**2.2. Irradiation procedure**

For the performance of photolysis experiments, a self-constructed illumination system was used which has been described in detail in the previous published survey of our laboratory [6]. In brief, the apparatus was a stainless steel illuminated box (chamber) irradiated by a set of UV low-pressure lamps (Hg-lamps manufactured in Philips TL-D, 4×18 W, λuv = 365 nm) that dominantly emitted radiation with maximum light intensity

with caps and photolyzed. Finally, in the case that H<sup>2</sup>

tures to yield the desired concentration (optimum H<sup>2</sup>

dropwise addition of 0.1 N HCl or 0.1 N NaOH solutions.

was accomplished by the addition of anhydrous Na<sup>2</sup>

**2.3. Analysis of the photolyzed solutions**

for the rest four OPPs 0.0083 μg L−1 [14].

phosphate (PO4

respectively [16].

**2.4. Evaluation of the extent of mineralization**

3−), nitrate (NO3

formed according to the ASTM 4500-NO3

ions according to ASTM 4500-NH3

according to ASTM 4500-SO4

−

TiO<sup>2</sup>

photolysis system, a certain volume of the stock solution of H<sup>2</sup>

O2

Photocatalytic Degradation of Selected Organophosphorus Pesticides Using Titanium Dioxide…

O2

erwise stated). Immediately, after adding hydrogen peroxide, the UV lamps were turned on, and irradiation started. The irradiated samples were magnetically stirred throughout the experiment. Photolysis was carried out at a constant ambient temperature of 30 ± 2°C in the photolysis apparatus. Prior to the photolysis tests, the pH value of the samples was not adjusted unless otherwise stated. In such cases, the adjustment was accomplished by

At specific time intervals, samples were removed from the photoreactor, and 5 mL aliquots of the photolyzed aqueous matrices were withdrawn for further analysis. In order to remove

was used for further analysis. Qualitative and quantitative determination of extracted pesticides' residues was performed on a Hewlett-Packard gas chromatographic (GC) system, model HP-5890, Series II, (Hewlett Packard, USA) equipped with a nitrogen-phosphorus detector (NPD). The chromatographic method and conditions applied are discussed in detail by Vagi et al. [14]. The validation of the method was carried out by the analysis of fortified water samples containing each target analyte at three different spiking levels in the range of 0.5–2 mg L−1 and prepared in triplicate individual solutions [15]. Percentage recoveries for the five selected insecticides were above 95.2 ± 2.4%, LOD values were 0.0050 μg L−1 for dimethoate and 0.0025 μg L−1 for the other four selected organophosphates (azinphos methyl, azinphos ethyl, disulfoton, and fenthion), and LOQ for dimethoate was 0.0165 μg L−1, while

Total organic carbon (TOC) measurements were performed with a Teledyne Instruments Tekmar TOC Combustion Analyzer (model Apollo 9000, Ohio, USA) calibrated with standard solutions of potassium phthalate [6]. Additionally, the release of inorganic anions containing the heteroatoms of the organophosphorus pesticides was monitored, such as

of irradiation time was performed spectrophotometrically by using UV/Vis spectrophotometer (Varian, model **C**ary 50, Australia) and by following the appropriate method of the American **S**tandard **T**est **M**ethod (ASTM). Specifically, nitrate ion determination was per-

2−-E turbidimetric and ASTM 4500-P−

+

), and sulfate (SO4



), ammonium (NH4

−

, samples were centrifuged at 8000 rpm for 15 min. OPP concentrations were determined after liquid-liquid extraction of the filtrated aqueous phases twice with 5 mL hexane or dichloromethane by vortex (1 min). Elimination of humidity in two combined extracts

SO4

O2

was applied as an oxidant in the

http://dx.doi.org/10.5772/intechopen.72193

concentration, 5 mM, unless oth-

, whereas 1 μL of the organic extract

2−) ions, as a function


was added into the mix-

245

**Table 1.** Chemical names, chemical formulas, and physicochemical properties of selected pesticides.

of 14.5 mW cm−2 at 15 cm distance. Pyrex glass bottles with a capacity of 250 mL were employed as photoreactors, containing the pesticide solutions and placed in the center of the photolysis construction. Substrates were dissolved in distilled water at ppm (mg L−1) levels, under their solubility levels by spiking the appropriate volume of a stock solution in methanol, so as to have a methanol content <0.05% [13]. Fortified aqueous solutions containing each tested insecticide individually (optimum pesticide's final concentration, 10 mg L−1, unless otherwise stated), in the presence and absence of the photocatalyst, were prepared by dissolving the appropriate quantities of each one of the substances in water. When TiO<sup>2</sup> was used (optimum TiO<sup>2</sup> concentration, 100 mg L−1, unless otherwise stated), the mixtures were magnetically stirred to obtain homogenization and a good dispersion of the catalyst and equilibrated in the dark for 30 min prior to illumination. Samples of 250 mL of the above-fortified solutions (in the absence of the photocatalyst) or suspensions (in the presence of the photocatalyst) were added to Pyrex bottles covered air tightly with caps and photolyzed. Finally, in the case that H<sup>2</sup> O2 was applied as an oxidant in the photolysis system, a certain volume of the stock solution of H<sup>2</sup> O2 was added into the mixtures to yield the desired concentration (optimum H<sup>2</sup> O2 concentration, 5 mM, unless otherwise stated). Immediately, after adding hydrogen peroxide, the UV lamps were turned on, and irradiation started. The irradiated samples were magnetically stirred throughout the experiment. Photolysis was carried out at a constant ambient temperature of 30 ± 2°C in the photolysis apparatus. Prior to the photolysis tests, the pH value of the samples was not adjusted unless otherwise stated. In such cases, the adjustment was accomplished by dropwise addition of 0.1 N HCl or 0.1 N NaOH solutions.
