Oxidation of Acetamide Herbicides in Natural Waters by Ozone and by

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Ind. Eng. Chem. Res. 2003, 42, 5762-5769

Oxidation of Acetamide Herbicides in Natural Waters by Ozone and by the Combination of Ozone/Hydrogen Peroxide: Kinetic Study and Process Modeling Juan L. Acero,* F. Javier Benitez, Francisco J. Real, and Cecilia Maya Departamento de Ingenierı´a Quı´mica y Energe´ tica, Universidad de Extremadura, 06071 Badajoz, Spain

The oxidation by ozone and by the combination O3/H2O2 of four herbicides included in the acetamide group (acetochlor, metolachlor, propachlor, and butachlor) was studied. In a first step, the rate constants were determined for the reactions of the selected herbicides with ozone (kO3) and OH radicals (kOH). The sequence of reactivities for solutions in ultrapure water were as follows: for ozonation, metolachlor < propachlor < acetochlor < butachlor; for oxidation by hydroxyl radicals, propachlor < acetochlor < metolachlor < butachlor. The rate constants for the reaction with ozone ranged from 1.1 to 5.3 L/(mol‚s), indicating that direct reactions with ozone will play a minor role during ozonation processes. Values for kOH ranged from 4.6 × 109 to 7.4 × 109 L/(mol‚s). Therefore, reactions with OH radicals will be the major pathway for the oxidative transformation of these four herbicides, even when conventional ozonation is applied. In a second step, the oxidation of the investigated herbicides during ozonation processes in some natural and mineral waters was studied, including the influence of the operating variables, and the herbicide oxidation levels were determined. Next, the oxidation process was characterized in terms of oxidant concentrations (ozone and OH radicals), and finally the evolution of the herbicide concentration in an ozonation process was modeled and predicted by applying a kinetics approach based on the rate constants for the reactions with ozone and OH radicals and the calculated concentrations of the active oxidants. 1. Introduction Modern agriculture is engaged in a continuous struggle against losses from pests such as weeds, insects, diseases, etc. To this end, pesticides in their different forms (insecticides, herbicides, fungicides, etc.) are widely used to increase the productivity of farms and croplands. However, concerns about the potential impacts of pesticides on human health have arisen because the extensive use of these substances leads to their presence together with their degradation byproducts in surface wastewaters from agricultural activities and in drinking waters.1 In addition to the toxic character of these substances in themselves, their potential to pollute is enhanced by the possibility of the generation of organohalogen compounds in the processes of disinfecting surface water and wastewaters through reactions with the most frequent agents used in those treatments, e.g., the chloro-derivative compounds. To control this hazard, the EU has set pesticide standards for drinking water at a maximum permissible concentration for any particular pesticide of 0.1 microgram/L and for the sum of all pesticides, including their degradation products, of 0.5 microgram/L.2 Several single agents, such as ozone, UV radiation, hydrogen peroxide, etc., are currently employed for the oxidation of pesticides and the subsequent purification of waters in which they are present.3,4 More powerful oxidation techniques are the so-called advanced oxidation processes (AOPs), which consist of oxidant combinations such as O3/H2O2, O3/UV, Fenton’s reagent, and * To whom correspondence should be addressed. Tel.: +34 924289384. Fax: +34 924289385. E-mail: [email protected].

the photo-Fenton system.5,6 Ozone, in particular, is an efficient oxidant for the treatment of drinking waters and some industrial wastewaters. In addition, it is a source of the hydroxyl radicals in the aforementioned AOPs (especially in the combination O3/H2O2), which are needed to eliminate refractory micropollutants, including most pesticides.7 Herbicides are among the pesticides that have been found to the greatest extent in water supplies near agricultural areas. Examples are triazines (atrazine, cyanazine, and simazine), phenoxyalkyl acid derivatives (2,4-D, mecoprop, and MCPA), and nitrogenous herbicides, mainly those included in the acetamide group (alachlor, acetochlor, metolachlor, propachlor, and butachlor). Specifically, alachlor, acetochlor, and metolachlor, together with atrazine, cyanazine, and simazine, have been reported as being among the most used herbicides in farming today.1 In view of these considerations, the group of the acetamide herbicides was selected for the present study with the aim of investigating their oxidation in natural and surface waters by conventional ozonation and by the AOP consisting of the combination O3/H2O2. The ozonation of alachlor has been studied recently by Beltran et al.,8 so that the present work focused on the oxidation of acetochlor (AC), metolachlor (MT), propachlor (PP), and butachlor (BT) by ozonation processes. The chemical structures of these four herbicides are shown in Figure 1. Although some of these herbicides have rate constants available for their reactions with ozone9,10 and OH radicals,10-12 there are no studies in the literature of their elimination from natural waters by the aforementioned oxidation processes, simulating drinking water treatment conditions. Thus, the first step

10.1021/ie030229y CCC: $25.00 © 2003 American Chemical Society Published on Web 10/11/2003

Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5763

Figure 1. Chemical structure of selected acetamide herbicides. Table 1. Water-Quality Parameters water sample

pH

NOM (mg/L)a

alkalinity (mM HCO3-)

Orellana reservoir Zujar reservoir mineral “Los Riscos”

7.5 8.0 7.0

8.7 15.1 4.4

1.7 1.4 0.6

a NOM measured as chemical oxygen demand (permanganate oxidation).

in the present study was to determine the rate constants for the single reactions of these herbicides with both oxidantssozone and OH radicalssin ultrapure water. These kinetic rate constants were then used to model and predict the kinetics of the oxidation of the herbicides during ozonation processes in natural or surface waters by applying the Rct concept.13 In addition, the influence of raw water-quality parameters (pH, organic matter, alkalinity, etc.) on the oxidation process and herbicide removal efficiency was established. 2. Experimental Section 2.1. Reagents and Natural Waters. All herbicides were dissolved in ultrapure water of high resistivity (18 MΩ‚cm) obtained from a Milli-Q water system (Millipore). Concentrated ozone stock solutions (8 × 10-4 M) were first prepared by bubbling a gas stream containing ozone and oxygen through ultrapure water in a flask that was cooled in an ice bath. Three different raw waters were selected to carry out the herbicide oxidation under realistic water treatment conditions. Two of them were natural waters collected from locations (Zujar and Orellana reservoirs) in the Extremadura Community (southwest Spain). These natural waters were filtered through a 0.45 µm cellulose nitrate filter within 24 h after collection and stored at 4 °C until use. The third was a purchased mineral water of the commercial brand “Los Riscos”. The main quality parameters of these waters are given in Table 1. 2.2. Analytical Methods. The ozone concentration of the stock solutions was determined directly by measuring their UV absorbance at 258 nm [ ) 3150 L/(mol‚cm)]. The ozone dissolved in the reaction samples

from the herbicide ozonation experiments was assayed by the Indigo method as proposed by Bader and Hoigne.14 The concentrations of herbicides and p-chlorobenzoic acid (pCBA, used as the reference or probe compound) were determined by high-performance liquid chromatography (HPLC) using a Waters chromatograph equipped with a 996 photodiode array detector and a Waters Spherisorb column (5 µm ODS2, 4.6 × 250 mm). Gradient elution was used with varying eluent ratios (acetonitrile and 10 mM phosphoric acid buffer) depending on the herbicide and the experiment. The wavelengths selected were 210 nm for herbicides and 235 nm for pCBA. Finally, the hydrogen peroxide concentration was assayed by the peroxidase-N,N-diethyl-p-phenylenediamine method.15 2.3. Experimental Procedures and Equipment. To determine the kinetic rate constant for the reactions of the herbicides with ozone, ozonation experiments of single herbicides dissolved in ultrapure water were performed under conditions of pseudo-first-order kinetics (excess of herbicide). The pH was adjusted to a value of 2 by means of a phosphoric acid/phosphate buffer (50 mM), and tert-butyl alcohol was added (to 0.1 M concentration) as the OH radical scavenger.16 Each experiment was initiated by injecting into the herbicide solution (64-500 µM) the volume of the ozone stock solution necessary to achieve the desired initial O3 dose (8-10 µM). The temperature was maintained at 20 °C. The flask was also provided with a bottle-top dispenser system,17 which was used to withdraw samples at regular time intervals. These samples were pumped directly into an Indigo solution to measure the remaining ozone concentration. The experiments were performed in duplicate. Competition kinetics was used to determine the second-order rate constants for the reactions of herbicides with OH radicals. The reference compound selected was pCBA. Because the reactivity of the selected herbicides with ozone was very low, it was possible to generate OH radicals by the AOP O3/H2O2. The experiments were performed in ultrapure water at pH 9 (50 mM phosphate buffer) with a high concentration of H2O2 (4.9 × 10-5 M), favoring fast ozone decomposition into OH radicals. These reactions were carried out in small glass vials of 20 mL. For every experiment, each vial was initially fed with the solutions of herbicide and pCBA (6-10 µM). After that, increasing volumes of the ozone stock solutions were injected in order to obtain the desired initial concentration in every vial, from 5 to 60 µM. After 1 h in a thermostatic bath at 20 °C, the remaining concentrations of herbicides and pCBA were measured by HPLC. The experiments were performed in duplicate. Finally, the experiments with natural and mineral waters were carried out under conditions similar to those applied in real drinking water treatment processes. The waters were buffered to pH 7 (10 mM borate buffer) and spiked with the herbicides and pCBA. Low concentrations (1.9 µM) were used to avoid interference effects in ozone decomposition, so that the main scavengers of OH radicals in solution were the substances present in the natural water. Hydrogen peroxide was also added in the combined O3/H2O2 experiments to give a molar ratio of [H2O2]/[O3] ) 0.5, which is reported as the optimum by Acero and von Gunten.18 For each experiment, the flask reactor with the natural water solution was immersed in a thermostatic bath at 20 °C,

5764 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 Table 2. Second-Order Rate Constants for the Reaction of Ozone (kO3) and OH Radicals (kOH,P) with the Selected Herbicides kO3 kOH,P kOH,P kO3 herbicide [L/(mol‚s)]a [L/(mol‚s)]b [109 L/(mol‚s)]a [109 L/(mol‚s)]b AC PP MT BT

2.39 ( 0.31 1.24 ( 0.25 1.11 ( 0.10 5.32 ( 0.25

0.94c 3c

6.3 ( 0.5 4.6 ( 0.3 6.7 ( 0.4 7.4 ( 0.5

7.5d 4.3c 5.1c

a This work. Errors ) 95% confidence intervals. b Published values. c From ref 10. d From ref 12.

starting the reaction after the ozone addition. At regular reaction times, two samples were withdrawn: one was directly introduced into an Indigo solution to determine the ozone concentration; the second was introduced into a vial containing potassium thiosulfate (0.1 M), which eliminated the residual ozone, and was subsequently assayed for the herbicide and pCBA concentrations. 3. Results and Discussion 3.1. Rate Constants for the Reaction of Ozone with the Selected Herbicides. To evaluate the rate constants for the direct reaction between ozone and each herbicide, several experiments were performed by varying the initial ozone and herbicide concentrations (with the herbicide concentration being kept 10 times higher than the ozone concentration). Under the applied conditions (pH 2 and in the presence of tert-butyl alcohol), molecular ozone attack is the only significant pathway for the oxidation of the selected herbicides. The overall reaction of ozone with organic compounds is known to be of second order, and first order with respect to each reactant.9,19 In the present case, because the initial herbicide concentrations were in excess with respect to ozone, the herbicide concentration over the course of each experiment can be considered to be almost constant. The reaction rate can then be reduced to pseudo-first-order kinetics with respect to the herbicide concentration. Table 2 lists the mean values of the second-order rate constants for the reaction of ozone with the four herbicides calculated following the procedures corresponding to these pseudo-first-order kinetics. The selected acetamide group herbicides are nonionizable compounds, which implies that the proposed rate constants should be independent of pH. The low values obtained for the second-order rate constants are due to the poor reactivity of acetamides with ozone and to the absence of activating groups for an electrophilic attack of ozone on the aromatic ring. Because of these low rate constants, direct reactions with ozone will occur only to a lesser degree during ozonation, and the oxidation of these compounds will be caused by OH radicals formed from ozone decomposition. Other values of these rate constants are found in the literature for some of these herbicides. For example, De Laat et al.10 reported 0.94 L/(mol‚s) for the ozone-PP reaction and 3 L/(mol‚s) for the ozone-MT reaction. These are of the same order of magnitude as the values obtained in the present work for these two herbicides. 3.2. Rate Constants for the Reaction of Hydroxyl Radicals with the Selected Herbicides. In the next step, the oxidation of the selected herbicides by means of the very reactive and oxidizing hydroxyl radicals was studied in order to determine the rate constants for this radical reaction at 20 °C. For this purpose, the OH

radicals were generated by the AOP O3/H2O2 at high pH and in the presence of a high concentration of hydrogen peroxide. Under these conditions, the ozone half-life is very short, and reactions with OH radicals by far predominate in the oxidation kinetics. These rate constants can be evaluated using the competitive kinetic model initially proposed by Gurol and Nekouinaini20 and later used with success by several workers21,22 for the determination of rate constants during the oxidation of some organic compounds by such oxidants as ozone, UV radiation, or hydroxyl radicals. In the present case, pCBA was selected as the reference compound because its rate constant for the reaction with hydroxyl radicals, 5 × 109 L/(mol‚s), is very well-known.23 Additionally, its reactivity with ozone is almost negligible, with the rate constant for the direct reaction with ozone being 0.15 L/(mol‚s).9 Applying this kinetic model gives the mean values also listed in Table 2 and which can be proposed as the rate constants for the reaction between each herbicide and the hydroxyl radicals. These results can be compared to the values found in the literature for the same reactions. Thus, Brekken and Brezonik,12 using the photolysis of KNO3 solutions as the source of hydroxyl radicals, report a value of 7.5 × 109 L/(mol‚s) for the rate constant of the reaction of AC with OH radicals, very close to that of 6.3 × 109 L/(mol‚ s) obtained in the present study. Also, De Laat et al.,10 in a study where the OH radicals were generated by the O3/H2O2 combination and atrazine was the reference compound, obtained values of 4.3 × 109 and 5.1 × 109 L/(mol‚s) for PP and MT, respectively, again very similar to those determined in this present work. The results (Table 2) for the reactivities of the different acetamide herbicides with OH radicals are more comparable because they were determined using the same reference compound. In general, the observed reactivities were high [rate constants in the range of (4.6-7.4) × 109 L/(mol‚s)], higher than those reported for other pesticides such as atrazine [3.0 × 109 L/(mol‚ s)]24 and carbofuran [4.0 × 109 L/(mol‚s)].25 This reveals that the selected herbicides are more efficiently oxidized by AOPs than many other major pesticides. Even if conventional ozonation is applied, these herbicides will be partially removed by the OH radicals generated from ozone decomposition. 3.3. Herbicide Degradation in Natural and Mineral Waters. In the following stage, experiments on the oxidation of the selected herbicides in some natural and mineral waters were carried out by the action of ozone alone or combined with hydrogen peroxide. Some operating conditions were maintained constant in this group of experiments: pH 7, T ) 20 °C, and a concentration of 1.9 × 10-6 M for the herbicides and pCBA. The initial concentrations of ozone and hydrogen peroxide were varied according to the values listed in Table 3. 3.3.1. Influence of Operating Variables. The influence of the initial ozone concentration and the additional presence of hydrogen peroxide on the consumption of ozone in a specific water can be observed in Figure 2, which shows the decay of ozone with reaction time in experiments performed with Zujar reservoir water taken as an example. An increase in the initial ozone concentration increased the ozone decomposition reaction rate during the first 10 min, and after that ozone was mostly consumed independently of the initial ozone concentration. Therefore, when typical

Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5765 Table 3. Degradation of Herbicides in Mineral and Natural Waters: Oxidant Doses and Herbicide Removala expt

[O3]0 (10-5 M)

[H2O2]0 (10-5 M)

XPP (%)

XBT (%)

MinO-1b MinO-2b MinOHb ZujO-1c ZujO-2c ZujOHc OreO-1d OreO-2d OreOHd

4.2 8.3 8.3 4.2 8.3 8.3 4.2 8.3 8.3

0 0 4.2 0 0 4.2 0 0 4.2

64 88 87 45 64 76 47 66 75

79 92 92 67 89 88 61 83 86

XAC (%)

XMT (%)

96 60

a Experimental conditions: pH 7, T ) 20 °C, [herbicide] ) 0 [pCBA]0 ) 1.9 µM. b Mineral water. c Zujar reservoir water. d Orellana reservoir water. For water-quality parameters, see Table 1.

Figure 3. Herbicide degradation curves during conventional ozonation of Zujar reservoir water (expt ZujO-1 in Table 4). Experimental conditions: pH 7, T ) 20 °C, [O3]0 ) 4.2 × 10-5 M, [herbicide]0 ) 1.9 µM. Symbols represent experimental data, and solid lines represent model calculations.

Figure 2. Experiments performed with Zujar reservoir water by varying the initial dose of ozone and the presence of H2O2. Experimental conditions: pH 7, T ) 20 °C, [herbicide]0 ) 1.9 µM, [H2O2]0 ) 4.2 × 10-5 M.

ozone dosages used in drinking water treatment (1-2 mg/L) are applied to this natural water, the ozone is completely consumed during the treatment process if the hydraulic retention time of the full-scale reactor is longer than 20 min. The presence of hydrogen peroxide increased the ozone decay rate to a greater extent: it can be seen that, at 5 min of reaction, the ozone was almost totally consumed. Similar trends were observed for the Orellana reservoir water and the mineral water. From another point of view, it is interesting to establish the influence of the different types of water used on the decomposition of ozone for a constant initial ozone dose. The ozone decomposition rate was highest for Orellana reservoir water, intermediate for Zujar reservoir water, and lowest for the mineral water (data not shown). These results can be explained by the amount of dissolved organic matter (DOM) present in these waters, which accelerates ozone decomposition. Thus, in the low DOM content mineral water, ozone decomposition is slower than that in the two natural waters where ozone depletion is faster as a result of the higher DOM content. Moreover, the higher ozone decomposition rate obtained for Orellana reservoir water with respect to Zujar reservoir water is probably due to a more pronounced promoting character of the natural organic matter (NOM) present in the former. Figure 3 depicts, by way of example, the degradation curves of the herbicides with reaction time in the single

ozonation of Zujar reservoir water (expt ZujO-1 in Table 3). The sequence of reactivities is BT > AC > PP, as would be expected, because this is the same trend as the sequence obtained for the rate constants of the direct ozone reaction (kO3 in Table 2) and also for the rate constants of the reaction with hydroxyl radicals in ultrapure water (kP,OH in Table 2). Table 3 also lists the final removals obtained for the four herbicides in this group of experiments. With regard to the influence of the initial ozone dose on the degradation of a specific herbicide, an increase in that dose led to a subsequent increase in the degradation of the herbicide. This effect was even more pronounced when hydrogen peroxide was present, especially in the two natural waters, because of the oxidizing power of the hydroxyl radicals generated by the combination O3/ H2O2 in the reacting medium. Finally, the effect of each type of water on the herbicide transformation is shown in Figure 4, which plots, by way of example, the concentration curves of BT versus reaction time. One observes that the degradation was fastest in the mineral water, intermediate in the Orellana reservoir water, and slowest in the Zujar reservoir water. The final BT degradation level was highest in the mineral water and similar in the two natural waters. Similar results were obtained for the other herbicides. The higher levels of herbicide oxidation obtained in the mineral water reflect the lower concentration of organic and inorganic compounds present that could consume both oxidants, ozone and hydroxyl radicals. On the contrary, in the natural waters, with their higher concentrations of organic compounds and bicarbonate ions, the concentration of OH radicals available to react with the herbicides is lower and, thus, there was a lower level of herbicide degradation. Therefore, in these natural waters, higher doses of oxidants must be used if one requires a greater degree of contaminant destruction. 3.3.2. Kinetic Study. As part of the general study of the chemical decomposition of some of the pollutants present in natural waters to make them fit for drinking, the final objective of a kinetic study will be the predic-

5766 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003

Figure 4. Concentration profiles of BT during conventional ozonation of natural waters. Experimental conditions: pH 7, T ) 20 °C, [O3]0 ) 4.2 × 10-5 M, [herbicide]0 ) 1.9 µM. Symbols represent experimental data, and solid lines represent model calculations.

tion and modeling of the oxidation rate for those contaminants during that chemical oxidation. For this purpose, in addition to the rate constants for the decomposition of the specific pollutants by the oxidants, it is also necessary to have an exact knowledge of the temporal evolution of the concentrations of those oxidants. In the present case, these oxidants are molecular ozone and OH radicals in both the conventional ozonation and the AOP O3/H2O2. With respect to molecular ozone, its evolution is easy to follow by measuring its concentration directly either colorimetrically as in the present work or electrochemically.26 Investigations of its decomposition in natural waters from several European locations have shown that this reaction follows first-order kinetics.13,18,27 Accordingly, the integrated ozone decomposition rate can be represented by the following equation:

ln

[O3] [O3]0

) -kt

(1)

where k is the first-order rate constant for this ozone decomposition. According to eq 1, a plot of the first term versus reaction time should be a straight line whose slope is the rate constant k. Figure 5 shows, as an example, this plot for the experiments carried out with the Orellana reservoir water. One observes that the points lie satisfactorily close to straight lines. Regression analysis gave the values of k listed in Table 4. These values were fairly similar in the two experiments performed with both the mineral and the Zujar reservoir waters by modifying the initial ozone concentration. Thus, mean values of 4.7 × 10-3 and 5.7 × 10-3 s-1 can be proposed for the first-order rate constant for ozone decomposition in the mineral and Zujar reservoir waters, respectively. For the Orellana reservoir water, however, this first-order rate constant was clearly dependent on the initial ozone concentration, which can be explained by a decline in the relative importance of the promoting effect of the organic matter in the presence of the higher initial ozone concentration. Therefore, the influence of the ozone dose on the ozone

Figure 5. Determination of the first-order rate constants for ozone decomposition in experiments conducted with Orellana reservoir water (eq 1). Experimental conditions: pH 7, T ) 20 °C, [pCBA]0 ) [herbicide]0 ) 1.9 µM, [H2O2]0 ) 4.2 × 10-5 M. Table 4. First-Order Rate Constants for Ozone Decomposition and Rct Values for Natural Water Treatmenta expt

water

k (10-3 s-1)

Rct (10-8)

MinO-1 MinO-2 MinOH ZujO-1 ZujO-2 ZujOH OreO-1 OreO-2 OreOH

mineral mineral mineral Zujar Zujar Zujar Orellana Orellana Orellana

4.8 4.7 10.6 6.1 5.3 15.4 9.6 5.7 19.3

2.8 2.6 6.7 1.9 1.6 6.4 3.5 1.8 9.5

a

For experimental conditions, see Table 3.

decomposition rate constant must be established separately for each particular natural water. Finally, in the experiments where H2O2 was present, the rate constants obtained were significantly greater, with values of 10.6 × 10-3, 15.4 × 10-3, and 19.3 × 10-3 s-1 for the mineral, Zujar reservoir, and Orellana reservoir waters, respectively. The determination of the evolution of the OH radical concentration is, however, a more complicated matter because there is no easy method for measuring this concentration in situ. Several indirect methods can be found in the literature that use computer simulations and predict the OH radical concentrations from the ozone decomposition.28,29 However, because of the complexity of the organic and inorganic matrix of natural waters such as those of the present research, even wellestablished models fail to adequately predict the OH radical concentrations. One proposal is the experimental approach of Elovitz and von Gunten13 designed to measure the transient concentrations of both the OH radical and O3 during an ozonation process. The method is based on measurements of the decrease of an ozone-resistant reference compound that reacts rapidly with OH radicals. In the present study, the compound selected was pCBA, which fulfills those requirements: its rate constant with ozone is 0.15 L/(mol‚s)9 and that with OH radicals is 5 × 109 L/(mol‚s).23 To evaluate the importance of ozone and •OH reactions, a new parameter, the Rct value, has been

Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5767

Figure 6. Determination of the Rct values in experiments performed with Zujar reservoir water (eq 3). Experimental conditions: pH 7, T ) 20 °C, [pCBA]0 ) [herbicide]0 ) 1.9 µM, [H2O2]0 ) 4.2 × 10-5 M. •OH

defined as the ratio of the exposures of and O3 (i.e., concentration of oxidant integrated over the reaction time):13

OH radical exposure Rct ) ) O3 exposure

∫[OH] dt ∫[O3] dt

(2)

The Rct value can be calculated from the experimentally measured pCBA and O3 concentrations:

ln

(

[pCBA]t

)

[pCBA]0

∫0t[OH] dt )

) -kOH,pCBA

-kOH,pCBARct

∫0t[O3] dt

(3)

Thus, a plot of the first term vs the exposure of ozone should be a straight line, from whose slope the Rct value can be deduced. If Rct remains constant during the ozonation process, it represents the ratio of the OH radical concentration to the ozone concentration. Under these conditions, the concentration of OH radicals can easily be determined from the experimentally measured ozone concentration or from the ozone concentration calculated using the kinetics equation for ozone decomposition (eq 1). Figure 6 shows this plot for the experiments carried out with Zujar reservoir water by way of example. The points lie satisfactorily close to straight lines, which confirms the goodness of the proposed model. A regression analysis gave the slopes that, divided by kOH,pCBA [5 × 109 L/(mol‚s)] gave the Rct values listed in Table 4. Mean values of 2.7 × 10-8 and 1.7 × 10-8 can be proposed for the mineral and Zujar reservoir waters, respectively. Different Rct values were obtained for the Orellana reservoir water depending on the initial ozone concentration, similar to the case for the first-order rate constant described above. Finally, in the combined O3/ H2O2 process, the following values were deduced for the Rct parameter: 6.7 × 10-8, 6.4 × 10-8, and 9.5 × 10-8 for the mineral, Zujar reservoir, and Orellana reservoir waters, respectively. These higher values confirm a greater production of OH radicals from the decomposition of ozone when an AOP is applied than with conventional ozonation.

Studies on the ozonation of natural waters in Europe27 and in South Korea30 have shown that the ozone decomposition rate consists of two stages: instantaneous ozone consumption and a slower ozone decay stage. However, only a single ozone decomposition stage was observed in the natural waters used in the present work, probably because of the fast ozone depletion obtained in these waters with their high NOM content. Likewise, a single value of Rct can be proposed for the whole ozonation process. 3.3.3. Micropollutant Oxidation Modeling. Once the evolution of the oxidant concentrations over the course of the oxidation process of a particular natural water has been established (the k and Rct values) and the second-order rate constants for the reaction with OH radicals (kOH) and ozone (kO3) are known, the relative decrease of a micropollutant P can be modeled and predicted by second-order kinetics as a function of Rct and the ozone exposure ∫[O3] dt:

ln

( )

[P] ) -kOH [OH] dt + kO3 [O3] dt ) [P]0

∫

∫

-(

∫0t[O3] dt)(kOHRct + kO ) 3

(4)

For that purpose, the ozone exposure at each selected time has to be determined previously from the calculated ozone concentration and eq 1. To test the application of this kinetic approach to the natural waters selected in the present study, the predictions were compared with the experimental results obtained for the oxidation of the selected herbicides. Using the values of kO3 and kOH given in Table 2 and of k and Rct given in Table 4, the calculations were performed with the computer program ACUCHEM, which solves complex systems of multiple chemical reactions.31 Figures 3 and 4 present this comparison for various experimental conditions and the two types of natural water. In these figures, the symbols represent the experimental results, and the dotted lines represent the predicted values calculated by means of computer simulations. The excellent agreement between predictions and experiment confirms the goodness of this kinetic approach to calculating the relative decrease of the studied herbicides present in natural and mineral waters. Therefore, for a given micropollutant, the evolution of its oxidation in a natural water by conventional ozonation or by the AOP O3/H2O2 can be predicted by a combination of kinetic parameters (reactivity with ozone and •OH) and the decomposition of ozone into •OH in the particular water (k and Rct). The application of the present model could be important in predicting the oxidation of micropollutants during drinking water treatment because, for example, EU regulations are becoming ever more restrictive with respect to the presence of micropollutants in drinking water.2 The ACUCHEM program also allows one to calculate the herbicide fraction that is degraded by the reaction with molecular ozone as well as the fraction oxidized by OH radicals. These percentages were determined for each herbicide and are listed in Table 5. The four herbicides were mainly eliminated by OH radicals (9899% in most cases), while the contribution of the reaction with molecular ozone was almost negligible (12%). This is a consequence of the low reactivity of these substances with ozone (kO3 ) 2.4, 1.1, 1.2, and 5.3

5768 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 Table 5. Partial Contributions of the Direct O3 and Radical Pathways to the Overall Oxidation of the Selected Acetamide Herbicidesa expt MinO-1 MinO-2 MinOH ZujO-1 ZujO-2 ZujOH OreO-1 OreO-2 OreOH a

herbicide PP BT PP BT PP BT MT PP BT AC PP BT PP BT PP BT PP BT PP BT

radical pathway (%)

direct O3 pathway (%)

99 98 99 98 99.6 99 99.8 99 96 98 98 96 99.6 99 99 98 98 96 99.7 99

1 2 1 2 0.4 1 0.2 1 4 2 2 4 0.4 1 1 2 1 4 0.3 1

For experimental conditions and nomenclature, see Table 3.

L/(mol‚s) for AC, MT, PP, and BT, respectively) in comparison with the high reactivity with OH radicals (kOH ) 6.3 × 109, 6.7 × 109, 4.6 × 109, and 7.4 × 109 L/(mol‚s) for AC, MT, PP, and BT, respectively). Only BT was even partially eliminated (up to 4%) by the direct pathway with molecular ozone when conventional ozonation was applied, because of its greater reactivity with ozone. The additional presence of H2O2 did not imply any significant enhancement in the herbicide fraction degraded by the radical pathway because the contribution of that pathway predominated even in the conventional ozonation. 4. Conclusions For the direct reaction between molecular ozone and the selected acetamide herbicides, the following secondorder rate constants were obtained: 2.4, 1.1, 1.2, and 5.3 L/(mol‚s) for AC, MT, PP, and BT, respectively. The reactions between hydroxyl radicals and these herbicides had much higher rate constants: 6.3 × 109, 6.7 × 109, 4.6 × 109, and 7.4 × 109 L/(mol‚s) for AC, MT, PP, and BT, respectively. In the oxidation of these herbicides in natural waters by ozone and hydroxyl radicals under drinking water treatment conditions, the influence of the operating variables (initial ozone and hydrogen peroxide concentrations, different types of waters and herbicides, etc.) was established. The ozonation process of a natural water was characterized in terms of the active oxidant concentration through the first-order rate constant for ozone decomposition and the Rct concept (i.e., Rct ) [OH]/ [O3]). The rate constants for the decomposition of ozone were 4.7 × 10-3 and 5.7 × 10-3 s-1 in the Zujar reservoir and mineral waters, respectively, and values of 2.7 × 10-8 and 1.7 × 10-8 s-1 were deduced for the Rct parameter. These values increased when the combination O3/H2O2 was used. However, in the Orellana reservoir water, both parameters were dependent on the initial ozone concentration. The application of a kinetic model that took into account the rate constants for the reactions of ozone and OH radicals with the selected herbicides and the

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Received for review March 10, 2003 Revised manuscript received September 5, 2003 Accepted September 5, 2003 IE030229Y