Photocatalytic Degradation of Acid Blue 80 in Aqueous Solutions

Among the large number of organic pollutants examined, dyes present in ... 44.4% w/w as Na2SO4), chloride (5.6% as NaCl), nitrate (2.2% as NaNO3), sod...
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Environ. Sci. Technol. 2001, 35, 971-976

Photocatalytic Degradation of Acid Blue 80 in Aqueous Solutions Containing TiO2 Suspensions ALESSANDRA BIANCO PREVOT,† CLAUDIO BAIOCCHI,† MARIA CARLA BRUSSINO,† E D M O N D O P R A M A U R O , * ,† PIERO SAVARINO,‡ VINCENZO AUGUGLIARO,§ G I U S E P P E M A R C `ı , § A N D LEONARDO PALMISANO§ Dipartimento di Chimica Analitica, Universita` di Torino, 10125 Torino, Italy, Dipartimento di Chimica Generale e Organica Applicata, Universita` di Torino, 10125 Torino, Italy, and Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita` di Palermo - 90128 Palermo, Italy

The photocatalytic degradation of the anthraquinonic dye Acid Blue 80 in aqueous solutions containing TiO2 dispersions has been investigated. The process has been monitored by following either the disappearance of the dye (via HPLC) and the formation of its end-products (via IC, GC, and TOC analysis). Although a relatively fast decolorization of the solutions has been observed, the mineralization is slower, and the presence of residual organic compounds was evidenced even after long term irradiation, confirming the relevant stability of anthraquinone derivatives. The identification of various unstable intermediates formed after low irradiation times was performed by HPLC-MS, allowing us to give insight into the early steps of the degradation process which mainly involve C-N bonds breaking and substrate hydroxylation. Complete and relatively fast mineralization of the substrate was achieved by irradiating the semiconductor dispersions in the presence of added K2S2O8.

Introduction The possible application of heterogeneous photocatalysis for wastewater remediation as an alternative to conventional methods has been the subject of a wide range of investigations carried out during the past 15 years (1-8). Among the large number of organic pollutants examined, dyes present in wastewaters originated from the textile industry are of particular environmental concern since they not only give an undesirable color to the waters but also in some cases are themselves harmful compounds and can originate dangerous byproducts through oxidation, hydrolysis, or other chemical reactions taking place in the waste phase. Studies on photocatalytic decolorization of different classes of textile dyes have been reported in the literature, most of them including a detailed examination of the so* Corresponding author fax: 011 6707615; e-mail: pramauro@ ch.unito.it. † Dipartimento di Chimica Analitica, Universita ` di Torino. ‡ Dipartimento di Chimica Generale e Organica Applicata, Universita` di Torino. § Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita` di Palermo. 10.1021/es000162v CCC: $20.00 Published on Web 01/27/2001

 2001 American Chemical Society

called primary process under different working conditions (9-14). However, less attention has been paid to the study of the degradation mechanism and to the identification of major transient intermediates, which have been more recently recognized as very important aspects of these processes, especially in view of their practical applications (15-17). Although the photocatalytic treatment of wastes containing anthraquinonic dyes should be particularly attractive due to the well-known resistance of such compounds to lightinduced fading, only a few studies have been reported in the literature (16, 18, 19). The degree of mineralization observed in these experiments seems to be largely dependent on the substrate molecular structure. In the present work we examined the degradation of the anthraquinonic dye Acid Blue 80 (C.I. 61585), also termed Blue Nylosan F.L., used for wool and nylon dyeing in slightly acidic media. Our attention was mainly focused on the examination of molecular breaking processes responsible for the color fading and on the search of suitable conditions for a quantitative and rapid mineralization of dye-containing aqueous wastes. The structure of Acid Blue 80 is shown below:

Experimental Section Reagents and Materials. Samples of the commercial dye, containing also inorganic salts, were obtained from Sandoz. The content of Acid Blue 80 in the mixture, determined by elemental analysis, was 45.7% w/w. The following inorganic ions were identified and determined: sulfate (major component, 44.4% w/w as Na2SO4), chloride (5.6% as NaCl), nitrate (2.2% as NaNO3), sodium and ammonium (ca. 2.3% as NH4Cl). NMR analysis confirmed the presence of Acid Blue 80 as the unique organic compound in the examined dyeing mixture. 1H NMR spectra were recorded with a JEOL EX 400 spectrometer in DMSO-d6 solution (3%). δ (ppm) ) 2.07 (s, 3 H, CH3); δ ) 2.44 (s, 3 H, CH3); δ ) 2.57 (s, 3 H, CH3); δ ) 6.60 (s, 1 H, 2′-phenyl proton); δ ) 6.99 (s, 1 H, 2-anthraquinone proton); δ ) 7.90 (m, 1 H, 7-anthraquinone proton); δ ) 8.38 (m, 1 H, 8-anthraquinone proton). The remaining part of the structure is symmetrical. Stock solutions containing 500 mg L-1 of Acid Blue 80 in water were prepared, protected from light, and stored at 5 °C. The following analytical-grade reagents were used as received: NaOH, HNO3, H2SO4, HCl, H3PO4, Na2CO3, NaHCO3, KNO3, ammonium acetate (all from Merck); acetonitrile (Lichrosolv, Merck); tetrahexylammonium bromide (Fluka); K2SO4 and NaCl (Carlo Erba); K2S2O8 (Merck). TiO2 P25 from Degussa having a surface area of ca. 55 m2 g-1 and a measured size of the primary particles around 2030 nm (20) was used in all the photocatalytic experiments. The semiconductor was preliminary irradiated overnight to eliminate adsorbed organics and successively washed repeatedly with water. After centrifugation, the solid material VOL. 35, NO. 5, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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was dried in an oven (ca. 12 h at 80 °C), and a stock suspension containing 2 g L-1 of the dry powder in water was prepared by sonication. Instruments. The following instruments were used: UVvis spectrophotometer (Uvikon 930, Kontron); HPLC MerckHitachi, equipped with L-6200 pumps and a UV-vis L-4200 detector; GC 4600 (Carlo Erba) equipped with a TCD; IC Biotronik 5000, equipped with a BT 0330 conductometric detector; HPLC Varian Star equipped with a Star 9010 ternary gradient pump interfaced to a Thermoquest LCQ Ion Trap Mass Spectrometer with APCI ion source. Degradation Experiments. Most irradiation runs were performed under aerobic conditions in stirred cylindrical closed cells (40 mm i.d. × 25 mm high) made of Pyrex glass, on 5 mL of aqueous dispersions. A 1500 W xenon source, equipped with a 340 nm cutoff filter was used (Solarbox, from Cofomegra, Milan). The temperature within the cell was ca. 60 °C. A number of experiments were also performed irradiating 1500 mL of aqueous dispersions in a photochemical reactor (from Helios Italquartz, Milan) equipped with a 500 W medium-pressure mercury lamp positioned inside a cylindrical Pyrex vessel. The lamp was surrounded by a recirculating water jacket (Pyrex). Oxygen flowed through the irradiated suspension, which was continuously mixed using a magnetic stirrer. The temperature was kept at 30 °C. Analytical Determinations. Samples were taken from the reaction vessels and filtered through a 0.45 µm cellulose membranes (Millex, Millipore). For the experiments performed at initial pH ) 3, the dispersion was previously brought to pH 6-7 by adding NaOH in order to completely recover the dye adsorbed onto the semiconductor. After each irradiation cycle, the amount of residual dye was thus determined by HPLC; a RP-C18 column (Merck) was used, eluent: phosphate buffer pH 7.0 10 mM plus tetrahexylammonium bromide 15 mM (40% v/v); acetonitrile (60% v/v). Detector wavelength: 626 nm. The formation of CO2 was followed by headspace gas chromatography (experiments performed in closed cells) after acidification of the suspension with 2 M H2SO4, injected through the cell Teflon stopcock, or by precipitation as BaCO3 (degradations runned in the photoreactor) after bubbling the oxygen gas stream coming from the reactor into a Ba(OH)2 solution. Analysis of most inorganic ionic species present in the dye sample or formed upon the treatment was performed on the filtered solutions using ionic chromatography. Ammonium was determined using the Berthelot colorimetric method. More experimental details can be found in ref 21. The analysis of organic transients was accomplished by HPLC-MS after readjustment of chromatographic conditions in order to make the mobile phase compatible with the working conditions of mass spectrometer. Solvent A was 5 mM aqueous ammonium acetate (pH 6.8) and solvent B was CH3CN. LC was carried out on a Lichrospher Merck RP-18 column (25 0.0 mm × 4.6 mm i.d., dp ) 5 µm): The mobile phase flow rate was 1.0 mL/min. A linear gradient was run as follows: t ) 0, A ) 80, B ) 20; t ) 30, B ) 100%. The column effluent was introduced in the APCI source of the mass spectrometer. APCI was carried out with the vaporizer at 450 °C and using nitrogen (Claind) as sheath (80 psi) and auxiliary (20 psi) gas to assist with the preliminary nebulization and to initiate the ionization process. A discharge current of 5 µA was applied. Tube lens and capillary voltages were optimized for maximum response during perfusion of Acid Blue 80 standard. When necessary MS/MS analysis was performed at a relative collision energy percent (REC %) ranging between 25 and 40% (in the arbitrary scale of the instrument). 972

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FIGURE 1. Acid Blue 80 bleaching followed via UV-vis analysis of the irradiated solution: TiO2, 400 mg L-1; Acid Blue 80, 20 mg L-1.

Results and Discussion It is known that, upon irradiation of TiO2 semiconductor particles with light of proper energy (wavelength < 400 nm), the formation of e-CB - h+VB pairs is produced, leading to the formation of hydroxyl radicals and superoxide radicals from water and dissolved oxygen, according to the following reactions:

e-CB + O2 f O2.-

(1)

2O2.- + H2O f H2O2 + O2 + 2OH-

(2)

e-CB + H2O2 f •OH + OH-

(3)

h+VB + H2O f •OH + H+

(4)

These radicals, which have been recognized as the primary oxidizing agents in photocatalysis (22, 23), can in turn attack and degrade the organic substrates allowing in most cases their complete mineralization. The mechanism of dye degradation in the presence of TiO2 particles irradiated by visible light is, however, different since in this case dyes are excited by the incident radiation and a successive electron injection from the excited dye to the conduction band of the semiconductor occurs, leaving the corresponding cationic dye radical (24, 25). The dye radicals in turn react with the same oxidizing species previously described and further degrade (26). Taking into account the nature of the light sources used in the reported experiments, which emit both in the UV and in the visible regions, the above-mentioned degradation mechanisms can be present simultaneously and are indistinguishable. Blank experiments performed in Solarbox without addition of TiO2 showed no appreciable decolorization of the irradiated solution, thus confirming the expected good stability of this anthraquinonic dye under solar light irradiation. Addition of 400 mg L-1 of TiO2 to solutions containing 20 mg L-1 of Acid Blue 80 did not alter the dye stability in the dark. On the contrary, a complete bleaching of the dye solution was observed upon irradiation with simulated solar light (see Figure 1). (A) Experiments Performed in Closed Cells. Decomposition of Acid Blue 80 (Primary Process). The dye degradation obeys to a pseudo-first-order kinetic law, as previously reported in the literature for most of the investigated organic substrates. A series of experiments was performed at three different initial pH values, which were above, below, and around the isoelectric point of the semiconductor oxide,

FIGURE 3. CO2 evolution: TiO2, 400 mg L-1; Acid Blue 80, 20 mg L-1. FIGURE 2. pH effect on the Acid Blue 80 degradation rate: TiO2, 400 mg L-1; Acid Blue 80, 20 mg L-1; (a) pH 6.4; (b) pH 9.0; (c) pH 3.0. respectively. A value of pI of ca. 6.3 was reported for the employed TiO2 (27). The degradation curves (see Figure 2) indicate that the process is faster at pH 6.4, is slightly slower at pH 9, and is markedly slower at pH 3. These results can be tentatively explained taking into account the effect of pH upon the OH radicals production, although other parameters not considered here could also influence the results. At higher pH values the formation of active •OH species is favored, due to improved transfer of holes to the adsorbed hydroxyls, but repulsive effects between the negatively charged TiO2 particle and the anionic dye are operating. At pH 3, on the contrary, electrostatic attraction between positively charged catalyst particles and the dye is operating (this is confirmed by the observed persistent blue color of the filtered oxide), but lower concentrations of active OH radicals are usually formed under these conditions. The effect of TiO2 concentration on dye degradation has been also examined. Experiments performed at pH 6.4 on different TiO2/substrate ratios (10, 20, and 40) showed, as expected, faster degradation rates in the presence of higher semiconductor concentrations. It is known, however, that a practical limit exists (around 2 g L-1) above which the increased light scattering reduces the photonic flux within the irradiated solution, thus lowering the degradation rate. Evolution of the Degradation Products. Formation of CO2. The complete mineralization of 1 mol of the dye molecule implies the formation of the equivalent amount (32 mol) of CO2 at the end of the treatment. However, the formation profile of CO2 (shown in Figure 3) clearly indicates that the reaction does not go to completion. In fact, after several hours irradiation only about 72% of the initial organic carbon has been transformed into CO2, thus implying that other organic compounds are still present in the irradiated solution. This is supported by the UV analysis, which suggests the presence of residual organic products even after 4-5 h irradiation, confirming the noticeable resistance to degradation of the examined dye. These findings are in agreement with those obtained in a study concerning the photocatalytic degradation of anthraquinone, where the persistence of various aromatic compounds was reported after long-term irradiation (28). Formation and Evolution of Nitrogen-Containing Species. Previous studies on photocatalytic degradation of nitrogen-containing aromatics demonstrated that either photogenerated electrons and hydroxyl radicals act concurrently to transform the nitrogen-containing groups. The relative abundance of the main mineralization end-products

FIGURE 4. Nitrate and ammonia evolution: TiO2, 400 mg L-1; Acid Blue 80, 20 mg L-1.

FIGURE 5. Sulfate ion evolution: TiO2, 400 mg L-1; Acid Blue 80, 20 mg L-1. (NH4+ and/or NO3-) largely depends on the initial oxidation state of nitrogen in the substituent group and on substrate structure (29, 30). Figure 4 shows that ammonium is the main product of nitrogen transformation for Acid Blue 80, with less than ca. 16% of the stoichiometric nitrogen found as NO3- after 6 h irradiation. At the beginning of the process the initial nitrate concentration (due to the presence of nitrate in the commercial dye) shows a decrease which could be attributed to a partial reduction to ammoniun, as reported in previous studies (29). The relatively high ammonium/nitrate ratio observed is not surprising since similar ratios were usually found starting VOL. 35, NO. 5, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. HPLC-MS analysis of Acid Blue 80, 40 mg L-1: (a) before the irradiation; (b) after 5 min irradiation. from organic substrates containing nitrogen at lower oxidation states. Further oxidation of ammonia to NO3- can be obtained by increasing the oxygen excess in the reaction medium, for example, by fluxing air or simply by opening the cells during the treatment (21). Since after 6 h irradiation the sum of nitrate and ammonium contributions arising from the dye degradation and from the initially present salts corresponds to ca. 86% of the expected stoichiometric value, it can be reasonably assumed that nitrogen-containing end-products more difficult to detect, such as nitrogen oxides (31), are formed. The formation of N2 from hydroxylamine (another possible nitrogen-containing product) was also demonstrated (32). The presence of nitrogen in residual organic products, although not impossible, seems less probable on the basis of HPLC-MS analysis, further described. Formation of SO42-. The evolution of the total concentration of this anion is shown in Figure 5. It can be seen that a plateau is reached after about 4 h irradiation, which corresponds to the formation of ca. 95% of the amount of sulfate expected assuming complete mineralization of the dye. Taking into account the uncertainty related to the IC determination of trace levels of this analyte, the removal of 974

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sulfonic groups can be considered practically complete. Moreover, it can be seen that most of the sulfate ion is formed after ca. 2 h irradiation, thus indicating a relatively fast kinetics of sulfonic group removal. These observations have been confirmed by the HPLC-MS analysis, reported in the next section. Analysis of Organic Transients. A group of intermediates formed during the earlier degradation steps (within 20 min irradiation), still containing the SO3- groups, has been identified by HPLC-MS. In Figure 6 chromatographic traces monitored at t ) 0 min (Figure 6a) and t ) 5 min (Figure 6b) by means of a MS detector with APCI ionization source in negative ions are reported. Similar chromatographic profiles were also obtained after 15 and 20 min irradiation, but the sensitivity was lower due to the decreased concentrations of the intermediates. The parent molecule of Acid Blue 80 elutes at 10.84 min and shows two quasi-molecular ions (see inset in Figure 6a). The m/z ) 633 amu corresponds to the singly unprotonated molecule, while m/z ) 316 corresponds to the completely dissociated one (doubly charged form). Before applying any irradiation it is the only one species present, as expected. After 5 min of irradiation other peaks appear, the major one

FIGURE 8. Primary degradation of 20 mg L-1 of Acid Blue 80 under oxygen flow: (a) only light; (b) light and TiO2; (c) light, TiO2 and K2S2O8; (d) light and K2S2O8. TiO2 ) 1 g L-1; K2S2O8 ) 1.1 g L-1.

FIGURE 7. Digital reconstruction at various m/z values of the chromatogram obtained after 5 min irradiation: (a) 649 amu; (b) 665 amu; (c) 231 amu. being that at 11.65 min (m/z ) 435 amu) corresponding to the loss of one of the side aromatic moiety containing a sulfate group (see Figure 6b). Figure 7 shows different digital reconstructions, at various significant m/z values, of the chromatogram obtained after 5 min irradiation. As it can be seen other significant m/z values corresponding to a doubly hydroxylated form (665 amu) and to a monohydroxylated form (649 amu) of the parent molecule are present in more than one peak due to the fact that the OH radicals may attack the parent molecule and hydroxylate it at different sites. Also the m/z ) 231 may be traceable back to a hydroxylate moiety of the parent molecule. The m/z values numerically associated to the reasonable structures above proposed were further analyzed in MS/MS conditions in order to check the reliability of the structural attributions. The prevalent fragmentation pathways corresponded to the loss of neutral molecules (typically SO2, SO3, or H2O). These results provide experimental evidence of the role of the OH radicals in the preliminary steps of degradation of the dye molecule. Moreover, the observed early break of the C-N bonds reveals that they should be weak points of the dye molecule under the examined photocatalytic conditions. The chromatographic analysis with UV detection of the sample after 2-3 h irradiation still shows evidence of the presence of residual organic products. Anyway the analysis of the same sample by HPLC-MS did not give any results both in negative and in positive ions detection mode, thus allowing to exclude the presence in the chemical structure of degradation products of sulfonated substituent groups and of any other groups able to take part to acid-base equilibria (necessary condition to be detected in APCI ionization mode). These results support the hypothesis that, after the irradiation time considered, the presence of nitrogen-containing organic products becomes negligible. The identification of residual compounds present at this reaction stage, a difficult task due to the complex nature of the mixture (various ring-opening aliphatic products are

usually simultaneously present together with aromatic byproducts) and to the low concentration levels of these components, was not undertaken. Our attention was successively mainly focused on the assessment of experimental conditions able to permit the quantitative mineralization of the residual products after few hours irradiation. (B) Experiments Performed in the Photochemical Reactor. Photocatalytic Degradation in the Presence of Added K2S2O8. The possibility of adding oxidizing species to heterogeneous TiO2 dispersions has been examined in order to trap e-CB, delaying electron-hole recombination, and to favor the formation of active oxidizing radicals (such as •OH, SO4•-, and others) thus increasing the rate of degradation of organic intermediates (33, 34). In particular, peroxydisulfate can generate strong oxidizing sulfate radicals upon reaction with photogenerated e-CB, according to the following reaction:

S2O8) + e- f SO4) + •SO4-

(5)

The sulfate radical anion can, in turn, react with water forming OH radicals

•SO4- + H2O f SO4) + •OH + H+

(6)

and with organic compounds by attacking them via (i) hydrogen atom abstraction from saturated carbon, (ii) addition to unsaturated or aromatic carbon, and (iii) removal of electrons from some reactive groups, such as carboxylate anion. These peculiar properties suggested the addition of peroxydisulfate to enhance the degradation rate and the mineralization extent in photocatalytic processes. The above-mentioned approach has been successfully applied to increase the photocatalytic degradation/mineralization rates of various pollutants, such as organophosphorus compounds (35), dioxins, chlorophenols, and atrazine (36), chlorinated hydrocarbons (37), aniline and pyridine (38), phenylcarbamates (39), and others. To increase the mineralization yield, photocatalytic experiments were performed on the investigated Acid Blue 80 solutions in the presence of added K2S2O8, operating in the previously described cylindrical photoreactor under oxygen flow. The evolution of the primary process was investigated (i) only in the presence of light, (ii) in the presence of light and TiO2, (iii) in the presence of light, TiO2 and K2S2O8, and (iv) in the presence of light and K2S2O8 (see Figure 8). Photodegradation slowly occurs in the first case (ca. 4% of the dye transformed after 30 min irradiation), whereas faster decomposition of Acid Blue 80 was observed either in the presence of TiO2 (complete disappearance of the dye occurs after ca. 30 min) or TiO2+K2S2O8 (complete degradation observed after 20 min). Although the literature reports a VOL. 35, NO. 5, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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general increase of the photocatalytic degradation rates when K2S2O8 is added, only minor effects were observed in the present case. Finally, the degradation of the dye induced by K2S2O8 (in the absence of TiO2) also occurs, but the reaction rate is markedly lower. The observed extent of mineralization was, however, very different in these experiments (performed at fixed initial pH 6.4). Indeed formation of CO2 was neither observed in the runs where only light was present nor in the experiments performed with K2S2O8 alone, even after 18 h irradiation. About 85% of stoichiometric CO2 was obtained, on the contrary, upon 5 h irradiation in the presence of TiO2, with no further CO2 evolution after longer irradiation times. Only the presence of both photocatalyst and K2S2O8 allows for achieving the complete mineralization of Acid Blue 80 after ca. 4 h irradiation. The reported results demonstrate the feasibility of the proposed oxidant-assisted photocatalytic procedure to treat aqueous wastes containing the examined dye. The use of this approach seems in particular useful when complete degradation of organic byproducts must be accelerated and/ or ensured.

Acknowledgments Financial support from M.U.R.S.T. (Rome) is gratefully acknowledged.

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Received for review July 25, 2000. Revised manuscript received November 17, 2000. Accepted November 21, 2000. ES000162V