**3. Results**

#### **3.1. Preliminary studies: control experiments**

Two control experiments were performed under the same experimental conditions that were employed for photolysis experiments which are mentioned in detail in Section 2.2. More specifically, in order to estimate the thermal (dark) reactions between the solute and TiO<sup>2</sup> , a first set of experiments were carried out over the same periods as those used in the photolysis experiments, but in the dark (bottles were covered with aluminum foil for the protection against light interference). The second set of experiments involved the irradiation of the pesticide aquatic solutions without the presence of catalyst to account any direct photolysis of the studied substances.

*3.1.2. Direct photolysis: photodegradation in the absence of TiO2*

absence of the catalyst TiO<sup>2</sup>

decomposition of these substances.

**3.2. Photocatalytic degradation of OPPs in UV-TiO2**

decomposition efficiencies of OPPs in UV-TiO<sup>2</sup>

methyl > fenthion > dimethoate. Semiconductor TiO<sup>2</sup>

methyl was achieved after 24 h of light exposure.

The photolytic decomposition of the tested pesticides in the presence of UV light and in the

methyl are illustrated in **Figure 1** (curve B). Direct photolysis of 10 mg L−1 of studied OPPs under illumination of 365 nm did not decrease significantly pesticides' original concentrations. Obviously, under these experimental conditions and at the end of irradiation, the observed disappearance of the OPP compounds occurred at very slow rates. According to the acquired results (not shown for azinphos ethyl, dimethoate, disulfoton, and fenthion), reduction in pesticides' initial concentration varied from 6.03 (for azinphos ethyl) to 10.68% (for dimethoate), depending on the physicochemical properties of the studied OPP individually. Moreover, these results are in agreement with TOC changes of initial TOC during direct

These results are conforming to other published data according to which direct photolysis is not expected to be an important process in water for several organophosphates, because their molecules do not absorb UV light at wavelengths greater than 290 nm, despite the fact that the most important wavelengths for the photolytic degradation of the majority of the organic pesticides are 280 and 320 nm [17, 19]. It should also be mentioned that the photocatalytic deterioration of numerous cases of organophosphates in the absence of several catalysts has been studied from researchers, such as fenitrothion [20], dimethoate [17], ethyl parathion, methyl parathion, ethyl bromophos, methyl bromophos, and dichlofenthion [13]. In all these data available in the literature, photolytic process was slower compared to photocatalytic

**Figure 2** depicts the photodecomposition of the compounds studied in the presence of the semiconducting catalyst under UV illumination. It is clear that all investigated organophos-

It is well established in the bibliography that the rates of the photocatalytic reaction depend on several experimental parameters among which included initial concentration of illuminated solute reactant, radiant flux, wavelength, type and mass of catalyst, type of photoreactor, pH, and temperature. As a consequence, only the comparison between data measured for

Obtained experimental results demonstrated that under the employed set of conditions the

ture of tested compounds and decreased in the order: disulfoton > azinphos ethyl > azinphos

generate highly reactive oxidizing agents caused the total decomposition (100%) of disulfoton after 12 h of illumination, whereas complete disappearance of azinphos ethyl and azinphos

phorus insecticides were sufficiently degraded in aqueous titanium dioxide (TiO<sup>2</sup>

sions (100 mg L−1) under illumination of UV light with wavelength of 365 nm.

a given set of experimental conditions is meaningful and valuable.

 **system**

system depended on the nature and the struc-

working as a catalyst with UV light to

) suspen-

photolysis tests conducted in the current study (presented in Section 3.4.1.).

was investigated. Experimental results for the case of azinphos

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Photocatalytic Degradation of Selected Organophosphorus Pesticides Using Titanium Dioxide…

#### *3.1.1. Adsorption in the dark*

As presented in a dotted line of **Figure 1** (curve A), the addition of the catalyst without UV radiation had a negligible effect on initial concentration of target analyte azinphos methyl (10 mg L−1). The same trend was followed for the other four tested compounds (data not shown), and these observations suggest that within 48 h, which was the total duration of the experiments conducted for all of the examined substances, no obvious degradation in dark reaction occurred. Therefore, it can be concluded that negligible adsorbance of the compounds on the catalyst's surface took place and that hydrolytic processes during the experimental course can be neglected. Similar results have been previously published by several other authors [17, 18]. For example, Evgenidou and her co-workers reported that the addition of two metal oxide catalysts, TiO<sup>2</sup> and ZnO, without UV radiation had a negligible effect on dimethoate's photooxidation rate [17].

**Figure 1.** Disappearance of target pesticide azinphos-methyl (10 mg L−1) in control as function of time. (Curve A) In the presence of the catalyst TiO<sup>2</sup> (100 mg L−1) and in the absence of light (in the dark); (Curve B) in the absence of the catalyst TiO<sup>2</sup> and in the presence of light (λUV = 365 nm).
