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p-Coumaric acid photodegradation with solar light, using a 2,4,6-triphenylpyrylium salt as photosensitizer
Applied Catalysis B: Environmental 23 (1999) 205–214
p-Coumaric acid photodegradation with solar light, using a 2,4,6-triphenylpyrylium salt as photosensitizer A comparison with other oxidation methods Ana Mar´ıa Amat a,1 , Antonio Arques a , Miguel Angel Miranda b,∗ a
b
Departamento de Ingenier´ıa Textil y Papelera, EPSA-UPV, Universidad Politécnica de Valencia, Paseo Viaducto 1, E-03801 Alcoy (Alicante), Spain Departamento de Qu´ımica e Instituto de Tecnolog´ıa Qu´ımica CSIC-UPV, Universidad Politécnica de Valencia, Apartado 22012, E-46071 Valencia, Spain Received 4 February 1999; received in revised form 29 June 1999; accepted 4 July 1999
Abstract p-Coumaric acid has been used as a probe in order to study the effect of solar light catalysed by 2,4,6-triphenylpyrylium salts on phenolic compounds present in olive oil industry wastewaters. The results are very satisfactory, and important degradation yields are achieved. Methylene blue has also been used as a photocatalyst, but it results in slower degradation. Other advanced oxidation methods (ozone and/or UV radiation) have been tested as well; as expected, p-coumaric acid abatement is much faster (100 times), but ozone and UV are dangerous and expensive for industrial uses. In contrast with other phenolic acids, ozone and UV do not show an important synergistic effect in p-coumaric acid oxidation. This could be due to differences in the absorption spectra. Major p-coumaric acid oxidation intermediates have been identified and quantitated by HPLC; on the basis of these data, a reaction mechanism is proposed. ©1999 Elsevier Science B.V. All rights reserved. Keywords: 2,4,6-Triphenylpyrylium; p-Coumaric acid; Photodegradation; Solar light; Oxidation; Ozone; Methylene blue; Wastewaters
1. Introduction Wastewaters from the olive and oil industry have a high concentration of phenolic compounds [1]. These compounds are not only difficult to degrade but also toxic for most micro-organisms present in water [2]. Both olive oil mills and olive table industries produce ∗ Corresponding author. Tel.: +34-96-387-7343; fax: +34-96-387-7349 E-mail addresses: [email protected] (A.M. Amat), [email protected] (M.A. Miranda) 1 Co-corresponding author.
the same kind of contaminants [3]; hence, the problems they cause are similar. As in the countries of the Mediterranean basin these industries are very important [4], the effects of these contaminants are serious. Some specific treatments are being developed. They include biological treatments with suitable micro-organisms [5], ultrafiltration [6], evaporation of water and incineration of dry residues, wet oxidation with oxygen at high pressures and temperatures [7,8] and advanced oxidation with ozone and/or ultraviolet (UV) radiation. Treatment with ozone is being studied by several groups and the results obtained are encouraging.
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Nevertheless, these studies are focused mainly on kinetics [9] and engineering [10], so many aspects of the degradation mechanism of these compounds [11–13] remain to be elucidated. The ozone effects can be enhanced by the use of catalysts like metal oxides, peroxides [14] and UV radiation [15]. It would be important to develop a treatment which could oxidise phenolic compounds using only solar light, air and a catalyst. This would allow to circumvent the use of ozone or UV light. Unfortunately, phenolic compounds do not absorb radiations within the visible range. So a photosensitizer would be needed in order to absorb radiations and act as a catalyst. Some studies have been published dealing with the use of semiconductor substances as photosensitizers [16], Fenton reagent [17,18] or organic compounds [19]. In our research, an organic compound, 2,4,6-triphenylpyrylium hydrogen sulfate, is used instead. This compound has an absorption band in the visible light wavelength, and upon light excitation, can work as an oxidation–reduction catalyst, in a mechanism that involves electron transfer [20,21,22]. In the present work, methylene blue has also been used as a photosensitizer. This compound does not involve electron transfer as the predominating mechanism, but rather energy transfer to oxygen, generating singlet oxygen [19,23]. We have also investigated oxidation with ozone for comparison as the reported results are very satisfactory (Scheme 1). p-Coumaric acid has been chosen as an example of a phenolic acid having the cinnamic acid structure whose degradative oxidation with ozone [11,24] oxygen [7] and radiation [17] has been investigated previously.
2. Experimental 2.1. Reagents and sensitizers p-Coumaric acid (Merck, predominantly trans), methylene blue (Fluka AG) and 2,4,6-triphenylpyrylium hydrogen sulfate (Aldrich) were used as received. In the case of the pyrylium salt, hydrogen sulfate was chosen because of its higher solubility in water.
Oxalic acid, 10, formic acid, oxalacetic acid, 15, glioxilic acid, glioxal, ketomalonic acid, 11, maleic acid, 12, pyruvic acid, malonic acid, 16, protocatechuic acid, 6, protocatechuic aldehyde, 4, p-hydroxybenzoic acid, 5, p-hydroxybenzaldehyde, 3, caffeic acid, 2, hydroquinone and benzoquinone, used as standards, were supplied by Aldrich. Ozone was supplied by a generator (ozogas, TRCE 4000), able to produce up to 8 g/h of ozone when it was fed with oxygen. In our experiments, production was fixed at 4 g/h of ozone and the gas flow was 3 Nl/min. Ozone in the gas stream was determined exactly by reaction with KI and subsequent titration of this solution with sodium thiosulfate.
2.2. Oxidative reactions The experiments were performed with different concentrations of p-coumaric acid. While in most cases, 0.001 M solutions were employed, saturated p-coumaric acid solutions (0.005 M) were more suitable for detecting minor intermediates. Ozonisations were carried out in a glass reactor similar to that described by Ben´ıtez et al. [25] which has approximately a 1 l capacity. The reactor was submerged in a thermostated bath and kept at the desired temperature. For all the experiments, the reactor was charged with 250 ml of p-coumaric acid solution. Periodically, samples were taken through an outlet to monitor the reactions. Eventually, the reactor was equipped with an axial immersion lamp Heraeus TNN 15/35 (low pressure mercury), which emits nearly monochromatic radiation at 254 nm. For the photosensitized reactions, in order to emulate solar light, experiments have been carried out in a glass reactor submerged in a cold water bath irradiated with an Osram Dulux EL lamp, whose emission spectrum is shown in Fig. 1. Solar experiments were conducted between 10 a.m. and 6 p.m. in order to achieve higher reproducibility in our results. Samples were analysed by HPLC (Perkin–Elmer Autosystem XL with diode-array detector and Split injector). Identification and quantitations were achieved by comparison with standards. HPLC is not able to resolve 10, glioxal, 11 and glioxilic acid. So their concentrations are given together and expressed as 10.
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Scheme 1.
Fig. 1. Emission spectrum of the Dulux Osram lamp.
3. Results and discussion
Fig. 2. p-Coumaric acid photodegradation with visible light (solar simulator) catalysed by pyrylium salt: N total p-coumaric acid, 䉬 trans p-coumaric acid, 䊏 cis p-coumaric acid.
3.1. Pyrylium salt As stated above [20–22], pyrylium salts have proved to be good catalysts for photochemical reactions. So we have tried it out to degrade p-coumaric acid, a phenolic compound present in olive industry wastewaters. First of all, an experiment was carried out with the solar simulator, which produces polychromatic visible light. A 0.001 M solution of p-coumaric acid, containing 2 mg of pyrylium salt (5% weight with respect to p-coumaric acid) was irradiated. In order to keep the desired temperature, the reactor was submerged in a cold water bath. Magnetic stirring was maintained throughout the experiment. As shown in Fig. 2, 38% of p-coumaric acid reacted after 8 h. Very low concentrations of 3 and 10 were detected and cis/trans isomerisation occurred during irradiation. This process was observed in all other photochemical reactions. As the results seemed to be encouraging, we decided to use solar light. Three different conditions were
Fig. 3. p-Coumaric acid photodegradation with solar light after 1 day under different conditions: A, absence of air, B, with no stirring, C, with magnetic stirring.
employed: (1) with magnetic stirring, (2) with no stirring and (3) in the absence of air. Results of p-coumaric acid abatement under all three conditions are shown in Fig. 3. These data show that oxygen is needed to oxidise p-coumaric acid as no reaction occurred in the
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Fig. 5. p-Coumaric photodegradation with solar light using 5% (weight) of pyrylium salt as a catalyst. Different initial concentrations of p-coumaric acid: 䉬0.0001 M, 䊏0.001 M, N0.005 M. Fig. 4. p-Coumaric acid photodegradation with solar light using pyrylium salt as a catalyst. Comparison with blanks; A, with no stirring, B, with magnetic stirring. A0 and B0 are the corresponding blanks without pyrylium salt.
absence of air. In the other two experiments, the results obtained did not seem to be significantly different, although the availability of air is not the same logically. A possible explanation of this behaviour could be the following: the rate of decomposition of p-coumaric acid is so slow that the air diffusing into the solution is enough to supply the oxygen consumed in the reaction. For experiments 1 and 2, controls were run. They involved solar irradiation of 250 ml of 0.001 M solution of p-coumaric acid, but without pyrylium salt. As the data in Fig. 4 indicate, the results were significantly different; this clearly shows the catalytic power of pyrylium salt. Different experiments have been performed changing p-coumaric acid concentration and keeping pyrylium salt always at 5% (weight) of p-coumaric acid. Data are shown in Fig. 5. The more diluted the solution was, the higher the rate of abatement achieved was. As the actual reagents are photons produced by sun, even lower concentrations of pyrilium salt acting in a catalytic way are able to bring about reaction. This explains why even if the substrate concentration is lower, the number of reacted substrate molecules can be the same and hence the percentage of reaction higher. In this context, it is worth mentioning that the usual concentrations of phenolic compounds in olive/oil industrial wastewaters is in the range of 100 mg/l [26]. In order to check the catalytic power of pyrylium salt, the photosensitizer/substrate ratio was reduced
Fig. 6. p-Coumaric photodegradation with solar light using pyrylium salt as a catalyst. Different concentrations of pyrylium salt: 䉬5%, 䊏2%, N1%, ×0.5%, 䊉 0.1%.
from 5 to 0.1%. Results shown in Fig. 6 prove that catalytic power decreases slowly only until 1% of pyrylium salt, but then, drops sharply at lower concentration. Intermediates formed during p-coumaric acid decomposition have also been detected. The major ones are represented in Fig. 7. In order to detect intermediates, saturated concentrations (0.005 M) were prepared, although the reaction was much slower under these conditions. 2 and 6 were also detected but we were not able to quantify them because of the low amounts formed. 10, 11 and glioxalic acid are given together, and their concentrations are expressed as 10. No substantial amounts of any intermediate were accumulated, and concentrations suffered very few changes while reaction was occurring. This can also be due to the slow rate of reaction of p-coumaric acid. Intermediates were formed very slowly, and as they were formed, they reacted, keeping a low constant concentration.
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Fig. 7. Intermediates of p-coumaric acid (0.005 M) photodegradation with solar light catalysed by pyrylium salt. 䉬 Oxalic acid, 䊏 maleic acid, N protocatechuic aldehyde, × p-hydroxybenzoic acid, 䊉 p-hydroxybenzaldehyde.
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Fig. 8. p-Coumaric acid abatement with solar light catalysed by methylene blue. Comparison with pyrylium salt. A, pyrylium salt. B, methylene blue.
The behaviour of this compound in neutral media has also been studied (pH of p-coumaric solution is around 3), but pyrylium salt was labile at this pH and the solution bleached. 3.2. Methylene blue Methylene blue can also act as a photosensitizer, but the mechanism of action is completely different. While pyrylium salt catalysis involves electron transfer, methylene blue acts predominantly via energy transfer to oxygen, generating singlet oxygen [19,23]. Solutions of approximately 0.001 M of p-coumaric acid were prepared and 5% methylene blue was added. 250 ml of the solution was employed for each experiment. The solution was submitted to solar exposure in order to check whether the sun can really decompose the compound. The results are shown in Fig. 8. From all these data, it is clear that methylene blue also gives rise to the decomposition of p-coumaric acid, but to a lower extent than pyrylium salt. So catalysis involving only generation of singlet oxygen appears to be effective, but less effective than electron transfer. Also, in this case, diluted solutions reacted much faster than saturated ones, as can be seen in Fig. 9. 3.3. Ozonisation of p-coumaric acid p-Coumaric acid was subjected to oxidation processes employing either ozone or UV radiation, and
Fig. 9. p-Coumaric photodegradiation with solar light using 5% (weight) of methylene blue as a catalyst. Different initial concentrations of p-coumaric acid: 䉬0.0001 M, 䊏0.001 M, N0.005 M
Fig. 10. p-Coumaric acid (0.001 M) abatement with ozone and/or UV radiation under different conditions, 䊊 ozone in acidic media, 䊏 UV in acidic media, 4 ozone and UV in acidic media, × ozone in basic media, 䊐 UV in basic media, 䊉 ozone and UV in basic media.
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Fig. 11. p-Coumaric acid abatement with ozone in acidic media: 䊐 formic acid, + maleic acid, 䊊 p-hydroxybenzaldehyde, – p-coumaric acid, 䊏 protocatechuic aldehyde, × p-hydroxybenzoic acid, 䊉 protocatechuic acid, 䉬 oxalic acid.
Fig. 12. p-Coumaric acid abatement with ozone and UV in acidic media. 䊐 formic acid, + maleic acid, 䊊 p-hydroxybenzaldehyde, – p-coumaric acid, 䊏 protocatechuic aldehyde, × p-hydroxybenzoic acid, 䊉 protocatechuic acid, 䉬 oxalic acid.
also a combination of both. Two series of experiments were performed, employing 250 ml of p-coumaric acid at two different concentrations: 0.005 M (to detect intermediates) and 0.001 M (to compare with the photosensitized experiments). Phenolic compounds are known to react faster with ozone in basic media than under acidic conditions [9,14,15,24,25,27]. So the experiments were performed under the natural acidic conditions of the solution (pH = 3.5) and at buffered basic pH (pH = 9). The following six sets were chosen: (a) ozone in acidic media, (b) UV in acidic media, (c) ozone/UV in acidic media, (d) ozone in basic media, (e) UV in basic media, (f) ozone/UV in basic media. Results obtained for 0.001 M solutions are shown in Fig. 10. p-Coumaric acid abatement with UV radi-
ation and ozone was much faster than when solar light and a photosensitizer were used. This was in general agreement with the expectations as the former conditions are more aggressive than the latter. The extent of p-coumaric acid degradation submitted to UV radiation without ozone (experiments b and e) was very low. Results were similar in acid and basic media. In both cases, concentrations of intermediates were very low. Data obtained in experiments a, c, d and f, employing 0.005 M solutions, are shown in Figs. 11–14. Ben´ıtez et al. [14,15] indicate that 6 and vanillic acid suffer a considerable abatement when they are irradiated with UV light (254 nm) and that when ozone and UV are combined, the reaction is strongly enhanced. These very authors have recently studied the effect of
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Fig. 13. p-Coumaric acid abatement with ozone in basic media. 䊐 caffeic acid, + maleic acid, 䊊 p-hidroxibenzaldehyde, – p-coumaric acid, 䊏 protocatechuic aldehyde, × p-hydroxybenzoic acid, 䊉 protocatechuic acid, 䉬 oxalic acid.
Fig. 14. p-Coumaric acid abatement with ozone and UV in basic media. 䉬 oxalic acid, 䊏 formic acid, N maleic acid, × protocatechuic aldehyde, 䊉 p-hydroxybenzaldehyde, - - caffeic acid, + p-coumaric acid.
UV/ozone on p-coumaric acid [24], and very small enhancement was achieved. In our experiments, UV light produced no noticeable effect on p-coumaric acid, and the results obtained with ozone and ozone/UV were not significantly different. This fact can be explained if we look at the UV spectra of all three acids and 3, the mayor intermediate in p-coumaric acid abatement (Fig. 15): vanillic acid and 6 have an absorption maximum at 260 nm, and the absorption at 254 nm is very high. On the other hand, p-coumaric acid has a minimum at 250 nm and very low absorption at 254 nm. All these data show that the phenolic compound has to absorb UV light in order to reach an activated state, and then, it reacts with ozone. The other possible mechanism [28,29], decomposition of ozone in • OH radicals caused by UV reaction and the attack of this radical on the phenolic compound, does not appear to fit in with the obtained data as, in this case, the
abatement of phenolic compound would not depend on its absorption spectrum. 3.4. Mechanism of the reaction With all the data obtained, we propose a mechanism for p-coumaric acid oxidative decomposition with ozone and UV and visible light irradiation. As the intermediates detected in all these experiences are the same, we assume that only one mechanism is needed to explain this reaction. Thus, the different reaction rates would account for the presence of higher or lower amounts of every compound under different conditions. Before the opening of the aromatic ring, three kind of reactions can happen: cleavage of exocyclic double bond, hydroxylation of the aromatic ring and oxidation of aldehydes to carboxylic acids (Scheme 2).
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Fig. 15. UV spectra of several phenolic compounds. — p-hydroxybenzaldehyde, — — protocatechuic acid, - - - p-coumaric acid. The spectrum of vanillic acid closely matches that of protocatechuic acid.
In acidic media, quantitative cleavage of exocyclic double bond occurs, to give 3. Then, this compound can be oxidized to 5, or hydroxylated to yield 4 or 6. In basic media, formation of 2 is observed, and the amounts of 3 are smaller than in acidic media. This fact can be due to the difference in the behaviour of ozone in acidic and basic media. Under acidic conditions, ozone is a stable compound and can easily give addition reactions and rupture of double bonds; attack on the aromatic ring is unlikely. As a consequence, 3 is easily formed but it is difficult to attack. So an accumulation of this compound occurs. On the other hand, ozone decomposes in basic media to give • OH radicals. Attack on the aromatic ring is easier for these radicals, and then, faster hydroxylation of aromatic ring occurs; this explains the formation of 2. p-Hydroxybenzaldehyde concentration is smaller as it reacts faster than in acidic media. The formation of • OH radicals also explains the higher reaction rates in basic media. Opening of the aromatic ring can happen in any of the aromatic intermediates formed. We assume that rupture of the C–C bond between the two carbon atoms linked to an OH group (C3 –C4 ) is easier because they are in a higher oxidation state. This would lead to compound 7 (Scheme 3). Compound 7 has two double bonds, and both of them can be broken to give 8 or 9 and 10 (Scheme 4). Both compounds 8 and 9 can undergo decarboxylation to give 12, and carbon dioxide, or rupture of double bond. In this case, 11 and 10 can be obtained (Scheme 5).
Scheme 2.
Scheme 3.
Finally, the decarboxylation of 11 and the rupture of the double bond of 12 would give 10, which in turn can be oxidized to two carbon dioxide molecules. Another possibility would be the opening up of the aromatic rings between C2 and C3 [11]. In this case, 15, 16 and piruvic acid, 17 should be formed. No 16 or 17 were found and only very small amounts of 15 were detected (Scheme 6).
4. Conclusions Solar light and air are able to degrade p-coumaric acid when pyrylium salts are present in the solution. Although the reaction is clearly slower than when ozone and UV radiation are used, the method is less expensive and hazardous as well as
A.M. Amat et al. / Applied Catalysis B: Environmental 23 (1999) 205–214
Scheme 4.
Scheme 5.
Scheme 6.
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environment-friendly. Hence, it deserves further investigations to check its generality, scope and limitations.
Acknowledgements Antonio Arques wants to thank Instituto de Cultura Juan Gil Albert for financial support. References [1] A. Cicheli, M. Solinas, Riv. Merceol. 23 (1984) 55–69. [2] M. Hamdi, J.L. Garc´ıa, R. Ellouz, Bioprocess Eng. 8 (1992) 79–84. [3] P. Garc´ıa, A. Garrido, A. Chakman, J.P. Lemonier, R.P. Overend, E. Chornet, Grasas y aceites 40 (1989) 291–295. [4] D. Ryan, K. Robards, Analyst 123 (1998) 31–44. [5] R. Borja, A. Garrido, M.M. Durán, Grasas y aceites 43 (1992) 317–321. [6] A. Garrido, P. Garc´ıa, M. Brenes, J. Food Eng. 17 (1992) 291–305. [7] D. Matzavinos, R. Hellenbrand, I.S. Metcalfe, A.G. Livingston, Water Res. 30 (1996) 2969–2976. [8] D. Matzavinos, R. Hellenbrand, I.S. Metcalfe, A.G. Livingston, Chem. Eng. Res. Des. 75 (1997) 87–91. [9] F.J. Ben´ıtez, J. Beltrán-Heredia, J.L. Acero, M.L. Pinilla, Ind. Eng. Chem. Res. 36 (1997) 638–644. [10] P. Bondioli, A. Lanzani, E. Fedeli, M. Sala, G. Gerali, Riv. Ital. Sostanze Grasse 69 (1992) 487–497. [11] R. Andreozzi, V. Caprio, M.G. D’Amore, A. Insola, Water Res. 29 (1995) 1–6.
[12] J.B. Gould, W.J. Weber, J. WPCF 48 (1976) 47–60. [13] Y. Yamamoto, E. Niki, H. Shiokawa, Y. Kamiya, J. Org. Chem. 44 (1979) 2137–2142. [14] F.J. Benitez, J. Beltrán-Heredia, J.L. Acero, T. Gonzalez, Water Res. 30(7) (1996) 1597–1604. [15] F.J. Ben´ıtez, J. Beltrán-Heredia, J.L. Acero, T. González, Toxicol. Environ. Chem. 47 (1995) 141–153. [16] P.A.S.S. Marques, M.F. Rosa, F. Mendes, M. Collares Pereira, J. Blanco, S. Malato, Desalination 108 (1997) 213–220. [17] F. Herrera, C. Pulgarin, V. Nadtochenko, J. Kiwi, Appl. Catal. B 17 (1998) 141–156. [18] R.F.P. Nogueira, R.M. Alberici, W.F. Jardim, Chem. Oxid. 6 (1996) 221–230. [19] R. Gerdes, D. Wohrle, W. Spiller, G. Schneider, G. Schnurpfeil, G. Schulz-Ekloff, J. Photochem. Photobiol. A 111 (1997) 65–74. [20] M.A. Miranda, H. Garc´ıa, Chem. Rev. 94 (1994) 1063–1089. [21] A. Sanjuán, M. Alvaro, G. Aguirre, H. Garc´ıa, J.C. Scaiano, J. Am. Chem. Soc. 120 (1998) 7351–7352. [22] A. Sanjuán, G. Aguirre, M. Alvaro, H. Garc´ıa, Appl. Catal. B 15 (1998) 247–257. [23] A.E.H. Machado, A.J. Gomes, C.M.F. Campos, M.G.H. Terrones, D.S. Perez, R. Ruggiero, A. Castellan, J. Photochem. Photobiol. A 110 (1997) 99–106. [24] F.J. Benitez, J. Beltran-Heredia, J.L. Acero, M. Pinilla, J. Chem. Technol. Biotechnol. 70 (1997) 253–260. [25] F.J. Benitez, J. Beltran-Heredia, J.L. Acero, T. González, Toxicol. Environ. Chem. 46 (1994) 37–47. [26] V. Balice, O. Cera, Grasas y aceites 35 (1984) 178–180. [27] L. Calvosa, A. Monteverdi, B. Rindoni, G. Riva, Water Res. 25 (1991) 985–992. [28] N. Takahasi, Ozone Sci. Eng. 12 (1989) 1–18. [29] W.H. Glaze, J.W. Kang, D.H. Chapin, Ozone Sci. Eng. 9 (1987) 335–352.
p-Coumaric acid photodegradation with solar light, using a 2,4,6-triphenylpyrylium salt as photosensitizer A comparison with other oxidation methods Ana Mar´ıa Amat a,1 , Antonio Arques a , Miguel Angel Miranda b,∗ a
b
Departamento de Ingenier´ıa Textil y Papelera, EPSA-UPV, Universidad Politécnica de Valencia, Paseo Viaducto 1, E-03801 Alcoy (Alicante), Spain Departamento de Qu´ımica e Instituto de Tecnolog´ıa Qu´ımica CSIC-UPV, Universidad Politécnica de Valencia, Apartado 22012, E-46071 Valencia, Spain Received 4 February 1999; received in revised form 29 June 1999; accepted 4 July 1999
Abstract p-Coumaric acid has been used as a probe in order to study the effect of solar light catalysed by 2,4,6-triphenylpyrylium salts on phenolic compounds present in olive oil industry wastewaters. The results are very satisfactory, and important degradation yields are achieved. Methylene blue has also been used as a photocatalyst, but it results in slower degradation. Other advanced oxidation methods (ozone and/or UV radiation) have been tested as well; as expected, p-coumaric acid abatement is much faster (100 times), but ozone and UV are dangerous and expensive for industrial uses. In contrast with other phenolic acids, ozone and UV do not show an important synergistic effect in p-coumaric acid oxidation. This could be due to differences in the absorption spectra. Major p-coumaric acid oxidation intermediates have been identified and quantitated by HPLC; on the basis of these data, a reaction mechanism is proposed. ©1999 Elsevier Science B.V. All rights reserved. Keywords: 2,4,6-Triphenylpyrylium; p-Coumaric acid; Photodegradation; Solar light; Oxidation; Ozone; Methylene blue; Wastewaters
1. Introduction Wastewaters from the olive and oil industry have a high concentration of phenolic compounds [1]. These compounds are not only difficult to degrade but also toxic for most micro-organisms present in water [2]. Both olive oil mills and olive table industries produce ∗ Corresponding author. Tel.: +34-96-387-7343; fax: +34-96-387-7349 E-mail addresses: [email protected] (A.M. Amat), [email protected] (M.A. Miranda) 1 Co-corresponding author.
the same kind of contaminants [3]; hence, the problems they cause are similar. As in the countries of the Mediterranean basin these industries are very important [4], the effects of these contaminants are serious. Some specific treatments are being developed. They include biological treatments with suitable micro-organisms [5], ultrafiltration [6], evaporation of water and incineration of dry residues, wet oxidation with oxygen at high pressures and temperatures [7,8] and advanced oxidation with ozone and/or ultraviolet (UV) radiation. Treatment with ozone is being studied by several groups and the results obtained are encouraging.
0926-3373/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 9 9 ) 0 0 0 8 0 - 6
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Nevertheless, these studies are focused mainly on kinetics [9] and engineering [10], so many aspects of the degradation mechanism of these compounds [11–13] remain to be elucidated. The ozone effects can be enhanced by the use of catalysts like metal oxides, peroxides [14] and UV radiation [15]. It would be important to develop a treatment which could oxidise phenolic compounds using only solar light, air and a catalyst. This would allow to circumvent the use of ozone or UV light. Unfortunately, phenolic compounds do not absorb radiations within the visible range. So a photosensitizer would be needed in order to absorb radiations and act as a catalyst. Some studies have been published dealing with the use of semiconductor substances as photosensitizers [16], Fenton reagent [17,18] or organic compounds [19]. In our research, an organic compound, 2,4,6-triphenylpyrylium hydrogen sulfate, is used instead. This compound has an absorption band in the visible light wavelength, and upon light excitation, can work as an oxidation–reduction catalyst, in a mechanism that involves electron transfer [20,21,22]. In the present work, methylene blue has also been used as a photosensitizer. This compound does not involve electron transfer as the predominating mechanism, but rather energy transfer to oxygen, generating singlet oxygen [19,23]. We have also investigated oxidation with ozone for comparison as the reported results are very satisfactory (Scheme 1). p-Coumaric acid has been chosen as an example of a phenolic acid having the cinnamic acid structure whose degradative oxidation with ozone [11,24] oxygen [7] and radiation [17] has been investigated previously.
2. Experimental 2.1. Reagents and sensitizers p-Coumaric acid (Merck, predominantly trans), methylene blue (Fluka AG) and 2,4,6-triphenylpyrylium hydrogen sulfate (Aldrich) were used as received. In the case of the pyrylium salt, hydrogen sulfate was chosen because of its higher solubility in water.
Oxalic acid, 10, formic acid, oxalacetic acid, 15, glioxilic acid, glioxal, ketomalonic acid, 11, maleic acid, 12, pyruvic acid, malonic acid, 16, protocatechuic acid, 6, protocatechuic aldehyde, 4, p-hydroxybenzoic acid, 5, p-hydroxybenzaldehyde, 3, caffeic acid, 2, hydroquinone and benzoquinone, used as standards, were supplied by Aldrich. Ozone was supplied by a generator (ozogas, TRCE 4000), able to produce up to 8 g/h of ozone when it was fed with oxygen. In our experiments, production was fixed at 4 g/h of ozone and the gas flow was 3 Nl/min. Ozone in the gas stream was determined exactly by reaction with KI and subsequent titration of this solution with sodium thiosulfate.
2.2. Oxidative reactions The experiments were performed with different concentrations of p-coumaric acid. While in most cases, 0.001 M solutions were employed, saturated p-coumaric acid solutions (0.005 M) were more suitable for detecting minor intermediates. Ozonisations were carried out in a glass reactor similar to that described by Ben´ıtez et al. [25] which has approximately a 1 l capacity. The reactor was submerged in a thermostated bath and kept at the desired temperature. For all the experiments, the reactor was charged with 250 ml of p-coumaric acid solution. Periodically, samples were taken through an outlet to monitor the reactions. Eventually, the reactor was equipped with an axial immersion lamp Heraeus TNN 15/35 (low pressure mercury), which emits nearly monochromatic radiation at 254 nm. For the photosensitized reactions, in order to emulate solar light, experiments have been carried out in a glass reactor submerged in a cold water bath irradiated with an Osram Dulux EL lamp, whose emission spectrum is shown in Fig. 1. Solar experiments were conducted between 10 a.m. and 6 p.m. in order to achieve higher reproducibility in our results. Samples were analysed by HPLC (Perkin–Elmer Autosystem XL with diode-array detector and Split injector). Identification and quantitations were achieved by comparison with standards. HPLC is not able to resolve 10, glioxal, 11 and glioxilic acid. So their concentrations are given together and expressed as 10.
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Scheme 1.
Fig. 1. Emission spectrum of the Dulux Osram lamp.
3. Results and discussion
Fig. 2. p-Coumaric acid photodegradation with visible light (solar simulator) catalysed by pyrylium salt: N total p-coumaric acid, 䉬 trans p-coumaric acid, 䊏 cis p-coumaric acid.
3.1. Pyrylium salt As stated above [20–22], pyrylium salts have proved to be good catalysts for photochemical reactions. So we have tried it out to degrade p-coumaric acid, a phenolic compound present in olive industry wastewaters. First of all, an experiment was carried out with the solar simulator, which produces polychromatic visible light. A 0.001 M solution of p-coumaric acid, containing 2 mg of pyrylium salt (5% weight with respect to p-coumaric acid) was irradiated. In order to keep the desired temperature, the reactor was submerged in a cold water bath. Magnetic stirring was maintained throughout the experiment. As shown in Fig. 2, 38% of p-coumaric acid reacted after 8 h. Very low concentrations of 3 and 10 were detected and cis/trans isomerisation occurred during irradiation. This process was observed in all other photochemical reactions. As the results seemed to be encouraging, we decided to use solar light. Three different conditions were
Fig. 3. p-Coumaric acid photodegradation with solar light after 1 day under different conditions: A, absence of air, B, with no stirring, C, with magnetic stirring.
employed: (1) with magnetic stirring, (2) with no stirring and (3) in the absence of air. Results of p-coumaric acid abatement under all three conditions are shown in Fig. 3. These data show that oxygen is needed to oxidise p-coumaric acid as no reaction occurred in the
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Fig. 5. p-Coumaric photodegradation with solar light using 5% (weight) of pyrylium salt as a catalyst. Different initial concentrations of p-coumaric acid: 䉬0.0001 M, 䊏0.001 M, N0.005 M. Fig. 4. p-Coumaric acid photodegradation with solar light using pyrylium salt as a catalyst. Comparison with blanks; A, with no stirring, B, with magnetic stirring. A0 and B0 are the corresponding blanks without pyrylium salt.
absence of air. In the other two experiments, the results obtained did not seem to be significantly different, although the availability of air is not the same logically. A possible explanation of this behaviour could be the following: the rate of decomposition of p-coumaric acid is so slow that the air diffusing into the solution is enough to supply the oxygen consumed in the reaction. For experiments 1 and 2, controls were run. They involved solar irradiation of 250 ml of 0.001 M solution of p-coumaric acid, but without pyrylium salt. As the data in Fig. 4 indicate, the results were significantly different; this clearly shows the catalytic power of pyrylium salt. Different experiments have been performed changing p-coumaric acid concentration and keeping pyrylium salt always at 5% (weight) of p-coumaric acid. Data are shown in Fig. 5. The more diluted the solution was, the higher the rate of abatement achieved was. As the actual reagents are photons produced by sun, even lower concentrations of pyrilium salt acting in a catalytic way are able to bring about reaction. This explains why even if the substrate concentration is lower, the number of reacted substrate molecules can be the same and hence the percentage of reaction higher. In this context, it is worth mentioning that the usual concentrations of phenolic compounds in olive/oil industrial wastewaters is in the range of 100 mg/l [26]. In order to check the catalytic power of pyrylium salt, the photosensitizer/substrate ratio was reduced
Fig. 6. p-Coumaric photodegradation with solar light using pyrylium salt as a catalyst. Different concentrations of pyrylium salt: 䉬5%, 䊏2%, N1%, ×0.5%, 䊉 0.1%.
from 5 to 0.1%. Results shown in Fig. 6 prove that catalytic power decreases slowly only until 1% of pyrylium salt, but then, drops sharply at lower concentration. Intermediates formed during p-coumaric acid decomposition have also been detected. The major ones are represented in Fig. 7. In order to detect intermediates, saturated concentrations (0.005 M) were prepared, although the reaction was much slower under these conditions. 2 and 6 were also detected but we were not able to quantify them because of the low amounts formed. 10, 11 and glioxalic acid are given together, and their concentrations are expressed as 10. No substantial amounts of any intermediate were accumulated, and concentrations suffered very few changes while reaction was occurring. This can also be due to the slow rate of reaction of p-coumaric acid. Intermediates were formed very slowly, and as they were formed, they reacted, keeping a low constant concentration.
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Fig. 7. Intermediates of p-coumaric acid (0.005 M) photodegradation with solar light catalysed by pyrylium salt. 䉬 Oxalic acid, 䊏 maleic acid, N protocatechuic aldehyde, × p-hydroxybenzoic acid, 䊉 p-hydroxybenzaldehyde.
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Fig. 8. p-Coumaric acid abatement with solar light catalysed by methylene blue. Comparison with pyrylium salt. A, pyrylium salt. B, methylene blue.
The behaviour of this compound in neutral media has also been studied (pH of p-coumaric solution is around 3), but pyrylium salt was labile at this pH and the solution bleached. 3.2. Methylene blue Methylene blue can also act as a photosensitizer, but the mechanism of action is completely different. While pyrylium salt catalysis involves electron transfer, methylene blue acts predominantly via energy transfer to oxygen, generating singlet oxygen [19,23]. Solutions of approximately 0.001 M of p-coumaric acid were prepared and 5% methylene blue was added. 250 ml of the solution was employed for each experiment. The solution was submitted to solar exposure in order to check whether the sun can really decompose the compound. The results are shown in Fig. 8. From all these data, it is clear that methylene blue also gives rise to the decomposition of p-coumaric acid, but to a lower extent than pyrylium salt. So catalysis involving only generation of singlet oxygen appears to be effective, but less effective than electron transfer. Also, in this case, diluted solutions reacted much faster than saturated ones, as can be seen in Fig. 9. 3.3. Ozonisation of p-coumaric acid p-Coumaric acid was subjected to oxidation processes employing either ozone or UV radiation, and
Fig. 9. p-Coumaric photodegradiation with solar light using 5% (weight) of methylene blue as a catalyst. Different initial concentrations of p-coumaric acid: 䉬0.0001 M, 䊏0.001 M, N0.005 M
Fig. 10. p-Coumaric acid (0.001 M) abatement with ozone and/or UV radiation under different conditions, 䊊 ozone in acidic media, 䊏 UV in acidic media, 4 ozone and UV in acidic media, × ozone in basic media, 䊐 UV in basic media, 䊉 ozone and UV in basic media.
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Fig. 11. p-Coumaric acid abatement with ozone in acidic media: 䊐 formic acid, + maleic acid, 䊊 p-hydroxybenzaldehyde, – p-coumaric acid, 䊏 protocatechuic aldehyde, × p-hydroxybenzoic acid, 䊉 protocatechuic acid, 䉬 oxalic acid.
Fig. 12. p-Coumaric acid abatement with ozone and UV in acidic media. 䊐 formic acid, + maleic acid, 䊊 p-hydroxybenzaldehyde, – p-coumaric acid, 䊏 protocatechuic aldehyde, × p-hydroxybenzoic acid, 䊉 protocatechuic acid, 䉬 oxalic acid.
also a combination of both. Two series of experiments were performed, employing 250 ml of p-coumaric acid at two different concentrations: 0.005 M (to detect intermediates) and 0.001 M (to compare with the photosensitized experiments). Phenolic compounds are known to react faster with ozone in basic media than under acidic conditions [9,14,15,24,25,27]. So the experiments were performed under the natural acidic conditions of the solution (pH = 3.5) and at buffered basic pH (pH = 9). The following six sets were chosen: (a) ozone in acidic media, (b) UV in acidic media, (c) ozone/UV in acidic media, (d) ozone in basic media, (e) UV in basic media, (f) ozone/UV in basic media. Results obtained for 0.001 M solutions are shown in Fig. 10. p-Coumaric acid abatement with UV radi-
ation and ozone was much faster than when solar light and a photosensitizer were used. This was in general agreement with the expectations as the former conditions are more aggressive than the latter. The extent of p-coumaric acid degradation submitted to UV radiation without ozone (experiments b and e) was very low. Results were similar in acid and basic media. In both cases, concentrations of intermediates were very low. Data obtained in experiments a, c, d and f, employing 0.005 M solutions, are shown in Figs. 11–14. Ben´ıtez et al. [14,15] indicate that 6 and vanillic acid suffer a considerable abatement when they are irradiated with UV light (254 nm) and that when ozone and UV are combined, the reaction is strongly enhanced. These very authors have recently studied the effect of
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Fig. 13. p-Coumaric acid abatement with ozone in basic media. 䊐 caffeic acid, + maleic acid, 䊊 p-hidroxibenzaldehyde, – p-coumaric acid, 䊏 protocatechuic aldehyde, × p-hydroxybenzoic acid, 䊉 protocatechuic acid, 䉬 oxalic acid.
Fig. 14. p-Coumaric acid abatement with ozone and UV in basic media. 䉬 oxalic acid, 䊏 formic acid, N maleic acid, × protocatechuic aldehyde, 䊉 p-hydroxybenzaldehyde, - - caffeic acid, + p-coumaric acid.
UV/ozone on p-coumaric acid [24], and very small enhancement was achieved. In our experiments, UV light produced no noticeable effect on p-coumaric acid, and the results obtained with ozone and ozone/UV were not significantly different. This fact can be explained if we look at the UV spectra of all three acids and 3, the mayor intermediate in p-coumaric acid abatement (Fig. 15): vanillic acid and 6 have an absorption maximum at 260 nm, and the absorption at 254 nm is very high. On the other hand, p-coumaric acid has a minimum at 250 nm and very low absorption at 254 nm. All these data show that the phenolic compound has to absorb UV light in order to reach an activated state, and then, it reacts with ozone. The other possible mechanism [28,29], decomposition of ozone in • OH radicals caused by UV reaction and the attack of this radical on the phenolic compound, does not appear to fit in with the obtained data as, in this case, the
abatement of phenolic compound would not depend on its absorption spectrum. 3.4. Mechanism of the reaction With all the data obtained, we propose a mechanism for p-coumaric acid oxidative decomposition with ozone and UV and visible light irradiation. As the intermediates detected in all these experiences are the same, we assume that only one mechanism is needed to explain this reaction. Thus, the different reaction rates would account for the presence of higher or lower amounts of every compound under different conditions. Before the opening of the aromatic ring, three kind of reactions can happen: cleavage of exocyclic double bond, hydroxylation of the aromatic ring and oxidation of aldehydes to carboxylic acids (Scheme 2).
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Fig. 15. UV spectra of several phenolic compounds. — p-hydroxybenzaldehyde, — — protocatechuic acid, - - - p-coumaric acid. The spectrum of vanillic acid closely matches that of protocatechuic acid.
In acidic media, quantitative cleavage of exocyclic double bond occurs, to give 3. Then, this compound can be oxidized to 5, or hydroxylated to yield 4 or 6. In basic media, formation of 2 is observed, and the amounts of 3 are smaller than in acidic media. This fact can be due to the difference in the behaviour of ozone in acidic and basic media. Under acidic conditions, ozone is a stable compound and can easily give addition reactions and rupture of double bonds; attack on the aromatic ring is unlikely. As a consequence, 3 is easily formed but it is difficult to attack. So an accumulation of this compound occurs. On the other hand, ozone decomposes in basic media to give • OH radicals. Attack on the aromatic ring is easier for these radicals, and then, faster hydroxylation of aromatic ring occurs; this explains the formation of 2. p-Hydroxybenzaldehyde concentration is smaller as it reacts faster than in acidic media. The formation of • OH radicals also explains the higher reaction rates in basic media. Opening of the aromatic ring can happen in any of the aromatic intermediates formed. We assume that rupture of the C–C bond between the two carbon atoms linked to an OH group (C3 –C4 ) is easier because they are in a higher oxidation state. This would lead to compound 7 (Scheme 3). Compound 7 has two double bonds, and both of them can be broken to give 8 or 9 and 10 (Scheme 4). Both compounds 8 and 9 can undergo decarboxylation to give 12, and carbon dioxide, or rupture of double bond. In this case, 11 and 10 can be obtained (Scheme 5).
Scheme 2.
Scheme 3.
Finally, the decarboxylation of 11 and the rupture of the double bond of 12 would give 10, which in turn can be oxidized to two carbon dioxide molecules. Another possibility would be the opening up of the aromatic rings between C2 and C3 [11]. In this case, 15, 16 and piruvic acid, 17 should be formed. No 16 or 17 were found and only very small amounts of 15 were detected (Scheme 6).
4. Conclusions Solar light and air are able to degrade p-coumaric acid when pyrylium salts are present in the solution. Although the reaction is clearly slower than when ozone and UV radiation are used, the method is less expensive and hazardous as well as
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Scheme 4.
Scheme 5.
Scheme 6.
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environment-friendly. Hence, it deserves further investigations to check its generality, scope and limitations.
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