Fenton Degradation of Organic Compounds Promoted by Dyes under

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Environ. Sci. Technol. 2005, 39, 5810-5815

Fenton Degradation of Organic Compounds Promoted by Dyes under Visible Irradiation JIAHAI MA, WENJING SONG, CHUNCHENG CHEN, WANHONG MA, JINCAI ZHAO,* AND YALIN TANG Key Laboratory of Photochemistry, Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China

The influence of dyes on the Fenton reaction of organic compounds under visible irradiation (λ > 450 nm) was examined. It was found that the presence of dyes could accelerate greatly the Fenton reaction of organic compounds such as salicylic acid, sodium benzenesulfonate, benzyltrimethylammonium chloride, and trichloroacetic acid under visible irradiation and that a complete mineralization of those compounds could also be achieved. The dyes such as Alizarin Violet 3B which has an anthraquinone structure unit showed much more significant effect on the reaction than the dyes such as malachite green without the quinone unit. A reaction mechanism of dye AV as a cocatalyst in the photo-Fenton reaction of organic compounds under visible irradiation is proposed based on the cycle of Fe3+/Fe2+ catalyzed by quinone species and an electron transfer from the excited dye molecule to Fe3+.

Introduction Fenton and photo-Fenton reactions have been proven effective methods to treat organic pollutants in wastewater, and the mechanism and kinetics have been studied by many researchers (1-10). The generally accepted free radical chain mechanism for the Fenton reaction is shown below (5, 11, 12), and the slow reaction (eq 2) is rate-determining step of Fenton reaction.

Fe2+ + H2O2 f Fe3+ + HO• + OH- k1 ) 76 M-1 s-1 (1)

are also important. The HO• quantum yield for reaction 6 is 0.14 at 313 nm and 0.017 at 360 nm (13). Visible light irradiation cannot lead to this reaction and hence cannot accelerate the Fenton reaction since the absorption wavelength of the Fe(OH)2+ species is less than 400 nm and general organic pollutants cannot absorb visible light. Dyes represent the principal pollutants in the textile and photographic industry (19). In China, more than 1.6 × 109 m3 of dye-containing wastewater per year drains into environmental water system without treatment. The dyes are usually difficult to biodegrade; and other conventional treatment methods such as activated carbon adsorption, coagulation and reverse osmosis are also not effective for treatment of those dye pollutants. During recent years, it was found that due to effective electron transfer from the visible light-excited dyes into Fe3+, which leads to regeneration of Fe2+ and an easy cycle of Fe3+/Fe2+, much faster degradation and mineralization of various dyes have been achieved in the photo-Fenton reaction under visible light irradiation (20-23), comparing with the Fenton reaction in the dark. The mechanism is shown below.

dye + visible light f dye*

(7)

dye* + Fe3+ f Fe2+ + dye+•

(8)

Fe2+ + H2O2 f Fe3+ + HO• + OH-

(1)

HO• + dye (or dye+•) f degraded products

(9)

However, could such a dye photosensitization principle promote Fenton degradation of other organic compounds coexisted in the solution? The objective of this study is to investigate how the presence of dyes influences the Fenton reaction of the other organic compounds under visible irradiation. Hydroquinone and catechol have been found to greatly catalyze the Fenton degradation of organic compounds (24, 25), which is attributed to the fact that they could reduce Fe3+ to Fe2+ rapidly at low pHs and hence make a rapid regeneration of Fe2+ bypassing eq 2, the slow step of the Fenton reaction (26).

Fe3+ + H2O2 f Fe2+ + HO2• + H+ Fe2+ + HO• f Fe3+ + OH-

k2 ) 0.02 M-1 s-1 (2) k3 ) 3 × 108 M-1 s-1 (3)

HO• + H2O2 f HO2• + H2O k4 ) 2.7 × 107 M-1 s-1 (4) HO2• + HO2• f H2O2 + O2

k5 ) 8.5 × 105 M-1 s-1

(5)

In recent years, many studies have shown that organic pollutants could be degraded much more rapidly under UV illumination of the Fe2+/H2O2 or Fe3+/H2O2 system than in the dark Fenton reactions (13-16). For such photo-Fenton reactions, the effect of UV light is attributed to the direct HO• formation and regeneration of Fe2+ from the photolysis of the complex Fe(OH)2+ in solution as follows (14, 17, 18).

FeIII(OH)2+ + UV f Fe2+ + HO•

The resulting quinone or semiquinone can rapidly react with HO2• generated through the Fenton reaction (eq 4), and thus the quinone cycle is built up.

(6)

In the presence of ROH and RCOO-, the reactions of FeIII(OR)2+ and FeIII(RCOO)2+ under UV illumination to give Fe2+ * Corresponding author phone: +86-10-8261-0080; fax: +86-108261-6495; e-mail: [email protected]. 5810

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10.1021/es050001x CCC: $30.25

 2005 American Chemical Society Published on Web 06/25/2005

More surprisingly, various aromatic compounds could catalyze the Fenton degradation of dye malachite green (MG) under visible irradiation or in the dark depending on their ability to be transformed into hydroquinone-like intermediates (27, 28). In this work, the effect of dyes with and without a quinone structure unit (see the molecular structures below) on the photo-Fenton reaction of organic compounds under visible irradiation was examined. It was found that the dyes such as Alizarin Violet 3B (AV) with a quinone structure unit exhibit much more effective catalytic effect on the Fenton reaction of various organic molecules under visible irradiation, which can accelerate significantly the Fenton degradation and mineralization of those compounds, comparing to the effect of the dyes without the quinone structure unit. On the basis of the cycle of Fe3+/Fe2+ catalyzed by the quinone structure unit and the electron transfer of excited dye molecules by visible light, a reaction mechanism is proposed. Better understanding of these reactions is very important for mechanism studies and the successful application of Fentontype technologies for the treatment of practical wastewaters containing complicated components.

Experimental Section Materials and Reagents. Malachite green (MG) was from Sigma. Alizarin Violet 3B (AV) was of analytical grade. Fe(ClO4)3 and HClO4 were from Aldrich. Salicylic acid, sodium benzenesulfonate, benzyltrimethylammonium chloride (BTAC), trichloroacetic acid, 1,4-hydroquinone, Rhodamine B, Acridine Orange, Alizarin Red, and hydrogen peroxide (30%) were of analytical grade and were used as received except where was noted. Water was distilled and then further purified in a Barnstead Nanopure system to a resistivity > 18 MΩ‚cm.

was placed outside the Pyrex jacket to completely remove wavelengths shorter than 450 nm to ensure that the irradiation only consisted of visible light. Analyses. At the given reaction time intervals, samples were taken out and immediately analyzed by measuring the UV-vis spectra of the dyes with a Lambda Bio20 UV-vis spectrophotometer (Perkin-Elmer Co). The reaction kinetics of dyes was determined by the changes in absorbance at 617 nm for MG and 561 nm for AV, respectively. Salicylic acid, sodium benzenesulfonate, and BTAC were analyzed by highperformance liquid chromatography on a 5-µm, 250 × 5 mm Diamonsil C-18 column and detected with a Dionex P580 pump and a built-in UVD 340s diode array UV-vis detector. The mobile phases were 70% methanol/30% water for the analyses of salicylic acid (monitored at 298 nm; retention time, tR, 2.8min), 30% methanol/70% water for benzenesulfonic acid sodium (monitored at 220 nm; tR, 4.4 min), and 60% methanol/40% water for BTAC (monitored at 215 nm; tR, 2.0 min). All the mobile phases were at 1.0 mL/min and water effluent contained 0.1% H3PO4. All samples were analyzed immediately to avoid further reactions. The TOC (total organic carbon) was measured with a Tekmar Dohrmann Apollo 9000 TOC analyzer. Electron paramagnetic resonance (EPR) experiments were performed on a Bruker model EPR 300E spectrometer equipped with an in situ irradiation source of Quanta-Ray ND: YAG laser system (λ ) 532 nm), and the same quartz capillary tube was used for all measurements to minimize errors. Cyclic voltammetry experiments were performed on a Princeton Applied Research Potentiostat/Galvanostat Model 283 (EG&G Instrucments) using a platinum electrode as working electrode with a reference electrode (SCE) and an auxiliary platinum electrode between -1.0 and +1.0 V. The scan rate was 50 mV/s.

Results and Discussion

Procedures. All experiments were conducted in aerated solution, and the initial pH of 2.90 was adjusted with HClO4. All solutions were freshly prepared daily. Stock solutions of Fe3+ (10 mM) were prepared in 0.1 M HClO4. All experiments were carried out in a cylindrical Pyrex vial (50 or 100 mL) with a small magnetic stir bar. The light source was a 500-W halogen lamp (Institute of Electric Light Source, Beijing) positioned inside a cylindrical Pyrex jacket and cooled by circulating water. A cutoff filter

Fenton Degradation of Organic Compounds in the Presence of Dyes. Figure 1 shows the (photo-)Fenton degradation of three organic compounds (salicylic acid, BTAC, and sodium benzenesulfonate) in the presence of two different dyes of AV and MG, respectively. The two dyes were selected for their clearly different molecular structures with or without a quinone structure unit. Because those substrates could not absorb visible light, visible irradiation has no effect on their Fenton degradation. Compared with the control reaction, the presence of MG decreased the degradation rates of those compounds (except BTAC, see discussion below) in the dark while accelerated their degradation under visible irradiation. Addition of AV greatly promoted the degradation of the three target compounds, especially under visible irradiation. At the reaction time of 20 min, about 90% salicylic acid still existed in the control reaction, but only about 10% salicylic acid remained in the system in the presence of AV under visible irradiation (Figure 1A). When salicylic acid was replaced by BTAC or sodium benzenesulfonate, the similar results were obtained (parts B and C of Figure 1). The two dyes greatly promoted degradation of the two target aromatic compounds under visible irradiation. In the presence of AV under visible irradiation, the degradation induction period of the two compounds declined very much compared with the control reaction. Noteworthy, the target compounds and the dyes were scarcely decomposed under visible irradiation in the Fe3+ homogeneous or H2O2 homogeneous solutions. As we have reported (20, 21, 28), visible light-excited dyes can effectively inject electrons into Fe3+ which leads to regeneration of Fe2+ and an easy cycle of Fe3+/Fe2+ in the presence of H2O2 and thus a continuous production of HO• (eqs 7, 8, and 1). So dyes could be degraded much faster in the Fenton reaction under visible light irradiation than in the dark. Now it was found that such a dye photosensitization VOL. 39, NO. 15, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Mineralization of sodium benzenesulfonate in the Fenton reactions: (a) control reaction, (b) in the presence of AV in the dark, (c) in the presence of AV under visible irradiation. Initial concentrations: 2 × 10-4 M Fe3+, 2 × 10-2 M H2O2, 4 × 10-4 M sodium benzenesulfonate, 2 × 10-5 M AV.

TABLE 1. Degradation of Sodium Benzenesulfonate (2 × 10-4 M) in the Presence of Different Dyes (2 × 10-5 M) in the Fenton Reaction (2 × 10-4 M Fe3+, 4 × 10-3 M H2O2) under Visible Irradiation

FIGURE 1. Fenton degradation of organic compounds: (a) in the presence of MG in the dark, (b) control reaction in the dark, (c) in the presence of AV in the dark, (d) in the presence of MG under visible irradiation, (e) in the presence of AV under visible irradiation. Initial concentrations: 2 × 10-5 M MG, 2 × 10-5 M AV; (A) 1 × 10-4 M Fe3+, 2 × 10-3 M H2O2, 4 × 10-4 M salicylic acid; (B) 2 × 10-4 M Fe3+, 4 × 10-3 M H2O2, 2 × 10-4 M BTAC; (C) 2 × 10-4 M Fe3+, 4 × 10-3 M H2O2, 2 × 10-4 M sodium benzenesulfonate. principle could also work in Fenton degradation of coexisted organic molecules, which could not absorb visible light. Both MG and AV could effectively promote the Fenton degradation of target compounds under visible irradiation (curves d and e). So the coexistence of dyes could effectively improve Fenton degradation of other organics that could not absorb visible light under visible irradiation. AV showed much more effective acceleration effects on the degradation of the target organic compounds than MG did under visible irradiation and worked even in the dark. It has been reported that quinone/hydroquinone analogues could greatly catalyze the Fenton degradation of organic compounds (24-27). Because of its quinone structure unit, the dye AV could play a role like hydroquinone to recycle Fe2+ from Fe3+ in the Fenton reaction, and thus the degradation of target organic compounds was greatly promoted in the presence of AV.

The HO2• produced in the initial Fenton reaction would quickly react with AV and gives a dye semiquinone radical 5812

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dye

control (no dye)

Rhodamine B

Acridine Orange

Alizarin Red

degradation%t)10min degradation%t)20min

3.5 8.1

13.1 78.8

11.2 79.2

48.0 100.0

of AVH•. Then the AVH• reduces Fe3+ to regenerate Fe2+, coming back to AV, and the cycle builds up. However the dye MG without the quinone structure unit did not show this effect in the dark and performed just as a substrate. So the dye AV has two catalysis effects on regenerating Fe2+ from Fe3+; one is by quinone unit cycle, another is by the dye*/ Fe3+/Fe2+ cycle, which accelerates greatly the Fenton reaction of the coexisted organics. The effect of other dyes, Rhodamine B, Acridine Orange, and Alizarin Red, on the Fenton degradation of sodium benzenesulfonate was also investigated; the similar results were observed (Table 1). All the dyes significantly promoted the Fenton degradation of the target compound under visible irradiation, and Alizarin Red with the quinone/hydroquinone structure showed much greater effect than the other two dyes. Mineralization of Substrates. Figure 2 shows the mineralization of sodium benzenesulfonate in Fenton reaction. The control reaction could reach about 40% TOC removal within 4 h. Without visible irradiation, addition of AV into the sodium benzenesulfonate solution only got about 32% TOC removal of the system (addition of AV changed the TOC of the system value very slightly), though the degradation of sodium benzenesulfonate was significantly accelerated. The results were consistent with those reported previously that the dark Fenton reaction always could not reach deep mineralization (2, 15, 20, 21, 29). But under visible irradiation, addition of AV nearly led to complete mineralization of the total system at the same time scale. We have reported that complete mineralization of various dyes could be achieved under visible light irradiation in the Fenton reaction (20, 21). And here due to photosensitization of dye AV under visible irradiation, deep mineralization of coexisted small organic molecules such as sodium benzenesulfonate was also achieved. Similar mineralization trendencies were also found in experiments for Fenton degradation of salicylic acid in the presence or absence of AV. So, as a cocatalyst in the photo-Fenton reaction, dye AV under visible irradiation must

FIGURE 3. Cyclic voltammetry of 2 × 10-4 M Fe3+ in the presence of 2 × 10-5 M AV under visible irradiation. The scan rate was 50 mV/s from -1.0 to +1.0 V. have changed the reaction pathway of organic molecules in favor of deep mineralization. Noting that this mechanism is distinct from other photo-Fenton methods to achieve total mineralization of organics such as direct photolysis of hydrated Fe(OH)2+ to give Fe2+ and HO• under UV irradiation (14) or Fe ligand complexes to give reactive ROO• (15, 16). We also performed the experiments of trichloroacetic acid as an example of aliphatic compounds; the presence of AV under visible irradiation also significantly enhanceded the mineralization of trichloroacetic acid in the Fenton reaction. Cyclic Voltammetry and EPR Measurements. To further evidence the catalysis effect of AV on the Fenton reaction under visible irradiation, cyclic voltammetry and EPR experiments were performed. Figure 3 gives the cyclic voltammetry results of an Fe3+ solution in the presence of AV under visible irradiation. As irradiation time increased, the cathodic peak current (ipc) appeared and increased and finally kept constant after 4 min of irradiation. It meant that Fe2+ was produced and accumulated in the solution. Therefore, owing to the electron transfer from the excited AV to Fe3+ under visible irradiation, Fe2+ could be easily regenerated, and this available pathway of recycling Fe ions is very important in the Fenton reaction system. The redox potential of the AV*/AV+• molecule which has a same chromophore with quinizarin is about -0.68 V against the Ag/AgCl electrode (30). The redox potentials of the Fe3+/ Fe2+ couple is 0.771 V vs NHE, i.e., 0.549 V against the Ag/ AgCl electrode. So the electron injection from AV* into Fe3+ is thermodynamically probable. Figure 4 shows the EPR spectra of 4-fold characteristic peaks of HO• radicals in the Fenton reaction in the presence of AV using DMPO as the radical scavenger (23, 31). Weak HO• and HO2• signals were observed in the dark Fenton reaction (Figure 4C). Compared with the control reaction, the DMPO-HO• signals became stronger in the presence of AV in the dark and increased more under visible irradiation. Further, due to the rapid reaction of eqs 13 and 14, HO2• signals were greatly obscured in the presence of AV (panels A and B of Figure 4). So the addition of AV to Fenton system increased generation of HO• radicals significantly due to regeneration of Fe2+ from Fe3+. By cyclic voltammetry and EPR experiments, we confirmed the catalysis effect of AV in the Fenton reaction. Degradation of Dyes Concomitantly with the Degradation of Target Compounds. Most organics decompose under HO• attack. Therefore, at the same time to catalyze the degradation of the target compounds, dyes themselves would

FIGURE 4. DMPO spin-trapping EPR spectra of Fenton reaction of 2 × 10-4 M Fe3+ and 4 × 10-3 M H2O2: (A) in the presence of 2 × 10-5 M AV under visible irradiation; (B) in the presence of 2 × 10-5 M AV in the dark reaction; (c) without AV in the dark. also be destructed and mineralized during the reaction under visible irradiation or in the dark. What is interesting is the case in the presence of salicylic acid. Figure 5 shows the degradation of MG and AV in the Fenton reaction of salicylic acid in the dark. The presence of phenol derivatives such as salicylic acid greatly catalyzed Fenton degradation of MG (Figure 5A). However, from Figure 5B, we found that the presence of salicylic acid could not accelerate the AV degradation but the degradation of salicylic acid was catalyzed by AV (Figure 1 A). So the promotion of the Fenton reaction by quinone structure unit is sensitive to the nature of coexisting compounds. When a dye molecule and a small aromatic molecule coexist in the Fenton reaction system, the one for which it is easier to become the hydroquinone analogue would play a catalyst role in the Fenton degradation of the other substrate. Under visible irradiation, the dyes degraded much faster than in the corresponding dark VOL. 39, NO. 15, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Cyclic Fenton degradation of sodium benzenesulfonate in the presence of MG in the dark. Initial concentrations: 2 × 10-4 M Fe3+, 5 × 10-3 M H2O2, 2 × 10-4 M sodium benzenesulfonate, 2 × 10-5 M MG. At the end of each run, H2O2, sodium benzenesulfonate, and MG at the same initial concentrations were added to the solution.

SCHEME 1. Mechanism of Fenton Degradation of Aromatic Compounds in the Presence of AV under Visible Irradiation

FIGURE 5. Fenton degradation of dyes in the dark: (a) control reaction, (b) in the presence of salicylic acid. Initial concentrations: (A) 2 × 10-4 M Fe3+, 2 × 10-3 M H2O2, 4 × 10-4 M salicylic acid, 2 × 10-5 M MG; (B) 4 × 10-5 M Fe3+, 2 × 10-2 M H2O2, 4 × 10-4 M salicylic acid, 2 × 10-4 M AV. reactions, and similar results were also obtained that showed the presence of salicylic acid delayed the degradation of AV and accelerated the MG degradation. In Figure 1B, even the addition of MG also quickened up the degradation of BTAC to some extent in the dark and the result was different from that of salicylic acid and sodium benzenesulfonate. It may be assumed that MG is more prone to give quinone analogues under HO• attack than BTAC, so the degradation of BTAC could even be accelerated a little by MG in the dark. Discussions on the Induction Period. The Fenton degradation of BTAC or sodium benzenesulfonate displayed an induction period followed by a fast decomposition. A similar result was also reported by Pignatello (26), in which an induction period existed in the Fenton degradation of phenol and was found to be sensitive to the presence of hydroquinone and quinone. We performed the cyclic degradation of sodium benzenesulfonate in the presence of MG in the dark in the same reactor. In Figure 6, the first cycle presented a long induction period at the beginning of the reaction and then followed by a rapid reaction. But the second cycle nearly became a linear reaction process and showed no lag period. The shape of the reaction curve of the third cycle was distinct from the former two; the initial reaction process became the fastest period. Compared with the corresponding AV experiments (see Figure 1, curve c) and others’ works (26-28), we suggested that the production of hydroquinone/quinone analogues controlled the reaction induction period. In the absence of AV, no hydroquinone/ quinone structure unit existed in the system, so the dark Fenton reaction was very slow. Once some hydroquinone/ quinone analogues are present, the quinone cycle works, so the reaction became dramatically fast. In Fenton reactions, HO• addition to the aromatic substrates to give mono- or multihydroxyl intermediates or products (hydroquinone/ quinone analogues) has been firmly established (7, 10, 26, 28). As the cycles increased, the hydroquinone/quinone analogues accumulated more and more in the reaction system, leading to form more and more Fe2+ and HO•, so the 5814

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sodium benzenesulfonate degradation was increasingly accelerated.

Mechanism Discussion A possible reaction mechanism is proposed in Scheme 1 based on all the information obtained above. The presence of dyes greatly promoted the Fenton degradation of organic compounds under visible irradiation. The dyes with a quinone structure showed a much more significant effect. We choose dye AV as an example to elucidate the reaction mechanism. The presence of AV dye successfully builds up two cycles (one quinone/hydroquinone analogue cycle, another cycle of Fe3+/Fe2+ induced by the excited dye) under visible irradiation, and the double effects greatly promote the HO• production and thus accelerate significantly the Fenton degradation of organic compounds. Concomitantly with the degradation of organics, the AV as a coexisting organic compound would also be decomposed. Therefore, by driving the cycle of iron and continuous HO• production, dye AV played a role of sacrificial cocatalyst in the Fenton reaction under visible irradiation. A more detailed mechanism needs further study. Better understanding of these reactions is helpful for practical wastewater treatment and revelation of Fenton-type reaction mechanism.

Acknowledgments This work was financially supported under grants from the Ministry of Science and Technology of China (No. 2003CB415006), the National Science Foundation of China (Nos.

20133010, 20371048, 20373074, 50221001, 20277038, and 20520120221) and by the Chinese Academy of Sciences.

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Received for review January 1, 2005. Revised manuscript received May 30, 2005. Accepted June 2, 2005. ES050001X

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