*3.2.3. Photodegradation kinetics*

possible. The downward trend in initial rates of pesticide's photocatalytic decomposition

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 heteroge-

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

The eventual leveling off of the initial rates of photocatalytic deterioration (**Figure 3**) could

centration 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

tion (>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

molecules due to collision with the ground-state molecules [25] as shown in the chemical reac-

• + TiO<sup>2</sup> → TiO<sup>2</sup>

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

In the present study, as it can be concluded from data illustrated in **Figure 3**, under the

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

ing the initial concentration of the solute from 5 to 60 mg L−1 (a concentration range that

), a set of photodegradation experiments was carried out by vary-

applied experimental conditions, the optimum value of catalyst's concentration TiO<sup>2</sup>

*3.2.2. Effect of the pesticides' initial concentration on photocatalytic degradation rate*

In order to investigate the dependence of the photolysis rate *(ro*

catalyst amount beyond 200 mg L−1 may result in the deactivation of activated

• is the active species of the catalyst that is adsorbed on its surface, whereas TiO<sup>2</sup>

such as particle-particle interactions (agglomeration) that may occur at high TiO<sup>2</sup>

(<10 mg L−1) was present, can

concentrations (>10 mg L−1),

were present at a high-enough con-
