Kinetic Studies on Black Light Photocatalytic Ozonation of Diclofenac

Article pubs.acs.org/IECR

Kinetic Studies on Black Light Photocatalytic Ozonation of Diclofenac and Sulfamethoxazole in Water Fernando J. Beltrán,* Almudena Aguinaco, Ana Rey, and Juan F. García-Araya Departamento de Ingeniería Química y Química Física, Universidad de Extremadura, 06006 Badajoz, Spain S Supporting Information *

ABSTRACT: Black light photocatalytic ozonation of two pharmaceutical compounds, sulfamethoxazole (SMX) and diclofenac (DCF), and the resulting total organic carbon (TOC) are studied. DCF and SMX removals from some mg·L−1 to 100 μg·L−1 are achieved in approximately 7 and 15 min ozonation, respectively, regardless of the ozone process, while the resulting TOC is eliminated via hydroxyl radical reactions. For initial concentrations lower than 50 μg·L−1, competition between direct ozonation and hydroxyl radical oxidation to eliminate SMX and DCF takes place. The initial reaction period for cases of high and low concentration is simulated through fast-moderate and slow gas−liquid second order reaction kinetics, respectively. For high starting concentrations the calculated results suggest the presence of ozone fast reacting intermediates. For low starting concentration, calculated results indicate the importance of hydroxyl radical oxidation and the synergic effect of ozone and photocatalytic oxidation. Kinetic data of TOC ozonation, photocatalytic oxidation, and photocatalytic ozonation are also presented.

1. INTRODUCTION Due to their extensive use for human health care, many pharmaceutical compounds are detected in different environmental waters.1,2 These emergent contaminants constitute a potential threat since many of them have been catalogued as carcinogenic, mutagenic, etc.3,4 Conventional unit operations of wastewater treatment plants (WWTP) can only partially remove these compounds, and in many cases, their presence in the WWTP effluents is very noticeable.5,6 The presence of these pharmaceuticals compounds is also a menace due to their potential endocrine disrupting character.7 Tertiary treatment technologies and, particularly, advanced oxidation processes (AOPs) are recommended to remove these contaminants.8,9 Among AOPs, combinations of ozone and hydrogen peroxide or different radiation sources in the presence of catalysts or photocatalytic ozonation processes (POPs) are presently of high research interest because these processes increase the hydroxyl radical formation and, hence, the rate of mineralization.10,11 In fact, POPs are characterized by the double mechanism of oxidation: the ozone direct attack to remove many initial specific functional group molecules and subsequent hydroxyl radical reactions to remove the remaining organic carbon, that is, to mineralize the water.12 In an attempt to search more environmentally friendly technologies, UVC/UVB sources have been replaced to use near UVA visible radiation, such as black light lamps or even solar radiation.13,14 According to this, a possible POP is the simultaneous application of black light with ozone in the presence of a semiconductor catalyst, such as TiO2. In this process, the radiation applied, closer to the visible spectrum, makes the oxidation a more environmentally benign and less expensive process, balancing the lack of ozone photolysis to yield hydroxyl radicals. In this work, 365 nm black light POP is applied to remove two already well-known pharmaceutical contaminants of wastewater: diclofenac (DCF) and sulfamethoxazole (SMX). © 2012 American Chemical Society

These two compounds, SMX, a sulphonamide synthetic type antibiotic, and DCF, a nonsteroidal anti-inflammatory drug, are frequently found in wastewater and even in surface water,15,16 and their elimination has been the focus of many works where different AOPs were used.17−20 However, as far as our knowledge is concerned, no work on their black light POP has already been reported. Due to its high efficiency in photocatalytic processes, Degusa P25 TiO2 has been chosen in this work as the catalyst. According to the TiO2 band gap energy (3.2 eV), wavelength radiations lower than 387 nm can be used to promote the transfer of electrons between the conduction and valence bands. Then, black light (mainly emitting at 365 nm) can potentially be used as the radiation source. Pharmaceutical compounds are usually present in wastewater with concentrations as high as some 10s or 100s μg·L−1. However, in this work, in most of cases, concentrations of some mg·L−1 have been applied in an attempt to compare different oxidation processes with black light photocatalytic ozonation, check any synergic mechanism of oxidation, study the kinetics and establish the importance of direct and free radical ways of reactions. Finally, with the information obtained, a few experiments at the 10ths of μg·L−1 level have also been conducted for kinetic simulation purposes.

2. MATERIALS AND METHODS DCF and SMX were obtained from Sigma-Aldrich and used as received. Ozone was generated from pure oxygen in a Sander laboratory ozonator able to produce 6 g of ozone per hour. Pure water was obtained from a Milli-Q Millipore system. Commercial TiO2 Degussa P25 (70% anatase and 30% rutile) Received: Revised: Accepted: Published: 4533

November 4, 2011 March 1, 2012 March 5, 2012 March 5, 2012 dx.doi.org/10.1021/ie202525f | Ind. Eng. Chem. Res. 2012, 51, 4533−4544

Industrial & Engineering Chemistry Research

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Concentrations of SMX, DCF, and PCBA (the latter only presented in experiments where the initial concentration of DCF and SMX was 50 μg·L−1) were determined by high pressure liquid chromatography (HPLC) (Elite La Chrom) with a Sinergi 4 μm Hydro-RP 80 A column. A 30:70 v/v methanol−water mixture was used as the mobile phase at a constant flow rate of 0.6 mL·min−1. In addition, the mobile phase was acidified at pH 2.5 with phosphoric acid (0.1% concentration). Detection was made with a L-2455 Hitachi Diode Array detector at 265, 277, and 238 nm for SMX, DCF, and PCBA, respectively. From standard solutions, analysis was repeated to establish the precision of the method, which was determined to be ±2% while accuracy was 1.3%. Detection limits for SMX and DCF were 100 μg·L−1 and for PCBA 0.5 μg·L−1. Hydrogen peroxide concentration was determined through the cobalt/bicarbonate method26 (accuracy was 3%). Total organic and inorganic carbon was monitored by a TOCVSCH Shimadzu carbon analyzer (detection limit, precision, and accuracy were 50 μg·L−1, 3%, and 1.5%, according to Shimadzu manufacturer, respectively). However, experimental precision was higher than 10% for TOC values lower than 0.5 mg·L−1). Aromaticity was followed by measuring the absorbance of the sample (50% diluted) at 254 nm (precision of the method was ±1.5%). Dissolved ozone concentration was measured by following the method proposed by Bader and Hoigné based on the decoloration of a 5,5,7 indigotrisulphonate solution. This method has 2% precision, according to the authors.27 Ozone in the gas phase was monitored by means of an Anseros Ozomat ozone analyzer, based on the absorbance at 254 nm.

was used as catalyst with an average particle size of 30 nm (the catalyst forms 300−400 nm aggregates in aqueous suspension) and BET surface area of 50 m2·g−1, according to the manufacturer. Benzoquinone, t-butanol, and potassium iodide, used as inhibitors of free radical reactions, were obtained from Panreac (Spain). PCBA or p-chlorobenzoic acid, used for calculating the hydroxyl radical concentration in experiments of DCF and SMX at μg·L−1 level, was obtained from Merck. Ozonation, photocatalytic oxidation, and catalytic and photocatalytic ozonation experiments were carried out in 1 L cylindrical reactor equipped with mechanical agitation and inlets for measuring temperature, feeding the gas (oxygen or ozone− oxygen) through a porous plate situated at the reactor bottom, sampling, and one outlet for the nonabsorbing gas. The reactor was situated in the center of a wooden box (45 cm × 48.5 cm each wall) where four 15 W black light lamps were placed in each of the corners inside the box. Also, the internal walls of the box were covered by aluminum foil in order to increase the flux of absorbed radiation due to reflection phenomena. Ferrioxalate actinometry21 was used to determine the incident photon flux, I0, in the photoreactor, which was found to be 5.08 × 10−5 Einstein·min−1. In these experiments the Fe(II) concentration was determined by the o-fenantroline method,22 using a Helios-α spectrophotometer at 510 nm (accuracy was 2.5%, precision 2%). The photon flux, Ia, absorbed by the catalyst was estimated through the determination of the quantum yield of hydroxyl radical generation (ϕOH) in the same photoreactor. This was obtained by applying the protocol of Serpone and Salinaro,23 using methanol as target compound, at the following operating conditions: methanol concentration from 0.5 to 2 M, TiO2 concentration from 0.01 to 3 g·L−1, pH0 = 7 (adjusted with NaOH), and oxygen saturated solution with 30 L·h−1 gas flow rate. The evolution of the reaction was followed through the determination of formaldehyde formed upon the reaction of methanol with hydroxyl radicals. For 0.5 g·L−1 of TiO2, Ia was found to be 4.79 × 10−5 Einstein·min−1. Formaldehyde was determined by the Nash method,24 based on the Hantzsch reaction. In this assay, 2 mL of reagent (0.2 mL of acetylacetone, 3 mL of acetic acid, and 25 g of ammonium acetate in 100 mL of water) are mixed with 5 mL of the sample and heated for 30 min at 50 °C in the dark.25 Spectrophotometric measurements were carried out at 412 nm (ε = 7890 M−1·cm−1) using a Helios-α Thermo Spectronic spectrophotometer (accuracy was 5%, precision was 1.5%). An aqueous solution containing the pharmaceutical compound and TiO2 (in catalytic experiments) at known concentrations was charged at the reactor. NaOH was used to obtain pH 7 in the starting aqueous solution. After this, in photocatalytic experiments, black light was turned on to start the run. In ozonation experiments, an ozone−oxygen gas mixture was also fed to the reactor. In some experiments carried out with DCF and SMX at μg·L−1 concentrations, pchlorobenzoic acid (PCBA) was also added to get information on hydroxyl radical concentration. In all runs, at regular intervals, samples were withdrawn from the reactor, centrifuged (5415D Eppendorf Centrifuge) and filtered through PET Chromafil filters (Chromafil, 0.20 μm) to retain TiO2 particles and concentrations of pharmaceutical compounds; dissolved ozone, hydrogen peroxide, aromaticity, and TOC were determined. Also, ozone concentration in the gas leaving the reactor was analyzed.

3. RESULTS AND DISCUSSION 3.1. Comparison of Processes Applied. A series of experiments of direct photolysis (UVA), adsorption (TiO2), ozonation (O3), photocatalytic oxidation (UVA/TiO2/O2), and catalytic and photocatalytic ozonation (O3/TiO2 and O3/ UVA/TiO2) were first carried out to compare the oxidation rates, pharmaceutical concentrations, and TOC reductions achieved from the different processes studied. Figures 1 and 2

Figure 1. Time evolution of the remaining dimensionless concentration of SMX during the black light radiation and oxidation systems applied. Conditions: 20 °C, initial pH 7, gas flow rate 30L·h−1, inlet ozone gas concentration 10 mg·L−1, initial SMX concentration 10−4 M (average value), TiO2 concentration 0.5 g·L−1. Systems: ⧫, UVA radiation; ▲, O3; □, O3/TiO2; Δ, O3/UVA; ◊, O2/TiO2/UVA; ■, O3/TiO2/UVA.

show some of the results obtained for the case of SMX. Regardless of the pharmaceutical compound, the effects of 4534

dx.doi.org/10.1021/ie202525f | Ind. Eng. Chem. Res. 2012, 51, 4533−4544


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humic substances also have this role.32,33 Since SMX has amine groups in the molecule, it should be expected this compound be also initiator of hydroxyl radicals. In the case of the O3/UVA system, removal of DCF or SMX should exclusively be due to their direct reaction with ozone and hydroxyl radicals produced in these reactions, since ozone does not absorb 365 nm radiation and direct photolysis is negligible. Then, experimental results clearly indicate that for DCF and SMX removal there is no need to apply AOPs, since ozone alone allows high oxidation rates, which explains the small differences observed between ozone involving processes. Regarding TOC abatement, Figure 2 presents the effects of the different oxidation processes applied to remove TOC during SMX oxidation runs (for results with DCF oxidation see Figure 2S of the Supporting Information). As can be seen from Figure 2 and as a difference to what was observed for the case of parent compounds, DCF and SMX, TOC oxidation rates are dependent on the type of ozone process applied. In any case, however, photocatalytic ozonation is the best process to mineralize the water in the lowest reaction time, since approximately 90 and 70% TOC abatement is achieved after 75 and 120 min for DCF and SMX oxidation processes, respectively. The oxidation rate is particularly important in the case of DCF, if compared to those observed with the other AOPs applied. For the other AOPs, the order of effectiveness to remove TOC is UVA/TiO2/O2 < O3 < O3/UVA ≈ O3/TiO2 for the case of DCF and UVA/TiO2/O2 ≈ O3 < O3/UVA ≈ O3/TiO2 for the case of SMX. TOC results obtained with the O3/UVA process are particularly surprising because ozone does not absorb 365 nm radiation, so results of this process should be close to the O3 process alone. Nonetheless, one possible explanation might be due to some colored intermediates formed during ozonation. These compounds were not identified but changes in color were visually observed in samples from the reaction mixture. It is well-known that ozonation of aromatic ring containing compounds gives colored quinone intermediates that absorb UVA and visible radiation.32 Direct photolysis of these intermediates could contribute to the decrease observed in TOC during O3/UVA experiments. In any case, further research is still needed to clarify this point. Regarding black light photocatalytic oxidation, as can be seen from Figure 2, after 2 h reaction, only 10% TOC removal from SMX oxidation is reached (30% in the case of DCF), which is a poor result compared to 93 and 85% TOC eliminated during the same processes (photocatalytic oxidation) but with 313 nm UVA radiation.29 The TOC removal rate after photocatalytic ozonation with 313 nm radiation is higher than that of 313 nm photocatalytic oxidation, so feeding ozone does not contribute to significant improvement of TOC removal. However, the use of ozone in black light photocatalytic oxidation is highly recommended, as can be deduced from Figure 2 (see also Figure 2S, Supporting Information). Since reactions were not buffered, pH was observed to decrease from 7 to approximately 3.5 at the end of experiments. This means that carboxylic acids, that is, more oxygenated organics, were formed, which are more refractory to free radical oxidation processes, thus explaining why the process eventually terminates. TOC results from Figure 2 (and Figure 2S, Supporting Information) confirm some sort of synergism between ozonation and photocatalytic oxidation. For example, after 120 min, a 20% total TOC reduction is obtained from the sum

Figure 2. Time evolution of TOC during the black light radiation and oxidation systems applied to SMX mineralization. Conditions are as in Figure 1. Systems: ⧫, UVA radiation; ▲, O3; □, O3/TiO2; Δ, O3/UVA; ◊, O2/TiO2/UVA; ■, O3/TiO2/UVA.

TiO2 adsorption (not shown) and direct photolysis were negligible, the latter as a logical consequence of the lack of DCF and the SMX absorbing capacity for wavelength radiations higher than 315 nm. As observed from Figure 1, removal rates due to ozone processes are so high that no differences are appreciated. Similar results were obtained in the case of DCF although removal rates in this case were faster than those of SMX. Thus, approximately 7 and 15 min are needed to reduce concentrations of DCF and SMX, respectively, to their detection limit (100 μg·L−1, 99.7% conversion), regardless of the ozone process applied (see corresponding results for DCF AOPs in Figures 1S and 2S of Supporting Information). These high oxidation rates are undoubtedly due to the very high reactivity of these pharmaceutical compounds with ozone, as reported previously. Rate constants of the direct ozone-DCF and ozone-SMX reactions, at pH 7, are 106,28 and 4.15 × 105 M−1s−1,29 respectively, accounting for the reactivity. Also, from Figure 1, it is seen that photocatalytic oxidation (UVA/TiO2/O2) allows some SMX removal, but oxidation rates are much lower than in ozone AOPs studied (see also Figure 1S, Supporting Information). In black light photocatalytic oxidation, 2 h are needed to reach about 70 and 45% DCF and SMX removal, respectively, at the conditions investigated. In previous works30,12 with 313 nm radiation, low differences were observed to remove SMX and DCF between photocatalytic oxidation and photocatalytic ozonation. However, as shown above, the use of black light in photocatalytic ozonation leads to important differences when compared to the same process without ozone. In photocatalytic oxidation, both hydroxyl radical and positive hole centers on the catalyst surface are responsible for oxidation.30 The rate constants of the reactions between the hydroxyl radical and DCF and SMX are quite similar: 7.5 × 109 and 5 × 109 M−1s−1, respectively;31 so, the differences observed in experimental oxidation rates of DCF and SMX during the photocatalytic oxidation can likely be due to the competitive action of other intermediates formed for the available free radicals. In the case of DCF and SMX photocatalytic ozonation, however, no clear way of oxidation (direct ozone reaction or free radical oxidation) can be established because the direct ozone reaction of DCF gives rise to the ozonide ion radical, precursor of the hydroxyl radical; that is, DCF is an initiator of the decomposition of ozone in hydroxyl radicals.17 In fact, some other complex organic compounds such as phenols, amines, 4535



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Aromaticity presents the highest removal rate during the first 15−30 min of the ozone process regardless of the presence of catalyst and/or black light, which supports the direct mechanism as the main way of oxidation. Then, the oxidation rate decreases with time, and even some differences in aromaticity removal rates are observed between ozone processes used. It seems that there is a second reaction mechanism with the oxidation rate depending on the formation and concentration of free radicals, that is, depending on the AOP applied. It should be highlighted, however, that a hydroxyl radical oxidation process such as black light photocatalytic oxidation was only able to remove 21% aromaticity (31% in the case of SMX, see Figure 3S, Supporting Information) after 2 h. Also, hydrogen peroxide concentration was determined, as can be seen in results shown in Figure 4 for the case of SMX oxidation.

of ozonation and photocatalytic oxidation contributions, while photocatalytic ozonation leads to nearly 80% TOC removal (see Figure 2). For the case of DCF the results are even better if applying black light photocatalytic ozonation, since at 75 min reaction ozonation and photocatalytic oxidation of DCF (Figure 2S, Supporting Information) lead to 20% and 30% TOC reductions, respectively, which would make a 50% total TOC elimination if the effects of both processes are summed. However, at the same reaction time, photocatalytic ozonation leads to 90% TOC elimination. It is evident that these results can only be due to some synergism mechanism (see the following section). 3.2. First Steps of the Reaction Mechanism. As previously commented, two types of reaction mechanisms contribute to the removal of compounds in any ozone AOP. These mechanisms usually develop in consecutive stages during the first minutes when unsaturated compounds, with certain electron-donating functional groups, react very fast with ozone (direct mechanism), and then the rest of the process, where hydroxyl radicals are formed and less ozone reactive compounds are eliminated by reacting with these radicals (indirect reaction mechanism). This double mechanism has been observed in previous works with O3, O3/catalyst and O3/ UVC-B/catalyst processes. 12 Also, in the presence of compounds such as DCF, the double mechanism simultaneously takes place because DCF is the initiator of the decomposition of ozone.17 Also, SMX is another potential initiator of ozone decomposition because its molecule presents some amine groups. As it is known, ozone decomposition initiators operate by reacting with ozone, yielding the ozonide ion radical as one of the subproducts, which immediately leads to the formation of hydroxyl radicals. In addition to this way, photocatalytic ozonation has other ways to generate hydroxyl radicals. Thus, in an attempt to check the development of these ways, in addition to concentrations of SMX and DCF, in the experiments carried out, aromaticity and hydrogen peroxide concentration were also followed. The changes with time of what can be defined as aromaticity, that is, 254 nm absorption of treated samples (50% diluted with ultrapure water in this case) during the DCF oxidation processes, are shown in Figure 3.

Figure 4. Time evolution of hydrogen peroxide concentration formed during different oxidation processes of SMX mineralization. Conditions: 20 °C, gas flow rate 30L·h−1, inlet ozone gas concentration 10 mg·L−1, initial pH 7, initial SMX concentration 10−4 M (average value), TiO2 concentration 0.5 g·L−1. Systems: □, O3/TiO2; ◊, O2/TiO2/UVA; ■, O3/TiO2/UVA.

As it can be observed, hydrogen peroxide concentration follows an already known trend in ozone AOPs:34 an increase of concentration during the first minutes up to a maximum value and a further concentration decrease. In the case of black light photocatalytic ozonation, the maximum concentration value is reached at the time when SMX has completely disappeared from water. It is reasonable to admit that hydrogen peroxide is formed from both ozone-unsaturated compound reactions,32,35 that are extremely important during the first minutes of the process, although possible recombination of superoxide ion radicals,36 in the case where TiO2/UVA is simultaneously applied, is also possible, but of less importance when ozone is also present.37 The decrease of hydrogen peroxide concentration is likely due to its reaction with ozone to yield hydroxyl radicals.38 Also, in Figure 4, hydrogen peroxide concentration with time during the UVA/TiO2/O2 process is presented. A different trend to that of ozone processes is seen with concentrations formed much lower. Because ozone is not present in this oxidizing system the low formation of hydrogen peroxide can likely be due to the combination of superoxide ion radicals.36 Similar results were found in the case of DCF (see Figure 4S, Supporting Information), although hydrogen peroxide concentrations during UVA/TiO2/O2 were found to be higher than those shown in Figure 4 for the case of SMX oxidation.

Figure 3. Time evolution of 254 nm absorbance during the black light radiation and oxidation systems applied to DCF mineralization. Conditions: 20 °C, initial pH 7, gas flow rate 30L·h−1, inlet ozone gas concentration 10 mg·L−1, initial diclofenac concentration 5 × 10−5 M (average value), TiO2 concentration 0.5 g L−1. Systems: ⧫, UVA radiation; ▲, O3; □, O3/TiO2; Δ, O3/UVA; ◊, O2/TiO2/UVA; ■, O3/TiO2/UVA. 4536



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respectively. In these experiments, the TOC was not measured because the presence of t-butanol and benzoquinone would

Another fact that supports the mechanism proposed is the evolution of dissolved ozone concentration. Thus, the consumption of ozone was so high during the first 15 min of reaction that no dissolved ozone was found, which supports the presence of fast ozone-direct reactions.39 After this first period, ozone starts to accumulate in water as a consequence of the slow ozone gas−liquid reactions taking place, which leads to hydroxyl radical formation (i.e., from the reaction between ozone and hydrogen peroxide formed during the first minutes). The absence of dissolved ozone during the first reaction period also allows us to conclude the lack of synergism between ozone and photocatalytic oxidation during this initial reaction period. In photocatalytic oxidation the first reaction is due to the semiconductor absorption of radiation that yields oxidant holes in the conduction band and electrons in the valence band,30 hν, λ= 365nm TiO2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ h+ + e−

(1) Figure 5. Variation of SMX dimensionless concentration with time. Effect of inhibitors on the black light photocatalytic oxidation process. Conditions: 20 °C, initial pH 7, gas flow rate 30 L·h−1, initial SMX concentration 10−4 M (average value), initial t-butanol concentration 10−3 M, initial benzoquinone concentration 10−4 M, initial iodide ion concentration 5 × 10−3 M, TiO2 concentration 0.5 g L−1. Symbols: ⧫, presence of t-butanol; ▲, presence of benzoquinone; ■, presence of iodide ion; Δ, absence of inhibitors.

+

These species, particularly the positive holes (h ), trigger a wellknown reaction mechanism involving the formation of hydroxyl radicals.30 To avoid the inhibition of the process due to the recombination reaction between holes and electrons, an oxidizing agent such as oxygen is used. Thus, oxygen captures the electrons to yield superoxide ion radicals: TiO2 + e− + O2 → TiO2 − O− 2·

(2)

When ozone is present in bulk water, it reaches the surface of the semiconductor and can also participate in capturing electrons, because of its higher oxidant character:40 TiO2 + e− + O3 → TiO2 − O− 3·

(3)

Also, ozone can react with the adsorbed superoxide ion radical to yield the adsorbed ozonide ion radical: − TiO2 − O− 2 ·+ O3 → TiO2 − O3 ·

(4)

Finally, this free radical decomposes to yield hydroxyl radicals: + TiO2 − O− 3 ·+ H → TiO2 − HO·+ O2

(5)

Reactions 3−5 constitute the synergism reactions between ozone and photocatalytic oxidation processes. During the first minutes of the black light photocatalytic ozonation of SMX and DCF, however, ozone is consumed through fast direct reactions in the proximity of the gas−water interface, so that no ozone reaches the bulk water or the surface of the semiconductor. As a consequence, reactions 3−5 cannot develop. However, during this initial reaction period, hydroxyl free radical reactions of DCF and SMX can also take place because these compounds are initiators of ozone decomposition.17,33 The importance of the direct ozone reaction and the hydroxyl radical oxidation during this period could be established from experiments in the presence of free radical inhibitors, as shown later (see section 3.3). On the contrary, when DCF and SMX have disappeared or are at ppb concentration level (see section 3.4.1), ozone starts to accumulate in water (as shown in this work) and reaches the surface of the semiconductor. Under these circumstances, photocatalytic ozonation is a clear synergic process because reactions 3−5 can take place. 3.3. Effects of Free Radical Inhibitors. Another series of reactions were carried out in the presence of iodide, t-butanol, and benzoquinone, which are well-known inhibitors of positive hole, hydroxyl, and superoxide ion radical oxidation, respectively.41 Figures 5 and 6 present the results obtained during UVA/TiO2/O2 and O3/UVA/TiO2 oxidation of SMX,

Figure 6. Variation of SMX dimensionless concentration with time. Effect of inhibitors on the black light photocatalytic ozonation process. Conditions: 20 °C, initial pH 7, gas flow rate 30 L·h−1, inlet ozone gas concentration 10 mg L−1, initial SMX concentration 10−4 M (average value), initial t-butanol concentration: 10−3 M, initial benzoquinone concentration 10−4 M, TiO2 concentration 0.5 g L−1. Symbols: ⧫, presence of t-butanol; ▲, presence of benzoquinone; Δ, absence of inhibitors.

mask the results. Also, the effect of iodide during the O3/UVA/ TiO 2 process was not studied because of its direct, instantaneous reaction with ozone. Regarding the O2/UVA/TiO2 process, as can be observed from Figure 5, the presence of any of the inhibitor agents used slows down the oxidation rate of SMX, which means that both positive hole, hydroxyl radical, and superoxide ion radical oxidation take place (similar results were found in the case of DCF). In the well-known mechanism of photocatalytic oxidation30 these reactions are

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h+ + M → P

(6)

TiO2 − HO·+M → P

(7)

TiO2 − O− 2· + M → P

(8)



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of reaction time in the experiments shown in Figures 1 and 3−6 (see also Figures 1S and 2S, Supporting Information). In these experiments, no significant differences were observed between ozone AOPs on the ozonation rate of SMX or DCF, which suggests that the direct ozone reaction is the main mechanism of pharmaceutical removal. Also, in this first period of time, dissolved ozone was not found, which means that these ozone direct reactions were moderate or fast.39 According to the literature,39 direct reactions between ozone and organics in water follow second order kinetics. Following gas−liquid absorption theories, the ozonation kinetic regime for a second order gas−liquid reaction can be confirmed by calculating the Hatta (Ha) number,43 defined as follows:

where free radicals are assumed to react in their adsorbed form with nonadsorbed DCF or SMX (M) (adsorption of these substances on TiO2 was negligible). After photoreaction 1, free radicals are formed in the following reactions:30 TiO2 + H2O → TiO2 − H2O TiO2 − H2O + h+ → TiO2 − HO· + H+

(9) (10)

and reaction 2 TiO2 + e− + O2 → TiO2 − O− 2·

Notice that results of Figure 5 also depend on the competitive effect of resulting intermediates (that is TOC from intermediates) to react with holes and free radicals. During the black light photocatalytic ozonation in the presence of inhibitors (in this case, only t-butanol and benzoquinone were investigated), the results were significantly different, as seen in Figure 6 compared to those of Figure 5. Thus, from Figure 6, it is seen that the presence of benzoquinone does not affect the oxidation rate of SMX, which seems to confirm the direct ozone reaction as the main way of SMX removal. According to the literature,29 reactions 3 or 4 and 5 are likely the main way of hydroxyl radical formation when ozone is simultaneously applied with UVA and TiO2. These reactions develop for TOC removal once SMX or DCF have disappeared. Regarding the effect of t-butanol, from Figure 6, it is seen that this HO radical scavenger improves the ozonation rate of SMX (similar results are found in the case of DCF) when a logical response is retardation or even no effect on SMX (or DCF) oxidation. However, this, a priori, anomalous result confirms that the removal of SMX and DCF is mainly due to direct ozone reactions and not to HO radical oxidation. It is known that t-butanol at the concentration applied in this work affects the mass transfer rate in gas−liquid reactions. In fact, the presence of t-butanol reduces both the surface tension and viscosity of the aqueous solutions. Thus, Tizaoui et al.42 reported 4% and 30% decrease of surface tension and viscosity, respectively, by adding t-butanol to water when ozone is being absorbed. These authors also visually observed formation of smaller gas bubbles when t-butanol was present and a significant gas hold-up increase, which results in a clear increase of the volumetric mass transfer coefficient. The increase of the ozone concentration in the film layer close to the gas−water interface will undoubtedly favor the ozonation rate due to direct ozone reactions, thus, reinforcing the conclusion about this mechanism during the first minutes of ozone AOPs. 3.4. Kinetic Aspects of the Processes Studied. According to the results obtained, ozone alone is able to remove SMX and DCF at a fast rate in the first minutes of the reaction time, when these compounds are present from mg·L−1 to 100s of μg·L−1 concentration. However, in real wastewater, the concentration of these pharmaceuticals is as high as 10ths of μg·L−1. Thus, aspects concerning the kinetics of photocatalytic ozonation of these compounds at this concentration level are necessary. In this work, the kinetics has been treated by separating the first minutes of the ozone processes, for both cases of high and low concentration, and the rest of the reaction period where free radical oxidation would mainly takes place. a. Kinetics of DCF and SMX Ozone AOPs. Pharmaceuticals at mg·L−1 to 100s of μg·L−1. These concentrations correspond to the first 15 min (case of SMX) and 7 min (case of DCF)

Ha =

k·DO3·CM kL

(11)

where CM is the concentration of DCF or SMX and k, kL, and DO3 are the rate constant of the ozone−M reaction, the individual mass transfer coefficient, and the ozone diffusivity in water, respectively. Values of all these parameters are given in Table 1S of the Supporting Information. They were experimentally determined or obtained from the literature. For the concentrations given above, Ha was always between 7 and 0.4 for the ozone−DCF reaction and between 4.6 and 0.3 for the ozone−SMX reaction, respectively, which confirms a fast to moderate kinetic regime of ozone absorption. For these kinetic regimes, the rate equation deduced by De Coursey44 can be applied. For a semicontinuous perfectly mixed gas−liquid reactor, such as the one used in this work, the mass balance of SMX or DCF is −z

CO g · R · T dCM = kLa · 3 ·E dt He

(12)

where z is the stoichiometric ratio of the direct ozone reaction, CO3g is the concentration of ozone in the gas leaving the reactor, kLa is the volumetric mass transfer coefficient, and He, R, and T are the Henry constant of the ozone−water system, the ideal gas constant, and the absolute temperature, respectively. E is the reaction factor for a second order gas−liquid reaction, which, following De Coursey equation, is defined as E=−

Ha + 2(Ei − 1)

Ha 2 2

+

4(Ei − 1)

Ei ·Ha +1 (Ei − 1) (13)

where Ei is the instantaneous reaction factor given in eq 14: Ei =

⎤ DO3 ⎡ ⎢1 + z ·DM ·CM ·He ⎥ DM ⎢⎣ DO3·CO3g ·R ·T ⎥⎦

(14)

with DM being the diffusivity of DCF or SMX in water. To solve eqs 11−14, the ozone gas mass balance equation is also needed. For the reactor type indicated above, this equation is (1 − β) ·V ·

dCO3g dt

= vg(CO3ge − CO3g) − kLa ·

CO3g ·R ·T He

·E ·β·V

(15)

where CO3ge is the ozone gas concentration at the reactor inlet, vg is the gas flow rate, β is the liquid hold-up, and V is the reac4538



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PCBA, which was also added to the water in these experiments at 50 μg·L−1 concentration (detection limit for PCBA was 0.5 μg·L−1). PCBA is a compound that only reacts with hydroxyl radicals,46 so eq 17 for this case reduces to eq 18:

tion volume (see Table 1S in the Supporting Information). The kinetic model of eqs 11−15 was solved with the fourth order Runge−Kutta method. Calculated results (not shown) were similar to the experimental ones for the first 2 min of reactions, and then, they were lower. Deviations observed are a clear consequence of the appearance of fast reacting intermediates that also consume ozone in competitive reactions of similar kinetic regime that were not dealt with in the kinetic model. For example, it is known that ozonation of DCF gives rise to 5-hydroxy-diclofenac or diclofenac 2,5-iminoquinone of reactivity toward ozone, similar to that of DCF17 and nitroaromatic compounds in the case of SMX.45 The results also give calculated ozone concentration in the gas (CO3g) higher than the experimental ones, which also confirms the presence of competitive fast ozone reactions. In any case, calculated and experimental concentrations of DCF or SMX, present in water at some mg·L−1 are reduced to 100s of μg·L−1 in a few minutes, as experimentally observed. b. Pharmaceuticals at Concentrations Lower than 50 μg·L−1. As stated, concentrations of pharmaceuticals in real urban wastewater are usually lower than some 100s of μg·L−1, so that the kinetics of photocatalytic ozonation of DCF and SMX at even lower concentrations are of high interest. In this work, some experiments with starting initial concentrations of DCF and SMX of 50 μg·L−1 have also been carried out, although due to analytical equipment limitations concentrations of these compounds could not be measured. However, data from these experiments on dissolved ozone concentration and estimated hydroxyl radical concentration (see below) allow the simulation of the evolution of DCF and SMX remaining concentrations with time. At 50 μg·L−1 concentrations, the Hatta number of the ozone direct reactions of DCF and SMX are 0.3 and 0.2, respectively, which means a slow kinetic regime for these reactions. which was confirmed with the presence of dissolved ozone in these experiments. Then, ozone adsorbs on the catalyst surface and participates in reactions 3 and 4 to immediately yield hydroxyl radicals through reaction 5. Also, other possible ways of forming hydroxyl radicals involving ozone are from the direct ozone reaction35 or from the ozone-hydrogen peroxide reaction;35 the latter formed when ozone breaks aromatic rings of SMX and DCF (see Figures 3 and 4). However, it is likely that these reactions, especially the ozone−H2O2 reaction, have little or low contribution to the formation of hydroxyl radicals because of the low concentration of SMX and DCF. Another source of hydroxyl radicals in photocatalytic processes are reactions 9 and 10. On the other hand, contribution of the ozone direct reactions to remove SMX and DCF cannot be neglected because of the high rate constant values of these reactions (see Table 1S in the Supporting Information). Then, in the semicontinuous photoreactor used in this work, the mass balance of SMX or DCF for slow kinetic regime would be −

dCM = (1/z)kCO3CM + kHOCHOCM dt

ln

t t CM = −(1/z)k CO3dt − kHO CHOdt C M0 0 0

∫

∫

∫

(18)

where CPCBA0 and CPCBA are the concentrations of PCBA at the start of experiment and time t, respectively. According to eq 18, values of the hydroxyl radical integrated concentration over different reaction time can be determined from the ratio between the left side of eq 18 and kHOPCBA, the rate constant of the hydroxyl radical−PCBA reaction (5 × 109 M−1s−1).46 Then, known the terms of the right side of eq 17 the remaining concentration of DCF and SMX could be estimated. From experiments of ozonation, photocatalytic oxidation, and photocatalytic ozonation of DCF or SMX in the presence of PCBA, at these low concentrations, similar PCBA concentration−time profiles were observed in the photocatalytic processes (see Figure 5S in the Supporting Information). In the ozonation process, the decomposition rate of PCBA in the absence of SMX and DCF was slower than in the presence of these compounds. This confirms an increase of hydroxyl radical concentration when PCBA, DCF, and SMX were simultaneously ozonated, likely as a result of the formation of radicals from the direct ozone reactions. In any case, once the integrated concentration of hydroxyl radical over different reaction times was determined (see eq 18), eq 17 was then applied to calculate the changes of DCF or SMX concentrations with time in the experiments of low concentration. Thus, Figure 7 shows, as example, the evolution of calculated DCF and SMX concentration with time from 50 μg·L−1 until 99.99% conversion is reached, that is, to get 5 ng·L−1. As can be seen from this figure, the model predicts that DCF and SMX are removed from water in 25 and 80 s, respectively, during the ozonation process, while 15 and 25 s are needed in the photocatalytic ozone process. Also, the calculated results predict photocatalytic oxidation as the slowest process because 90 s are needed for 99.99% and 50% DCF and SMX removal, respectively. Although, no experimental data on SMX and DCF concentration was available in this work from these experiments, the literature47 reports that these compounds present in a real wastewater at these concentration levels disappear in less than 4 min of ozonation (no data is reported on photocatalytic oxidation and photocatalytic ozonation). Given the fact that, during ozonation in a real wastewater, the resistance of DCF or SMX to be degraded is undoubtedly higher than it is in ultrapure prepared water, as in this work, the results predicted from eq 18 could be considered close to the actual ones in the laboratory prepared water used here. Finally, Table 1 gives DCF and SMX percentage removals due to direct ozonation and hydroxyl radical oxidation for experiments at low concentrations. These have been determined from experimental data and eq 17, taking into account the time-integrated concentrations of ozone and hydroxyl radicals. As can be seen from Table 1, the contribution of hydroxyl radicals is always higher in photocatalytic ozonation than in ozonation, regardless of reaction time and pharmaceutical compound. Also, for any ozone process, contribution of direct ozonation to remove DCF and SMX is increasing with time which is a consequence of the increase of dissolved ozone concentration and the increase of inhibitors that reduces the contribution of free radicals to the whole oxidation process. In

(16)

where CHO and kHO are the concentration of hydroxyl radicals and the rate constant of their reaction with DCF or SMX, respectively. Integration of eq 16 is ln

t CPCBA = −kHOPCBA CHOdt CPCBA 0 0

(17)

In eq 17, the integrated concentration of hydroxyl radicals over reaction time can be determined from the experimental concentrations of a reference compound, p-chlorobenzoic acid, 4539



Article

a. Kinetics of TOC Oxidation with the UV/TiO2/O2 System. For the photocatalytic oxidation system, both direct photolysis and free radical oxidation (advanced oxidation) are usually the ways of contaminant removal in water. In the cases studied here, direct photolysis can be neglected, as only free radical oxidation is taking place. Because mineralization is the major aim of advanced oxidation, TOC oxidation kinetics has been considered. According to literature,48 photocatalytic oxidation kinetics follows a Langmuir type equation that, combined with the TOC mass balance equation, taking into account the photoreactor used, leads to: −

k′ ·TOC dTOC = UVA dt 1 + K TOC

(19)

where k′UVA and K are the apparent rate constant of the reaction and TOC equilibrium constant, respectively. However, adsorption of TOC was observed to be negligible, so that it can be admitted that 1 ≫ KTOC, as eq 19 becomes one of pseudo-first order kinetics that, integrated, leads to ⎛ TOC ⎞ ln⎜ ⎟ = −k′UVA t ⎝ TOC0 ⎠

(20)

Kinetics of this process is due to the development of reactions 1, 2 and 4−10 because both hydroxyl radicals, from positive holes and the superoxide ion radical, reaction 2, likely contribute to TOC removal (see effects of inhibitors). Experimental results confirmed eq 20 (see Figure 6S in the Supporting Information). Values of k′UVA for the cases of DCF and SMX photocatalytic oxidation are shown in Table 2. Furthermore, following the literature49 for the intensity of radiation, TiO2 absorbs (Ia = 4.8 × 10−5 Einstein·min−1, see the experimental part), a direct proportionality between Ia and k′UVA should be expected, so that the rate constant finally should be

Figure 7. Simulated concentration−time evolution of (a) SMX and (b) DCF during (■) ozonation, (•) photocatalytic oxidation, and (▲) photocatalytic ozonation at low concentrations. Conditions: 20 °C, initial pH 7, gas flow rate 30L·h−1, inlet ozone gas concentration 10 mg·L−1, initial pharmaceutical concentration 50 μg·L−1, TiO2 concentration 0.5 g·L−1.

photocatalytic ozonation hydroxyl radical contribution is more important to remove DCF than SMX. However, explanation of this fact is not an easy task since, on one hand, direct ozone reaction rate of DCF is higher than that of SMX because of the rate constant values of both reactions (see Table 1S in the Supporting Information); however, on the other hand and due to the same reason, formation of hydroxyl radicals from the ozone-DCF reaction is also higher. The sum of both contributions likely makes the hydroxyl radical way more important to remove DCF than SMX. 3.4.2. Kinetics of TOC Oxidation in AOPs Studied. In most of experiments of this work, an AOP was carried out on aqueous solutions of DCF and SMX, starting with concentrations of some mg·L−1. In these experiments, once DCF and SMX were removed, TOC was also followed with time to establish the kinetics of mineralization achieved in the AOP applied: photocatalytic oxidation, ozonation, and photocatalytic ozonation.

kUVA =

k′UVA Ia

(21)

Values of kUVA are also presented in Table 2 for UVA/TiO2/O2 oxidation of DCF and SMX. b. Kinetics of TOC Ozonation. For the ozonation process and reaction times higher than the ones needed for the removal of the parent compound and fast reacting intermediates (from approximately 15 min of reaction), the kinetic regime of ozone absorption becomes slow (dissolved ozone starts to accumulate in water) and free radical oxidation is the principal way of oxidation. TOC kinetics in this case is exclusively due to free radical oxidations. The mass balance of TOC reduces to −

dTOC = kHO·CHO·TOC dt

(22)

Table 1. Ozone and Hydroxyl Radical Contributions to the SMX and DCF Removal from Water during Ozonation and Black Light Photocatalytic Ozonationa O3 (%) SMX DCF a

ozonation photocatalytic ozonation ozonation photocatalytic ozonation

OH (%)

5s

15 s

30 s

90 s

5s

15 s

30 s

90 s

62 29 45 20

79 56 75 42

84 70 88 59

93 87 97 78

38 71 55 80

21 44 25 58

16 30 12 41

7 13 3 22

Calculated results from eq 17 for experiments starting with 50 μg·L−1 concentration of DCF and SMX. 4540



Article

Table 2. Apparent Rate Constant Values of TOC Ozonation, Black Light TiO2 Photocatalytic Oxidation, and Black Light TiO2 Photocatalytic Ozonation of DCF and SMXa kT, M−2·min−1

oxidation process

kt, min−1

O2/UVA/TiO2 1.36 ± 0.07 × 1015 (1.22 ± 0.05 × 1013) 9.4 ± 0.1 × 1015 (3.9 ± 0.1 × 1014)

O3 O3/UVA/TiO2 a

1.1 ± 0.2 × 104 (1.2 ± 0.1 × 106) 1.5 ± 0.3 × 103 (3.7 ± 0.2 × 104)

k′UVAx103, min−1

kUVA, L·Einstein−1

2.65 ± 0.05 (1.07 ± 0.03)

54 ± 1 (23 ± 1)

8.3 ± 3.0 (7.8 ± 2.5)

163 ± 4 (154 ± 2)

Values in bold face correspond to DCF processes. Values in parentheses correspond to SMX processes.

where kHO is the rate constant of the reaction between the hydroxyl radical and TOC, that can take an average value of 109 M−1·s−150 and CHO is the concentration of hydroxyl radicals that can be expressed as a function of the rate of initiation (likely the reaction between ozone and the ionic form of hydrogen peroxide) and the inhibiting factor, kt, which is the sum of the products between the concentration of any substance present in water that scavenges hydroxyl radicals and the corresponding rate constant of their reaction.39 Thus, eq 22 becomes −

2k i·CHO2 −·CO3 dTOC = k OH· ·TOC dt kt = kT CHO2−·CO3·TOC

(23) Figure 8. Checking eq 25 for TOC removal in the single ozonation of a SMX aqueous solution. Conditions are the same as in Figure 1.

where CHO2− and CO3 are the concentrations of the hydroperoxide ion and dissolved ozone, respectively, and ki is the rate constant of the initiation reaction (between ozone and HO2−).38 Notice that CHO2− is pH dependent: CHO2− =

10 pH − pK 1 + 10 pH − pK

C H2O2T

Values of kt resulted to be 1.06 × 104 and 1.22 × 106 min−1 for DCF and SMX, respectively (see Table 2). These results suggest that byproducts present in the water in the case of SMX ozonation were more reactive toward hydroxyl radicals to finalize the chain mechanism than the ones formed in the case of DCF ozonation, which confirms the higher TOC oxidation rates achieved in this latter case. c. Kinetics of TOC Oxidation with the UV/TiO2/O3 System. For the case of photocatalytic ozonation kinetics, a similar procedure is followed. Now, after the first 15 min reaction, hydroxyl radical oxidation is the way of TOC removal. However, in this case, two contributions have to be considered: the reaction between ozone and hydrogen peroxide in bulk water (as in the ozonation process) and the photocatalytic process that involves the action of hydroxyl radicals formed in reactions 5 and 10. Contribution of superoxide ion radicals can be considered negligible because ozone is a stronger oxidant than oxygen and reactions 3 and/or 4 are more important than reaction 2 to take the electrons of the conduction band. Thus, the mass balance of TOC in the photocatalytic reactor used is

(24)

where pK is 10.3 for the hydrogen peroxide equilibrium in water. Separation of variables and integration of eq 23 leads to ln

t TOC = −kT CHO2−·CO3·dt TOC15 15

∫

(25)

where TOC15 corresponds to the TOC value at 15 min reaction during ozonation. Integral values of eq 25 were numerically determined at different reaction time by taking into account the experimental concentrations of ozone and hydrogen peroxide and the pH of reaction medium. In Figure 8 the left-hand side of eq 25 has been plotted against the integral value of the right side at different reaction times for the case of the ozonation of SMX. As can be seen, points follow a straight line confirming eq 25. Similar results were obtained in the case of DCF (Figure 7S of Supporting Information). Values of kT obtained from leastsquares fitting of straight lines are also given in Table 2. It has to be noted, however, that these kT values should be taken with caution because of the low differences between the standard deviation of TOC analysis (0.02) and maximum variation of parameter ln(TOC/TOC15) (0.07), as observed in Figure 8. According to eq 23 the scavenging factor of the water containing the ozonated aqueous solutions of DCF or SMX solution, is

kt =

2k i kT

−

dTOC = k′UVA ·TOC + kHO·CHO·TOC dt

(27)

In eq 27, the first term of the right side should be of Langmuir type as in the UVA/TiO2/O2 process, but adsorption of TOC here was also negligible. Thus, eq 27 reduces to −

(26) 4541

dTOC = (k′UVA + kT·CHO2−·CO3)TOC dt

(28)



Article

Separation of variables and integration lead to ln

t TOC = −[k′UVA ·t + kT · CHO2−·CO3·dt ] TOC15 15

∫

(29)

Once the integral of eq 29 was numerically evaluated for different reaction times, TOC experimental results were fitted to eq 29 by multiple regression analysis (see Figure 9 and Figure 8S, Supporting Information). Values of k′UVA, kT, and their corresponding kUVA and kt values (see eqs 22 and 27) are shown in Table 2. As can be seen from Table 2, values of kUVA are higher than those of the photocatalytic oxidation process, which is due to the contribution of reaction 3, more important than reaction 2. Also, the scavenging factor, kt, is now lower than those of ozona-

Figure 9. (A) Checking eq 29 for TOC removal in the black light photocatalytic ozonation of a SMX aqueous solution. Conditions are the same as in Figure 1. (B) Comparison of experimental with calculated Y values.

■

• For 10s of μg·L−1 to ng·L−1 concentrations, data on dissolved ozone concentration and estimation of hydroxyl radical concentration allow the simulation of DCF and SMX concentration−time evolution. These data confirm that ozonation alone is able to remove these compounds in a few seconds, although contribution of hydroxyl radicals can represent up to 55% of the oxidation rate. Also, using photocatalytic ozonation improves the oxidation rate, and hydroxyl radical contribution increases up to 80% of the removal rate. At this concentration level, the synergic effect between ozone and photocatalytic oxidation is evident. • Regarding TOC resulting from the oxidation of DCF and SMX when present in water at mg·L−1 to 100s of μg·L−1 concentration, from the mechanism proposed, rate constants for the free radical TOC oxidation process in ozonation alone, photocatalytic oxidation, and photocatalytic ozonation have been determined. These rate constants confirm the importance of ozone addition in the presence of TiO2 and black light to speed up the removal of TOC and the synergic effect of ozone and photocatalytic oxidation. • The kinetics presented also allows determination of the water scavenging factor. This parameter confirms the high TOC oxidation rate during DCF ozonation processes compared to SMX processes (intermediates are of lesser hydroxyl radical scavenging character) and also the different nature or concentration that these intermediates have during ozonation alone and TiO2 black light photocatalytic ozonation.

ASSOCIATED CONTENT

S Supporting Information *

tion alone, regardless of the initial pharmaceutical compound, which indicates changes in composition (nature or concentration) of intermediates of the process.

Additional figures and a table containing the kinetic parameters and physical properties used in the kinetic studies. This information is available free of charge via the Internet at http:// pubs.acs.org/.

■

4. CONCLUSIONS In this work, the following conclusions have been reached: • For the removal of the parent compounds, SMX and DCF, from mg·L−1 to 100s of μg·L−1 concentration, the presence of any agent (chemical or light) in the ozone process does not add any significant increase of the oxidation rate. Then ozonation alone is the recommended process for the removal of DCF and SMX, regardless of their concentration in water. • Once the initial products have disappeared, ozone starts to accumulate in water as a consequence of the presence of more refractory compounds. These compounds can be eliminated in a great extent through advanced oxidation, so the addition of black light and a semiconductor as TiO2 accelerates the oxidation rates. In this regard, black light lamps mainly emitting at 365 nm, near the visible light spectrum, are shown to be very effective to significantly reduce the TOC of the water. In fact, TiO2 black light photocatalytic ozonation is much better than ozone alone, ozone photolytic, TiO2 catalytic ozonation, and photocatalytic oxidation processes. • For mg·L−1 to 100s of μg·L−1 concentration and for the first minutes of any ozone process, the results were simulated through the kinetics of a fast-moderate second order gas− liquid reaction. The results suggest the appearance of intermediates that also react fast with ozone.

AUTHOR INFORMATION

Corresponding Author

*Telephone: 34924289387. Fax: 34924289385. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work has been supported by the CICYT of Spain and the European Region Development Funds of the European Commission (Project CTQ2009/13459/C05/05). Almudena Aguinaco thanks the Spanish Ministry of Education for providing her a FPU research grant. Ana Rey thanks Universidad de Extremadura for a postdoctoral research contract.

■

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Kinetic Studies on Black Light Photocatalytic Ozonation of Diclofenac

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