Reaction Mechanism of Photoreduction of 2,4-Dichlorophenoxyacetic

The direct photolysis of 2,4-dichlorophenoxyacetic acid (2,4-D) in water is slow and ineffective, but surfactant micelles can greatly enhance the phot...
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Ind. Eng. Chem. Res. 2005, 44, 1645-1651

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Reaction Mechanism of Photoreduction of 2,4-Dichlorophenoxyacetic Acid in Surfactant Micelles C. Y. Kwan and W. Chu* Department of Civil and Structural Engineering, Research Centre for Environmental Technology and Management, The Hong Kong Polytechnic University, Kowloon, Hong Kong

The direct photolysis of 2,4-dichlorophenoxyacetic acid (2,4-D) in water is slow and ineffective, but surfactant micelles can greatly enhance the photochemical reaction of organic contaminants. The effect of an anionic (sodium dodecyl sulfate), a cationic (cetyltrimethylammonium bromide), and two nonionic (Brij 35, Tween 80) surfactants on the phototransformation of 2,4-D at 253.7 nm was examined. Except for the anionic surfactant, the other micelles were significant facilitators in both removal efficiencies and decay rates of 2,4-D. This herbicide was then irradiated in the presence of Brij 35 micelles at five different initial pH levels. 2,4-D is an organic acid, and its speciation (molecular or ionic form) is controlled by pH. It was found that the photoreduction of 2,4-D was more effective at lower initial pH than that in basic conditions. The transformation of 2,4-D in micelles occurred predominantly through reductive chlorination, resulting in generation of less chlorinated products. The presence of hydroxylated products in low yields indicated photohydrolysis was a minor competitive reaction pathway. The measurable intermediates were 2-chlorophenoxyacetic acid, 4-chlorophenoxyacetic, chlorohydroxyphenoxyacetic acid, phenoxyacetic acid, hydroxyphenyoxyacetic acid, and phenol. The transformed products were identified by comparing the HPLC retention times of the standards, and confirmed by LC-MS. Introduction A surfactant is an amphiphilic compound which possesses both hydrophilic and hydrophobic functional groups. The application of surfactants to contaminated soils or sediments was found to improve the solubilization of hydrophobic compounds and their partition from the soil phase to the liquid phase.1-4 The extracted pollutants in the aqueous phase were generally treated by a subsequent chemical, biological, and/or physical treatment, which draws wide interest in the study of degrading various organic contaminants in micellar medium. For example, surfactants have been demonstrated to increase the biodegradation efficiency of contaminants when the surfactant concentration is above the critical micelle concentration (CMC).5 The cationic surfactant cetyltrimethylammonium chloride (CTAC) has been shown to catalyze the electrochemical oxidation of 1,3,7,9-tetramethyluric acid by providing a favorable electron-transfer environment.6 Schmelling et al.7 found the degradation rate of polychlorinated biphenyl by ionizing radiation was higher in a 2% Triton X-100 solution than those in petroleum ether and diethyl ether. Various surfactants have been shown useful in photolysis processes to decompose organic pollutants.8-10 The chlorinated aromatics (ArCl) are of special interested in such a process due to the possible detoxification effect via photodechlorination in the presence of micelles. It has been suggested that major routes of photodechlorination are through direct triplet homolytic fission and fragmentation of radical anion/radical cation ion pairs: ArCl•δ-ArCl•δ+.11,12 Subsequently, the ion pair * To whom all correspondence should be addressed. Email: [email protected]. Fax: (852)2334 6389.

fragments to give an unstable aryl radical anion (ArCl•-) which then cleaves to an aryl radical (Ar•). As in homolysis, Ar-H and chloride ion are the products after hydrogen abstraction. Freeman et al.13 reported that the sum of the quantum yields of these two pathways accounted for over 90% of the total quantum yield of the phototransformation of pentachlorobenzene, in which the former pathway was dominant at low concentrations of the reactants. As the reactant concentration increased, more and more ground-state molecules were excited, and the chance of forming the triplet excimer increased. In surfactant solutions the phototransformation of 2,4-dichlorophenoxyacetic acid (2,4-D), a herbicide commonly observed in the environment, was investigated through the reaction kinetics and mechanism data in this study. The impact of pH levels on the process was also examined; due to the transforming characteristics of 2,4-D structures in acidic or basic solutions, such information is very limited and has seldom been discussed before. In addition, the selectivity of surfactants and the reaction pathways of photodechlorination of 2,4-D in surfactant solutions was proposed. Methodologies Materials. The chemicals 2,4-D, 2,4-dichlorophenol (2,4-DCP), and phenol were purchased from Riedel-de Haen, and 2-chlorophenoxyacetic acid was obtained from Tokyo Chemical Industries. The 4-chlorophenoxyacetic acid (extra pure grade) and phenoxyacetic acid (99%) were both bought from International Laboratory, U.S.A. The nonionic surfactants, Brij 35 (polyoxyethylene (23) lauryl alcohol) and Tween 80 (polyoxyethylene (20) sorbitan monooleate), were purchased from the Warenzeichen der ICI America, Inc., and CNCIEC, respectively. The anionic surfactant sodium dodecyl

10.1021/ie049509j CCC: $30.25 © 2005 American Chemical Society Published on Web 02/01/2005

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Table 1. Summary of Physical and Chemical Characteristics of Surfactants and the Initial Decay Rates of 2,4-D in Various Solutions medium

typea

chemical name

chemical formula

CMC (M)

reaction rateb (s-1)

water SDS Tween 80 CTAB Brij 35

A N C N

sodium dodecyl sulfate polyoxyethylene sorbitan monooleate cetyltrimethyl-ammonium bromide polyoxyethylene (23) lauryl ether

H2O C12H25SO4Na C18S6c(CH2CH2O)20 C19H42BrN C12(CH2CH2O)23

8.4 × 10-3 9.9 × 10-6 1.0 × 10-3 4.3 × 10-5

0.0002 0.0002 0.0005 0.0006 0.0010

a A: anionic surfactant; N: nonionic surfactant; C: cationic surfactant ring.

sulfate (SDS) was obtained from Farco Chemical Supplies. The cationic surfactant cetyltrimethylammonium bromide (CTAB) was purchased from ACROS. The characteristics of the selected surfactants are summarized in Table 1. All the stock solutions of 2,4-D, standards, and surfactants were prepared in deionized, distilled water, while HPLC-grade acetonitrile, methanol, HPLC-water and acetic acid were purchased from LAB-SCAN. All chemicals were used as received, without undergoing further purification. Method. For each batch of experiment, 2,4-D and 30 mL of 0.0167 mol/L surfactant solution were added to a 50-mL volumetric flask. The solution was adjusted to the desired pH levels (i.e., 2.02, 2.98, 5.03, 6.97, 10.03) by sodium hydroxide and/or sulfuric acid, and the flask was filled up to the mark with distilled water. The final concentrations of 2,4-D and surfactant were 0.79 mmol/L and 0.01 mol/L, respectively. After the solution was stirred by a magnetic stirrer for 15 min, 5 mL of wellmixed solution was delivered to a number of 15-mL cylindrical quartz cuvettes and sealed with Teflon-lined caps to minimize the volatilization effect. These cuvettes were placed inside an RPR-200 Rayonet photochemical reactor and exposed to a light source consisting of eight 253.7 nm phosphor-coated low-pressure mercury lamps with a total photon intensity of 1.2 × 10-5 Einstein/s/ L. The irradiated sample was removed from the photoreactor at predetermined time intervals. The parent compound and intermediates were then quantified by liquid chromatography (LC) analysis with a ThermoQuest Hypersil ODS column. A linear gradient of 0.15% acetic acid and acetonitrile increased from a ratio of 100:0 to 50:50 in 35 min at a flow rate of 1.0 mL/min was used to separate 2,4-D and all of the intermediates. A Finnigan LCQ DUO ion-trap mass spectrometer coupled to the LC was used to identify and confirm the reaction intermediates through electrospray ionization (ESI) associated with a negative operational mode. Helium was used as the collision gas during the collision-induced dissociation (CID) process in the MS/MS analysis. As some of the intermediates were not available from the supplier, the quantifications of chlorohydroxyphenoxyacetic acid and 4-hydroxyphenoxyacetic acid were calculated from the UV absorptivity of 2,4-D and 4-chlorophenoxyacetic acid, correspondingly, because of their similar molecular structures.14 Results and Discussion Effect of Surfactant Type. Photodecay of 2,4-D in water and in micellar solutions of four different surfactants including the anionic SDS, the cationic CTAB, and the nonionic Brij 35 and Tween 80 was investigated, and the pseudo-first-order decay curves are shown in Figure 1. The decay rate constants of 2,4-D transformation in various solutions at pH 3.0 are summarized Table 1. As seen in Figure 1, the direct photolysis of

b

Reaction rates are obtained from Figure 1. c S6 is a sorbitan

Figure 1. Pseudo-first-order decay curves of 2,4-D irradiated at 253.7 nm in water or various surfactant micelles at pH level of 3.

2,4-D in water was a slow process (reaction coefficient ) 0.0002 s-1), and the use of anionic surfactant (SDS) exhibited no additional effect on the 2,4-D degradation. However, the removal efficiencies could be greatly increased by 2.5 (Tween 80) to 5 (Brij 35) times if cationic or neutral surfactant micelles were used. The performance variation is apparently oriented from the surfactant-structure differences. The photochemical reactivity may depend on the properties of micelles such as cage effects, polarity and counterion effects, chain effects, localization and compartmentalization effects, and the effect of the charged interface.15 For nonionic surfactants, Brij 35 is a linear structure, and Tween 80 carries a sorbitan ring; the quantity of hydrophilic polyoxyethylene (POE) chains is 23 and 20, respectively. Since 2,4-D is generally a polar compound with a water solubility of 0.089 g/100 mL, the higher reactivity in Brij 35 is likely due to the higher affinity between 2,4-D and the longer hydrophilic POE chain in Brij 35. To quantify and verify such an effect, the hydrophilic-lipophilic balance (HLB) number could be used. This index was designed for matching surfactant structure to an organic chemical to be emulsified, and a higher HLB number indicates a higher affiliation of soluble chemical with the surfactant.5 According to Jafvert et al.,2 the HLBs of Brij 35 and Tween 80 were 16.9 and 15.0, respectively, which confirmed our observation. Because the surfactant is the dominant hydrogen source,16 to complete the photodechlorination process, the higher affiliation between 2,4-D and Brij 35 molecules reduces the distance among the reactants and therefore accelerates the process.

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Figure 2. Plots of natural logarithms of the decay ratio (C/C0) of 2,4-D and proton concentration ([H+]) versus time during the photoreduction tests conducted at pH: (a) 2.02, (b) 2.98, (c) 5.03, (d), 6.97, and (e) 10.03 in Brij 35 micelles. The insets show the drop of solution pH throughout the reaction. The solid line indicates the first stage, and the dashed lines represent the second and the third stages, accordingly.

For ionic surfactants, the solubilization of polar or charged substances within a micelle was suggested to occur in the surface region (or Stern layer) of a micelle.17 The anionic DS- heads of the Stern layer of SDS was likely the reason for the failure in promoting 2,4-D photodecomposition. First, the electrostatic repulsion between the anionic micelles and negatively charged 2,4-D (ionic form) restricts the formation of ion pairs (ArCl•δ-ArCl•δ+). Then, the subsequently formed anion radical (ArCl•-) of the triplet excimer, which is the key intermediate of C-Cl bond fragmentation, may also repel the anionic micelles present in the solution. Third, the highest CMC value of SDS among all the tested surfactants leads to the lowest concentration of surfactant micelles in the solutions. As a result, the addition of SDS has a photoreduction rate of 2,4-D similar to that when using water alone. In contrast, the anionic 2,4-D and the anionic radicals should be attractive to the cationic micelles of CTAB. This could be the reason that 2,4-D is transformed faster in CTAB than in the nonionic Tween 80. However, the Stern layer of the cationic heads is theoretically surrounded by elevated bromide ions (counterions) that originally come with CTAB which may become a barrier to hinder the diffusion of the anionic radical intermediates to the hydrogen source (at the micellar core). Thus, the rate enhancement by using CTAB is about 40% less than that with Brij 35. On the basis of these results, the best performer, Brij 35, was selected exclusively for the remaining tests in this study.

Effect of Initial pH. A series of experiments was conducted to photodegrade 0.79 mM 2,4-D in 0.01 M Brij 35 solutions at 253.7 nm at various pH levels. Parts a-e of Figure 2 show the changes of the natural logarithms of 2,4-D decay (in C/C0) and proton concentration over time for each tested initial pH. The inserts illustrate the corresponding decrease of pH levels during each reaction. It is interesting to note that different pH level can alter the photochemical reaction of 2,4-D significantly. At pH 2.02 and 2.98 (Figure 2a and b), the solution pH dropped slightly, and the change in [H+] was calculated to be 1.0 and 1.1 mM, respectively. The generated protons are stoichiometrically close to the order of total chlorine available in the 2,4-D molecules (i.e., 1.58 mM), indicating the photodechlorination is likely to be the dominant mechanism for 2,4-D decay, in which the chloride and proton ions were generated simultaneously.18 However, if the initial pH levels were further increased above 5 (Figure 2c-e), the solution pH dropped significantly during the reaction, while the accumulation of [H+] was much lower (from 0.45 to 0.17 mM) due to the neutralization from the higher [OH-] in the solution. Although the process can essentially be depicted by the pseudo-first-order kinetics; from weak acid to basic ranges (pH 5.03, 6.97, and 10.03), a slight increment of the rate was observed, and the trend was assumed to be separated by three stages. A slowest initial decay rate (k1) in the first stage was followed by a slightly faster transition stage (with a rate k2), and then reached the

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Table 2. Effect of Solution pH on the Photoreduction Rate Constants of 2,4-D initial pH 2 3 5 7 10

k1a (s-1)

k2a (s-1)

10-3

1.1 × (2.02-1.98)b 1.0 × 10-3 (2.98-2.64) 4.4 × 10-4 (5.03-4.22) 3.4 × 10-4 (6.97-5.02) 3.1 × 10-4 (10.03-5.36)

k3a (s-1)

-

-

-

-

7.7 × 10-4 (4.22-3.68) 5.4 × 10-4 (5.02-4.15) 5.2 × 10-4 (5.36-4.29)

1.2 × 10-3 (3.68-3.35) 9.5 × 10-4 (4.15-3.63) 7.7 × 10-4 (4.29-3.77)

a k , k , and k represent the reaction rate constants of the first, 1 2 3 the second and the third stage, respectively. All tests were in duplicate, and the variation was within 9%. a The numbers in the brackets indicate the drop of the solution pH during each stage.

fastest decay rate (k3) in the final stage. In addition, the lower the initial pH level, the higher the corresponding reaction rates in all three stages. The rate constants and their associated solution pH ranges of each stage are summarized in Table 2. Since the solution pH drops throughout the reaction as well (see Figure 2), it implied that 2,4-D photoreduction was strongly pH dependent and the proton concentration in the solution might indirectly accelerate the reaction. However, these results are not consistent with those of the previous study for chlorinated aromatics,16 where the photolysis was mostly accelerated as the pH rose. Nevertheless, pH or proton concentration is a key factor that affects the photodegradation of aryl halides in micellar solutions. It is known that 2,4-D has a pKa of 2.9.19 Thus, speciation of 2,4-D is varied according to the solution pH, in which the distribution of the dissociated (ionic) and undissocaited (molecular) forms of 2,4-D depends on the following equation:

Ka )

[R-COO-][H +] [R-COOH]

(1)

where Ka is the acid dissociation constant (i.e., 1.26 × 10-3) and R-COOH and R-COO- represent the molecular and ionic 2,4-D, respectively. Assuming the fraction of the molecular and ionic 2,4-D species to be x (%) and 100 - x, then x can be incorporated into eq 1 as:

Ka [H+]

)

100 - x x

(2)

After rearranging, the x becomes pH dependent, see eq 3:

x)

100[H +] Ka + [H +]

(3)

The fraction of molecular 2,4-D during the reaction was therefore estimated and shown in Figure 3. The dominant species at low and high initial pH levels was molecular and ionic 2,4-D, respectively. When the initial pH (2.98) was slightly higher than its pKa at 2.90, the transformation of dominant species was observed. In addition, since all the reaction pH levels are declining over time, the fraction of molecular species was increased during the reaction accordingly.

Figure 3. Increased formation of molecular species of 2,4-D due to the reduction in solution pH during the photochemical reduction.

Because a faster degradation of 2,4-D was observed at lower pH levels and the rate acceleration was prevalently observed due to proton generation (and therefore the increase of molecular 2,4-D in the solution), in the photodestruction of 2,4-D, the molecular species of 2,4-D is likely the more preferred species than is the ionic one. Recall that the two major mechanisms of photodechlorination are direct homolytic cleavage of the aromatic C-Cl bond and the electron transfer of the radical anion-like moiety of the excimer. Although the relative contributions of the two reaction pathways are not known, the latter route is very likely to be affected by solution pH especially be retarded in alkaline medium. This is because the predominant anionic 2,4-D at pH level above pKa repels the anion-like radical and hinders the generation of the triplet excimer. As such, the neutral molecular 2,4-D can be dechlorinated through both the direct homolysis and the electron-transfer process, while the ionic 2,4-D will transform via the former route only. As a consequence, the photodecay of 2,4-D is slower in a basic environment where the dissociated 2,4-D species is dominant. Assuming no other major degradation pathways in the solution, the photochemical reduction at pH 10.03 encounters 99.9% negatively charged 2,4-D ions, and the corresponding initial rate constants (3.1 × 10-4 s-1) should be close to that of the direct fission of the triplet state. Similarly, the speciation at pH 2.02 consists of 89% neutral molecular 2,4-D, and its decay rate constant (1.1 × 10-3 s-1) should be the sum of the two mechanisms. Therefore, the rate constant solely due to the molecular 2,4-D (about 7.9 × 10-4 s-1) is 2.5 times faster than the ionic one. Therefore, the photodegradation process in acidic medium occurred in a faster rate than that at higher pH levels. Reaction Products and Pathways. The reaction intermediates of direct photolysis of 2,4-D in pure water, identified by LC/MS, were 2,4-dichlorophenol (2,4-DCP), 2-chloro-4-hydroxyphenoxyacetic acid (2-Cl-4-OH-PAA), 4-chloro-2-hydroxyphenoxyacetic acid (4-Cl-2-OH-PPA),

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Figure 4. Formation and destruction of the reaction products of 2,4-D photolysis in Brij 35 micellar solution at initial pH of 2.98. Total I is the sum of all the detectable reaction intermediates.

and 4-chlorocatechol. The yields of these intermediates were quite low because less than 30% of 2,4-D was transformed in water at both acidic (pH of 3.03) and basic (10.05) conditions. It is believed that the hydroxylated products result from photohydrolysis.9,20 Water was apparently involved in the reaction in substituting the chlorine atom or the -OCH2COOH group on the aromatic ring. Since the identical intermediates are observed at both low and high pH levels, the photohydrolysis process is effective to both ionic and molecular forms of 2,4-D. In the presence of surfactant micelles, however, additional intermediates including 2-chlorophenoxyacetic acid (2-Cl-PAA), 4-chlorophenoxyacetic acid (4-Cl-PAA), phenoxyacetic acid (PAA), 4-hydroxyphenoxyacetic acid (4-OH-PAA), and phenol were identified other than those of chlorohydroxyphenoxyacetic acids. Yet the 2,4-DCP and its secondary product 4-chlorocatechol are no longer measurable. This suggests that the pathway leading to 2,4-DCP is more trivial than other competitive mechanisms in micellar solutions. To study the details of the phototransformation of 2,4-D in micellar medium, the evolution of the 2,4-D and its derivatives during the UV irradiation in Brij 35 at pH levels of 2.98 and 10.03 are illustrated in Figures 4 and 5, respectively, where the sum of all measurable intermediates (total I) and the mass of aromatic rings were also shown for comparison. Due to the lower decay rate of 2,4-D at basic pH levels, the total I keeps increasing throughout the reaction, while the same curve declines after the midpoint of the reaction at acidic conditions, while the mass of the aromatic ring shows a continuous decline for both cases. Since the ring-mass was composed of the residual of 2,4-D and aromatic derivatives in the solution, the fractions of lost ringmass in low (0.63) and high pH levels (0.19) indicate the levels of ring-opening formation of nondetectable aromatic end-products. Benzene is likely to be one of such compounds, because the reduction of phenol to benzene is possible as justified by Moulton and Wade.21 By interpreting and organizing the possible degradation mechanisms from the intermediate data, the evolu-

Figure 5. Concentrations of the intermediates obtained at pH 10.

Figure 6. Proposed reaction mechanism for photodegradation of 2,4-D in surfactant micelles.

tion profiles of different degradation products, and the results of double mass spectrum (MS/MS), a schematic reaction pathway of 2,4-D photolysis in surfactant micelle is proposed as shown in Figure 6, where the photoreduction and photohydrolysis are both observed in the process. The much higher total yield of the dechlorinated products (2-Cl-PAA and 4-Cl-PAA) than

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that of the hydroxylated products (chlorohydroxyphenoxyacetic acid) suggests the photoreduction (via photodechlorination) is the dominant pathway. In addition, as 2,4-D bears two chlorine atoms, the reduction may occur at either the ortho or para site to yield 4-Cl-PAA and 2-Cl-PAA, respectively. In an acidic condition, the [4-Cl-PAA] was about twice higher than the [2-Cl-PAA]; however, in a basic condition, it was interesting to observe that the [2-Cl-PAA] became more dominant. This is likely because the major dechlorination route of molecular 2,4-D is through the fragmentation of triplet excimers at the ortho position; however, as the ionic 2,4-D is prevailing in the solution at high pH level, the negative charge of -OCH2COO- group inhibits the formation of aryl radical anion (ArCl•-) at the nearby ortho site, and thus,a the para substitution becomes dominant due to less repulsion. Nevertheless, those two primary intermediates subsequently undergo further photodechlorination resulting in phenoxyacetic acid as one of the secondary intermediates; this can be justified by the development of the lag of its initial concentration as shown in Figures 4 and 5. The phenoxyacetic acid is then further reduced to phenol as the final detectable product. Simultaneously, a minor pathway via photohydrolysis is also observed with the generation of low levels of hydroxylated products including chlorohydroxyphenoxyacetic acid (from 2,4-D) and 4-OH-PAA (from 4-Cl-PAA). In micellar solution, 2,4-D may undergo a nucleophilic displacement of chloride at either the ortho or para positions to yield chlorohydroxyphenoxyacetic acids, which is also observed in the direct photolysis of 2,4-D in pure water. Only one peak exhibited a clear 3:1 cluster at m/z 201, and 203 in the mass spectrum indicates that photohydrolysis is selective toward the para position. This is because the water molecule adduct at the para site may experience less steric strain than that at the ortho position. Similarly, the 4-Cl-PAA may also be transformed through photohydrolysis to give 4-OH-PAA, but the 2-Cl-PAA cannot be further photohydrolyzed to 2-hydroxyphenoxyacetic acid (2-OH-PAA) due to the steric hindrance at the ortho site. To verify the schemes of the proposed reaction mechanism, the individual photoreduction tests of PAA, 2-ClPAA, and 4-Cl-PAA in Brij 35 solutions were carried out at pH 3.0. The major intermediate of PAA was found to be phenol; no hydroxyphenoxyacetic acids were detected. This implies that addition of OH groups to unoccupied sites of the aromatic ring through photohydrolysis is not possible. For the reaction initiated with 2-Cl-PAA, phenol and PAA were identified as the photoproducts. The absence of 2-OH-PAA confirms that the water molecule is unlikely to replace chlorine at the ortho position through photohydrolysis. Furthermore, the photodegradation of 4-Cl-PAA gave high yields of 4-OH-PAA, PAA, and phenol with small amounts of 2-Cl-PAA. The formation of 2-Cl-PAA should be a result of isomerization of 4-ClPAA upon UV absorption. Finally, hydroxylated chlorophenoxyacetic acids are not detected in reactions starting with either 2-Cl-PAA or 4-Cl-PAA, suggesting that the OH group can only substitute at a site where an electron-withdrawing group (such as a chlorine atom) is attached rather than at an unoccupied site. This is likely due to the presence of surfactant micelles that establish a reductive environment for the reaction; hence, the generation of oxidative products is inhibited.

Conclusions The photodegradation of 2,4-D in water and/or various (anionic, cationic, and nonionic) micellar solutions shows that the presence of nonionic surfactant Brij 35 significantly improves the removal rate of 2,4-D to be five times greater than that in water alone. The reaction mechanism of this process is also extensively investigated at different pH levels, where higher transformation efficiency was observed in the acidic solution than in the alkaline one due to specification of 2,4-D under different reaction mechanisms as stated in the previously. The major reaction pathway is the photoreduction process (via photodechlorination) by using surfactant as an additional hydrogen source while a minor pathway, photohydrolysis, was also observed by giving low yields of hydroxylated products. Acknowledgment The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. PolyU 5038/02E). Literature Cited (1) Clarke, A. N.; Mutch, R. D.; Wilson, D. J.; Oma, K. H. Design and Implementation of Pilot Scale Surfactant Washing/Flushing Technologies Including Surfactant Reuse. Water Sci. Technol. 1992, 26, 127-135. (2) Jafvert, C. T.; Health, J. K.; Van Hoof, P. L. Solubilization of Nonpolar Compounds by Nonionic Surfactant Micelles. Water Res. 1994, 28, 1009-1017. (3) Jafvert, C. T.; Van Hoof, P. L.; Chu, W. The Phase Distribution of Polychlorobiphenyl Congeners in Surfactant-amended Sediment Slurries. Water Res. 1995, 29, 2387-2397. (4) Roy, D.; Kommalapati, R. R.; Mandava, S. S.; Valsaraj, K. T.; Constant, W. D. Soil Washing Potential of a Natural Surfactant. Environ. Sci. Technol. 1997, 31, 670-675. (5) Doong, R. A.; Lei, W. G. Solubilization and Mineralization of Polycyclic Aromatic Hydrocarbons by Pseudomonas putida in the Presence of Surfactant. J. Hazard. Mater. 2003, 96, 15-27. (6) Goyal, R. N.; Jain, N.; Gupta, P. Electrochemical Studies of 1,3,7,9-Tetramethyluric Acid in Aqueous and Micellar Media. Colloid Surf., A 1999, 162, 239-247. (7) Schmelling, D. C.; Poster, D. L.; Chaychian, M.; Neta, P.; Silverman, J.; Al-Sheikhly, M. Degradation of Polychlorinated Biphenyls Induced by Ionizing Radiation in Aqueous Micellar Solutions. Environ. Sci. Technol. 1998, 32, 270-275. (8) Epling, G. A.; Florio, E. M.; Bourque, A. J.; Qian, X. H.; Stuart, J. D. Borohydride, Micellar, and Exciplex-Enhanced Dechlorination of Chlorobiphenyls. Environ. Sci. Technol. 1988, 22, 952-956. (9) Shi, Z.; Sigman, M. E.; Ghosh, M. M.; Dabestani, R. Photolysis of 2-Chlorophenol Dissolved in Surfactant Solutions. Environ. Sci. Technol. 1997, 31, 3581-3587. (10) Diehl, C. A.; Jafvert, C. T.; Marley K. A.; Larson, R. A. Surfactant-assisted UV-photolysis of Nitroarenes. Chemosphere 2002, 46, 553-560. (11) Bunce, N. J.; Hayes, P. J.; Lemke M. E. Photolysis of Polychlorinated Benzenes in Cyclohexane Solution. Can. J. Chem. 1983, 61, 1103-1104. (12) Freeman, P. K.; Ramnath, N.; Richardson, A. D. Photochemistry of polyhaloarenes. 8. The photodechlorination of pentachlorobenzene. J. Org. Chem. 1991, 56, 3643-3646. (13) Freeman, P. K.; Ramanath, N.; Richardson, A. D. Photochemistry of polyhaloarens. 9. Characterization of the radical anion intermediate in the photodehalogenation of polyhalobenzenes. J. Org. Chem. 1991, 56, 3646-3651.

Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005 1651 (14) Zona, R.; Solar S.; Gehringer, P. Degradation of 2,4Dichlorophenoxyacetic Acid by Ionizing Radiation: Influence of Oxygen Concentration. Water Res. 2002, 36, 1369-1374. (15) Saha, S.; Mukherji, D.; Sarkar, S. K.; Aditya, S. Photolytic Behaviour of [Co(NH3)4C2O4] in Micellar and Inverse Micellar Solutions. J. Photochem. Photobiol. A: Chem. 1997, 103, 127131. (16) Chu, W.; Jafvert, C. T. Photodechlorination of Polychlorobenzene Congeners in Surfactant Micelle Solutions. Environ. Sci. Technol. 1994, 28, 2415-2422. (17) Whitten, D. G.; Russell, J. C.; Schmehl, R. H. Photochemical Reactions in Organized Assemblies: Environmental Effects on Reactions Occurring in Micelles. Vesicles, Films and Multilayer Assemblies and at Interfaces. Tetrahedron 1982, 38, 2455-2487. (18) Chu, W.; Jafvert, C. T.; Diehl, C. A. Phototransformation of Polychlorobiphenyls in Brij 58 Micellar Solutions. Environ. Sci. Technol 1998, 32, 1989-1993.

(19) Peller, J.; Wiest, O.; Kamat, P. V. Sonolysis of 2,4Dichlorophenoxyacetic Acid in Aqueous Solutions. Evidence for •OH-Radical-Mediated Degradation. J. Phys. Chem. A 2000, 105, 3176-3181. (20) Boule, P.; Guyon, C.; Tissot, A.; Lemaire, J. Specific Phototransformation of Xenobiotic Compounds: Chlorobenzenes and Halophenols. In Photochemistry of Environmental Aquatic System; American Chemical Society: Washington, D.C., 1987. (21) Moulton, W.; Wade, C. Reduction of Phenols to Aromatic Hydrocarbons. J. Org. Chem. 1961, 26, 2528-2529.

Received for review June 7, 2004 Revised manuscript received December 12, 2004 Accepted December 17, 2004 IE049509J