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H2O2 treatments
Chemosphere 81 (2010) 114–119
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Technical Note
Degradation of thiacloprid in aqueous solution by UV and UV/H2O2 treatments Biljana F. Abramovic´ *, Nemanja D. Banic´, Daniela V. Šojic´ Faculty of Sciences, Department of Chemistry, Biochemistry and Environmental Protection, Trg D. Obradovic´a 3, 21000 Novi Sad, Serbia
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Article history: Received 26 April 2010 Received in revised form 9 July 2010 Accepted 12 July 2010 Available online 9 August 2010 Keywords: Neonicotinoid pesticides Thiacloprid Photolysis AOP Pollution control Natural water
a b s t r a c t Although some studies concerning flash photolysis and photocatalytic ozonation of thiacloprid have already been published, no complete investigation and explanation of the effects of thiacloprid photodegradation under the conditions of UV and UV/H2O2 (high-pressure mercury lamp and H2O2) have been reported yet. The photochemical degradation of thiacloprid (0.32 mM) was studied under a variety of solution conditions, by varying the initial concentrations of H2O2 from 0 to 162 mM and the pH from 2.8 to 9. In the UV/H2O2 system, thiacloprid reacted rapidly, the maximum first-order rate constant (2.7 102 min1, r = 0.9996) being observed at the H2O2/thiacloprid molar ratio of 220 and pH 2.8. Under these conditions, 97% of the thiacloprid was removed in about 120 min. The thiacloprid degradation is accompanied by the formation of a number of ionic byproducts (Cl, acetate, formate, SO2 4 , and NHþ 4 ) and organic intermediates, so that after 35 h of irradiation, 17% of organic carbon remained nondegraded. The application of UV radiation, or H2O2 alone, yielded no significant thiacloprid degradation. The study of the rate of removal of thiacloprid from natural water showed that it is dominantly influenced by the presence of HCO 3. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Thiacloprid (Z)-3-(6-chloro-3-pyridylmethyl)-1,3-thiazolidin2-ylidenecyanamide, Bayer CropScience’s second neonicotinoid launched under the brand name Calypso in 2000 (Elbert et al., 2008), belongs to a relatively new class of insecticide. Like other chloronicotinyl insecticides, thiacloprid acts selectively on the insect nervous system as an agonist of the nicotinic acetylcholine receptor (Tomlin, 2009). This unique mode of action makes this compound highly applicable for controlling the biological effect of insects in cases when these developed resistance to conventional chlorinated hydrocarbons, organophosphate, carbamate and pyrethroid insecticides (Jeschke et al., 2001). Several studies of environmental behavior of thiacloprid showed that the molecule is resistant 6 and more months to the degradation in water by hydrolysis at acidic or neutral medium, whereas at pH 10, only 10% of thiacloprid degraded in the aerated water after 6 months ˇ ernigoj et al., 2007). (Guzsvány et al., 2006; C To minimize the risk of pesticide pollution, it is advisable to develop new technologies that would promote the easy degradation of these biorecalcitrant organic compounds (Farré et al., 2005). A possible solution would be the application of advanced oxidation processes (AOPs), which are able to produce HO under mild experimental conditions (Gaya and Abdullah, 2008; Thiruvenkatachari * Corresponding author. Tel.: +381 21 4852753; fax: +381 21 454065. E-mail addresses: [email protected] (B.F. Abramovic´), n.banic@ yahoo.com (N.D. Banic´), [email protected] (D.V. Šojic´). 0045-6535/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2010.07.016
et al., 2008). Ozone and H2O2 are the most often used substances to generate HO . UV irradiation, catalysts or a combined application of ozone and H2O2 are used to initiate the reaction (GonzalezOlmos et al., 2009). One of alternatives to increase the HO production is the photocatalytic ozonation (both homogeneous and heterogeneous) by using photo-Fenton reaction (Løgager et al., 1992; Piera et al., 2000) or TiO2-photocatalysis (Sánchez et al., 1998; Piera et al., 2000). However, the high energy consumption for generating ozone by silent electrical discharge makes its use expensive. Additionally, its limited water solubility and short lifetime make its application inconvenient (Gonzalez-Olmos et al., 2009). However, the UV/H2O2 combination has shown great potential for the destruction of a wide range of persistent organic contaminants. The process comprises direct photolysis, where the target compound is transformed through absorbing UV photons, and indirect photolysis, where the compounds react with HO radical produced via photolysis of H2O2 (Crittenden et al., 1999; Nienow et al., 2008; Wu and Linden, 2008). Flash photolysis (Dell’Arciprete et al., 2009) and photocatalytic ozonation (Cˇernigoj ˇ ernigoj et al., 2010) of thiacloprid has been reported et al., 2007; C by several authors. The objective of this study was to investigate the kinetics of thiacloprid photodegradation during treatment with UV and UV/ H2O2, using high-pressure mercury lamp, over a wide range of H2O2 concentration and pH. Also, the kinetics of appearance/disappearance of some intermediates of the photodegradation was investigated. Finally, the effect of dissolved inorganic and organic matter from natural water system on photolytic removal of
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thiacloprid was also studied at two different pH (2.8 and natural – 8.2), as well as at two different H2O2 concentrations. 2. Materials and methods 2.1. Chemicals and solutions All chemicals were of reagent grade and were used without purification. Thiacloprid (C10H9ClN4S, 99.9% purity), manufactured by Riedel-de Häen; 30% H2O2, was obtained from Centrohem (Stara Pazova, Serbia); 85% H3PO4 (Lachema, Neratovice, Czech Republic); 99.8% acetonitrile (ACN) (J.T. Baker); NaOH (ZorkaPharm, Šabac, Serbia); and H2SO4 (Merck). All solutions were made using doubly distilled water. Concentration of the aqueous stock solution of thiacloprid was 0.32 mM. 2.2. Photolysis experiments A typical photolysis experiment was carried out in a batch reactor made of Pyrex glass (total volume of ca 100 mL, solution depth 46 mm). The irradiation was provided by a Philips HPL-N 125 W high-pressure mercury lamp (k > 290 nm), which has emission bands in the UV region at 304, 314, 335 and 366 nm, with maximum emission at 366 nm. The overall photonic flux in the cell was measured using the potassium ferrioxalate actinometry method and estimated to 2.54 107 E s1. Experiments were carried out using 30 mL of stock solution of thiacloprid. Prior to the irradiation, an appropriate amount of H2O2 was added, and the pH adjusted by adding H2SO4 or NaOH. Dark experiments were conducted under identical conditions but without UV exposure. During the experiment, oxygen was continuously introduced at the bottom of the reactor, at a constant flow of 5.0 mL min1. Apart from oxygen bubbling, the solution was homogenized with the aid of a stir bar, to ensure completely mixed batch conditions. The temperature was kept at 40 ± 0.5 °C by water circulation through the reactor jacket because in treatment of wastewater by photocatalytic processes in the reactors with solar irradiation results in an important heating of the reactors, reaching 40–45 °C (Zahra et al., 2006). To study the influence of natural water constituents on photodegradation use was made of a water sample from the Begej river at Itebej, Serbia (pH 8.2; total organic carbon (TOC) 2.7 mg L1; conductivity 287 lS cm1). The composition of environmental electrolytes was as follows (in mg L1): Ca2+ – 28.0, Mg2+ – 7.0, Na+ – 10.2, K+ – 3.1, Cl – 12.0, SO2 4 – 21.0, HCO3 – 118, NO3 – þ 1.8, and NH4 – 2.1. The same experiment was performed using distilled water. To avoid microbial degradation, natural water was sterilized by filtration and all glassware was sterilized by autoclaving for 30 min at 140 °C.
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Absorbance spectra were recorded on a double-beam T80+ UV– Vis Spectrometer (UK), at a fixed slit width (2 nm), using 1 cm quartz cell and computer-loaded UV Win 5 data software. TOC was measured by using irradiated samples of 800 lL, which were taken from the cell at different times during the irradiation and diluted to 25 mL. Before measurements samples were acidified. All measurements were performed on an Elementar Liqui TOC II (Germany), and made in accordance with Standard US EPA Method 9060A. The pH measurements were made using an Iskra combined glass electrode, on a previously calibrated pH-meter (Jenway, UK). For ion chromatographic (IC) determinations, aliquots of 1 mL of the reaction mixture were taken at regular time intervals and diluted to 6 mL. After dilution, the solutions were analyzed on an IC Dionex ICS-3000, equipped with a Dionex AS 19 column for anion determination or with a CS 12A column for cation determination, and a conductometric detector. The mobile phase for cation determination was 20 mM solution of methanesulfonic acid, and for anion determination 40 mM solution of KOH, the flow rate in both cases being 1 mL min1. 3. Results and discussion 3.1. Direct photolysis of thiacloprid Thiacloprid strongly absorbs UV irradiation over a wide range of wavelengths, from 200 to 280 nm (Fig. 1), with the maximum absorption band at 242 nm. Molar absorptivity was found to be 1.86 104 M1 cm1. Hence, only those UV sources with good output below approximately 280 nm can be useful for the direct photolysis of thiacloprid. Since the emission spectrum of high-pressure mercury lamp has no significant peaks in the wavelength range of 200–280 nm, no direct photolysis of thiacloprid could be expected. To investigate the direct photolysis of thiacloprid, the first experiments were performed with thiacloprid dissolved in distilled water and natural pH of 6. After 240 min of irradiation, no significant change in the thiacloprid concentration could be observed (Fig. 2), which is in agreement with the above discussion. 3.2. H2O2 assisted oxidation 3.2.1. H2O2 concentration The effect of initial concentration of H2O2 on photodegradation of thiacloprid is shown in Fig. 2. As can be seen, the process was greatly enhanced due to the production of the strong oxidizing
2.3. Analytical methods The HPLC–diode array detection analyses were made on an Agilent Chromatograph, Series 1100 (Agilent Technologies, USA), equipped with C-18 column (Zorbax Eclipse XDB-C18; 4.6 mm 150 mm 5 lm), and Rheodyne injector (20 lL loop). For the kinetic studies of thiacloprid removal, 1 mL aliquots of the reaction mixture were taken at the beginning of the experiment and at regular time intervals (volume variation ca 10%), and the solution was diluted to 10 mL with doubly distilled water. Aliquots of 20 lL were injected manually through an injection loop. Column temperature was held at 25 °C, mobile phase was a mixture of 0.1% H3PO4–ACN (7:3, v/v), pH 2.56, and the flow rate was 1 mL min1. Thiacloprid elution was monitored at 242 nm (thiacloprid absorption maximum), the retention time being 5.9 min.
Fig. 1. Molar absorption spectrum of thiacloprid (C0 = 3.8 102 mM) and H2O2 (45.3 mM) (left axis) and UV emission spectra of HPL-N lamp (right axis).
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Fig. 2. Effect of initial concentration of H2O2 on the kinetics of thiacloprid (C0 = 0.32 mM) photodegradation at pH 2.8. The inset represents the thiacloprid reaction rate constants at different molar ratios of H2O2/thiacloprid determined for the first 120 min of irradiation.
species, HO . Indeed, the absorption of UV radiation by H2O2 causes its dissociation into HO which react rapidly and non-selectively with most organic compounds, either by H-abstraction or by addition to C–C unsaturated bonds (Stefan et al., 1996). The effect of the initial H2O2 concentration is an important parameter in the UV/H2O2 AOP (Fig. 2). The increased addition of H2O2, however, did not result in a linear increase of the rate of thiacloprid removal. While the rapid increase in the reaction rate occurred at the H2O2 concentrations from 0 to 45 mM, in the concentration range of 45–162 mM only a slight increase was observed, remaining practically constant afterwards (Fig. 2, inset). Thus, an optimal H2O2/thiacloprid molar ratio was 220. Two competing effects may contribute to the relationship between the molar ratio of H2O2/thiacloprid and the resulting rate constant. First, a higher concentration of H2O2 results in a higher steady-state HO concentration, and thus increases the availability of HO to degrade thiacloprid. Second, H2O2 also acts as a HO scavenger, producing much less reactive hydroperoxyl radical (HO2 ) (Eqs. (1) and (2)). The scavenging effect becomes significant at higher H2O2 concentrations, so that less HO is available for removal of thiacloprid. The combination of such effects results in an optimal H2O2/thiacloprid ratio corresponding to the highest reaction rate constant. Similar trends have also been observed for the UV/H2O2 process of acetone (Stefan et al., 1996), metolachlor (Wu et al., 2007) and nitroaromatics (García Einschlag et al., 2002).
H2 O2 þ HO ! HO2 þ H2 O
ð1Þ
HO2 þ HO ! H2 O þ O2
ð2Þ
removal was studied in the pH range between 2.8 and 9. Fig. 3b indicates that the reaction rate of decay increased with decreasing pH. A linear empirical expression (k = 3.7 102–3.4 103 pH) could fit the k data reasonably well (r = 0.9758). On the other hand, the experiments in dark showed that the most pronounced decay of the reaction rate was observed under alkaline conditions, although the differences in the rates are very small, which could be expected since the difference in absorption spectra recorded in the pH range from 2 to 9 was small. Nevertheless, the reaction rates which include irradiation were much faster than those derived for dark conditions, indicating that the contribution of hydrolysis to the overall rate is negligible. The changes of the pH during the disappearance of thiacloprid in dependence of the initial pH (Fig. 3a) show that more acidic products are formed during the degradation process. It is also evident that for the initial pH in the interval 2.8–7, the pH after 120 min of irradiation was practically the same, which is understandable bearing in mind that thiacloprid has been removed almost completely. The effect of the solution pH on thiacloprid removal rate in the UV/H2O2 system is manifested as two competing processes: the HO production and scavenging. On the other hand, the hydroper oxide anion (HO 2 ) is also a scavenger of the HO (Eq. (3)) and can cause decomposition of H2O2 (Eq. (4)). It has been reported that the reaction rate of HO with HO 2 is about 100 times faster than that with H2O2 (Aleboyeh et al., 2005). Furthermore, the rate of H2O2 self-decomposition (Eq. (5)) also increases strongly with increasing solution pH.
HO2 þ HO ! HO2 þ HO
ð3Þ
HO2 þ H2 O2 ! H2 O þ O2 þ OH
ð4Þ
2H2 O2 ! 2H2 O þ O2
ð5Þ
Together, these competing processes control the HO concentration in the UV/H2O2 system, which causes a decrease of k with increase of the solution pH. Dark experiments signify that the degradation goes only under alkaline conditions, but at a lower rate, as has already been mentioned.
As can be seen from Fig. 2, at 45 mM of H2O2 and pH 2.8, the removal of thiacloprid occurred approximately in 120 min. Nevertheless, the degree of mineralization of the thiacloprid, quantified by TOC evaluation, was only 15% in the same time period. To achieve the quasi-total mineralization, a longer irradiation time was needed; after 35 h of irradiation, 17% of organic carbon remained non-degraded. The TOC decrease was rather non-linear (data not shown), indicating that very resistant compounds are formed during the reaction (López Cisneros et al., 2002). Therefore, a much longer period of irradiation is needed to achieve the total mineralization and avoid toxicity problems. 3.2.2. pH Bearing in mind that the reactivity of H2O2 and the reaction rates of compounds degradation depend highly on the solution pH (Aleboyeh et al., 2005), the influence of initial pH on thiacloprid
Fig. 3. Changes in the pH for its different initial values (a), and the effect of the initial pH on the kinetics (b) during the thiacloprid (C0 = 0.32 mM) degradation in the presence of 71 mM H2O2.
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3.3. Effect of natural waters In view of the fact that natural water systems contain natural organic matter, carbonate radicals and other species that can complicate the photodegradation process (Nienow et al., 2008; Wu and Linden, 2008), the rate of thiacloprid removal from a natural water sample was investigated at two H2O2 concentrations, i.e. 5 and 45 mM, and at two pH, i.e. natural – 8.2 (Fig. 4b) and 2.8, at which the rate in distilled water was the highest (Fig. 4a). It was found that at pH 8.2 the process in natural water followed also the pseudo-first-order reaction kinetics, but the rate was lower than in distilled water, which was especially pronounced at 45 mM H2O2. Also, it was found that during the photodegradation of thiacloprid in natural water the pH changed by 0.7 pH units at 45 mM H2O2, and only by 0.2 at 5 mM H2O2. However, in distilled water (the pH adjusted to pH 8.2 by adding the necessary amount of NaOH) the pH dropped by even 4.8 units (45 mM H2O2) and by 4.1 (5 mM H2O2). Supposing that such large changes in the pH may be a primary cause of the higher rate of thiacloprid removal, we followed the kinetics of disappearance of thiacloprid in distilled water to which NaHCO3 was added in an amount equivalent to that in river water, making thus the pH 8.2. As can be seen, there is no practically difference between the rates of thiacloprid degradation.
Fig. 4. Photodegradation of thiacloprid (C0 = 0.32 mM) in natural and distilled water in the presence of 5 and 45 mM H2O2 at the pH 2.8 (a), and 8.2 (b).
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These results indicate that in the presence of HCO 3 the change in the pH during thiacloprid removal is small, which decreases the degradation rate (see Fig. 3), although the presence of HCO 3 also always reduces the overall reaction rates due to scavenging of HO , despite the contribution of carbonate radicals to the decay of the target pollutant (Wang et al., 2000; Wu and Linden, 2010). However, by following the rate of thiacloprid removal from the natural water after adjusting its pH to 2.8 it was found that the process also followed pseudo-first-order reaction kinetics, but in this case the rate was somewhat higher than in distilled water (Fig. 4a). At the pH 2.8, HCO 3 is not present in natural water, and the pH does not change during the degradation. Taking into account composition of natural water, the enhanced photodegradation can be probably attributed to the presence of naturally occurring photosensitizers. Since the mentioned pH is lower than the natural one, the reason for the increased rate of thiacloprid removal from a natural water was not the subject of our further study. 3.4. Formation of ionic byproducts Thiacloprid is the organic compound that contains covalently bounded chlorine, sulfur and nitrogen so that the release of inorganic ionic degradation byproducts during the photodegradation process is to be expected. Monitoring of these anions can provide information about the mineralization of the organic matter. The re þ lease of inorganic ions such as, Cl, SO2 4 , NO3 , NO2 and NH4 during phototransformation process of thiacloprid was monitored by IC, as well as of acetate and formate. Formation curves for different ions at pH 2.8 were studied for two initial H2O2 concentrations of 5 and 45 mM (Fig. 5a and b). As expected (Fig. 5), the evolution of inorganic anions was slower than the disappearance of thiacloprid. At the lower H2O2 concentrations (Fig. 5a), the rates of formation of Cl and SO2 4 , as end degradation products, are the highest compared to the other inorganic ions, although, as already mentioned, the rate of removal of thiacloprid is significantly higher, suggesting the formation of intermediates with chloro and sulfur as substituents. However, at higher H2O2 concentration (Fig. 5b), the rate of Cl release is the highest, and that of SO2 is significantly lower. Under the given 4 conditions, a stoichiometric release of Cl takes place during 240 min of photolysis, i.e. there are no any more chlorinated intermediates. This probably indicates the different dominant reaction pathways of degradation at the two H2O2 concentrations. As is known (Abramovic´ et al., 2004), in the course of photodegradation, the organic nitrogen is transformed mainly to NHþ 4 and/or þ NO 2 /NO3 , and sometimes to nitrogen. The ratio of NH4 /NO3 depends on the chemical structure of the substrate and reaction conditions (Alberici et al., 2001). From the start of the photodegradation process, the NO 2 and NO3 appeared at the same time, and their concentrations did not change noticeably for the first 240 min of irradiation. As can be seen from the curves (Fig. 5), the dominant process is the evolution of NHþ 4 . Here, it is necessary to remind that there are four nitrogen atoms in the molecule and only one of chlorine and sulfur. Hence, it follows that the stability of the intermediates with nitrogen is higher compared to those involving Cl and SO2 4 . In this case too, the reaction rate was lower than the rate of Cl release (that is the pyridine moiety degradation), which suggests that nitrogen-containing intermediates in the acyclic structure were formed. This is in agreement with the previous results obtained in the study of the degradation mechanism of 2-amino-5-chloropyridine (Abramovic´ et al., 2003), 3-amino-2-chloropyridine (Abramovic´ et al., 2004) and clopyralid (Šojic´ et al., 2009). The concentrations of acetate and formate ions increased steadily, especially that of formate. The same trend continued even
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of natural water to 2.8, the rate of thiacloprid removal was higher by about 1.5 times (45 mM H2O2), i.e. 1.2 times (5 mM H2O2), compared to that for distilled water. Since natural water at pH 2.8 does not contain, and bearing in mind composition of the natural water, the enhanced photodegradation could be probably attributed to the presence of naturally occurring photosensitizers, i.e. dissolved organic matter. Acknowledgment This work was financially supported by the Ministry of Science and Technological Development of the Republic of Serbia (Project: ON142029). References
Fig. 5. Formation of ionic byproducts during the photodegradation of thiacloprid (C0 = 0.32 mM) at 5 mM H2O2 (a), and 45 mM H2O2 (b).
after all thiacloprid was degraded at higher H2O2 concentration, which indicates that the rate of thiacloprid disappearance is significantly higher than the rate of its mineralization.
4. Conclusions The process of thiacloprid (0.32 mM) disappearance from water is fast under UV/H2O2 treatment using high-pressure mercury lamp. In the investigated pH range from 2.8 to 9 the rate of its removal decreases with the increase in pH. At the optimal H2O2/thiacloprid molar ratio of 220 and pH 2.8 the first-order rate constant is 2.7 102 min1 (r = 0.9996). Under these conditions, 97% of the thiacloprid is removed in about 120 min. The thiacloprid disappearance is accompanied by the release of ionic byproducts – þ Cl, acetate, formate, SO2 4 and NH4 . However, the mineralization of thiacloprid is much slower, and after 35 h of irradiation, 17% of organic carbon remained non-degraded. No significant thiacloprid degradation was observed by applying alone UV irradiation or H2O2 oxidation alone. The reaction rate constant of thiacloprid removal from natural water of pH 8.2 is by about 2.4 times (45 mM H2O2), and 1.5 times (5 mM H2O2) lower compared to that measured for distilled water under the conditions of the same ini tial pH and in the absence of HCO 3 . However, if HCO3 was added in the amount equivalent to that present in river water the reaction rate constants were almost identical, which indicates that this ion plays a major role. On the other hand, after adjusting the pH
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Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Technical Note
Degradation of thiacloprid in aqueous solution by UV and UV/H2O2 treatments Biljana F. Abramovic´ *, Nemanja D. Banic´, Daniela V. Šojic´ Faculty of Sciences, Department of Chemistry, Biochemistry and Environmental Protection, Trg D. Obradovic´a 3, 21000 Novi Sad, Serbia
a r t i c l e
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Article history: Received 26 April 2010 Received in revised form 9 July 2010 Accepted 12 July 2010 Available online 9 August 2010 Keywords: Neonicotinoid pesticides Thiacloprid Photolysis AOP Pollution control Natural water
a b s t r a c t Although some studies concerning flash photolysis and photocatalytic ozonation of thiacloprid have already been published, no complete investigation and explanation of the effects of thiacloprid photodegradation under the conditions of UV and UV/H2O2 (high-pressure mercury lamp and H2O2) have been reported yet. The photochemical degradation of thiacloprid (0.32 mM) was studied under a variety of solution conditions, by varying the initial concentrations of H2O2 from 0 to 162 mM and the pH from 2.8 to 9. In the UV/H2O2 system, thiacloprid reacted rapidly, the maximum first-order rate constant (2.7 102 min1, r = 0.9996) being observed at the H2O2/thiacloprid molar ratio of 220 and pH 2.8. Under these conditions, 97% of the thiacloprid was removed in about 120 min. The thiacloprid degradation is accompanied by the formation of a number of ionic byproducts (Cl, acetate, formate, SO2 4 , and NHþ 4 ) and organic intermediates, so that after 35 h of irradiation, 17% of organic carbon remained nondegraded. The application of UV radiation, or H2O2 alone, yielded no significant thiacloprid degradation. The study of the rate of removal of thiacloprid from natural water showed that it is dominantly influenced by the presence of HCO 3. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Thiacloprid (Z)-3-(6-chloro-3-pyridylmethyl)-1,3-thiazolidin2-ylidenecyanamide, Bayer CropScience’s second neonicotinoid launched under the brand name Calypso in 2000 (Elbert et al., 2008), belongs to a relatively new class of insecticide. Like other chloronicotinyl insecticides, thiacloprid acts selectively on the insect nervous system as an agonist of the nicotinic acetylcholine receptor (Tomlin, 2009). This unique mode of action makes this compound highly applicable for controlling the biological effect of insects in cases when these developed resistance to conventional chlorinated hydrocarbons, organophosphate, carbamate and pyrethroid insecticides (Jeschke et al., 2001). Several studies of environmental behavior of thiacloprid showed that the molecule is resistant 6 and more months to the degradation in water by hydrolysis at acidic or neutral medium, whereas at pH 10, only 10% of thiacloprid degraded in the aerated water after 6 months ˇ ernigoj et al., 2007). (Guzsvány et al., 2006; C To minimize the risk of pesticide pollution, it is advisable to develop new technologies that would promote the easy degradation of these biorecalcitrant organic compounds (Farré et al., 2005). A possible solution would be the application of advanced oxidation processes (AOPs), which are able to produce HO under mild experimental conditions (Gaya and Abdullah, 2008; Thiruvenkatachari * Corresponding author. Tel.: +381 21 4852753; fax: +381 21 454065. E-mail addresses: [email protected] (B.F. Abramovic´), n.banic@ yahoo.com (N.D. Banic´), [email protected] (D.V. Šojic´). 0045-6535/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2010.07.016
et al., 2008). Ozone and H2O2 are the most often used substances to generate HO . UV irradiation, catalysts or a combined application of ozone and H2O2 are used to initiate the reaction (GonzalezOlmos et al., 2009). One of alternatives to increase the HO production is the photocatalytic ozonation (both homogeneous and heterogeneous) by using photo-Fenton reaction (Løgager et al., 1992; Piera et al., 2000) or TiO2-photocatalysis (Sánchez et al., 1998; Piera et al., 2000). However, the high energy consumption for generating ozone by silent electrical discharge makes its use expensive. Additionally, its limited water solubility and short lifetime make its application inconvenient (Gonzalez-Olmos et al., 2009). However, the UV/H2O2 combination has shown great potential for the destruction of a wide range of persistent organic contaminants. The process comprises direct photolysis, where the target compound is transformed through absorbing UV photons, and indirect photolysis, where the compounds react with HO radical produced via photolysis of H2O2 (Crittenden et al., 1999; Nienow et al., 2008; Wu and Linden, 2008). Flash photolysis (Dell’Arciprete et al., 2009) and photocatalytic ozonation (Cˇernigoj ˇ ernigoj et al., 2010) of thiacloprid has been reported et al., 2007; C by several authors. The objective of this study was to investigate the kinetics of thiacloprid photodegradation during treatment with UV and UV/ H2O2, using high-pressure mercury lamp, over a wide range of H2O2 concentration and pH. Also, the kinetics of appearance/disappearance of some intermediates of the photodegradation was investigated. Finally, the effect of dissolved inorganic and organic matter from natural water system on photolytic removal of
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thiacloprid was also studied at two different pH (2.8 and natural – 8.2), as well as at two different H2O2 concentrations. 2. Materials and methods 2.1. Chemicals and solutions All chemicals were of reagent grade and were used without purification. Thiacloprid (C10H9ClN4S, 99.9% purity), manufactured by Riedel-de Häen; 30% H2O2, was obtained from Centrohem (Stara Pazova, Serbia); 85% H3PO4 (Lachema, Neratovice, Czech Republic); 99.8% acetonitrile (ACN) (J.T. Baker); NaOH (ZorkaPharm, Šabac, Serbia); and H2SO4 (Merck). All solutions were made using doubly distilled water. Concentration of the aqueous stock solution of thiacloprid was 0.32 mM. 2.2. Photolysis experiments A typical photolysis experiment was carried out in a batch reactor made of Pyrex glass (total volume of ca 100 mL, solution depth 46 mm). The irradiation was provided by a Philips HPL-N 125 W high-pressure mercury lamp (k > 290 nm), which has emission bands in the UV region at 304, 314, 335 and 366 nm, with maximum emission at 366 nm. The overall photonic flux in the cell was measured using the potassium ferrioxalate actinometry method and estimated to 2.54 107 E s1. Experiments were carried out using 30 mL of stock solution of thiacloprid. Prior to the irradiation, an appropriate amount of H2O2 was added, and the pH adjusted by adding H2SO4 or NaOH. Dark experiments were conducted under identical conditions but without UV exposure. During the experiment, oxygen was continuously introduced at the bottom of the reactor, at a constant flow of 5.0 mL min1. Apart from oxygen bubbling, the solution was homogenized with the aid of a stir bar, to ensure completely mixed batch conditions. The temperature was kept at 40 ± 0.5 °C by water circulation through the reactor jacket because in treatment of wastewater by photocatalytic processes in the reactors with solar irradiation results in an important heating of the reactors, reaching 40–45 °C (Zahra et al., 2006). To study the influence of natural water constituents on photodegradation use was made of a water sample from the Begej river at Itebej, Serbia (pH 8.2; total organic carbon (TOC) 2.7 mg L1; conductivity 287 lS cm1). The composition of environmental electrolytes was as follows (in mg L1): Ca2+ – 28.0, Mg2+ – 7.0, Na+ – 10.2, K+ – 3.1, Cl – 12.0, SO2 4 – 21.0, HCO3 – 118, NO3 – þ 1.8, and NH4 – 2.1. The same experiment was performed using distilled water. To avoid microbial degradation, natural water was sterilized by filtration and all glassware was sterilized by autoclaving for 30 min at 140 °C.
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Absorbance spectra were recorded on a double-beam T80+ UV– Vis Spectrometer (UK), at a fixed slit width (2 nm), using 1 cm quartz cell and computer-loaded UV Win 5 data software. TOC was measured by using irradiated samples of 800 lL, which were taken from the cell at different times during the irradiation and diluted to 25 mL. Before measurements samples were acidified. All measurements were performed on an Elementar Liqui TOC II (Germany), and made in accordance with Standard US EPA Method 9060A. The pH measurements were made using an Iskra combined glass electrode, on a previously calibrated pH-meter (Jenway, UK). For ion chromatographic (IC) determinations, aliquots of 1 mL of the reaction mixture were taken at regular time intervals and diluted to 6 mL. After dilution, the solutions were analyzed on an IC Dionex ICS-3000, equipped with a Dionex AS 19 column for anion determination or with a CS 12A column for cation determination, and a conductometric detector. The mobile phase for cation determination was 20 mM solution of methanesulfonic acid, and for anion determination 40 mM solution of KOH, the flow rate in both cases being 1 mL min1. 3. Results and discussion 3.1. Direct photolysis of thiacloprid Thiacloprid strongly absorbs UV irradiation over a wide range of wavelengths, from 200 to 280 nm (Fig. 1), with the maximum absorption band at 242 nm. Molar absorptivity was found to be 1.86 104 M1 cm1. Hence, only those UV sources with good output below approximately 280 nm can be useful for the direct photolysis of thiacloprid. Since the emission spectrum of high-pressure mercury lamp has no significant peaks in the wavelength range of 200–280 nm, no direct photolysis of thiacloprid could be expected. To investigate the direct photolysis of thiacloprid, the first experiments were performed with thiacloprid dissolved in distilled water and natural pH of 6. After 240 min of irradiation, no significant change in the thiacloprid concentration could be observed (Fig. 2), which is in agreement with the above discussion. 3.2. H2O2 assisted oxidation 3.2.1. H2O2 concentration The effect of initial concentration of H2O2 on photodegradation of thiacloprid is shown in Fig. 2. As can be seen, the process was greatly enhanced due to the production of the strong oxidizing
2.3. Analytical methods The HPLC–diode array detection analyses were made on an Agilent Chromatograph, Series 1100 (Agilent Technologies, USA), equipped with C-18 column (Zorbax Eclipse XDB-C18; 4.6 mm 150 mm 5 lm), and Rheodyne injector (20 lL loop). For the kinetic studies of thiacloprid removal, 1 mL aliquots of the reaction mixture were taken at the beginning of the experiment and at regular time intervals (volume variation ca 10%), and the solution was diluted to 10 mL with doubly distilled water. Aliquots of 20 lL were injected manually through an injection loop. Column temperature was held at 25 °C, mobile phase was a mixture of 0.1% H3PO4–ACN (7:3, v/v), pH 2.56, and the flow rate was 1 mL min1. Thiacloprid elution was monitored at 242 nm (thiacloprid absorption maximum), the retention time being 5.9 min.
Fig. 1. Molar absorption spectrum of thiacloprid (C0 = 3.8 102 mM) and H2O2 (45.3 mM) (left axis) and UV emission spectra of HPL-N lamp (right axis).
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Fig. 2. Effect of initial concentration of H2O2 on the kinetics of thiacloprid (C0 = 0.32 mM) photodegradation at pH 2.8. The inset represents the thiacloprid reaction rate constants at different molar ratios of H2O2/thiacloprid determined for the first 120 min of irradiation.
species, HO . Indeed, the absorption of UV radiation by H2O2 causes its dissociation into HO which react rapidly and non-selectively with most organic compounds, either by H-abstraction or by addition to C–C unsaturated bonds (Stefan et al., 1996). The effect of the initial H2O2 concentration is an important parameter in the UV/H2O2 AOP (Fig. 2). The increased addition of H2O2, however, did not result in a linear increase of the rate of thiacloprid removal. While the rapid increase in the reaction rate occurred at the H2O2 concentrations from 0 to 45 mM, in the concentration range of 45–162 mM only a slight increase was observed, remaining practically constant afterwards (Fig. 2, inset). Thus, an optimal H2O2/thiacloprid molar ratio was 220. Two competing effects may contribute to the relationship between the molar ratio of H2O2/thiacloprid and the resulting rate constant. First, a higher concentration of H2O2 results in a higher steady-state HO concentration, and thus increases the availability of HO to degrade thiacloprid. Second, H2O2 also acts as a HO scavenger, producing much less reactive hydroperoxyl radical (HO2 ) (Eqs. (1) and (2)). The scavenging effect becomes significant at higher H2O2 concentrations, so that less HO is available for removal of thiacloprid. The combination of such effects results in an optimal H2O2/thiacloprid ratio corresponding to the highest reaction rate constant. Similar trends have also been observed for the UV/H2O2 process of acetone (Stefan et al., 1996), metolachlor (Wu et al., 2007) and nitroaromatics (García Einschlag et al., 2002).
H2 O2 þ HO ! HO2 þ H2 O
ð1Þ
HO2 þ HO ! H2 O þ O2
ð2Þ
removal was studied in the pH range between 2.8 and 9. Fig. 3b indicates that the reaction rate of decay increased with decreasing pH. A linear empirical expression (k = 3.7 102–3.4 103 pH) could fit the k data reasonably well (r = 0.9758). On the other hand, the experiments in dark showed that the most pronounced decay of the reaction rate was observed under alkaline conditions, although the differences in the rates are very small, which could be expected since the difference in absorption spectra recorded in the pH range from 2 to 9 was small. Nevertheless, the reaction rates which include irradiation were much faster than those derived for dark conditions, indicating that the contribution of hydrolysis to the overall rate is negligible. The changes of the pH during the disappearance of thiacloprid in dependence of the initial pH (Fig. 3a) show that more acidic products are formed during the degradation process. It is also evident that for the initial pH in the interval 2.8–7, the pH after 120 min of irradiation was practically the same, which is understandable bearing in mind that thiacloprid has been removed almost completely. The effect of the solution pH on thiacloprid removal rate in the UV/H2O2 system is manifested as two competing processes: the HO production and scavenging. On the other hand, the hydroper oxide anion (HO 2 ) is also a scavenger of the HO (Eq. (3)) and can cause decomposition of H2O2 (Eq. (4)). It has been reported that the reaction rate of HO with HO 2 is about 100 times faster than that with H2O2 (Aleboyeh et al., 2005). Furthermore, the rate of H2O2 self-decomposition (Eq. (5)) also increases strongly with increasing solution pH.
HO2 þ HO ! HO2 þ HO
ð3Þ
HO2 þ H2 O2 ! H2 O þ O2 þ OH
ð4Þ
2H2 O2 ! 2H2 O þ O2
ð5Þ
Together, these competing processes control the HO concentration in the UV/H2O2 system, which causes a decrease of k with increase of the solution pH. Dark experiments signify that the degradation goes only under alkaline conditions, but at a lower rate, as has already been mentioned.
As can be seen from Fig. 2, at 45 mM of H2O2 and pH 2.8, the removal of thiacloprid occurred approximately in 120 min. Nevertheless, the degree of mineralization of the thiacloprid, quantified by TOC evaluation, was only 15% in the same time period. To achieve the quasi-total mineralization, a longer irradiation time was needed; after 35 h of irradiation, 17% of organic carbon remained non-degraded. The TOC decrease was rather non-linear (data not shown), indicating that very resistant compounds are formed during the reaction (López Cisneros et al., 2002). Therefore, a much longer period of irradiation is needed to achieve the total mineralization and avoid toxicity problems. 3.2.2. pH Bearing in mind that the reactivity of H2O2 and the reaction rates of compounds degradation depend highly on the solution pH (Aleboyeh et al., 2005), the influence of initial pH on thiacloprid
Fig. 3. Changes in the pH for its different initial values (a), and the effect of the initial pH on the kinetics (b) during the thiacloprid (C0 = 0.32 mM) degradation in the presence of 71 mM H2O2.
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3.3. Effect of natural waters In view of the fact that natural water systems contain natural organic matter, carbonate radicals and other species that can complicate the photodegradation process (Nienow et al., 2008; Wu and Linden, 2008), the rate of thiacloprid removal from a natural water sample was investigated at two H2O2 concentrations, i.e. 5 and 45 mM, and at two pH, i.e. natural – 8.2 (Fig. 4b) and 2.8, at which the rate in distilled water was the highest (Fig. 4a). It was found that at pH 8.2 the process in natural water followed also the pseudo-first-order reaction kinetics, but the rate was lower than in distilled water, which was especially pronounced at 45 mM H2O2. Also, it was found that during the photodegradation of thiacloprid in natural water the pH changed by 0.7 pH units at 45 mM H2O2, and only by 0.2 at 5 mM H2O2. However, in distilled water (the pH adjusted to pH 8.2 by adding the necessary amount of NaOH) the pH dropped by even 4.8 units (45 mM H2O2) and by 4.1 (5 mM H2O2). Supposing that such large changes in the pH may be a primary cause of the higher rate of thiacloprid removal, we followed the kinetics of disappearance of thiacloprid in distilled water to which NaHCO3 was added in an amount equivalent to that in river water, making thus the pH 8.2. As can be seen, there is no practically difference between the rates of thiacloprid degradation.
Fig. 4. Photodegradation of thiacloprid (C0 = 0.32 mM) in natural and distilled water in the presence of 5 and 45 mM H2O2 at the pH 2.8 (a), and 8.2 (b).
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These results indicate that in the presence of HCO 3 the change in the pH during thiacloprid removal is small, which decreases the degradation rate (see Fig. 3), although the presence of HCO 3 also always reduces the overall reaction rates due to scavenging of HO , despite the contribution of carbonate radicals to the decay of the target pollutant (Wang et al., 2000; Wu and Linden, 2010). However, by following the rate of thiacloprid removal from the natural water after adjusting its pH to 2.8 it was found that the process also followed pseudo-first-order reaction kinetics, but in this case the rate was somewhat higher than in distilled water (Fig. 4a). At the pH 2.8, HCO 3 is not present in natural water, and the pH does not change during the degradation. Taking into account composition of natural water, the enhanced photodegradation can be probably attributed to the presence of naturally occurring photosensitizers. Since the mentioned pH is lower than the natural one, the reason for the increased rate of thiacloprid removal from a natural water was not the subject of our further study. 3.4. Formation of ionic byproducts Thiacloprid is the organic compound that contains covalently bounded chlorine, sulfur and nitrogen so that the release of inorganic ionic degradation byproducts during the photodegradation process is to be expected. Monitoring of these anions can provide information about the mineralization of the organic matter. The re þ lease of inorganic ions such as, Cl, SO2 4 , NO3 , NO2 and NH4 during phototransformation process of thiacloprid was monitored by IC, as well as of acetate and formate. Formation curves for different ions at pH 2.8 were studied for two initial H2O2 concentrations of 5 and 45 mM (Fig. 5a and b). As expected (Fig. 5), the evolution of inorganic anions was slower than the disappearance of thiacloprid. At the lower H2O2 concentrations (Fig. 5a), the rates of formation of Cl and SO2 4 , as end degradation products, are the highest compared to the other inorganic ions, although, as already mentioned, the rate of removal of thiacloprid is significantly higher, suggesting the formation of intermediates with chloro and sulfur as substituents. However, at higher H2O2 concentration (Fig. 5b), the rate of Cl release is the highest, and that of SO2 is significantly lower. Under the given 4 conditions, a stoichiometric release of Cl takes place during 240 min of photolysis, i.e. there are no any more chlorinated intermediates. This probably indicates the different dominant reaction pathways of degradation at the two H2O2 concentrations. As is known (Abramovic´ et al., 2004), in the course of photodegradation, the organic nitrogen is transformed mainly to NHþ 4 and/or þ NO 2 /NO3 , and sometimes to nitrogen. The ratio of NH4 /NO3 depends on the chemical structure of the substrate and reaction conditions (Alberici et al., 2001). From the start of the photodegradation process, the NO 2 and NO3 appeared at the same time, and their concentrations did not change noticeably for the first 240 min of irradiation. As can be seen from the curves (Fig. 5), the dominant process is the evolution of NHþ 4 . Here, it is necessary to remind that there are four nitrogen atoms in the molecule and only one of chlorine and sulfur. Hence, it follows that the stability of the intermediates with nitrogen is higher compared to those involving Cl and SO2 4 . In this case too, the reaction rate was lower than the rate of Cl release (that is the pyridine moiety degradation), which suggests that nitrogen-containing intermediates in the acyclic structure were formed. This is in agreement with the previous results obtained in the study of the degradation mechanism of 2-amino-5-chloropyridine (Abramovic´ et al., 2003), 3-amino-2-chloropyridine (Abramovic´ et al., 2004) and clopyralid (Šojic´ et al., 2009). The concentrations of acetate and formate ions increased steadily, especially that of formate. The same trend continued even
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of natural water to 2.8, the rate of thiacloprid removal was higher by about 1.5 times (45 mM H2O2), i.e. 1.2 times (5 mM H2O2), compared to that for distilled water. Since natural water at pH 2.8 does not contain, and bearing in mind composition of the natural water, the enhanced photodegradation could be probably attributed to the presence of naturally occurring photosensitizers, i.e. dissolved organic matter. Acknowledgment This work was financially supported by the Ministry of Science and Technological Development of the Republic of Serbia (Project: ON142029). References
Fig. 5. Formation of ionic byproducts during the photodegradation of thiacloprid (C0 = 0.32 mM) at 5 mM H2O2 (a), and 45 mM H2O2 (b).
after all thiacloprid was degraded at higher H2O2 concentration, which indicates that the rate of thiacloprid disappearance is significantly higher than the rate of its mineralization.
4. Conclusions The process of thiacloprid (0.32 mM) disappearance from water is fast under UV/H2O2 treatment using high-pressure mercury lamp. In the investigated pH range from 2.8 to 9 the rate of its removal decreases with the increase in pH. At the optimal H2O2/thiacloprid molar ratio of 220 and pH 2.8 the first-order rate constant is 2.7 102 min1 (r = 0.9996). Under these conditions, 97% of the thiacloprid is removed in about 120 min. The thiacloprid disappearance is accompanied by the release of ionic byproducts – þ Cl, acetate, formate, SO2 4 and NH4 . However, the mineralization of thiacloprid is much slower, and after 35 h of irradiation, 17% of organic carbon remained non-degraded. No significant thiacloprid degradation was observed by applying alone UV irradiation or H2O2 oxidation alone. The reaction rate constant of thiacloprid removal from natural water of pH 8.2 is by about 2.4 times (45 mM H2O2), and 1.5 times (5 mM H2O2) lower compared to that measured for distilled water under the conditions of the same ini tial pH and in the absence of HCO 3 . However, if HCO3 was added in the amount equivalent to that present in river water the reaction rate constants were almost identical, which indicates that this ion plays a major role. On the other hand, after adjusting the pH
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