Degradation Pathways of the Commercial Reactive Azo Dye Procion

Environ. Sci. Technol. 2008, 42, 6663–6670

Degradation Pathways of the Commercial Reactive Azo Dye Procion Red H-E7B under Solar-Assisted Photo-Fenton Reaction J U L I A G A R C ´I A - M O N T A Ñ O , † FRANCESC TORRADES,‡ LEONIDAS A. ´ REZ-ESTRADA,§ ISABEL OLLER,§ PE SIXTO MALATO,§ MANUEL I. MALDONADO,§ AND ´ P E R A L * ,† JOSE Departament de Qu´imica, Edifici Cn, Universitat Auto`noma de Barcelona, E-08193 Bellaterra (Barcelona), Spain, Departament d’Enginyeria Qu´imica, ETSEIA de Terrassa (UPC), C/Colom, 11, E-08222 Terrassa (Barcelona), Spain, and ´ km4, Plataforma Solar de Almer´ia-CIEMAT, Carretera Senes 04200 Tabernas (Almer´ia), Spain

Received March 10, 2008. Revised manuscript received May 16, 2008. Accepted June 23, 2008.

Reactive azo dye Procion Red H-E7B solutions have been submitted to solar-assisted photo-Fenton degradation. The solution color quickly disappears, indicating a fast degradation of the azo group. Nevertheless, complete DOC removal was not accomplished, in accordance with the presence of resistant triazine rings at the end of the reaction. The intermediates generated along the reaction time have been identified and quantified. LC-(ESI)-TOF-MS analysis allowed the detection of 18 aromatic compounds of different size and complexity. Some of them shared the same accurate mass, and consequently, the same empirical formula, but appeared at different chromatographic retention times, evidencing their different molecular structures. Heteroatom oxidation products like NH4+, NO3-, Cl-, and SO42- have also been quantified and explanations of their release are proposed. Short chain carboxylic acids are also detected at long reaction times, as a previous step to complete dye mineralization. A link between the disappearance of the largest intermediate products and the increase of the solutions biodegradability has been established. Finally, taking into account all the findings of the present study and previous related works, the evolution from the original dye to the final products (triazine and CO2) is proposed in a general reaction scheme.

Introduction Azo dyes are of a nonbiodegradable nature and their disposal in water streams poses an important environmental threat. The main problem associated with dyes containing water is their strong color that involves both consumer rejection and * Corresponding author phone: 34 93 581 2772; fax 34 93 581 2920); e-mail:[email protected]. † Departament de Quı´mica, Edifici Cn, Universitat Auto`noma de Barcelona. ‡ Departament d’Enginyeria Quı´mica, ETSEIA de Terrassa (UPC). § Plataforma Solar de Almerı´a-CIEMAT. 10.1021/es800536d CCC: $40.75

Published on Web 08/07/2008

 2008 American Chemical Society

the difficulties of light propagation that affects the metabolism of aquatic phototrophic organisms. Among conventional technologies for the removal of pollutants, biological methods are the cheapest ones and, thus, they are usually considered the best choice. Nevertheless, the microorganisms used in conventional biological treatments are affected by the toxicity of some pollutants, or they simply do not assimilate nonbiodegradable substances like dyes. Thus, the development of new technologies that pursue the straightforward degradation of such substances is of interest. In recent years, the coupling between Advanced Oxidation Processes (AOP) and biological systems for the treatment of polluted effluents has been proposed (1-3). The goal is to perform the AOP as a first step to partially degrade the original pollutant, enhancing the biodegradability and generating a new effluent able to be treated in a biological plant. AOPs are based on the production of the highly reactive hydroxyl radical. This radical can react with organic matter giving CO2 as the final product. Due to their high reactivity, their attack is nonselective, something that is useful for the treatment of wastewater containing many different pollutants. Photo-Fenton is preferred among other AOPs because it can achieve high reaction yields with low treatment costs, mainly due to the possibility of an effective use of solar light as photon source (4). The process involves the reaction between Fe(II) and hydrogen peroxide (reaction 1, Fenton process). Fe(II) + H2O2 f Fe(III) + HO- + HO·

(R1)

Under irradiation of λ e 410 nm, Fe(III) can be reduced to Fe(II) closing a loop mechanism where the Fe species act as catalyst, giving rise to additional HO• (reaction R2, photoFenton process). (R2) Fe(III)(OH)2+ + hν f Fe(II) + HO The optimal combination of a chemical-biological coupled system is achieved by using the lowest amount of reactants in the chemical step to produce biocompatible effluents. In this sense, the monitoring of the intermediates of the chemical treatment is essential to understand and predict the biological compatibility of the phototreated effluents. The chemical analysis is not an easy task in the case of dye molecules, since they usually are large units that produce complex intermediates after one oxidation event, or they may produce low concentrations of small polar compounds difficult to detect. Procion Red H-E7B, is a widely used homobireactive dye that contains two monochlorotriazine groups. Its degradation in aqueous solution through the combined effect of the photo-Fenton reaction and a biological treatment has been previously reported by our group both at laboratory scale (3) and at pilot plant scale (5). The goal of the present work, carried out at pilot plant, has been to gain knowledge and to characterize the chemical substances that are involved along the photo-Fenton degradation of a complex azo dye molecule. To date, the only partial reaction mechanism of Procion Red H-E7B degradation, proposed by O’Neill et al., (6) is related to an anaerobic-aerobic treatment. Also, an attempt has been made to establish a relationship between those intermediates and the biodegradable character of the solutions, something that would help to predict the best coupling conditions among chemical and biological treatments.

Experimental Section Chemicals. Commercial Procion Red H-E7B reactive dye (C.I. Reactive Red 141, C52H34O26S8Cl2N14, DyStar) with a purity VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Procion Red H-E7B chemical structure. degree of 41%, the rest being of inorganic nature (Na2SO4 and NaCl), was used as received. Its chemical structure is shown in Figure 1. To use the chemical form normally found in industrial effluents, the solutions were hydrolyzed by adjusting the pH to 10.6 and heating to a constant temperature of 80 °C for 6 h, obtaining the following composition: DOC ) 40 mg L-1 C; COD ) 108 mg L-1 O2; and BOD5/COD ) 0.10). Mass balance calculations were based on the DOC in solution after hydrolysis. The rest of the chemicals used were of the highest commercially available grade and used as received. Sulphuric acid and sodium hydroxide solutions were used for pH adjustments. Pilot plant water was obtained from a distillation plant (conductivity

in the aromatic rings (they may be indistinctly linked to either the benzene or naphthalene ring). Additionally, isomers due to different lactone location in C3 and C3′ naphthalene structures, as well as different possibilities for the remaining sulfonic and/or chlorine substituent groups in C1, C1′, C1′′, C2, C2′, C2′′, C3, and C3′ byproduct must also be taken into consideration. With the available information, it is not possible to exactly know the molecular structures and the differences between the detected intermediates. Figure 2 only gives indications of the general structure among the several possible isomers that could justify MS data. To reveal the precise structures, more analysisse.g., comparison with commercial standards (if available), Nuclear Magnetic Resonance (NMR), and/or further tandem Mass Spectrometry (MS/MS) detection-would be necessary. The identified intermediates appeared neither in all collected samples nor with the same time profile. Figure 3 shows the time course of each compound. An early appearance of the large C1, C1′, C1′′, C2, C2′, C2′′, C3, and C3′ compounds, with maximum abundances at t30W between 0 and 6 min is observed. Smaller C4, C5, and C6 compounds presented a similar chronological distribution, with maxima located at t30W ≈ 0 min. C7, C8, C9, and C10 compounds were detected later, with maximum abundances at t30W between 6 and 13 min. After attainment of their maximum concentrations, all of the above intermediates react until complete disappearance. This decrease happened simultaneously with the appearance of the smallest C11, C12, and C13 compounds, whose presence continuously increased along the rest of the reaction time. The order of appearance of those intermediates helps to envisage the Procion Red H-E7B photodegradation mechanism. Hydroxyl radicals, the main oxidant species involved in photo-Fenton process, are strong electrophilic oxidants. Consequently, Procion Red H-E7B reactive azo dye degradation should be initiated by the attack of HO · upon an electronrich site, i.e., near the nitrogen atoms of the azo group or near the amino groups (7). In view of this, the fast decolorisation of the dye solution seems to suggest a sequenced oxidation mechanism in which hydroxyl radical preferably attacks the chromophore center of the dye molecules (i.e., the azo groups, sNdNs) cleaving them in three parts: the two lateral substituted naphthalene rings and the central body. The non detection of intermediates containing the original azo groups also points to this direction. Azo groups may be attacked at two positions (8). One is at the CsN single bond between the azo group and

633.0661 621.0217 365.0539 347.0878 366.0379 381.0488 395.0281 377.062 238.049 146.0227 128.0566 147.0068

the naphthalene ring, generating N2 gas according to eqs 2 and 3 (9, 10). R1sNdNsR2+HO· f R1sNdN · + R2sOH

(2)

R1sNdN· f R1 · + NtN

(3)

The second attack would be to the chromophore double bond, giving the formation of primary aromatic amines. By further degradation, the amino group would be finally released into aqueous solution as NH4+. The secondary amino group placed between the naphthalene or benzene and the triazine rings would be the target of the next HO · attack. The rupture between the N and the naphthalene or benzene rings is easier than that between the N and the triazine (11). In this way, the secondary amino group degradation would generate a primary amine linked to the triazine ring. A simplified scheme of this reaction would be as follows (eq 4):

(4)

The nearly simultaneous appearance of C1, C1′, C1′′, C2, C2′, C2′′, C3, C3′, C4, and C5 compounds in solution indicates that the above-described decomposition pathways (attack of HO · to the azo and amino groups) were both initially involved. Nevertheless, the molecular structures of some of those intermediate compounds show that additional degradation mechanisms should have also taken place. C3 and C3′ compounds would have been originated from subsequent C1, C1′, and C1′′ compounds degradation via a complex series of reactions. Alternatively, the addition of HO · to one of the carbon atoms bearing the sulfonic groups in structures C1, C1′, C1′′, C2, C2′, C2′′, C3, and C3′, would lead to the release of sulfate anion (10). Thus, SO42- presence in solution would clearly increase after the concentration of those intermediates begin their decline, in agreement with data about SO42evolution shown in Figure 4. At the end of the experiment, and after subtracting the sulfur initially present in the sample as impurity and the one introduced in the form of sulfuric acid, 30.7 mg L-1 (92% of the maximum stoichiometrically expected 33.4 mg L-1 of sulfur taking into account eq 1) were detected in solution indicating that total desulphuration had VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2 6666

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FIGURE 2. Procion Red H-E7B identified degradation intermediates.

FIGURE 3. Time course of the 18 detected Procion Red H-E7B degradation products (250 mg L-1 of hydrolyzed Procion Red H-E7B solution; 2 mg L-1 Fe (II); 250 mg L-1 H2O2; pH ) 2.8). been probably achieved even with a considerable DOC still remaining in solution.

Analogously, part of the substituting chlorine groups were replaced along the photodegradation. As can be seen in Figure VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Time course of sulfur (SO42-), chlorine (Cl-), and nitrogen (NH4+ and NO3-) concentrations (250 mg L-1 of hydrolyzed Procion Red H-E7B solution; 2 mg L-1 Fe (II); 250 mg L-1 H2O2; pH ) 2.8). For reference, maximum stoichiometrically expected concentrations are as follows: 43.6 mg L-1 of sulfur (taking into account the added H2SO4), 9.26 mg L-1 of chlorine, and 25.6 mg L-1 of nitrogen. 2, some of the intermediates have lost one or two of the original dye chlorines, while others still have both of them. Figure 4 also shows the Cl- formation (after substracting the Cl- originally present in the dye sample) along the reaction time. It should be pointed out that part of the original chlorine atoms linked to the triazine groups of the dye had already been released to solution during the preliminary hydrolysis process, and their concentration was neither included in the figure (the concentration of Cl- at the beginning of the dark Fenton reaction was taken as zero). The maximum attained concentration stands for 45% of the total stoichiometrically expected Cl- (9.3 mg L1, as calculated from eq 1), whereas the rest was lost during the hydrolysis or it should still be linked to the triazine moiety. The stable nature of chlorine atoms bonded to triazine rings had already been reported by other authors (8). On the other hand, C1, C1′, C1′′, C2, C2′, C2′′, C3, C3′, and C7 compounds show the direct addition of HO · to unsaturated aromatic bonds of the naphthalene and/or benzene structures, giving place to the formation of mono- or poly hydroxylated derivatives. Further oxidation of those groups would lead to degradation intermediates containing quinonelike structures (C8 and C9). The formation of those compounds is of special interest in Fenton chemistry since they facilitate the Fe (II) regeneration and, consequently, they enhance the catalytic character of the whole process by means of the following general equations (12):

(5)

(6)

As stated in the literature, benzene and naphtalene group hydroxylation ends up with the ring opening, giving short chain carboxylic acids (9, 13). In the present study, formic, 6668

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FIGURE 5. Time course of short chain carboxylic acid concentration (250 mg l-1 of hydrolyzed Procion Red H-E7B solution; 2 mg L-1 Fe (II); 250 mg L-1 H2O2; pH ) 2.8). acetic, oxalic, and maleic acids were detected in solution, and the evolution of their concentration with time is shown in Figure 5. As can be seen, all carboxylic acids appear at the outset of the photo-Fenton process. In fact, their presence was evident even before the beginning of solar irradiation (t30W ) 0 min). This behavior is indicative of the fast and easy hydroxylation and breaking of the naphthalene and/or benzene rings. Other studies have already reported the generation of formic acid as an initial intermediate of the degradation of large azo dye molecules (9). However, although the acids were early present in solution, their concentration reached a maximum around t30W ) 30 min, when C1, C1′, C1′′, C2, C2′, C2′′, C3, C4, C5, C6, C7, C8, and C9 compounds had disappeared (see Figure 3) and free sulfate and chloride anions had attained their maximum concentration (Figure 4). As previously known, (9) acetic acid is quite resistant to the hydroxyl radical attack and its degradation requires longer reaction times (Figure 5). The fate of Procion Red H-E7B triazine groups was quite different. The amino group degradation mechanism illustrated in Equation 3 would gradually transform large intermediates to smaller structures, like C10, C11, C12, and C13. As shown in Figure 3, the C11, C12, and C13 byproduct

FIGURE 6. Proposed reaction mechanism for Procion Red H-E7B degradation; XtCl or OH, YtSO3H or OH, Zt possible mono- or poly hydroxylations of naphthalene or benzene rings. continuously increased their concentration along reaction, a fact that suggests their accumulation and permanence in the final phototreated solution, in accordance with previous studies that report the difficult mineralization of the triazine moiety by photo-Fenton and other AOPs treatments (8, 14, 15). In those previous studies, cyanuric acid: is

recognized as the final degradation product, although it has not been detected in the present work. The degradation of

C11, C12, and C13 intermediates to cyanuric acid would involve the hydroxylation of the carbons holding the chlorine or the primary amino group. It is worth noting that, analogously to Cl- release, the substitution of primary amino groups linked to triazine rings by HO · is a particularly slow process (15), where the amino nitrogen would be directly transformed into NO3-, with just a minor formation of NH4+ (8). Among the identified intermediates, C13 and C6 could undergo such an amino group degradation mechanism. Finally, Figure 4 also shows the total nitrogen concentration in solution, corresponding to the NH4+ and NO3- ions released during Procion Red H-E7B mineralization. The mass balance of N was by no means completed, scarcely attaining 9% of the expected stoichiometric quantity (25.6 mg L-1 as calculated from Equation 1). In accordance with the previous discussion, the detected NH4+ may be generated from either VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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azo bonds or amino group degradation. On the other hand, the detected nitrate would correspond to both the direct transformation of amino groups into NO3- and the oxidation of NH4+. Nonetheless, it should be noted that ammonium intermediate oxidation is a slow process in acid media (16). The presence at the end of the reaction of triazine derivatives (e.g., C11, C12, C13, which may contain up to 5 nitrogen atoms in their structure), in addition to the possibility of N2 gas generation as a result of azo group attack by HO · (17), would justify the incomplete nitrogen mass balance. The presence of long-living triazine species would also give an explanation for the remaining DOC detected at the end of the photoFenton process. The knowledge of the reaction pathways is a key issue in those cases where a partial chemical treatment is designed to precede a biological process; a relationship between the chemical nature of the generated byproduct and the biocompatibility of the reaction mixture could be established. As reported in a previous work (5), 2 mg L-1 Fe (II) and 65 mg L-1 H2O2 were enough to generate biodegradable Procion Red H-E7B intermediate solutions with 20% DOC removal. In the present work, such a mineralization percentage corresponded to t30W ) 19 min, just when C11, C12, C13, and carboxylic acids intermediates were gaining importance and the large compounds had nearly completely disappeared (see Figures 3 and 5). This is an evidence of the higher biodegradability of the triazine-like species with regard to their precursors. In fact, cyanuric acidsthe expected final degradation product of the triazine ringssis reported to be biodegradable and non-toxic (18). Alternatively, the biodegradable nature of short chain carboxylic acids is also wellknown. Overall, the whole Procion Red H-E7B solar assisted photoFenton degradation can be described by a series of consecutive steps, schematically depicted in Figure 6. In order to completely identify some of the degradation intermediates that appear in the reaction scheme, further experimentation making use of complementary analytical tools should be necessary.

Acknowledgments The authors are thankful for the economic support of the Spanish Ministry of Education and Science through project CTQ 396 2005-02808, and the “Programa de Acceso y Mejora de Grandes Instalaciones Cientı´ficas Espan ˜ olas” (Plataforma Solar de Almerı´a, GIC-05-17).

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