Polyoxometallates as effective photocatalysts in water purification from pesticides ∗

Polyoxometallates (POM), i.e., metal oxide clusters of mainly tungsten are effective homogeneous photocatalysts for the mineralization of organic pollutants. POM are, at least, as effective as the well studied TiO2. OH·radicals formed by the reaction of the photo-excited polyoxometallates with H2O, appear to play a key role in the process. In this study, the photocatalytic activity of POM have been tested on some diversified pesticides of interest such as lindane, bentazone and fenitrothion to determine the feasibility of decontamination of aqueous solutions from these pollutants. The results show that aqueous solutions containing the above pesticides, upon photolysis in the presence of polyoxotungstates undergo effective degradation to CO2, H2O and the corresponding inorganic anions. The intermediates identified give rise to complicated degradation mechanisms prior to mineralization. The characteristic reactions that take place consist of hydroxylation, H-abstraction, dehalogenation, denitration, desulfurization, breaking of the C-C bond etc.


INTRODUCTION
The phocatalytic mineralization of several organic pollutants, i.e., decomposition to CO 2 , H 2 O and inorganic ions has been demonstrated using polyoxometallates (POM) [1][2][3]. These are acid condensation products, mainly of molybdenum and tungsten [4][5][6], that upon excitation with near visible and UV light become powerful oxidizing reagents capable of destroying a great variety of pollutants in the aquatic environment. OH · radicals generated by reaction of POM with H 2 O seem to play a key role in the process [7].
POM are at least as effective as the widely published TiO 2 . Dioxygen is important in that it oxidizes (regenerates) the catalyst and through reductive activation may or may not participate further in the process, depending on the substrate [8].
Lindane is a typical organochlorine insecticide with a remarkable stability in the environment [9]. Although it has been replaced by more polar or photodegradable pesticides, it seems that will be used for many years to come in the developing countries. Bentazone belongs to acidic herbicides that are of great interest within the European Union since they are extensively used in agriculture. Due to their chemical and physical properties they do not attach to soil particles and as a result they leach and may pose problems for resources such as drinking water [10]. Fenitrothion is an insecticide with a typical OP stucture. Like other organophosphorous pesticides fenitrothion is known for its acute toxicity * Part of this work was presented in the 2nd Mediterranean Meeting on Photochemistry, Sicily Italy, 28th-2nd July 2003. towards nontarget organisms [11]. The chemical structure of these three pesticides are given in Scheme 1.
In this work, we report the complete photocatalytic degradation of the above diversified pesticides, i.e., lindane, bentazone, and fenitrothion, in the presence of POM, by monitoring the final products of the process. We also report on the intermediates involved in the photocatalytic degradation process of fenitrothion, as identified by HPLC-DAD and GC-MS. The three pesticides were chosen as model substrates because they were extensively used in agriculture, belong to the most usual pesticide families and they have very different chemical structures. Lindane [18]. Extra pure dioxygen and nitrogen were used for oxygenation and evaporation of solutions.

Instrumentation.
Photolysis experiments were performed with a laboratory constructed "illumination box" equipped with four F15W/T8 black light tubes (Sylvania). The maximum emission of these tubes is around 375 nm, emitting 71.7 µW cm −2 at a distance of 25 cm.
HPLC analysis was carried out using an HPLC apparatus consisting of a Waters model TM 600 gradient pump, a Waters Model 996 photodiode array detector (DAD) and a Waters 717 Autosampler controlled by the Millenium (Waters) software. The mobile phase composition for the determination of fenitrothion was acetonitrile-water (50:50 v/v) and the column used was a Novapak C 18 (4.6 × 250 mm, i.d. 4 µm) cartidge column. In the case of bentazone the mobile phase consisted of a gradient eluent 15% A (ACN/methanol = 1/1 + 0.001% CH 3 COOH) − 85% B (5 mM KH 2 PO 4 + 0.001% CH 3 COOH) at 0 min, 50% A at 16 min, 55% A at 20 min, 15% A at 21 min, at a flow rate of 0.35 ml/min. GC analysis was carried out using a Hewlett Packard Model 5890 gas chromatograph equipped with ECD, split-splitless injection port and a DB-l fused silica capillary column by J&W Scientific Inc. (30 m × 0.32 mm l.D., 0.25 µm film thickness). The temperature program applied in GC-ECD, for the determination of lindane, was as follows: 80 • C for 1 min, 80 • C-218 • C at 8 • C/min, 218 • C for 18 min, 218 • C-250 • C at 4 • C/min and 250 • C for 10 min. Detector's temperature was held at 300 • C. The injection was carried out splitless at 250 • C and the injection volume was 1 µl.
Identification of intermediates were performed using an Agilent 6890 Series gas chromatograph equipped with an HP-5 MS capillary column (30 m × 0.25 mm i.d., 0.25 mm film thickness), interfaced to an Agilent 5973 mass selective detector. Data acquisition, processing and instrument control were performed by the Agilent MSD Chem-Station software. The flow rate was 1.0 ml/min. The temperature programme applied in GC-MS was as follows: 50 • C for 4 min, 5 • C/min to 150 • C for 1 min, 5 • C/min to 280 • C for 10 min. Detector's temperature was held at 280 • C. The injection was carried out splitless at 250 • C and the injection volume was 2.0 µl.
The CO 2 formation was followed by head space gas chromatographic analysis of the gas phase of pyrex cells by GC/MS. The temperature programme was: 40 • C for 1 min, 4.2 • C/min to 50 • C, 16.7 • C/min to 80 • C for 1.2 min. The injector temperature was 210 • C. The injection was carried out splitless in full scan mode. 100 µl were sampled from the headspace and injected for analysis. Extracted chromatograms on ion 44 were used for the detection and quantitation of the CO 2 peak.
For the monitoring of inorganic ions an Ionic Chromatograph of Dionex DX-300 which consists of a pulsed electrochemical detector PED-2 dionex and an IonPack AG 14(4 × 250 mm) analytical column was used. The eluent was 3.5 mM sodium carbonate, 1.0 mM sodium bicarbonate, flow rate 1.2 ml/min, injection volume 25 µl. Samples of 5 ml of the above fortified solutions were added to a glass cell. After 20 min of deaeration or oxygenation, the cell was covered airtightly with a cap. Photolysis was performed at ambient temperature in the photolysis apparatus. The solution was magnetically stirred throughout the experiment. This process takes place in the presence and absence of dioxygen but no photodegradation of lindane takes place in the absence of catalyst under these conditions. The complete photodegradation of lindane leads to its mineralization with CO 2 and Cl − as final products (Figure 1). The induction period involved prior to evolution of CO 2 indicates the formation of intermediate products.

Photocatalytic degradation of lindane in
The main reactions occurred is dechlorination of the substrate followed by C-C bond cleavage, as resulted from the kind of the final products.

Photocatalytic degradation of bentazone in the presence of POM (PW 12 O 40 3− and SiW 12 O 40 4− ).
Effective photodecomposition of bentazone occurs in the presence of POM (PW 12  In the absence of dioxygen, no regeneration (reoxidation) of the catalyst takes place, at least in the first stages of photolysis. Thus the accumulation of the blue colour of the one-electron reduced POM, with photolysis time, could be followed. Photoreduction of catalyst results in concomitant oxidation of bentazone. Figure 2 shows the formation of the reduced catalyst (PW 12  ( Figure 3). As evidenced by others for TiO 2 photocatalysis [19] this is due to ammonium ions also present in variable amounts in most cases or to the existence of other nitrogen-containing species, such as hydroxylamine. Nitrite ions could not detected in our system.    [17]).

Photocatalytic degradation of fenitrothion in the presence of
The total mineralization of fenitrothion, i.e., formation of CO 2 and inorganic ions (PO 4 3− , occurred within a few hours ( Figure 5). A satisfactory mass balance was found among the quantities of fenitrothion consumed and CO 2 , PO 4 3− , SO4 2− , and NO 3 − evolved. The final, almost total, recovery of nitrate anions took place through the formation of nitrites that afterwards were oxidized to nitrates. The main reactions were denitration and desulfurization followed by C-C bond cleavage, in accordance with the photocatalytic degradation of the other target pesticides.

Intermediates in the photodecomposition of fenitrothion.
A detail study of the photodegradation of fenitrothion (I) reveals the formation and decay of several intermediates, prior to the final decomposition to CO 2 and inorganic ions (Scheme 2). Five of these photodegradation intermediates, i.e., fenitrooxon (II), 3-methyl-4-nitrophenol (IV), 3-methyl-4nitroanisole (V), 2-methyl-hydroquinone (VI), and 2methyl-1,4-benzoquinone (VII), were identified with HPLC-DAD by comparing their retention times and spectral data (obtained by DAD) with those of commercially available standards. The presence of II, IV, V, and VII was confirmed with GC-MS/EI by comparing their mass spectra with those of commercially available standards. Two others intermediates, formylfenitrothion (III), and O,O,S-trimethyl phosphorothioate (VIII) were identified with GC-MS/EI by comparing their mass spectra with those reported in the literature.
The formation of several intermediates prior to mineralization suggests a rather complicated mechanism of decomposition, where OH · radicals seem to play an important role. This conclusion comes from the verification of OH addition (intermediates IV and VI) and H abstraction (intermediate III) which are typical processes for OH · radicals.

CONCLUSIONS
The photocatalytic degradation of diversified pesticides can been achieved in the presence of polyoxometallates, as photocatalysts, upon illumination in the UV/Vis light. The decomposition process is not selective driving to mineralization all target substrates, i.e., production of CO 2 , H 2 O and the corresponding inorganic anions. The induction period prior to CO 2 evolution, indicates the formation of intermediates, through a rather complicated mechanism. Many of these intermediates were identified in details in the case of fenitrothion. The characteristic reactions that take place in all studied cases consist of hydroxylation, H-abstraction, dehalogenetion, denitration, desulfurization, and breaking of the C-C bond, as resulted from the intermediates and final products.