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Production of Reactive Oxygen Species by Photoactive Anthraquinone Compounds and Their Applications in Wastewater Treatment Ning Liu and Gang Sun* Division of Textiles and Clothing, University of California, Davis, California 95616, United States ABSTRACT: 2-Anthraquinone sulfonate (2-AQS) was found to exhibit an abnormally high singlet-oxygen quantum yield in aqueous solution after UVA irradiation based on a widely used testing method for singlet oxygen. To discover the cause of the abnormal results, several possible reactive oxygen species were also measured in the system. Results revealed that 2-AQS and two other similar compounds produced singlet oxygen, hydroxyl radicals, and superoxide, so-called reactive oxygen species (ROS), under UVA exposure. The production of hydroxyl radical contributed to the abnormally high quantum yield. Quantifications of the ROS produced by these compounds were conducted by oxidation of p-nitrosodimethylaninline in the presence and absence of either L-histidine or superoxide dismutase. EPR results also confirmed the formation of singlet oxygen by these compounds under UVA irradiation. The influence of different exposure times on the formation of ROS was measured for all three photosensitive compounds. The effects of different additives on the production of ROS were also investigated. The results showed that some compounds traditionally known as “hydroxyl radical scavengers” instead served as hydroxyl radical “generation promoters” under UVA exposure in the system. Moreover, for the first time, these photosensitive compounds were used to degrade a textile colorant, namely, Reactive Black 5, achieving the total decoloration of this compound within 10 min.
’ INTRODUCTION Many compounds can absorb ultraviolet and visible light and become activated upon light exposure. The photoactivated compounds can produce radicals and, in the presence of oxygen, even reactive oxygen species (ROS) such as singlet oxygen, hydroxyl radicals, and superoxide radicals under such photoactivation. These ROS could provide functions such as inactivating microorganisms and decomposing toxic compounds.16 Such photoinduced functions are economically and environmentally attractive and can be employed in different applications.7,8 The UV-induced radicals on the compounds could also initiate radical polymerizations, which can be employed in surface modifications of polymers and incorporation of novel functions into the polymers.9,10 For these UV-activated reactions, photosensitizers are the key compounds to induce the desirable reactions. Benzophenone is a common photosensitizer and can photoinitiate graft polymerizations and antibacterial effects on polymers.914 Because of their similar structural features, anthraquinone compounds were expected to exhibit similar photoinduced activities. Indeed, three aromatic ketone compounds, namely, 2-anthraquinone sulfonate (2-AQS), 2,6-anthraquinone sulfonate (2,6AQS), and 2,7-anthraquinone sulfonate (2,7-AQS), have shown the ability to initiate polymerization of vinyl monomers under UVA irradiation.1519 Recently, fabrics treated with these three compounds were evaluated against microorganisms under UVA and fluorescent irradiation, and the results indicated that the compounds provided excellent photoinduced antibacterial functions to the polymers.20 Compared with the most frequently used photocatalyst, TiO2,8 these anthraquinone compounds could be easily incorporated into certain polymers having reactive functional groups, whereas TiO2 is mostly employed in ultrafine particles.21 Two different mechanisms have been proposed for the photoinduced antibacterial functions of these compounds. According to the first mechanism, superoxide, serving as the major biocide, is r 2011 American Chemical Society
produced through electron-transfer reactions between the excited triplet state of the photosensitive compounds and oxygen molecules, with H2O2 and reactive oxygen radicals formed during the photoirradiation process. Based on the second mechanism, singlet oxygen is the major antibacterial agent.22 The formation of superoxide radical, singlet oxygen, and hydroxyl radical has been detected23 and found to be affected by addition of different solvents in quinone solutions.23 Research results indicate that the oxygen uptake by AQS compounds increases significantly in the presence of hydrogen donors,24 which consequently affects the formation of all three reactive oxygen species. Therefore, in this study, we studied the influence of different solvents on the production of reactive oxygen species, especially hydroxyl radicals. In addition, because hydroxyl radicals are formed during the photolysis of anthraquinone compounds, this phenomenon was explored for the degradation of potential pollutants.8 Thus, the feasibility of using these anthraquinone derivatives to degrade a representative colorant, Reactive Black 5, was studied.
’ MATERIALS AND METHODS Materials. Anthraquinone-2-sulfonate sodium (2-AQS), anthraquinone-2,6-sulfonate sodium (2,6-AQS), anthraquinone2,7-sulfonate (2,7-AQS), superoxide dismutase (SOD) from bovine liver (4800 unit/mg), L-histidine (g99.5%), p-nitrosodimethylaniline (p-NDA), benzoic acid (g99.5%), Reactive Black 5, and dimethyl sulfoxide (DMSO) were purchased from Aldrich (Milwaukee, WI). All other chemicals were supplied by major chemical vendors and used as received. All water used in this study was deionized water. UV irradiation was obtained from a Spectrolinker XL-1000 UV cross-linker (Spectroline, IN), with Received: July 4, 2010 Accepted: March 13, 2011 Revised: March 11, 2011 Published: April 04, 2011 5326
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Industrial & Engineering Chemistry Research five 8-W lamps at a wavelength of 365 nm. The distance between the UV lamps and the sample material was 12 cm, and the intensity was between 1.3 and 2.0 mW/cm2. Generation and Quantification of Singlet Oxygen. The formation of singlet oxygen and the corresponding quantum yields of these anthraquinone compounds were initially measured by a widely used method.25,26 According to this method, pNDA was used as a selective scavenger to measure the formation of singlet oxygen, whereas the formation of other reactive oxygen species (ROS) was neglected. Although p-NDA absorbs UVA radiation at 365 nm, according to the literature,26 it does not generate ROS. Specifically, 1.5 104 M 2-AQS or rose bengal phosphate buffer (pH = 7.4) solution containing 0.74 mM Lhistidine was mixed with an equal amount of 4 104 M p-NDA aqueous solution containing 10% ethanol. Because rose bengal is known to have a quantum yield of singlet oxygen of 0.76,27 it was chosen as a reference standard to calculate the quantum yield of 2-AQS. UVvis absorption for p-NDA was measured with an Evolution 600 UVvisible spectrophotometer (Thermo Scientific, Waltham, MA) at a wavelength of 440 nm with polystyrene disposable cuvettes after 60 min of UVA exposure. Under constant conditions, changes in UVvis absorbance are proportional to the amount of singlet oxygen formed. Detection of Hydroxyl Radicals. Because singlet oxygen itself and superoxide radicals could not directly oxidize p-NDA,26 the production of hydroxyl radicals was tested based on a photometric method.28 Generation of hydroxyl radicals was quantified by determination of the bleaching rate of p-NDA (change in concentration with time). The relative quantum yield of hydroxyl radicals was obtained by mixing 2 105 M solutions of photoactive 2-AQS, 2,6AQS, or 2,7-AQS in phosphate buffer (0.01 M, pH 7.4) with equal amounts of 2 105 M p-NDA solution. UVvis absorption of pNDA was also measured with the UVvisible spectrophotometer at a wavelength of 440 nm with polystyrene disposable cuvettes. EPR Detection of Singlet Oxygen. An electron paramagnetic resonance (EPR) spectrum of 2-AQS was recorded at room temperature (25 ( 5 °C) with the X-band at 9 GHz frequency on a Bruker ECS 106 spectrometer (Bruker Corporation, Billerica, MA) using 2,2,6,6-tetramethyl-4-piperidone (TEMP) as a spin-trapping reagent.29 The EPR flat cell containing 100 μL of 2 105 M 2-AQS phosphate buffer solution (pH 7.4) and 20 μL of 1 M TEMP was photoactivated with UVA radiation for 10 min. After that, the contents of the EPR tube were immediately subjected to EPR measurement. Degradation of Reactive Black 5. The molecular structure of Reactive Black 5 (Aldrich) is shown here.
It was made into an aqueous solution in 200 μM concentration. To obtain the AQS-catalyzed photobleaching rate, each of six centrifuge tubes (10 cm in diameter) was filled with 10 mL of a 0.2 mM solution of the dye, 1 mL of a 2 g/L solution of AQS, and 0 or 2 mL of isopropanol or DMSO. The tubes were put in the cross-linker and were exposed to UVA light (365 nm) for 10, 30, 60, 90, or 120 min. Immediately after each exposure, the UVvisible absorbance of each solution mixture was measured, and the absorbance in the UVvisible range (300800 nm) was recorded. The initial dye concentration before exposure to the
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light served as the initial concentration (C0) for calculating the ratio with the concentration at time t, Ct/C0. For the influence of oxygen on the degradation of Reactive Black 5, a similar procedure was used, except that the solution of Reactive Black 5, 2-AQS, and isopropanol in a volume ratio of 20:2:1 was purged with oxygen gas before irradiation. The UVvis spectra were recorded after different durations of UVA exposure. Computational Methods. All calculations were performed with the Gaussian 03 suite of programs.30 The structures of the three photoactive dyes were optimized at the B3LYP/3-21G level using density functional theory (DFT). Frequency calculations were performed on the optimized geometries to ascertain the stationary points. Time-dependent density functional theory (TD-DFT) was used to calculate the excited energies. The excited energies were still calculated at the B3LYP/3-21G level.
’ RESULTS AND DISCUSSION Photochemical Reaction Mechanism. Photosensitive anthraquinone compounds can be excited to a singlet state upon UVA (365-nm) irradiation. Because of the high efficiency (1011) of intersystem crossing (ISC) for diaryl ketones,31 the excited anthraquinone compounds mainly exist in triplet states, instead of their singlet states. Triplet-state AQS could collide with ground-state oxygen to form singlet oxygen (eq 2).31,32 It is also very easy for triplet-state AQS to abstract a hydrogen atom from another molecule, solvent, or polymer substrate to produce AQS radical and solvent/polymer radicals (•R0 ) (eq 3).19,20,31
The AQS radical could further react with oxygen (O2) to form superoxide radical and an intermediate cationic structure under 5327
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Table 1. Energies Calculated Using Gassian 03: Single-Point Energies (E) at Optimized Geometries, First Triplet Excitation Energies (et1), First Singlet Excitation Energies (es1), Total Energies in the Triplet State (Et1), and Total Energies in the Singlet State (Es1) E (au)
et1 (eV)
es1 (eV)
Et1 (au)
Es1 (au)
2-AQS
1466.186
2.399
2.872
1463.787
1463.314
2,6-AQS 2,7-AQS
2247.462 2247.461
2.376 2.371
2.851 2.842
2245.086 2245.090
2244.611 2244.619
light exposure (eq 4). The superoxide radical could react with this AQS intermediate (eq 5) to form a peroxide radical. Two peroxide radicals can combine together to form H2O2 and oxygen. Under light exposure, H2O2 could produce hydroxyl radicals or could react with superoxide radical to produce hydroxyl radical.8 Based on eqs 38, the formation of hydroxyl radical, superoxide radical, and H2O2, also called reactive oxygen species (ROS), is dependent on the formation of AQS radicals of these photosensitive compounds. Basically, if there are oxygen and hydrogen donors (R0 H) in the system, these anthraquinone radicals will abstract hydrogen atoms from these donors and produce ROS under UVA irradiation. Reactivity of Excited AQS Compounds Based on Computational Modeling. The three photosensitive AQS compounds investigated have very similar structures. However, their abilities to produce ROS are significantly different because of differences in their electronic configurations. Therefore, their single-point energies (E) at optimized geometries, first excitation triplet energies (et1), first excitation singlet energies (es1), total energies in the triplet state (Et1), and total energies in the singlet state (Es1) were calculated and are listed in Table 1. Based on the calculation results by using DFT, all three compounds are more stable in their triplet states than in their singlet states. Because direct photoexcitation from a ground state to a triplet state is forbidden by molecular orbital theory, upon light initiation, molecules in the ground state will initially be excited to the singlet state, and then the singlet molecule will transfer to a triplet state through an intersystem crossing process. Aromatic ketone triplet-state molecules can easily abstract hydrogen from a hydrogen donor in solution. Compared with 2,6AQS and 2,7-AQS, 2-AQS has the highest energy in its triplet state; therefore, it is the most reactive compound among these three photosensitive dyes. Thus, 2-AQS can more readily abstract hydrogen from the environment and form a more stable structure, 2-AQS radical, that can further react with oxygen molecules and form reactive oxygen species (eqs 48). This might explain why 2-AQS was found to produce the largest amount of ROS under the same conditions. Because of the small basis set of 3-21G applied in the computational process, the difference in the formation energies for 2,6-AQS and 2,7-AQS in their triple states is trivial, but 2,6-AQS still shows a slightly higher energy in its triplet state. Thus, it is easier to abstract hydrogen to continue on the following reactions with oxygen and, consequently, to produce more ROS than for 2,7-AQS. Besides revealing the reactivities of the three photosensitive compounds, the calculation results also predicted the formation of singlet oxygen. Specifically, when these compounds form their triplet states, the triplet dye molecules can collide with tripletground-state oxygen molecules and form singlet oxygen, as shown in eq 2.32 The first triplet excitation energies of the three
photoactive compounds are much higher than the excitation energy of oxygen, which was reported as 1.05 eV;32,33 thus, the production of singlet oxygen is highly possible from this perspective. Quantum Yield of Singlet Oxygen. Researchers have reported a quantum yield of 0.41 for 2-AQS after 5 min of irradiation of air-saturated samples at their maximum absorption wavelength (>310 nm),23 based on a comparison with the UVvis absorbance changes of p-NDA in a rose bengal solution.23,34 However, we obtained a much higher quantum yield of singlet oxygen for 2-AQS solution after a longer exposure time (60 min) to UVA (365-nm) light. The initial quantum yield at short exposure time was very close to the literature value (0.41), but the quantum yield of singlet oxygen reached 1.87 when the exposure time was extended to 60 min based on this measurement.25,26 Such a high quantum yield of singlet oxygen is absolutely abnormal for such a system, which means that other oxidative species could have formed during the measurement. The agent, p-NDA, used in the measurement of singlet oxygen could react rapidly with hydroxyl radicals (k = 1.25 1010 M 1 s1),28 but will not react with superoxide radical. Thus, the oxidation of p-NDA in the presence of L-histidine and 2-AQS could be caused by both singlet oxygen and hydroxyl radicals in the system, which could explain the abnormal quantum yield of singlet oxygen. In an attempt to differentiate the two species, ethanol, generally reported to be a hydroxyl radical scavenger,35 was added to the system. However, the results indicated that the oxidative effect of hydroxyl radicals was not inhibited by ethanol. Because photoexcited 2-AQS can easily abstract hydrogen from the substrate or solution, the presence of ethanol possibly served as a hydrogen donor to the excited 2-AQS. Thus, the addition of ethanol could not reduce the high quantum yield of singlet oxygen in the system. However, the formation of hydroxyl radicals in the system and relationship to UVA exposure time were verified in subsequent experiments (described in the next section), confirming that the very high quantum yield was due to the formation of hydroxyl radicals. Generation of Hydroxyl Radicals. Based on the above discussion, we predicted the formation of hydroxyl radicals in the 2-AQS system during UVA light exposure. To obtain solid experimental evidence for hydroxyl radicals, the changes in the concentration of p-NDA before and after light exposure were determined. A calibration curve of p-NDA is shown in Figure 1. In the concentration range from 0 to 2 105 M, there is a linear relationship between p-NDA concentration and optical density of the solution. During the experiments, the initial concentration of p-NDA was controlled in this range. Before and after different durations of light exposure, the p-NDA concentration was obtained through the calibration curve. In the absence of Lhistidine, the bleaching rate of p-NDA can be directly related to the production of hydroxyl radicals. For the three photosensitizers 2-AQS, 2,6-AQS, and 2,7-AQS, the amounts of produced hydroxyl radicals after different times of UVA exposure are compared in Figure 2. According to Figure 2, under the same conditions, 2-AQS is the most effective generator of hydroxyl radicals, followed by 2,6-AQS and then 2,7-AQS. The amount of hydroxyl radicals formed increased as the UVA exposure time was extended. These results explain the reason for the abnormal quantum yield of singlet oxygen measured and also indicate the limitation of the method. In addition, the experimental results are consistent with the computational modeling results. 5328
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Figure 1. Calibration curve of p-nitrosodimethylaniline from 0 to 20 μmol/L.
Figure 2. Quantification of hydroxyl radicals produced by 2-AQS, 2,6AQS, and 2,7-AQS under different UVA (365-nm) exposure times.
Influence of Different Solvents on the Generation of Hydroxyl Radicals. The formation of hydroxyl radicals from
photoactive anthraquinone compounds was confirmed by the oxidation of p-NDA. Theoretically, when hydroxyl radical scavengers were added to the solution, oxidation of p-NDA should have been inhibited. However, abnormal results were observed upon addition of different potential hydroxyl radical scavengers to the system. The influence of different additives on the rates of p-NDA bleaching caused by the three photoactive compounds is shown in Figure 3. In 2-AQS solution, addition of DMSO (10 vol %) greatly increased the photobleaching rate, and in 40 min, the formed hydroxyl radicals oxidized all of the p-NDA in the solution, with its color turned from yellow to colorless. It has been reported that benzophenone and its derivatives in DMSO can exhibit biphotonic photochemistry.36 DMSO could participate in the reactions either by energy and polarization transfer or by electron transfer between benzophenone derivatives and DMSO. At the same time, excited triplet benzophenone derivatives could either return to their original form or produce an intermediate radical.36 Considering the
Figure 3. Quantification of hydroxyl radicals produced by (a) 2-AQS, (b) 2,6-AQS, and (c) 2,7-AQS in different solutions.
structural similarity of benzophenone and anthraquinone, it is reasonable that 2-AQS, 2,6-AQS, and 2,7-AQS might react with DMSO in a similar way and form reactive oxygen species. 5329
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Industrial & Engineering Chemistry Research Compared with addition of DMSO to 2-AQS aqueous solution, the presence of isopropanol (10 vol %) during the first 20 min basically served as a hydroxyl radical scavenger, and not much decolorization of p-NDA was observed. However, after 2040 min of irradiation, there was a significant change in p-NDA concentration. This change was not observed for 2,6-AQS and 2,7AQS solutions. Unlike DMSO, isopropanol could serve as a very good hydrogen donor for these photosentive anthraquinone dyes.37 According to our computational modeling, triplet 2-AQS is the most active species among the three photosensitive compounds; thus, it can more easily abstract hydrogen from isopropanol than can triplet 2,6-AQS and 2,7-AQS, generating secondary isopropanol radical and 2-AQS radical in the system. 2-AQS radical could either react with oxygen through eqs 48, forming reactive oxygen species, or couple with isopropanol radical in the system at the beginning, producing a possible structure formed between the secondary isopropanol radical and the anthraquinone ring without the sulfonate group.38 The hydroxyl radicals formed could be quenched by isopropanol in the system. However, the quenching effect could disappear when isopropanol was gradually consumed. At this time, the formed adduct compounds could be further photoexcited, generating more 2-AQS radical and isopropanol radicals,38 in which case more hydroxyl radicals would be produced and the oxidation of p-NDA would be accelerated . The inhibition effect of benzoic acid (10 mM) on the oxidation of p-NDA can be observed in Figure 3a. The reaction rate of hydroxyl radicals with benzoic acid is reported to be 5.9 1012 M1 s1, which is comparable to the rate of reaction between hydroxyl radical and pNDA.39 In 2,6-AQS solution, addition of DMSO produced results similar to those observed in 2-AQS solution (Figure 3). The oxidation rate of 2,6-AQS in the first 20 min was slightly higher than that of 2-AQS, and after 20 min, the curve in Figure 3b became gradually saturated as a result of the full consumption of p-NDA. When isopropanol was added, the 2,6-AQS system did not show the same effect as observed in 2-AQS solution. This could be caused by the difficulty of having a possible coupling reaction between isopropanol radical and 2,6-AQS or 2,7-AQS radicals because of the presence of sulfonate groups on both sides of the anthraquinone rings. Thus, the formed isopropanol radicals could couple with each other, producing a pinocol structure that is relatively more photostable than the coupling product of 2-AQS with isopropanol. Therefore, fewer AQS radicals could exist, and consequently, fewer hydroxyl radicals were observed in 2,6-AQS and 2,7-AQS solutions when isopropanol was added. The inhibition effects of benzoic acid were very similar in all three cases. Figure 3c shows that the addition of DMSO still promotes the formation of hydroxyl radicals in 2,7-AQS solution. Because 2,7AQS is the least reactive compound among the three investigated chemicals, it takes the longest time to reach saturation. Meanwhile, the inhibition effects caused by isopropanol and benzoic acid were significant. The concentration change of p-NDA was small even after 1 h of irradiation. Compared with 2-AQS and 2,6-AQS, 2,7-AQS produced fewer hydroxyl radicals, which is consistent with the results in Figure 2. Overall, the observed differences in photoactivities among the three photoreactive compounds are caused by differences in their electron distributions and the positions of their sulfonate substituents. Evidence for Production of Singlet Oxygen and Superoxide Radicals. A spin-trapping method was used to determine the formation of singlet oxygen after UVA irradiation. Figure 4
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Figure 4. EPR spectrum of singlet oxygen observed after irradiation at 365 nm of an aqueous solution containing 100 μL of 2 105 M 2-AQS phosphate buffer solution (pH = 7.4) and 20 μL of 1 M 2,2,6,6tetramethyl-4-piperidine (TEMP).
shows a typical EPR spectrum of an adduct between 2,2,6,6tetramethyl-4-piperidine (TEMP) and singlet oxygen that was composed of triplet lines having a peak distance of 32 G.40 The formation of singlet oxygen was further confirmed by a photometric method. As discussed earlier, in the presence of L-histidine, singlet oxygen will react with L-histidine and form a transanular intermediate that can oxidize p-NDA. Compared with the system without L-histidine, more changes in p-NDA concentration were observed, meaning that, in addition to hydroxyl radicals, singlet oxygen was also formed in the system (Figure 5). The formation of superoxide radicals was also verified photometrically (Figure 5). When 100 U/mL of superoxide dismutase (SOD) was added to the 2-AQS/p-NDA mixture solution, superoxide radicals reacted with enzyme and formed H2O2, which further formed hydroxyl radicals under UVA exposure. Thus, more p-NDA was oxidized after addition of SOD. The photometric evidence for both singlet oxygen and superoxide formed by 2-AQS, 2,6-AQS, and 2,7-AQS is shown in Figure 5. Because the reaction ratios between singlet oxygen and L-histidine, the reaction intermediate and p-NDA, and hydroxyl radicals and p-NDA are still unknown, we cannot quantify individual reactive oxygen species produced in the systems. The noticeable concentration changes of p-NDA observed in the presence of L-histidine in these graphs represent increases of both singlet oxygen and superoxide. For the systems containing SOD, slight increases in p-NDA concentration changes were observed for all three photosensitive compounds when compared with the systems without SOD. Degradation of Reactive Black 5. The formation of large amounts of reactive oxygen species (ROS) during the irradiation of 2-AQS could possibly be employed in the degradation of chemicals. Thus, Reactive Black 5, a common colorant, was selected as an example in this investigation. Figure 6a shows the results of photodegradation of a solution mixture containing 2 104 M Reactive Black 5 and 2 g/L 2-AQS in a volume ratio of 10:1, indicating that about 60% of the Reactive Black 5 was decomposed in 2 h. However, when a small amount of DMSO or isopropanol was added to the solution, the degradation rate greatly increased. The dark color of Reactive Black 5 was completely eliminated within 10 min when the mixture ratio of Reactive Black 5, 2-AQS, and DMSO was 20:2:1. Similarly to the production of hydroxyl radicals, the degradation of Reactive Black 5 in the presence of isopropanol was not as fast as that in the DMSO solution. However, because all ROS could participate in the reaction with the dye, no inhibition effect of isoproponal was observed. 5330
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Figure 6. Photodegradation of Reactive Black 5 in different solutions: (a) Concentration changes of Reactive Black 5 solutions containing (9) 2-AQS only, (b) 2-AQS and isopropanol, and(2) 2-AQS and DMSO under normal air conditions. (b) Comparison of the Reactive Black 5 solution containing 2-AQS and isopropanol (9) before and (b) after it had been purged with oxygen for 20 min.
of comparing the mixture solution of Reactive Black 5, 2-AQS, and isopropanol purged with or without oxygen for 20 min before the irradiation. Degradation of Reactive Black 5 was accelerated by purging in oxygen, which also led to the formation of more ROS in the system. Such a result further confirms the proposed mechanisms of photoactivated AQS solutions.
Figure 5. Quantifications of hydroxyl radical, singlet oxygen, and superoxide produced by (a) 2-AQS, (b) 2,6 -AQS, and (c) 2,7-AQS in different solutions.
According to the proposed photochemistry reaction mechanism, the presence of oxygen played an important role in the production of all reactive oxygen species (eqs 48). Figure 6b shows the results
’ CONCLUSIONS In this research, reactive oxygen species, including singlet oxygen, hydroxyl radicals, and superoxide radicals, produced by three photosensitive anthraquinone compounds were detected and quantified using different methods. The oxidations of pNDA under different conditions could confirm the existence of all reactive oxygen species, and EPR analysis also showed the existence of singlet-oxygen radicals. When the UVA exposure time was increased, more hydroxyl radicals were produced in solution. Different additives to the solutions had significant effects on the formation of reactive oxygen species. The presence of DMSO could promote formation of the reactive species. However, additives such as benzoic acid could inhibit the 5331
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Industrial & Engineering Chemistry Research formation of hydroxyl radicals in the system. Computational modeling revealed that 2-AQS is the most powerful ROS producer because of the highest formation energy of its triplet state, followed by 2,6-AQS and then 2,7-AQS. 2-AQS was able to degrade a common colorant, Reactive Black 5, with addition of either DMSO or isopropanol significantly increasing the degradation rate of Reactive Black 5. When oxygen was purged into the solution before the irradiation, the photodegradation rate of the dye also increased.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: 530-752-0840. Fax: 530-7527584.
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