may concomitantly The resulting materials are termed AC-N-D and AC-Cl-D, modify most of its textural and chemical properties. For respectively. instance, a classical treatment of carbon with nitric acid not Temperature programmed desorption experiments folonly introduces both acidic (i.e. carboxylic) and basic (i.e. lowed by a mass spectrometer (TPD MS) were performed carbonyl) oxygen groups at the surface of carbon, but also 6 3 under vacuum (10 10 mmhg or Pa) at a may modify the nature of preexisting oxygen groups [11]. heating rate of 5 K/ min. The apparatus has been described In addition, even if carried out under mild conditions to elsewhere [17]. CO and CO desorption fluxes were avoid a destruction of the porous structure of carbon, followed by monitoring the heights of the mass peaks carboxylic groups created by nitric treatment may block m/e 5 44 and m/e 5 8 amu, respectively. Ionization micropore entrances and consequently prevent the access voltage was fixed to 5 ev instead of the conventional 70 of phenol molecules to adsorption sites [1 14]. ev in order to minimize CO fractionation within the MS This paper presents a study of p-nitrophenol (PNP) source and, hence, make negligible the contribution of CO adsorption from aqueous solutions on a variously oxidized to the mass peak m/e 5 8 amu. activated carbon. The aim of this study was to get BET specific surface areas of carbons were determined additional information on the influence of oxygen species by nitrogen adsorption at 77 K. Micropore volumes V p on the adsorption of substituted phenols. were obtained by Dubinin Radushkevich transforms of isotherms. Pore size distributions were determined using a mercury porosimeter (maximum pressure 400 kg/ cm ). Experimental allowing the mercury penetration in pores with a size larger than 3.7 nm. This technique provides the external An activated carbon, denoted AC, was prepared from specific surface area Sext that corresponds to pores with a olive stones as described below. The raw vegetal material size larger than 3.7 nm, the volume V1 of pores with a size was washed in sulfuric acid to eliminate most inorganic ranging from 3.7 to 50 nm and the volume V of pores elements and then washed with distilled water up to sulfate larger than 50 nm (macropores). elimination. The resulting material was carbonized at 173 The adsorption of p-nitrophenol (PNP) was carried out K for 1 h under a nitrogen flow. The activation was then by contacting 100 mg of carbon and 100 ml of variously carried out in a stream of carbon dioxide at 1113 K for 16 concentrated aqueous solutions of PNP in tightly closed h. Carbon AC was then used in the form of grains, the size glass flasks. Carbon samples were weighed after they were of which were in the range mm. kept at 383 K under air for one night. Flasks were shaken In order to introduce surface oxygen groups, carbon AC at 98 K in a thermostated bath for 1 week. PNP solutions was separated into two parts. The first part was oxidized were then separated by sedimentation and their concenwith nitric acid, the second one by sodium hypochlorite. trations were evaluated by UV absorption at l5317 nm. The resulting materials were respectively termed AC-N No buffer was used in PNP solutions to avoid a possible and AC-Cl. influence of foreign ions on the adsorption measurements. 3 Nitric oxidation was carried out as follows: 80 cm of a The free ph of the solutions during adsorption was in the concentrated HNO3 solution (15 M) was slowly poured on range g of AC located in a vessel provided with a magnetic stirrer and cooled in an ice bath. Carbon AC was then left in contact with nitric acid at ambient temperature for 4 h. 3. Results The vessel contents were diluted with water and filtered. The carbon was washed with distilled water until neutral Table 1 shows the elemental analysis of the samples of ph and separated by filtration. It was finally dried under activated carbon. The oxygen content was not obtained by vacuum at 33 K for one night. difference but by an analytical method. This method Hypochlorite oxidation treatment has been proposed in consisted of a complete pyrolysis of the sample at 1353 K early studies for carbon oxidation [15,16]. In this study, it under nitrogen. The products of this pyrolysis were was carried out by contacting 4 g of AC with a NaOCl transformed into CO by flowing through an active carbon solution (0 volumes) in conditions similar to those used bed at 1393 K. Carbon monoxide was finally evaluated by for nitric oxidation. At the end of this operation, hypochlo- means of a specific IR detector. The sum of the per-

3 S. Haydar et al. / Carbon 41 (003) Table 1 Oxidized carbons partly outgassed present intermediate Results of elemental analysis of carbons oxygen contents. C% H% N% O% Sum (%) Table gives textural data obtained from nitrogen isotherms (type I) on the one hand, and from mercury AC porosimetry on the other hand. The BET surface area of AC-N AC-N-D carbon AC decreases on oxidation by HNO3 or NaOCl. It AC-Cl is partly restored by the outgassing following the oxidation AC-Cl-D but it does not reach its original value. DR micropore volume Vp follows the same trends. The external surface area Sext is lower after each subsequent carbon treatment. centages is always around 98.5%. The missing 1.5% is The same trends are also followed by V 1, the volume of very likely due to the presence of impurities in carbons, pores the size of which is in the range nm. Volume which were not considered in this analysis. The nitrogen V, corresponding to pores larger than 50 nm, decreases content remains 0.3% except for the sample treated with also on subsequent treatments. This volume is strongly nitric acid that shows a higher content. This is not affected by the treatment with NaOCl. surprising because it has been noted that nitrate or nitro Fig. 1 shows the TPD profiles of CO from the various surface groups are present in small amounts on the surface samples studied. It is seen that CO desorbs between 350 after such a treatment [18]. The oxygen content of carbon and 1000 K. As expected, oxidation of carbon by HNO3 or AC as prepared is low, i.e. 0.6%, while carbons oxidized NaOCl dramatically increases the amount of CO-evolving with nitric acid or sodium hypochlorite present the highest oxygen groups. However, CO TPD profiles are not oxygen contents, i.e. 11.7% and 10.06%, respectively. proportional for samples AC-N and AC-Cl. This means Table Textural characteristics obtained from N adsorption at 77 K (columns and 3) and mercury porosity (columns 4 6) Surface area Vp V1 V Ext. surface (m /g) (cm /g) (cm /g) (cm /g) area (m /g) AC AC-N AC-N-D AC-Cl AC-Cl-D Fig. 1. CO TPD profiles; d, carbon AC; j, carbon AC-N; h, carbon AC-N-D; m, carbon AC-Cl; ^, carbon AC-Cl-D.

4 390 S. Haydar et al. / Carbon 41 (003) Fig.. CO TPD profiles; d, carbon AC; j, carbon AC-N; h, carbon AC-N-D; m, carbon AC-Cl; ^, carbon AC-Cl-D. that the oxygen entities, or their populations, are not D) the ratios CO/ CO are strongly increased. The amounts identical on both samples. Outgassing of samples AC-N of atomic oxygen evaluated from the TPD profiles of CO and AC-Cl up to 83 K strongly reduces the amounts of and CO are always smaller than those found by elemental CO-evolving groups. analysis. This is mainly due to the contribution of adsorbed Fig. shows the TPD profiles of CO from the various water to elemental analysis results. samples studied. It is seen that CO-evolving oxygen Fig. 3 shows the PNP adsorption isotherms obtained groups are not completely decomposed at the final tem- with carbon AC as prepared, oxidized with nitric acid perature of the TPD runs, i.e. at 113 K. The amount of (sample AC-N), and partly outgassed after this oxidation these groups also strongly increases during the oxidation of (sample AC-N-D). It is seen that, for concentrations carbon AC by HNO3 or NaOCl. The partial outgassing of ranging from 1 to 140 mg/l, the isotherm for the carbon samples AC-N and AC-Cl up to 83 K not only decreased AC-N is clearly located under that for carbon AC. The the amount of CO desorbed up to this temperature, as isotherm for carbon AC-N-D is located in between. As expected, but also increased the amount of CO desorbed in shown in Fig. 4, exactly the same behavior is observed the range K for carbon AC-N and in the range when carbon AC is oxidized with sodium hypochlorite K for carbon AC-Cl. This means that, apart from the decomposition of CO-evolving entities at lower temperatures during the partial outgassing, there are some 4. Discussion transformations among these entities. Such transformations have been observed previously [19] Intermediate plateau Table 3 provides quantitative results obtained by integration of the TPD profiles shown in Figs. 3 and 4. The shape of isotherms shown in Figs. 5 and 6 suggests Oxidized carbons AC-N and AC-Cl desorbed more CO the existence of an intermediate plateau at PNP conthan CO. After outgassing (samples AC-N-D and AC-Cl- centrations close to 10 mg/ l, at least for oxidized samples Table 3 Comparison of the amounts of carbon oxides and of elemental oxygen calculated from TPD and from elemental analysis Sample TPD Elem. analysis CO (mmol/ g) CO (mmol/ g) CO/ CO O (mmol/ g) O (mmol/ g) AC AC-N AC-N-D AC-Cl AC-Cl-D

5 S. Haydar et al. / Carbon 41 (003) Fig. 3. PNP isotherms; d, carbon AC; j, carbon AC-N; h, carbon AC-N-D. Fig. 4. PNP isotherms; d, carbon AC; m, carbon AC-Cl; ^, carbon AC-Cl-D. Fig. 5. PNP uptakes per unit surface of carbon; d, carbon AC; j, carbon AC-N; h, carbon AC-N-D; m, carbon AC-Cl; ^, carbon AC-Cl-D.

6 39 S. Haydar et al. / Carbon 41 (003) Fig. 6. PNP uptakes per unit surface of carbon in the lower equilibrium concentrations region; (a) d, carbon AC; j, carbon AC-N; h, carbon AC-N-D; (b) d, carbon AC; m, carbon AC-Cl; ^, carbon AC-Cl-D. AC-N and AC-Cl. This confirms the early finding of General interpretations for the existence of two plateaus Mattson et al. [11] who showed that PNP isotherms on an in solute isotherms have been proposed by Giles [1]: activated carbon indeed exhibit such a plateau. In a recent firstly, the formation a double layer, the saturation of each work by Finqueneisel et al. [0] about PNP adsorption on layer corresponding to a plateau, secondly, a different carbons from lignites of relatively low surface area (300 orientation, i.e. parallel or perpendicular to the surface, of 400 m / g), this plateau is also visible in some isotherms adsorbed molecules and, thirdly, the adsorption on differthough the authors did not mention it. The intermediate ent types of sites. Mattson et al. [11] have followed this plateau, the existence of which is more firmly established third interpretation, proposing that the lower plateau is due in the case of phenol [10,1 3], has been only rarely to the adsorption of PNP on carbonyl groups (according to mentioned in the case of PNP because it is less marked for the donor acceptor argument) while the higher plateau this compound than for phenol [11]. It is also seen in PNP results from an adsorption of PNP on graphene basal isotherms shown in Figs. 5 and 6 that the intermediate planes (in agreement with the p p argument). More plateau appears more clearly for oxidized samples than for recently, Nevskaia et al. [3] also assumed that the donor the original and the partly outgassed samples. This ob- acceptor complex mechanism is operating in the case of servation may be compared to that made by Nevskaia et al. phenol adsorption on HNO3-treated activated carbons. [3] in the adsorption isotherms of phenol which exhibit Besides, these authors intended to explain the presence of this plateau only for their sample oxidized with nitric acid. the two plateaus as follows: the lower concentration part of This confirms the qualitative similarity between the iso- the isotherm corresponds to a displacement of water from therms of PNP and those of phenol. carbon surfaces free of functional groups while the higher

7 concentration part results from a displacement of water bonded to oxygen functional groups. These different assumptions show the interest of studying the low concentration part of the isotherms. Despite numerous efforts already dedicated to understand the adsorption of substituted phenol on carbons, more work is needed to clarify the behavior of isotherms at these concentrations. S. Haydar et al. / Carbon 41 (003) by TPD, have an electron withdrawal effect less important than those created by HNO 3. Anyway, as in the case of nitric acid treatment, no favorable effect of CO-evolving groups is observed for carbon AC-Cl-D, either Lower part of isotherms Fig. 6a shows the PNP isotherms, expressed per unit surface, for samples AC, AC-N and AC-N-D, at equilib- 4.. Textural versus chemical effects rium concentrations lower than 10 mg/ l. Again, PNP uptakes support the p p argument, since they decrease Higher part of isotherms when CO-evolving groups are massively introduced in The role of the chemical properties of carbon surfaces carbon and then increase without reaching their original has been evidenced by numerous authors in the case of values when an important part of CO-evolving groups are phenol adsorption, which has been much more investigated removed by the partial outgassing. than that of PNP adsorption. In the present work, the Isotherms for carbon AC-Cl and AC-Cl-D are shown in qualitative parallelism between the amounts of PNP ad- Fig. 6b. Despite the dispersion of the experimental points, sorbed under a given solution concentration and the it is seen that the effect of oxygen groups on PNP uptakes surface area of carbon samples is striking. Such a parallel- is much less important than for carbons AC-N and AC-Nism is also evident when the microporous volume Vp of D. The possible reasons for this property are the same for samples is considered. In an attempt to assess the respec- low and high concentrations and they have been given tive role of surface chemistry and textural properties, we divided the amounts of PNP adsorbed by the BET surface areas. The resulting isotherms are presented in Fig. 5. It is seen that all isotherm points move toward a single curve except those for sample AC-N. The same behavior is observed when the amounts of adsorbed PNP are divided by the microporous volume V p, this volume being virtually proportional to the surface area. earlier. Besides, it is clear from Fig. 6 that no enhancement of the adsorption with the strong increase of CO-evolving groups for carbons AC-N-D and AC-Cl-D are observed in the lower concentration part of the isotherms. Therefore, our results do not support the donor acceptor argument for the lower part of the PNP isotherms, either Adsorption sites The decrease in adsorption capacity, expressed per surface unit of the sample after a humid oxidation treat- The donor acceptor argument implies the presence of a ment, has been often observed in the case of phenol sufficient amount of basic oxygen group for binding the [4,5]. The comparison in Fig. 5 of the results we aromatic cycle of substituted phenols. According to a obtained for the original carbon AC and the HNO3-treated recent review by Boehm [6], candidates for such basic carbon AC-N show that the adsorption of PNP follows the centers are carbonyl groups, in particular when these same trend. A partial outgassing of the HNO3-treated groups are engaged in a polycyclic pyrone-like structure. sample restores the initial capacity per unit surface. This In Fig. 3, it is seen that the maximum PNP adsorption behavior is in favor of the p p argument. Thus, the capacity at high concentration is roughly 500 mg/ g, i.e. treatment by HNO3 strongly increases the number of 3600 mmol/ g. This amount is about 40 times higher than CO -evolving groups that withdraw electrons from the that of CO-evolving groups reported in Table 3. This graphene layers, and consequently decreases the number of means that the donor acceptor model may be definitely adsorption sites for PNP. On the other hand, the partly ruled out for the entire isotherm for carbon AC. The p p outgassed sample AC-N-D has lost a large part of the argument seems then relevant to PNP adsorption on a CO-evolving groups but contains about 1 times more non-oxidized carbon. This conclusion is similar to that CO-evolving groups than the original carbon AC (see reached by Nevskaia et al. [3] in the case of phenol. Table 3). Nevertheless, these oxygen groups have virtually The intermediate plateaus observed in the frames inno effect on the PNP uptakes. This result disagrees with cluded in Figs. 5 and 6 correspond roughly to PNP uptakes the expected consequences of the donor acceptor argu- of 50 mg/ g of carbon, i.e mmol of PNP per gram. ment. Considering the donor acceptor argument, the same Results for samples AC-Cl and AC-Cl-D depicted in amount of basic oxygen groups would be required to Fig. 4 show virtually no effect of the oxygen groups on adsorb PNP molecules. This is not verified for carbon AC PNP uptakes. This is in part understandable, in the frame as stressed before. There is also a lack of CO-evolving of the p p argument, because the oxidation by NaOCl groups for carbons AC-N-D and AC-Cl-D. The amount of created less CO-evolving groups than the oxidation by CO-evolving groups for carbons AC-N and AC-Cl-D HNO 3 (see Table 3). It is also possible that the oxygen indeed reach 1867 and 1553 mmol/g. Thus, in these cases, groups created by the oxidation by NaOCl, which are it is not possible to rule out the role of carbonyl groups as different to those created by the nitric treatment as shown adsorption sites for PNP on the single basis of the possible

8 394 S. Haydar et al. / Carbon 41 (003) amount of CO-evolving groups. However, it is likely that being observed for oxidized carbons, the donor acceptor these last figures remain insufficient because carbonyl argument may be rejected in this case too. Consequently, functional groups are only a part of CO-evolving groups our findings are in favor of an adsorption mechanism among esters, phenols and anhydride species for instance involving an interaction between some basic groups lo- [7]. Anyway, the qualitative arguments developed in cated in graphene layers and the aromatic ring of PNP, in Section 4. showed that it is unlikely that carbonyl groups agreement with the early proposal of Coughlin et al. [10]: are the adsorption sites for PNP. the negative role of superficial CO-yielding groups on The ph of the solutions was left free during adsorption PNP adsorption is attributed to the withdrawal of electrons experiments and therefore these solutions were slightly from these adsorption sites by these groups. acidic (ph 5 6). Consequently, PNP that has a pka equal to 7.13 was virtually not ionized in the solutions used. Then, it is expected that an electrostatic interaction does References not play a significant role in PNP adsorption under our experimental conditions. At this point, the present discus- [1] Radovic LR, Moreno-Castilla C, Rivera-Utrilla J. Carbon sion allows us to conclude that, definitely ruled out for materials as adsorbents in aqueous solutions. In: Radovic carbon AC because of the evident lack of adsorption sites, LR, editor, Chemistry and physics of carbon, vol. 7, New the donor acceptor argument is quite unlikely for oxidized York: Marcel Dekker, 000, pp and partly outgassed carbons for the whole PNP isotherms. [] Giles CH, D Silva AP, Trivedi AS. Use of p-nitrophenol for Franz et al. [1] recently arrived at a similar conclusion specific surface measurement of granular solids and fibers. J for the adsorption of aqueous and organic solutions of Appl Chem 1970;0: [3] Caturla F, Martın-Martınez JM, Molina-Sabio M, Rodphenol, nitrobenzene, aniline and benzoic acid. The present riguez-reinoso F, Torregrosa R. Adsorption of substituted study deals with PNP possessing the strong deactivating phenols on activated carbon. J Colloid Interf Sci group NO attached to the aromatic ring. A priori, one 1988;14(): could have thought that the presence of this group is in [4] Paprowicz JT. Activated carbons for phenols removal from favor of the formation of a donor acceptor bond formation wastewater. Environ Technol 1990;11:71 8. between surface carboxylic groups and the electron-poor [5] Rivera-Utrilla J, Utrera-Hidalgo E, Ferro-Garcia MA, aromatic ring of PNP. Our results show that, even in this Moreno-Castilla C. Comparison of activated carbons pre- case, the donor acceptor mechanism is not operating. pared from agricultural raw materials and Spanish lignites There are other important differences between the study of when removing chlorophenols from aqueous solutions. Car- Franz et al. [1] and the present study. Thus, in the work bon 1991;9(4/ 5): [6] Moreno-Castilla C, Rivera-Utrilla J, Lopez-Ramon MV, described by these authors, carbon samples were oxidized Carrasco-Marın F. Adsorption of some substituted phenols with oxygen while they were oxidized with HNO3 or on activated carbons from a bituminous coal. Carbon NaOCl in the present work. In addition, oxidized carbons 1995;33(6): were only partly outgassed in the present study while the [7] Daifullah AAM, Girgis BS. Removal of some substituted original carbon was virtually completely outgassed in the phenols by activated carbon obtained from agricultural work of Franz et al. Finally, the concentration range we waste. Water Res 1998;3(4): used goes to lower concentrations, allowing the apparition [8] Cases JM, Villieras F, Michot L. Les phenomenes ` d adsorp- of the intermediate plateau in PNP isotherms. Thus, results tion, d echange ou de retention a` l interface solide-solution presented here complete those found by Franz et al. aqueuse. 1 Connaissance des proprietes structurales, tex- Our results also show that the presence of carboxylic turales et superficielles des solides. CR Acad Sci Paris, Sciences de la terre et des planetes ` 000;331: groups, which thermally decompose into CO, inhibits [9] Hu Z, Srinivasan MP. Mesoporous high-surface-area acti- PNP adsorption. This observation shows that H-bonding of vated carbon. Micropor Mesopor Mater 001;43: PNP molecules to these groups does not play any pre- [10] Coughlin RW, Ezra FS. Role of surface acidity in the dominant role in the aqueous adsorption of this adsorbate. adsorption of organic pollutants on the surface of carbon. This is likely due to the strong competition of water versus Environ Sci Technol 1968;(4):91 7. PNP molecules to form such a bond [1]. [11] Mattson JS, Mark Jr. HB, Malbin MD, Weber Jr. WJ, Crittenden JC. Surface chemistry of active carbon: specific adsorption of phenols. J Colloid Interf Sci 1969;31(1): Conclusion 30. [1] Franz M, Arafat HA, Pinto NG. Effect of chemical heterogeneity on the adsorption mechanism of dissolved aromatics The determination of p-nitrophenol adsorption isotherms on activated carbon. Carbon 000;38: from aqueous solutions on a carbon variously oxidized led [13] Dubinin MM. Porous structure and adsorption properties of us to definitely rule out the donor acceptor argument for activated carbons. In: Walker Jr. PL, editor, Chemistry and carbon AC because of the evident lack of carbonyl physics of carbon, vol., New York: Marcel Dekker, 1966, adsorption sites. No relationship between the amounts of pp CO-evolving groups and the PNP adsorption capacity [14] Biniak S, Kazmierczak J, Swiatkowski A. Adsorption of

9 S. Haydar et al. / Carbon 41 (003) phenol from aqueous solutions on activated carbons with adsorption. Part XI. A system of classification of solution different oxygen contents. Adsorp Sci Technol adsorption isotherms, and its use in diagnosis of adsorption 1990;6(4): mechanisms and in measurements of specific surface areas of [15] Donnet JB, Hueber F, Reitzer C, Oddoux J, Riess G. Etude solids. J Chem Soc 1960;10: de l action chimique des oxydants sur le noir de carbone. [] Giles CH, Smith D. A general treatment and classification of Bull Soc Chim France 196;94: the solute adsorption isotherm/ I theoretical. J Colloid Interf [16] Richard D, Gallezot P. Preparation of highly dispersed, Sci 1974;47(3): carbon supported, platinum catalysts. In: Delmon B, Grange [3] Nevskaia DM, Santianes A, Munoz V, Guerrero-Ruız A. P, Jacobs PA, Poncelet G, editors, Preparation of catalysts, Interaction of aqueous solutions of phenol with commercial vol. IV, Amsterdam: Elsevier Science, 1987, pp activated carbons: an adsorption and kinetic study. Carbon [17] Joly JP, Perrard A. Determination of the heat of adsorption of 1999;37: ammonia on zeolites from temperature-programmed desorp- [4] Mahajan OP, Moreno-Castilla C, Walker Jr. PL. Surfacetion experiments. Langmuir 001;17: treated activated carbon for removal of phenol from water. [18] Moreno-Castilla C, Ferro-Garcıa MA, Joly JP, Bautista- Sep Sci Technol 1980;15(10): Toledo I, Carrasco-Marın F, Rivera-Utrilla J. Activated [5] Mota JPB, Silva IF, Dias SMP, Ramos SJB, Jamnadas JS. A carbon surface modifications by nitric acid, hydrogen perox- theoretical and experimental study of the adsorption of ide, and ammonium peroxydisulfate treatments. Langmuir organic compounds on chemically modified activated car- 1995;11: bons. In: Meunier F, editor, Fundamentals of adsorption, vol. [19] Carrasco-Marın F, Rivera-Utrilla J, Joly JP, Moreno-Castilla 6, Amsterdam: Elsevier, 1998, pp C. Effects of ageing on the oxygen surface complexes of an [6] Boehm HP. Surface oxides on carbon and their analysis: a oxidized activated carbon. J Chem Soc Faraday Trans critical assessment. Carbon 00;40: ;9(15): [7] Dandekar A, Baker RTK, Vannice MA. Characterisation of [0] Finqueneisel G, Zimmy T, Vogt D, Weber JV. Feasibility of activated carbon, graphitized carbon fibers and synthetic the preparation of effective cheap adsorbents from lignites in diamond powder using TPD and DRIFTS. Carbon rotary kiln. Fuel Process Technol 1998;57: ;36(1): [1] Giles CH, MacEwan TH, Nakhwa SN, Smith D. Studies in