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

**3. Results**

*3.1.1. Adsorption in the dark*

of two metal oxide catalysts, TiO<sup>2</sup>

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

and in the presence of light (λUV = 365 nm).

TiO<sup>2</sup>

dimethoate's photooxidation rate [17].

**3.1. Preliminary studies: control experiments**

246 Titanium Dioxide - Material for a Sustainable Environment

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 specifi-

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.

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

**Figure 1.** Disappearance of target pesticide azinphos-methyl (10 mg L−1) in control as function of time. (Curve A) In the

(100 mg L−1) and in the absence of light (in the dark); (Curve B) in the absence of the catalyst

and ZnO, without UV radiation had a negligible effect on

, a first set of

cally, in order to estimate the thermal (dark) reactions between the solute and TiO<sup>2</sup>

The photolytic decomposition of the tested pesticides in the presence of UV light and in the absence of the catalyst TiO<sup>2</sup> was investigated. Experimental results for the case of azinphos 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 photolysis tests conducted in the current study (presented in Section 3.4.1.).

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 decomposition of these substances.

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

**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 organophosphorus insecticides were sufficiently degraded in aqueous titanium dioxide (TiO<sup>2</sup> ) suspensions (100 mg L−1) under illumination of UV light with wavelength of 365 nm.

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 a given set of experimental conditions is meaningful and valuable.

Obtained experimental results demonstrated that under the employed set of conditions the decomposition efficiencies of OPPs in UV-TiO<sup>2</sup> system depended on the nature and the structure of tested compounds and decreased in the order: disulfoton > azinphos ethyl > azinphos methyl > fenthion > dimethoate. Semiconductor TiO<sup>2</sup> working as a catalyst with UV light to 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 methyl was achieved after 24 h of light exposure.

0 to 500 mg L−1 were conducted, whereas the initial concentration of the pesticide tested was maintained the same, 10 mg L−1. The reaction rate (*r*) of the photocatalytic reactions was expressed as the change (decrease) in concentration of photolyzed pesticide reactants divided by the time interval during which this change was observed. Values of *r* were cal-

Photocatalytic Degradation of Selected Organophosphorus Pesticides Using Titanium Dioxide…

where *r* is the reaction rate of photolysis (in mg L−1 h−1), *C* is the level of concentration of the degraded reactant (in mg L−1), and *t* is the illumination time (in h). The initial reaction rates as

Obviously, as shown in **Figure 3**, the initial rate of photocatalytic degradation of all stud-

level off. These trends and observations correspond with those of other researchers who have reported that this may be due to different contributions of distinct homogenous and

concentration for all cases investigated are depicted in **Figure 3**.

<sup>d</sup>*<sup>t</sup>* (1)

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249

concentrations [17, 24–26]. More

concentration was increased from 0 to 10 mg L−1 and

, only the homogeneous photochemical reduction was

concentration was increased to 100 mg L−1, where it seemed to

concentration on initial rate of photocatalytic degradation of studied OPPs (pesticide's

, 6.06; λUV, 365 nm, in the presence of methanol).

culated by Eq. 1:

a function of TiO<sup>2</sup>

**Figure 3.** Effect of TiO<sup>2</sup>

concentration, 10 mg L−1; catalyst's concentration, 0–500 mg L−1; pH<sup>o</sup>

ied analytes decreased when TiO<sup>2</sup>

specifically, in the absence of TiO<sup>2</sup>

then increased as the TiO<sup>2</sup>

*r* = −\_\_\_\_\_\_\_\_\_\_\_\_\_\_ <sup>d</sup> *<sup>C</sup>*

heterogeneous photochemical reactions at different TiO<sup>2</sup>

**Figure 2.** Photodegradation rates of studied OPPs in aqueous suspensions of the catalyst TiO<sup>2</sup> under UV light (pesticide's concentration, 10 mg L−1; catalyst's concentration, 100 mg L−1; pH<sup>o</sup> , 6.06; λUV, 365 nm, in the presence of methanol).

On the contrary, fenthion and dimethoate proved to be very resistant to photocatalytic reduction, and even after 48 h of light exposure, 95 and 60% of their original concentration decomposed, respectively. That is the main reason that those two substances were selected for the study of photodegradation in the UV-TiO<sup>2</sup> -H<sup>2</sup> O2 oxidation systems.

In general, the comparison of obtained results with other experimental data previously reported for other organic pesticide micropollutants shows that degradation rates estimated in the present survey are lower. This phenomenon can be ascribed to differences in the laboratory conditions used by other researchers. For instance, Echavia and co-workers [21] reported a complete (100%) decomposition of dimethoate within 60 min of irradiation, using TiO<sup>2</sup> immobilized on silica gel and UV light emitting mostly at 365 nm with light intensity of 1.4 mW cm−2. Evgenidou and co-workers reported 65% reduction of dimethoate after 10 min of irradiation, using UV light with λ > 290 nm in the presence of TiO<sup>2</sup> 100 mg L−1 [17].

#### *3.2.1. Effect of the catalyst's concentration on photocatalytic degradation rate*

The reaction rate of heterogeneous photodegradation as a function of the catalyst's concentration is very important and has been investigated in several other cases reported in the literature [5, 10, 22, 23]. Hence, the purpose of these experiments was to determine the optimum TiO<sup>2</sup> concentrations for subsequent experiments and to investigate how the TiO<sup>2</sup> concentration affected the photocatalytic decomposition pathway of selected toxicants. Therefore, photolytic procedures employing different concentrations of TiO<sup>2</sup> ranging from 0 to 500 mg L−1 were conducted, whereas the initial concentration of the pesticide tested was maintained the same, 10 mg L−1. The reaction rate (*r*) of the photocatalytic reactions was expressed as the change (decrease) in concentration of photolyzed pesticide reactants divided by the time interval during which this change was observed. Values of *r* were calculated by Eq. 1:

$$r = -\frac{d\,C}{dt} \tag{1}$$

where *r* is the reaction rate of photolysis (in mg L−1 h−1), *C* is the level of concentration of the degraded reactant (in mg L−1), and *t* is the illumination time (in h). The initial reaction rates as a function of TiO<sup>2</sup> concentration for all cases investigated are depicted in **Figure 3**.

Obviously, as shown in **Figure 3**, the initial rate of photocatalytic degradation of all studied analytes decreased when TiO<sup>2</sup> concentration was increased from 0 to 10 mg L−1 and then increased as the TiO<sup>2</sup> concentration was increased to 100 mg L−1, where it seemed to level off. These trends and observations correspond with those of other researchers who have reported that this may be due to different contributions of distinct homogenous and heterogeneous photochemical reactions at different TiO<sup>2</sup> concentrations [17, 24–26]. More specifically, in the absence of TiO<sup>2</sup> , only the homogeneous photochemical reduction was

On the contrary, fenthion and dimethoate proved to be very resistant to photocatalytic reduction, and even after 48 h of light exposure, 95 and 60% of their original concentration decomposed, respectively. That is the main reason that those two substances were selected for the study of

 oxidation systems. In general, the comparison of obtained results with other experimental data previously reported for other organic pesticide micropollutants shows that degradation rates estimated in the present survey are lower. This phenomenon can be ascribed to differences in the laboratory conditions used by other researchers. For instance, Echavia and co-workers [21] reported a complete (100%) decomposition of dimethoate within 60 min of irradiation,

intensity of 1.4 mW cm−2. Evgenidou and co-workers reported 65% reduction of dimethoate after 10 min of irradiation, using UV light with λ > 290 nm in the presence of TiO<sup>2</sup>

The reaction rate of heterogeneous photodegradation as a function of the catalyst's concentration is very important and has been investigated in several other cases reported in the literature [5, 10, 22, 23]. Hence, the purpose of these experiments was to determine the

concentration affected the photocatalytic decomposition pathway of selected toxicants.

Therefore, photolytic procedures employing different concentrations of TiO<sup>2</sup>

immobilized on silica gel and UV light emitting mostly at 365 nm with light

concentrations for subsequent experiments and to investigate how the TiO<sup>2</sup>

ranging from

under UV light (pesticide's

, 6.06; λUV, 365 nm, in the presence of methanol).


**Figure 2.** Photodegradation rates of studied OPPs in aqueous suspensions of the catalyst TiO<sup>2</sup>

concentration, 10 mg L−1; catalyst's concentration, 100 mg L−1; pH<sup>o</sup>

248 Titanium Dioxide - Material for a Sustainable Environment

*3.2.1. Effect of the catalyst's concentration on photocatalytic degradation rate*

photodegradation in the UV-TiO<sup>2</sup>

using TiO<sup>2</sup>

100 mg L−1 [17].

optimum TiO<sup>2</sup>

**Figure 3.** Effect of TiO<sup>2</sup> concentration on initial rate of photocatalytic degradation of studied OPPs (pesticide's concentration, 10 mg L−1; catalyst's concentration, 0–500 mg L−1; pH<sup>o</sup> , 6.06; λUV, 365 nm, in the presence of methanol).

possible. The downward trend in initial rates of pesticide's photocatalytic decomposition that is shown in **Figure 3**, when only a small amount of TiO<sup>2</sup> (<10 mg L−1) was present, can be explained by the fact that the photocatalyst acted mainly to absorb and/or scatter UV light, thus inhibiting the homogenous reaction but not yet causing a significant heterogeneous reaction of photodegradation. However, at higher TiO<sup>2</sup> concentrations (>10 mg L−1), an increase in the rate of the process was observed as the heterogeneous reaction increased in importance. Actually, from the obtained data, it became obvious that in this range of concentrations the reaction rate is directly proportional to the mass of the photocatalyst. This can be explained on the basis that with the increase in catalyst dosage, the total active surface area increases; hence, availability of more active sites on catalyst surface increases as well. However, above a certain value, the reaction rate leveled off and became independent of the TiO<sup>2</sup> concentration.

The eventual leveling off of the initial rates of photocatalytic deterioration (**Figure 3**) could be explained by the fact that suspended particles of TiO<sup>2</sup> were present at a high-enough concentration to block UV light passage to the interior parts of the reactor, increased the light scattering, and made the homogeneous reaction insignificant. Moreover, other phenomena such as particle-particle interactions (agglomeration) that may occur at high TiO<sup>2</sup> concentration (>100 mg L−1) could result in a loss of surface area available for light harvesting and thus lead in a decrease of the photoreaction rates [17, 24]. Chen and Chou reported that further increase in TiO<sup>2</sup> catalyst amount beyond 200 mg L−1 may result in the deactivation of activated molecules due to collision with the ground-state molecules [25] as shown in the chemical reaction (Eq. (2)):

$$\text{TiO}\_2\text{"+TiO}\_2 \rightarrow \text{TiO}\_2\text{"+TiO}\_2\tag{2}$$

facilitated the procedure of LLE method), whereas the catalyst loading was maintained constant (100 mg L−1). Throughout the performed tests, it was assumed that, in the first stages of irradiation (≤20% reduction), no variations took place resulting from other parameters such as competitive effects of intermediates, pH changes, etc. [6, 17]. The effect of the insecticides'

was independently obtained and illustrated in **Figure 4** (the inset of **Figure 4** is the linear

From the depicted experimental data acquired in current investigation and are shown in **Figure 4**, it is obvious that the degradation rate increased with the increase of the concentration of all of the studied compounds until it reached a saturation value (10 mg L−1). At higher concentrations above that value (>10 mg L−1), the initial rate started to become constant and

According to several laboratory studies that have conducted, the use of Langmuir-Hinshelwood kinetics model and first-order rate equations provided reasonable simulations to the observed photocatalysis process of various organic pollutants over illu-

**Figure 4.** Effect of initial concentration of selected OPPs on photocatalytic degradation rate (pesticide's concentration,

, 6.06, λUV, 365 nm, in the presence of methanol).

[5, 6, 13, 17]. Langmuir-Hinshelwood model is described by the following

) of each individual pesticide reactant on the initial reaction rate *(ro*

Photocatalytic Degradation of Selected Organophosphorus Pesticides Using Titanium Dioxide…

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251

initial concentration (*Co*

*3.2.3. Photodegradation kinetics*

versus 1/*Co*

independent of target solutes' concentration.

5–60 mg L−1; catalyst's concentration, 100 mg L−1; pH<sup>o</sup>

values).

transform of 1/*ro*

minated TiO<sup>2</sup>

relationship:

where TiO<sup>2</sup> • is the active species of the catalyst that is adsorbed on its surface, whereas TiO<sup>2</sup> # is the deactivated form of the catalyst [24].

The photocatalytic degradation of other organic pollutants has also exhibited the same dependency on catalyst dose [17, 24]. According to previous studies, the concentration of optimum catalyst was found to be dependent on the initial solute concentration of the photocatalyzed compound [17, 26].

In the present study, as it can be concluded from data illustrated in **Figure 3**, under the applied experimental conditions, the optimum value of catalyst's concentration TiO<sup>2</sup> P-25 on photocatalytic decomposition of investigated insecticides is 100 mg L−1, and consequently, this amount was selected to work throughout the study. This result is in agreement with other studies [17].
