**1. Introduction**

Several human activities may result in the presence of numerous and various types of emerging chemical contaminants and toxicants in water or wastewater. Pesticide is one of the most common groups of chemical pollutants found in wastewater effluents due to their widespread use in agriculture in order to maintain crop quality and quantity [1, 2]. Their widespread use for the prevention, control, or elimination of pests has led to public health concerns in recent years. According to an extended published literature, pesticides are usually detected in trace concentrations (ng L−1 or μg L−1) and consequently are considered as micropollutants, which among other characteristics are specific water and wastewater constituents that cannot be removed by primary or secondary conventional treatment. As a result, the adaptation and application of advanced oxidation processes (AOPs) are necessary to decompose these persistent compounds from contaminated environmental matrices [3].

experiments. More specifically, the objectives of this work were (i) to evaluate and compare the kinetics of pesticide disappearance, (ii) to examine the influence of oxidant reagent (H<sup>2</sup>

Photocatalytic Degradation of Selected Organophosphorus Pesticides Using Titanium Dioxide…

added into the photocatalytic system of selected OPPs, and (iii) to investigate the potential of the catalytic systems studied to be applied in a small scale for the removal of OPPs, as, for

Disappearance of the investigated organophosphates contained in spiked water samples (that were irradiated) was detected by a gas chromatographic (GC) system equipped with a nitrogen-phosphorus detector (GC-NPD means). Degradation kinetics and obtained kinetic parameters, expressed as rate constant and half-life values, are presented. Moreover, mineralization of parent compounds during the batch photocatalytic experiments was assessed by quality and quantity determination of formed and released inorganic end products (evolu-

TOC analyzer measurements. Based on the acquired results, the applicability of the UV-TiO<sup>2</sup> method that was employed in the present survey is discussed and compared with other suc-

High-purity analytical standards (>97.7%) of the five selected organophosphorus pesticides (azinphos methyl, azinphos ethyl, dimethoate, disulfoton, and fenthion) were purchased from Dr. Ehrenstorfer-Schäfers (Augsburg, Germany) and used without further purification. Their chemical structures and data for selected physicochemical properties of stud-

from Merck & Co (Darmstadt, Germany). Pesticide-grade and HPLC-grade solvents (acetone, hexane, methanol, and dichloromethane) were supplied from Labscan Ltd. (Dublin, Ireland). Organic-free water used for all aquatic solutions was of specific resistance greater than 18.2 MΩ cm (25°C) and was produced by a Milli-Q/Milli-RO water purification system (Millipore, USA). Individual pesticide stock solutions were prepared by gravimetric weighting of high-purity standards (to concentrations of approximately 1000 mg L−1) in methanol (HPLC-grade) and were stored at -10°C in the dark. Working standard solutions of individual compounds were prepared by appropriate dilutions of the stock solutions. Other

SO4

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

, and C8

H4 K2 O4

ied OPPs, taken from Ref. [12], are shown in **Table 1**. Titanium dioxide (TiO<sup>2</sup>

Degussa P-25 (Frankfurt, Germany). Hydrogen peroxide (30% solution H<sup>2</sup>

2−, NO3 −

, and PO4

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

instance, by research laboratories for purifying the wastewater they produce.

tion of heteroatoms at their highest oxidized states, such as SO4

cessful applications and treatments reported in the literature.

**2. Materials and methods**

**2.1. Test chemicals, reagents, and standards**

chemical reagents used (HCl, NaOH, Na<sup>2</sup>

procured from Merck (Merck, Germany).

**2.2. Irradiation procedure**

O2 ) 243

3−) along with

) catalyst was

) was obtained

O2

) were of analytical grade and

AOP is a class of oxidation techniques and procedures that are based on the in situ generation of highly reactive and oxidizing radical species (mainly powerful hydroxyl radicals (•OH)), which interact with the molecules of the organic pollutants and lead to their progressive degradation. AOPs can be classified as photochemical or non-photochemical processes that furthermore can be categorized either as homogeneous or heterogeneous. More specifically, heterogeneous AOPs require the addition of a solid semiconductor (such as metal oxides and sulfurs of Ti, Al, Zn, V, Cr, Mn, etc., or organometallic catalysts) to produce a colloidal suspension that is stable under radiation and is required to stimulate a photochemical reduction reaction in the solid/liquid interface (occurrence of accelerated photoreaction). In particular, illumination of the catalyst with radiation of the proper wavelength (≥E<sup>g</sup> , bandgap energy) generates electron and hole pairs (e− and h+ , respectively, acting as energy carriers) that can recombine or dissociate (both reactions take place in competition); when dissociation occurs, conduction band electrons and valence band holes are produced, which are able to migrate to the particle surface and interact with adsorbed electron acceptors (oxygen) and oxidize electron donors (− OH and H<sup>2</sup> O) yielding in hydroxyl radicals. Compared with the homogeneous AOPs, the heterogeneous AOPs have the advantage of the easier separation from the product (meaning the treated effluents) [4, 5].

The use of heterogeneous photocatalysis has been shown as an ideal methodology for the decontamination and restoration of water contaminated with persistent organic pollutants (POPs) in developing countries [6–8]. Nowadays, among the most promising and successful applications of heterogeneous photocatalysis applied for the removal of various toxicants from water, photocatalysis over titanium dioxide (TiO<sup>2</sup> ) is included, since it has been demonstrated as one of the most frequently used methodologies employed for the treatment of chlorinated phosphate esters and carbamic, thiocarbamic, and triazine pesticides [4–6, 9–11].

The focus of the present chapter is to provide the results of the photocatalytic degradation study conducted with five selected organophosphorus pesticides (OPPs) (azinphos methyl, azinphos ethyl, disulfoton, dimethoate, and fenthion) by heterogeneous photochemical process using UV light and TiO<sup>2</sup> . The process of UV-TiO<sup>2</sup> system was applied to the photooxidation of all selected OPPs, while the UV-TiO<sup>2</sup> -H<sup>2</sup> O2 system was applied only to degradation of dimethoate and fenthion as they proved to be more resistant in the previous set of photolysis experiments. More specifically, the objectives of this work were (i) to evaluate and compare the kinetics of pesticide disappearance, (ii) to examine the influence of oxidant reagent (H<sup>2</sup> O2 ) added into the photocatalytic system of selected OPPs, and (iii) to investigate the potential of the catalytic systems studied to be applied in a small scale for the removal of OPPs, as, for instance, by research laboratories for purifying the wastewater they produce.

Disappearance of the investigated organophosphates contained in spiked water samples (that were irradiated) was detected by a gas chromatographic (GC) system equipped with a nitrogen-phosphorus detector (GC-NPD means). Degradation kinetics and obtained kinetic parameters, expressed as rate constant and half-life values, are presented. Moreover, mineralization of parent compounds during the batch photocatalytic experiments was assessed by quality and quantity determination of formed and released inorganic end products (evolution of heteroatoms at their highest oxidized states, such as SO4 2−, NO3 − , and PO4 3−) along with TOC analyzer measurements. Based on the acquired results, the applicability of the UV-TiO<sup>2</sup> method that was employed in the present survey is discussed and compared with other successful applications and treatments reported in the literature.
