Reaction Mechanism and Kinetic Modeling of DEET Degradation by

For this purpose, the degradation of DEET (N,N-diethyl-3-methylbenzamide), ... FAFT model was developed to fit all of the degradation data of DEET and...
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Environ. Sci. Technol. 2006, 40, 4488-4494

Reaction Mechanism and Kinetic Modeling of DEET Degradation by Flow-Through Anodic Fenton Treatment (FAFT) HUICHUN ZHANG† AND ANN T. LEMLEY* Graduate Field of Environmental Toxicology, TXA, MVR Hall, Cornell University, Ithaca, New York 14853-4401

The previously developed batch anodic Fenton treatment (AFT) technology has been successfully applied to degrade various pesticides in aqueous solution. The goal of this work is the development of a flow-through AFT system (FAFT) which is critical to bringing this technology into practical general use in the field. For this purpose, the degradation of DEET (N,N-diethyl-3-methylbenzamide), an insect repellent, and nine model amides was studied. Oxidation products of these compounds in FAFT were identified by GC/MS, and the results revealed that various -OH additions (most likely on the aromatic ring), quinone/keto product formation, and dimerization/bimolecular disproportionation are the major reaction pathways. This proposed overall reaction mechanism was then combined with the basic Fenton’s mechanism to model the kinetics of various active species in FAFT including DEET, Fe2+, H2O2, and total iron under different reaction conditions. In addition, both initial and steady-state hydroxyl radical concentrations were measured in FAFT using benzoic acid as a chemical probe; the measured •OH concentrations were best-fitted exponentially. On the basis of the obtained [•OH] trend and the mass balance of the FAFT system, a simple FAFT model was developed to fit all of the degradation data of DEET and the model amides.

Introduction Pesticide-containing wastewaters have been widely generated, mainly from applicators’ container rinsates and from agrochemical formulating and manufacturing plants (1). To eliminate the potential contamination of the aqueous environment through improper disposal of these wastes, an environmentally sound and cost-effective on-site treatment technology is critically needed. Among various cleanup technologies, advanced oxidation processes (AOP) using the highly reactive hydroxyl radical generated via the Fenton reaction have been widely recognized as an effective treatment technology (2-4), among which anodic Fenton treatment (AFT) has recently been studied systematically at the benchtop scale (5, 6). AFT has been proven to be effective at treating high-concentration pesticide-containing wastewaters with a broad spectrum of target pollutants. On the basis of the established batch AFT system, the development * Corresponding author phone: (607)255-3151; fax: (607)255-1093; e-mail: [email protected]. † Current address: National Exposure Research Laboratory, U.S. Environmental Protection Agency, 960 College Station Road, Athens, GA 30605. 4488

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of a flow-through AFT system (FAFT) is critical to bringing this technology into practical general use in the field. Recent studies (7-9) have developed a well-accepted mechanism for the Fenton process as shown in Table S1 in the Supporting Information. The reaction kinetics may be controlled by a chain reaction in which the hydroxyl radical (•OH) is identified as an active oxidizing agent. The hydroxyl radical is known to react unselectively with organic compounds with a fast rate constant in the range of 107 to 1010 M-1 s-1 (4). Both hydrogen abstraction from alkyl groups and •OH addition to unsaturated bonds are the typical reaction mechanisms for •OH leading to the generation of organic radicals (3, 4, 10, 11). As a strongly electrophilic species, the reactivity of •OH is significantly enhanced by electron donating groups (e.g., R-OH, R-OR, and amide N) and inhibited by electron-withdrawing groups (e.g., >CdO, >COOH, -X where X ) -F or -Cl). The generated organic radicals from the above reactions can be oxidized by oxygen and/or Fe3+, reduced by Fe2+, or undergo other reactions such as dimerization or bimolecular disproportionation to form a variety of products (3, 4, 10). Depending on the structure of the organic species, the extent of these reactions may vary. For example, carbon-centered radicals will react with oxygen at very high rate constants (∼109 to 1010 M-1 s-1) leading to peroxyl radicals (RCOO‚) (10). Radicals can also be reduced by Fe2+ in the presence of electron-withdrawing groups with a reaction rate constant >107 M-1 s-1 (4). In addition, C-centered organic radicals containing R-OH or R-OR substituents can reduce Fe3+ rapidly to Fe2+ with the rate constant estimated to be >4 × 108 M-1 s-1 for •CH2OH (4). All of the above information, together with the products identified in this study (see the Results and Discussion), will be used to propose a reaction mechanism for DEET degradation in FAFT. To date, most modeling work on the Fenton reaction is based on the reaction mechanism of either the Fenton system (7, 12) or the organic compound(s) (3, 13) because of the complex nature of the reactions involving the latter. Only one recent paper (8) successfully applied the basic Fenton’s mechanism (as shown in Table S1 in the Supporting Information) to model the degradation of a simple organic compound (i.e., formic acid) under a wide range of reaction conditions. A kinetic model that combines the reaction mechanism of the Fenton system with that of the organic compound will likely provide kinetic profiles for all species of interest. This knowledge may be helpful for further toxicological studies and can provide a basis for the treatment of FAFT effluent for the final discharge. It can also serve as a mechanistic basis for future FAFT studies on other organic pollutants. DEET (N,N-diethyl-3-methylbenzamide, see Scheme 1 for structure information) was selected in this study as a chemical probe to investigate the Fenton system because it is the most often used insect repellent applied on the skin (14) and, similar to other pesticides, may pollute the environment because of improper disposal and container rinsing associated with its frequent use (1). The objectives of this work were (i) to investigate the reaction mechanism of DEET degradation in the FAFT system, (ii) to model the species profiles by combining the basic Fenton’s mechanism with the DEET degradation mechanism under a range of reaction conditions, (iii) to develop a simple kinetic model for FAFT in a continuous flow stirred tank reactor (CSTR), and (iv) to test the kinetic model using model amide compounds including benzadox (BDX), hexamide (HXD), 2-methylbenzamide (o-MBD), 3-methyl benzamide (m-MBD), p-toluamide (p-MBD), N,N-diethylnicotinamide 10.1021/es060515b CCC: $33.50

 2006 American Chemical Society Published on Web 06/07/2006

SCHEME 1. Proposed Oxidation Mechanism of DEET in FAFT

(DND), N-methyl-o-toluamide (MTD), 2-fluoro-3-(trifluoromethyl)benzamide (FBD), and 3,5-dinitro-o-toluamide (DTD) (see Figure S1 in the Supporting Information for structure information).

Materials and Methods Chemicals. DEET (98% purity) was purchased from Chem Services (West Chester, PA). BDX, o-MBD, m-MBD, p-MBD, DND, MTD, FBD, DTD, benzoic acid (BA), and p-hydroxybenzoic acid (HBA) were all purchased from Sigma-Aldrich (St. Louis, MO) at greater than 98% purity. HXD was obtained from Interbioscreen (Russia). Acetonitrile (HPLC grade), ammonia acetate buffer, ferrous ammonium sulfate, HCl, H2O2, hydroxylamine hydrochloride (10%, APHA for iron), methanol (HPLC grade), o-phenanthroline, H3PO4, KMnO4, NaCl, Na2SO4, H2SO4, and water (HPLC grade) were obtained from Fisher (Fair Lawn, NJ) at certified grade unless otherwise specified. All chemicals were used directly without further purification. Degradation of DEET in CSTR by FAFT. A detailed description of the experimental setup and the experimental apparatus (Figure S2) is available in the Supporting Information. Briefly, all experiments were carried out in two 500mL glass half-cells that served as the anode and cathode which were constantly stirred. After pumping influents (200 µM DEET solution with 0.02 M NaCl or 0.08 M NaCl) into the respective half-cells, reaction was initiated by turning on the power supply (to deliver ferrous iron into the anodic halfcell) when the first drop of H2O2 entered the anodic half-cell. At given time intervals, 1 mL of anodic effluent was collected and added to a 2-mL HPLC vial containing 100 µL of methanol (to quench the subsequently generated hydroxyl radicals) for HPLC analysis. A volume of 20 mL of anodic effluent was also collected and added into a 25-mL glass vial containing

2 mL of methanol for ferrous ion and H2O2 analyses when needed. Treatments were repeated for a total of three replicates, and all data are shown in the format of average ( standard deviation. Analytical Methods. Decrease in the concentration of DEET or model amides was monitored by a reversed-phase high-performance liquid chromatography (HPLC) system with a Restek ultra C18 column (4.6 mm × 250 mm, 5 µm) and a diode-array UV-vis detector (1100, Agilent Technology). The detector wavelength was set at 220-265 nm for DEET, amides, BA, and HBA. The mobile phase consisted of acetonitrile and water (pH adjusted to 3 using H3PO4). The concentration of H2O2 was determined by titration using standard 1 mM KMnO4 solution (15). The concentrations of Fe2+ and total iron were analyzed using the 1,10-phenanthroline method (16). Products of DEET and model amides were analyzed by an Agilent GC/MS (6890/5973) system with a SUPELCO Equity-5 capillary column (30 m × 250 µm × 0.25 µm) (see the Supporting Information for more details). Kinetic Modeling. Application of the basic Fenton’s reaction mechanism to DEET degradation in the FAFT system was achieved by use of the Kintecus modeling software (17). Previously, this program was successfully used in modeling formic acid oxidation in a classic Fenton system (8). Tables S1 and S2 in the Supporting Information show the reaction sets used as input. The reaction set in Table S1 was based on the kinetic model proposed by De Laat and Gallard (7) for the Fenton system with minor modifications as described by Duesterberg et al. (8). The reaction set in Table S2 was based on the DEET degradation mechanism proposed in this paper (see Scheme 1). Generally the rate constants in Tables S1 and S2 were taken from the literature. For reactions with unknown rate constants, the k values were obtained by VOL. 40, NO. 14, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Mass Spectra of DEET and Its Oxidation Products in FAFT compd

RT (min)

m/z

fragments m/z (relative abundance)

DEET a3 a3 a3 or b2′ unknown unknown a2 or b2 a2 or b2 a7 a5 or b4′ a2′ or b4 a2′ or b4 a2′ or b4 a5 or b4′ a7 a6 a5 or b4′ a5 or b4′ a6

12.58 13.53 13.62 13.92 14.02 14.10 14.60 14.73 15.07 15.44 15.50 15.57 15.64 15.97 16.11 16.18 16.47 16.63 17.85

190 207a 207b 207c 207d 205 204a 204b 241a 223a 220a 220b 220c 223b 241b 353 223c 223d 352

190 (50), 176 (3), 162 (4), 148 (2), 119 (100), 91 (35) 207 (65), 206 (60), 190 (10), 178 (5), 135 (100), 119 (4), 107 (10), 91 (2), 77 (30) 207 (55), 206 (50), 190 (13), 178 (5), 135 (100), 119 (6), 107 (13), 91 (3), 77 (25) 207 (30), 206 (35), 190 (88), 179 (8), 162 (12), 134 (100), 119 (40), 106 (88), 91 (20), 77 (30) 207 (6), 198 (90), 190 (6), 135 (12), 119 (15), 105 (2), 91 (7), 77 (5), 72 (100) 205 (24), 198 (10), 190 (75), 162 (7), 150 (5), 134 (100), 119 (25), 106 (25), 77 (20), 72 (60) 205 (30), 204 (45), 190 (4), 176 (4), 162 (4), 133 (100), 119 (4), 105 (25), 91 (2), 77 (30) 205 (14), 204 (17), 190 (4), 176 (15), 162 (8), 133 (100), 119 (7), 105 (28), 91 (3), 77 (21) 241 (100), 223 (20), 206 (15), 190 (10), 179 (11), 169 (82), 162 (5), 150 (40), 119 (30), 72 (94) 223 (10), 206 (40), 190 (6), 180 (8), 163 (7), 150 (17), 135 (43), 119 (20), 105 (8), 91 (14), 72 (100) 221 (79), 220 (81), 206 (19), 192 (15), 176 (5), 149 (100), 135 (24), 119 (21), 91 (15), 72 (67) 221 (28), 220 (35), 206 (5), 192 (5), 178 (3), 163 (2), 149 (100), 135 (9), 119 (7), 105 (2) 221 (30), 207 (8), 193 (29), 178 (4), 165 (5), 149 (100), 135 (12), 119 (14), 91 (8), 72 (40) 223 (1), 206 (38), 192 (1), 178 (4), 164 (1), 135 (100), 119 (1), 107 (12), 91 (1), 72 (18) 241 (16), 221 (10), 206 (40), 178 (3), 169 (35), 162 (4), 150 (28), 135 (100), 119 (7), 107 (10) 353 (28), 281 (78), 223 (10), 206 (38), 135 (100), 150 (20), 119 (15) 223 (8), 207 (9), 150 (9), 135 (100), 119 (4), 107 (7), 91 (3), 77 (15) 223 (81), 163 (6), 149 (100), 135 (65), 119 (12), 91 (8) 352 (44), 281 (100), 225 (8), 207 (30), 193 (6), 179 (10), 162 (6), 149 (7), 133 (28), 119 (16), 105 (7)

use of a numerical routine built into the Kintecus program in order to accurately simulate species profiles. The optimized k values were chosen based on visual comparison of the model and the experimental data across the range of reaction conditions investigated (see the Supporting Information for details).

Results and Discussion Oxidation Products of DEET and Model Amides. Analyses of DEET reaction extracts indicated the generation of a variety of oxidation products (see Figure S3 in the Supporting Information for the GC chromatogram, Table 1 for spectral information, and Scheme 1 for the proposed structures). The molecular ion of DEET is m/z 190, and its fragmentations are related to losses from the aliphatic chain: m/z 176 (-CH2), m/z 162 (-C2H4), m/z 148 (-C3H6), m/z 119 (-NC4H10), and m/z 91 (-CONC4H10). Compounds at 13.53 and 13.62 min are believed to be isomers of monohydroxylated DEET (a3 in Scheme 1) because (i) the aromatic ring is known to react with •OH (•OH addition at the ortho or para position) at a fast rate (3, 4) and (ii) they exhibit a fragmentation pattern similar to that of DEET except for a mass increase of 16 (Table 1). With the use of a similar approach (see the Supporting Information for detailed spectral analysis), the compound at 13.92 min is a product resulting from •OH addition at the meta position of the benzene ring (a3) or at the side chain (b2′); compounds at 14.60 and 14.73 min are monoquinone or keto analogues of DEET (a2 or b2); compounds at 15.07 and 16.11 min are DEET analogues with three •OH additions; compounds at 15.44, 15.97, 16.47, and 16.63 min are DEET analogues with two •OH additions (a5 or b4′); compounds at 15.50, 15.57, and 15.64 min correspond to quinone or keto analogues of DEET (a2′ or b4); compounds at 16.18 and 17.85 min are dimeric products of DEET and/or its oxidation products (a6). Compounds at 14.02 and 14.10 min cannot be identified based on their spectral information. By comparison with the GC chromatogram of the DEET control (data not shown), the peak at 12.70 min is believed to be due to an impurity and is thus not identified. Also, the mass analysis of the small GC peaks is not reliable when the GC signals are so weak that they are similar to noise in the baseline, so the identities of peaks at 13.92, 14.73, 15.44-15.64, and 16.1117.85 min are not as reliable as the others. Nevertheless, the presence of these small peaks reflects the presence of a variety of DEET oxidation products (see more discussion in the next section). Note that all of the generated oxidation products 4490

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can further react with •OH to form various breakdown products and may eventually mineralize, because all of these products disappeared when the reaction time was longer than 15-20 min (data not shown). Degradation products of selected amide compounds (mMBD, p-MBD, MTD, and FBD) were also analyzed by GC/ MS (see Tables S3-S6 in the Supporting Information for spectral information). All of these amides exhibited a similar product pattern as that of DEET, i.e., various -OH additions (most likely on the ring), quinone/keto product formation, and dimeric products. However, MTD (with a methyl group on the side chain) degradation generated a larger number of products than the other amides (without any substituents on the side chain), suggesting oxidation occurred on the side chain in addition to the benzene ring. Moreover, HPLC chromatograms of amide FAFT effluents showed the presence of a number of minor product peaks that could not be identified using GC/MS (data not shown). All of the amide product peaks also disappeared in extended studies (reaction time >10-30 min). These results strongly suggest that the amides degraded in FAFT via a mechanism similar to that of DEET. Reaction Mechanism. On the basis of the reported reaction mechanisms of the hydroxyl radical (3, 4, 13, 18) and the product identification, a reaction scheme for the oxidation of DEET in FAFT is proposed (Scheme 1 and Table S2). In summary, both the benzene ring and the aliphatic chain of DEET can react with •OH through either -OH addition (forming a1) or hydrogen abstraction (forming b1). Radical a1 can then be oxidized by oxygen or Fe3+ to form either quinone analogues of DEET (a2 and a2′) or hydroxylated DEET (a3). The reaction between radical a1 and oxygen has been well documented (3, 18), while reactivity of a1 toward Fe3+ is because of the electron-rich environment of the aromatic ring (4). Similar to DEET, a3 reacts with •OH forming the a4 radical which can be further oxidized to a5. In addition to the above reactions, radical a1 can undergo either dimerization to form product a6 or bimolecular disproportionation to form a3 and DEET (3). Radical b1 can be oxidized by oxygen to form b2 and other intermediates such as b2′ (10). Because of the presence of an electronwithdrawing keto group on the side chain of b1, radical b1 is not expected to be oxidized by Fe3+ (4). Similar to the formation of radical b1, a3 can also undergo H‚ abstraction to form radical b3 which can be oxidized by oxygen to form b4 and other intermediates such as b4′ (10). Furthermore, in the presence of electron-withdrawing groups, the generated radicals such as a1, a4, b1, and b3 can be reduced by

FIGURE 1. (A) Experimental (symbols) and Kintecus model (lines) data for the degradation of DEET in FAFT. [DEET]0 ) 200 µM; electrolysis I ) 0.050 A; input H2O2/Fe2+ ) 10:1 (curves a-d), 5:1 (curve e, ×), 15:1 (curve f, +), and 20:1 (curve g, b); FAFT flow rate ) 9.4 ( 0.8 mL/min (curve a), 16.4 ( 1.2 mL/min (curve b), 27.4 ( 1.4 mL/min (curves c, e-g), and 37.4 ( 1.0 mL/min (curve d). (B-D) Concentrations of H2O2, Fe2+, and total Fe in the same experiments.

Fe2+ to the corresponding starting compound (DEET) or a3 (4). Because of the high reactivity of •OH, all of the formed oxidation intermediates can further react with •OH to form various breakdown products that were not detected by the current analytical method. Degradation of DEET in FAFT under Different Reaction Conditions. In an effort to verify the proposed reaction mechanism and to model the species profiles in FAFT, FAFT experiments were conducted to degrade DEET. Control experiments were conducted to test the system, and no significant FAFT degradation of DEET was found in reactions with only electrolysis (Fe2+ delivery) or only H2O2 delivery (data not shown). In contrast, fast DEET degradation occurred in FAFT in the presence of both Fe2+ and H2O2 under different reaction conditions (Figure 1A). The conditions that were varied were the input H2O2/Fe2+ ratio and the FAFT flow rate. On the basis of the proposed reaction mechanism for the FAFT system (Tables S1 and S2 and Scheme 1), the reaction kinetics of DEET were modeled using the Kintecus modeling program. Kinetic evolution of H2O2, Fe2+, and total Fe (Figure 1B-D) was also monitored and modeled under the same reaction conditions. Note that the total Fe measurement was only conducted at two reaction conditions. The kinetic trends for Fe2+ were quite similar between the studied conditions. To show them clearly, only two trends are included in Figure 1C for two extreme reaction conditions. As the modeling results show in Figure 1, the proposed reaction mechanism can adequately predict the experimental results for DEET, H2O2, and total Fe. For the kinetic trend of Fe2+, however, the model results overestimated the generation of Fe2+ in the initial reaction period. We do not have a good explanation for this disagreement, but we speculate that a delay in the mass transfer of the generated Fe2+ from the

electrode into solution might be part of the reason because this overestimated generation of Fe2+ agrees with its constant delivery rate (Figure 1C). Alternatively, the current method for Fe2+ analysis may not be able to capture true “instant” ferrous iron concentrations since methanol only quenches radical reactions and reactions 1 and 2 in Table S1 are still active in the presence of high concentrations of H2O2 in the course of analysis. Depending on the levels of Fe2+ and Fe3+, [Fe2+] can be either underestimated (consumed in reaction 1) or overestimated (generated from reaction 2). Note the much larger rate constant of reaction 1 than that of reaction 2. Nevertheless, the model can predict the overall trend of Fe2+ very well (reaction time from 6 to 200 min, see Figure S4 in the Supporting Information). As also shown in Figure S4, the model prediction agrees well with the experimental results for both H2O2 and DEET in the long-term study (total Fe was not monitored in this case). The slightly lower DEET effluent concentration under steady-state conditions as predicted by the mechanism is likely related to the presence of various unknown breakdown products that compete with DEET for the available •OH. In addition to the above modeling, kinetic profiles for all reaction intermediates can be predicted in a similar way (data not shown). This information may be particularly useful for the treatment of organic pollutants whose oxidation intermediates are still toxic and thus deserve tracking during the treatment process without costly and labor-intensive laboratory analysis. Overall, the above results support the proposed reaction mechanism in Scheme 1 for DEET oxidation in the FAFT system. Determination of •OH Concentrations in FAFT. Because of the nonselectivity and high reactivity of •OH as shown in Tables S1 and S2, it is difficult to propose a single equation for changes in •OH concentrations. Its low concentration VOL. 40, NO. 14, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (A) Degradation of BA in FAFT in the presence of 200 µM DEET at different initial concentrations: 27 µM (a), 10 µM (b), and 20 µM (c and d). BA was added at treatment time zero (a and b), 10 min (c), and 20 min (d). Note that the time axis in the figure is not treatment time but starts from the point that BA was added. Other reaction conditions are as follows: flow rate ) 27.4 ( 1.4 mL/min, input H2O2/Fe2+ ) 10:1, I ) 0.05 A. (B) Calculated [•OH] in FAFT (symbols) based on the probe method and the corresponding exponential fit (line). also prevents direct measurement; therefore indirect methods must be used. From different indirect detection methods available in the literature (19-21), BA was chosen as a probe to target the concentration of •OH in FAFT owing to the convenience of the method and the similarity in matrixes between the FAFT system and the system in reported work by Lindsey and Tarr (20), which has detailed the measurement of •OH concentrations under non-steady-state conditions. It was proposed that the reaction between •OH and any organic compound be regarded as second-order kinetics (4) and that when the probe concentration [P] is sufficiently low (product yield 0.999 for all the cases. The FAFT model can also fit the long-term DEET degradation (up to 200 min) and accurately predicted the steady-state DEET effluent concentration with

radicals and oxidation byproducts. Final discharge of the effluent is only possible after removal of these pollutants. Biodegradation will be one potential method to remove the organic compounds, as it has been shown that the biodegradability of pesticide wastewater can be greatly improved after AFT treatment (22).

Acknowledgments This work was supported by the Cornell University Agricultural Experiment Station federal formula funds, Project 329423 (Regional Project W-045), received from CSREES, USDA. Professor Allen Back is acknowledged for valuable input in solving the differential equations. The authors thank Dr. Eric J. Weber and Dr. Lingjun Kong for reviewing the manuscript. The authors are especially grateful to Dr. Qiquan Wang for helpful discussion in the beginning stage of this study and for critically reviewing the manuscript.

Supporting Information Available Mechanism of Fe(II)-initiated chain reactions (Table S1), reaction mechanism of DEET oxidation in FAFT (Table S2), mass spectra information for m-MBD, p-MBD, MTD, and FBD, respectively (Tables S3-S6), structures of DEET and model amide compounds (Figure S1), CSTR FAFT apparatus (Figure S2), GC chromatogram of DEET reaction mixture in FAFT (Figure S3), long-term time course of Fe2+, H2O2, and DEET in FAFT and the corresponding Kintecus modeling results (Figure S4), and FAFT modeling fits of model amides degradation (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited FIGURE 3. Experimental (symbols) and FAFT model (lines) data for degradation of DEET in FAFT under the same conditions as in Figure 1. [DEET]0 ) 200 µM; input H2O2/Fe2+ ) 10:1 (curves a-d), 5:1 (curve e), 15:1 (curve f, +), and 20:1 (curve g, b ); electrolysis I ) 0.050 A; flow rate ) 9.4 ( 0.8 mL/min (curve a), 16.4 ( 1.2 mL/min (curve b), 27.4 ( 1.4 mL/min (curves c, e-g), and 37.4 ( 1.0 mL/min (curve d). r2 > 0.999 (data not shown). A study is currently underway to optimize the FAFT system under various operating conditions using the FAFT model and will provide more insight into the physical meanings of parameters a and z. To test the developed FAFT model, degradation of various model amide compounds was conducted in FAFT under conditions similar to that of DEET. The results are shown in Figure S5 in the Supporting Information. All of the amide degradation kinetics can be fitted by the FAFT model well with r2 > 0.999. Visually the reactivity of the amides follows this order: DEET ≈ o-MBD ≈ m-MBD ≈ p-MBD ≈ MTD ≈ BDX ≈ HXD > FBD > DND . DTD. Because •OH is an electrophilic species, the presence of the electron-withdrawing groups (i.e., -CF3 and -F in FBD, aromatic N in DND, and -NO2 in DTD) likely causes the lower reactivity of the last three compounds. This study converts the batch AFT system to a flowthrough system, a major step forward toward its final application in treating small-scale, high-concentration pesticide wastewaters for individual farmers and commercial agrochemical applicators. Further testing of the reaction mechanism and the FAFT model with different pesticides and pesticide mixtures under more environmentally relevant conditions (e.g., using real pesticide wastewater instead of artificial water, adding cosolutes such as ions and natural organic matter, etc.) would help scale up the FAFT system to industrial applications. The FAFT effluent is acidic (pH ∼ 3.0) and contains a large amount of ferric ions and various

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Received for review March 5, 2006. Revised manuscript received April 18, 2006. Accepted April 19, 2006. ES060515B