The Hydrogen Peroxide-Assisted Photocatalytic Degradation of

At 300 nm of light illumination, eight different dosages of H2O2 (0.05, 0.24, 0.74, 1.24, 4.94, 9.88, 66.7, and 124.0 mmol/L) were applied to the reac...
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Environ. Sci. Technol. 2003, 37, 2310-2316

The Hydrogen Peroxide-Assisted Photocatalytic Degradation of Alachlor in TiO2 Suspensions C. C. WONG AND W. CHU* Department of Civil and Structural Engineering, Research Center for Urban Environmental Technology and Management, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

Photocatalytic degradations of alachlor in TiO2 suspensions with and without the use of hydrogen peroxide were studied using two different monochromatic UV irradiations (300 and 350 nm). Direct photolysis of alachlor was a rather slow process, but the addition of TiO2 enhanced the reaction rates by 12 and 26 times using 300 and 350 nm UV irradiation, respectively. The results showed that a low H2O2 dosage in photocatalysis using 300 nm UV would enhance the rates by 3.3 times, but an overdose of H2O2 will retard the rate due to the hydroxyl radicals are consumed. However, this process is impracticable at 350 nm due to the absorption characteristic of H2O2. A neutral initial pH level was found to favor the H2O2 assisted photocatalysis at 300 nm UV illumination. Eleven major intermediates were identified by liquid chromatography-electrospray ionizationmass spectrometry (LC-ESI-MS) and MS/MS. The major degradation mechanisms of H2O2-assisted alachlor photocatalysis include dechlorination, dealkylation, hydroxylation, cyclization, scission of C-O bond, and N-dealkylation. Bell-shaped evolution profiles of different intermediates were observed. Degradation pathways were proposed accordingly to illustrate series of degradation steps. The TOC analysis revealed the different stages of the reaction.

alization process resulting from the TiO2 photocatalyst. The recombination of holes (h+) and electrons (e-) has been regarded as the unfavorable or limiting process in photocatalysis using titanium dioxide suspensions. Over the past 10 years, research has focused on reducing the effect of recombination of charges or enhancing photocatalysis performance by applying different techniques including the electrochemical method (10) and surface modification of TiO2 (11) and adding an external electron acceptor such as hydrogen peroxide (H2O2) (2).

TiO2 + hν f TiO2 (e- + h+)

(1)

h+ + OH- f HO•

(2)

e- + O2 f •O2

(3)

Alachlor (2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide) is a widely used herbicide; it has been detected in groundwater and rivers in the United States (12, 13), and about 26 million pounds were used for agricultural purposes from 1990 to 1993 in the United States. The maximum contaminant level of alachlor for drinking water established by the USEPA is 2 µg/L (14). This study focused on the photocatalytic degradation of alachlor by TiO2 with the addition of hydrogen peroxide, which has been reported to exhibit appreciable rate increases when it is added to a photocatalysis process (2); however, negative effects have also been reported on the use of H2O2 on the photocatalytic degradation of dichlorvos, trinitrotoluene, and trinitrobenzene (15, 16). Broadband UV lamps are normally used for H2O2-assisted photocatalysis (15, 17-23), yet the use of monochromatic UV lamps is seldom discussed and may be interesting for such applications. The pH effect on the degradation rate during H2O2-catalyzed photocatalysis was also investigated. Identifications of reaction intermediates were performed using LCMS and MS/MS, which reveals the degradation mechanism and reaction pathways of the process. TOC changes were also monitored simultaneously, so that the relationship between mineralization and alachlor degradation could be realized.

Introduction

Methodology

Photocatalytic reaction has been shown to be useful in destructing a wide range of air- and/or water-originated contaminants (1-3). Titanium dioxide is broadly used as a photocatalyst because it is photochemically stable, nontoxic, and low cost (4, 5). One of the examples using titanium dioxide-induced photocatalytic reactions in aqueous systems is the mineralization of halogenated organic compounds, in which they are converted to carbon dioxide, water, and halide ions (6). Photocatalytic degradation using UV/TiO2 appears to be an effective strategy for degrading and mineralizing chlorinated pesticides (7-9). In general, photoinduced electrons (e-) and positive holes (h+) (see eq 1) are produced from TiO2 under the irradiation of UV light (λ < 380 nm), which has an energy greater than the band gap (3.2 eV) of TiO2. These charged species can further generate free radicals under appropriate conditions, as shown in eqs 2 and 3. The highly oxidizing positive hole h+ has been considered to be the dominant oxidizing species contributing to the miner-

Chemicals. Alachlor (99.7% HPLC grade) was purchased from Riedel-de Hae¨n, and the initial concentration of alachlor in all the experiments was 2.2 × 10-5 mol/L. Titanium dioxide TiO2-P25 (containing 70% Anatase and 30% Rutile) with average particle size of 30 nm and BET surface areas of 50 m2/g (24) obtained from Degussa Corp. Japan, was used in this study. A. R. grade hydrogen peroxide H2O2 (30%) was purchased from APS Chemicals Ltd. Since the initial alachlor concentration was relative low, it was necessary to use a lower TiO2 dosage such that the kinetic of the alachlor decay could be monitored more accurately. The concentration of titanium dioxide used in all experiments was 5 mg/L and not the optimum concentration at 50 mg/L (25) so that a better resolution of reaction mechanism can be realized. Acetonitrile of HPLC grade from LAB-SCAN was used in the preparation of the mobile phase in the HPLC analysis without further purification. All solutions were prepared using distilleddeionized water (resistivity 18 MΩ-cm) by a Barnsted NANO pure water system. A. R. grade hydrochloric acid and sodium hydroxide were used to adjust the initial pH of the solutions to the predetermined levels.

* Corresponding author phone: (852)2766-6075; fax: (852)23346389; e-mail: [email protected]. 2310

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10.1021/es020898n CCC: $25.00

 2003 American Chemical Society Published on Web 04/12/2003

Methods All direct photolysis and photocatalysis experiments were conducted using an RPR-200 Rayonet photochemical reactor purchased from the South New England Ultraviolet Company. In each experiment, a quartz vessel (52 mm i.d. × 470 mm length) filled with 500 mL sample solutions was placed inside the reactor and illuminated with monochromatic UV lamps. A magnetic stirrer was located at the reactor’s base such that a homogeneous TiO2 suspension could be maintained throughout the reaction; a cooling fan was also installed at the reactor base such that the experimental temperature maintained at 24-25 °C. Near the UV irradiation, either ten 300 nm “sunlight” UV lamps (210 W) or ten 350 nm “blacklight” UV lamps (240 W) were used in this study, so that a comparable photocatalysis rate of alachlor for better process evaluation could be achieved. From the manufacturer’s specifications, the total photon-intensity for ten 300 and 350 nm lamps are 6.64 × 10-3 and 3.24 × 10-2 Einstein L-1 min-1, respectively. Right after the addition of H2O2 (if any) to the reactor, the UV lamps were turned on to initiate the reaction. Samples were taken from the quartz vessel at different reaction times until the total degradation of the alachlor achieved 80% or higher. The collected samples were first centrifuged at 3000 rpm for 10 min using a Hettich ZENTRIFUGEN centrifuge to remove the titanium dioxide prior to chemical quantification. The remaining alachlor was then quantified by liquid chromatography (LC). The LC system comprised a Spectra SYSTEM P4000 Gradient pump, a Spectra SYSTEM AS3000 Autosampler with a 20 µL injection loop, a Thermo Quest Hypersil ODS (3 µm 150 × 2.1 mm) column, and a Spectra SYSTEM UV6000LP photodiode array UV detector. The maximum absorption wavelength (λmax) selected to detect the alachlor was 265 nm. A Spectronic Genesys 5 UV-vis spectrophotometer was used to obtain the UV spectrum of the hydrogen peroxide. The mobile phase consisted of 70% acetonitrile and 30% distilled-deionized water and was delivered at a flow rate of 0.2 mL/min. The TOC decay was determined using a SHIMADZU TOC 5000A equipped with an ASI-5000A autosampler. The study of intermediates in the H2O2-assisted photocatalysis was performed using a Finnigan LCQ DUO ion trap mass spectrometer by electrospray ionization (ESI), which was coupled to the LC. As suggested by other researchers (26, 27), it was operated in positive ionization mode. To minimize unwanted metal adducts formation and increase the detection sensitivity/ionization performance, the entering of LC eluent into the MS detector was set to be delayed by 0.5 min in order to remove the salts. Nitrogen generated by the NITROX nitrogen generator was used as both a sheath and auxiliary gas. The capillary temperature was 200 °C, and the rates of flow of sheath and auxiliary gas into the atmospheric pressure ionization region were 0.6 and 0.0 L/min, respectively. The damping gas in the mass analyzer was helium, which was purchased from Hong Kong Special Gas Co. Ltd. Helium also acted as the collision gas during the collision-induced dissociation (CID) process in the MS/MS analysis. By direct infusion of alachlor into the mass spectrometer, the ion optics setting was automatically optimized using the LCQ Tune plus software.

Results and Discussion Direct Photolysis and Photocatalysis of Alachlor at Different UV Wavelengths. The decay of alachlor by either 300 or 350 nm UV irradiation with and without the presence of TiO2 was investigated, and the results are shown in Figure 1. The initial decays may be described by pseudo-zero-order kinetics (common for optical-dense reactions) due to the higher initial

FIGURE 1. The direct photolysis and photocatalysis of alachlor at two different monochromatic UV irradiations: 300 and 350 nm. concentrations. Because it failed to describe the kinetics after 10 min of reaction, the pseudo-first-order kinetics was used to describe the reaction rates with good correlations of r2 g 0.99 throughout the reaction. The photocatalysis rate constants (with TiO2) for 300 and 350 nm were 0.120 and 0.129 min-1, which were higher than their corresponding direct photolysis rates (without TiO2) by 12 and 26 times, respectively. It should be noted that although the photocatalytic degradation of alachlor at 350 nm seemed to be slightly faster than that at 300 nm, the photon intensity of 300 nm is smaller than that of 350 nm by about 5 times. Therefore, under the condition of equal photon intensity, higher conversion of alachlor is expected at 300 nm. Besides, the test of alachlor decay in the dark (with TiO2, but without the exposure to UV light) showed that the variation of alachlor concentrations after 30 min of mixing was insignificant, suggesting that the adsorption (or loss) of alachlor onto the surface of the TiO2 particles could be neglected. Since the photocatalysis rates at the two different wavelengths were of about the same order, it is worthwhile comparing the possible rate improvement of photocatalysis assisted by H2O2 at both wavelengths. Effect of H2O2 in Improving and Retarding the Photocatalysis Process. To investigate the effect of adding H2O2 to the photocatalysis of alachlor, experiments with different dosages of H2O2 were carried out using two near UV wavelengths of 300 and 350 nm. At 300 nm of light illumination, eight different dosages of H2O2 (0.05, 0.24, 0.74, 1.24, 4.94, 9.88, 66.7, and 124.0 mmol/L) were applied to the reaction, and the results are shown in Figure 2 at either low (a) or high (b) ranges of H2O2 dosages. The reaction rate was found to increase with H2O2 dosages at low H2O2 dosages. At maximum 3.3 times rate increment was achieved at the optimal H2O2 dosage of 4.94 mmol/L compared to that of sole photocatalysis without H2O2. Above the optimal dosage, however, the overdose of H2O2 retards the alachlor photocatalytic rates. The rate improvement at lower H2O2 dosages is probably due to the following reasons. First, the direct photolysis of H2O2 by UV light can generate free radicals (28), which are likely to be the dominant rate-improving mechanism in this process (eq 4). Another minor mechanism proposed by Ollis et al. (2) and Ilisz et al. (29) may partially contribute to the rate enhancement, in which H2O2 is suggested to be a better electron acceptor than oxygen. This would reduce the chances of electrons-holes recombination (i.e. the termination of excited components), an undesirable process in the photocatalytic process, which may generate one hydroxyl radical as shown in eq 5 (21, 30, 31), rather than VOL. 37, NO. 10, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Variation of pseudo-first-order decay constant k (min-1) of H2O2-assisted photocatalysis at various H2O2 dosages under either 300 or 350 nm at an initial pH level of 6.

FIGURE 2. The photocatalytic decay of alachlor at (a) low (0.05 to 1.24 mmol/L) and (b) high (4.94 to 124.0 mmol/L) H2O2 dosages at 300 nm, where the [TiO2] was 5 mg/L and the initial pH level was 6. the weaker •O2- radical (as indicated in eq 3).

H2O2 + hv f 2HO•

(4)

e- + H2O2 f HO• + OH-

(5)

At high H2O2 dosage, however, the excess H2O2 molecules scavenge the valuable HO• that was generated by either the direct photolysis of H2O2 (eq 4) or the photooxidation of OH- by h+ (eq 2) and form a much weaker oxidant HO2• (eq 6). Therefore, the total oxidation capabilities of the system are largely reduced (eq 7) and the rates retarded (20, 32).

H2O2 + HO• f H2O + HO2•

(6)

HO2• + HO• f H2O + O2

(7)

Similarly, different dosages of H2O2 were applied to the alachlor photocatalysis process at 350 nm. It is interesting to note that no rate improvement was observed at all; instead, the higher the H2O2 dose, the lower the reaction rate; the comparison of the photocatalytic rates at two wavelengths is summarized in Figure 3. At a very low H2O2 dosage of 0.0024 mmol/L, the rate constant was about the same as that of sole photocatalysis without H2O2 (0.129 min-1). Any further addition of H2O2 could retard the reaction. This observation is likely attributed to the 50 times difference of molar absorptivity of H2O2 at 300 and 350 nm, which are 80 and 1.6 2312

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FIGURE 4. The alachlor decay at (a) low (3.3-6.0) and (b) high initial pH levels (7.5-11.1). cm-1 L mol-1, respectively. As H2O2 has extremely low absorption at 350 nm, the critical direct photolysis of H2O2 generating HO• (eq 4) is insignificant, and hence free H2O2 is most likely to be present at relatively high levels (i.e. overdosed), which embedded other minor rate-improving mechanisms (e.g. eq 5), and a rate retardation was observed. The use of H2O2 in photocatalytic reaction at 300 nm is therefore used exclusively for the rest of this study.

TABLE 1. Summary of MS/MS Analysis Showing the Daughter Ions, Relative Abundances, Collision Energy Applied, and Proposed Fragments. protonated parent ion [M + H+]

compd no.

[M + H+] of major daughter ions (relative abandances in %)

collision energy (eV)

270 286 258 252 244 242 238 228 219 216 212 198

alachlor 1 2 3 4 5 6 7 8 9 10 11

238(100%) 268(40%), 254(45%), 242(65%) 240(35%), 226(60%) 234(5%), 220(12%), 176(100%) 226(100%), 195(10%) 198(100%), 147(50%) 220(55%), 162(88%) 196(40%), 151(100%) 201(100%), 191(5%) 198(80%), 161(60%) 180(75%) and 168(100%) 180(75%) and 170(30%)

20 23 20 23 27 25 25 20 25 25 25 30

FIGURE 5. Variation of photocatalytic decay rate at different initial pH levels at 300 nm with 4.94 mmol/L of H2O2. Effect of Initial pH on H2O2-Assisted Photocatalysis. The effect of initial pH on alachlor photocatalysis with the addition of an optimal dosage of H2O2 was then studied as illustrated in Figures 4 and 5. In general, the reaction rates gradually increased with increments of initial pH level, and an optimal pH was found at 6. However, the further elevation of the solution pH retarded the rates. At low to optimal pH levels, the rate increment is likely due to the increase of hydroxyl ions in the solution, which is suggested to be critical and positively related to the determination of the concentration of hydroxyl radicals in a UV photolysis process (33) or an oxidation process (34). Besides, an increased amount of OHat elevated pH could enhance free radical generation through photooxidation by holes as indicated in eq 2. However, in an alkaline medium, H2O2 becomes unstable and easily and rapidly decays into water and oxygen, even in the presence of UV light (35). In addition, the CO2 level in the solution keeps increasing during the reaction, either generated by the decay of alachlor or by the dissolution form the air; the former is justified by the TOC analysis (to be discussed) and the latter confirmed by a blank test with a contribution up to 2 mg/L. Because the first and second pka of H2CO3 is 6.4 and 10.3, when the solution pH is higher than 6, the dominant species of carbonate system will either be bicarbonate ions or carbonate ions. As both of them are radical scavengers, the presence of these ions will scavenge the free radicals and therefore retard the reactions at medium to high pH levels.

proposed fragments for first daughter ion CH3OH H2O H2O H2O H2O CHOCH3 H2O CH3 + OH H2O H2O CH3 + OH H2O

proposed fragments for second daughter ion

proposed fragments for third daughter ion

CH3OH CH3OH CH3OH ClCH2 CH2OCH3 + ClCH2 HOCH2CO + OH ClCH2CO CHCH3 HCl + H2O CH3 + CH2CH3 CHCH3

CHOCH3 HOCH2 + CH2OCH3

The Mechanism of H2O2-Assisted Photocatalytic Decay of Alachlor. GC-MS analysis has been used a lot in the intermediate studies; however, a solvent extraction and/or sample derivatization procedure was usually required, and the escaping of some polar intermediates from the MS detection was inevitable. Therefore, the identification of intermediates in this study was achieved by using the most recent LC-MS to avoid the problem. A total of 11 major intermediates were identified in the H2O2-assisted alachlor photocatalysis at 300 nm by interpreting and organizing the possible degradation mechanisms from analogous studies (36-38), the evolution profiles of different degradation products, and the results of double mass spectrum (MS/ MS). The MS/MS results, including the mass of protonated ion ([M + H+]) of the daughter compounds, the relative abundances, the collision energy applied, and the proposed fragments are summarized in Table 1, where the intermediates were presumably subjected to cleavage of the side chains attached to the nitrogen atom and the aromatic ring. The common fragmentation pattern includes loss of mass units of 18, 28, 32, 44, 49, and 77, corresponding to the loss of H2O, CHCH3, CH3OH, CHOCH3, ClCH2, and ClCH2CO, respectively. Figure 6 illustrates the proposed degradation mechanism of the H2O2-assisted photocatalytic process, in which the decay pathways included dechlorination, hydroxylation, cyclization, dealkylation, scission of C-O bond, and Ndealkylation. The results have been compared to those from previous research with analogous degradation mechanisms such as alachlor photocatalysis (36, 37) and photolysis/ photocatalysis of a similar herbicide, propachlor (38, 39), which supported our findings. For specific intermediates, compounds 1 and 3 are considered to be the major primary intermediates in the process. Hydroxyl free radicals react rapidly at unsaturated carbons (40), and hydroxyl aromatic products were often found to be the initial oxidation products for the degradation of aromatic compounds in •OH-based oxidation processes (41-43). In addition, as the aromatic ring is electron-rich in nature, the electrophilic attack of the aromatic ring of alachlor by hydroxyl radicals formed compound 1 (36). The subsequent loss of an ethyl group on the aromatic rings of compound 1 formed compound 2. The dechlorination and hydroxylation of alachlor formed compound 3 (37), in which the photodechlorination has been suggested to be a major pathway in a UV photolysis process (44). The scission of the C-O bonds of compounds 2 and 3 formed compounds 4 and 6, respectively. The different scission positions of the C-O bond of compound 5 formed compounds 10 and 7. Another primary intermediate, compound 8, formed through a cyclization process, was also characterized by Chiron et al. (45) as a major intermediate in the photolysis VOL. 37, NO. 10, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Proposed degradation mechanism of H2O2-assisted alachlor photocatalysis. of alachlor. Dealkylation was initiated by hydrogen abstraction and normally observed in hydroxyl radical reactions (40), leading to the loss of the ethyl group from the benzene ring of alachlor and compound 4, resulting in the formation of compounds 5 and 9, respectively. An analogous Ndealkylation mechanism was previously reported in the photocatalytic degradation of propachlor (38), pendimethalin (46), and atrazine (47); likewise in the H2O2-assisted photocatalytic process, the N-dealkylation of compounds 7 and 10 are observed and generate compound 11. Verification of the Proposed Degradation Pathways and End Products. The proposed mechanism was further verified by Figure 7, which shows the decay and generation of alachlor, the 11 identified reaction intermediates, and some unidentified low-molecular-weight intermediates and/or end products during the hydrogen peroxide catalyzed photocatalytic reaction in terms of the protonated ion [M + H+] intensity. A study of the evolution profiles offers useful information in supporting the proposed mechanism. For instance, in Figure 7, the decay of alachlor leads to the generation of primary intermediates such as compounds 1 and 3, whereas the decay 2314

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of these two primary intermediates generates secondary intermediates such as compounds 2 and 6, respectively. The gradual increases of the ion intensity of the small molecular weight compounds (i.e. the protonated masses of 60, 83, and 100) were probably the polar end products formed after the cleavage of the benzene ring. These end products are apparently resistant to free radical attack, as their ion intensity was maintained at high levels even at an extended reaction time of 95 min. TOC analysis was carried out simultaneously with LC-MS analysis to verify the conversions (decay/generation) of different molecular size species (see Figure 8). The decay of TOC could be divided into three stages: the first stage was the lag phase, where the TOC reduction was insignificant because the dominant reaction was the decay of alachlor to primary intermediates with high molecular weights. The lag phase was followed by a mild TOC decay, in which 90% of the alachlor was decayed; the decay of high molecular weight intermediates (to lower ones) was initiated and became the dominant process. In the final stage, the TOC decay rate increased significantly, indicating that the dominant decaying

FIGURE 7. The variation of protonated ion intensity of alachlor, the reaction intermediates and end products during the H2O2-assisted photocatalysis at 300 nm, where the [H2O2] is 4.94 mmol/L and the initial pH level is 6.

Literature Cited

FIGURE 8. Comparison of the photocatalytic decay of alachlor and TOC at 300 nm, where the [H2O2] is 4.94 mmol/L and the initial pH level is 6. species were small molecules. About 90% of the TOC was removed (or mineralized) at 95 min.

Acknowledgments The work described in this paper was supported by a grant from the University Research Fund of the Hong Kong Polytechnic University (GW-045).

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Received for review August 26, 2002. Revised manuscript received January 15, 2003. Accepted March 12, 2003. ES020898N