M E EL HACHEMI, E NAFFRECHOUX, J SUPTIL, R HAUSLER Bicarbonate effect in the ozone-UV process in the presence of nitrate, Ozone Science & Engineering, accepted for publication, 2013
Bicarbonate effect in the ozone-UV process in the presence of nitrate M. Errachid El Hachemi 1,*, Emmanuel Naffrechoux 2, Joël Suptil 2, Robert Hausler 1 1
École de Technologie Supérieur, STEPPE, 1100 rue Notre Dame Ouest, H3C 1K3 Montréal, Qc, Canada
2
Université de Savoie, Laboratoire de Chimie Moléculaire et Environnement, Campus Scientifique de Savoie Technolac,
73376 Le Bourget du Lac cedex, France *Email :
[email protected]
Abstract : Ozonation and advanced oxidation processes (AOP) are very efficient methods for the destruction of refractory organic matters. These virtues have always been related to the production of hydroxyl radicals HO°, which are extremely powerful and non selective oxidants. In this study, O3-UV process is used as an AOP, where hydroxyl radicals are generated from the photodecomposition of ozone by short wavelength ultraviolet radiation. The obtained results indicated a weak scavenging effect of tertbutanol proving that hydroxyl radicals and ozone are not the only oxidants existing in the medium. Moreover, bicarbonate, known for a long time as effective HO° radical scavengers, does not slow down the oxidation of benzoic acid, but surprisingly increases it. Chlorides significantly decrease the degradation of organic compounds through their reaction with HO° radicals to produce chlorine. Carbonate radicals, nitrate and nitrogenated species as peroxynitrite/ peroxynitrous acid are involved in the oxidative mechanisms. Keywords : Ozone, AOP, Refractory Matters, Hydroxyl Radical, Ultraviolet, Scavenger, Bicarbonate, Nitrate, Peroxynitrite
Introduction : 1
In the last 20 years, a fast evolution of research activities devoted to environmental protection has been recorded as the consequence of the special attention given to the environment by social, political and legislative international authorities. One of the most important components of the environment is water, which is undergoing different kinds of adulteration due to human activities. The most hazardous causes of water pollution are industrial wastewaters and landfill leachates, which are a complex non biodegradable mixture of organic and inorganic compounds (Pétrier et al., 2002). These waters are characterized by high concentrations of organic toxic compounds, and must be treated by other non biological technologies (Andreozzi et al., 1999). Currently, the main processes used to eliminate these refractory organic compounds are incineration, membrane separation (reverse osmosis - nanofiltration) or ozonation. In view of their expensive costs and relative efficiency, many works are carried out to improve the yields of purification (Naffrechoux et al., 2001).
In our research, we try to test the performance of ozonation coupled with UV irradiation, and the formation and reactivity of oxidizing species such as peroxynitrite. The ozone/UV technique is one of the advanced oxidation processes (AOP) that are characterised by a common chemical feature : the capability of exploiting the high reactivity of HO° radicals in driving oxidation processes (Andreozzi et al., 1999), which are suitable for achieving the complete abatement and mineralization of even less reactive pollutants. In the process ozone/UV, ozone produces hydroxyl radicals by absorbing UV light at 254 nm (Andreozzi et al., 2000). The initiation step of the radical mechanism is the direct photolysis of ozone to produce hydrogen peroxide whose reaction with ozone and/or dissociation under UV radiation generates HO° radicals.
Ozone/UV is the most complex reaction system of the advanced oxidation technologies (AOTs) because the dissolved matter can be degraded in different ways : direct ozonation, photolysis reaction and hydroxyl radical oxidation (Beltran et al., 1997). Yet, it seems that HO° radical is the main reagent that reacts with organic matter (Naffrechoux et al., 2000).
This radical entity is very efficient because it is a powerful instable oxidant and is not selective (it can react with any organic pollutant, which is not the case of molecular ozone). The oxidation potential is 2
2.80 V for the couple (HO°/OH-) (Edelahi, 2004). It is well known that there are other species involved in the oxidation (Andreozzi et al., 2000) such as HO2-, H2O2, HO2°, O3°-. However, a very powerful oxidant has been neglected : peroxynitrite. ONOO- is a nitrogen oxyanion containing an O-O peroxo bond that is a structural isomer of the nitrate ion. It is a powerful oxidant that has been shown to react with a wide variety of inorganic and organic reductants, as well as hydroxylate and nitrate aromatic compounds, including benzene (Hurst, 2008).
Peroxynitrite is also capable of reacting through a variety of oxidative mechanisms. Protonation of peroxynitrite yields HOONO (pKa = 6,8). The trans configuration of peroxynitrous acid is thought to react with organic matter via an energetic intermediate complex with hydroxyl-like oxidant properties (Gonzalez and Braun, 1996). The lifetime of ONOOH is about 1 s at normal conditions (25°C and 1 atm) (Bartlett et al., 1995).
In 1964, a solution containing 500 g/L of sodium nitrate was irradiated at pH 12, and then a yellow coloration due to peroxonitrite had been noticed (Papee and Petriconi, 1964). There are several methods to form peroxynitrous acid in water, the most widely used are : reaction of hydrogen peroxide with nitrous acid at low pH and quenching of peroxynitrite with alkali, reaction of NO° with O2°-, flash photolysis of nitrate solutions and pulse radiolysis of nitrate or nitrite solutions (Logager and Sehested, 1993). The oxidation potential of the different couples are : (ONOO-, H2O/NO2°, OH-) = 2,1 V; (ONOOH, H+/NO2°, H2O) = 1,4 V; and (ONOOH, H+/NO2-, H2O) = 0,99 V (Hurst, 2008; Bartlett et al., 1995; Denicola et al., 1996; Ramezanian et al., 1996).
Synergetic effects were considered in order to suggest a reaction mechanism for O3-UV process. Figure 1
presents
the
proposed
mechanism
for
the
reactivity
of
peroxynitrite
in
O3-UV process, where the main species involved in organic matter oxidation are portrayed, according to Peyton and Glaze 1988, and Denicola et al. 1996. R : organic radical
3
ROO° : organic peroxide M : organic matter Ar : aromatic compound FIGURE 1. Involved mechanisms in O3-UV process in the presence of nitrate and carbonates (Denicola
et al., 1996; Peyton and Glaze 1988).
The aim of this work is to assess the effect of bicarbonates on the oxidation of benzoic acid. The involvement of other oxidants is investigated in order to check if ozone and hydroxyl radicals are the only oxidizing species in the medium. The intervention of nitrogenated species such as peroxynitrite is discussed, since peroxynitrite is formed during UV ozonation in presence of nitrate.
Experimental section : Chemicals : All solutions were prepared using ultra high quality (UHQ) water (resistivity more than 18.1 MΩ). Benzoic acid, NaHCO3, NaNO3, NaNO2 and HgSO4 were purchased from Prolabo. NaCl and K2Cr2O7 were bought from Acros. Tert-butanol and Ag2SO4 were acquired from Fluka, KI from Fischer Scientific, NaOH from Sigma-Aldrich and H2SO4 from Merck. Analytical Methods : The retention time for benzoic acid and its derivatives was completely different, therefore it was possible to detect them by a high performance liquid chromatography (model Waters 515) with a photodiode array detector (model Waters 996). The column was a Hypersil ODS 5µm C18 (250 mm × 4,6 mm) and the eluent was a mixture of acetonitrile/water-phosphoric acid (50/50/0,2; V/V/V). Chemical Oxygen Demand was conducted according to the method of Thomas et al. (1986). Nitrates and nitrites were analysed by ionic chromatography (Metrosep Anion Dual 1). Apparatus : The system described in Figure 2 is constituted of a cylindrical reactor. Ozone is generated from air by a trailigaz “Labo 5LO” generator (130 W electric power and 100 L/h of air flow rate) and is fed in the 4
reactor with a Sulzer static mixer. UV irradiation stage is built around a cylindrical low pressure mercury vapour lamp (40 W electric power) emitting at 185 and 254 nm.
FIGURE 2. Experimental ozone-UV reactor.
Results and Discussion : Benzoic acid (BA) has been chosen as a model molecule due to its weak reactivity towards molecular ozone (k = 1,2 ± 0,2 M-1s-1 for benzoate anion) (Doré, 1989). It is also representative of organic macromolecules in landfill leachates. Benitez et al. (2000) consider that benzoic acid is an industrial pollutant found in olive oil factory wastewaters. It is also present in chemical and petrochemical industries (Andreozzi et al., 2004). The rate constants of HO° radical with benzoic acid and benzoate anion are respectively 4,3. 109 M-1s-1 and 5,9. 109 M-1s-1 (Oturan and Pinson, 1995). Figure 3 shows the destruction of benzoic acid in O3-UV process. FIGURE 3. Effect of chloride and bicarbonate on the oxidation of benzoic acid. 5
[BA]0 = 3. 10-3 M ; [NaCl] = 0,1 M ; [NaHCO3] = 3. 10-2 M.
Chlorides have a negative effect because they quickly react with hydroxyl radicals (Liao et al., 2001). Cl- + HO° -
HOCl°+
HOCl° + H Cl° + Cl°
k = 4,3 ± 0,4. 109 M-1s-1
Cl° + H2O Cl2
(1) (2)
(3)
The apparent rate constant of benzoic acid oxidation is 7,5. 10-3 min-1. The effect of hydrogen carbonate is surprising. In fact, multiple authors showed that bicarbonate is a strong scavenger for hydroxyl radicals (Hoigné and Bader, 1976; Staehelin and Hoigné, 1985; Hausler et al., 1990): HCO3- + HO° CO32- + HO°
HCO3° + OHCO3°- + OH-
k = 1,5. 107 M-1s-1
(4)
k = 4,2. 108 M-1s-1
(5)
Yet, in our experiments, the pseudo first order rate constants for benzoic acid degradation in presence and absence of bicarbonate are respectively 2,25. 10-2 min-1 and 1,27. 10-2 min-1. The fast oxidation in presence of HCO3- has been explained by the more important rate constant with benzoate anion at pH > 8 (Oturan and Pinson, 1995). FIGURE 4. Effect of bicarbonate and chloride on COD removal in O3-UV treatment. [BA]0 = 3. 10-3 M ; COD0 = 720 mg O2/L.
The COD elimination (Figure 4) is faster in presence of HCO3-. Thus, the oxidation of byproducts is also enhanced in the presence of bicarbonate. One would think that hydroxyl radicals are not the only oxidants present in the medium, and other species, which are not scavenged by HCO3- would be responsible for the degradation of benzoic acid and its byproducts. For a better evaluation of hydroxyl radicals’ contribution in the oxidation of benzoic acid, a comparison of kinetic rates was done to assess the effect of tert-butanol. FIGURE 5. Effect of tert-butanol on the oxidation of benzoic acid. [BA]0 = 10-3 M ; [tertbutanol]0 = 10-2 M.
Figure 5 shows that benzoic acid is completely eliminated after 30 minutes, while 60 minutes are required to oxidize all the benzoic acid in presence of tert-butanol. 6
The degradation of benzoic acid in the presence of an excess of tert-butanol (10-2 M) proves that hydroxyl radicals are not the only species, which oxidize organic pollutants in O3-UV system. The shape of the degradation curves suggests a slower degradation of benzoic acid in the beginning of the treatment. This slowing down is attributed to the formation of oxidizing species that are not scavenged by tert-butanol. Rivas et al. (2001) explained this kinetic by the presence of other species such as organic peroxides and Fe(IV) (in Fenton process). In our study, this slow degradation is probably due to the formation of organic peroxides and nitrogenated molecules such as ONOOH, NO° and NO2°. Other tests were carried out with a low pressure mercury vapour lamp emitting at 254 nm only. FIGURE 6. Effect of tert-butanol and bicarbonate on benzoic acid degradation.
[BA]0
= 10-3 M ; [t-butanol] = 10-2 M ; [NaHCO3] = 3. 10-2 M ; pH adjusted between 6 and 7 by regular adding of NaOH 10-3 M.
Figure 6 shows that the benzoate anion is eliminated very quickly in presence of HCO3(k = 1,77. 10-1 min-1). Yet, in the medium with pH > 6 where the benzoate form is predominant, the oxidation is significantly slower (k = 1,25. 10-1 min-1). The degradation of the protonated form is slower (k = 9,48. 10-2 min-1) and more slowly in the presence of tert-butanol. TABLE . Apparent rate constants of benzoic acid oxidation under different conditions : concentration of ozone in the gas 40 mg/L, gas flow rate 1,6 L/min, in presence of bicarbonate pH = 8,5, in its absence pH = 3,3 Benzoic acid concentration (mol.L-1)
Conditions
Apparent rate constant
7
3. 10-3
Benzoic acid alone
1,27. 10-2 min-1
3. 10-3
NaHCO3 3. 10-2 M
2,25. 10-2 min-1
3. 10-3
NaCl 10-1 M
0,75. 10-2 min-1
10-3
Benzoic acid alone
9,48. 10-2 min-1
10-3
NaHCO3 3. 10-2 M
17,7. 10-2 min-1
10-3
6 < pH < 7
12,5. 10-2 min-1
10-3
Tert-butanol 10-2 M
#not a pseudo-first order law
The results in table 1 demonstrate that the different rate constants of hydroxyl radicals towards benzoic acid and benzoate anion (kBA = 4,3. 109 ; kB- = 5,9. 109 M-1s-1) can’t explain the enhancement due to hydrogen carbonate. This study demonstrates that in the presence of bicarbonate ions famous as HO° scavengers, the oxidation kinetic of benzoic acid is improved. This suggests that ozone and HO° are not the only oxidizing species in the medium in the ozone/UV process. Virtually, in this study of the ozone/UV process, all the conditions for creating peroxynitrite (and/or peroxynitrous acid) were fulfilled; energetic UV radiation and nitrates formed in parallel with ozonation. From the results recorded in Figure 7, we observed the formation of about 160 mg/L of nitrates in ozone/UV process. FIGURE 7. Nitrates formation in the O3-UV reactor. pH0 = 6,7 ; pH240 = 2,7.
Nitrate is produced by nitrogen oxidation in the ozone generator. Indeed, the oxidation of nitrogen gives nitrogen pentoxide (N2O5), which can combine with water vapour, and then produces nitric acid (Doré, 1989) according to the reaction (6) : N2O5 + H2O
2 HNO3
(6)
Sarakha et al. (1993) showed that the formation of nitroaromatic intermediates was not influenced by the presence of HO° radical scavengers. Ramezanian et al. (1996) studied the nitration and hydroxylation of phenolic compounds by peroxynitrite. They showed that nitration is important at pH 1,8 and 6,8, while hydroxylation is dominant between these two pH values. Denicola et al. (1996) showed that the presence of bicarbonate significantly enhanced peroxynitritemediated nitration of aromatics. Moreover, HCO3° reacts with various molecules 102 to 107 times slower than hydroxyl radical, but it will diffuse longer and may selectively reach and oxidize critical targets. Also, the generation rate of HO° and CO3°- in the presence of NaHCO3 would be higher than that of HO° alone in its absence (Vione et al., 2009). Also, Niang-Gaye and Karpel Vel Leitner (2005) showed that the contribution of carbonate radicals to atrazine degradation can reach more than 40 % during ozonation in the presence of bicarbonate ions (7 mM). In our experiments, the pH in the presence and absence of bicarbonates was 8,5 and 3 respectively, then both hydroxylation and nitration can occur. Nevertheless, it is well known that proxynitrite provokes the oxidative damage of DNA, especially for guanine (Liu et al., 2006), and the nitration of organic compounds generates toxic byproducts. Then, the scientists should be careful when UV disinfection is 8
completed in presence of nitrate. And other studies will be conducted in order to assess the oxidation mechanisms of the UV/NO3- system. Conclusion : This study shows that in the presence of bicarbonates deemed to be hydroxyl radical scavengers, the oxidation kinetic of benzoic acid is improved. This suggests that ozone and hydroxyl radicals and ozone are not the only oxidants present in the medium in the ozone/UV system. Nitrogen species are involved in the system coupling the ultraviolet and ozonation in the presence of nitrates. Carbonate radicals have also a contribution in the oxidation of benzoic acid because they diffuse longer than hydroxyl radicals, and they improve the oxidation pathway by peroxynitrite. References Andreozzi R., Caprio V., Insola A., Marotta R., Advanced oxidation processes (AOP) for water purification and recovery, Catalysis Today, 1999, 53, 51-59. Andreozzi R., Caprio V., Insola A., Marotta R. and Sanchirico R., Advanced oxidation processes for the treatment of mineral oil-contaminated wastewaters, Wat. Res., 2000, 34, 620-628. Andreozzi R., Marotta R., Removal of benzoic acid in aqueous solution by Fe (III) homogenous catalysis, Wat. Res., 2004, 38, 1225-1236. Bartlett D., Church D. F., Bounds P. L. and Koppenol W. H., The kinetics of the oxidation of Lascorbic acid by peroxynitrite, Free radical biology and medicine, 1995, 18, 85-92. Beltran F. J., Encinar J. M. and Gonzalez J. F., Industrial wastewater advanced oxidation – Ozone combined with hydrogen peroxide or UV radiation, Wat. Res., 1997, 31, 2415-2428. Benitez F. J., Beltran-Heredia J., Peres J. A., Dominguez J. R., Kinetics of p-hydroxybenzoic acid photodecomposition and ozonation in a batch reactor, Journal of hazardous materials, 2000, B73, 161178. Denicola A., Freeman B. A., Trujillo M. and Radi R., Peroxynitrite reaction with carbon dioxide/bicarbonate: kinetics and influence on peroxynitrite-mediated oxidations, Archives of Biochemistry and Biophysics, 1996, 333, 49-58. Doré M., Chimie des oxydants et traitement des eaux, LAVOISIER, Tec et Doc, 1989. Edelahi M. C., Contribution à l’étude de dégradation in situ des pesticides par procédés d’oxydation avancés faisant intervenir le fer. Application aux herbicides phénylurées, Thèse de Doctorat, Université de Marne La Vallée, France, 2004. 9
Gonzalez M. C., Braun A. M., Vacuum-UV photolysis of aqueous solutions of nitrate : effect of organic matter I. Phenol, Journal of photochemistry and photobiology A : Chemistry, 1996, 93, 7-19. Hausler R., Béron P. et Brière F., Mécanismes d’ozonation dans le traitement des eaux, Sciences et techniques de l’eau, 1990, novembre, 351-364. Hoigné J. and Bader H., The role of hydroxyl radical reactions in ozonation processes in aqueous solutions, Wat. Res., 1976, 10, 377-386. Hurst
J.
K., "Peroxynitrite," in
AccessScience,
©McGraw-Hill
Companies,
2008,
http://www.accessscience.com Liao C. H., Kang S. F., Wu F. A., Hydroxyl radical scavenging role of chloride and bicarbonate ions in the H2O2/UV process, Chemosphere, 2001, 44, 1193-1200. Liu N., Ban F., and Boyd R. J., Modeling Competitive Reaction Mechanisms of Peroxynitrite Oxidation of Guanine, J. Phys. Chem., A 2006, 110, 9908-9914. Logager T. and Sehested K., Formation and decay of peroxynitrous acid : a pulse radiolysis study, J. Phys. Chem., 1993, 97, 6664-6669. Lymar S. V. and Hurst J. K., Rapid reaction between peroxonitrite ion and carbon dioxide : implications for biological activity, J. Am. Chem. Soc., 1995, 117, 8867-8868. Mack J., Bolton J. R., Photochemistry of nitrite and nitrate in aqueous solution : a review, Journal of photochemistry and photobiology A : Chemistry, 1999, 128, 1-13. Naffrechoux E., Chanoux S., Pétrier C., Suptil J., Sonochemical and photochemical oxidation of organic matter, Ultrasonics Sonochemistry, 2000, 7, 255-259. Naffrechoux E., Fachinger C., Pétrier C., Suptil J., Muller P., Nobili E., Rostan A., Traitement d’oxydation avancé O3-UV-US des lixiviats d’ordures ménagères, Congrès international du GRUTTEE, Limoges, France, 2001. Niang-Gaye P., Karpel Vel Leitner N., Contribution of carbonate radicals to atrazine oxidation during ozonation of aqueous solutions containing bicarbonate ions, Rev. Sci. Eau, 18/1/2005, 65-86. Oturan M. A. and Pinson J., Hydroxylation by electrochemically generated OH° radicals. Mono and polyhydroxylation of benzoic acid : Products and isomers’ distribution, J. Phys. Chem., 1995, 99, 13948-13954. Papee H. M. and Petriconi G. L., Formation and decomposition of alkaline pernitrite, Nature, 1964, 204, 142-144. Pétrier C., Naffrechoux E.,
Hurtado Gimeno C., El Hachemi M. E., Combined processes
ultrasound/ozone and ultrasound/UV for liquid waste treatment, Ultrasound in Environmental 10
Engineering II, Technical University of Hamburg-Harburg reports on sanitary engineering, 2002, 35, 17-30. Peyton G. R. and Glaze W. H., Destruction of pollutants in water with ozone in combination with ultraviolet radiation – Photolysis of aqueous ozone, Environment Science and Technology, 1988, 22, 761-767. Ramezanian M. S., Padmaja S., and Koppenol W. H., Nitration and hydroxylation of phenolic compounds by peroxynitrite, Chem. Res. Toxicol., 1996, 9, 232-240. Rivas F. J., Beltran F. J., Frades J. and Buxeda P., Oxidation of p-hydroxybenzoic acid by Fenton’s reagent, Wat. Res., 2001, 35, 387-396. Sarakha M., Boule P., Lenoir D., Phototransformation of 2-phenylphenol induced in aqueous solution by excitation of nitrate ions, Journal of Photochemistry and Photobiology A: Chemistry, 1993, 75, 6165. Staehelin J. et Hoigné J., Decomposition of ozone in water in the presence of organic solutes acting as promoters and inhibitors of radical chain reactions, Environment Science and Technology, 1985, 19, 1206-1213. Thomas O. et Mazas N., La mesure de la demande chimique en oxygène dans les milieux faiblement pollués, Analusis, 1986, 14, 300-302. Vione D., Khanra S., Cucu Man S., Maddigapu P. R., Das R., Arsene C., Olariu R. I., Maurino V., Minero C., Inhibition vs. enhancement of the nitrate-induced phototransformation of organic substrates by the °OH scavengers bicarbonate and carbonate, Wat. Res., 2009, 43, 4718-4728.
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