Role of reactive intermediates in the radiolytic degradation of Acid Red 1 in aqueous solution

ARTICLE IN PRESS Radiation Physics and Chemistry 78 (2009) 13–18

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Role of reactive intermediates in the radiolytic degradation of Acid Red 1 in aqueous solution Cs.M. Fo¨ldva´ry , L. Wojna´rovits Institute of Isotopes, Hungarian Academy of Sciences, P.O. Box 77, H-1525 Budapest, Hungary

a r t i c l e in fo

abstract

Article history: Received 2 June 2008 Accepted 31 July 2008

d d d d The role of reactive intermediates of water radiolysis (e aq, H , HO , O2 /HO2 ) in decoloration and mineralization of aqueous solutions of Acid Red 1 dye was investigated. The decoloration is highly d d d d effective in the reactions of e aq and H , and less effective in HO reactions. The O2 /HO2 pair does not take part in decoloration. For mineralization, which is an oxidative degradation, HOd radicals are d needed: the efficiency increases with the dissolved oxygen concentration. The reactions of the Od 2 /HO2 radical pair slightly increase the rate of mineralization. Iron and copper ions (possible constituents of waste waters) in low concentration do not influence the reactions. & 2008 Elsevier Ltd. All rights reserved.

Keywords: Acid Red 1 Azophloxine Azo dyes Radiolysis Superoxide anion

1. Introduction Since the 1980s, there has been a great interest towards the ionizing radiation-induced degradation of non-biodegradable azo dye molecules in aqueous solutions. The researches in the field have led to laboratory demonstration of the applicability of technique, and to industrial pilot plant and full-scale established technologies for treating effluents of dying factories. The research with dye molecules is continued in the direction involving dye molecules with different structures in the investigations, clarifying the mechanism of the undergoing reactions, finding the most probable sites of radical attack on dye molecules and identifying the short-lived transients and stable intermediates of degradation (Kim et al., 2007; Wojna´rovits and Taka´cs, 2008; Emmi and Taka´cs, 2008). From the three short-lived intermediates of water radiolysis, as is shown in reaction 1 (hydroxyl radical (HOd), hydrated electron d (e aq) and hydrogen atom (H )), the hydroxyl radical is considered to cause the degradation. It is because (i) it reacts with doublebonded compounds with practically diffusion-controlled rate coefficients, (ii) HOd is the only oxidizing radical of the three, (iii) in the presence of dissolved oxygen the degradation is an oxidative degradation with intermediate products with progressively higher oxygen to carbon ratios as the degradation proceeds towards complete mineralization and (iv) in oxygen-saturated or air-free solutions, the ratio of acting free radicals is e aq (44%), Hd atom (10%) and HOd radical (46%) (Schittl et al., 2007), but in  Corresponding author.

E-mail address: [email protected] (Cs.M. Fo¨ldva´ry). 0969-806X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2008.07.009

d the presence of air e react with O2 in fast reaction, aq and H d forming the relatively inert O2 /HOd2 pair (reactions (2)–(4)):

radiation

H2 O ! OHd ; eaq  ; Hd ; H2 O2 ;

(1)

GdOH ¼ 0:29; GHd ¼ 0:06; Geaq ¼ 0:28; GH2 O2 ¼ 0:07 m mol J1 ; eaq  þ O2 ! O2d Hd þ O2 ! HO2d

k ¼ 1:9  1010 mol1 dm3 s1 ; k ¼ 2:1  1010 dm3 s1 ;

HO2d þ H2 O2O2d þ H3 Oþ

pka ¼ 4:8:

(2) (3) (4)

At the same time, several papers in the literature report on d effective decoloration by e aq and H under favorable conditions (e.g. in deoxygenated systems) (Zhang et al., 2005; Sharma et al., 2002, 2003; Hihara et al., 2003). In this paper, using UV–vis and chemical oxygen demand (COD) measurements, we investigate the role of the different reactive intermediates in the decoloration and mineralization of Acid Red 1 (AR1, Azophloxine). In a previous conference paper, we have already published some preliminary data on the degradation of this compound (Fo¨ldva´ry and Wojna´rovits, 2007).

2. Experimental section AR1 powder (Sigma–Aldrich Co.) was used without further purification. The concentration was 2.5  104 mol dm3 and 2.5  103 mol dm3 in UV–vis and COD measurements, respectively. The concentrations were optimized for the well-measurable

ARTICLE IN PRESS ´ry, L. Wojna ´rovits / Radiation Physics and Chemistry 78 (2009) 13–18 ¨ldva Cs.M. Fo

range: applying 2.5  104 mol dm3 concentration the absorbance remained below 2 needed for the validity of the Lambert–Beer law. In COD measurement, the 2.5  103 mol dm3 concentration solution results in relative high COD value (850 mg O2 dm3) as the starting point, so the decrease caused by the degradation can be easily measured. The COD was measured according to the ISO Standard no. 6060:1989 by a Behrotest TRS 200 COD system. For COD experiments, the following chemicals were used: (NH4)2Fe (SO4)2  6H2O (Mohr-salt), K2Cr2O7 (Reanal Co.), cc. H2SO4, Ag2SO4, HgSO4, KH(C4H2O2)2 (Spektrum-3D), Ferroin-solution indicator (Fluka). UV–vis spectra were taken by the JASCO 550 UV–vis spectrophotometer in the 200–700 nm range, using 1 cm cell. The AR1 solutions were irradiated by Co-60 gamma rays up to 250 kGy dose, the dose rate was 2 kGy h1 under conditions when hydroxyl radical, hydrated electron, hydrogen atom or the superoxide ion–perhydroxyl radical pair is the main reacting radical. In the latter case, 0.05 mol dm3 Na-formate (Fluka) was added to the solutions. The effect of iron or copper ions on the degradation was also investigated using oxygen-saturated AR1 solution containing FeSO4  7H2O (Reanal Co., Spektrum-3D) or CuSO4 (Reanal Co.). When the H-atom reactions were studied, the pH of the solution was set to 2, for converting the hydrated electron to H-atom (reaction (7)) and the solution here and in e aq reaction investigation contained tert-butanol in order to convert the HOd radicals to less-reactive dCH2(CH3)2COH radicals in reaction (5) (Buxton et al., 1988). In the case of H-atom and e aq, the AR1 solutions were saturated with N2 gas. HOd radical measurements were carried out in N2O-saturated solution in order to transform the hydrated electron to hydroxyl radical in reaction (6): HOd þ ðCH3 Þ3 COH ! d CH2 ðCH3 Þ2 COH þ H2 O; k ¼ 6  108 mol1 dm3 s1 ;

(5)

eaq  þ N2 Oþ H2 O ! HOd þ N2 þ OH ; k ¼ 7  109 mol1 dm3 s1 ;

(6)

eaq  þ H3 Oþ ! Hd þ H2 O; k ¼ 2:3  1010 mol1 dm3 s1 :

(7)

3. Results 3.1. Decoloration As Fig. 1 shows, there is a strong decrease in the visible absorbance when the AR1 solution is irradiated. The initial 2.5  104 mol dm3 AR1 concentration decreases by about 50%, applying an absorbed dose of 0.4 kGy in acidic solution, when the H-atom is the main reaction partner (curve c). In air-saturated d neutral solution, when HOd radicals and Od 2 /HO2 pair are present, 0.6 kGy dose (curve d) is needed for reaching similar fading effects. In H-atom reaction at 1.5 kGy dose (curve g), the color completely disappears. When HOd radicals were the main reaction partners, the decrease of absorbance at the 531 nm peak was about 40% only at 0.5 kGy dose (curve b) and about 77% at 1.5 kGy (curve f). The stronger effect of the H-atom reaction on fading is clear from Fig. 1. In the case of HOd radical reaction in N2O-saturated solution (curves b and f) or air-saturated solution (curves d and e), the decoloration effect is smaller even at higher dose compared with H-atom reactions. When the hydrated electron is the main reactant (not shown), the decoloration process is very similar to

2.0

2.5x10-4 mol dm-3 AR1 a) 0 Gy

1.5 Absorbance

14

b) 0.5 kGy, OH-radical c) 0.4 kGy, H-atom

1.0

d) 0.6 kGy, air e) 1 kGy, air

0.5

f) 1.5 kGy, OH-radical g) 1.5 kGy, H-atom 0.0 300

400

500

600

700

Wavelength (nm) Fig. 1. Absorption spectrum of the 2.5  104 mol dm3 AR1 solution irradiated under different conditions. Natural pH (6.2) except for the H-atom reaction, in the latter case the pH was set to 2.0.

the reaction of H-atom. A small red shift of the peak at 531 nm is observable in the case of air-saturated solutions, the shift was higher in N2O-saturated solution (curves b, d–f). This shifting increases with increasing dose. In Fig. 2, the dose dependence curves of Hd, e aq and HOd reactions followed by measuring the absorbance at 531 nm (curves b–d) show some similarities at lower doses, although the yield of HOd radicals (in N2O-saturated solution) is twice the 1 yield of e ). The H-atom and aq (G ¼ 0.56 and 0.29 m mol J e aq decoloration curves are close to linear dependence below the 0.5 kGy dose, which is in agreement with our previous result (Wojna´rovits and Taka´cs, 2008). The HOd radical-induced decoloration curve clearly shows the lower efficiency of the HOd radical reaction, it deviates from the linearity. Curve b has almost perfect linearity (r2 ¼ 0.9963) in the logarithmic scale, which means the fading reaction with HOd radical has an exponential shape, again similar to some previous observations with several dye molecules. In Fig. 2, curves a and e represent the dose dependences of decoloration in air- or O2-saturated neutral solutions. In O2-saturated solution due to the high dissolved oxygen concend tration (1.4  103 mol dm3), e atom practically aq and H d entirely convert to the Od 2 /HO2 pair in reactions (2)–(4). In airsaturated solution, the O2 and AR1 concentrations are practically the same, 2.7  and 2.5  104 mol dm3; therefore, some part of the hydrated electrons reacts with the dye. In both solutions, the HOd radicals react with dye molecules with a yield of G ¼ 0.29 HOdm mol J1. Comparing curve b (N2O-saturated solution, HOd d d and Hd, G ¼ 0.56 and 0.06), curve a (HOd and e aq+O2 /HO2 , G ¼ 0.29 and 0.34) and curve e (N2-saturated, tert-butanold containing solution, HOd and Od 2 /HO2 , G ¼ 0.29 and 0.34) we 4 can conclude that (i) at 2.5  10 mol dm3 AR1 concentration in air-saturated solution, the e aq reactions contribute to the dye d degradation and (ii) the contribution of the Od 2 /HO2 pair to the decoloration is small. We came to the latter conclusion also based on curve g. In this case, the O2-saturated solution contained 0.05 mol dm3 HCOONa: the HOd radicals reacting with formate d ions in the two-step process transform also to Od 2 /HO2 (reactions (8) and (9)). Finally, all the three intermediates of water radiolysis d d d (HOd, e aq and HO ) end up as O2 /HO2 (Yamashita et al., 2008). HCO2  þ HOd ! CO2d þ H2 O; k ¼ 3:2  109 mol1 dm3 s1 ;

(8)

ARTICLE IN PRESS ´ry, L. Wojna ´rovits / Radiation Physics and Chemistry 78 (2009) 13–18 ¨ldva Cs.M. Fo

Absorbance (%)

100 80 2.5x10-4

60

mol

1.0

1.5 2.0 2.5 Dose (kGy)

OH-radical

c)

H-atom

d)

eaq

AR1 e) λ = 531 nm f)

20

0.5

irradiated in air

dm-3

40

0 0.0

a) b)

3.0

15

saturated with O2 O2-sat. with tert-but.

g)

O2-sat. with Na-form.

h)

O2-sat. with Na-form., Fe2+

i)

O2-sat. with Na-form., Cu2+

3.5

Fig. 2. Absorbance at 531 nm in 2.5  104 mol dm3 AR1 solutions irradiated under different conditions.

CO2 d þ O2 ! CO2 þ O2d ; 9

k ¼ 4:2  10 mol

1

3 1

dm s

(9)

:

The very low decrease in the visible absorbance, when solution (g) is irradiated, clearly shows the inefficiency of this pair in decoloration. In case of curve f, tert-butanol was given to the d d O2-saturated solution: in this system, Od 2 /HO2 and CH2(CH3)2COH radicals are present. The low decoloration found here shows that d d not only the Od 2 /HO2 pair but also the CH2(CH3)2COH radicals react with the low rate coefficient with the dye. The starting G-values of the fading reaction are as follows in the case of curves b–d: GE0.23, 0.27 and 0.16 m mol J1, respectively. Comparing these results with the yields of water radicals, the efficiencies of water radiolysis intermediates in the decoloration process of the dye are determined. The efficiencies are 41%, 80% and 60%, in the case of HOd radical (curve b), H-atom (curve c) and hydrated electron (curve d), respectively. We repeated the experiments of HCOONa+O2 with adding small amounts of iron or copper ions (2  105 mol dm3) to the solutions (curves h and i). These ions are expected to induce the catalytic decomposition of H2O2 formed in reactions (1) and (10)–(12) in Fenton or Fenton-like processes (reactions (13)–(16)) (Galindo et al., 2000; Suzuki et al., 1975). We show the reactions in the example of iron ions: 2HO2d ! H2 O2 þ O2 ; k ¼ 8  105 mol1 dm3 s1 ;

(10)

2O2d þ 2H2 O ! H2 O2 þ O2 þ 2HO ; ko3  101 mol1 dm3 s1 ;

(11)

HO2d þ O2d ! H2 O2 þ O2 þ HO ; k ¼ 9  107 mol1 dm3 s1 ;

(12)

Fe2þ þ HO2d þ H3 Oþ ! Fe3þ þ H2 O2 þ H2 O; k ¼ 1:2  106 mol1 dm3 s1 ;

(13)

Fe2þ þ H2 O2 ! Fe3þ þ HO þ HOd ; k ¼ 5  101 mol1 dm3 s1 ;

(14)

Fe3þ þ H2 O2 þ H2 O ! Fe2þ þ HO2d þ H3 Oþ ;

(15)

Fe3þ þ HO2d þ H2 O ! Fe2þ þ O2 þ H3 Oþ :

(16)

All of these reactions are very slow, especially the HO radical forming reaction (14); the slowness can be the reason why no increase in the dye decoloration was observed in the presence of iron or copper ions. (The slight increase in the absorbencies in the d

presence of copper ion (curve i) may be due to some complexation reactions.) 3.2. Mineralization The decoloration merely shows the disintegration of the conjugated electron structure giving the color. If the irradiation is continued after decoloration, there is a possibility of decomposition of the aromatic rings forming smaller molecules, e.g. in the presence of oxygen, aldehydes, ketones and carboxylic acids. The final result can be a complete mineralization to H2O, CO2, N2 or nitrogen oxides. The amount of oxygen needed for the complete oxidation of organic compounds, namely the process of mineralization, can be followed by COD measurements. The COD value of a 2.5  103 mol dm3 AR1 solution is ca. 850 mg O2 dm3; this value agrees with the value calculated based on the chemical formula. The calculated COD values were determined from the oxidation state of atoms in the dye molecules and the products (CO2, H2O). There is a good linear relationship between the concentration of AR1 solution and the COD value (Fo¨ldva´ry and Wojna´rovits, 2007). These findings demonstrate the applicability of the method for our purposes. Unfortunately, it is not practical to carry out COD measurements in tert-butanol-containing solutions, since the very high COD value of tert-butanol at the concentration applied masks the changes in the COD value of the dye. In Na-formate-containing solution, the oxidation of one formate ion needs two electrons. The ratio between the COD value of Na-formate and the dye is lower than in the case of tert-butanol. The difference in the COD value of the AR1 solution with Na-formate and the pure Naformate gives the same result as the AR1 solution without Na-formate, in agreement with the expectations. In Fig. 3, we show the dose dependence of the COD value in large number solutions. Curves a and c show the COD removal in air- and oxygen-saturated solutions. It is obvious that, as opposed to the decoloration, the dye degradation in O2-saturated solution is much more effective than in air-saturated solution. Hosono et al. (1993) came to the same conclusion based on the irradiation of Acid Red 265 dye. They found a further increase in the degradation efficiency when they supersaturated the solution with oxygen by applying a pressurized reaction vessel. At an initial 2.5  103 mol dm3 AR1 and 0.05 mol dm3 HCOONa concentration about 25% of the HOd radicals react with the dye and about 75% with formate in reaction (8). (The COd 2 radical formed in reaction (8) is a reducing radical and even at 2.5  103 mol dm3 dye concentration is expected to react predominantly with oxygen in reaction (9).) Due to the incomd d plete HOd radical conversion to COd 2 and then to O2 /HO2 in our

ARTICLE IN PRESS ´ry, L. Wojna ´rovits / Radiation Physics and Chemistry 78 (2009) 13–18 ¨ldva Cs.M. Fo

COD (%)

16

110 100 90 80 70 60 50 40 30 20 10 0

Na-formate addition

0

50

100 150 Dose (kGy)

a) 2.5x10-3 mol dm-3 AR1 in air b) saturated with N2, at 15 kGy Na-formate addition and saturated with O2 c) saturated with O2 d)

sat. with O2, 0.05 mol dm-3 Na-formate

e)

sat. with N2, 0.05 mol dm-3 Na-formate

f)

sat. with O2, 0.05 mol dm-3 Na-formate, 2x10-5 mol dm-3 CuSO4

g)

sat. with O2, 0.05 mol dm-3 Na-formate, 2x10-5 mol dm-3 FeSO4

200

Fig. 3. Dose dependence of the COD values in 2.5  104 mol dm3 AR1 solutions irradiated under different conditions.

formate-containing solutions, always there is some contribution from the HOd radical reaction to the degradation. That is true for solution (e) that was saturated with N2. In such solution e aq and d d Od 2 /HO2 pair are the main radical species, but and H are also expected to contribute to the processes taking place. In the case of curve b, the solution was saturated with N2 (for hydrated electron and hydroxyl radical being the main reactant) and irradiated with the 15 kGy dose to induce fading reaction. After the 15 kGy dose, the solution was saturated with O2 for investigating the degradation. At the 15 kGy dose, 0.05 mol dm3 HCOONa was also given to the solution, which caused a jump in the COD value. The degradation effect HOd radical alone was also investigated (curve not shown); in this case, the solution was saturated first with N2O and then with O2 (at 15 kGy), but we obtained the same curve as in the case of hydrated electron reaction+hydroxyl radical reaction (curve b). Curve b shows a stronger decrease of the COD value. The low efficiency of degradation in the case of curve e is certainly due to the absence d of oxygen, which is needed for oxidative degradation. e aq, CO2 d and H as reducing radicals are not expected to induce COD removal and we may think that the small removal observed irradiating these solutions is due to the contribution of HOd radicals, not scavenged by the formate. This means that hydrated electrons or the hydroxyl radicals do not produce such intermediate products or do not open new decay routes that are d especially sensitive to Od 2 /HO2 reactions. The COD removal efficiency was higher when the formate containing solution d was saturated with oxygen, than irradiating in air-saturated solutions without the formate (curve a). Since in d solution d, the main radical intermediate is the Od 2 /HO2 radical d pair, and the contribution of HO radical reactions is small, we may conclude that the radical pair takes part in degradation during the mineralization process. As it is obvious from solutions f and g, copper or iron ions present in the solution—similar to the decoloration—do not influence considerably the mineralization (compare curves d, g and f).

4. Discussion From the conducted experiments, it is clear that different factors determine decoloration and mineralization. Their sensitivities to the reactions of the reactive intermediates of water radiolysis and to the conditions of irradiation are different. The dose requirement of mineralization is higher by about one order of magnitude than the dose requirement of decoloration. As it was mentioned in connection with Figs. 1 and 2, the efficiency of decoloration is the highest when reducing radicals e aq

or Hd are the reaction partners. These water radicals are suggested to react with the azo group, causing a destruction of the intensive electron conjugation through the azo bridge with high probability. This intensive conjugation is the reason of the vivid color of the azo dyes. The light absorption of the decolored molecule falls in the UV region; therefore, the visible absorbance gradually decreases with the absorbed dose, but there is no change in the shape of the absorption spectrum. Flamigni and Monti (1985), by studying the e aq reactions with azobenzene, suggested an electron capture by the azo bond and a subsequent protonation: + d e aq+–NQN–+H -–NH–N –. The hydrazyl radical thus formed may disappear in bimolecular radical–radical or radical–molecule reactions. Hydrazyl radicals directly form in the reaction of Hd atoms with the azo double bond. From the high efficiency d of decoloration by e and the nearly linear dose aq and H dependence, we can conclude that these intermediates react with low rate coefficients with the other parts of the dye and also they react with low efficiency with the transformation products of decoloration. It is clear from Fig. 1, when the HOd radical is the reactant, the visible absorption band of the chromophore group shifts to higher wavelengths upon irradiation, which means formation of new, modified chemical structures of the chromophore. As it was suggested in our former works, the HOd radicals add to the aromatic rings in the molecule (Wojna´rovits et al., 2007; Wojna´rovits and Taka´cs, 2008; Fo¨ldva´ry and Wojna´rovits, 2007). In Fig. 1, the change of the shape and the position of the peak at 531 nm is due to the reaction of radical intermediates, such as cyclohexadienyl or phenoxyl radicals. The cyclohexadienyl radicals in bimolecular disproportionation reactions may reform the aromatic structure; however, the reformed molecules have extra OH groups. If this extra OH group is on the part of the molecule, which is involved in the extensive conjugation, the color shifts to red. The combination of two phenoxyl radicals may also lead to colored products, with a modified structure. d As Fig. 2 shows, the Od 2 /HO2 radical anion–radical pair practically does not react with the aromatic part of the dye and also does not react with the azo bridge between two aromatic moieties. This is in agreement with findings published in the literature (Krapfenbauer et al., 2000; Getoff, 2002; Wojna´rovits et al., 2007), e.g. the superoxide radical anion is unreactive with p-phenylazoaniline, which is regarded as a model dye compound. d Owing to the low pKa of the Od 2 /HO2 pair (4.8) in neutral and d basic solutions, the O2 form is dominant. Od is a reducing 2 species with a standard reduction potential of E(O2/Od 2 ) ¼ 0.33 V vs. normal hydrogen electrode (NHE) (Wardman, 1989). Od reacts in one-electron transfer process with quinine-type 2 molecules, e.g. the rate coefficient with benzoquinone is k ¼ 9  108 mol1 dm3 s1 (Simic and Hayon, 1973). In the case of the dyes,

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one-electron reduction takes place when the hydrated electron (a strongly reducing species (E(aq/e aq) ¼ 2.9 V vs. NHE) or the 2-hydroxy-2-propyl radical (E(CH3COCH3,H+/CHd3C(OH)CH3) ¼ 1.9 V vs. NHE) reacts with dye molecules. Seemingly the d Od 2 /HO2 pair cannot induce such reaction. This must be due to the reduction potential conditions. In the literature, there are few reduction potentials published for adding one electron to dye molecules. The values for azobenzene, Orange I and Methylorange are around 0.6 V (Hunt et al., 2003; Sharma et al., 2003; Yadav et al., 2005). The low value makes the one-electron transfer from Od 2 thermodynamically impossible. However, some electron donating substituents are known to increase the reduction potential, e.g. for 4,40 -dihydroxyazobenzene, the value is 0.25 V (Bragger et al., 1997). From the negative result of decoloration of AR1 by O2d, we can conclude that its reduction potential must be much more negative than 0.33 V. Based on the above analysis, we may say that one-electron reduction Od 2 is possible with some dyes. It is known that in many free radical reactions addition dominate over one-electron transfer reaction despite the fact that the reduction potential is high enough to allow electron transfer (Jin et al., 1993). For instance, the superoxide radical anion and the phenoxyl-type radical can add to one another, and the electron transfer in certain cases is of minor process (Jin et al., 1993; Winterbourn and Kettle, 2003). The superoxide anion reacts with the phenoxyl-type radical formed from tyrosine with a rate coefficient of 1.5  109 mol1 dm3 s1. In the process, oxidized compounds, e.g. hydroperoxides may form. We show this reaction on the example of the phenoxyl radical (reaction (17)) (Winterbourn and Kettle, 2003). In the case of dyes, the formed dye-hydroperoxide products are also colored.

(17)

In irradiated systems, the mineralization is determined by the reactions of hydroxyl radicals that, by addition to aromatic rings, produce cyclohexadienyl-type radicals. In the reaction of two cyclohexadienyl-type radicals, as it was discussed formerly, either a dimer or a partly saturated molecule plus a new dye molecule with extra OH group forms. When the HOd radical addition takes place to the ring, which carries the hydroxyl group substituent, there is a possibility for water elimination forming phenoxyl-type radical (Roder et al., 1999). The phenoxyl-type radicals are not sensitive to oxygen. In the case of mineralization, the presence of oxygen is critical: the efficiency increases with O2 concentration. The cyclohexadienyl-type radicals readily react with dissolved oxygen, forming peroxyl radicals and peroxides as final products. (The O2 scavenging reaction is in competition with the unimolecular transformation and bimolecular reactions of these radicals, as mentioned before.) Getoff and co-workers showed in several publications that the peroxyl radicals and the peroxides formed in their stabilization in radiolysis systems undergo a large number of reactions, including ring opening with the formation of dicarboxylic acids, carboxylic acids, aldehydes, etc. (Getoff, 1998, 2002; Krapfenbauer et al., 2000). Mineralization starts through these reactions. The mineralization—followed by measuring the COD value—shows nearly linear dose dependence. At very high conversions, the trend changes and the COD removal slows down. Galindo et al. (2000) reported similar result in connection with aminoazo dyes mineralization. This is suggested to be due to the

17

formation of some aliphatic by-products, which have low reactivity with the oxidizing dOH radicals. d As we mentioned before (Fig. 3), the Od 2 /HO2 pair slightly enhances mineralization. The dye-hydroperoxides formation in the reaction between the superoxide anion and phenoxyl-type radicals is an oxidation process; further reactions of dyehydroperoxides probably also enhance the oxidation of the transformed dye molecules.

Acknowledgment The authors thank the Hungarian Science Foundation (OTKA, no.: K 60096) for the support. References Bragger, J.L., Lloyd, A.W., Soozandehfar, S.H., Bloomfield, S.F., Marriott, C., Martin, G.P., 1997. Investigations into the azo reducing activity of a common colonic microorganism. Int. J. Pharm. 157, 61–71. Buxton, G.V., Greenstock, C.L., Helman, W.P., Ross, A.B., 1988. Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (dOH/Od) in aqueous solution. J. Phys. Chem. Ref. Data 17, 518–886 updated version: /http://www.rcdc.nd.eduS. Emmi, S.S., Taka´cs, E., 2008. Water remediation by electron-beam treatment. In: Spotheim-Maurizot, M., Mostafavi, M., Douki, T., Belloni, J. (Eds.), Radiation Chemistry, from Basics to Applications in Material and Life Sciences. EDP Sciences, France, pp. 87–103. Flamigni, L., Monti, S., 1985. Primary processes in the reduction of azo dyes in alcohols studied by pulse radiolysis method. J. Phys. Chem. 89, 3702–3707. Fo¨ldva´ry, Cs.M., Wojna´rovits, L., 2007. The effect of high-energy radiation on aqueous solution of Acid Red 1 textile dye. Radiat. Phys. Chem. 76, 1485–1488. Galindo, C., Jacques, P., Kalt, A., 2000. Photodegradation of the aminoazobenzene acid orange 52 by three advanced oxidation processes: UV/H2O2, UV/TiO2 and VIS/TiO2. Comparative mechanistic and kinetic investigations. J. Photochem. Photobiol. A: Chem. 130, 35–47. Getoff, N., 1998. The role of peroxyl radicals and related species in the radiationinduced degradation of water pollutants. In: Cooper, W.J., Curry, R.D., O’Shea, K.E. (Eds.), Environmental Applications of Ionizing Radiation. Wiley, New York, pp. 231–246. Getoff, N., 2002. Factors influencing the efficiency of radiation-induced degradation of water pollutants. Radiat. Phys. Chem. 65, 437–446. Hihara, T., Okada, Y., Morita, Z., 2003. Reactivity of phenylazonaphthol sulfonates, their estimation by semiempirical molecular orbital PM5 method, and the relation between their reactivity and azo-hydrazone tautomerism. Dyes Pigments 59, 201–222. Hosono, M., Arai, H., Aizawa, M., Yamamoto, I., Shimizu, K., Sugiyama, M., 1993. Decoloration and degradation of azo dye in aqueous solution supersaturated with oxygen by irradiation of high-energy electron beams. Appl. Radiat. Isot. 44, 1199–1203. Hunt, P., Worrall, D.R., Wilkinson, F., Batchelor, S.N., 2003. Quantitative rate constants for the reaction of dyes and alkenes with a-hydroxyalkyl radicals, measured by laser flash photolysis. Photochem. Photobiol. Sci. 2, 518–523. Jin, F.M., Leitich, J., von Sonntag, C., 1993. The superoxide radical reacts with tyrosine-derived phenoxyl radicals by addition rather than electron transfer. J. Chem. Soc. Perkin Trans. 2 9, 1583–1588. Kim, T.H., Lee, J.K., Lee, M.J., 2007. Biodegradability enhancement of textile wastewater by electron beam irradiation. Radiat. Phys. Chem. 76, 1037–1041. Krapfenbauer, K., Wolfger, H., Getoff, N., Hamblett, I., Navaratnam, S., 2000. Pulse radiolysis and chemical analysis of azo dyes in aqueous solution I. p-Phenylazoaniline. Radiat. Phys. Chem. 58, 21–27. Roder, M., Wojna´rovits, L., Fo¨ldia´k, G., Emmi, S.S., Beggiato, G., D’Angelantonio, M., 1999. Addition and elimination kinetics in OH radical induced oxidation of phenol and cresols in acidic and alkaline solutions. Radiat. Phys. Chem. 54, 475–479. Schittl, H., Quint, R.M., Getoff, N., 2007. Products of aqueous vitamin B5 (pantothenic acid) formed by free radical reactions. Radiat. Phys. Chem. 76, 1594–1599. Sharma, K.K., Rao, B.S.M., Mohan, H., Mittal, J.P., Oakes, J., O’Neill, P., 2002. Freeradical-induced oxidation and reduction of 1-arylazo-2-naphthol dyes: a radiation chemical study. J. Phys. Chem. A 106, 2915–2923. Sharma, K.K., O’Neill, P., Oakes, J., Batchelor, S.N., Rao, B.S.M., 2003. One-electron oxidation and reduction of different tautomeric forms of azo dyes: a pulse radiolysis study. J. Phys. Chem. A 107, 7619–7628. Simic, M., Hayon, E., 1973. Comparison between the electron transfer reactions from free radicals and their corresponding peroxy radicals to quinones. Biochem. Biophys. Res. Commun. 50, 364–369. Suzuki, N., Nagai, T., Hotta, H., Washino, M., 1975. The radiation-induced decoloration of azo dye in aqueous solution. Bull. Chem. Soc. Jpn. 48, 2158–2163.

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´ry, L. Wojna ´rovits / Radiation Physics and Chemistry 78 (2009) 13–18 ¨ldva Cs.M. Fo

Wardman, P., 1989. Reduction potentials of one-electron couples involving free radicals involving free radicals I aqueous solution. J. Phys. Chem. Ref. Data 18, 1637–1755. Winterbourn, C., Kettle, A.J., 2003. Radical–radical reactions of superoxide: a potential route to toxicity. Biochem. Biophys. Res. Commun. 305, 729–736. Wojna´rovits, L., Taka´cs, E., 2008. Irradiation treatment of azo dye containing wastewater: an overview. Radiat. Phys. Chem. 77, 225–244. Wojna´rovits, L., Pa´lfi, T., Taka´cs, E., 2007. Kinetics and mechanism of azo dye destruction in advanced oxidation processes. Radiat. Phys. Chem. 76, 1497–1501.

Yadav, P., Rao, B.S.M., Batchelor, S.N., O’Neill, P., 2005. From alcohols to sugars—how does reactivity of a-hydroxyalkyl radicals change with structure? A quantitative examination by pulse radiolysis. J. Phys. Chem. 109, 2039–2042. Yamashita, S., Katsumura, Y., Lin, M., Muroya, Y., Miyazaki, T., Murakami, T., 2008. Water radiolysis with heavy ions of energies up to 28 GeV. 1. Measurements of primary g values as track segment yields. Radiat. Phys. Chem. 77, 439–446. Zhang, S.-J., Yu, H.-Q., Li., Q.-R., 2005. Radiolytic degradation of acid orange 7: a mechanistic study. Chemosphere 61, 1003–1011.