Ultrasonic Induced Decomposition of Methidathion Pesticide

RESULTS AND DISCUSSION

Effect of ultrasonic double amplitude: Typical variation of the aqueous concentration, C/C₀ of methidathion. Where C₀ is the initial concentration of methidathion and C is the concentration of methidathion at time t. The degradation of methidathion was more effective at greater amplitude of ultrasound. There was 5% degradation ratio with amplitude 72 μm of ultrasound, but increased to 10% with ultrasonic amplitude 120 μm (Fig. 1). Both were better than control (without sonication), which has only 2% degradation. In the literature no author assessed the effect of ultrasonic amplitude on the decomposition of hazardous materials. The sonicator (Micron XL 2020) of the type used in this research has a constant frequency output and the power of the ultrasound is set with the ultrasound amplitude. Greater ultrasound amplitude means greater energy supply so as to cause cavitation bubbles at a higher temperature. In relation to power our results resembled to those in which the reaction rate increased with greater ultrasonic power (Pandey et al., 1980). H₂O₂ generated in the ultrasonic process, measured by titration, indirectly caused the OH radical to be produced. However, the concentration was much small about 120 mg L^-1. Possibly the frequency is varied or the power supplied is insufficient. The limited H₂O₂ generated might be the reason for the small degradation efficiency.

Effect of H₂O₂ addition: Ultrasound alone poorly decomposes omethoate, so H₂O₂ is added initially to enhance the ultrasonic process. This combination resulted in significant degradation efficiency. The concentration of H₂O₂ increased the decomposition efficiency of omethoate. The more H₂O₂ was added the greater the removal of omethoate (Fig. 2).

After reaction for 60 min, the improvement in methidathion decomposition with H₂O₂ (500 mg L^-1) was as great as 57% over the control, i.e., without H₂O₂. The results revealed that there were more OH radicals generated on addition of H₂O₂, which was effective for degradation.

Fig. 1:	The effect of variable ultrasonic waves amplitudes on the degradation of methidathion at 20 kHz

Fig. 2:	Effect of dose of H2O2 on the decomposition of methidathion

However there was no significance difference between concentrations 200 and 500 mg L^-1 of H₂O₂, with regard to decomposition of methidathion (45 and 48%). Possibly the concentration of OH radicals was saturated; further H₂O₂ failed to increase the OH radicals, so was useless to reaction. Thus a significant dosage of H₂O₂ was found to be 200 mg L^-1 according to this experiment.

Effect of pH: The effect of pH on the decomposition of methidathion vs. time (Fig. 3), which indicates that a smaller pH value is more effective.

The decomposition of methidathionattained 99% at pH 3, 85% at pH 5 and 62% at pH 7. For samples controlled at pH 9 and 11, the extents of methidathion decomposition were only 22 and 15%. The obvious variation among them arises because the pk_a value of methidathionis 8.49 at 25°C, but in a molecular form. The fraction in the molecular state of methidathion was larger when the pH was smaller.

Fig. 3:	Effect of pH on the ultrasound/H2O2 process at amplitude 120 μm and H2O2 (200 mg L-1)

The methidathion ions cannot vaporizes into the cavitation bubbles; they can react only outside the bubble film with OH radicals cleaved from water. However, in the molecular state methidathioncan vaporize into cavitation bubbles and react both inside by thermal cleavage and outside with OH radicals as an oxidation reaction (Hua and Thompson, 2000). Hence the smaller pH (especially less than the pk_a) was effective for decomposition. There might exist other reasons such as that OH radical can easily contact with molecular omethoate. The state of methidathion had a large influence on the degradation efficiency of the ultrasound/H₂O₂ process.

Kinetics: In analyzing the experimental results, decomposition of methidathion appeared to follow first-order kinetics. The predicted decomposition of methidathion is compared with the observed data (Fig. 3) for various pHs. The squared correlation coefficients, R², of the model exceeded 0.9 at various pHs. The rate of coefficients increased with decreased pH. We also used logarithmic regression with rate coefficients and pH and obtained a close relationship (Fig. 4). The relationship between pH and k is:

k = 0.0422 exp (-0.476 (pH))

Effect of ultrasonic frequency on decomposition: Exposure of methidathion solution of different concentrations to ultrasound at 20 and 100 kHz shows a higher rate of decomposition for the higher frequency (Fig. 5).

The rates of degradation have been determined when the proportion of degraded methidathion does not surpass 46% at 20 kHz (Fig. 2). This could be attributed to the competition between methidathion and products of the reaction.

Fig. 4:	Relationship between pH and k on the ultrasound/H2O2 process

Fig. 5:	Effect of ultrasonic frequency on the decomposition of methidathion

However at 100 kHz, the degradation has increased to more than 74% in the same period of time.

OH^● production and methidathion decomposition: The physical phenomena leading to sonochemical effects in water are complex and not fully elucidated. Nevertheless, whatever the theoretical model describing the origin of the molecular activation (thermal and/or electrical), the place where the molecules are brought to an excited state and dissociate is the interior of the bubble of cavitation, which is filled with gas and vapor. In the cleavage of water saturated with air, the first step appears to be the cleavage of water and the dioxygen molecule (Misik et al., 1995).

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Inside the bubble or in the liquid shell surrounding the cavity, these radicals can combine in various ways or react with gases and vapor present, leading to the detection in the medium of HNO₃, HNO₂ and H₂O₂.

Fig. 6:	Concentration of H2O2 produced as a function of insonation time in aerated water

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The main fraction of the H₂O₂ formed during water sonolysis seem to come from the OH^● and ^●OOH radicals, which combine in the bubble or in the layer surrounding the bubble of cavitations in the absence of substrate (Okouchi et al., 1992) (reactions 10, 11). The amount of H₂O₂ produced at each of the two frequencies was determined (Fig. 6). The concentration of hydrogen peroxide increases linearly vs. time and the rate of formation were found higher at 100 kHz than at 20 kHz. With these results in hand H₂O₂ production was examined in the presence of various concentrations of methidathion at 100 kHz. There is a close relationship between methidathion degradation and H₂O₂ production (Fig. 7). The hydrogen peroxide yield decreases when the methidathion concentration increases. There is always H₂O₂ formation, this may be due to the fact that part of the hydrogen peroxide originates through reaction 11 is consumed by methidathion added.

Fig. 7:	Methidathion degradation and rate of hydrogen per oxide formation at different frequencies

The sonochemical methidathion degradation which proceeds more rapidly at high than at low frequency can be related to a better release of OH^● in the solution in the former case. The basis of this discrepancy is not yet well understood, but there must be some differences between the behavior of a 20 kHz and a 100 kHz bubble. The more important cavitational effects occur when the frequency of the ultrasonic wave is equal to the resonance frequency of bubble. The duration of collapse is shorter at 100 kHz than at 20 kHz. In such conditions, at high frequency, ^●OH radicals could be ejected more efficiently in the solution before they have time to combine in the bubble of cavitation (Christen and Anne, 1997).

^●OH localization in an ultrasonic field:
There are several reports in the literature, which demonstrate that the sonochemical yields are related to the configuration of the reactor. For instance, iodide oxidation and carbon tetrachloride degradation were reported to proceed faster when a standing wave system is created in the column of liquid above the emitting surface (Sun et al., 2007; Hua and Thompson, 2000). The difference between high and low-frequency ultrasonic field was also observed. The high-frequency wave generates a directive acoustic beam which induces convection currents and foundation effects at the surface of the liquid.

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The acoustic field spread at low frequency, it was then important to determine where the site of the sonochemical reaction in the 20 and 100 kHz reactors were located. Since the ^●OH radical is the reagent involved in the sonochemical methidathion degradation, the place where it is produced was studied with the help of sonochemically induced luminescence of luminol. In basic medium, luminol reacts with hydroxyl radicals, giving aminophalate anion and a blue fluorescence (Eq. 1) (Koteonarou et al., 1996).

The two very different patterns are observed with this method for the two frequencies (Koteonarou et al., 1996). In the two cases, the surface of the solution is heavily disturbed, which prevents the standing waves in the medium. At 20 kHz the luminescence is located near the surface of the transducer. At 100 kHz, most of the fluorescence originates from a wider volume close to the surface of the liquid, without any light near the surface of the emitter. It can be noted that modifications on the height of the liquid do not change the location of the luminescence zone or the yield of methidathion degradation.

CONCLUSIONS

Ultrasound/H₂O₂ process gave better decomposition than ultrasound alone, although the decomposition of methidathion was effective with ultrasound at greater amplitude. When the pH was controlled bellow the pk_a value of omethoate, significantly better decomposition resulted than for greater pH. The decomposition of methidathion at 200 mg L^-1 was 88% at about amplitude 120 μm with H₂O₂ and at pH 3, after reaction for 90 min. The decomposition of methidathion appeared to follow first-order reaction kinetics. The reaction rate constant (k) was greater at smaller pH and there is an exponential relation between them. The experimental data of this work demonstrated that the ultrasonic frequency has an important effect on the yield of the sonochemical degradation of methidathion. Because of the phenomenon, it appears difficult to provide a complete interpretation of the experimental data. For economic consideration and from a fundamental point of view, studies of frequency effects seem to be of interest.

Significant findings of the research

This research has the capacity to launch it in industries as a treatment technique for effluent treatment.