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active carbon catalyst
Applied Catalysis B: Environmental 65 (2006) 261–268 www.elsevier.com/locate/apcatb
Catalytic wet peroxide oxidation of phenol with a Fe/active carbon catalyst J.A. Zazo *, J.A. Casas, A.F. Mohedano, J.J. Rodrı´guez Ingenierı´a Quı´mica, Universidad Auto´noma de Madrid, Ctra. de Colmenar km 15, 28049 Madrid, Spain Received 19 October 2005; received in revised form 1 February 2006; accepted 9 February 2006 Available online 20 March 2006
Abstract A Fe on activated carbon catalyst has been prepared and tested for phenol oxidation with H2O2 in aqueous solution at low concentration (100 mg/L). Working at 50 8C, initial pH 3 and a dose of H2O2 corresponding to the stoichiometric amount (500 mg/L) complete removal of phenol and a high TOC reduction (around 85%) has been reached. Oxidation of phenol gives rise to highly toxic aromatic intermediates which finally disappear completely evolving to short-chain organic acids. Some of these last showed to be fairly resistant to oxidation being responsible for the residual TOC. In long-term continuous experiments the catalyst undergoes a significant loss of activity in a relatively short term (20–25 h) due to Fe leaching, this being related with the amount of oxalic acid produced. Deactivation may also be caused by active sites blockage due to polymeric deposits on whose formation some evidences were found. Washing with 1N NaOH solution allows to recover the activity although complete restoration was not achieved. # 2006 Elsevier B.V. All rights reserved. Keywords: CWPO; Hydrogen peroxide; Phenol oxidation; Active carbon iron supported catalyst; Fe/AC
1. Introduction Effective cleaning of industrial wastewaters has become an increasingly concerning problem in the last decades. Modern legislation in many countries imposes environmental regulations and health quality standards that steadily become more restrictive. These effluents frequently contain pollutants which are toxic and resistant to conventional wastewater treatments. Therefore, development of efficient technologies is a need and considerable research efforts are being devoted in this field. Phenol and phenolic compounds are starting and/or intermediate compounds in petrochemical, chemical and pharmaceutical industries and also are formed in the oxidation pathway of higher-molecular-weight aromatic hydrocarbons [1]. Thus, phenol is usually taken as a model compound for advanced wastewater treatment studies. Wet air oxidation (WAO) and catalytic wet air oxidation processes (CWAO) using air or pure oxygen as oxidant have been used to treat industrial wastewaters containing phenols. However, high pressures (20–200 bar) and temperatures (200–
* Corresponding author. Tel.: +34 91 497 8774; fax: +34 91 497 3516. E-mail address: [email protected] (J.A. Zazo). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.02.008
320 8C) are needed thus increasing the treatment cost [2]. An alternative in order to reduce the severity of these reaction conditions is the use of hydrogen peroxide in the so-called catalytic wet peroxide oxidation (CWPO) [3]. Among the different processes using hydrogen peroxide, Fenton’s reagent is one of the most interesting options. In this process, H2O2 decomposes catalytically in the presence of Fe(II) giving rise to hydroxyl radicals with a high oxidation capacity. The application of Fenton’s process in wastewater treatment is attractive, in principle, due to the fact that iron is a widely available and non-toxic element and hydrogen peroxide is easy to handle and the excess decomposes to environmentally safe products [4]. Some advantages of Fenton’s process with regard to other oxidation techniques are that it requires a relatively simple installation and operates under mild conditions (atmospheric pressure and room temperature). Consequently it has been postulated as the most economic alternative by several authors [5,6]. However, this treatment shows some inconveniences derived from the high hydrogen peroxide consumption and the need of removing iron after the treatment, which requires additional separation steps increasing the treatment cost [7]. These drawbacks can be overcome by the use of heterogeneous catalysts. In this case, the process is usually
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Nomenclature dp Ph SBET T TOC W X
particle diameter phenol BET surface area (m2/g) temperature (8C) total organic carbon catalyst concentration (g/L) conversion
Subscript 0 relative to initial conditions
called heterogeneous Fenton. Transition metals, mainly iron, are used as active phase. Alumina [8], silica [9], mesoporous molecular sieves [10,11], zeolites [12–14], pillared clays [7,15– 17] and ion-exchange resin [18] have been used as supports to prepare the catalysts. The best results reported in the bibliography have been obtained with zeolites and pillared clays. The main problem associated to these catalysts comes from the leaching of the active phase when oxidation is carried out at pH near 3, the optimum value in the case of the homogeneous Fenton process. With zeolites a substantial leaching of iron was observed at pH values below 5 and the efficiency increased when pH decreased, whereas at neutral pH no significant activity was observed, which may suggest significant contribution of homogeneous catalysis at low pH. Pillared clays are more stable than zeolites at low pH, although the leaching of iron increases when pH decreases. However, the activity of the catalyst is less sensible to pH, allowing high TOC reduction even at a pH value of 5 [17]. The aim of this work is to study the catalytic oxidation of phenol with hydrogen peroxide using a home-made active carbon supported iron catalyst. Although carbon materials have been widely used as catalysts and as catalytic supports [19] there are no works in the literature dealing with active carbon supported catalysts for CWPO. 2. Experimental 2.1. Catalyst preparation and characterization The Fe/AC catalyst was prepared by incipient wetness impregnation of an activated carbon supplied by Merck (Cod. 102514100; dp: 1.5 mm) with an aqueous solution of iron nitrate. The volume of solution was 0.83 mL/g catalyst, which represents a 10% excess with respect to the pore volume of the active carbon. The Fe concentration was adjusted to obtain a 4% Fe (w/w) on the catalyst. After impregnation, the solid was left 2 h at room temperature, dried during 12 h at 60 8C and calcinated at 200 8C for 4 h. To characterize the porous structure of both, the active carbon and the catalyst, N2 adsorption–desorption isotherms at 77 K were obtained using a Quantachrome Autosorb-1 instrument. The samples were outgassed at 548 K for 12 h to
Table 1 Porous structure of the active carbon (AC) and the Fe/AC catalyst
2
SBET (m /g) Pore volume (cm3/g) Micropore Mesopore Macropore
AC
Fe/AC
974 0.75 0.34 0.19 0.22
781 0.67 0.27 0.16 0.24
a residual pressure < 10 5 Torr. Mercury porosimetry measurements were also accomplished using a Carlo Erba Porosimeter 4000 which allows to cover the pore range above 3.7 nm diameter. The total iron content of the catalyst was determined by inductively coupled plasma mass spectrometry (ICP-MS) using an ICP-MS Elan 6000 Perkin-Elmer Sciex instrument. X-ray photoelectron spectroscopy (XPS) using a Physical Electronics 5700C Multitechnique System was employed for iron analysis on the surface of the catalyst. The deconvolution of the spectra was performed using Multipak V 5.0A software. Finally, scanning electron micrographs (SEM) of the fresh and used catalyst were obtained by means of a JEOL JSM-840 apparatus. Table 1 shows the characterization of the porous structure of the activated carbon (AC) and the Fe/AC catalyst. As can be seen impregnation leads to a decrease of surface area in good agreement with that of micropore volume. Analysis by ICP-MS confirmed the 4% Fe content of the catalyst. The main iron species on the surface corresponds to Fe2O3 according with the XPS spectra which showed a well defined peak at 710.9– 711.1 eV and its doublet separated 13.6 eV. 2.2. Oxidation procedure To study the activity of the catalyst batch experiments were carried out in 100 mL stoppered glass bottles shaken in a thermostatic bath at an equivalent stirring velocity around 200 rpm. A volume of 50 mL of a 100 mg/L phenol aqueous solution and a weight of catalyst of 25 mg were used in all the experiments. The dose of H2O2 was 500 mg/L which means the calculated stoichiometric amount of H2O2 for complete oxidation of phenol up to CO2 and H2O. The catalyst was used in powdered form (dp < 100 mm). Most of the experiments were carried out at 50 8C and initial pH 3 which is the optimum value for homogeneous Fenton oxidation [4] but other temperature (25 and 70 8C) and pH (5 and 7) values were also checked. Long-term activity tests were also carried out. Experiments were performed for this purpose in a 1 L continuous stirred tank reactor at a 5 mL/min flow rate using 5 g of catalyst, 50 8C temperature and initial pH 3. The starting phenol concentration and the H2O2 dose were maintained in the aforementioned values. Reaction samples were taken and immediately analyzed after filtration through fiber glass filters (Albet FV-C). Phenol and aromatic intermediates were identified and quantified by means of HPLC (Varian Pro-Start 240) with a diode array detector (330 PDA). A Microsorb C18 5 mm column (MV 100,
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15 cm long, 4.6 mm diameter) was used as stationary phase and 1 mL/min of 4 mM aqueous sulfuric solution as mobile phase. UV detector at 210 nm wavelength was used for phenol, catechol and hydroquinone and at 246 nm for p-benzoquinone. Short-chain organic acids were analyzed in a ion chromatograph with chemical suppression (Metrohm 790 IC) using a conductivity detector. A Metrosep A supp 5-250 column (25 cm long, 4 mm diameter) was used as stationary phase and 0.7 mL/min of an aqueous solution of 3.2 mM Na2CO3 and 1 mM NaHCO3 as mobile phase. TOC was measured in a OI TOC Analyzer (model 1010). The residual H2O2 and the iron concentration in the liquid phase were determined by colorimetric titration with a Simadzu UV/Vis spectrophotometer using the titanium sulfate method [20] and the ophenantroline method [21], respectively. 3. Results and discussion
263
Fig. 2. Evolution of TOC and H2O2 with the active carbon and the Fe/AC catalyst ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/L, Wcatalyst: 500 mg/L, 50 8C, pH0 3).
3.1. Catalyst activity A previous set of experiments were performed to compare the behaviour of the Fe/AC catalyst and the AC support and to discriminate the effect of adsorption and reaction. These experiments were carried out at 50 8C and initial pH 3 and the rest of the operating conditions as indicated in Section 2.2. The results obtained are shown in Fig. 1. As can be seen the experiments carried out with the nonimpregnated active carbon gave rise to fairly similar results in presence and in absence of H2O2. This suggests that oxidation of phenol is almost negligible and phenol removal takes place essentially by adsorption. On the opposite, in the case of Fe/AC catalyst substantially different results were obtained with and without H2O2. In the absence of this reactant the evolution of phenol is comparable to that observed with the active carbon, the small differences being explainable from the lower surface area of the Fe/AC catalyst. Thus, again, practically only
Fig. 1. Phenol removal upon treatment with the AC and the Fe/AC catalyst in the presence and absence of H2O2 ([phenol]0: 100 mg/L; [H2O2]0: 500 mg/L; Wcatalyst: 500 mg/L; 50 8C, pH0 3).
adsorption must be occurring. When H2O2 is added phenol removal is greatly enhanced and complete conversion is finally reached, allowing to conclude that phenol is being transformed through oxidation and the Fe/AC catalyst is active for this reaction. To have a better knowledge on the action of the catalyst, Fig. 2 shows the results obtained in terms of TOC as well as the evolution of the H2O2 concentration in the experiments carried out with the active carbon and the Fe/AC catalyst. A substantially higher TOC removal and H2O2 conversion is reached with the Fe/AC catalyst. The evolution of TOC is in agreement with that of phenol in the case of the active carbon whereas, in the case of the Fe/AC catalyst the TOC reduction is lower than the corresponding to phenol removal, confirming that this compound is being transformed upon oxidation to intermediate products which evolve towards CO2 and H2O as the reaction proceeds. Nevertheless, complete mineralization is not attained and a residual TOC value is remaining even after 4 h reaction when phenol has been completely converted. Looking at the H2O2 concentration profile it can be seen that, although in less amount with respect to the Fe/AC catalyst, the active carbon also decomposes H2O2. The almost practical absence of reaction in the case of AC suggest that the decomposition of H2O2 proceeds through a different pathway that with the Fe/AC catalyst. This second promotes the formation of highly oxidant OH radicals whereas with the active carbon alone, decomposition of H2O2 proceeds majorly through O2 formation, which under the mild operating conditions of the process does not oxidize phenol. In fact, the concentration of the primary oxidation intermediates, such as catechol, hydroquinone and p-benzoquinone detected were always very low when using AC (0.52, 0.06 and 0.12 mg/L, respectively, as the highest analyzed values). To learn more on the potential feasibility of this Fe/AC catalyst in H2O2 oxidation of phenolic wastewaters it is necessary to know the evolution of the oxidation intermediates, because some of them (the aromatics, especially p-benzoquinone) are much more toxic than phenol and thus their presence
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Fig. 3. Evolution of aromatic intermediates in the oxidation of phenol with H2O2 using the Fe/AC catalyst ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/L, Wcatalyst: 500 mg/L, 50 8C, pH0 3).
even at low concentration would mean high values of ecotoxicity [22]. Figs. 3 and 4 show the distribution curves of the identified oxidation intermediates. The first one refers to aromatic species and the second to short-chain acids. The TOC values calculated from the sum of all the identified intermediates were always at the least in the vicinity of 90% of the measured TOC values throughout the 4 h reaction time investigated. The concentration versus time curves for phenol (Fig. 1) and the oxidation products are characteristic of a series reaction pathway where the primary products correspond to aromatics resulting from phenol hydroxylation which then evolve to short-chain organic acids and finally a high degree of mineralization, namely complete oxidation to CO2 and H2O, is reached. Hydroxylation takes place predominantly in the ortho position since catechol is the most abundant aromatic intermediate. The concentration of p-benzoquinone is higher
than that of hydroquinone as a result of the effect of active carbon on the redox equilibrium. Maleic and its isomer fumaric acid are the primary products from ring opening and oxidation of the aromatic intermediates. These evolve to acetic, oxalic and formic acids which show to be more resistant to oxidation under these operating conditions and are responsible for the residual TOC. Nevertheless, due to the total depletion of phenol and aromatic intermediates the final values of ecotoxicity were substantially lower than the starting ones but it is important to notice that the reaction time has to be the necessary to reach sufficiently low concentrations of aromatic intermediates, specially p-benzoquinone whose ecotoxicity is about three orders of magnitude higher than that of phenol itself [22]. The concentration values of phenol and the oxidation intermediates in Figs. 1, 3 and 4 suggest that the aromatic intermediates probably remain in a great part adsorbed on the catalyst surface where they undergo further oxidation to organic acids. An important question with regard to the application of this catalyst in practice is the possible leaching of iron under the operating conditions. In the experiments described up to this point Fe leaching was always very low. After 2 h of reaction the concentration of Fe in solution was around 1 mg/L and never raised above 2.5 mg/L after 4 h. In these conditions some, although relatively low, contribution of homogeneous phase reaction may take place. To check this, two experiments were carried out at the same operating conditions but using 1 and 3 mg/L of dissolved Fe instead of the Fe/AC catalyst. The TOC reduction after 4 h was 21 and 25%, respectively, much lower than the 84% value obtained with the Fe/AC catalyst. Furthermore, during the first hour of reaction in presence of Fe/AC catalyst, phenol and aromatic intermediates were practically depleted whereas the amount of Fe leached was practically negligible. Even when using a quantity of dissolved Fe equal to the total weight of this metal in the catalyst, the TOC reduction obtained through homogeneous phase oxidation was around one-half that reached in the heterogeneous process. 3.2. Effect of temperature
Fig. 4. Evolution of short-chain organic acids in the oxidation of phenol with H2O2 using the Fe/AC catalyst ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/L, Wcatalyst: 500 mg/L, 50 8C, pH0 3).
Table 2 compares the results obtained at 25, 50 and 70 8C under the same operating conditions already described. As can be seen the rate of disappearance of phenol as well as that of TOC abatement increases at increasing temperature. At 25 8C, a 75% TOC reduction was reached although depletion of phenol was not complete. Moreover, the residual concentration of p-benzoquinone means a substantially higher ecotoxicity than that of the starting phenol solution. Increasing the temperature up to 50 8C produces a remarkable increase on the rate of the oxidation process. Phenol and the aromatic intermediates are completely removed and only organic acids, mainly formic and oxalic, remained in the liquid after 4 h. Thus, ecotoxicity was drastically reduced. The loss of Fe from the catalyst by leaching is somewhat higher although it does not represent more than 12% of the initial iron weight after 4 h. Working at a higher temperature allows to reach high phenol and TOC removal in a significantly shorter
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Table 2 Results obtained at different temperatures ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/L, Wcatalyst: 500 mg/L, pH0 3) Temperature (8C)
Time (h)
Xphenol
XTOC
XH2 O2
Saromaticsa (mg/L)
Sacids (mg/L)
Feleached (mg/L)
25
0.5 1 2 4
0.49 0.61 0.84 0.93
0.46 0.50 0.61 0.75
0.18 0.26 0.35 0.45
0.78 3.01 5.08 2.88
3.8 7.0 10.4 11.2
0.1 0.1 0.2 0.3
50
0.5 1 2 4
0.84 0.94 0.98 1
0.58 0.72 0.78 0.84
0.44 0.59 0.72 0.86
6.59 0.83 0.26 0.00
14.3 16.4 18.5 21.8
0.4 0.7 1.1 2.4
70
0.5 1 2 4
0.95 0.99 1 1
0.81 0.86 0.87 0.88
0.79 0.96 0.99 1
5.01 0.42 0.00 0.00
23.8 27.4 33.7 31.2
0.7 2.4 6.5 7.8
a
Other than phenol.
reaction time but some inconveniences can be also observed. A complete removal of aromatics is reached at 70 8C in about half the reaction time needed at 50 8C but at that time the loss of Fe from the catalyst be leaching is almost three times higher at the former temperature. A high reduction of TOC is observed in a short term but then the rate of mineralization slows down due to the lower oxidation rate of organic acids and finally at 4 h reaction time fairly similar values of TOC removal are obtained. It is noticeable that the amount of organic acids is significantly higher as the working temperature increases. The relative distribution of these acids changes with temperature. At the end of the experiments, oxalic acid, which showed to be the more resistant to oxidation represents almost 80% of the total amount of organic acids at 70 8C whereas at 50 8C is less than 40%. This species has been related with Fe leaching as will be discussed later. From these results a temperature around 50 8C can be recognized as the optimal one for this process.
and an initial pH of 3 is clearly the advisable operating value in spite of the fact that a higher Fe leaching is observed in these conditions. This trend is similar to that observed for the homogeneous Fenton oxidation where a pH around 3 is the optimum one. The results for phenol and TOC removal at initial pH values of 5 and 7 were similar to those obtained by adsorption in absence of H2O2. This indicates that at these pH the activity of the Fe/AC catalyst is almost negligible, as well as OH production, despite the fact that higher H2O2 conversion values were obtained. Therefore, at pH 5 and 7, the decomposition of H2O2 leads predominately to O2 not capable to oxidize phenol under the mild operating conditions of these experiments. A similar trend was observed by Fajerwerg et al. [14] and by Gou and Al-Dahhan [16] using Fe-ZSM-5 and pillared clays as catalyst, respectively.
3.3. Effect of pH
To learn on the stability of the catalyst, long-term experiments were carried out in a 1 L continuous stirred tank reactor at 50 8C. A 100 mg/L phenol and 500 mg/L H2O2
Table 3 summarizes the results obtained at 50 8C and two initial pH values different than 3. Comparing these results with those of Table 2 at 50 8C (pH0 3) it can be seen that the efficiency of the oxidation process is remarkably affected by pH
3.4. Catalyst deactivation
Table 3 Results obtained at different initial pH ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/ L, Wcatalyst: 500 mg/L, 50 8C) pH
Time (h)
Xphenol
XTOC
XH2 O2
Saromaticsa (mg/L)
Sacids (mg(L)
Feleached (mg/L)
5
0.5 1 2 4
0.40 0.42 0.45 0.54
0.37 0.39 0.39 0.40
0.55 0.72 0.91 0.99
0.83 1.38 2.08 2.56
0.0 0.31 0.47 4.34
0.0 0.0 0.1 0.3
7
0.5 1 2 4
0.40 0.41 0.43 0.45
0.37 0.40 0.41 0.42
0.61 0.77 0.92 0.99
1.05 1.29 1.64 1.67
0.0 0.1 0.2 0.2
0.0 0.0 0.0 0.0
a
Other than phenol.
Fig. 5. Results of a long-term continuous experiment ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/L, Wcatalyst: 5 g/L, tr: 200 min, 50 8C, pH0 3).
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Fig. 6. Fe leached vs. oxalic acid concentration.
aqueous solution at pH 3 was fed at a 5 mL/min flow rate over 170 h. A weight of 5 g of catalyst was initially added to the reactor. The results are shown in Fig. 5 where, for the sake of comparison, the results obtained in the same conditions but in absence of H2O2 have been included. During the initial 20 h a phenol conversion higher than 90% and a TOC reduction of at least 80% were maintained. Beyond that time an increasing loss of activity was observed until the removal percentages set at about 65% for phenol and 25% for TOC. The H2O2 conversion value was always higher than 90%. One possible cause of deactivation is the loss of Fe from the catalyst by leaching which reached almost 50% of the initial weight at the end of the 170 h run. From a complete analysis of our results in all the discontinuous runs carried out we have found a relation between the Fe leaching and the concentration of oxalic acid in the liquid phase. The results of these observations are shown in Fig. 6. These results fit well a simple relationship given by the expression included in that figure. The action of oxalic acid could be the result of Fe complexation. In fact, the Fe to oxalate weight ratio in that expression is very close to the corresponding to a trivalent Fe–oxalate complex.
Although the presence of oxalic acid seems to play a significant role, future research is needed to better understand the causes of Fe lixiviation. It is important to study the way of achieving a stronger anchorage of Fe on the active carbon surface. In that sense oxidation treatments of the AC with some reagents, like HNO3 or (NH4)2S2O8, are well know methods to modify the carbon surface through the introduction of oxygen groups varying in amount and nature [23–25]. To learn more on the possible causes of deactivation, a characterization of the porous structure of the catalyst after the 170 h continuous run was carried out as well. The SBET decreased to 460 m2/g. It means a reduction of more than 40% with respect to the fresh catalyst (see Table 1). The total pore volume decreased from 0.67 to 0.36 cm3/g and the micropore volume from 0.27 to 0.15 cm3/g. Thus, there is an important surface blockage which not only affects the micropores but also the rest of the porous structure. This suggests the possible formation of some deposits on the surface of the catalyst, probably of polymeric nature, as a result of oxidative coupling of phenol and phenolic intermediates on the active carbon surface [26]. The existence of these deposits may also explain the aforementioned higher amount of organic acids detected at increasing temperature (see Table 2). An increase of temperature could lead to the oxidation of such polymer-like material in a higher extent to yield organic acids. SEM micrographs of the fresh and used catalyst were obtained trying to gather some evidence on the formation of such deposits. Fig. 7 shows two of those micrographs when some differences in the external appearance of the fresh and used catalyst can be observed. To gain more conclusive information on this point, the catalyst from the long-term experiment was submitted to several washing tests. When contacted with 1N NaOH aqueous solution a brownish liquid was obtained. The catalyst recovered in a great part its original SBET and pore volume (SBET = 738 m2/g; total pore volume = 0.60 cm3/g; micropore volume = 0.24 cm3/g). The Fe content after NaOH followed by distilled water washing was practically equal to that of the catalyst at the end of the long-term experiments.
Fig. 7. SEM micrographs of the catalyst before (a) and after (b) 170 h reaction time.
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267
Table 4 Comparison of the results of phenol oxidation with H2O2 using different catalyst Catalyst support Experimental conditions
Results
% Fe (w/w) Wcatalyst (g/L) [Ph]0 mg/L H2O2/Ph Silica Alumina Zeolite
Pillared clays
Resin Active carbon
1.5 7.7 2
4.9
0.35 1 0.4
65 94 66
0.35
65
5 10
50
1.8
0.5 1 1
250
21.6 4
3 0.5
1000 100
1.5 1.1 1.5
a
T (8C) pH0 Reaction time (h) Xphenol XTOC XH2 O2
Feleached (mg/L) Reference
70 – 90
3.5 – 3 3.5 5
3 2 6
0.65 1 1 1 1
0.19 0.6 0.58 0.48 0.50
0.30 0.5 0.66 0.54 0.56
5 6.5 5 1.5 1
[12] [8] [13,14]
70
3.5
3
0.81
0.17
0.27
1
[12]
1.2
40 70
3.5 3.5
4 2
1 1
0.72 0.78
– –
0.8 0.2
[7]
1
50
3
2 3
1 1 1
0.87 0.89 0.88
– – –
– 1.2 <1
0.7 1
80 50
3 3
2 4
1 1
0.75 0.84
1 0.86
– 2.4
[17]
[18] This work
(–) Data not shown. a Respect to the stoichiometric.
The washed catalyst was tested in discontinuous runs as the ones previously described. Fig. 8 shows the results obtained where it can be observed an important recovery of the activity although complete restoration was not reached. Further research is needed to enlighten the causes of catalyst deactivation in order to diminish this problem and to develop some efficient regeneration procedure. 3.5. Comparison with other catalysts Table 4 summarizes a comparison of the results obtained in this work with other reported in the bibliography with different catalysts for phenol oxidation with H2O2. As can be seen, this Fe/AC catalyst compares favourably with all of them except for one of the two Fe on pillared clays catalyst [17], which shows a slightly better performance.
4. Conclusions Catalysts based on Fe supported on active carbon are interesting candidates to be used in catalytic wet peroxide oxidation for wastewaters treatment. A complete removal of phenol and aromatic intermediates can be achieved at 50 8C and pH 3. TOC was lowered in more than 80%, the remaining corresponding to low weight organic acids. Thus, the toxicity was drastically reduced. The catalyst showed an acceptable stability, although some Fe leaching was observed, which has been related with the formation of oxalic acid. In long-term continuous experiments, a TOC reduction higher than 80% was maintained over 20–25 h. Beyond this operation time a loss in activity was observed, which can be attributed to iron leaching and active sites blockage by polymeric deposits. A treatment with 1N NaOH solution to remove these deposits practically restores the activity of the catalyst. Research in course is addressed to improve the anchorage of Fe on the active carbon surface and reduce the Fe leaching. Acknowledgements The authors wish to thank PhD. E. Gonza´lez and PhD. A. Bahamonde for their help in the characterization of porous structure of AC and Fe/AC. This work has been supported by the Spanish Plan Nacional de I + D through the project CTQ2004-02912/PPQ and PETRI under project PTR95.0716.OP. References
Fig. 8. Comparison of the fresh and NaOH-regenerated catalyst ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/L, Wcatalyst: 500 mg/L, 50 8C, pH0 3).
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Catalytic wet peroxide oxidation of phenol with a Fe/active carbon catalyst J.A. Zazo *, J.A. Casas, A.F. Mohedano, J.J. Rodrı´guez Ingenierı´a Quı´mica, Universidad Auto´noma de Madrid, Ctra. de Colmenar km 15, 28049 Madrid, Spain Received 19 October 2005; received in revised form 1 February 2006; accepted 9 February 2006 Available online 20 March 2006
Abstract A Fe on activated carbon catalyst has been prepared and tested for phenol oxidation with H2O2 in aqueous solution at low concentration (100 mg/L). Working at 50 8C, initial pH 3 and a dose of H2O2 corresponding to the stoichiometric amount (500 mg/L) complete removal of phenol and a high TOC reduction (around 85%) has been reached. Oxidation of phenol gives rise to highly toxic aromatic intermediates which finally disappear completely evolving to short-chain organic acids. Some of these last showed to be fairly resistant to oxidation being responsible for the residual TOC. In long-term continuous experiments the catalyst undergoes a significant loss of activity in a relatively short term (20–25 h) due to Fe leaching, this being related with the amount of oxalic acid produced. Deactivation may also be caused by active sites blockage due to polymeric deposits on whose formation some evidences were found. Washing with 1N NaOH solution allows to recover the activity although complete restoration was not achieved. # 2006 Elsevier B.V. All rights reserved. Keywords: CWPO; Hydrogen peroxide; Phenol oxidation; Active carbon iron supported catalyst; Fe/AC
1. Introduction Effective cleaning of industrial wastewaters has become an increasingly concerning problem in the last decades. Modern legislation in many countries imposes environmental regulations and health quality standards that steadily become more restrictive. These effluents frequently contain pollutants which are toxic and resistant to conventional wastewater treatments. Therefore, development of efficient technologies is a need and considerable research efforts are being devoted in this field. Phenol and phenolic compounds are starting and/or intermediate compounds in petrochemical, chemical and pharmaceutical industries and also are formed in the oxidation pathway of higher-molecular-weight aromatic hydrocarbons [1]. Thus, phenol is usually taken as a model compound for advanced wastewater treatment studies. Wet air oxidation (WAO) and catalytic wet air oxidation processes (CWAO) using air or pure oxygen as oxidant have been used to treat industrial wastewaters containing phenols. However, high pressures (20–200 bar) and temperatures (200–
* Corresponding author. Tel.: +34 91 497 8774; fax: +34 91 497 3516. E-mail address: [email protected] (J.A. Zazo). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.02.008
320 8C) are needed thus increasing the treatment cost [2]. An alternative in order to reduce the severity of these reaction conditions is the use of hydrogen peroxide in the so-called catalytic wet peroxide oxidation (CWPO) [3]. Among the different processes using hydrogen peroxide, Fenton’s reagent is one of the most interesting options. In this process, H2O2 decomposes catalytically in the presence of Fe(II) giving rise to hydroxyl radicals with a high oxidation capacity. The application of Fenton’s process in wastewater treatment is attractive, in principle, due to the fact that iron is a widely available and non-toxic element and hydrogen peroxide is easy to handle and the excess decomposes to environmentally safe products [4]. Some advantages of Fenton’s process with regard to other oxidation techniques are that it requires a relatively simple installation and operates under mild conditions (atmospheric pressure and room temperature). Consequently it has been postulated as the most economic alternative by several authors [5,6]. However, this treatment shows some inconveniences derived from the high hydrogen peroxide consumption and the need of removing iron after the treatment, which requires additional separation steps increasing the treatment cost [7]. These drawbacks can be overcome by the use of heterogeneous catalysts. In this case, the process is usually
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Nomenclature dp Ph SBET T TOC W X
particle diameter phenol BET surface area (m2/g) temperature (8C) total organic carbon catalyst concentration (g/L) conversion
Subscript 0 relative to initial conditions
called heterogeneous Fenton. Transition metals, mainly iron, are used as active phase. Alumina [8], silica [9], mesoporous molecular sieves [10,11], zeolites [12–14], pillared clays [7,15– 17] and ion-exchange resin [18] have been used as supports to prepare the catalysts. The best results reported in the bibliography have been obtained with zeolites and pillared clays. The main problem associated to these catalysts comes from the leaching of the active phase when oxidation is carried out at pH near 3, the optimum value in the case of the homogeneous Fenton process. With zeolites a substantial leaching of iron was observed at pH values below 5 and the efficiency increased when pH decreased, whereas at neutral pH no significant activity was observed, which may suggest significant contribution of homogeneous catalysis at low pH. Pillared clays are more stable than zeolites at low pH, although the leaching of iron increases when pH decreases. However, the activity of the catalyst is less sensible to pH, allowing high TOC reduction even at a pH value of 5 [17]. The aim of this work is to study the catalytic oxidation of phenol with hydrogen peroxide using a home-made active carbon supported iron catalyst. Although carbon materials have been widely used as catalysts and as catalytic supports [19] there are no works in the literature dealing with active carbon supported catalysts for CWPO. 2. Experimental 2.1. Catalyst preparation and characterization The Fe/AC catalyst was prepared by incipient wetness impregnation of an activated carbon supplied by Merck (Cod. 102514100; dp: 1.5 mm) with an aqueous solution of iron nitrate. The volume of solution was 0.83 mL/g catalyst, which represents a 10% excess with respect to the pore volume of the active carbon. The Fe concentration was adjusted to obtain a 4% Fe (w/w) on the catalyst. After impregnation, the solid was left 2 h at room temperature, dried during 12 h at 60 8C and calcinated at 200 8C for 4 h. To characterize the porous structure of both, the active carbon and the catalyst, N2 adsorption–desorption isotherms at 77 K were obtained using a Quantachrome Autosorb-1 instrument. The samples were outgassed at 548 K for 12 h to
Table 1 Porous structure of the active carbon (AC) and the Fe/AC catalyst
2
SBET (m /g) Pore volume (cm3/g) Micropore Mesopore Macropore
AC
Fe/AC
974 0.75 0.34 0.19 0.22
781 0.67 0.27 0.16 0.24
a residual pressure < 10 5 Torr. Mercury porosimetry measurements were also accomplished using a Carlo Erba Porosimeter 4000 which allows to cover the pore range above 3.7 nm diameter. The total iron content of the catalyst was determined by inductively coupled plasma mass spectrometry (ICP-MS) using an ICP-MS Elan 6000 Perkin-Elmer Sciex instrument. X-ray photoelectron spectroscopy (XPS) using a Physical Electronics 5700C Multitechnique System was employed for iron analysis on the surface of the catalyst. The deconvolution of the spectra was performed using Multipak V 5.0A software. Finally, scanning electron micrographs (SEM) of the fresh and used catalyst were obtained by means of a JEOL JSM-840 apparatus. Table 1 shows the characterization of the porous structure of the activated carbon (AC) and the Fe/AC catalyst. As can be seen impregnation leads to a decrease of surface area in good agreement with that of micropore volume. Analysis by ICP-MS confirmed the 4% Fe content of the catalyst. The main iron species on the surface corresponds to Fe2O3 according with the XPS spectra which showed a well defined peak at 710.9– 711.1 eV and its doublet separated 13.6 eV. 2.2. Oxidation procedure To study the activity of the catalyst batch experiments were carried out in 100 mL stoppered glass bottles shaken in a thermostatic bath at an equivalent stirring velocity around 200 rpm. A volume of 50 mL of a 100 mg/L phenol aqueous solution and a weight of catalyst of 25 mg were used in all the experiments. The dose of H2O2 was 500 mg/L which means the calculated stoichiometric amount of H2O2 for complete oxidation of phenol up to CO2 and H2O. The catalyst was used in powdered form (dp < 100 mm). Most of the experiments were carried out at 50 8C and initial pH 3 which is the optimum value for homogeneous Fenton oxidation [4] but other temperature (25 and 70 8C) and pH (5 and 7) values were also checked. Long-term activity tests were also carried out. Experiments were performed for this purpose in a 1 L continuous stirred tank reactor at a 5 mL/min flow rate using 5 g of catalyst, 50 8C temperature and initial pH 3. The starting phenol concentration and the H2O2 dose were maintained in the aforementioned values. Reaction samples were taken and immediately analyzed after filtration through fiber glass filters (Albet FV-C). Phenol and aromatic intermediates were identified and quantified by means of HPLC (Varian Pro-Start 240) with a diode array detector (330 PDA). A Microsorb C18 5 mm column (MV 100,
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15 cm long, 4.6 mm diameter) was used as stationary phase and 1 mL/min of 4 mM aqueous sulfuric solution as mobile phase. UV detector at 210 nm wavelength was used for phenol, catechol and hydroquinone and at 246 nm for p-benzoquinone. Short-chain organic acids were analyzed in a ion chromatograph with chemical suppression (Metrohm 790 IC) using a conductivity detector. A Metrosep A supp 5-250 column (25 cm long, 4 mm diameter) was used as stationary phase and 0.7 mL/min of an aqueous solution of 3.2 mM Na2CO3 and 1 mM NaHCO3 as mobile phase. TOC was measured in a OI TOC Analyzer (model 1010). The residual H2O2 and the iron concentration in the liquid phase were determined by colorimetric titration with a Simadzu UV/Vis spectrophotometer using the titanium sulfate method [20] and the ophenantroline method [21], respectively. 3. Results and discussion
263
Fig. 2. Evolution of TOC and H2O2 with the active carbon and the Fe/AC catalyst ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/L, Wcatalyst: 500 mg/L, 50 8C, pH0 3).
3.1. Catalyst activity A previous set of experiments were performed to compare the behaviour of the Fe/AC catalyst and the AC support and to discriminate the effect of adsorption and reaction. These experiments were carried out at 50 8C and initial pH 3 and the rest of the operating conditions as indicated in Section 2.2. The results obtained are shown in Fig. 1. As can be seen the experiments carried out with the nonimpregnated active carbon gave rise to fairly similar results in presence and in absence of H2O2. This suggests that oxidation of phenol is almost negligible and phenol removal takes place essentially by adsorption. On the opposite, in the case of Fe/AC catalyst substantially different results were obtained with and without H2O2. In the absence of this reactant the evolution of phenol is comparable to that observed with the active carbon, the small differences being explainable from the lower surface area of the Fe/AC catalyst. Thus, again, practically only
Fig. 1. Phenol removal upon treatment with the AC and the Fe/AC catalyst in the presence and absence of H2O2 ([phenol]0: 100 mg/L; [H2O2]0: 500 mg/L; Wcatalyst: 500 mg/L; 50 8C, pH0 3).
adsorption must be occurring. When H2O2 is added phenol removal is greatly enhanced and complete conversion is finally reached, allowing to conclude that phenol is being transformed through oxidation and the Fe/AC catalyst is active for this reaction. To have a better knowledge on the action of the catalyst, Fig. 2 shows the results obtained in terms of TOC as well as the evolution of the H2O2 concentration in the experiments carried out with the active carbon and the Fe/AC catalyst. A substantially higher TOC removal and H2O2 conversion is reached with the Fe/AC catalyst. The evolution of TOC is in agreement with that of phenol in the case of the active carbon whereas, in the case of the Fe/AC catalyst the TOC reduction is lower than the corresponding to phenol removal, confirming that this compound is being transformed upon oxidation to intermediate products which evolve towards CO2 and H2O as the reaction proceeds. Nevertheless, complete mineralization is not attained and a residual TOC value is remaining even after 4 h reaction when phenol has been completely converted. Looking at the H2O2 concentration profile it can be seen that, although in less amount with respect to the Fe/AC catalyst, the active carbon also decomposes H2O2. The almost practical absence of reaction in the case of AC suggest that the decomposition of H2O2 proceeds through a different pathway that with the Fe/AC catalyst. This second promotes the formation of highly oxidant OH radicals whereas with the active carbon alone, decomposition of H2O2 proceeds majorly through O2 formation, which under the mild operating conditions of the process does not oxidize phenol. In fact, the concentration of the primary oxidation intermediates, such as catechol, hydroquinone and p-benzoquinone detected were always very low when using AC (0.52, 0.06 and 0.12 mg/L, respectively, as the highest analyzed values). To learn more on the potential feasibility of this Fe/AC catalyst in H2O2 oxidation of phenolic wastewaters it is necessary to know the evolution of the oxidation intermediates, because some of them (the aromatics, especially p-benzoquinone) are much more toxic than phenol and thus their presence
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Fig. 3. Evolution of aromatic intermediates in the oxidation of phenol with H2O2 using the Fe/AC catalyst ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/L, Wcatalyst: 500 mg/L, 50 8C, pH0 3).
even at low concentration would mean high values of ecotoxicity [22]. Figs. 3 and 4 show the distribution curves of the identified oxidation intermediates. The first one refers to aromatic species and the second to short-chain acids. The TOC values calculated from the sum of all the identified intermediates were always at the least in the vicinity of 90% of the measured TOC values throughout the 4 h reaction time investigated. The concentration versus time curves for phenol (Fig. 1) and the oxidation products are characteristic of a series reaction pathway where the primary products correspond to aromatics resulting from phenol hydroxylation which then evolve to short-chain organic acids and finally a high degree of mineralization, namely complete oxidation to CO2 and H2O, is reached. Hydroxylation takes place predominantly in the ortho position since catechol is the most abundant aromatic intermediate. The concentration of p-benzoquinone is higher
than that of hydroquinone as a result of the effect of active carbon on the redox equilibrium. Maleic and its isomer fumaric acid are the primary products from ring opening and oxidation of the aromatic intermediates. These evolve to acetic, oxalic and formic acids which show to be more resistant to oxidation under these operating conditions and are responsible for the residual TOC. Nevertheless, due to the total depletion of phenol and aromatic intermediates the final values of ecotoxicity were substantially lower than the starting ones but it is important to notice that the reaction time has to be the necessary to reach sufficiently low concentrations of aromatic intermediates, specially p-benzoquinone whose ecotoxicity is about three orders of magnitude higher than that of phenol itself [22]. The concentration values of phenol and the oxidation intermediates in Figs. 1, 3 and 4 suggest that the aromatic intermediates probably remain in a great part adsorbed on the catalyst surface where they undergo further oxidation to organic acids. An important question with regard to the application of this catalyst in practice is the possible leaching of iron under the operating conditions. In the experiments described up to this point Fe leaching was always very low. After 2 h of reaction the concentration of Fe in solution was around 1 mg/L and never raised above 2.5 mg/L after 4 h. In these conditions some, although relatively low, contribution of homogeneous phase reaction may take place. To check this, two experiments were carried out at the same operating conditions but using 1 and 3 mg/L of dissolved Fe instead of the Fe/AC catalyst. The TOC reduction after 4 h was 21 and 25%, respectively, much lower than the 84% value obtained with the Fe/AC catalyst. Furthermore, during the first hour of reaction in presence of Fe/AC catalyst, phenol and aromatic intermediates were practically depleted whereas the amount of Fe leached was practically negligible. Even when using a quantity of dissolved Fe equal to the total weight of this metal in the catalyst, the TOC reduction obtained through homogeneous phase oxidation was around one-half that reached in the heterogeneous process. 3.2. Effect of temperature
Fig. 4. Evolution of short-chain organic acids in the oxidation of phenol with H2O2 using the Fe/AC catalyst ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/L, Wcatalyst: 500 mg/L, 50 8C, pH0 3).
Table 2 compares the results obtained at 25, 50 and 70 8C under the same operating conditions already described. As can be seen the rate of disappearance of phenol as well as that of TOC abatement increases at increasing temperature. At 25 8C, a 75% TOC reduction was reached although depletion of phenol was not complete. Moreover, the residual concentration of p-benzoquinone means a substantially higher ecotoxicity than that of the starting phenol solution. Increasing the temperature up to 50 8C produces a remarkable increase on the rate of the oxidation process. Phenol and the aromatic intermediates are completely removed and only organic acids, mainly formic and oxalic, remained in the liquid after 4 h. Thus, ecotoxicity was drastically reduced. The loss of Fe from the catalyst by leaching is somewhat higher although it does not represent more than 12% of the initial iron weight after 4 h. Working at a higher temperature allows to reach high phenol and TOC removal in a significantly shorter
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265
Table 2 Results obtained at different temperatures ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/L, Wcatalyst: 500 mg/L, pH0 3) Temperature (8C)
Time (h)
Xphenol
XTOC
XH2 O2
Saromaticsa (mg/L)
Sacids (mg/L)
Feleached (mg/L)
25
0.5 1 2 4
0.49 0.61 0.84 0.93
0.46 0.50 0.61 0.75
0.18 0.26 0.35 0.45
0.78 3.01 5.08 2.88
3.8 7.0 10.4 11.2
0.1 0.1 0.2 0.3
50
0.5 1 2 4
0.84 0.94 0.98 1
0.58 0.72 0.78 0.84
0.44 0.59 0.72 0.86
6.59 0.83 0.26 0.00
14.3 16.4 18.5 21.8
0.4 0.7 1.1 2.4
70
0.5 1 2 4
0.95 0.99 1 1
0.81 0.86 0.87 0.88
0.79 0.96 0.99 1
5.01 0.42 0.00 0.00
23.8 27.4 33.7 31.2
0.7 2.4 6.5 7.8
a
Other than phenol.
reaction time but some inconveniences can be also observed. A complete removal of aromatics is reached at 70 8C in about half the reaction time needed at 50 8C but at that time the loss of Fe from the catalyst be leaching is almost three times higher at the former temperature. A high reduction of TOC is observed in a short term but then the rate of mineralization slows down due to the lower oxidation rate of organic acids and finally at 4 h reaction time fairly similar values of TOC removal are obtained. It is noticeable that the amount of organic acids is significantly higher as the working temperature increases. The relative distribution of these acids changes with temperature. At the end of the experiments, oxalic acid, which showed to be the more resistant to oxidation represents almost 80% of the total amount of organic acids at 70 8C whereas at 50 8C is less than 40%. This species has been related with Fe leaching as will be discussed later. From these results a temperature around 50 8C can be recognized as the optimal one for this process.
and an initial pH of 3 is clearly the advisable operating value in spite of the fact that a higher Fe leaching is observed in these conditions. This trend is similar to that observed for the homogeneous Fenton oxidation where a pH around 3 is the optimum one. The results for phenol and TOC removal at initial pH values of 5 and 7 were similar to those obtained by adsorption in absence of H2O2. This indicates that at these pH the activity of the Fe/AC catalyst is almost negligible, as well as OH production, despite the fact that higher H2O2 conversion values were obtained. Therefore, at pH 5 and 7, the decomposition of H2O2 leads predominately to O2 not capable to oxidize phenol under the mild operating conditions of these experiments. A similar trend was observed by Fajerwerg et al. [14] and by Gou and Al-Dahhan [16] using Fe-ZSM-5 and pillared clays as catalyst, respectively.
3.3. Effect of pH
To learn on the stability of the catalyst, long-term experiments were carried out in a 1 L continuous stirred tank reactor at 50 8C. A 100 mg/L phenol and 500 mg/L H2O2
Table 3 summarizes the results obtained at 50 8C and two initial pH values different than 3. Comparing these results with those of Table 2 at 50 8C (pH0 3) it can be seen that the efficiency of the oxidation process is remarkably affected by pH
3.4. Catalyst deactivation
Table 3 Results obtained at different initial pH ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/ L, Wcatalyst: 500 mg/L, 50 8C) pH
Time (h)
Xphenol
XTOC
XH2 O2
Saromaticsa (mg/L)
Sacids (mg(L)
Feleached (mg/L)
5
0.5 1 2 4
0.40 0.42 0.45 0.54
0.37 0.39 0.39 0.40
0.55 0.72 0.91 0.99
0.83 1.38 2.08 2.56
0.0 0.31 0.47 4.34
0.0 0.0 0.1 0.3
7
0.5 1 2 4
0.40 0.41 0.43 0.45
0.37 0.40 0.41 0.42
0.61 0.77 0.92 0.99
1.05 1.29 1.64 1.67
0.0 0.1 0.2 0.2
0.0 0.0 0.0 0.0
a
Other than phenol.
Fig. 5. Results of a long-term continuous experiment ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/L, Wcatalyst: 5 g/L, tr: 200 min, 50 8C, pH0 3).
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Fig. 6. Fe leached vs. oxalic acid concentration.
aqueous solution at pH 3 was fed at a 5 mL/min flow rate over 170 h. A weight of 5 g of catalyst was initially added to the reactor. The results are shown in Fig. 5 where, for the sake of comparison, the results obtained in the same conditions but in absence of H2O2 have been included. During the initial 20 h a phenol conversion higher than 90% and a TOC reduction of at least 80% were maintained. Beyond that time an increasing loss of activity was observed until the removal percentages set at about 65% for phenol and 25% for TOC. The H2O2 conversion value was always higher than 90%. One possible cause of deactivation is the loss of Fe from the catalyst by leaching which reached almost 50% of the initial weight at the end of the 170 h run. From a complete analysis of our results in all the discontinuous runs carried out we have found a relation between the Fe leaching and the concentration of oxalic acid in the liquid phase. The results of these observations are shown in Fig. 6. These results fit well a simple relationship given by the expression included in that figure. The action of oxalic acid could be the result of Fe complexation. In fact, the Fe to oxalate weight ratio in that expression is very close to the corresponding to a trivalent Fe–oxalate complex.
Although the presence of oxalic acid seems to play a significant role, future research is needed to better understand the causes of Fe lixiviation. It is important to study the way of achieving a stronger anchorage of Fe on the active carbon surface. In that sense oxidation treatments of the AC with some reagents, like HNO3 or (NH4)2S2O8, are well know methods to modify the carbon surface through the introduction of oxygen groups varying in amount and nature [23–25]. To learn more on the possible causes of deactivation, a characterization of the porous structure of the catalyst after the 170 h continuous run was carried out as well. The SBET decreased to 460 m2/g. It means a reduction of more than 40% with respect to the fresh catalyst (see Table 1). The total pore volume decreased from 0.67 to 0.36 cm3/g and the micropore volume from 0.27 to 0.15 cm3/g. Thus, there is an important surface blockage which not only affects the micropores but also the rest of the porous structure. This suggests the possible formation of some deposits on the surface of the catalyst, probably of polymeric nature, as a result of oxidative coupling of phenol and phenolic intermediates on the active carbon surface [26]. The existence of these deposits may also explain the aforementioned higher amount of organic acids detected at increasing temperature (see Table 2). An increase of temperature could lead to the oxidation of such polymer-like material in a higher extent to yield organic acids. SEM micrographs of the fresh and used catalyst were obtained trying to gather some evidence on the formation of such deposits. Fig. 7 shows two of those micrographs when some differences in the external appearance of the fresh and used catalyst can be observed. To gain more conclusive information on this point, the catalyst from the long-term experiment was submitted to several washing tests. When contacted with 1N NaOH aqueous solution a brownish liquid was obtained. The catalyst recovered in a great part its original SBET and pore volume (SBET = 738 m2/g; total pore volume = 0.60 cm3/g; micropore volume = 0.24 cm3/g). The Fe content after NaOH followed by distilled water washing was practically equal to that of the catalyst at the end of the long-term experiments.
Fig. 7. SEM micrographs of the catalyst before (a) and after (b) 170 h reaction time.
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267
Table 4 Comparison of the results of phenol oxidation with H2O2 using different catalyst Catalyst support Experimental conditions
Results
% Fe (w/w) Wcatalyst (g/L) [Ph]0 mg/L H2O2/Ph Silica Alumina Zeolite
Pillared clays
Resin Active carbon
1.5 7.7 2
4.9
0.35 1 0.4
65 94 66
0.35
65
5 10
50
1.8
0.5 1 1
250
21.6 4
3 0.5
1000 100
1.5 1.1 1.5
a
T (8C) pH0 Reaction time (h) Xphenol XTOC XH2 O2
Feleached (mg/L) Reference
70 – 90
3.5 – 3 3.5 5
3 2 6
0.65 1 1 1 1
0.19 0.6 0.58 0.48 0.50
0.30 0.5 0.66 0.54 0.56
5 6.5 5 1.5 1
[12] [8] [13,14]
70
3.5
3
0.81
0.17
0.27
1
[12]
1.2
40 70
3.5 3.5
4 2
1 1
0.72 0.78
– –
0.8 0.2
[7]
1
50
3
2 3
1 1 1
0.87 0.89 0.88
– – –
– 1.2 <1
0.7 1
80 50
3 3
2 4
1 1
0.75 0.84
1 0.86
– 2.4
[17]
[18] This work
(–) Data not shown. a Respect to the stoichiometric.
The washed catalyst was tested in discontinuous runs as the ones previously described. Fig. 8 shows the results obtained where it can be observed an important recovery of the activity although complete restoration was not reached. Further research is needed to enlighten the causes of catalyst deactivation in order to diminish this problem and to develop some efficient regeneration procedure. 3.5. Comparison with other catalysts Table 4 summarizes a comparison of the results obtained in this work with other reported in the bibliography with different catalysts for phenol oxidation with H2O2. As can be seen, this Fe/AC catalyst compares favourably with all of them except for one of the two Fe on pillared clays catalyst [17], which shows a slightly better performance.
4. Conclusions Catalysts based on Fe supported on active carbon are interesting candidates to be used in catalytic wet peroxide oxidation for wastewaters treatment. A complete removal of phenol and aromatic intermediates can be achieved at 50 8C and pH 3. TOC was lowered in more than 80%, the remaining corresponding to low weight organic acids. Thus, the toxicity was drastically reduced. The catalyst showed an acceptable stability, although some Fe leaching was observed, which has been related with the formation of oxalic acid. In long-term continuous experiments, a TOC reduction higher than 80% was maintained over 20–25 h. Beyond this operation time a loss in activity was observed, which can be attributed to iron leaching and active sites blockage by polymeric deposits. A treatment with 1N NaOH solution to remove these deposits practically restores the activity of the catalyst. Research in course is addressed to improve the anchorage of Fe on the active carbon surface and reduce the Fe leaching. Acknowledgements The authors wish to thank PhD. E. Gonza´lez and PhD. A. Bahamonde for their help in the characterization of porous structure of AC and Fe/AC. This work has been supported by the Spanish Plan Nacional de I + D through the project CTQ2004-02912/PPQ and PETRI under project PTR95.0716.OP. References
Fig. 8. Comparison of the fresh and NaOH-regenerated catalyst ([phenol]0: 100 mg/L, [H2O2]0: 500 mg/L, Wcatalyst: 500 mg/L, 50 8C, pH0 3).
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