**4. Conclusions**

latter. Furthermore, these results are confirming the kinetic data acquired in the current study

photocatalytic media of dimethoate and fenthion not only enhanced the removal of TOC in both cases but also resulted in almost total decomposition of target pesticide pollutants.

In general, the trend in TOC reduction was similar to that observed in pesticide disappearance. However, when the obtained *kobs* values of these two studied processes were compared, the reduction in TOC content of irradiated solutions was found to be a slower phenomenon than the photodecomposition of the parent pesticides. This observation can be explained by the fact that the photocatalytic decomposition of the parent compounds occurred through the formation of various organic intermediates and not instantaneously. Moreover, taking into consideration the complex nature of photocatalysis and the wide variety of stable and unstable photoproducts that can be formed, rate of TOC reduction depended on the individual tested organophosphate. The same behavior has been observed in numerous irradiated pesticide solutions reported in the available literature; for instance, the formation and evolution of several carboxylic acids (such as formic, acetic, glycolic, and cyanuric acids) as transient intermediates of photocatalytic reaction, which could eventually undergo complete mineralization as irradiation progresses, have been published [4, 5, 9, 21]. Formation of oxon derivatives (such as paraoxon ethyl, pirimiphos-oxon, fenthion-oxon), corresponding phenols (e.g., nitrophenol), various and different trialkyl and dialkyl phosphorothioate or phosphate esters, and quinonidal compounds has also been observed and detected as major intermediate photoproducts that subsequently underwent mineralization [5]. On the contrary, in the absence of UV light (dark condition, not presented in TOC reduction data), no significant percent reduction in TOC of studied compounds occurred,

surface.

+

O2

4−, NO3 −

) ions, originating from photocata-

−4 and PO4

suspensions. It is well documented that the pesticides

ions as well. These findings are consistent

, and PO4

2−), phosphate

3−, while that of the

3−

in the irradiated

from the application of Langmuir-Hinshelwood model. The addition of H<sup>2</sup>

256 Titanium Dioxide - Material for a Sustainable Environment

suggesting negligible adsorbance of the pesticides on TiO<sup>2</sup>

Evolution of the heteroatoms at their highest oxidation states such as SO<sup>2</sup>

surveyed. More specifically in the present study, the formation of sulfate (SO4

lytic degradation of selected toxicants under UV irradiation, was investigated.

−

Decomposition of all five tested organophosphates released SO<sup>2</sup>

− , NO3 −

provides evidence that pesticide degradation occurred primarily through photocatalytic oxidation reactions [21]. Therefore, in order to further confirm the extent of photocatalytic reduction and better understand the reaction mechanisms involved, the formation of inorganic anions containing the heteroatoms of the selected organophosphorus compounds was

), and ammonium (NH4

As illustrated in **Figure 6**, photocatalytic treatment of target pesticides resulted in the destruction of the parent molecules as evidenced by the evolution of monitored inorganic anions.

three nitrogen-containing molecules, azinphos methyl, azinphos ethyl, and dimethoate (chemical

with published works involving mineralization studies of other organophosphorus pesticides

containing sulfur atoms are mineralized into sulfate ions [5, 17, 20, 21]. Overall, the monitoring of

−4 ions showed that a rapid increase in their concentration was observed achieving finally (in the final stages of irradiation treatment) their expected amounts according to the stoichiometry proposed in the reaction (10). Formation of sulfate ions took place by the rupture of the sulfur

+

, and NH4

*3.4.2. Mineral inorganic ions*

3−), nitrite (NO<sup>2</sup>

−

formulas in **Table 1**), liberated NO<sup>2</sup>

during heterogeneous photolysis over TiO<sup>2</sup>

), nitrate (NO3

(PO4

SO<sup>2</sup>

Based on the results of the current study concerning the photocatalytic decomposition of five selected organophosphorus insecticides contained individually in aqueous solutions and by using the heterogeneous systems of UV-TiO<sup>2</sup> and UV-TiO<sup>2</sup> -H<sup>2</sup> O2 , it appeared that TiO<sup>2</sup> is a semiconductor with high catalytic activity; photodegradation of all studied compounds proceeded at higher reaction rates in its presence than in its absence (direct photolysis). Total decomposition in UV-TiO<sup>2</sup> system was accomplished for the three cases of azinphos ethyl, azinphos methyl, and disulfoton after illumination time that depended on the tested organophosphate, whereas longer irradiation time for the cases of dimethoate and fenthion is probably needed. With the addition of H<sup>2</sup> O2 into illuminated TiO<sup>2</sup> suspensions, a synergistic effect was observed, which led to an enhancement of the photolytic process, achieving total disappearance of dimethoate and fenthion. Finally, the experimental data revealed that both catalytic systems investigated have good potential for small-scale applications, such as the wastewater purification systems of research laboratories or agrochemical manufacturer, formulators, and producer companies. The advantages of the proposed photocatalytic systems among others include simplicity in design because they give the opportunity to employ UV lamps easily found in the market and semiconductor powder (TiO<sup>2</sup> ) of low cost as catalyst.

[6] Petsas AS, Vagi MC, Kostopoulou MN, Lekkas TD. Photocatalytic degradation of the

[7] Zwiener C, Frimmel FH.Water quality. In: Kleiböhmer W, editor. Environmental Analysis, Vol 3. Handbook of Analytical Separations, 1st ed. Amsterdam, The Netherlands: Elsevier

[8] Ollis DF, Pelizzetti E, Serpone N. Destruction of water contaminants. Environmental

[9] Tamimi M, Qourzal S, Assabbane A, Chovelon JM, Ferronato C, Ait-Ichou Y. Photocatalytic degradation of pesticide methomyl: Determination of the reaction pathway and identification of intermediate products. Photochemical & Photobiological Sciences.

[10] Bahnemann D, Bockelmann D, Goshlich R. Mechanistic studies of water detoxification

[11] Vagi MC, Petsas AS. Advanced oxidation processes for the removal of pesticides from wastewater: recent review and trends. 2017. Proceedings of the 15th International Conference of Environmental Science and Technology. Available from https://cest. gnest.org/sites/default/files/presentation\_file\_list/cest2017\_01225\_oral\_paper.pdf

[12] Tomlin CDS. The Pesticide Manual: A World Compendium. 11th ed. Farnham, Surrey,

[13] Konstantinou IK, Sakellarides TM, Sakkas VA, Albanis TA. Photocatalytic degradation of selected s-triazine herbicides and organophosphorus insecticides over aqueous TiO<sup>2</sup>

[14] Vagi MC, Petsas AS, Kostopoulou MN, Lekkas TD. Adsorption and desorption processes of the organophosphorus pesticides, dimethoate and fenthion, onto three Greek agricultural soils. International Journal of Environmental Analytical Chemistry. 2010;

[15] Huber L. Validation of analytical methods: Review and strategy. LC-GC International.

[16] APHA, AWWA, WEF. Standard Methods for Examination of Water and Wastewater. 22nd ed. Washington: American Public Health Association; 2012, 1360 pp. ISBN 978- 087553-013-0. Available from: http://www.standardmethods.org/ [Accessed 2017-20-9]

[17] Evgenidou E, Fytianos K, Poulios I. Photocatalytic oxidation of dimethoate in aqueous solutions. Journal of Photochemistry and Photobiology A: Chemistry. 2005;**175**:29-38

UK: British Crop Protection Council; 1997. 1606 pp. ISBN:1901396118

suspensions. Environmental Science & Technology. 2001;**35**:398-405

suspensions. Sol. Energy Maternité. 1991;**24**:564-583

diation. Proceedings of the 13rd International Conference of Environmental Science and Technology. 2013. Available from http://www.gnest.org/proceedings/cest2013/public\_

Photocatalytic Degradation of Selected Organophosphorus Pesticides Using Titanium Dioxide…

under UV irra-

259

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

organophosphorus pesticide fenthion in aqueous suspensions of TiO<sup>2</sup>

html/papers/0514.pdf? [Accessed 2017-30-10]

Science; 2001. pp. 277-318. ISBN: 9780444500212

Science & Technology. 1991;**25**(9):1522-1529

2006;**5**:477-482

in illuminated TiO<sup>2</sup>

[Accessed 2017-30-10]

**90**(3-6):369-389

1998;**11**(2):96-105
