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research 46 (2012) 3859e3867

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Degradation pathway of the azo intermediate 1-diazo-2- naphthol-4-sulfonic using Fenton’s reagent

Nanwen Zhu a,*, Lin Gu a,**, Haiping Yuan a, Ziyang Lou a, Liang Wang b, Xin Zhang a a School of Environmental Science and Engineering, Shanghai Jiao Tong University, Dongchuan road 800#, Shanghai 200240, PR China b Energy and Environment Division, Shanghai Advanced Research Institute, the Chinese Academy of Sciences, 99 Haike Rd, Pudong District, Shanghai 201210, PR China article info abstract

Article history: Degradation of naphthalene dye intermediate 1-diazo-2- naphthol-4-sulfonic acid (1,2,4- Received 17 September 2011 Acid) by Fenton process has been studied in depth for the purpose of learning more Received in revised form about the reactions involved in the oxidation of 1,2,4-Acid. During 1,2,4-Acid oxidation, the 11 April 2012 solution color initially takes on a dark red, then to dark black associated with the formation Accepted 22 April 2012 of quinodial-type structures, and then goes to dark brown and gradually disappears, Available online 2 May 2012 indicating a fast degradation of azo group. The observed color changes of the solution are a result of main reaction intermediates, which can be an indicator of the level of oxi- Keywords: dization reached. Nevertheless, complete TOC removal is not accomplished, in accordance Fenton’s reaction with the presence of resistant carboxylic at the end of the reaction. The intermedi- ates generated along the reaction time have been identified and quantified. UPLCe(ESI) 1,2,4-Acid eTOFeHRMS analysis allows the detection of 19 aromatic compounds of different size and complexity. Some of them share the same accurate mass but appear at different retention time, evidencing their different molecular structures. Heteroatom oxidation products like 2- SO4 have also been quantified and explanations of their release are proposed. Short-chain carboxylic acids are detected at long reaction time, as a previous step to complete the process of dye mineralization. Finally, considering all the findings of the present study and previous related works, the evolution from the original 1,2,4-Acid to the final products is proposed in a general reaction scheme. ª 2012 Elsevier Ltd. All rights reserved.

1. Introduction (usually exceeding 3 w.%), poor decolorization and biodegra- dation (Fu, 2002; Li, 1997; Lv et al., 2001). Various attempts Naphthalene and its derivatives are important industrial have been made to treat such kinds of wastewater, among chemicals and used extensively in dye and pharmacy indus- which, evaporation, polymeric absorption (Lv et al., 2001) and tries. The produced wastewater is always characterized by extraction (Hu et al., 2005) have been reported so far. intense color, high , concentrated substrate and However, there are still many drawbacks that limit the large-

* Corresponding author. Tel./fax: þ86 21 34203732. ** Corresponding author. Tel./fax: þ86 21 34200769. E-mail addresses: [email protected] (N. Zhu), [email protected] (L. Gu). 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2012.04.038 3860 water research 46 (2012) 3859e3867

scale application of these methods in real wastewaters. For example, polymeric adsorption is confronted with higher 2. Experimental section consumption rate, heavy contamination of the resin by some pollutants, as well as the difficulties of resin regeneration after 2.1. Chemicals adsorption saturation. Evaporation often produces volatile organic compounds (VOCs), causing the problem of secondary Commercial 1-diazo-2- naphthol-4-sulfonic acid (C10H6N2O4S, environmental pollution. SanFeng) with a purity degree of 98%, the rest being of inor- Naphthalene ring has delocalization-conjugated bond ganic nature, was used as received. Its chemical structure is composed of ten carbon atoms and this structure is quite shown in Fig. 1. All chemicals were prepared using high-purity U stable. These compounds are generally recalcitrant to biolog- water from a Millipore system with a resistivity of 18.2 M cm. ical treatment and constitute a source of pollution due to both Perchloric acid, ferrous sulfate, and peroxide (33% their toxic and mutagenic effect on humans, fish, algae and w/v) were purchased from Merck. microorganisms (Fedorak and Clemente, 2005). Advanced oxidation processes (AOPs) are an interesting treatment 2.2. Experimental setup option for this type of wastewaters, because of their great potential to oxidize, partially or totally, numerous organic All experiments were carried out at batch mode using an compounds (Pandey et al., 2011). This process is based on the acrylic reactor with a working volume of 250 mL. The formation of hydroxyl radical (HO$), which is a more powerful temperature was 28 2 C during the reaction. The reactor oxidant (E0 2.8 V) than the chemical reagents commonly used was also equipped with a mixer to ensure appropriate agita- 0 0 tion. Synthetic wastewater containing 1 mM 1,2,4-Acid was for this purpose, such as (E 2.0 V) or H2O2 (E 1.8 V). Rate constant in AOPs for organic compounds is several orders of dissolved with high-purity water and then pH-adjusted with magnitude higher than those reported for other processes 0.2 N NaOH or 0.2 N H2SO4 solutions. The initial pH was such as ozonation (Szpyrkowicz et al., 2001; Gotvajn et al., adjusted to 2.5 based on the results of preliminary experi- 2011; Duran-Moreno et al., 2011). Due to its high reactivity, ments (Gu et al., 2011). After pH adjustment, a calculated the hydroxyl radical is very unstable and must be continu- amount of catalyst ferrous sulfate (1.5 mM) was added as the 2þ ously produced in situ by means of chemical or photochem- source of Fe in this experiment. Then, H2O2 (30 mM) was ical reactions (Nogueira et al., 2011; Min et al., 2011). added into the reactor to start the reaction. At selected time One of the most effective AOPs consists of the utilization of intervals, 1 mL of the reaction mixture was taken and imme- 2þ diately injected into 0.1 N NaOH to increase the pH to 10 to Fenton’s reagent, a combination of H2O2 and Fe . In this þ process, H O decomposes catalytically by means of Fe2 at terminate the reaction (Lu et al., 2005; Anotai et al., 2006). The 2 2 m acid pH, giving rise to hydroxyl radicals samples were filtered through 0.45 m membrane filters to remove the precipitates formed. Filtered samples were then analyzed for 1,2,4-Acid removal and TOC removal. 2þþ ! 3þ þ $ þ Fe H2O2 Fe HO OH (1) All the experiments were conducted at least three times The application of Fenton’s reagent as an oxidant for and the experimental data are the average of at least three wastewater treatment is attractive, however, in recent years, measurements with an accuracy of 5%. the goal to perform the AOPs has been converted from one step elimination to partially degrade the pollutants. This is 2.3. Chemical analysis aimed to enhance the biodegradability and generate a new effluent able to be treated in a biological plant. Therefore, TOC was determined with a TOC Shimadzu 5000 A analyzer. A monitoring of the intermediates of the chemical treatment is Sulfate and carboxylic acids were measured essential to understand and predict the biological compati- with a Dionex DX-600 chromatograph using a Dionex bility of the Fenton-treated effluents. The mechanism in Ionpac AS11-HC4 250 nm column. Fenton’s reaction is complicated and schemes of reaction are The original dye degradation was monitored by High generally complex (Peral et al., 2008). The chemical analysis is Performance Liquid Chromatography (C-18 Phenomenex e not an easy task in the case of dye , since they LUNA column) and UV Vis detection (Agilent, Series 1100) at 1 usually are large units that produce complex intermediates 254 nm wavelength. The mobile phase were 5 g L KH2PO4 (A), 1 1 after one oxidation event, or they may produce low concen- 1.2 g L NaH2PO4 and 1 ml L . The column tration of small polar compounds that are difficult to detect. temperature was 25 C. The goal of the present work, carried out at a batch scale, is The elution gradient program for anions quantification to gain knowledge and characterize the chemical substances was a 5 min prerun with 20 mM NaOH, followed by of that are involved along the Fenton degradation of complex azo dye , especially that of naphthalene dye interme- diate. To date, literatures concerning the oxidation of 1,2,4- Acid using AOPs are scarcely reported elsewhere. This study also analyzes the color changes during the Fenton’s oxidation of 1,2,4-Acid in order to evaluate the relations between the intermediates evolution and the development of the color observed in the solution. Finally, a degradation pathway of 1,2,4-Acid by Fenton oxidation is proposed. Fig. 1 e Chemical structure of 1,2,4-Acid. water research 46 (2012) 3859e3867 3861

þ 20 mM NaOH for 8 min with 35 mM NaOH for 7 min. The flow (Lu et al., 2009), for a given Fe2 and parent , 1 rate was 1.5 mL min . The gradient program for carboxylate increasing the starting concentration of H2O2 does not always anions analysis consisted of a 10 min prerun with 1 mM NaOH, correspond to the faster oxidation of the target substance. As 10 min with 15 mM NaOH, 10 min with 30 mM NaOH and the reaction Kinetic Model predicted (Ledakowicz et al., 2000), 10 min with 60 mM NaOH. The flow rate was 1.5 mL min 1. For the oxidation of 1,2,4-Acid proceeded to a significantly lower sulfate calculations, previously introduced sulfate ion for pH extent than the condition in which 1,2,4-Acid was the only adjusting was removed from the results obtained from IC. substrate for hydroxyl radicals, suggesting the formation of Organic byproduct identification was carried out by means slowly degraded by-products that competed with 1,2,4-Acid of Ultra Performance Liquid Chromatography-(Electrospray and a limited organic carbon mineralization (Reardon and Ionization)-Time of Flight-High Resolution Zimbron, 2009). Researchers (Tang et al., 2008) came to (UPLCe(ESI)eTOFeHRMS) analysis of the different Fenton- a similar conclusion when examining the oxidation of treated samples. Solid-phase extraction (SPE) with oasis HLB a variety of chlorinated aliphatic compounds by Fenton’s cartridges from was employed for samples pre- reagent, presenting experimental evidence for an optimal concentration prior to the analysis. Separation was made H2O2 concentration around 20 mM. using reversed phase liquid chromatography (flow rate Purgeable total organic carbon (TOC) was also measured in 0.5 mL/min, injection volume 20 mL) in an HPLC (Agilent series these experiments (Fig. 2). TOC removal efficiency (refers to 1100) equipped with a 150 mm 4.6 mm C-18 analytical TOCt/TOC0) followed the different trend as 1,2,4-Acid removal m column of 5 m particle size. efficiency (refers to Ct/C0 of 1,2,4-Acid). The increase in TOC removal efficiency was due to the presence of higher concen-

tration of H2O2, which formed nonhydroxyl radicals and 3. Results and discussion hydroxyl radicals, while H2O2 concentrations were varied from 5 to 20 mM. However, decreased TOC removal efficiency was 3.1. Degradation of 1,2,4-acid observed at the beginning of 30 mM of initial H2O2 concentra- tion, which was probably due to the competition of interme- diates with 1,2,4-Acid on hydroxyl radicals. The higher The extent of 1,2,4-Acid degradation at different H2O2 doses during the experiments is presented in Fig. 2. This experiment concentration of H2O2 might have favored the side reactions $ was designed to provide complementary data on reaction that scavenged the OH to form radicals via stoichiometry at longer time scales, and the reactions were reaction 2, which has a constant of about e 1 1 allowed to proceed to completion (15 h) before analysis (1.2 4.5) 107 M S (Watts and Teel, 2005). The highest TOC removal was 25% when 30 mM of was used: (Reardon and Zimbron, 2009). At lower H2O2 doses (up to 20 mM), higher extent of 1,2,4-Acid degradation was achieved.

As the initial H2O2 concentration was increased (20 mM and OH þ H2O2/H2O þ HOO$ (2) higher), the enhanced 1,2,4-Acid removal efficiency was observed. 27% removal efficiency of 1,2,4-Acid could be ach- Although it may be assumed that 1,2,4-Acid degradation was complete, the total TOC removal did not occur, which was ieved when 5 mM initial H2O2 was added. Continuously a sign that organic substances highly recalcitrant to the Fen- increasing the initial H2O2 concentration up to 30 mM could degrade 90% of 1,2,4-Acid. This was attributed to the presence ton process had been formed (the analysis of intermediates formed is discussed in detail below) (Malato et al., 2009). In of excess of H2O2 available that can produce more hydroxyl radicals, as illustrated in reaction 1. However, only marginal view of reaction 3,20mMofH2O2 should be enough for the increase in 1,2,4-Acid’s degradation was observed when initial mineralization of 1 mM of 1,2,4-Acid. The H2O2 efficiency for 1,2,4-Acid was also determined in this study as illustrated in H2O2 concentration exceeded 20 mM. As previously reported Fig. 2. The H2O2 efficiency for 1,2,4-Acid removal was calcu- 1 lated using the amount of H2O2 consumed (mg L ) for the amount of 1,2,4-Acid removed (mg L 1)(Zhang et al., 2007). The result shown in Fig. 2 indicated that increasing the

initial H2O2 concentration from 3 to 10 mM could increase H2O2

efficiency. Continuously increasing the H2O2 concentration

would decrease the H2O2 efficiency. Theoretically, the highest

H2O2 efficiency of 100% for 1,2,4-Acid removal would mean that

1 moL of 1,2,4-Acid was removed by 1 moL of H2O2. The H2O2

efficiency for 1,2,4-Acid removal at 13 mM of H2O2 was 41%.

This means that not all of the H2O2 consumed was used to degrade 1,2,4-Acid only. Decreased efficiency can be explained by more aliphatic intermediates, favoring the side reaction of Fenton process reported by others (Malato et al., 2009).

Control experiments using initial concentration of H2O2 up to 20 mM (in the absence of iron salt), and initial concentration 2þ of Fe at 1 mM (in the absence of H2O2) resulted in no e Fig. 2 Effect of initial H2O2 on 1,2,4-Acid and TOC removal degradation of 1,2,4-Acid. Thus, 1,2,4-Acid degradation could 2þ efficiency by Fenton’s reagent. not occur when using H2O2 or Fe alone. 3862 water research 46 (2012) 3859e3867

3.2. Production of organic intermediate it is not possible to know the exact molecular structures and the differences between the detected intermediates. Table 1 In a single experiment of 1,2,4-Acid mineralization by Fenton’s only gives indications of the general structure among the oxidation, the TOC and 1,2,4-Acid concentration decreased several possible isomers that could justify MS data. To reveal from 110 mg L 1 and 1 mM to 82 mg L 1 and 0.08 mM respec- the precise structures, more analysis, e.g., comparison with tively, using 30 mM hydrogen peroxide after 200 min of Fenton commercial standards (if available), Nuclear Magnetic Reso- 2þ treatment (molar ratio of H2O2:Fe equaled to 20: 1 according nance (NMR), and/or further tandem Mass Spectrometry (MS/ to the preliminary experiments) (Fig. 3). It is important that the MS) detection- would be necessary (Peral et al., 2008). TOC removal reached a steady state before the analysis. The The identified intermediates appeared neither in all complete oxidation stoichiometry follows Eq. (3): collected samples nor with the same time profile. The evolu- tion of some of these intermediates along reaction time is þ / þ þ þ C10H6N2O4S 13 O2 10CO2 3N2 3H2O SO4 (3) shown in Fig. 4aed. These substances were determined by After that, the 82 mg L 1 TOC remained in solution, indi- accurate mass measurements, which allowed information cating the presence of unknown organic compounds of about the elemental composition of the intermediates and recalcitrant nature even with prolonged oxidation times. their fragment to be found with very low mass errors However, as previously stated, complete dye and color (Aguera et al., 2007). depletion took place even before the beginning of minerali- It is observed that during the 15 min of the Fenton reaction, zation (removal of TOC). several break-down products of 1,2,4-Acid appeared, Degradation samples corresponding to t ¼ 0, 5, 15, 20, 30, including 4-hydroxynaphthalene-1-sulphonic acid (1), 1,2- 40, 60e200 min were taken to detect oxidation intermediates Dihydronaphthalene (2), 2-hydroxyl-1,4-naphthoquinone (3) and to characterize their evolutions with reaction time. A (Fig. 4a). These naphthalene-type compounds might be large number of different aromatic compounds were found by considered as primary degradation products of the 1,2,4-Acid, means of UPLC e (ESI) eTOF e HRMS technique, and 19 of originating from the oxidative cleavage of the molecule in the them were identified based on accurate mass measurements. vicinity of the azo bond. Hydroxyl radicals, the main oxidant The proposal of the best chemical structures was supported species involved in Fenton process, are strong electrophilic by prior knowledge of the molecule pattern and the oxidative oxidants. Consequently, 1,2,4-Acid should be initiated by the treatment. Additional information was subtracted from the attack of HO$ upon an electron rich site, i.e., near the characteristic mass spectra isotopic distribution of certain atoms of the azo group or near the amino group (Galindo et al., molecules. Table 1 shows data related to the experimental and 2000). calculated masses of the deprotonated ions (mass to charge In Fenton’s oxidation process, the fast decolorization of the ratio; m/z), and proposed empirical formula corresponding to dye solution seems to suggest a sequenced oxidation mech- the mainly identified compounds. In all cases, the resulting anism in which hydroxyl radical preferably attacks the chro- accurate masses were found with an error lower than 1.7 mDa mophore center of the dye molecules (i.e., the azo groups, or 3.1 ppm. eN¼Ne) cleaving them into the lateral substituted naphtha- Among the identified intermediates, some molecules lene ring. The non detection of intermediates containing the shared the same accurate mass and the same empirical original azo group also pointed to this direction. Azo groups formula. Nevertheless, their different retention times evi- might be attacked at two positions (Wong et al., 2003). One is denced their different molecular structures, which probably the CeN single bond between the azo group and the naph- was a consequence of isomerism due to the trans isomers like thalene ring, generating N2 gas according to Eqs. (4) and (5) fumaric acid and maleic acid. With the available information, (Karkmaz et al., 2004; Lachheb et al., 2002).

N N N N O OH + HO (4)

O S O O S O H H

N N

OH OH (5) + NN

O S O O S O H H

Fig. 3 e Effect of time on 1,2,4-Acid degradation and TOC removal efficiency. water research 46 (2012) 3859e3867 3863

Table 1 e Accurate masses obtained from the LC-(ESI)-TOF-MS analysis of 1,2,4-acid degradation products. Compound Retention time min Detected m/z Formula Error mDa

e 1-diazo-2-naphthol-4-sulfonic acid 11.37 248.981 C10H5N2O4S 0.1 e 4-hydroxynaphthalene-1-sulphonic acid 0.60 223.140 C10H7O4S 1.1 e 1,2-Dihydronaphthalene 3.79 159.041 C10H7O2 1.5 e 2-hydroxyl-1,4-naphthoquinone 8.17 173.340 C10H5O3 1.4 e Phthalic anhydride 6.06 147.129 C8H3O3 0.2 e Phthalide 6.13 133.421 C8H5O2 1.1 e Phthalic acid 7.92 165.018 C8H5O4 0.2 e 2-acetyl-benzoic acid 8.96 163.572 C9H7O3 0.1 e 2-hydroxy-benzoic acid 7.56 137.334 C7H5O3 0.5 e 2-methyl-benzoic acid 6.74 135.113 C8H7O2 0.1 Benzoic acid 5.51 121.442 C7H5O2- 0.4 Phenol 2.12 93.187 C6H5O- 1.5 2,5-cyclohexadiene-1,4-dione 3.77 107.090 C6H3O2- 0.3

Fumaric acid 2.98 115.171 C4H3O4- 1.5 Maleic acid 2.08 115.398 C4H3O4- 0.1 Malonic acid 3.02 103.250 C3H3O4- 1.2

Ethanedioic acid () 1.76 89.075 C2HO4- 0.5 Acetic acid 1.29 58.997 C2H3O2- 0.1 Methanoic acid (formic acid) 0.79 44.977 CHO2- 0.2

The fragments produced by the cleavage of the azo bond to degradation intermediates containing -like of the dye molecule are the primary reaction intermediates. structures such as 2-hydroxyl-1,4-naphthoquinone. The The compounds above are further degraded to give some of formation of those compounds is of special interest in Fenton þ the lower-molecular-weight products presented in Table 1, chemistry since they facilitate the Fe2 regeneration and, via a series of complicated oxidation reactions, which can not consequently, they enhance the catalytic character of the be identified in detail. Further oxidation of 4-hydroxynaph- whole process by means of the following general equations thalene-1-sulphonic acid or 1,2-dihydronaphthalene would (Ma et al., 2006):

Fig. 4 e Time course of some main detected 1,2,4-Acid degradation products. 3864 water research 46 (2012) 3859e3867

OH O

3+ Fe + + Fe2+ + H+ (6)

OH OH

(7)

Fig. 5 e Concentration of low-molecular weight acids in solution (Full line) and sulfate ions (dotted line), as function of time.

Smaller aromatic intermediates containing a six- and 40 min, indicating a progressive oxidation of higher molecular a five-atom ring, such as phthalic anhydride (4), phthalide (5) weight species. appear almost together with the early naphthalene pieces In the case of their mineralization, literature (Rodriguez (Fig. 4b). Fragments of the compounds above, including et al., 2005) have reported that a complete oxidation of for- phthalic acid (6), 2-acetyl-benzoic acid (7), 2-methyl-benzoic mic acid would be achieved in the event that initial concen- acid (8) and benzoic acid (9), appear already in small quantity trations of H2O2 was 14 times the stoichiometric amount at the beginning of the reaction. The concentrations of these required to oxidize formic acid to CO2. Oxalic acid is a stable aromatic acids increase with the function of time and go intermediate and its concentration remained constant once as through maxima at 30 min of the reaction (Fig. 4c). All these it reached a maximal value as Fig. 5 shows. Alegrı´a et al. þ intermediates are detectable for up to 1 h. Aliphatic acids, (Alegria et al., 2003) have proposed a formation of Fe2 and þ such as fumaric acid (10), malonic acid (11) maleic acid, (12) Fe3 complexes. In the case of acetic acid, it was evidenced appear at measurable concentrations after 5 min, and their that negligible (less than 5%) oxidation was observed when concentration goes through relatively sharp maxima at 35min temperature and catalyst dose were increased (Rodriguez (Fig. 4d). et al., 2005). No intermediates were detected in the course of The order of appearance of those intermediates helps to formic and acetic acids oxidation (Kim et al., 2008). envisage the 1,2,4-Acid degradation mechanism in Fenton The addition of hydroxyl radicals to one of the carbon induced system. atoms bearing the sulfonic groups in structures would lead to the release of sulfate anion (Wang et al., 2006). The subse- e 3.3. Evolution of low-molecular-weight substances quent elimination of SO3 is a non-probable pathway due to the electron withdrawing effect of sulphonate and steric 2- The other primary intermediate originating from the initial hindrance. Thus the SO4 present in solution would clearly cleavage of the naphthalene group is the ring. This increase after the concentration of those intermediates began 2- species may undergo further degradation in the aqueous to decline, in agreement with the data about SO4 evolution 1 phase to form products such as benzoic acid or phenol. All shown in Fig. 5. At the end of the experiment, 91 mg L of these intermediates are subjected to further oxidation in order was detected in solution (94.7% of the maximum 1 to form the lower-molecular-weight acids. As stated in liter- stoichiometrically expected 96 mg L of sulfur taking into ature, benzene and naphthalene group hydroxylation ends up account Eq. (3)), indicating that total desulphuration had been with the ring opening, giving short-chain carboxylic acids probably achieved even with a considerable TOC remaining in (Stylidi et al., 2003). In the present study, formic, acetic, and solution. oxalic were detected in solution, and the evolution of their concentration with time is shown in Fig. 5. As can be seen, all 3.4. Decolorization carboxylic acids appear at the outset of the Fenton reaction. The behavior indicated the fast and easy hydroxylation and The color shown by wastewaters during oxidation treatment breakage of the naphthalene and/or benzene rings (Peral et al., could be used as an overall indicator of their oxidation state. 2008). Other studies have already reported the generation of Color generation during 1,2,4-Acid oxidation is a fast reaction formic acid as an initial intermediate of the degradation of where the naphthalene solution changes initially from red to large azo dye molecules (Rodriguez et al., 2005). Although dark black, and later the colors decays to minimum. these compounds are present in the solution from the early Representative UV/Vis spectra obtained from the solution stage of the reaction, their concentration is maximized at (data not shown) indicates that the absorption spectrum of water research 46 (2012) 3859e3867 3865

the formation of intermediates, resulting from the Fenton’s oxidation of the azo dye, which still contains benzoic- and naphthalene-type rings. The pathway followed by 1,2,4-Acid reaction and the many experimental results reported here show that the color directly depends on the main reaction intermediates. In the first minutes of the reaction, the 1,2,4-Acid solution undergoes a fast color change from dark red to dark black, reaching a peak level. Their color comes from their quinodial structures, which contains chromophore groups substituted in naphthalene rings. 2-hydroxyl-1,4-naphthoquinone achieves their peak level during the first minutes, then disappears slowly. These highly colored species are well known in the literature (Mijangos et al., 2006). The interaction between reacting species would be the other reason for the dark appearance. On the other hand, Fenton reaction involves a reaction between iron and hydrogen peroxide where ferric ions are Fig. 6 e Normalized intensities of the three main UV/Vis generated (Sheu et al., 1989). These species can bind to inter- absorbance bands of 1,2,4-Acid plotted as function of time. mediates with hydroxyl group substituted in naphthalene rings generating metal complexes (Lindsey et al., 2003). The effect of iron on the fading rate of water is not so clear. The final 1,2,4-Acid in water at pH 2.5 is characterized by a band in light brown color probably comes from the formation of metal þ visible region located at 495 nm, and by two bands in ultra- complexes between the oxidized Fe3 and various organic violet region located at 230 and 305 nm, that are due to the substrates, such as ferrioxalate complex. These complexes are benzene and naphthalene rings of 1,2,4-Acid, respectively. colored species that persist throughout the process. The normalized intensities of the absorption bands at 230 nm (benzene ring), 305 nm (naphthalene ring) and 495 nm (azo 3.5. Proposed route for 1,2,4-acid oxidation linkage) are plotted as function of reaction time (Fig. 6). It is observed that the intensity of the band due to the visible light Based on those intermediates findings, a general pathway for chromophore decreases exponentially with time and almost degradation of 1,2,4-Acid with Fenton process was proposed. disappears after 30 min. The intensity of the absorbance peaks The proposed pathway is shown in Fig. 7. As can be seen, the at 230 nm and 305 nm decreases rather slowly during the first degradation process can be initialed either by the attack of 10e30 min, when the solution is still colored, and more rapidly HO$ on 1,2,4-Acid, resulting in the destruction of azo bond or in the continue, when decolorization is almost complete. The by the direct hydroxylation of 1,2,4-Acid’s N-position to give slower decrease of the intensity of the bands at 230 and 1,2-dihydronaphthalene. 2-hydroxyl-1,4-naphthoquinone is 305 nm, with respect to that of azo bond, can be attributed to continuously formed from HO$ attack which further breaks

Fig. 7 e Proposed reaction mechanism for 1,2,4-Acid degradation. 3866 water research 46 (2012) 3859e3867

into phthalic acid. The further degradation of aromatic Fedorak, P.M., Clemente, J.S., 2005. A review of the occurrence, compounds to ring cleavage reaction generating analyses, toxicity, and biodegradation of naphthenic acids. e a mixture of maleic acid and fumaric acid. These acids are Chemosphere 60 (5), 585 600. Fu, C., 2002. Development of treatment technology of transformed into formic acid, acetic acid and oxalic acid. The naphthalene dye intermediate wastewater in China. Dyestuff ultimate , oxalic acid, is slowly converted to Industry 39, 35e38 (in Chinese). $ CO2 by HO in the Fenton process. As far as we know, the Galindo, C., Jacques, P., Kalt, A., 2000. Photodegradation of the proposed degradation pathway of 1,2,4-Acid using Fenton aminoazobenzene acid orange 52 by three advanced e process has not yet been reported in the literature. oxidation processes: UV/H2O2 UV/TiO2 and VIS/TiO2 comparative mechanistic and kinetic investigations. Journal of Photochemistry and Photobiology A-Chemistry 130 (1), 35e47. 4. Conclusions Gotvajn, A.Z., Nakrst, J., Bistan, M., Tisler, T., Zagorc-Koncan, J., Derco, J., 2011. Comparison of Fenton’s oxidation and 1,2,4-Acid can be completely degraded in ozonation for removal of estrogens. Water Science and using Fenton’s reagent, but a comparatively low TOC Technology 63 (10), 2131e2137.

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