Water pollution is one the major environmental impacts of irregular and wasteful consumption of water [1–4]. Nowadays, huge quantities of different pesticides are used in agricultural and hygienic aspects, which highly contaminate water bodies. Since the majority of organic pesticides are non-degradable and carcinogenic, they are considered as a powerful category of water contaminants. Some health problems related to these substances like immunosuppressive, cytotoxic, mutagenic and endocrine-disrupting impacts have been documented. Huge amounts of effluents containing various pesticides are discharged into the environment before being treated well [5, 6]. It has been claimed that many receiving waters have been polluted by these kinds of substances. The European Economic Community advices that the concentrations of pesticides in drinking water should be less than 0.1 g.L–1 . Diazinon, which is an organophosphorus (PO), is employed to control different kinds of insects [8, 9]. It is classified by the World Health Organization (WHO) as “moderately hazardous” Class II. Low concentrations of diazinon (even 350 ng.L–1) can be highly toxic to aquatic organisms. It has been found that fatal human doses are in the range of between 90 and 444 mgkg–1. It is, thus, essential to use effective chemical and biological methods for treatment of wastewaters containing pesticides, particularly diazinon. Since most conventional ways cannot be applied to remove compounds like organic products existing in wastewaters from chemical, agrochemical and textile industries, application of advanced oxidation processes (AOPs), in which high reactive HO radicals in driving oxidation processes capable of mineralizing even less reactive contaminates are created, is entirely vital [10–12]. Cheapness, complete degradation, decomposition of pollutants even at very low amounts and degradation of toxic materials without pollution transfer to another phase are a few benefits of AOPs [13–15]. WO3 is defined as a material with good photochromic, electrochromic, gas ochromic, and photocatalytic properties [16–18]. Further, compared to TiO2 and ZnO, WO3 is capable of absorbing more solar light in the visible region because its band gap is about 2.7 eV [16, 17]. However, when WO3 is used separately, its photocatalytic activity is weak because of its relatively low conduction-band level (0.5 V vs) [16, 17]. For improving the photocatalytic characteristic of WO3, Fe, Pt, Cu, and CuBi2O4 are employed for synthesis and modification of it [16, 17, 19, 20]. It should be pointed that the multielectron O2 reduction can increase the photocatalytic activity of these materials. The magnetic separation, which is an environmentally benign method, has been used by many researchers as it creates no pollutants and can remove huge volumes of effluents in a very short time . A magnetic composite, which is widely used, is Fe3O4 having superparamagnetic characteristics and can be recovered by means of an external magnetic field and reused without losing the active sites. Thus, because of the high conductivity of Fe3O4 and CB level (1 V vs. NHE), we were convinced to apply it in concert with WO3 for improving photocatalytic efficiency through increasing charge transport. In recent years, many researchers have used the nanoparticles such as surface-modified Fe3O4 for treatment of effluents containing dyes even though the preparation of these magnetic adsorbents is not comfortable [22, 23]. Monolayers can be formed on several surfaces by aminosilane coupling agents; by doing this, the properties of the surface are changed and NH2 groups are introduced into the modified surfaces. It has been reported that an aminosilane was employed to modify the surfaces of magnetite particles for lipase immobilization, wear properties improvement, and DNA purification. It should be noted that more studies are needed to survey the photocatalytic characteristics of aminosilane modified Fe3O4 and other nanoparticles for wastewaters containing pesticides. The aminopropyltriethoxysilane (APTES) polymers are used for gaining cationic amine groups on magnetic particle. The grafting reaction conditions of APTES are easier and simpler in comparison with most organosilane molecules.
In this research, in the presence of UV irradiation Fe3O4-WO3-APTES nanoparticles were first synthesized as a new catalyst. The nanoparticles were then used for photocatalytic degradation of diazinon from synthetic and real water samples. Also, the effects of operating variables like solution pH, catalyst dosage, initial diazinon concentration, different purging gases, H2O2 concentration, and type of organic compounds on the removal efficiency of diazinon were studied in presence of UV irradiation. Furthermore, a kinetic study was performed and simulated with the zero, first, second and Langmuir–Hinshelwood kinetic models. In order to investigate the cost-efficiency of the processes, the electrical energy per order (EEo) was calculated.
2 Materials and methods
All chemicals: tungsten chloride, FeCl3.6H2O, FeCl2.4H2O, 3-aminopropyltriethoxysilane (APTES), sodium hydroxide, folic acid, citric acid, humic acid, oxalic acid, phenol, EDTA and hydrochloric acid were of the highest available quality and procured from Merck (Darmstadt, Germany) and used without further purification. Diazinon (O,O-diethyl O-[6-methyl-2-(1-methylethyl)-4-pyrimidinyl] was obtained from Chem-service (USA). Table 1 presents the chemical structure and properties of diazinon. Figure 1 shows the absorption spectra of diazinon. The light source was supplied with a 125- W medium-pressure UVC lamp emitting a maximum wavelength at 247.3 nm. The light intensity of the UVC lamp was 1,020 μwcm–2 recorded by a digital radiometer (Model DRC-100X, SPECTROLINE) combined with a DIX-365 radiation sensor (ShokofanTosee, Iran). The photocatalytic reactor utilized in a previous study was applied to remove diazinon . The pH values were measured by a Metrohm model 713 pH-meter and NaOH or HCl (0.1 molL–1) were used for adjusting pH.
2.2 Synthesis of Fe3O4-WO3- APTES nanoparticles
The Fe3O4-WO3-APTES nanoparticles were synthesized by means of two simultaneous methods: co-precipitation and hydrothermal [21, 25, 26]. A schematic diagram of the preparation of the Fe3O4-WO3- APTES nanoparticles has been shown in Figure 2. X-ray diffraction (XRD, Siemens D-5000, Germany), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM, Mira3, Tescan, Czech Republic), energy dispersive X-ray (EDX), vibrating sample magnetometer (VSM, MDKFD, Iran), pHpzc techniques were used for characterization of the prepared nanoparticles.
2.3 Experimental procedure and analysis
The fresh stock solutions of the pesticide containing 1,000 mg.L–1 of diazinon were prepared weekly and stored at 4 °C and the working samples were prepared daily by dilution of the stock solutions in doubled distilled water. A certain dose of the photocatalyst (0.1–1 g.L–1) was added to 1 L of the diazinon solution (10–50 mg.L–1) and, within all runs, the pH values were set from 3 to 11. All experiments were performed under ambient conditions for 2 h. The batch experiments were conducted at 25±1oC and continuously stirred. For the photocatalyst removal of the pesticide, a solution, containing known contents of diazinon and the nanocatalyst, was prepared and allowed to equilibrate for 30 min. A 125- W medium-pressure UVC lamp, placed into the batch photoreactor, was used for irradiation. At each stage, 10 mL of the suspension was taken at definite time intervals. The nanocatalyst was separated from the mixture solution by permanent magnet for 2 min. The residual contents of diazinon in all samples were detected via a spectrophotometer (UV/Vis Spectrophotometer, Hach-DR 5000, USA) at λmax= 300 nm by a calibration curve depicted based on the Beer-Lambert law . It should be noted that a few experiments were performed in triplicate and mean values of the data were reported. The error bars have not been shown in the figures because all standard deviations never exceeded ±1.5 %.
3 Results and discussion
3.1 Characterization of nanocatalyst
3.1.1 XRD analysis
The XRD patterns of the nanoparticles: WO3, Fe3O4, Fe3O4-WO3 and Fe3O4-WO3-APTES have been illustrated in Figure 3. The patterns exhibit the crystalline structure of both WO3 and Fe3O4 even after coating the Fe3O4 nanoparticles onto WO3. The main peaks at 2θ values of 18.27, 21.16, 30.11, 30.21, 35.42, 35.53, 37.03, 37.18, 43.12 and 57.09 corresponded to the (011), (002), (112), (200), (121), (103), (022), (202), (004) and (321) planes of orthorhombic Fe3O4 (JCPDS card no. 031156) . And, the main peaks at 2θ values of 23.707, 24.099, 26.587, 28.776, 34.022, 35.525, 41.524, 50.494, 54.302, 55.116, 57.677 and 62.446 corresponded to the (020), (200), (120), (111), (220), (121), (221), (112), (041), (401), (331) and (430) planes of orthorhombic crystalline WO3 (JCPDS card no. 36-1451) . As illustrated in Figure 3, the XRD peaks related to the nanoparticles of Fe3O4 and WO3 are still observed after the modification with APTES. These results indicate the formation of mixture of Fe3O4 and WO3 nanoparticles and APTES. The average crystalline size of the WO3, Fe3O4, Fe3O4-WO3 and Fe3O4-WO3-APTES nanoparticles were calculated with the Debye–Sherrer’s equation. The mean crystallite size of the nanoparticles: WO3, Fe3O4, Fe3O4-WO3 and Fe3O4-WO3-APTES were estimated to be 23, 8, 19 and 19.3 nm, respectively.
3.1.2 FT-IR analysis
Since photocatalytic reactions often take place on nanocatalysts’ surface, the functional groups on the surface of the nanocatalysts are very important. Thus, the FT-IR analysis on the surface of the nanoparticles: WO3, Fe3O4, Fe3O4-WO3 and Fe3O4-WO3-APTES nanoparticles was performed in the range of 400–4,000 cm–1 (Figure 4). The Fe3O4 nanoparticles showed significant absorption peaks at 447, 580, 860, 1,403, 1,623, 3,378, 3,788, and 3,850 cm–1 . Also, the FT-IR spectrum of the WO3 nanoparticles showed significant absorption peaks at 817.40, 1,449.94, 1,639.29, 2,360.37 and 3,398.37 cm–1 . The FT-IR spectrum of the Fe3O4-WO3 nanoparticles presented significant absorption peaks at 445.85, 573.99, 813.4, 940.17, 1,395.19, 1,624.45, 2,360.99 and 3,385.3 cm–1. FT-IR spectrum of the Fe3O4-WO3-APTES nanoparticles indicated significant absorption peaks at 444.19, 577.47, 793.93, 944.77, 1,114.47, 1,221.31, 1,385.18, 1,507.55, 1,629.27, 2,361.09, 3,410.10, 3,741.3 and 3,853.91 cm–1. The two distinct absorption peaks at 580 and 447 cm–1 are attributed to the vibrations of Fe3+-O2– and Fe2+-O2–, respectively . The peaks between 700 and 900 cm–1 are assigned to the aromatic C-H stretching. The peak at ~3,400 cm–1 belongs to vibration of -OH group. For the Fe3O4 nanoparticles, the band observed between 600 and 800 cm–1 corresponds to ν(W-Ointer-W) and ν (W-Ointra-W) . For the WO3 nanoparticles, the band observed between 2,800 and 3,500 cm–1 corresponds to the ν(HOH) and ν(OH) . The band observed between 1,600 and 1,650 cm–1 corresponds to δ H2O . The FT-IR analysis supported coating the Fe3O4 nanoparticles onto the WO3 nanoparticles. After amine-modified Fe3O4-WO3 nanoparticles with APTES, the new peaks at 1,507.55, 1,385.18, 557.47 and 600–800 cm–1 corresponding to N–H, C–N bending vibration, Fe–O vibration of Fe3O4 and ν(W-Ointer-W) and ν (W-Ointra-W) were seen in the spectrum of the Fe3O4-WO3-APTES nanoparticles. These observations evidence that amine groups have been successfully introduced on the surface of the Fe3O4-WO3 nanoparticles, forming the APTES - Fe3O4-WO3 nanocatalyst.
3.1.3 SEM, EDX and VSM analysis
Scanning electronic microscopy (SEM) was used to investigate the morphology, shape and size of the WO3, Fe3O4, Fe3O4-WO3 and Fe3O4-WO3-APTES nanoparticles. The SEM images of the WO3, Fe3O4, Fe3O4-WO3 and Fe3O4-WO3-APTES nanoparticles have been shown in Figure 5a-d, respectively. Figure 5d clearly illustrates the distribution of APTES on the surface of the Fe3O4-WO3 nanoparticles. Also, Figure 5d shows the sphere structure for the Fe3O4-WO3-APTES nanoparticles. The EDX patterns of the Fe3O4, Fe3O4-WO3 and Fe3O4-WO3-APTES nanoparticles have been presented in Figure 6a. According to the results of EDX analysis, weight percentages of C, O and Fe in Fe3O4 nanoparticles were 20.94, 26.07 and 45.28 %, weight percentages of O, Fe and W in Fe3O4-WO3 nanoparticles were 12.53, 78.29 and 11.68 %, and weight percentages of C, O, Fe and W in Fe3O4-WO3-APTES nanoparticles were 2.06, 21.19, 64.29 and 9.98 %, respectively. Therefore, the synthesized compound was composed of Fe, O, and W, indicating the formation of the Fe3O4-WO3-APTES nanoparticles. VSM magnetization curve of the samples at room temperature has been depicted in Figure 6b. The saturated magnetization values were 58.97, 43.83, 42.74 and 0.0043 emug–1, respectively, for the Fe3O4, Fe3O4-WO3-APTES, Fe3O4-WO3 and WO3 nanoparticles.
3.2 Effects of operational parameters on the photocatalytic degradation of diazinon
3.2.1 Effect of initial pH
In this study, pH levels were set between 3 and 11 for studying the pH impact on diazinon removal at the constant dose of 0.25 g.L–1 of the Fe3O4-WO3-APTES nanoparticles. It should be noted that the pH value was 5.63 at the beginning of the reaction. Generally, the degradation efficiency fluctuated widely by changing the levels of pH; at pH values of 3, 5, 6, 7, 8, 9 and 11, respectively, 58.42, 88.9, 91.4, 96.4, 75.15, 45.15 and 33.88 % of diazinon was removed (Figure 7). As can clearly be seen, the highest removal efficiency happened at pH 7 because, in either acidic or basic conditions, the APTES nanoparticles are corroded. The values of pHpzc of the WO3, Fe3O4, Fe3O4-WO3 and Fe3O4-WO3-APTES nanoparticles were 5.09, 6, 7.03 and 8.22, respectively. It can be pointed that at different pH levels various electrostatic reactions take place between on the surface of the Fe3O4-WO3-APTES nanoparticles and the pesticide [8, 28]. The value of pKa of diazinon was found to be 2.6 , in which the nanoparticles of Fe3O4-WO3-APTES are positively charged and negatively charged at pH values of bellow 8.2 and above 2.6, respectively. The positively charged Fe3O4-WO3-APTES nanoparticles and negatively charged diazinon molecules can quickly attract together in an optimal condition in which pH ranges from pKadiaiznon to . A lower extent of adsorption was observed on the nanoparticles’ surface, at high pHs, as both diazinon and the Fe3O4-WO3-APTES nanoparticles are negatively charged and, in turn, the electrostatic repulsion happens between them [8, 28]. As mentioned above, the highest performance was seen at the pH value of 7; therefore, all other experiments were carried out at this level.
3.2.2 Effect of photocatalyst dosage
In the present research, different doses (0.1–1 g.L–1) of the nanocatalyst were used to evaluate the removal efficiency of the pesticide at the constant amount of diazinon (20 mg.L–1) and pH of 7. As can be seen in Figure 8, a marginal efficiency was observed at time zero (without UV irradiation) for different dosages. The removal efficiency improved between 68.7 and 96.4 % with raising the dose of the nanocatalyst between 0.1 to 0.25 g.L–1. After that, a downward trend was seen in efficiency. The main reasons for this improvement in removal efficiency are as follows: when the dose of the nanocatalyst is raised, there will be more diazinon molecules that can be adsorbed on the surface of the catalyst and the density of catalyst particles increases in the area of illumination [8, 9, 28, 29]. In the case of catalyst doses higher than 0.25 g.L–1, a downward trend was experienced in diazinon removal as light scattering decreasing the aggregation and the activity of the catalyst particles takes place. In this study, the dose of 0.25 g.L–1 was selected as the optimal value for the Fe3O4-WO3-APTES nanoparticles. The highest removal efficiency happened at the photocatalyst dose of 0.25 g.L–1. Therefore, more examinations were performed at this value. It should be stated that the photocatalytic reduction of diazinon went up between 65.32 and 96.4 % with rising time of irradiation from 15 to 120 min.
3.2.3 Kinetics and electrical energy studies
For investigation of the kinetic information, some examinations were done under the following conditions: initial diazinon concentration of 10, 20, 30, 40, 50 mg.L–1, initial pH of 7 and constant nanocatalyst dose of 0.25 g.L–1. In order to achieve the kinetic information, the zero, first and second order equations (eqs (1)–(3)) were used [8, 30]: (1) (2) (3)
where [Diazinon]0 is the initial concentration of diazinon (mg.L–1), (mol.L–1 min–1), kobs (min–1) and (L mol–1 min–1). So as to attain kinetic variables for the photocatalytic destruction of the pesticide, ln[C0/Ct] and versus t was plotted. The kobs values at different initial concentrations (Table 2) were calculated from the slope between ln([Diazinon]0/[Diazinon]) versus reaction time. Table 2 presents a breakdown of the kinetic variables of zero, first and second-order reactions for the removal efficiency at different initial diazinon concentrations. The first-order model explained better the photocatalytic degradation rate.
As can be seen from eqs (4) and (5), the relationship between the first-order rate constant (kobs) and initial diazinon concentration was described better by the Langmuir–Hinshelwood (L-H) model [8, 30]. (4) (5)
where kc (mg.L–1 min–1) is the kinetic rate constant of surface reaction and KDiazinon (mg.L–1)–1 is the Langmuir adsorption constant. KDiazinon and kc were 0.085 (Lmg–1) and 0.2016 (mg.L–1 min–1) by plotting (kobs)–1 versus initial concentration of diazinon, respectively.
EEo value, which is the number of kWh of electrical energy required to decline the content of a contaminant by 1 order of magnitude (90 %) in 1 m3 of polluted water, of the photocatalytic reduction of the pesticide was studied in terms of economic views (eq. (6) and (7)). (6) (7)
where P is the sum of input power (kW), t is the irradiation time (min), V is the volume (L) of the wastewater, and kobs is the first-order rate constant (min–1) for the decay of the pollutant. The EEo values for the processes studied in this research have been shown in Table 2. As can be seen, the EEo value raised between 47.62 and 941.18 kWhm–3 by rising diazinon content between 10 and 50 mg.L–1. Furthermore, the process of UV/Fe3O4-WO3-APTES had the lowest EEO value (172.44 kWhm–3) as compared with other methods: UV/WO3 (672.74 kWhm–3), UV/Fe3O4 (845.02 kWhm–3), UV (1,016.11 kWhm–3) and UV/Fe3O4-WO3 (18,898.92 kWhm–3).
3.2.4 Effect of different purging gases
In order to study the impact of the variable of purging gases, oxygen and nitrogen gases at 2 L.min–1 flow rate were used under the condition: Fe3O4-WO3-APTES nanoparticle dosage of 0.25 g.L–1, constant diazinon concentration of 20 mg.L–1 and pH of 7. The impacts of purging gases and change of DO content have been presented in Figure 9. As can be clearly seen, in the presence of both gases, the process experienced a lower efficiency compared the method in which no gas was utilized. In ambient condition, the removal efficiency boosted by 31.08 % when time was raised from 15 to 120 min. And, in the case of oxygen and nitrogen, the efficiencies ranged between 53.44 to 83.83 % and 27.29 to 61.68 %, respectively. Since O2 and N2 gases serve as an electron scavenger competing with diazinon ions for capturing electrons, using these gases results in a decrease in efficiency .
3.2.5 Effect of hydrogen peroxide
It is imperative that the optimum value of H2O2 be selected because it raises the photocatalytic performance. It should be pointed that the type and quantity of contaminants influence the selection of this optimum point. In this study, the effect of different H2O2 concentrations (2–50 mM) on the removal efficiency was investigated under the conditions: diazinon concentration of 20 mg.L–1, Fe3O4-WO3-APTES nanoparticle dosage of 0.25 g.L–1, and pH of 7. It can be seen in Figure 10 that a rise in the content of H2O2, the efficiency went up between 96.4 and 99.23 %, but it dropped to 47.23 % at the concentration of 50 mM. H2O2 can react with electron in the conduction band of Fe3O4-WO3-APTES nanoparticles, leading to an enhancement in diazinon removal. Equations (8) and (9) state that H2O2 can prevent the electron–hole recombination [31, 32]. H2O2 can act as an alternative electron acceptor to oxygen because it is a better electron acceptor than dissolved oxygen. Hence, a significant improvement in diazinon removal is expected in the case of low contents of H2O2 on account of the inhibition of the electron–hole recombination. Since H2O2 is a powerful scavenger of electron and •OH, at high concentrations of H2O2, the reaction between diazinon and positive holes or •OH in the valence band of the Fe3O4-WO3-APTES nanoparticles can be inhibited.
3.2.6 Effect of organic compounds
A few organic combinations: citric acid, folic acid, EDTA, oxalic acid, phenol and humic acid (equal to 20 mg.L–1) were used in the present study to investigate the influence of these matters on the efficiency under the following conditions: initial diazinon content of 20 mg.L–1, Fe3O4-WO3-APTES nanoparticles dosage of 0.25 g.L–1 and initial pH of 7. Figure 11 illustrates that 96.4, 70.3, 69.32, 67.37, 65.23, 60.12 and 3.22 % of the pesticide was removed in the case of without organic compounds, citric acid, folic acid, EDTA, oxalic acid, phenol and humic acid, respectively. Addition of an organic compound results in a decrease in the efficiency because of the fact that the interference happens among adsorbed organic molecules; as a result, the diazinon molecules do not have access to photocatalyst’s surface [8, 13]. The structure of organic compounds is comprised of many aliphatic and aromatic organic molecules that can occupy the photoactive sites on the surface of the nanocatalyst. Also, these molecules that are subject to the same chemistry as the target analyte can leave fewer sites capable of removing diazinon molecules. Therefore, a slow degradation of diazinon is expected to happen due to competition among molecules.
3.2.7 The comparison of each process and reusability
In this research, the performances of some photocatalytic methods: Fe3O4-WO3-alone, UV/Fe3O4-WO3, Fe3O4-WO3-APTES-alone, WO3-alone, Fe3O4-alone, UV-alone, UV/Fe3O4, UV/WO3 and UV/Fe3O4-WO3-APTES were studied and compared at the same optimum reaction conditions at initial diazinon concentration of 20 mg.L–1, nanoparticle dose of 0.25 g.L–1 and initial pH of 7. Removal efficiencies for the processes were 4.97, 2.97, 2.98, 17.97, 23.52, 43.27, 49.38, 57.48, and 96.4 % respectively (Figure 12). When adsorption was applied separately, it could not remove the pesticide dramatically. But the highest removal efficiency was achieved by the UV/Fe3O4-WO3-APTES method. The results of the photocatalytic reduction of diazinon via the UV/Fe3O4-WO3-APTES method extracted in this research were compared with other reported data. The efficiencies and kinetics parameters of the processes have been compared in Table 3. The findings indicated that the catalyst of UV/Fe3O4-WO3-APTES is suitable to remove this pesticide from aqueous media as compared with Fe3O4 and WO3-alone. In other words, the synergistic effect between Fe3O4 and WO3 is entirely useful in the photocatalytic characteristics of UV/Fe3O4-WO3-APTES. Since the CB level of Fe3O4 is lower, the photogenerated electrons are transferred from the surface of WO3 to Fe3O4 [13, 21, 26]. Moreover, since the conductivity of Fe3O4 is by far higher than that of WO3, the transport rate of the electrons is quick, thereby declining the recombination of the photogenerated electrons and holes. Also, owing to the interface effects (like potential barrier) of the hybrid process of Fe3O4/WO3, a decline in recombination of photogenerated electron–hole pairs is expected. The collected electrons may be used by a multielectron O2 reduction mechanism.
Based on the above experiments and analysis, a plausible mechanism for the photocatalysis can be proposed as follows:
When Fe3O4-WO3-APTES is illuminated with the UV light (λ< 390 nm), an electron excites from the valence band to the conduction band to give separation of electron and hole (eq. 13): (13)
Next, the electron can react with O2 and H2O2. Further, positive hole reacts with H2O or OH– to create highly reactive radical species like hydroxyl radicals, which can oxidize unselectively organic compounds and their degradation intermediates. Additionally, the holes can oxidize directly many contaminants [13, 21, 26, 30]. (14) (15) (16) (17)
Another reason for the mechanism of performance improvement degradation of diazinon is, the APTES behaves as Si–N co-doped in Fe3O4-WO3 that can generate intermediate level between valence band and conduction band, thus not only reducing in the band gap but also restraining the electron–hole recombination. Further, the photocatalytic activity can be enhanced via the monolayer and/or multilayer of amino-silane, which can increase the oxygen chemisorption on the surface of Fe3O4-WO3 .
Several researchers have stated the mechanism of diazinon decomposition; GC–MS tests have claimed that a few molecules and single ring structures are formed during the photocatalytic decomposition. Also, in many studies, the temporal distribution of species over the phtocatalytic degradation of diazinon in aqueous environments via AOPs like ozonation and UV/H2O2 at different pH values has been surveyed [34, 35]. For instance, in the study by Shemer and Linden, 2-isopropyl-6-methyl-pyrimidin-4-ol and diethylthiophosphoric acid were found to be the products of the process: UV and UV/H2O2 . And, Nakaoka et al. concluded that species such as 2-isopropyl-6-methyl-pyrimidin-4-ol, IMP and diazoxo are generated during the removal of this pesticide through Xe lamp/platinized TiO2 . Shirzad-Siboni et al. also applied the UV/Cu doped ZnO process for degradation of diazinon from aqueous environments and they reported that the following products are created: N-Benzyl-N-ethyl-p-isopropylbenzamide, Pentasiloxane, dodecamethyl-, Silane, cyclohexyldimethoxymethyl-, Tridecane, 1-iodo-, Hexadecane, Silane, (4-methoxyphenoxy)trimethyl-, Heptadecane, 8-methyl-, Decane, 2,4,6-trimethyl-, Tetradecane, 4-methyl-, Phenol, 2,4-bis(1,1-dimethylethyl)-, Dodecane, Tridecane, and 1-iodo .
Some authors have stated that sulfate ions are formed at a very early period of the diazinon degeradation and then phosphate, carbonate and nitrate ions are generated. The structure of diazinon can express these results. The diazinon molecules detach sulfur groups, which are oxidized to sulfate ions. Many researchers have also reported similar findings and complete degradation pathways of organophosphorus pesticides in aqueous environments by AOPs [15, 30, 37, 38].
As the reusability of a photocatalyst is of great importance after reaction, the photocatalyst loaded with the pollutant was placed in the 2 M NaOH solution as a desorbing agent; all photocatalytic runs were repeated five times. It can be seen in Figure 13 that the UV/Fe3O4-WO3-APTES showed quite similar photocatalytic activity after 120 min over five repeated experiments.
3.3 Removal of diazinon from real water samples
So as to evaluate the performance of the method: UV/Fe3O4-WO3-APTES for removing the pesticide from a real water sample, 20 mg.L–1 of diazinon was added to a real water sample from the drinking water distribution system in Rasht City, Iran. Table 4 shows a breakdown of the properties of the real water. Since real water consists of various materials like sulphate, chloride, carbonate, bicarbonate, nitrate and nitrite, a decrease in photodegradation of the pesticide is expected (see Figure 14). Active sites on the surface of Fe3O4-WO3-APTES can be blocked by these ions, which may deactivate the catalysts for diazinon and intermediates . Although the created radical anions have oxidation properties, they are considered as weak oxidants as compared with hydroxyl radicals. In comparison with distilled water, lower removal efficiency was seen in diazinon photodegradation; the reason for this may be because of carbonate and bicarbonate ions in real water increasing the level of pH. In the presence of Fe3O4-WO3-APTES, the pH of the real water comprised the pesticide declined from 7.9 to 6.8, probably due to CO2 formation. But the specific conductivity of the solution improved from 0.9 to 0.99 during the process of UV/Fe3O4-WO3-APTES because of the photocatalytic destruction of the pesticide form inorganic ions like SO42−, PO43− and NO3−, thereby reducing the conductivity.
In the current research, we synthesized the Fe3O4-WO3-APTES by means of a facile co-precipitation technique and the effects of some operating parameters: pH, Fe3O4-WO3-APTES nanparticles dosage, initial diazinon concentration, oxygen and nitrogen gases, H2O2 concentration, and type of organic compounds were investigated. In order to characterize the nanparticles of Fe3O4-WO3-APTES, XRD, FT-IR, SEM, EDX, VSM and pHpzc techniques were applied. Among all processes used in this study, the highest removal efficiency was achieved by the UV/Fe3O4-WO3-APTES process at pH 7.The photocatalytic destruction of the pesticide increased with raising the dose of the nanocatalyst up to 0.25 g.L–1. KDiazinon and kc were found to be 0.085 (Lmg–1) and 0.2016 (mg.L–1min–1), respectively, by using the first-order kinetic assumption. In comparison with the UV/WO3 and UV/Fe3O4 process, the UV/Fe3O4-WO3-APTES processes had lower electrical energy consumption per order of magnitude. In addition, the photodegradation of diazinon declined in both purging gases of oxygen and nitrogen and organic compounds. It should be noted that the photocatalytic activity remained unchanged even after five successive cycles. Approximately 86.17 % of diazinon in a real water sample was removed after 120 min under the optimal circumstances.
This paper is issued from an integrated research of IR.GUMS.REC.1395.170 as a project number MS student of Kobra Ayagh. Financial support was provided by Guilan University of Medical Sciences, Rasht, Iran.
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Published Online: 2017-04-01