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The insecticides are widely used for the protection of crops from harmful insects. However, there is a negative impact for their use on the environment, beside their effects on human health. Insecticides are hazardous chemical compounds, because they are carcenogenous, and bioaccumulative. Hence, there presence in the food-chain is indeed a major risk to human health. Many studies were carried out in trying to minimize dangerous chemical substances, such as chlorophenols and dyes using catalysts in an aqueous environment. Phenol derivatives are involved in the synthesis of many chemical compounds and are frequently found as pollutants of natural waters. Photochemical transformation can be useful to eliminate phenolic compounds. The photo catalytic transformation of a phenyl-urea herbicide was investigated using tio2 and zno. Many researches were carried out in trying to find ways to photo degrade organic pollutants.
The photo catalytic degradation of insecticide methomyl in water, using TiO2 and ZnO (Merck), under UV (366 nm) was studied. The influence of the catalyst concentration and pH was investigated. The optimal concentration of the catalyst was found to be 2.0 g/l. It was found that ZnO is a better catalyst than TiO2 under the same reaction conditions. Also, the influence of NaCl was studied. The presence of Clâˆ' significantly affects the photo degradation of the pollutant.
Methomyl is a broad spectrum insecticide. It is a very toxic and hazardous compound and a pollutant of environmental concern because of its high solubility in water (57,9 g/l at 25 °C) . Since the sorption affinity of methomyl to soils is rather low, it can easily cause contamination of both ground and surface water sources.
IUPAC name: S-methyl N-(methylcarbamoyloxy) thioacetimidate.
Although biological processes are often most economical for a contaminated site cleanup and an industrial waste treatment, the advanced oxidation processes (AOPs) provide an effective means of rapidly treated compounds with the efficient process control . Methomyl has also been photodegradated by using AOP. A different catalyst has been used, mostly TiO2 [4-8]. Photo-Fenton reaction was also employed.
The aim of the present work is to study the photo catalytic degradation of insecticide methomyl in water using TiO2 and ZnO (Merck) under UV light (366 nm).The influence of the catalyst concentration, pH and NaCl concentration was studied.
Hydrochloric acid, sodium chloride and sodium hydroxide (all p.a.) were obtained commercially.TiO2, Merck Eusolex® T (anatase modification), and ZnO (Merck) were used as received. Methomyl was obtained from DuPont (analytical grade, 99.8 %). Deionised water was obtained from a Milipore Waters Milli Q purification unit. Irradiations under UV light (366 nm) were performed in an open flask made of a quartz glass (20 ml volume) with the UV lamp (2x8 W) placed 5 cm from the surface of the reaction mixture.
The photo degradation of methomyl was studied by preparing a solution containing 16.4 mg/l of methomyl and a certain amount of the catalyst. In a typical experiment, 15 ml of the solution was used. Then, the lamp was switched on and during the irradiation the agitation was applied, and after an appropriate time of irradiation the suspension was sampled. The reaction mixtures were kept at 20 °C. The concentration of methomyl was determined after the centrifugation of a sample by UV-Vis spectrophotometer at λmax = 234 nm, and by HPLC. The characteristics of the HPLC instrument are as follows: HPLC Instrument GBC, a pump LC 1120, a UV-Vis detector LC 1205, a manual injector RHEODYNE 7725i, a column ZORBAX Eclipse XDB-C8 (4.6×150 mm, 5μm), a mobile phase acetonitrile: water (25:75, flow rate 1.0 cm3 min-1), wavelength 234 nm. pH of the samples was adjusted by adding a dilute NaOH and HCl and measured using a pH meter (PHM93 reference pH meter, Radiometer.
Result and discussion:-
UV-Vis spectra changes
The changes in the absorption spectra of the methomyl solution during the photocatalytic degradation at different irradiation times are presented in Fig 2. The insecticide shows a band with a maximum absorption at 234 nm. The decrease of the absorption peak actually indicates a rapid degradation of the insecticide.
Fig2. UV-Vis spectra changes of methomyl (16.4 mg/l) inaqueous TiO2 dispersion (concentration of TiO2: 1 g/l) irradiated with UV lamp (366 nm).
The effect of the catalyst concentration
The effect of the catalyst concentration (TiO2) on the photo degradation efficiency was shown in Figure3. The photo degradation efficiency increased with increased concentration of the photo catalyst, reached
the highest value at 2.0 g/l and than decreased. The possible explanation for this phenomenon is the fact that when all insecticide molecules are adsorbed on TiO2, the addition of higher quantities of TiO2 would have no effect on the photo degradation efficiency. Negligible degradation effect was observed if catalyst or irradiation were applied separately.
Figure 3. The effect of the concentration of TiO2 on the photodegradation
efficiency (X = (C0-C)/C0) of methomyl at irradiation
time of 6.0 h (concentration of methomyl: 16.4 mg/l).
The effect of pH
It is well known that pH value has an influence on the rate of degradation of some organic compounds in photo catalytic processes [10,11]. The photo degradation of methomyl was studied at three different pH values (3.5, 5.6 and 9.0). The pH was adjusted the addition of HCl (3.5) or NaOH (9.0). The third value is the pH of the pure insecticide solution in deionised water. The obtained results (Figure 4) imply that the photo degradation rate is highest in the acidic solution and lowest in the alkaline solution.
Figure 4. The effect of pH on the photodegradation rate of methomyl (insecticide concentration: 16.4 mg/l; catalyst concentration: 2.0 g/l of TiO2).
The effect of NaCl
A common inorganic ion, chloride ion, was employed as sodium chloride to study the photodegradation rate of methomyl. The influence of different concentrations of sodium chloride (0-5 % w/v) on the pho to degradation rate of methomyl is presented in Figure 5. The decrease of the photodegradation of methomyl in the presence of chloride ions is due to the hole scavenging properties of chloride ions .
Figure 5. The effect of NaCl concentration on the photodegradation
rate of methomyl (insecticide concentration: 16.4 mg/l,
catalyst concentration: 2.0 g/l of TiO2).
The effect of the catalyst type
ZnO is also frequently used in AOP. The biggest advantage of ZnO in comparison to TiO2 is that it absorbs over a larger fraction of UV spectrum. On the other hand, ZnO exhibits a tendency to dissolve and photo decompose . The obtained results (Figure 6) showed that the photodegradation was much faster when ZnO was used in comparison to the reaction with TiO2.
Figure 6. The effect of the catalyst type on the photodegradation rate of methomyl (insecticide concentration: 16.4 mg/l, catalyst concentration: 2.0 g/l).
The obtained results of the photodegradation of methomyl by UV light (366 nm) indicated that the photodegradation was affected by the initial catalyst concentration, pH value and the type of catalyst. The presence of NaCl led to the inhibition of the photo degradation process.