Impact of Photolysis and TiO 2 on Pesticides Degradation in Wastewater

: Pesticide residues are harmful to the environment and human and animal health even at low levels because of long-term bioaccumulation. In this study, photolysis was applied to treat three representative water samples: aqueous atrazine and dimethoate solutions as target pesticides, as well as wastewater and agriculture wastewater containing pesticide residue. It was performed using ultraviolet (UV) irradiation at two wavelengths (254 and 306 nm) with exposure times ranging from 2 to 12 h in the presence and absence of a photocatalyst to identify the optimal degradation conditions. Extraction and analyzation process were performed by the Quick Easy Cheap Effective Rugged Safe (QuEChERS) methods and gas chromatography–tandem mass spectrometry with triple quadrupole detector (GC–MSMS/TQD), respectively. Photodegradation increased with an increase in exposure time and the TiO2 catalyst was beneﬁcial for degradation. Both selected irradiation wavelengths were effective, although the wavelength of λ = 306 nm was the most efﬁcient.


Introduction
Pesticides are used to improve crop plant quality and reduce losses because of insects. They are applied at various stages of cultivation and during post-harvest storage of various crops [1]. The wide-ranging use of pesticides has significant advantages in agriculture, but the inappropriate selection of pesticides, their over-use, and harvesting crops before washing off the residues can result in negative effects and undesirable pesticide levels in the produce that reaches consumers [2,3]. Exposure to pesticides can be direct, e.g., oral, inhalation, and dermal or indirect, e.g., through food, drinking water, residential, and occupational exposure. The consumption of pesticide-laden food crops is a major concern [4]. The persistent nature of pesticides can cause damage to the environment via transportation in water, soil, and air, leading to widespread environmental contamination [5][6][7][8][9][10].
Furthermore, the use of pesticides has been linked to human health risks, including acute effects such as nausea and headaches and chronic effects such as reproductive harm, endocrine system disruption, and cancer [11]. The potential damage of pesticides has been outlined in detail by Walker et al. [12].
Currently, many kinds of commodity pesticides are commercially available and are commonly used in agricultural practices. A commonly used pesticide worldwide is dimethoate (O, O-dimethyl-S-(N-methylcarbamoylmethyl) phosphorodithioate), an organophosphorus insecticide first registered in 1962 [13]. It can be applied to control Natural water samples (10 mL) and those spiked with 2000 PPb of pesticides (2 ppm) were incubated for 12 h under UV light and aliquots (10 mL) were removed at intervals of 2 h to determine the remaining quantity of pesticides. The same procedure was applied after adding 0.001 g of TiO 2 as a photocatalyst to each 10 mL of water samples before UV remediation for 2 to 12 h.

Samples Preparation and Extraction and Cleanup by Quick Easy Cheap Effective Rugged Safe (QuEChERS)
First, 10 mL of the water sample was transferred into a 50 mL centrifuge tube, and vortexed briefly. Then, 10 mL of acetonitrile was added to each sample. Samples were shaken (manually or mechanically) or vortexed for 5 min to extract pesticides. (In this study a Spex SamplePrep Geno/Grinder 2010 operated at 1500 rpm was used). After that, the contents of an ECQUEU750CT-MP (citrate salts) Mylar pouch were added to each centrifuge tube. Immediately samples were shaken for at least 2 min and centrifuge for 5 min at ≥3500 rcf. We transferred a 1 mL aliquot of supernatant to a 2 mL CUMPSC18CT (MgSO 4 , PSA, C18) dSPE tube. Samples were vortexed for 0.5-1 min. They were centrifuged for 2 min at high rcf (e.g., ≥5000). Purified supernatant was filtered through a 0.2 µm syringe filter directly into a GC sample vial. Finally, the samples were analyzed by GC-MS/MSTSQ 8000/SRM.

QAQC Strategies and Method Performance
Quality control samples were prepared and analyzed in triplicate samples, blank and spiked, and certified reference material (CRM) was prepared for this purpose and processed with each batch (5-10 samples). QuEChERS and the GC-MSMS/TSQ 8000 method limit of detection (LOD) and limit of quantification (LQD), repeatability, reproducibility, accuracy and precession also were determined for each compound of the pesticides (Table 2). Table 2. Parameters of retention time (RT), limit of detection (LOD) and limit of quantification (LOQ), recovery % and GC-MS/TSQ target mass of SRM scanning mode.

Results and Discussion
Photoremediation of the pesticide residues for atrazine and dimethoate in different water media was studied: aqueous solutions (AS), wastewater (WW) and agriculture wastewater (Ag.W). The samples were irradiated at two different wavelengths (λ = 254 and 306 nm) in the presence and absence of a catalyst. The results are shown in Figures 1 and 2 and listed in Table 3. Figures 1 and 2a show the effect of exposure time of irradiation on the degradation of pesticides in aqueous solutions at two different wavelengths (λ = 254 and 306 nm) in the presence and absence of a catalyst. It is clear that the amounts of pesticides decreased with increased UV exposure time, indicating the degradation increases with increasing time, irrespective of the presence of the catalyst.
A similar trend is observed for the photoremediation of the pesticide residues in real water samples such as wastewater and agriculture wastewater (Figures 1 and 2b,c, respectively). To explain the obtained results, the mechanism of the process must be understood, and it can be described as follows: For a non-catalytic process, the light energy from the radiation is absorbed by the pesticide molecules, which subsequently become activated and attain the excited state. These undergo homolysis, heterolysis, or photoionization, as illustrated in Figure 3a [38].  Table 3. Mean of concentration of pesticide residues degradation (R(ppb)) at two different wavelengths and irradiation time in the presence and absence of a catalyst (± standard deviation (SD)). (spiked with 2000 PPb) and for catalytic process: 0.001 gm TiO 2 /10 mL. Khan et al. observed that atrazine could be degraded by direct photolysis at 254 nm by absorbing the photons from UV light and transforming into the excited state, which subsequently decomposes [22]. The mechanism for the direct photolysis of atrazine involves photoinduced solvolysis via the heterolytic cleavage of the C-Cl bond [39,40].
In a photocatalytic process, a semiconductor catalyst is photoexcited by absorbing electromagnetic UV radiation. Upon light absorption, the electrons in the valence band are excited to the conduction band, leaving a positive hole in the valance band [33]. The empty hole in the valence band and the electron in the conduction band can induce reduction or oxidation of the pesticide adsorbate [41], as illustrated in Figure 3b [31].
Thus, the increase in the photodegradation of pesticides with increasing the contact time could be attributed to the higher opportunities of electron transforming into the excited state, and in the case of the catalytic process, leads to generation of more free radicles which are powerful for degradation.
Previous studies reported the importance of the contact time as an effective factor in the photodegradation process. A previous study showed a result in agreement with our results, demonstrating that this phenomenon occurs because of more process of oxidation, and the chance of atrazine photodegradation is higher with increasing exposure time to radiation [42]. The study pointed to the importance of determination of the required time to reach to equilibrium stage wherein the reaction rate reached a plateau at this stage [42]. from [27]) and (b). photocatalysis process (the scheme is quoted from [31]).

204
Furthermore, it is observed that the addition of a catalyst enhances the photoremedi-205 ation over the irradiation time. After 12 h of irradiation, Pesticides were not detected at 206 either tested wavelengths, but only atrazine was detected at 254 nm. These results agree 207 with the findings reported in the literature. The potential of common pollutants like di-208 methoate, chlorpyrifos, and malathion in the complete mineralization by photodegrada-209 tion was investigated by direct and indirect sunlight exposure [23], and it was found that 210 the degradation efficiencies of the various methods of photolysis follow the order, benzo-211 phenone sensitized > β-carotene sensitized > direct exposure. Evgenidou et al. observed 212 that the degradation of dimethoate occurred at a very low rate in the absence of the cata-213 lyst, while it was positively influenced by the presence of semiconducting oxides (TiO2 214 and ZnO) as catalysts [43]. It was reported in another study [24] that the efficiency of at-215 razine degradation by a combination of UV and chlorine was better than that by UV or 216 chlorine alone. Pesticides such as atrazine, thiobencarb, dimethoate, lindane, dipterex, 217 malathion, and bentazone were photodegraded effectively in the presence TiO2 ceramics 218 as catalysts [44]. Atrazine also underwent an effective photocatalytic degradation via mi-219 crowave-assisted photocatalysis in the presence of nanocatalyst of titania-coated multi-220 wall carbon nanotube [45]. 221 The role is played by the photocatalyst is to facilitate degradation, as suggested by 222 Yola et al. [46]. According to them, the separation of electrons (e-) and holes (h+) is en-223 hanced by the catalyst molecules (TiO2), which help in producing •O2− and •OH radicals. 224 Figure 3. Scheme of photoremediation process; (a) direct photolysis process (the scheme is quoted from [27]) and (b) photocatalysis process (the scheme is quoted from [31]). Furthermore, it is observed that the addition of a catalyst enhances the photoremediation over the irradiation time. After 12 h of irradiation, Pesticides were not detected at either tested wavelengths, but only atrazine was detected at 254 nm. These results agree with the findings reported in the literature. The potential of common pollutants like dimethoate, chlorpyrifos, and malathion in the complete mineralization by photodegradation was investigated by direct and indirect sunlight exposure [23], and it was found that the degradation efficiencies of the various methods of photolysis follow the order, benzophenone sensitized > β-carotene sensitized > direct exposure. Evgenidou et al. observed that the degradation of dimethoate occurred at a very low rate in the absence of the catalyst, while it was positively influenced by the presence of semiconducting oxides (TiO 2 and ZnO) as catalysts [43]. It was reported in another study [24] that the efficiency of atrazine degradation by a combination of UV and chlorine was better than that by UV or chlorine alone. Pesticides such as atrazine, thiobencarb, dimethoate, lindane, dipterex, malathion, and bentazone were photodegraded effectively in the presence TiO 2 ceramics as catalysts [44]. Atrazine also underwent an effective photocatalytic degradation via microwave-assisted photocatalysis in the presence of nanocatalyst of titania-coated multiwall carbon nanotube [45].
The role is played by the photocatalyst is to facilitate degradation, as suggested by Yola et al. [46]. According to them, the separation of electrons (e-) and holes (h+) is enhanced by the catalyst molecules (TiO 2 ), which help in producing •O2− and •OH radicals. The •OH radical attacks the pesticides, causing their degradation in a stepwise manner that ends up in the generation of small molecules such as Cl − , NO 3− , CO 2 , and H 2 O [45].
The effect of the wavelength of irradiation (254 and 306 nm) on the degradation of pesticides in the different water media is presented in Figure 4 and Table 3. Although both of them were effective, it was observed that the photoremediation at λ = 306 nm was better than that at λ = 254 nm, in the presence and absence of a catalyst, in agreement with previous studies [47]. This indicates that the increase in wavelength increases the photodegradation because of the destructive effect of UV radiation on molecular bonds, and the enhancement of the electron transmission between the valence and conduction bands, which makes the degradation process easier and faster.  In addition, the results reveal that the tendency of degradation in dimethoate upon irradiation is greater than that in atrazine in both the presence and absence of the photocatalyst. This can be an attribute of the pesticide structure. The difference in the structure of the two pesticides is distinct: atrazine is an aromatic compound, while dimethoate is an aliphatic compound (Figure 5), which can be degraded more easily than the aromatic ring. In addition, the results reveal that the tendency of degradation in dimethoate irradiation is greater than that in atrazine in both the presence and absence of the p catalyst. This can be an attribute of the pesticide structure. The difference in the stru of the two pesticides is distinct: atrazine is an aromatic compound, while dimethoat aliphatic compound (Figure 5), which can be degraded more easily than the aromati (b) (a) Furthermore, and although the effect of dissolved constituents in water samp photodegradation of pesticides in the water samples was out of the scope of our stu is worth indicating the role which is played by this kind of matter. In general, UV rad has a destructive effect on molecular bonds. Therefore, it is expected that the organi ter in water samples can be affected the radiation by absorbing the UV light to be e and entering in series of reactions [48][49][50][51]. Despite the fact that this matter might be for our experiment because most of pesticide residues is adsorbed or absorbed in t ganic part in the targeted wastewater samples, an inhibitory effect of them on the p degradation process is also possible [52,53]. A study reported that the dissolved or matter in the excitation state act as a photosensitizer, enhancing phototransformat pesticides in addition; it may act as an antioxidant decreasing phototransformation through various mechanisms [52]. Inhibition mechanisms by natural organic matter also reported in another study [53].
The differences in the response of the different water media towards photodeg tion could be explained by this factor although it needs to be confirmed by further e iments ( Figure 6). However, previous published works highlighted the effect of dif organic species on the photodegradation process of pesticides in some aquatic sy Furthermore, and although the effect of dissolved constituents in water samples on photodegradation of pesticides in the water samples was out of the scope of our study, it is worth indicating the role which is played by this kind of matter. In general, UV radiation has a destructive effect on molecular bonds. Therefore, it is expected that the organic matter in water samples can be affected the radiation by absorbing the UV light to be excited and entering in series of reactions [48][49][50][51]. Despite the fact that this matter might be good for our experiment because most of pesticide residues is adsorbed or absorbed in the organic part in the targeted wastewater samples, an inhibitory effect of them on the photodegradation process is also possible [52,53]. A study reported that the dissolved organic matter in the excitation state act as a photosensitizer, enhancing phototransformation of pesticides in addition; it may act as an antioxidant decreasing phototransformation rates through various mechanisms [52]. Inhibition mechanisms by natural organic matter were also reported in another study [53].
The differences in the response of the different water media towards photodegradation could be explained by this factor although it needs to be confirmed by further experiments ( Figure 6). However, previous published works highlighted the effect of different organic species on the photodegradation process of pesticides in some aquatic systems and discussed this factor in detail [48,54]. In a recently published review [55], challenges in a photocatalytic process of natural water were compiled by analyzing the cyanotoxins' remediation, and the critical influence of natural organic matter was discussed.
Water 2021, 13, x FOR PEER REVIEW 11 of 15 of the two pesticides is distinct: atrazine is an aromatic compound, while dimethoate is an 242 aliphatic compound ( Figure 5), which can be degraded more easily than the aromatic ring. 243 (b) (a) Furthermore, and although the effect of dissolved constituents in water samples on 245 photodegradation of pesticides in the water samples was out of the scope of our study, it 246 is worth indicating the role which is played by this kind of matter. In general, UV radiation 247 has a destructive effect on molecular bonds. Therefore, it is expected that the organic mat-248 ter in water samples can be affected the radiation by absorbing the UV light to be excited 249 and entering in series of reactions [48][49][50][51]. Despite the fact that this matter might be good 250 for our experiment because most of pesticide residues is adsorbed or absorbed in the or-251 ganic part in the targeted wastewater samples, an inhibitory effect of them on the photo-252 degradation process is also possible [52,53]. A study reported that the dissolved organic 253 matter in the excitation state act as a photosensitizer, enhancing phototransformation of 254 pesticides in addition; it may act as an antioxidant decreasing phototransformation rates 255 through various mechanisms [52]. Inhibition mechanisms by natural organic matter were 256 also reported in another study [53]. 257 The differences in the response of the different water media towards photodegrada-258 tion could be explained by this factor although it needs to be confirmed by further exper-259 iments ( Figure 6). However, previous published works highlighted the effect of different 260 organic species on the photodegradation process of pesticides in some aquatic systems 261 and discussed this factor in detail [48,54]. In a recently published review [55], challenges 262 in a photocatalytic process of natural water were compiled by analyzing the cyanotoxins' 263 remediation, and the critical influence of natural organic matter was discussed.

270
In this study, the photodegradation of atrazine and dimethoate in aqueous solutions 271 and in real water samples such as wastewater and agricultural wastewater collected from 272 Riyadh, Saudi Arabia, was investigated. UV irradiation at λ = 254 and 306 nm was used 273 to induce the photodegradation in the presence and absence of TiO2 as a photocatalyst. 274 The study of effect of time and selected wavelength on the photodegradation was the aim 275 of this work. The samples were extracted by the QuEChERS method and analyzed using 276 gas chromatography-mass spectrometry.

Conclusions
In this study, the photodegradation of atrazine and dimethoate in aqueous solutions and in real water samples such as wastewater and agricultural wastewater collected from Riyadh, Saudi Arabia, was investigated. UV irradiation at λ = 254 and 306 nm was used to induce the photodegradation in the presence and absence of TiO 2 as a photocatalyst. The study of effect of time and selected wavelength on the photodegradation was the aim of this work. The samples were extracted by the QuEChERS method and analyzed using gas chromatography-mass spectrometry. The findings show that photoremediation was successfully achieved; the conclusions are the photodegradation increased with the increase of process time under the selected wavelengths of irradiation, particularly at λ = 306 nm. The catalyzed process with TiO 2 was more efficient than the uncatalyzed process, although the applied conditions of photodegradation were effective for both the selected pesticides; dimethoate was more easily degradable than atrazine. The photodegradation process conditions are applicable for pesticides residues in real environments. The effect of the structure of the pesticides on the photodegradation process needs to be confirmed by further studies in future. Studying other factors such as the effect of the dissolved constituents in water on the photodegradation process and the effect of other catalysts with evaluating their recyclability is recommended.