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Degradation of olive oil mill effluents by catalytic wet air oxidation
Applied Catalysis B: Environmental 63 (2006) 68–75 www.elsevier.com/locate/apcatb
Degradation of olive oil mill effluents by catalytic wet air oxidation 1. Reactivity of p-coumaric acid over Pt and Ru supported catalysts Doan Pham Minh, Pierre Gallezot, Miche`le Besson * Institut de Recherches sur la Catalyse-CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France Received 12 July 2005; received in revised form 29 August 2005; accepted 2 September 2005 Available online 26 October 2005
Abstract The objective of this work was to investigate the catalytic wet air oxidation of p-coumaric acid, a biorecalcitrant phenolic compound typically found in olive oil wastewaters in the presence of Pt and Ru supported catalysts. The influence of the operating variables were established. The most important intermediates determined by HPLC measurements suggest a rapid attack by oxygen of the side-chain of p-coumaric acid, and the mineralization proceeds through different aromatic compounds reacting further to aliphatic intermediates (mainly acids). Important mineralization yields were achieved in the presence of the catalysts at 140 8C and 50 bar air. The importance of the nature of the support (TiO2, ZrO2) on the adsorption of p-coumaric acid was demonstrated. # 2005 Elsevier B.V. All rights reserved. Keywords: Catalytic wet air oxidation; Platinum and ruthenium heterogeneous catalysts; p-Coumaric acid; Olive oil mill effluents
1. Introduction Olive oil manufacturing is an important economic activity of many countries particularly throughout the Mediterranean Sea (the annual world olive oil production, estimated at about 1.5– 1.7 Mt/year in the eighties, reached around 2.5 Mt/year in the recent seasons) [1]. However, olive oil extraction is one of the most pollution intensive food-processing industries. It involves a high consumption of water and large volumes of strongly polluted wastewaters known as olive mill wastewater (OMW) are generated. The amount of olive oil mill wastewater depends on the milling process, ranging from about 0.6 m3/t of olives processed for classical mills to about 1.7 m3/t olives in centrifugal mills [2]. Furthermore, OMW is characterized by a very high Chemical Oxygen Demand (COD up to 200 g l 1), a high content in phenol-like substances (in the range 1–5 g l 1 measured as phenol) and acidity. Because of its highly phytotoxicity and strong antimicrobial properties [3,4], the classical biological treatment cannot be applied. Alternative appropriate treatments have to be considered for the management of these wastewaters. Most physical and physicochemical
* Corresponding author. Tel.: +33 4 72 44 53 58; fax: +33 4 72 44 53 99. E-mail address: [email protected] (M. Besson). 0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.09.009
methods, such as precipitation, flocculation/clarification, coagulation, filtration, evaporation in open ponds [5] give only partial solution to the problem. Reverse osmosis or ultrafiltration are usually costly. Anaerobic biological digestion with production of biogas is increasingly being used, but is not yet completely satisfactory [6,7]. A possible solution would be the chemical oxidative degradation of OMW used as a pre-treatment process to decrease its toxicity prior to its biological treatment. Ozone alone or combined with hydrogen peroxide or UV radiation [8–10], photo-Fenton treatment [11–13], Fenton’s reagent [14], oxidation with polymer supported FeCl3 [15], wet air oxidation (WAO) with the addition of H2O2 [16] have been tested. WAO is increasingly used for the elimination of organic pollutants in a variety of wastewaters from different chemical plants [17,18]. It consists of oxidizing pollutants under air or oxygen pressure under elevated temperature (200–350 8C) and pressure (50– 150 bar). The organic matter is totally mineralized to CO2 and water or partially oxidized to smaller intermediate products (typically low-molecular-weight organic acids). Catalytic WAO operates at lower temperatures and pressures, in the presence of, e.g., noble metal based catalysts [19,20]. Typical OMW contains high concentrations of tyrosol, cinnamic and/or benzoic acid derivatives. The scope of this work was to investigate the use of catalytic WAO to treat model
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molecules representative of OMW. p-Coumaric acid (trans-phydroxycinnamic acid) was chosen as a representative of the phenolic fraction. Previously, the reactivity of this compound has been studied using ozone [21], UV radiation and combination of ozone and UV radiation [22,23], photo-assisted Fenton reaction [24] or photocatalysis [25]. Fe-ZSM5 catalysts and Fe-containing pillared clays in combination with hydrogen peroxide were found more active than the homogeneous (Fe3+salt) [26]. The wet air oxidation of p-coumaric acid has been investigated without catalyst [27] and using various homogeneous (Fe2+, Cu2+, Zn2+, Co2+) and heterogeneous (CuOZnO-Al2O3) catalysts [28]. Some leaching of the heterogeneous catalyst was measured whose extent was strongly dependent on the operating conditions. Fe- and Zn-promoted ceria prepared by co-precipitation were also investigated in the wet air oxidation [29]. In the present investigation, p-coumaric acid was reacted with air using noble metal supported catalysts. The influence of the operating reaction conditions and of the catalyst on the extent of degradation are reported. 2. Experimental 2.1. Catalyst preparation The supports were commercial titanium and zirconium oxides, known as leaching resistant materials under the reaction conditions [19,30,31]. The textural properties of solids were determined from nitrogen adsorption–desorption isotherms at liquid nitrogen temperature by using a Micromeritics ASAP 2010 instrument. The pH of the point of zero charge (pHPZC) was measured according to the method described in [32]. The characteristics of the supports are given in Table 1. A series of Pt and Ru catalysts were prepared by incipientwetness impregnation of the supports with an aqueous solution of H2PtCl6 or RuCl3 to obtain a 3 wt.% Pt or Ru content. The dried mixture was reduced by heating up in a hydrogen flow (15 l h 1) at 1 8C min 1 up to 300 8C. After cooling and purging with argon, passivation of the catalyst was carried out in a flow of 1%O2/N2.
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grid. Samples were also prepared by the extractive replica procedure. Carbonaceous species deposited on the solid during reaction were quantified by measuring the percentage of carbon at Service Central d’Analyse du CNRS. 2.3. Oxidation reaction CWAO experiments were carried out in a batch reactor made of Hastelloy C22 previously described elsewhere [19]. The 280 ml autoclave is equipped with a magnetically driven stirrer. One hundred and fifty milliliter of an aqueous solution containing the organic pollutant (4.5 mmol l 1 p-coumaric acid, maximum solubility at room temperature) was introduced into the reactor with the catalyst, purged with argon, heated to the desired temperature. After introduction of air at the desired pressure, the stirring speed was set at 1500 rpm. This point was taken as ‘‘zero time’’. Liquid samples were periodically withdrawn from the reactor and analysed. Total organic carbon (TOC) in the supernatant was measured using a Shimadzu TOC 5050A analyser for establishing the degree of conversion. The organic compounds both aromatic compounds and aromatic ring cleavage products (mainly acids) could be analysed after separation of the catalyst by a 0.45 mm Millipore filter, using HPLC and UV-absorption detection at 210 nm, using a CarboSep Coregel-87H3 column. The mobile phase was constituted by a 0.005 M aqueous sulphuric acid solution eluted at a flow rate 0.5 ml min 1. The products were identified by comparison of their retention times with those of authentic compounds. p-Coumaric acid in the starting material gave two products identified as the cis and trans form. It appeared that the trans–cis isomerisation occurred during the preparation of the aqueous solution in the ultrasound bath used to improve solubilization of the substrate in water. It was verified that this process did not cause any oxidation nor bond cleavage of p-coumaric acid. The mass balance for TOC and HPLC results were compared to verify that all intermediates were correctly identified by HPLC. 3. Results and discussion
2.2. Catalyst characterization
3.1. Characterization of catalysts
X-ray diffraction (XRD) of the solids was carried out using a Siemens D5005 diffractometer with Cu Ka1+2 radiation at 0.154184 nm. Transmission electron microscopy (TEM) studies was performed on selected samples using a JEOL-JEM 100 electron microscope. Samples were suspended in ethanol with ultrasonic agitation and dispersed on a carbon-coated copper
The XRD patterns of 3%Pt/TiO2 P25 and 3%Pt/ZrO2 Mel are shown in Fig. 1. The unlabeled peaks were assigned to rutile and anatase in TiO2 and to monoclinic and tetragonal zirconia. In 3%Pt/ZrO2 Mel, the Pt crystallites were unobservable due to their small size, while in 3%Pt/TiO2 P25 the most intense X-ray diffraction peaks due to platinum could be recorded.
Table 1 Characteristics of titanium and zirconium oxides Support
Supplier
SBET (m2 g 1)
Crystalline phase
Mean pore diameter (nm)
pHPZC
TiO2 Eng TiO2 DT51 TiO2 P25 ZrO2 Mel ZrO2 EP
Engelhard Thann and Mulhouse Degussa MEL Chemicals Degussa
50 92 55 90 40
Anatase Anatase Anatase + rutile Monoclinic + tetragonal Monoclinic + tetragonal
20 9 Non-porous 9 Non-porous
6.1 4.4 5.6 6.1 6.3
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Fig. 1. XRD patterns of 3%Pt/ZrO2 Mel (upper curve) and 3%Pt/TiO2 P25 (down curve). The most intense X-ray diffraction peaks due to platinum are recorded.
Fig. 2. Size distribution of supported nanoparticles for 3%Ru/TiO2 DT51, 3%Ru/ZrO2 Mel, as determined from transmission electron microscopy.
TEM photographs of the 3%Ru/TiO2 DT51 and 3%Ru/ZrO2 Mel indicate a relatively narrow size distribution of metal particles dispersed on TiO2 and ZrO2 with an average particle size of 3.5 and 2.9 nm, respectively (Fig. 2). From XRD and TEM results it can be concluded that the catalysts are well dispersed using the incipient-wetness impregnation method. 3.2. Oxidation of p-coumaric acid 3.2.1. Influence of operating variables A series of experiments was conducted at 120, 130, 140 and 150 8C at a total pressure of air of 50 bar over the 2.8%Ru/TiO2
Eng catalyst. Fig. 3 shows the conversion curves for p-coumaric acid (Fig. 3a) and TOC (Fig. 3b) for the experiments performed at these different temperatures, maintaining all the other parameters constant. Degradation rate increased with temperature. It is clear that the treatment of the p-coumaric acid solution in the presence of the catalyst leads to its total disappearance. At 120 and 130 8C, a rapid conversion of p-coumaric acid occurred, with a significant initial conversion of TOC. Prolonged treatment lead to the total disappearance of the initial substrate, but had little effect on TOC abatement (ca. 55%) suggesting that the intermediates formed (dicarboxylic acids, vide infra) are not further oxidized under these reaction conditions. On the other hand, at higher temperatures (140 and 150 8C), the rate of oxidation of pcoumaric acid increased, the intermediates were further oxidized and nearly total conversion of TOC was observed at 140 and 150 8C after 7 and 4 h, respectively. The degradation of p-coumaric acid is a complicated process involving a great number of intermediates. Thus, Fig. 4 shows the concentration profiles for p-coumaric acid and the main intermediate compounds detected versus reaction time at 140 8C, 50 bar air in the presence of the ruthenium catalyst. While oxidation of p-coumaric acid was occurring, the major intermediates formed were p-hydroxybenzaldehyde and p-hydroxybenzoic acid. Low concentrated aromatic intermediates, such as phenol, catechol and hydroquinone, and traces of hydroxylation products such as 3,4-dihydroxycinnamic acid (caffeic acid), 3,4-dihydroxybenzaldehyde and 3,4-dihydroxybenzoic acid were detected. Maleic acid, and, in lower amounts, fumaric acid were formed, as expected from the opening of the aromatic ring of phenol for instance. A wide range of other low concentrated compounds were detected, but not identified. On the basis of our experimental results and previous publications on CWAO of p-coumaric acid [27,28] we established a simplified oxidation mechanism given in Scheme 1. During the oxidative decomposition of p-coumaric acid 1, the initial step is the rapid attack of the very reactive exocyclic double bond to yield p-hydroxybenzaldehyde 2 and glyoxylic acid 3, which can be oxidized to p-hydroxybenzoic acid 4 and oxalic acid 5. 5 is very reactive under these conditions, and when formed it can rapidly be oxidized to carbon dioxide [33,34]. Glyoxylic acid 3 may also be directly oxidized to CO2. Decarboxylation of p-hydroxybenzoic acid 4 to phenol 6 occurs
Fig. 3. Conversion of (a) p-coumaric acid and (b) TOC as a function of time at different temperatures. Catalyst 2.8%Ru/TiO2 Eng, total air pressure 50 bar.
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Fig. 5. Evolution of (a) p-hydroxybenzaldehyde (OH-BZAL) and (b) p-hydroxybenzoic acid (OH-BZOIC) as a function of TOC conversion at various temperatures in the presence of 2.8%Ru/TiO2 Eng. Fig. 4. Concentration profile as a function of time during oxidation of pcoumaric acid in the presence of 2.8%Ru/TiO2 Eng. Reaction conditions: 150 ml of an aqueous solution of 4.5 mmol l 1 p-coumaric acid, 0.5 g catalyst, 140 8C, 50 bar air. COUM, p-coumaric acid; OH-BZAL, p-hydroxybenzaldehyde; OH-BZOIC, p-hydroxybenzoic acid; HQ, hydroquinone; FUM, fumaric acid; MAL, maleic acid; diOH-BZAL, 3,4-dihydroxybenzaldehyde; diOHCINNAM, 3,4-dihydroxycinnamic acid; PhOH, phenol.
and all expected compounds derived from oxidation of phenol (dihydroxylated benzenes 7 and 8, maleic and fumaric acids, oxaloacetic acid, . . .) are further detected. Hydroxylation of pcoumaric acid to 3,4-dihydroxycinnamic acid 9 was also observed. A possible path to 3,4-dihydroxybenzaldehyde 10 and 3,4-dihydroxybenzoic acid 11 is via this intermediate by attack of the C C double bond, rather than by hydroxylation of p-hydroxybenzaldehyde 2 and p-hydroxybenzoic acid 4. Indeed, separate experiments performed on catalytic oxidation of these two latter substrates did not evidence the occurrence of the hydroxylation reaction. Opening of the aromatic ring and further oxidation of the intermediates can happen in any of the phenolic intermediates formed. These observations are in general agreement with the data obtained in the literature for non-catalytic or catalytic wet air oxidation experiments [27,28]. Discrepancies concern p-(1-hydroxyethyl)phenol, an intermediate suggested in these reports, that was not observed in our experiments. We verified that this latter compound synthesized by catalytic hydrogenation of the corresponding ketone exhibited a retention time that did not match any peaks of our chromatograms.
Fig. 5a and b show the concentration of the major intermediates ( p-hydroxybenzaldehyde and p-hydroxybenzoic acid) as a function of TOC conversion at various temperatures. In the range of temperatures considered, no significant differences were observed on the shape of the curves, either for p-hydroxybenzaldehyde or for p-hydroxybenzoic acid. Only an accumulation of the compounds originating from the ring cleavage was observed at the lower temperatures. It is also shown that p-hydroxybenzoic acid can be produced from the oxidation of p-hydroxybenzaldehyde. Some experiments were also conducted to determine the influence of air pressure. In the range 30–50 bar, no effect in either p-coumaric acid removal rate or TOC reduction rate was noticed. Since the cleavage of the exocyclic double bond may be pH sensitive, the catalytic oxidation of p-coumaric acid at 140 8C on a 3%Ru/ZrO2 Mel catalyst has also been performed by increasing the initial pH from 3.3 to 7.9 and 11.1 by adding sodium hydroxide. The differences observed were very small. Whatever the initial pH, p-coumaric acid was rapidly converted to CO2 and p-hydroxybenzaldehyde and p-hydroxybenzoic acid as the major intermediates (similar maxima concentrations ca. 1 mmol l 1). The rate of oxidation of these aromatic compounds to ring opening compounds was slightly higher when the initial pH was basic and yielded higher concentrations of maleic and fumaric acids. The formation of these acids rapidly lowered the value of pH. Finally, similar final TOC conversions were obtained (ca. 90% after 7 h of reaction).
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Scheme 1. Proposed simplified oxidation of p-coumaric acid during CWAO.
3.2.2. Influence of the support and the metal Fig. 6 shows the results of TOC abatement in the presence of the TiO2 or ZrO2 supports without metal deposited on them (Fig. 6a and b, respectively), in the presence of platinum catalysts (Fig. 6c) and of ruthenium catalysts (Fig. 6d). In the absence of catalyst, oxidation of p-coumaric acid started immediately after the addition of air and continues up to total disappearance after 7 h. All intermediates mentioned earlier were detected, while the TOC conversion increased slowly up to 27% (Fig. 6a). The same experiment performed under argon yielded almost no change in TOC detected, indicating that p-coumaric is relatively thermally stable under these conditions and the degradation occuring under air is due to oxidation. In the presence of the TiO2 and ZrO2 supports, it was observed that, after the period of heating under argon to the desired temperature, at time zero, both the concentration of p-coumaric acid (not shown) and the TOC value (Fig. 6a and b) are different from the nominal value introduced in the reactor. This deficit in mass balance in the liquid phase is dependent on the support used for experiment. Thus, an apparent TOC conversion of 3 and 7.2% was observed over TiO2 P25 and TiO2 DT51, respectively, and 13.3 and 26.4% over ZrO2 EP and ZrO2 Mel, respectively. The reason of these differences may be due to the strong adsorption of p-coumaric acid on the solids. Given the point of zero charge of the supports (4.4 and 5.6 for TiO2 DT51 and TiO2 P25, respectively, and 6.1 and 6.3 for ZrO2 Mel and ZrO2 EP, respectively) and the acidic pH of the initial solution (pH 3.3), the surface of the supports is positively
charged (Ti-OH2+ and Zr-OH2+) and should be suitable for adsorption of anion groups. However, at that pH, the carboxyl group should be mainly in the uncharged COOH form (the pKa of –OH and –COOH groups of p-coumaric acid are 9.0 and 4.4, respectively) [35,36]. Thus, other interactions than purely anionic interactions must be invoked, such as hydrogen bonding between the atoms of p-coumaric acid bearing negative charges and the groups at the surface of the support. A much higher amount of TOC was retained on ZrO2 Mel than on TiO2 DT51. This difference cannot be explained on the basis of specific surface area and porosity of the supports which are both of the same order, but on the different pHPZC. To support this interpretation in terms of the high affinity of p-coumaric acid for the oxide surface, two experiments were set up in which the p-coumaric acid solution was heated in the presence of supports TiO2 DT51 or ZrO2 Mel up to 140 8C under argon (as in a standard experiment), but the reaction was stopped before introduction of air (time zero of a standard oxidation reaction). The solids were recovered, washed with water, dried and analysed for carbon content. The percentages of carbon of the supports were measured at 0.29 and 3.38%, which correspond to an extent of adsorption of the initial TOC concentration of 2 and 23.1%, respectively. These data are in good agreement with the analysis of the supernatant solution at time zero (TOC abatement of 7.2 and 26.4%, respectively), showing a good carbon balance. Upon further reaction under air, a small increase of TOC conversion was then observed throughout the course of the reaction with comparable or lower rate than the uncatalysed reaction. The final carbon content of both supports
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Fig. 6. TOC abatement in the presence of (a) TiO2 supports, (b) ZrO2 supports, (c) supported platinum catalysts and (d) ruthenium supported catalysts. Reaction conditions: 150 ml of an aqueous solution of 4.5 mmol l 1 p-coumaric acid, 140 8C, 50 bar air, 0.5 g solid.
after 7 h of reaction (2.21 and 3.68%) correspond to an adsorption of 15.1 and 25.2% of the initial TOC, respectively. In the uncatalysed reaction and in experiments performed with supports, it was also observed that the solution initially colourless became coloured to yellow-brown. Fig. 7a shows a comparison between the profiles of TOC conversion measured
Fig. 7. Comparison of TOC conversion determined from TOC measurements (TOCm) and calculated from HPLC analysis (TOCc) in the presence of (a) ZrO2 Mel and (b) 3%Ru/ZrO2 Mel.
from the analysis of the TOC in solution (TOCm) and the TOC conversion calculated from the organic carbon contribution from all the compounds detected by HPLC (TOCc) for support ZrO2 Mel. Fig. 7b shows the same comparison for the corresponding ruthenium catalyst. The results obtained for ZrO2 Mel (Fig. 7a) show an initial good agreement between TOCm and TOCc, but as reaction proceeds large discrepancies between both are noted. Evidently, intermediates which may be oligomers of p-coumaric acid are formed throughout the reaction under air that are not analysed by HPLC. The results in presence of the other supports lead to the same conclusion. A possible explanation for this behaviour could be the formation of dimers identified as dilactone (Scheme 2) as evidenced by studies on the oxidation of p-coumaric acid by periodate or Fenton reactant [35]. Dimer formation appeared to be related to the fact that the C C double bond is conjugated with the aromatic ring, and conjugation with
Scheme 2. Possible dimer derived from p-coumaric acid in the absence of a metallic catalyst.
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the 4-hydroxyl group may also be important. On the other hand, in the case of ruthenium supported catalyst (Fig. 7b), a good agreement was obtained indicating that most of the intermediates formed were analyzed by HPLC. In the presence of Pt supported catalysts (Fig. 6c), a decrease in TOC was detected at time zero, caused by adsorption. It shows that p-coumaric acid is adsorbed strongly on 3%Pt/ZrO2 Mel (33.5% of initial TOC), but very little adsorbed on 3%Pt/ TiO2 P25, as it was on the bare supports (Fig. 6a and b). A higher reaction rate was observed on 3%Pt/ZrO2, due to a higher dispersion of the metallic phase. TOC abatement was then limited because p-hydroxybenzoic acid and aliphatic products from aromatic ring cleavage were not further oxidized. On the other hand, the ruthenium catalysts were able to oxidize all compounds present in the reaction medium at high extent. The TOC conversion at time zero (in fact as shown previously, TOC adsorption) on 3%Ru/ZrO2 Mel and 3%Ru/ TiO2 DT51 from TOC measurements and the carbon loading of catalysts at time zero determined after heating them in the p-coumaric acid aqueous solution under argon give consistent values: the extent of adsorption of initial p-coumaric acid was found to be 28 and 26% on 3%Ru/ZrO2 Mel and 7.4 and 5.8% on 3%Ru/TiO2 DT51 of the initial TOC concentration. Then, after an initial rapid conversion (Fig. 6d) corresponding to the degradation of p-coumaric acid to (CO2 + H2O) and aromatic intermediates, a lower rate of TOC abatement of these intermediates was observed, due to the lower reactivity of the intermediate carboxylic acids. A very good carbon balance was obtained when comparing TOC measured and TOC calculated from HPLC, as exemplified for 3%Ru/ZrO2 Mel (Fig. 7b), indicating that most of the intermediates formed were analysed by HPLC. The carbon content of the used catalysts 3%Ru/TiO2 DT51 and 3%Ru/ZrO2 Mel was also determined. The final carbon loading was 1.6 and 3.1%, respectively, very close to C loading at time zero (0.85 and 3.8%, respectively). It clearly shows that the carbonaceous species are still strongly adsorbed after 7 h of reaction (11.2 and 21.3% of initial TOC). Though this high carbon content, the ruthenium particles are still accessible to the reaction and no deactivation seems to occur. High carbon loadings on Pt/CeO2 and Ru/CeO2 catalysts have also been measured during CWAO of stearic acid, without deactivation [37]. Finally, no metal leaching (Pt and Ru <0.2 mg l 1) was detected in all the treated aqueous solutions. 4. Conclusion CWAO in the presence of noble metal catalysts (Pt, Ru) deposited on TiO2 or ZrO2 seems to be a suitable method to efficiently remove p-coumaric acid, a biorecalcitrant phenolic compound found in olive oil processing, from the aqueous solutions of waste to make more suitable a biological secondary treatment. Some reaction intermediates have been identified. In the suggested mechanism of oxidation, p-coumaric acid reacts quickly through cleavage of the exocyclic double bond. Further degradation of p-hydroxybenzaldehyde is slower with cleavage
of the aromatic ring. Strong adsorption of p-coumaric acid was noted, which depended on the support used. Although there was a high carbonaceous deposit on the oxide supports, the catalysts were active throughout the reaction and high TOC conversion could be obtained. Acknowledgement We are indebted to the European Union for financial support (contract ICA3-2002-10096 ‘‘Novel catalytic Technologies for the Treatment of wastewater from Agro-Food and Industrial Productions in MED Countries’’). References [1] International Olive Oil Council, http://www.internationaloliveoil.org/. [2] M. Hamdi, Bioprocess Eng. 8 (1993) 209. [3] J. Perez, T. de la Rubia, E. Moreno, J. Martinez, Environ. Toxicol. Chem. 11 (1992) 489–495. [4] R. Capasso, A. Evidente, L. Schivo, G. Orru, M. Marchialis, G. Cristinzio, J. Appl. Bacteriol. 79 (1995) 393. [5] A. Rozzi, F. Malpei, Int. Biodeterior. Biodegr. 38 (1996) 135. [6] D. Dalis, K. Anagnostidis, A. Lopez, I. Letsiou, L. Hartmann, Bioresour. Technol. 57 (1996) 237. [7] M. Beccari, F. Bonemazzi, M. Majone, C. Riccardi, Water Res. 30 (1996) 183. [8] F.J. Benitez, J. Beltran-Heredia, J. Torregrosa, J.L. Acero, Toxicol. Environ. Chem. 61 (1997) 173. [9] R. Andreozzi, G. Longo, M. Majone, G. Modesti, Water Res. 32 (1998) 2357. [10] F.J. Beltran, J.F. Garcia-Araya, J. Frades, O. Gimeno, Water Res. 33 (1999) 723. [11] F.J. Rivas, F.J. Beltran, O. Gimeno, J. Frades, J. Agric. Food Chem. 49 (2001) 1873. [12] C. Pulgarin, M. Invernizzi, S. Parra, V. Sarria, R. Polania, P. Peringer, Catal. Today (1999) 341. [13] W. Gernjak, M.I. Maldonado, S. Malato, J. Caceres, T. Krutzler, A. Glaser, R. Bauer, Solar Energy 77 (2004) 567. [14] F.J. Rivas, F.J. Beltran, O. Gimeno, J. Frades, J. Agric. Food Chem. 49 (2001) 1873. [15] A. Fiorentino, A. Gentili, M. Isidori, M. Lavorgna, A. Parrella, F. Temussi, J. Agric. Food Chem. 52 (2004) 5151. [16] M. Chakchouk, M. Hamdi, J.N. Foussard, H. Debellefontaine, Environ. Technol. 15 (1994) 323. [17] F. Luck, Catal. Today 53 (1999) 81. [18] S.T. Kolaczkowski, P. Plucinski, F.J. Beltran, F.J. Rivas, D.B. McLurgh, Chem. Eng. J. 73 (1999) 143. [19] J.C. Be´ziat, M. Besson, P. Gallezot, S. Durecu, J. Catal. 182 (1999) 129. [20] Y.I. Matatov-Meytal, M. Sheintuch, Ind. Eng. Chem. Res. 37 (1998) 309. [21] R. Andreozzi, V. Caprio, M.G. D’Amore, A. Insola, Water Res. 29 (1995) 1. [22] F. Javier Benitez, J. Beltran-Heredia, J.L. Acero, M.L. Pinilla, J. Chem. Technol. Biotechnol. 70 (1997) 253. [23] M.A. Miranda, A.M. Amat, A.A. Arques, Water Sci. Technol. 44 (2001) 325. [24] F. Herrera, C. Pulgarin, V. Nadtochenko, J. Kiwi, Appl. Catal. B 17 (1998) 141. [25] I. Poulios, G. Kyriacou, Environ. Technol. 23 (2002) 179. [26] S. Perhatoner, G. Centi, Top. Catal. 33 (2005) 207. [27] D. Mantzavinos, R. Hellenbrand, I.S. Metcalfe, A.G. Livingston, Water Res. 30 (1996) 2969. [28] D. Mantzavinos, R. Hellenbrand, A.G. Livingston, I. Metcalfe, Appl. Catal. B 7 (1996) 379–396. [29] G. Neri, A. Pistone, C. Milone, S. Galvagno, Appl. Catal. B 38 (2002) 321.
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Degradation of olive oil mill effluents by catalytic wet air oxidation 1. Reactivity of p-coumaric acid over Pt and Ru supported catalysts Doan Pham Minh, Pierre Gallezot, Miche`le Besson * Institut de Recherches sur la Catalyse-CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France Received 12 July 2005; received in revised form 29 August 2005; accepted 2 September 2005 Available online 26 October 2005
Abstract The objective of this work was to investigate the catalytic wet air oxidation of p-coumaric acid, a biorecalcitrant phenolic compound typically found in olive oil wastewaters in the presence of Pt and Ru supported catalysts. The influence of the operating variables were established. The most important intermediates determined by HPLC measurements suggest a rapid attack by oxygen of the side-chain of p-coumaric acid, and the mineralization proceeds through different aromatic compounds reacting further to aliphatic intermediates (mainly acids). Important mineralization yields were achieved in the presence of the catalysts at 140 8C and 50 bar air. The importance of the nature of the support (TiO2, ZrO2) on the adsorption of p-coumaric acid was demonstrated. # 2005 Elsevier B.V. All rights reserved. Keywords: Catalytic wet air oxidation; Platinum and ruthenium heterogeneous catalysts; p-Coumaric acid; Olive oil mill effluents
1. Introduction Olive oil manufacturing is an important economic activity of many countries particularly throughout the Mediterranean Sea (the annual world olive oil production, estimated at about 1.5– 1.7 Mt/year in the eighties, reached around 2.5 Mt/year in the recent seasons) [1]. However, olive oil extraction is one of the most pollution intensive food-processing industries. It involves a high consumption of water and large volumes of strongly polluted wastewaters known as olive mill wastewater (OMW) are generated. The amount of olive oil mill wastewater depends on the milling process, ranging from about 0.6 m3/t of olives processed for classical mills to about 1.7 m3/t olives in centrifugal mills [2]. Furthermore, OMW is characterized by a very high Chemical Oxygen Demand (COD up to 200 g l 1), a high content in phenol-like substances (in the range 1–5 g l 1 measured as phenol) and acidity. Because of its highly phytotoxicity and strong antimicrobial properties [3,4], the classical biological treatment cannot be applied. Alternative appropriate treatments have to be considered for the management of these wastewaters. Most physical and physicochemical
* Corresponding author. Tel.: +33 4 72 44 53 58; fax: +33 4 72 44 53 99. E-mail address: [email protected] (M. Besson). 0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.09.009
methods, such as precipitation, flocculation/clarification, coagulation, filtration, evaporation in open ponds [5] give only partial solution to the problem. Reverse osmosis or ultrafiltration are usually costly. Anaerobic biological digestion with production of biogas is increasingly being used, but is not yet completely satisfactory [6,7]. A possible solution would be the chemical oxidative degradation of OMW used as a pre-treatment process to decrease its toxicity prior to its biological treatment. Ozone alone or combined with hydrogen peroxide or UV radiation [8–10], photo-Fenton treatment [11–13], Fenton’s reagent [14], oxidation with polymer supported FeCl3 [15], wet air oxidation (WAO) with the addition of H2O2 [16] have been tested. WAO is increasingly used for the elimination of organic pollutants in a variety of wastewaters from different chemical plants [17,18]. It consists of oxidizing pollutants under air or oxygen pressure under elevated temperature (200–350 8C) and pressure (50– 150 bar). The organic matter is totally mineralized to CO2 and water or partially oxidized to smaller intermediate products (typically low-molecular-weight organic acids). Catalytic WAO operates at lower temperatures and pressures, in the presence of, e.g., noble metal based catalysts [19,20]. Typical OMW contains high concentrations of tyrosol, cinnamic and/or benzoic acid derivatives. The scope of this work was to investigate the use of catalytic WAO to treat model
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molecules representative of OMW. p-Coumaric acid (trans-phydroxycinnamic acid) was chosen as a representative of the phenolic fraction. Previously, the reactivity of this compound has been studied using ozone [21], UV radiation and combination of ozone and UV radiation [22,23], photo-assisted Fenton reaction [24] or photocatalysis [25]. Fe-ZSM5 catalysts and Fe-containing pillared clays in combination with hydrogen peroxide were found more active than the homogeneous (Fe3+salt) [26]. The wet air oxidation of p-coumaric acid has been investigated without catalyst [27] and using various homogeneous (Fe2+, Cu2+, Zn2+, Co2+) and heterogeneous (CuOZnO-Al2O3) catalysts [28]. Some leaching of the heterogeneous catalyst was measured whose extent was strongly dependent on the operating conditions. Fe- and Zn-promoted ceria prepared by co-precipitation were also investigated in the wet air oxidation [29]. In the present investigation, p-coumaric acid was reacted with air using noble metal supported catalysts. The influence of the operating reaction conditions and of the catalyst on the extent of degradation are reported. 2. Experimental 2.1. Catalyst preparation The supports were commercial titanium and zirconium oxides, known as leaching resistant materials under the reaction conditions [19,30,31]. The textural properties of solids were determined from nitrogen adsorption–desorption isotherms at liquid nitrogen temperature by using a Micromeritics ASAP 2010 instrument. The pH of the point of zero charge (pHPZC) was measured according to the method described in [32]. The characteristics of the supports are given in Table 1. A series of Pt and Ru catalysts were prepared by incipientwetness impregnation of the supports with an aqueous solution of H2PtCl6 or RuCl3 to obtain a 3 wt.% Pt or Ru content. The dried mixture was reduced by heating up in a hydrogen flow (15 l h 1) at 1 8C min 1 up to 300 8C. After cooling and purging with argon, passivation of the catalyst was carried out in a flow of 1%O2/N2.
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grid. Samples were also prepared by the extractive replica procedure. Carbonaceous species deposited on the solid during reaction were quantified by measuring the percentage of carbon at Service Central d’Analyse du CNRS. 2.3. Oxidation reaction CWAO experiments were carried out in a batch reactor made of Hastelloy C22 previously described elsewhere [19]. The 280 ml autoclave is equipped with a magnetically driven stirrer. One hundred and fifty milliliter of an aqueous solution containing the organic pollutant (4.5 mmol l 1 p-coumaric acid, maximum solubility at room temperature) was introduced into the reactor with the catalyst, purged with argon, heated to the desired temperature. After introduction of air at the desired pressure, the stirring speed was set at 1500 rpm. This point was taken as ‘‘zero time’’. Liquid samples were periodically withdrawn from the reactor and analysed. Total organic carbon (TOC) in the supernatant was measured using a Shimadzu TOC 5050A analyser for establishing the degree of conversion. The organic compounds both aromatic compounds and aromatic ring cleavage products (mainly acids) could be analysed after separation of the catalyst by a 0.45 mm Millipore filter, using HPLC and UV-absorption detection at 210 nm, using a CarboSep Coregel-87H3 column. The mobile phase was constituted by a 0.005 M aqueous sulphuric acid solution eluted at a flow rate 0.5 ml min 1. The products were identified by comparison of their retention times with those of authentic compounds. p-Coumaric acid in the starting material gave two products identified as the cis and trans form. It appeared that the trans–cis isomerisation occurred during the preparation of the aqueous solution in the ultrasound bath used to improve solubilization of the substrate in water. It was verified that this process did not cause any oxidation nor bond cleavage of p-coumaric acid. The mass balance for TOC and HPLC results were compared to verify that all intermediates were correctly identified by HPLC. 3. Results and discussion
2.2. Catalyst characterization
3.1. Characterization of catalysts
X-ray diffraction (XRD) of the solids was carried out using a Siemens D5005 diffractometer with Cu Ka1+2 radiation at 0.154184 nm. Transmission electron microscopy (TEM) studies was performed on selected samples using a JEOL-JEM 100 electron microscope. Samples were suspended in ethanol with ultrasonic agitation and dispersed on a carbon-coated copper
The XRD patterns of 3%Pt/TiO2 P25 and 3%Pt/ZrO2 Mel are shown in Fig. 1. The unlabeled peaks were assigned to rutile and anatase in TiO2 and to monoclinic and tetragonal zirconia. In 3%Pt/ZrO2 Mel, the Pt crystallites were unobservable due to their small size, while in 3%Pt/TiO2 P25 the most intense X-ray diffraction peaks due to platinum could be recorded.
Table 1 Characteristics of titanium and zirconium oxides Support
Supplier
SBET (m2 g 1)
Crystalline phase
Mean pore diameter (nm)
pHPZC
TiO2 Eng TiO2 DT51 TiO2 P25 ZrO2 Mel ZrO2 EP
Engelhard Thann and Mulhouse Degussa MEL Chemicals Degussa
50 92 55 90 40
Anatase Anatase Anatase + rutile Monoclinic + tetragonal Monoclinic + tetragonal
20 9 Non-porous 9 Non-porous
6.1 4.4 5.6 6.1 6.3
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Fig. 1. XRD patterns of 3%Pt/ZrO2 Mel (upper curve) and 3%Pt/TiO2 P25 (down curve). The most intense X-ray diffraction peaks due to platinum are recorded.
Fig. 2. Size distribution of supported nanoparticles for 3%Ru/TiO2 DT51, 3%Ru/ZrO2 Mel, as determined from transmission electron microscopy.
TEM photographs of the 3%Ru/TiO2 DT51 and 3%Ru/ZrO2 Mel indicate a relatively narrow size distribution of metal particles dispersed on TiO2 and ZrO2 with an average particle size of 3.5 and 2.9 nm, respectively (Fig. 2). From XRD and TEM results it can be concluded that the catalysts are well dispersed using the incipient-wetness impregnation method. 3.2. Oxidation of p-coumaric acid 3.2.1. Influence of operating variables A series of experiments was conducted at 120, 130, 140 and 150 8C at a total pressure of air of 50 bar over the 2.8%Ru/TiO2
Eng catalyst. Fig. 3 shows the conversion curves for p-coumaric acid (Fig. 3a) and TOC (Fig. 3b) for the experiments performed at these different temperatures, maintaining all the other parameters constant. Degradation rate increased with temperature. It is clear that the treatment of the p-coumaric acid solution in the presence of the catalyst leads to its total disappearance. At 120 and 130 8C, a rapid conversion of p-coumaric acid occurred, with a significant initial conversion of TOC. Prolonged treatment lead to the total disappearance of the initial substrate, but had little effect on TOC abatement (ca. 55%) suggesting that the intermediates formed (dicarboxylic acids, vide infra) are not further oxidized under these reaction conditions. On the other hand, at higher temperatures (140 and 150 8C), the rate of oxidation of pcoumaric acid increased, the intermediates were further oxidized and nearly total conversion of TOC was observed at 140 and 150 8C after 7 and 4 h, respectively. The degradation of p-coumaric acid is a complicated process involving a great number of intermediates. Thus, Fig. 4 shows the concentration profiles for p-coumaric acid and the main intermediate compounds detected versus reaction time at 140 8C, 50 bar air in the presence of the ruthenium catalyst. While oxidation of p-coumaric acid was occurring, the major intermediates formed were p-hydroxybenzaldehyde and p-hydroxybenzoic acid. Low concentrated aromatic intermediates, such as phenol, catechol and hydroquinone, and traces of hydroxylation products such as 3,4-dihydroxycinnamic acid (caffeic acid), 3,4-dihydroxybenzaldehyde and 3,4-dihydroxybenzoic acid were detected. Maleic acid, and, in lower amounts, fumaric acid were formed, as expected from the opening of the aromatic ring of phenol for instance. A wide range of other low concentrated compounds were detected, but not identified. On the basis of our experimental results and previous publications on CWAO of p-coumaric acid [27,28] we established a simplified oxidation mechanism given in Scheme 1. During the oxidative decomposition of p-coumaric acid 1, the initial step is the rapid attack of the very reactive exocyclic double bond to yield p-hydroxybenzaldehyde 2 and glyoxylic acid 3, which can be oxidized to p-hydroxybenzoic acid 4 and oxalic acid 5. 5 is very reactive under these conditions, and when formed it can rapidly be oxidized to carbon dioxide [33,34]. Glyoxylic acid 3 may also be directly oxidized to CO2. Decarboxylation of p-hydroxybenzoic acid 4 to phenol 6 occurs
Fig. 3. Conversion of (a) p-coumaric acid and (b) TOC as a function of time at different temperatures. Catalyst 2.8%Ru/TiO2 Eng, total air pressure 50 bar.
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Fig. 5. Evolution of (a) p-hydroxybenzaldehyde (OH-BZAL) and (b) p-hydroxybenzoic acid (OH-BZOIC) as a function of TOC conversion at various temperatures in the presence of 2.8%Ru/TiO2 Eng. Fig. 4. Concentration profile as a function of time during oxidation of pcoumaric acid in the presence of 2.8%Ru/TiO2 Eng. Reaction conditions: 150 ml of an aqueous solution of 4.5 mmol l 1 p-coumaric acid, 0.5 g catalyst, 140 8C, 50 bar air. COUM, p-coumaric acid; OH-BZAL, p-hydroxybenzaldehyde; OH-BZOIC, p-hydroxybenzoic acid; HQ, hydroquinone; FUM, fumaric acid; MAL, maleic acid; diOH-BZAL, 3,4-dihydroxybenzaldehyde; diOHCINNAM, 3,4-dihydroxycinnamic acid; PhOH, phenol.
and all expected compounds derived from oxidation of phenol (dihydroxylated benzenes 7 and 8, maleic and fumaric acids, oxaloacetic acid, . . .) are further detected. Hydroxylation of pcoumaric acid to 3,4-dihydroxycinnamic acid 9 was also observed. A possible path to 3,4-dihydroxybenzaldehyde 10 and 3,4-dihydroxybenzoic acid 11 is via this intermediate by attack of the C C double bond, rather than by hydroxylation of p-hydroxybenzaldehyde 2 and p-hydroxybenzoic acid 4. Indeed, separate experiments performed on catalytic oxidation of these two latter substrates did not evidence the occurrence of the hydroxylation reaction. Opening of the aromatic ring and further oxidation of the intermediates can happen in any of the phenolic intermediates formed. These observations are in general agreement with the data obtained in the literature for non-catalytic or catalytic wet air oxidation experiments [27,28]. Discrepancies concern p-(1-hydroxyethyl)phenol, an intermediate suggested in these reports, that was not observed in our experiments. We verified that this latter compound synthesized by catalytic hydrogenation of the corresponding ketone exhibited a retention time that did not match any peaks of our chromatograms.
Fig. 5a and b show the concentration of the major intermediates ( p-hydroxybenzaldehyde and p-hydroxybenzoic acid) as a function of TOC conversion at various temperatures. In the range of temperatures considered, no significant differences were observed on the shape of the curves, either for p-hydroxybenzaldehyde or for p-hydroxybenzoic acid. Only an accumulation of the compounds originating from the ring cleavage was observed at the lower temperatures. It is also shown that p-hydroxybenzoic acid can be produced from the oxidation of p-hydroxybenzaldehyde. Some experiments were also conducted to determine the influence of air pressure. In the range 30–50 bar, no effect in either p-coumaric acid removal rate or TOC reduction rate was noticed. Since the cleavage of the exocyclic double bond may be pH sensitive, the catalytic oxidation of p-coumaric acid at 140 8C on a 3%Ru/ZrO2 Mel catalyst has also been performed by increasing the initial pH from 3.3 to 7.9 and 11.1 by adding sodium hydroxide. The differences observed were very small. Whatever the initial pH, p-coumaric acid was rapidly converted to CO2 and p-hydroxybenzaldehyde and p-hydroxybenzoic acid as the major intermediates (similar maxima concentrations ca. 1 mmol l 1). The rate of oxidation of these aromatic compounds to ring opening compounds was slightly higher when the initial pH was basic and yielded higher concentrations of maleic and fumaric acids. The formation of these acids rapidly lowered the value of pH. Finally, similar final TOC conversions were obtained (ca. 90% after 7 h of reaction).
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Scheme 1. Proposed simplified oxidation of p-coumaric acid during CWAO.
3.2.2. Influence of the support and the metal Fig. 6 shows the results of TOC abatement in the presence of the TiO2 or ZrO2 supports without metal deposited on them (Fig. 6a and b, respectively), in the presence of platinum catalysts (Fig. 6c) and of ruthenium catalysts (Fig. 6d). In the absence of catalyst, oxidation of p-coumaric acid started immediately after the addition of air and continues up to total disappearance after 7 h. All intermediates mentioned earlier were detected, while the TOC conversion increased slowly up to 27% (Fig. 6a). The same experiment performed under argon yielded almost no change in TOC detected, indicating that p-coumaric is relatively thermally stable under these conditions and the degradation occuring under air is due to oxidation. In the presence of the TiO2 and ZrO2 supports, it was observed that, after the period of heating under argon to the desired temperature, at time zero, both the concentration of p-coumaric acid (not shown) and the TOC value (Fig. 6a and b) are different from the nominal value introduced in the reactor. This deficit in mass balance in the liquid phase is dependent on the support used for experiment. Thus, an apparent TOC conversion of 3 and 7.2% was observed over TiO2 P25 and TiO2 DT51, respectively, and 13.3 and 26.4% over ZrO2 EP and ZrO2 Mel, respectively. The reason of these differences may be due to the strong adsorption of p-coumaric acid on the solids. Given the point of zero charge of the supports (4.4 and 5.6 for TiO2 DT51 and TiO2 P25, respectively, and 6.1 and 6.3 for ZrO2 Mel and ZrO2 EP, respectively) and the acidic pH of the initial solution (pH 3.3), the surface of the supports is positively
charged (Ti-OH2+ and Zr-OH2+) and should be suitable for adsorption of anion groups. However, at that pH, the carboxyl group should be mainly in the uncharged COOH form (the pKa of –OH and –COOH groups of p-coumaric acid are 9.0 and 4.4, respectively) [35,36]. Thus, other interactions than purely anionic interactions must be invoked, such as hydrogen bonding between the atoms of p-coumaric acid bearing negative charges and the groups at the surface of the support. A much higher amount of TOC was retained on ZrO2 Mel than on TiO2 DT51. This difference cannot be explained on the basis of specific surface area and porosity of the supports which are both of the same order, but on the different pHPZC. To support this interpretation in terms of the high affinity of p-coumaric acid for the oxide surface, two experiments were set up in which the p-coumaric acid solution was heated in the presence of supports TiO2 DT51 or ZrO2 Mel up to 140 8C under argon (as in a standard experiment), but the reaction was stopped before introduction of air (time zero of a standard oxidation reaction). The solids were recovered, washed with water, dried and analysed for carbon content. The percentages of carbon of the supports were measured at 0.29 and 3.38%, which correspond to an extent of adsorption of the initial TOC concentration of 2 and 23.1%, respectively. These data are in good agreement with the analysis of the supernatant solution at time zero (TOC abatement of 7.2 and 26.4%, respectively), showing a good carbon balance. Upon further reaction under air, a small increase of TOC conversion was then observed throughout the course of the reaction with comparable or lower rate than the uncatalysed reaction. The final carbon content of both supports
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Fig. 6. TOC abatement in the presence of (a) TiO2 supports, (b) ZrO2 supports, (c) supported platinum catalysts and (d) ruthenium supported catalysts. Reaction conditions: 150 ml of an aqueous solution of 4.5 mmol l 1 p-coumaric acid, 140 8C, 50 bar air, 0.5 g solid.
after 7 h of reaction (2.21 and 3.68%) correspond to an adsorption of 15.1 and 25.2% of the initial TOC, respectively. In the uncatalysed reaction and in experiments performed with supports, it was also observed that the solution initially colourless became coloured to yellow-brown. Fig. 7a shows a comparison between the profiles of TOC conversion measured
Fig. 7. Comparison of TOC conversion determined from TOC measurements (TOCm) and calculated from HPLC analysis (TOCc) in the presence of (a) ZrO2 Mel and (b) 3%Ru/ZrO2 Mel.
from the analysis of the TOC in solution (TOCm) and the TOC conversion calculated from the organic carbon contribution from all the compounds detected by HPLC (TOCc) for support ZrO2 Mel. Fig. 7b shows the same comparison for the corresponding ruthenium catalyst. The results obtained for ZrO2 Mel (Fig. 7a) show an initial good agreement between TOCm and TOCc, but as reaction proceeds large discrepancies between both are noted. Evidently, intermediates which may be oligomers of p-coumaric acid are formed throughout the reaction under air that are not analysed by HPLC. The results in presence of the other supports lead to the same conclusion. A possible explanation for this behaviour could be the formation of dimers identified as dilactone (Scheme 2) as evidenced by studies on the oxidation of p-coumaric acid by periodate or Fenton reactant [35]. Dimer formation appeared to be related to the fact that the C C double bond is conjugated with the aromatic ring, and conjugation with
Scheme 2. Possible dimer derived from p-coumaric acid in the absence of a metallic catalyst.
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the 4-hydroxyl group may also be important. On the other hand, in the case of ruthenium supported catalyst (Fig. 7b), a good agreement was obtained indicating that most of the intermediates formed were analyzed by HPLC. In the presence of Pt supported catalysts (Fig. 6c), a decrease in TOC was detected at time zero, caused by adsorption. It shows that p-coumaric acid is adsorbed strongly on 3%Pt/ZrO2 Mel (33.5% of initial TOC), but very little adsorbed on 3%Pt/ TiO2 P25, as it was on the bare supports (Fig. 6a and b). A higher reaction rate was observed on 3%Pt/ZrO2, due to a higher dispersion of the metallic phase. TOC abatement was then limited because p-hydroxybenzoic acid and aliphatic products from aromatic ring cleavage were not further oxidized. On the other hand, the ruthenium catalysts were able to oxidize all compounds present in the reaction medium at high extent. The TOC conversion at time zero (in fact as shown previously, TOC adsorption) on 3%Ru/ZrO2 Mel and 3%Ru/ TiO2 DT51 from TOC measurements and the carbon loading of catalysts at time zero determined after heating them in the p-coumaric acid aqueous solution under argon give consistent values: the extent of adsorption of initial p-coumaric acid was found to be 28 and 26% on 3%Ru/ZrO2 Mel and 7.4 and 5.8% on 3%Ru/TiO2 DT51 of the initial TOC concentration. Then, after an initial rapid conversion (Fig. 6d) corresponding to the degradation of p-coumaric acid to (CO2 + H2O) and aromatic intermediates, a lower rate of TOC abatement of these intermediates was observed, due to the lower reactivity of the intermediate carboxylic acids. A very good carbon balance was obtained when comparing TOC measured and TOC calculated from HPLC, as exemplified for 3%Ru/ZrO2 Mel (Fig. 7b), indicating that most of the intermediates formed were analysed by HPLC. The carbon content of the used catalysts 3%Ru/TiO2 DT51 and 3%Ru/ZrO2 Mel was also determined. The final carbon loading was 1.6 and 3.1%, respectively, very close to C loading at time zero (0.85 and 3.8%, respectively). It clearly shows that the carbonaceous species are still strongly adsorbed after 7 h of reaction (11.2 and 21.3% of initial TOC). Though this high carbon content, the ruthenium particles are still accessible to the reaction and no deactivation seems to occur. High carbon loadings on Pt/CeO2 and Ru/CeO2 catalysts have also been measured during CWAO of stearic acid, without deactivation [37]. Finally, no metal leaching (Pt and Ru <0.2 mg l 1) was detected in all the treated aqueous solutions. 4. Conclusion CWAO in the presence of noble metal catalysts (Pt, Ru) deposited on TiO2 or ZrO2 seems to be a suitable method to efficiently remove p-coumaric acid, a biorecalcitrant phenolic compound found in olive oil processing, from the aqueous solutions of waste to make more suitable a biological secondary treatment. Some reaction intermediates have been identified. In the suggested mechanism of oxidation, p-coumaric acid reacts quickly through cleavage of the exocyclic double bond. Further degradation of p-hydroxybenzaldehyde is slower with cleavage
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