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Ind. Eng. Chem. Res. 2009, 48, 3380–3388
Kinetics of the Chemical Oxidation of the Pharmaceuticals Primidone, Ketoprofen, and Diatrizoate in Ultrapure and Natural Waters Francisco J. Real,* F. Javier Benitez, Juan L. Acero, Juan J. P. Sagasti, and Francisco Casas Departamento de Ingenierı´a Quı´mica y Quı´mica Fı´sica, UniVersidad de Extremadura, 06071 Badajoz, Spain
Oxidation of three pharmaceuticals (primidone, ketoprofen, and diatrizoate sodium) by means of several advanced oxidation processes including such as ozonation, UV radiation, and Fenton’s reagent has been studied. The influence of operating variables was established, and first-order rate constants were determined in most of the oxidation systems. Specifically, for the reactions with ozone, the following rate constants were obtained by means of a competition kinetic model: 1.0 ( 0.1, 0.40 ( 0.07, and 0.05 ( 0.01 M-1 · s-1, for primidone, ketoprofen, and diatrizoate, respectively. The results showed that reaction with OH radicals was the major pathway for the oxidative transformation of these compounds. In the photodegradation experiments by 254 nm radiation, the quantum yields were also determined for every compound at different pH and temperatures. Additionally, the competition kinetic model, which was also used in Fenton’s reagent experiments, allowed to evaluate the rate constants for the reaction with hydroxyl radicals whose values were the following: (6.7 ( 0.2) × 109 M-1 · s-1 for primidone, (8.4 ( 0.3) × 109 M-1 · s-1 for ketoprofen, and (5.4 ( 0.3) × 108 M-1 · s-1 for diatrizoate. Furthermore, the simultaneous oxidation of these selected pharmaceuticals in some natural water systems (a commercial mineral water, a groundwater, and surface water from a reservoir) was studied. The influence of the operating conditions on the removal efficiency was established. Finally, a kinetic model was proposed for the prediction of the elimination of the selected pharmaceuticals by ozone in these natural waters, with theoretical results that agreed well with the experimental ones. 1. Introduction Pharmaceuticals have recently become of great aquatic environmental concern due to their biological activity.1 Pharmaceuticals used for human medical care are not entirely metabolized in the human body, and they are excreted to water effluents to be treated at wastewater treatment plants. Several studies have revealed that these compounds are not removed quantitatively in conventional water treatment processes,2,3 and they can persist in drinking waters and constitute a potential risk for human health. Because of the low removal levels of current wastewater treatments toward some recalcitrant pharmaceuticals, it is advisable to develop technologies that promote an easier degradation of these pollutants. Among different treatments, ozonation4 and advanced oxidation processes (AOPs) like O3/H2O2, UV/H2O2,5 and Fenton’s reagent6 have demonstrated high effectiveness in the degradation of pharmaceuticals and X-ray contrast media in drinking water and wastewater. Within a wide research program focused on the removal of pharmaceuticals from waters by chemical treatments, the anticonvulsant primidone, the anti-inflammatory ketoprofen, and the X-ray contrast medium sodium diatrizoate were selected as model compounds for the study of their degradation by several oxidizing systems such as ozone, UV radiation, and AOPs (Fenton’s reagent, photo-Fenton system, O3/H2O2 and UV/H2O2). Primidone is an anticonvulsant, which is used to treat disorders of movement such as tremors. This drug is metabolized in the liver to phenobarbital, which is also an anticonvulsant and excreted in the urine.7 Ketoprofen is a nonsteroidal antiinflammatory drug (NSAD) used to treat the pain and inflammation of arthritis. Diatrizoate is an anionic tri-iodinated X-ray * To whom correspondence should be addressed. E-mail: fjreal@ unex.es. Fax number: +34 924289385. Tel. number: +34 924289384.
contrast medium used to enhance the visibility of internal body structures by X-ray imaging technologies. The published information in this research field is not abundant. Thus, Ternes et al.8 reported moderate reactivity of primidone toward ozonation. About 87% of 1 mg/L of primidone spiked in a flocculated surface water sample was converted by ozonation at an applied ozone dose of 3 mg/L. Diatrizoate has been shown particularly resistant to ozone-based treatment. No conversion of 5.7 mg/L diatrizoate in a sewage treatment plant effluent was observed by ozonation at an applied ozone dose of 5 mg/L at pH 7.2.1 Increasing the ozone dose to 15 mg/L, addition of hydrogen peroxide or UV irradiation slightly enhanced the degradation of diatrizoate to 14%, 25%, and 36%, respectively. Huber et al.9 also noted the negligible and very little reactivity of diatrizoate toward molecular ozone and hydroxyl radicals, respectively. No studies were found on the degradation of ketoprofen by ozonation or AOPs. In spite of the existence of the mentioned studies in the literature of the elimination of these compounds by chemical oxidation, neither rate constants for their reactions with ozone and OH radicals nor quantum yields for photodegradation reactions have been reported. Therefore, the first step of the present study is focused on the determination of the rate constants and quantum yields for the individual reactions between each one of these pharmaceuticals with ozone, OH radicals, and UV radiation, separately, in ultrapure water, as well as the establishment of the influence of the main operating conditions. Additionally, a study of the oxidation of these compounds during ozonation and UV radiation processes in several natural water systems was also conducted. 2. Experimental Section 2.1. Experimental Procedures. The ozonation experiments were carried out in heterogeneous conditions with respect to ozone (which was fed in a gas stream). The ozone was produced
10.1021/ie801762p CCC: $40.75 2009 American Chemical Society Published on Web 03/02/2009
Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3381
from a bottled synthetic air stream in a laboratory ozone generator (Sander, mod. 300.5; Ingetecsa, Barcelona, Spain). The ozone-air gas flow in every experiment was set at 40 L · h-1, the inlet ozone partial pressure was adjusted to the desired value which ranged between 0.2 and 0.9 kPa, and the gas stream was introduced through a porous plate into the reactor containing a solution of 350 mL of the corresponding pharmaceutical (50 µM) buffered at the selected pH by adding phosphoric acid/phosphate solution (0.01 M). At regular reaction times, samples were taken from the reactor. The ozonation experiments performed to determine the second-order rate constants were conducted at 20 °C and pH ) 3, as well as in the presence of tert-butylalcohol (0.1 M) as an OH radical scavenger.10 The kinetic rate constants were determined by using the well-known competition kinetic model widely described in the literature.11,12 In this study, linuron and even primidone were used as reference compounds, once their rate constants were previously determined. In contrast, in the ozonation experiments carried out in natural waters, homogeneous conditions were used. They were conducted in a 500 mL flask reactor containing the mixture of the three pharmaceuticals. Each run was initiated by injecting a variable volume of the ozone stock solution (80 µM, after saturating ultrapure water for an hour at 5 °C), necessary to achieve the desired initial O3 dose into the flask, which also contained a volume of the pharmaceutical solution with an initial concentration of 1 µM of every compound. At regular reaction times, two samples were withdrawn: one was directly introduced into an Indigo solution to determine the remaining ozone concentration; the second one was introduced into a vial containing potassium thiosulfate (0.1 M) to quench the residual ozone and was subsequently assayed for the pharmaceutical concentrations. The photodegradation experiments were conducted in the same conditions, using a low pressure vapor mercury lamp (TNN 15/32, nominal electrical power 15 W; Heraeus, Madrid, Spain) which emitted monochromatic radiation at 254 nm and was located inside the reactor at an axial position. In these experiments, a nitrogen gas stream was used in order to homogeneize the solution. For the photodegradation experiments in natural waters, the same procedure was used, introducing into the reactor a solution of the three pharmaceuticals with an initial concentration of 50 µM of every compound. Finally, the experiments of oxidation by Fenton’s reagent were carried out in 250 mL Erlenmeyer flasks, where the temperature was kept constant at 20 °C and the solutions were homogenized by means of a magnetic stirrer. For every experiment conducted, each flask was filled with the aqueous solution containing the selected pharmaceutical and buffered at the selected pH by adding a perchloric acid/perchlorate solution (0.01 M), which has been confirmed as the most appropriate pH regulator in a previous research.13 The required amounts of ferrous ion and hydrogen peroxide were also added to the reactor. The experiments using the photo-Fenton reagent were conducted in the same reactor as the photodegradation ones. Fenton’s reagent experiments were also performed for the determination of the rate constants of the reaction between each compound and hydroxyl radicals. Once more, the competition kinetics was used for this purpose. The reference compound selected in these experiments was p-chlorobenzoic acid (pCBA), whose rate constant with OH radicals is known in advance.14
Table 1. Characterization of the Natural Water Systems Used type of water
pH
absorbance at 254 nm (cm-1)
TOC (mg · L-1)
alkalinity × 103 (M in HCO3-)
MIN GW PA
7 7.5 7.5
0.004 0.064 0.125
1.2 2.6 6.7
5.2 1.15 0.47
2.2. Analytical Methods. The three selected pharmaceuticals, as well as linuron and p-CBA (used as reference compounds), were analyzed by HPLC in a Waters chromatograph equipped with a 2487 dual λ detector and a Waters Nova-Pak C18 column (5 µm 150 × 3.9 mm). The detection was performed at 218 nm for primidone, 259 nm for ketoprofen, 238 nm for diatrizoate, and p-CBA and 248 nm for linuron. The mobile phase was a mixture of methanol and 0.01 M aqueous phosphoric acid solution in different proportions, depending on the mixture analyzed. The elution flow rate was 1 mL min-1, and the injection volume was 50 µL in all samples. The ozone concentration in the inlet gas ozone-oxygen stream was determined iodometrically,15 while the ozone concentration of the stock solutions was determined directly by measuring their UV absorbance at 258 nm (ε ) 3150 L · mol-1 · cm-1). 2.3. Natural Water Systems. Three different waters were used in the present study for the oxidation of the selected pharmaceuticals under realistic water treatment conditions. The first one was a commercial mineral water of the brand “Los Riscos”. The two remaining waters were natural waters collected from locations in the Extremadura community (southwest Spain): a groundwater and a surface water from the public reservoir “Pen˜a del Aguila”. In this paper, they will be called MIN, GW, and PA, respectively. Absorbance at 254 nm, total organic content (TOC), and alkalinity for these waters are listed in Table 1. Both parameters, UV absorbance and TOC, constitute a significant indication of the total dissolved organic matter (DOM) present in these waters. 3. Results and Discussion 3.1. Oxidation of the Pharmaceuticals Individually in Ultrapure Water. The oxidation of the three target pharmaceuticals (primidone (PRM), ketoprofen (KPF), and diatrizoate (DTZ)) was carried out in a first stage in ultrapure water by using different oxidizing systems on each compound individually: ozone, UV radiation, and Fenton’s reagent. For every set of experiments, the influence of the main operating variables was established and the main kinetic parameters were evaluated. 3.1.1. Ozonation of the Pharmaceuticals. The removal by ozone of the pharmaceuticals primidone, ketoprofen, and diatrizoate was studied in experiments carried out at 20 °C in heterogeneous conditions, by varying the pH from 3 to 9 and the ozone partial pressure in the ozone-oxygen mixture fed to the reactor (from 0.24 to 0.90 kPa). An additional experiment for each compound was conducted by adding 0.1 mM of hydrogen peroxide, in order to achieve an increase in the oxidation levels. Table 2 summarizes these experiments, as well as the pseudofirst-order rate constants for the ozonation of each organic compound B, kO3-B′. From their values, it can be established that a positive influence of the pH and the ozone partial pressure on the oxidation of each compound. In the case of the pH influence (experiments O-1-O-4), a great increase of the rate constant for primidone can be observed (from 0.14 min-1 at pH 3 to 0.54 min-1 at pH 9), even greater for ketoprofen (from 0.07 min-1 at pH 3 to 0.69 min-1 at pH 9) and for diatrizoate (from 0.003 min-1 at pH 3 to 0.10 min-1 at pH 9), noticing that this compound is the most recalcitrant to ozonation. In the
3382 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009
to the experimental results obtained in this study. The application of the competition kinetic model leads to the following relationship: ln
Figure 1. Influence of the pH on the ozonation of ketoprofen: experimental conditions T ) 20 °C; pO3 ) 0.24 kPa. Table 2. Ozonation Experiments in Ultrapure Water expt
pO3 (kPa)
pH
O-1 O-2 O-3 O-4 O-5 O-6 OH-1
0.24 0.24 0.24 0.24 0.57 0.90 0.24
3 5 7 9 7 7 7
[H2O2]0 (mM)
kO3-PRM′ (min-1)
kO3-KPF′ (min-1)
kO3-DTZ′ (min-1)
0.1
0.14 ( 0.01 0.44 ( 0.03 0.51 ( 0.05 0.58 ( 0.11 1.30 ( 0.05 2.01 ( 0.30 0.70 ( 0.04
0.07 ( 0.01 0.29 ( 0.01 0.38 ( 0.01 0.69 ( 0.03 1.08 ( 0.05 2.10 ( 0.18 0.60 ( 0.03
0.003 ( 0.001 0.008 ( 0.001 0.049 ( 0.01 0.10 ( 0.01 0.17 ( 0.02 0.27 ( 0.03 0.11 ( 0.01
two first pharmaceuticals, a pronounced skip is produced between pH 3 and 5 (from 0.14 to 0.44 min-1 for primidone and from 0.07 to 0.29 min-1 for ketoprofen), as can be seen in Figure 1, which shows this influence for the pharmaceutical ketoprofen, taken as example. This fact can be explained in the case of the ketoprofen attending to its pKa, whose value is 4.45.16 It indicates that the deprotonated species would be more reactive toward ozone favoring the electrophilic attack and reaching removals higher than 50% at 3 min of treatment for pH g 5. The contribution of OH radicals generated from ozone decomposition at high pH could also explain the increment of reactivity with pH. In the same way, the ozone partial pressure exerts a positive influence on the pharmaceuticals degradation (expts O-3, O-5, and O-6), as can be expected. Thus, the pseudo-first-order rate constants at pH 7 increase their values among 4 and 5.5 times when increasing the ozone partial pressure in the gas phase from 0.24 to 0.90 kPa. For primidone and ketoprofen, removals of 100% were obtained after 5 min of reaction when using a higher pO3. Finally, the improvement due to the addition of a radical promoter such as H2O2 was investigated (comparison of expts O-3 and OH-1). The kO3-B′ values were increased in all cases when 0.1 mM of hydrogen peroxide was added (from 0.51 to 0.70 min-1 for primidone, from 0.38 to 0.60 min-1 for ketoprofen, and from 0.049 to 0.11 min-1 for diatrizoate). In a second step of the kinetic study, the second order rate constants for the direct reaction between each pharmaceutical and ozone, kO3-B, was determined by conducting experiments as described in the Experimental Section. Under operating conditions of pH ) 3 and the presence of tert-butylalcohol, molecular ozone attack is expected to be the only significant pathway for the oxidation of the three compounds. The competition kinetics initially proposed by Gurol and Nekouinaini,17 and later successfully used in other studies,11,12 with the final goal of determining rate constants for the direct oxidation of some organic compounds with ozone, was applied
kO3-B [R]0 [B]0 ) ln [B] kO3-R [R]
(1)
where B and R are the target and the reference compound, respectively, and kO3-R is the known rate constant for the reference compound. From a representation of the logarithmic terms a straight line is obtained from whose slope the rate constant can be determined. The target compounds were the selected pharmaceuticals and the reference compound used for primidone and ketoprofen was linuron. This pollutant was selected because previous experiments revealed that it presents a reactivity toward ozone in the same order of magnitude than those of primidone and ketoprofen. Additionally, its ozonation process was previously studied,18 and the rate constant for its reaction with ozone was reported, its value being 1.9 M-1 · s-1. On the contrary, diatrizoate presents a much lower reactivity. So, in that case, primidone was used as reference compound, once its rate constant was previously evaluated. Two experiments were conducted for every target compound in which very similar slopes were obtained, from whose values the following rate constants for kO3-B were determined: 1.0 ( 0.1 M-1 · s-1 for primidone, 0.40 ( 0.07 M-1 · s-1 for ketoprofen, and 0.05 ( 0.01 M-1 · s-1 for diatrizoate. As can be noticed, these values are really low in all cases, which indicates that the direct ozonation of these pharmaceuticals is negligible, and the degradation of these compounds in the ozonation process take place mainly through the radical pathway. 3.1.2. Oxidation of the Pharmaceuticals by UV Radiation. The photodegradation of the selected pharmaceuticals was conducted by using a monochromatic radiation, varying the pH from 3 to 9 and the temperature between 10 and 40 °C. Table 3 summarizes the operating conditions for these experiments, including those carried out by using the UV/H2O2 system (expts PH-1-PH-4). Figure 2 shows the evolution of the concentration of each compound in the experiments carried out at pH 3 and 9. As can be seen, the three pharmaceuticals show very different reactivity toward the photodegradation, being ketoprofen the most reactive, followed by diatrizoate and finally primidone. Additionally, the effect of the pH on each compound can also be revised from this Figure 2. This effect seems to be less important the more reactive the compound. Thus, there is a significant improvement from pH 3 to 9 for primidone (reaching at 10 min of reaction a removal of 4.8% at pH 3 and 9.4% at pH 9) and very slight for the other compounds (for diatrizoate 91.9% and 96.6% at pH 3 and 9, respectively, and almost the same values for ketoprofen). Due to this fact, the study of the effect of the pH on the pharmaceutical removal has been focused mainly on primidone, as appears in Table 3 (expts P-1-P-4). The kinetic study of the photodegradation process can be carried out in a first approach by considering that the photochemical oxidation reaction also follows first order kinetics, where kUV-B is the rate constant for every compound. These first order rate constants, also reported in Table 3, can be used again to settle down the influence of the operating variables. In the case of the pH, it can be observed that the kUV-PRM values for primidone are increasing with pH from 0.006 to 0.012 min-1, the values being very similar for ketoprofen and diatrizoate. On the other hand, different experiments have been conducted at 10, 20, and 40 °C, in order to evaluate the effect of the
Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3383 Table 3. Photodegradation Experiments in Ultrapure Water expt
T (°C)
pH
P-1 P-2 P-3 P-4 P-5 P-6 P-7 P-8 PH-1 PH-2 PH-3 PH-4
20 20 20 20 10 40 10 40 20 20 20 20
3 5 7 9 3 3 7 7 3 3 7 7
[H2O2]0 (mM)
0.1 0.2 0.1 0.2
kUV-PRM (min-1)
φPRM × 102 (mol/Ein)
kUV-KPF (min-1)
φKPF × 102 (mol/Ein)
kUV-DTZ (min-1)
φDTZ × 102 (mol/Ein)
0.006 0.009 0.011 0.012
4.8 ( 0.2 7.5 ( 0.6 8.2 ( 0.3 9.4 ( 0.3
2.23
36 ( 3
0.34
3.1 ( 0.2
36 ( 2 26 ( 2 55 ( 3
0.36 0.38
3.5 ( 0.3 3.7 ( 0.4
0.006 0.016
4.5 ( 0.3 10.9 ( 0.8
2.63 1.63 4.58
0.11 0.91
2.3 ( 0.3 6.8 ( 0.7
2.27 2.41 0.063 0.126
temperature on the photodegradation of primidone and diatrizoate at pH 7 (expts P-7, P-3, and P-8) and ketoprofen at pH 3 (expts P-5, P-1, and P-6). As can be observed in Table 3, the rate constants were increasing with temperature for the three pharmaceuticals, from 0.006 to 0.016 min-1 for primidone, from 1.63 to 4.58 min-1 for ketoprofen, and from 0.11 to 0.91 min-1 for diatrizoate. These values also reflect the order of reactivity of the three pharmaceuticals previously commented. Finally, in Table 3 the first order rate constants for the experiments conducted by using the UV/H2O2 system are also reported. It can be observed that for ketoprofen and diatrizoate the addition of hydrogen peroxide up to 0.2 mM does not imply an improvement in the overall photodegradation process as the kUV-B values show (expts P-1, PH-1, and PH-2 for ketoprofen and P-3, PH-3, and PH-4 for diatrizoate). This fact can be explained in the case of ketoprofen due to its intrisic high reactivity toward UV radiation alone. In the case of diatrizoate, this influence will be explained later, once its rate constant is determined with OH radicals, the species responsible for the oxidation by the AOP UV/H2O2. On the other hand, primidone has been affected positively by the addition of hydrogen peroxide, increasing its kUV-PRM from 0.011 to 0.126 min-1 (P3, PH-3, and PH-4). Figure 3 shows the decay of this compound in the three experiments, in which the increase in its photodegradation with the addition of hydrogen peroxide can also be observed. At a more advanced level, the main kinetic parameter in the photodegradation reaction is the quantum yield (φ), which is defined as the number of molecules decomposed per photon absorbed.19 The determination of the quantum yields was carried
Figure 2. Oxidation curves of the selected pharmaceuticals with reaction time in the photodegradation experiments. Experimental conditions: T ) 20 °C.
0.32 0.35
out by assuming the model proposed by previous authors in similar studies on the photooxidation of organic substances.20,21 This model provides the following final equation for the disappearance rate of the selected compound: -
φ d[B] ) Wabs dt V
(2)
where Wabs represents the absorbed radiation flow rate. The integration of eq 2 leads to the following: [B] ) [B]0 -
φ V
∫W t
0
abs
dt
(3)
The determination of Wabs is carried out by using the line source spherical emission model, initially proposed by Jacob and Dranoff,22 as a radiation source model. For its application, the knowledge of the molar extinction coefficients ε of each compound at 254 nm is required, which were determined to be 220 L · mol-1 · cm-1 for primidone, 15227 L · mol-1 · cm-1 for ketoprofen, and 31200 L · mol-1 · cm-1 for diatrizoate. Once the Wabs is determined, the quantum yields for every experiment can be evaluated according to eq 3 following the procedure described in a previous paper.23 Following this procedure, the φ values determined for the photodegradation of every pharmaceutical are detailed in Table 3. Then, the following average values are proposed for the quantum yields at 20 °C: for primidone, pH-influenced compound, they are in a range of 0.048-0.094 mol · Einstein-1; 0.36 mol · Einstein-1 for ketoprofen; and 0.035 mol · Einstein-1 for diatrizoate. The fact that the quantum yields for diatrizoate is lower than for primidone is only due to their different molar extinction coefficients and,
Figure 3. Influence of the additional presence of H2O2 on the photooxidation of primidone. Experimental conditions: T ) 20 °C; pH ) 7.
3384 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 Table 4. Fenton’s Reagent System Experiments in Ultrapure Water [Fe2+]0 [H2O2]0 UV XPRM, XKPF, XDTZ, expt pH (mM) (mM) radiation % (5 min) % (5 min) % (5 min) F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8 F-9 F-10 PF-1 PF-2
3 3 3 3 3 3 3 3 2 5 3 3
0.025 0.05 0.1 0.1 0.1 0.1 0.2 0.5 0.1 0.1 0.1 0.1
0.1 0.1 0.1 0.2 0.3 0.5 0.5 0.5 0.1 0.1 0.1 0.5
254 nm 254 nm
29.7 47.6 76.2 81.0 85.8 100
29.5 40.9 64.3 78.8 85.7
73.9 57.7 88.5
59.7 50.9 100
17.2 20.6 28.8 41.5
64.7
especially, the very low value of this parameter for primidone. In fact, if a comparison of the terms ε · φ is made instead of only quantum yields, the same order of reactivity previously stated is obtained. Finally, the temperature also has an effect on the quantum yields as can be noted in Table 3 (expts P-7, P-3, and P-8 for primidone and diatrizoate and expts. P-5, P-1, and P-6 for ketoprofen). 3.1.3. Oxidation of the Pharmaceuticals by Fenton’s Reagent. Individual oxidation experiments of each pharmaceutical using Fenton’s reagent were performed at 20 °C as described in the Experimental Section, by varying the initial concentrations of ferrous ions and hydrogen peroxide and the pH. Table 4 depicts the operating conditions as well as the removal obtained after 5 min of reaction for each compound. A comparison of the removals reached leads to the discovery that primidone and ketoprofen have similar reactivity levels toward Fenton’s reagent but diatrizoate presents a quite small reactivity, which is the reason why the initial concentrations of Fe2+ and H2O2 were increased for its study (see expts F-6-F-8 in Table 4). The effect of the initial Fe2+ concentration was established from experiments with the same initial concentration of H2O2 and increasing initial concentration of Fe2+, that is, expts F-1-F-3 for primidone and ketoprofen and expts F-6-F-8 for diatrizoate. It can be observed in all cases an increase in the removal (from 29.7% to 76.2% for primidone, from 29.5% to 64.3% for ketoprofen, and from 20.6% to 41.5% for diatrizoate) when [Fe2+]0 was increased. In the same way, experiments conducted with increasing initial concentrations of hydrogen peroxide can be compared from Table 4. Thus, for primidone, a removal of 76.2% was obtained when 0.1 mM of H2O2 was used; and it increased along expts F-3-F-6 (from 0.1 to 0.5 mM of H2O2) up to 100%. In case of ketoprofen, expts F-3-F-5 (from 0.1 to 0.3 mM of H2O2) show a skip in the removal from 64.3% to 85.7%, while for diatrizoate a slight improvement from 17.2% to 20.6% was stated when increasing the H2O2 initial concentration from 0.2 to 0.5 mM (expts F-4 and F-6). In all these experiments, a constant initial concentration of ferrous ions of 0.1 mM was used. From the results contained in Table 4, it can be concluded that both species, Fe2+ and H2O2, promote a direct effect on the compound oxidation removal, which is consequent with the most important reaction of the Fenton’s reagent, responsible of the generation of the oxidant species, the OH radical.24 In addition, decomposition experiments of the pharmaceuticals primidone and ketoprofen were performed by varying the pH between 2 and 5, proposed as the optimum range of pH by other authors.25,26 The removals after 5 min of reaction shown in Table 4 (expts F-3, F-9, and F-10), confirm pH 3 as the optimum one for both compounds, the removal obtained being higher than that at pH 2 with these results adding to those at
pH 5. The decrease in degradation at pH 5 can be attributed to a decrease of the free iron species in the solution, due to the precipitation of ferric oxyhydroxides. On the other hand, the decrease in the degradation at pH below 3 is due to the inhibition of the formation Fe(III)-peroxy complexes, which are the precursors of iron(II) regeneration.27 Finally, an additional experiment was carried out with every pharmaceutical by using the photoassisted Fenton reaction in order to state the enhancement reached with this AOP. Once again, the removals depicted in Table 4 permit the visualizatio of this increase in the degradation of the three selected pharmaceuticals. Thus, from expts F-3 and PF-1, an improvement from 76.2% to 88.5% for primidone and a complete degradation of ketoprofen at 5 min of reaction are observed. Similarly, for diatrizoate, expts F-6 and PF-2 show an increase from 20.6% to 64.7%. According to the reaction mechanism, the rate equation for the removal of organic compound by means of hydroxyl radicals can be written in the form: -
d[B] ) kOH-B[•OH][B] dt
(4)
where kOH-B is the second-order rate constant. These rate constants present a very high value in the range of 107-1010 M-1 · s-1.14,28 With these extremely fast reaction rates and the absence of analytical methods to directly measure OH radicals concentrations, it is not possible to determine the unknown rate constants by direct meassurements. Then, these radical rate constants can be again determined by the same competition kinetic model used in advance for the evaluation of the direct ozonation rate constants. The model proposes a relationship similar to eq 1 which leads to the evaluation of the rate constants forthereactionbetweenhydroxylradicalsandeverypharmaceutical: ln
kOH-B [R]0 [B]0 ) ln [B] kOH-R [R]
(5)
According to the competition kinetic model, two experiments at 20 °C of the simultaneous oxidation of each pharmaceutical and p-CBA by Fenton’s reagent as an oxidant were performed. In this study, p-chlorobenzoic acid (p-CBA) was used as reference compound for the evaluation of the rate constants, whose kOH-R value is 5 × 109 M-1 · s-1.14 With the experimental results obtained and according to eq 5, plots of the logarithmic terms led to straight lines with similar slope kOH-B/kOH-R for each pharmaceutical, from which the rate constants for the reaction with hydroxyl radicals can be evaluated, their values being (6.7 ( 0.2) × 109 M-1 · s-1 for primidone, (8.4 ( 0.3) × 109 M-1 · s-1 for ketoprofen, and (5.4 ( 0.3) × 108 M-1 · s-1 for diatrizoate. As can be seen, these rate constants are very high for primidone and ketoprofen, indicating that any AOP in which OH radicals are involved can be a good process for their elimination but is much lower for diatrizoate. That is the reason why, in the previously commented UV/H2O2 system, the addition of H2O2 does not imply an enhancement of the degradation, as well as the poor results obtained in AOPs such as the O3/H2O2 combination or ozone at higher pH. 3.2. Oxidation of the Pharmaceuticals in Natural Water Systems. In order to study the overall oxidation processes in real aqueous systems, several experiments on the simultaneous oxidation of the selected pharmaceuticals in some natural waters were carried out. The oxidation systems used were UV radiation and ozone, individually and combined with hydrogen peroxide. Tables 5 and 6 show the conditions applied for every experi-
Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3385 Table 5. Photodegradation Experiments in Natural Waters expt
[H2O2]0 (mM)
MINP-1 MINP-2 MINP-3 GWP-1 GWP-2 GWP-3 PAP-1 PAP-2 PAP-3
XPRM, % (3 min)
XKPF, % (3 min)
XDTZ, % (3 min)
6.0 12.6 15.9 3.6 6.4 9.3 3.7 8.8 14.9
99.1 98.5 98.3 97.9 98.0 98.0 98.3 99.0 97.2
23.5 24.9 23.8 21.6 22.9 25.4 23.5 23.8 24.8
1 5 1 5 1 5
Table 6. Ozonation Experiments in Natural Waters expt MINO-1 MINO-2 MINO-3 GWO-1 GWO-2 GWO-3 PAO-1 PAO-2 PAO-3
[O3]0 [H2O2]0 XPRM, XKPF, XDTZ, Xp-CBA, (µM) (mM) % (3 min) % (3 min) % (3 min) % (3 min) 40 70 40 40 70 40 40 70 40
5
5
5
88.0 93.6 58.7 83.4 99.2 10.2 35.1 88.5 36.8
93.2 94.0 62.8 85.6 100 9.8 49.7 92.9 40.7
26.6 34.0 12.9 1.3 20.6 1.1 0.1 21.4 8.9
84.1 84.7 52.5 70.3 88.1 9.1 32.6 83.8 38.5
ment. The waters used were described in the Experimental Section: a commercial mineral water (MIN), a groundwater (GW), and a surface water from a public reservoir (PA). The experimental procedures for the photodegradation were the same as the individual UV radiation in ultrapure water. In contrast, the simultaneous ozonation of the compounds in natural waters was conducted in homogeneous conditions as described in section 2.1. For the ozonation experiments, an additional compound (p-CBA) was used as reference compound, with the same initial concentrations as the pharmaceuticals. Several conditions were maintained constant in all these experiments: the temperature was kept at 20 °C, and the initial concentrations of the pharmaceuticals were 50 µM for photodegradation experiments and 1 µM for ozonation experiments in homogeneous conditions, in this case even for p-CBA. The pH of the natural waters was not modified, being in the range 7-7.5 for all the types of waters. 3.2.1. Effect of Operating Variables, Water Types, and Nature of Pharmaceuticals. Tables 5 and 6 also report the removals obtained for every compound at 3 min of reaction taken as an example. As can be seen in Table 5, the simultaneous photodegradation of the three pharmaceuticals leads to the order of reactivity already stated in section 3.1.2, that is, ketoprofen . diatrizoate > primidone. From the evolution with time of the removal data for every experiment, it can also be determined the reaction time necessary for the 50% removal, t1/2, revealing average values for the experiments MINP-1, GWP-1, and PAP-1 of 0.9 min for ketoprofen, 6.5 min for diatrizoate, and 92 min for primidone. On the other hand, the influence of the addition of hydrogen peroxide is newly confirmed, showing a positive effect for the primidone removal and negligible in the photodegradation of the other compounds. Finally, and according to Table 5, the influence of the type of water is not significant on the pharmaceutical removal by photodegradation. Only for the least reactive primidone, and regarding the experiments with UV radiation alone (MINP-1, GWP-1, and PAP-1), can a slightly higher removal be observed in mineral water which can be explained from its lower 254 nm absorbance, as exposed in Table 1 (water absorbance values: 0.004, 0.064, and 0.125 cm-1 for MIN, GW, and PA, respectively), indicating that the organic matter in that type of water could absorb less UV radiation.
Figure 4. Influence of the type of water on ozone decomposition in experiments with an initial ozone dose of 70 µM. Experimental conditions: T ) 20 °C; pH ) 7-7.5; pharmaceuticals and p-CBA initial concentration ) 1 µM.
On the other hand, Table 6 shows the removals obtained for each compound in the ozonation experiments. Once again, the relative reactivity of every pharmaceutical toward ozonation is reproduced even in homogeneous conditions; that is, a slightly higher reactivity of primidone over ketoprofen, diatrizoate being the least reactive compound. The increase in the ozone dose (expts 1 and 2 for every type of water) leads to a logical increase in the removals for every compound, this fact being more evident for the least reactive to ozonation diatrizoate. Nevertheless, the addition of hydrogen peroxide to the system yields to a decrease of the removal of every compound. This fact can be explained from the higher decomposition undergone by the ozone dissolved in the mass reaction in presence of H2O2. That decomposition usually would cause a higher hydroxyl radical generation that could favor the degradation of organic compounds as occurred in heterogeneous conditions, but in homogeneous conditions, the ozone is not fed into the reactor once the reaction started and the decomposition of the initial ozone in radical ways does not lead to an effective removal of the pharmaceuticals. Finally the influence of the type of water on the removal of the three compounds was also established from Table 6 data. It can be observed that the degradation was faster in the mineral water, intermediate in the groundwater (especially for diatrizoate and also for the reference compound p-CBA), and slower in the reservoir water in all cases. The differences of reactivity in each type of water are a consequence of the different concentration of organic matter present which consumes part of the oxidant. According to the UV absorbance values and TOC given in Table 1, this concentration is lower for mineral water and higher for reservoir water. Therefore, in water systems with more amount of dissolved organic matter (DOM), the amount of oxidant available to react with the organic compounds is lower, requiring higher doses of oxidant to reach the desired degree of pollutant elimination. In order to settle down this fact, Figure 4 shows the evolution of ozone in three experiments carried out in different types of water and by using similar ozone doses (expts MINO-2, GWO-2, and PAO-2). As can be seen, there is a faster ozone decomposition rate in the groundwater in relation to the mineral water, and this is much faster in the reservoir water. This effect confirms the influence of the DOM present in each type of water on the ozone consumption. 3.2.2. Modeling the Ozonation of Pharmaceuticals. A kinetic model has been proposed in order to predict the oxidation of the selected pharmaceuticals in natural waters from the ozone decay, OH radical concentration, and the presviously calculated
3386 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 Table 7. Ozonation Experiments in Natural Watersa [O3]0 (µM) Rct1 × 108 Rct2 × 108 pharmac fOH1 (%) fOH2 (%)
expt MINO-1
40
20.8
1.5
MINO-2
70
10.6
0.9
GWO-1
40
16.0
0.06
GWO-2
70
14.7
2.1
PAO-1
40
0.9
0.05
PAO-2
70
14.6
0.03
PRM KPF DTZ PRM KPF DTZ PRM KPF DTZ PRM KPF DTZ PRM KPF DTZ PRM KPF DTZ
99.9 100 100 99.9 100 99.9 99.9 100 99.9 99.9 100 99.9 99.8 99.9 99.9 99.9 100 99.9
99.0 99.7 99.4 98.4 99.5 99.1 79.2 92.5 87.0 99.3 99.8 99.6 74.8 90.6 83.9 68.8 87.7 79.5
a Rct values and fraction of the pharmaceuticals degraded by OH radicals.
kO3 and kOH rate constants. Since the concentration of hydroxyl radicals cannot be directly measured, Elovitz and von Gunten29 proposed an experimental approach in order to measure the transient OH radical and O3 concentrations during ozonation processes. For this purpose, they introduced the Rct parameter, defined as the ratio between the OH radicals and O3 exposures. This method consists of the measurement of the concentration decay of a reference compound which reacts rapidly with OH radicals and has a very low reactivity toward ozone. As the reaction between the OH radicals and this reference compound follows second-order kinetics, the integration of its rate equation and the introduction of the Rct parameter definition leads to the following expression for the probe compound concentration profile:
( )
ln
[R]t ) -kOH-R [R]0
∫ [OH] dt ) -k t
0
OH-RRct
∫ [O ] dt (6) t
0
3
where [R]0 and [R]t represent the reference compound concentrations at the initial and at any reaction times, respectively, and kOH-R is the rate constant for the direct reaction between the reference compound and OH radicals. As was previously mentioned, the reference compound selected was again p-CBA, whose rate constants with ozone and OH radicals are 0.15 and 5 × 109 M-1 · s-1, respectively.11,14 According to eq 6, the Rct parameter can be experimentally calculated from the decreases in the concentrations of p-CBA and ozone. Once the Rct value and the ozone exposure were experimentally obtained, the evaluation of the OH radicals exposure at any reaction time, and consequently, the knowledge of the OH radicals concentration profile in a ozonation process were deduced. Following this procedure, the Rct values listed in Table 7 were determined. Two Rct values were deduced for each experiment, corresponding to two periods of ozone decay: instantaneous ozone consumption and a slower ozone decay stage.29 The higher Rct values in the first period indicate a higher amount of OH radicals formed from ozone decomposition than in the second period. It was not possible to obtain the Rct values for the experiments with the combination O3/H2O2 due to the extremely fast consumption of ozone. These Rct values determined are useful for predicting and modeling the oxidation of the pharmaceuticals in natural waters by ozone and OH radicals as oxidant species. As Elovitz and von Gunten pointed out,29 and according to the previous
Figure 5. Comparison of the predicted (lines) and experimental (symbols) concentrations of the pharmaceuticals in the ozonation experiment performed in groundwater with an initial ozone dose of 70 µM. Experimental conditions: T ) 20 °C; pH ) 7.5; pharmaceutical initial concentration ) 1 µM.
considerations, the reaction rate for any micropollutant present in any type of water can be written in the form:
( )
ln
[B] ) - kOH-B [B]0
(
∫ [OH] dt + k ∫ [O ] dt) ) -(k R + k )( ∫ [O ] dt) (7) O3-B
3
t
OH-B ct
O3-B
0
3
where kOH-B and kO3-B are the rate constants for the reactions of the pharmaceuticals with OH radicals and ozone, respectively. Equation 7 was applied for the determination of the theoretical concentrations of each compound in the experiments performed, these values being compared with the experimental results. Figure 5 presents this comparison, for each pharmaceutical in the experiment GWO-2 taken as example, where the symbols represent the experimental results and the lines represent the calculated values by means of eq 7. It must be noted that the two Rct values are used in the theoretical prediction (Rct1 for the first minutes and Rct2 for longer times), according to the previously stated with the reference compound. The fair agreement between predictions and experiments confirms the goodness of this kinetic approach. As similar plots are obtained for the other experiments, it can be concluded that the application of the present model can be of great interest in predicting the oxidation of these compounds during drinking water treatments. Finally, the Rct parameter is also useful for determining the relative importance of OH radicals and O3 reaction pathways in the oxidation of a pollutant present in water systems. Thus, the fraction of B degraded by OH radicals can be expressed in the form: fOH )
kOH-B[•OH][B] •
kOH-B[ OH][B] + kO3-B[O3][B]
)
kOH-BRct kOH-BRct + kO3-B (8)
Equation 8 was applied to the results obtained in the experiments carried out with the pharmaceuticals of the present study in the waters tested, the percentages obtained for each compound being compiled in Table 7. From these values it is clearly deduced that the radical pathway dominates over the direct ozonation pathway, more especially in the first period, where the global degradation due to the radical pathway was
Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3387
higher than 99.7%, and being totally negligible the direct ozone pathway. Moreover, it is in this first stage where the main degradation of every compound takes place. In the second period, the radical pathway also dominates, especially in the mineral water, with percentages around 99% being the lowest values for the reservoir water. Regarding the fOH values for each compound in this second period, primidone presents moderate ozone pathway proportion, reaching values between 20% and 30% of the total degradation in several experiments (except in the mineral water). However, in the main oxidation stage (first period), all the pharmaceuticals are degraded almost exclusively by means of the radical pathway with no evident influence of the type of water. 4. Conclusions In the ozonation of the selected pharmaceuticals in ultrapure water, the pH and the ozone partial pressure present a positive influence on the degradation rate of the three compounds. The second-order rate constants of the reaction between each pharmaceutical and ozone were evaluated by competition kinetics. The following values were obtained: 1.0 ( 0.1 M-1 · s-1 for primidone, 0.40 ( 0.07 M-1 · s-1 for ketoprofen, and 0.05 ( 0.01 M-1 · s-1 for diatrizoate. Therefore, the reactivity of molecular ozone toward these pharmaceuticals is very low, the reactions with OH radicals being the major pathway in ozonation processes. In the oxidation of the compounds by a monochromatic UV radiation in ultrapure water, the influence of pH, temperature, and presence of hydrogen peroxide has been established. Firstorder rate constants and quantum yields were also determined at different pH and temperature values. Primidone was found to be the most recalcitrant to photodegradation, while ketoprofen was extremely reactive. Finally, oxidation experiments with Fenton’s reagent showed the positive influence of the initial concentrations of ferrous ions and hydrogen peroxide on the pharmaceutical removal, as well as with the presence of UV radiation. From experiments using a competition kinetic model, the rate constants between each pharmaceutical and OH radicals were evaluated. The rate constants obtained were the following: (6.7 ( 0.2) × 109 M-1 · s-1 for primidone, (8.4 ( 0.3) × 109 M-1 · s-1 for ketoprofen, and (5.4 ( 0.3) × 108 M-1 · s-1 for diatrizoate. Diatrizoate was very much less reactive toward oxidation processes involving radical pathways. Simultaneous oxidation experiments of these pharmaceuticals by UV radiation and by ozonation were performed in three natural water systems: a commercial mineral water, a groundwater, and a surface water from a reservoir. The influence of the operating conditions (presence of hydrogen peroxide, nature of pharmaceuticals, type of water, and initial ozone dose) on the pharmaceutical removal efficiency was established. For the ozonation of these compounds, the application of a kinetic model which considered the evaluated rate constants and the Rct parameter provided predictions for the pharmaceuticals decay that agreed well with the experimental values. Finally, the partial contributions of radical pathways in the overall ozonation process were calculated in all the experiments in the natural water systems, finding a almost exclusive contribution of the radical pathway for every pharmaceutical, as could be expected from the rate constants determined. Acknowledgment The authors wish to gratefully acknowledge financial support from the Ministerio de Educacion y Ciencia of Spain through
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ReceiVed for reView November 18, 2008 ReVised manuscript receiVed January 29, 2009 Accepted January 30, 2009 IE801762P
