*3.4.1. TOC content*

systems

O2

can act as an

(5 mM) system, in the

and the het-

−

O2

in the aqueous suspensions of

was 0.71 h (42.6 min) for fenthion and

<sup>−</sup> + H<sup>2</sup> O<sup>2</sup> → OH− + OH• (8)

(100 mg L−1)-Η<sup>2</sup>

Ο2

.− + H<sup>2</sup> O<sup>2</sup> → OH− + OH• + O<sup>2</sup> (9)

O2

/10 mg L−1 OPPs was more effective on the photocatalytic oxidation of fen-

+ [(2x + 3θ + 4z + 4φ + 1 − ω)/2] O<sup>2</sup> xCO<sup>2</sup> + θNO3

<sup>2</sup><sup>−</sup> + H<sup>2</sup> O + (ψ − 2) H<sup>+</sup> (10)

O2

4.04-fold increase in rate constants, kobs, compared to the results obtained in UV-TiO<sup>2</sup>

**Pesticide** *R<sup>2</sup> kobs* **(h−1)** *t1/2* **(h)** Dimethoate 0.9907 0.0744 9.32 Fenthion 0.9838 0.9739 0.71

containing pesticides [27, 28]. This observation is explained by the fact that H<sup>2</sup>

The magnitude of enhancement is in agreement with other studies concerning phosphorus-

alternative electron acceptor to oxygen (Eq. (8)) that is a thermodynamically more favorable reaction than oxygen reduction [29]. This should consequently promote the charge separation and accelerate the heterogeneous photocatalysis [30]. At the same time, hydroxyl radicals able to oxidize organic pollutants, such as pesticides, are generated by either the reduction of H<sup>2</sup>

at the conductance band [29] or the acceptance of an electron from superoxide again (chain reactions) (Eq. (9)) [31]. As a consequence, and regardless which conductance band reaction overrules, additional •OH oxidizing species may be produced resulting in the increase of the

thion (100% decomposition in 5 h) rather than in the case of dimethoate (95% decomposition

The general stoichiometric reaction proposed for the photocatalytic oxidation of the studied

eroatoms into inorganic anions at their highest oxidation states which remain in the solution is described by the following reaction (Eq. (10)). Consequently, in order to assess the extent of mineralization during photocatalysis of selected organic pollutants, TOC measurements were carried out along with determination of released inorganic anions containing the heteroatoms

pesticides that leads to the conversion of all of their carbon atoms to gaseous CO<sup>2</sup>

3− + φSO4

and H<sup>2</sup>

for fenthion and dimethoate, respectively.

254 Titanium Dioxide - Material for a Sustainable Environment

**Table 3.** Photocatalytic kinetic parameters of selected pesticides in UV-TiO<sup>2</sup>

presence of methanol, using a radiant UV energy of 14.5 mW cm−2.

oxidizing power of the system [30]:

eCB

O<sup>2</sup>

100 mg L−1 TiO<sup>2</sup>

in 36 h). The *t*

9.32 h for dimethoate.

**3.4. Mineralization studies**

of the selected organics (**Figure 6**):

Cx Hψ Νθ Οω P<sup>z</sup> Sφ

+ zPO4

Experimental data obtained proved that the addition of H<sup>2</sup>

*1/2* in the presence of both TiO<sup>2</sup>

As shown in **Figure 6**, in the presence of TiO<sup>2</sup> and under UV irradiation, the TOC concentration of the insecticides exhibited a constant decrease with time, reaching TOC reduction that ranged between 58 and 100% (total mineralization) after the end of illumination time. In addition, it can be seen that the rates of TOC reduction during irradiation of solutions of azinphos methyl, azinphos ethyl, and disulfoton were higher compared to those of dimethoate and fenthion, suggesting that former ones exhibit greater susceptibility to photocatalysis than the

**Figure 6.** Pesticide and TOC reduction and evolution of sulfate (SO4 2−), phosphate (PO4 3−), nitrite (NO<sup>2</sup> − ), nitrate (NO3 − ), and ammonium (NH4 + ) ions originating from photocatalytic degradation of selected pesticides as a function of irradiation time (pesticide's concentration level, 10 mg L−1; TiO<sup>2</sup> , 100 mg L−1; H<sup>2</sup> O2 , 5 mM; λUV, 365 nm, in the presence of methanol).

latter. Furthermore, these results are confirming the kinetic data acquired in the current study from the application of Langmuir-Hinshelwood model. The addition of H<sup>2</sup> O2 in the irradiated 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.

group from the P═S and P─S bonds, which occurred firstly in the case of phosphorothioates, leading to the formation of oxon intermediate derivatives as reported in previous studies [5, 13]. Acquired results are in accordance with several experimental results, which have shown that

sions produces phosphate ions [5, 17, 20]. Release of phosphate ions occurred by the cleavage of the phosphorus atom from the P═S, P─S, and P─O bonds caused by the continuous oxidation attack of the •OH radicals. However, according to other reported studies, non-detection

phenomenon in the pH range used, that partially inhibits the reaction rate of deterioration

The formation of nitrite, nitrate, and ammonium ions was also monitored for the three selected nitrogen-containing molecules, since these compounds could give rise to these ions [5, 32], based


observed in the first stage of irradiation can be ascribed to the fact that these ions are formed as

anions, probably formed through oxidation pathways, have little effect on the kinetics of reaction [5], whereas ammonium ions are relatively stable cations released from reduction processes [5].

the N-containing organic compounds and the initial oxidation state of nitrogen and some experimental conditions, including the irradiation time, the pH of the solution, and the substance concentration [5, 32]. Under the employed experimental conditions, higher nitrate-to-ammonium concentration ratio was produced for the cases of azinphos methyl and azinphos ethyl that con-

> − ([NO3 −

Mineralization studies of dimethoate and fenthion during photocatalysis were also conducted

ance of mineral ions, it is obvious that the addition of the oxidant enhanced the transformation of both parent compounds and was able to achieve higher mineralization. This is in agreement with previous studies supporting that the final products of organophosphorus pesticides photooxidation are eventually carbon dioxide and inorganic ions containing the heteroatoms [18].

Based on the results of the current study concerning the photocatalytic decomposition of five selected organophosphorus insecticides contained individually in aqueous solutions and by

conductor 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

and UV-TiO<sup>2</sup>


concentration (NO3

, which was demonstrated by the disappearance of NO<sup>2</sup>

− /NH4 +

pounds, such as azinphos methyl and azinphos ethyl. Obviously, the release of NO<sup>2</sup>

3− ions in the irradiated solutions of phosphorus-containing pesticides, such as acephate and dimethoate, is possible due to incomplete mineralization of the pesticides as well as com-

3− on the photoactivated reaction sites of the catalyst TiO<sup>2</sup>

Photocatalytic Degradation of Selected Organophosphorus Pesticides Using Titanium Dioxide…

gas can be generated mainly from the photodegradation of the

after longer irradiation periods. Generally, it has been found that nitrate

]> > [NH4

+

) (**Figure 6**). Based on the kinetics of formation and disappear-

+

suspen-

257

, a strong

that was

−

and concomi-

−

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

) depends mainly on the nature of

] than for dimethoate in which

−

] < <[NH4

is a semi-

+ ].

species ([NO3

, it appeared that TiO<sup>2</sup>

photodecomposition of several organophosphorus substances in the presence of TiO<sup>2</sup>

(20–70% reduction at levels of greater than 10−3 mol dm−3) [5, 21].

of PO4

petitive adsorption of PO4

on relative literature, even N<sup>2</sup>

−

−

tained ring nitrogen converted mainly into NO3

using the heterogeneous systems of UV-TiO<sup>2</sup>

and NH4

+

the contained amino group was predominantly mineralized into NH4

O2

−

an intermediate of NO3

tant formation of NO3

The molar ratio of NO3

in the presence of oxidant (H<sup>2</sup>

**4. Conclusions**

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

#### *3.4.2. Mineral inorganic ions*

Evolution of the heteroatoms at their highest oxidation states such as SO<sup>2</sup> 4−, NO3 − , and PO4 3− 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 surveyed. More specifically in the present study, the formation of sulfate (SO4 2−), phosphate (PO4 3−), nitrite (NO<sup>2</sup> − ), nitrate (NO3 − ), and ammonium (NH4 + ) ions, originating from photocatalytic degradation of selected toxicants under UV irradiation, was investigated.

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. Decomposition of all five tested organophosphates released SO<sup>2</sup> −4 and PO4 3−, while that of the three nitrogen-containing molecules, azinphos methyl, azinphos ethyl, and dimethoate (chemical formulas in **Table 1**), liberated NO<sup>2</sup> − , NO3 − , and NH4 + ions as well. These findings are consistent with published works involving mineralization studies of other organophosphorus pesticides during heterogeneous photolysis over TiO<sup>2</sup> suspensions. It is well documented that the pesticides containing sulfur atoms are mineralized into sulfate ions [5, 17, 20, 21]. Overall, the monitoring of SO<sup>2</sup> −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 group from the P═S and P─S bonds, which occurred firstly in the case of phosphorothioates, leading to the formation of oxon intermediate derivatives as reported in previous studies [5, 13].

Acquired results are in accordance with several experimental results, which have shown that photodecomposition of several organophosphorus substances in the presence of TiO<sup>2</sup> suspensions produces phosphate ions [5, 17, 20]. Release of phosphate ions occurred by the cleavage of the phosphorus atom from the P═S, P─S, and P─O bonds caused by the continuous oxidation attack of the •OH radicals. However, according to other reported studies, non-detection of PO4 3− ions in the irradiated solutions of phosphorus-containing pesticides, such as acephate and dimethoate, is possible due to incomplete mineralization of the pesticides as well as competitive adsorption of PO4 3− on the photoactivated reaction sites of the catalyst TiO<sup>2</sup> , a strong phenomenon in the pH range used, that partially inhibits the reaction rate of deterioration (20–70% reduction at levels of greater than 10−3 mol dm−3) [5, 21].

The formation of nitrite, nitrate, and ammonium ions was also monitored for the three selected nitrogen-containing molecules, since these compounds could give rise to these ions [5, 32], based on relative literature, even N<sup>2</sup> gas can be generated mainly from the photodegradation of the -N = N- double bond moieties contained in aliphatic or heterocyclic nitrogen-containing compounds, such as azinphos methyl and azinphos ethyl. Obviously, the release of NO<sup>2</sup> − that was observed in the first stage of irradiation can be ascribed to the fact that these ions are formed as an intermediate of NO3 − , which was demonstrated by the disappearance of NO<sup>2</sup> − and concomitant formation of NO3 − after longer irradiation periods. Generally, it has been found that nitrate anions, probably formed through oxidation pathways, have little effect on the kinetics of reaction [5], whereas ammonium ions are relatively stable cations released from reduction processes [5]. The molar ratio of NO3 − and NH4 + concentration (NO3 − /NH4 + ) depends mainly on the nature of the N-containing organic compounds and the initial oxidation state of nitrogen and some experimental conditions, including the irradiation time, the pH of the solution, and the substance concentration [5, 32]. Under the employed experimental conditions, higher nitrate-to-ammonium concentration ratio was produced for the cases of azinphos methyl and azinphos ethyl that contained ring nitrogen converted mainly into NO3 − ([NO3 − ]> > [NH4 + ] than for dimethoate in which the contained amino group was predominantly mineralized into NH4 + species ([NO3 − ] < <[NH4 + ].

Mineralization studies of dimethoate and fenthion during photocatalysis were also conducted in the presence of oxidant (H<sup>2</sup> O2 ) (**Figure 6**). Based on the kinetics of formation and disappearance of mineral ions, it is obvious that the addition of the oxidant enhanced the transformation of both parent compounds and was able to achieve higher mineralization. This is in agreement with previous studies supporting that the final products of organophosphorus pesticides photooxidation are eventually carbon dioxide and inorganic ions containing the heteroatoms [18].
