Degradation of Dazomet by Thermal Fenton and Photo-Fenton Processes under UV and Sun lights at Different Temperatures

In this research, the degradation of Dazomet has been studied by using thermal Fenton process and photo-Fenton processes under UV and lights sun. The optimum values of amounts of the Fenton reagents have been determined (0.07g FeSO4 .7H2O, 3.5μl H2O2) at 25 ° C and at pH 7 where the degradation percentages of Dazomet were recorded high. It has been found that solar photo Fenton process was more effective in degradation of Dazomet than photo-Fenton under UV-light and thermal Fenton processes, the percentage of degradation of Dazomet by photo-Fenton under sun light are 88% and 100% at 249 nm and 281 nm respectively, while the percentages of degradation for photo-Fenton under UV-light are 87%, 96% and for thermal Fenton are 70% and 66.8% at 249 nm and 281 nm respectively. In this research the effect of temperature on all the reactions has been studied in the range 25°C-45°C, it has been noticed that the reaction rate constant (k) has increased with increasing temperature, and the best percentage degradation of Dazomet was at 45°C in all processes, so, the thermodynamic functions ΔG * , ΔH * , ΔS * have been calculated.


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The photo-Fenton processes consist of in the combination of ferrous ions (Fe +2 ), hydrogen peroxide (H2O2), and UV irradiation (18,19). The photo-Fenton process can be divided into the following steps; the first stage is the so-called Fenton reaction, in which ferrous ions (Fe +2 ) are oxidized to ferric ions (Fe +3 ), giving rise to hydroxyl radicals (HO • ) as shown in eq.1 The ferric ions, represented by the complex [Fe (OH)] +2 , is reduced back to Fe +2 by absorption UV irradiation, according to following equation The generated hydroxyl radicals can degrade organic pollutants (RH) present in aqueous solutions. Parameters such as pH and reagent concentration ratios must be controlled, in order to enhance the efficiency of the degradation process, as shown in the following equation ( Hydrogen peroxide concentration plays a more crucial role in deciding the overall efficiency of the degradation process. (3) Recently, the possibility of combining the solar photo-Fenton and biological process has also been proposed considering the increase in biodegradability after photo-Fenton process. (20,21,22) In this paper the degradation of the Dazomet has been studied by Fenton reactions using UV-visible spectrophotometer to observe the degradation of the Dazomet. Table 1 shows all chemicals used in this work, their purity available and supplier companies.  Table 2 illustrates the list of instruments used in this work, their manufactures, brand, and country of supplier.

Irradiation System
In this work irradiation system ( Fig. 2) consists of Pyrex photoreaction cell 75 ml capacity with a quartz window fitted with a focusing lens to ensure parallel beam of light. The mercury lamp 125 watt was placed 10 cm apart from the photoreaction cell. The cell is supplied with two openings of 0.5 cm in diameter and one of these was used for sampling processes. The magnetic stirrer was used to keep the catalyst aqueous suspension in homogeneous form during the photolysis experiments, and the circulating thermostat was used to control the reaction temperature.  According to the results of calibration curve, it has been found that the best concentration to use in experiments is 25 ppm where above the concentration the absorbance is higher than 2 as (beer-lambert law) and the absorbance values recorded through this work transferred to concentration using calibration curve.

Procedure of Degradation of Dazomet by Thermal Fenton, Photo-Fenton Reaction and Solar Fenton.
All of the experiments were carried out in the photoreaction cell using (irradiation system) Fig. 2. Firstly several concentrations of H2O2 (30%) were added, secondly different concentrations of FeSO4 .7H2O were used, thirdly different buffer solutions pH (3,5,7,9) (prepared by using HCl, NaOH, C8H5KO4, K2HPO4, C4H11NO3) were used to find the best media for the reaction, circulating thermostat was used to control the reaction temperature, while magnetic stirrer was used to mix the mixture, the solutions was irradiated with medium mercury lamp (in photo-Fenton processes). Then 5ml aqueous solution of mixture from each experiment is taken at various time intervals (10 min during 90 min), then FeSO4 .7H2O was removed from the samples by using centrifuge. After that, the absorbance of Dazomet was measured by using UV-Visible spectrophotometer at wavelengths 249 nm and 281 nm to determine the optimum values of FeSO4 .7H2O , H2O2 and pH . A series experiments was repeated using different temperatures (25,30,35,40 and 45°C) to determine the best temperature for degradation of pesticide at optimum values of catalysts. A solar Fenton experiment was carried out by putting 75 ml of Dazomet solution 25ppm on Pyrex beaker and optimum values of FeSO4 .7H2O, H2O2 were added at pH 7 then covered by thick quartz plate which the sun light could enter through quartz plate. The experiment was performed at 12 pm on mid July 2017. The percentage degradation of dazomet was calculated by using the following equation.   irradiation time 90 min, pesticide removal increases with increasing the amount of H2O2 till 3.5 µl , where the percentages of degradation was 78% and 89% at 249 and 281 nm respectively. After that, the percentage of removal decreases with increasing the amount of H2O2. From the results, it can be noticed that the excess H2O2 does not improve the removal efficiency, because H2O2 coverts to oxygen and water. (23)

Effect of Ferrous Sulphate Heptahydrate FeSO4 .7H2O Amount
Various amounts of Ferrous sulphate heptahydrate (FeSO4 .7H2O) were added to aqueous solution of Dazomet (25 ppm) to determine the optimum value of ferrous sulphate heptahydrate at fixed amount of H2O2 (3.5 µl) and 25 0 C within irradiation time 90 min. Fig. 6 shows the degradation percentage (D%) of Dazomet, it can be observed that the degradation of Dazomet increases with increasing the amount of FeSO4 .7H2O till attaining an optimum value of FeSO4 .7H2O, which is 0.07 g, where the percentages of degradation reach to 87% and 96% at 249 and 281 nm respectively. After that, the degradation will be decreased, owing to the fact that when Fe +2 concentrations become higher, most of the HO • radicals were used in the side reactions effectively. (23)  Fig.7 shows the effect of pH on the degradation of Dazomet in aqueous solution at different pH ranging from 3-9. It can be easily depicted that the maximum degradation achieved at pH 7 (optimum pH) where the percentages of degradation was 54% and 63% at 249 and 281 nm respectively at fixed amounts of FeSO4 .7H2O (0.07g) and H2O2 (3.5 µl). This may happen because at higher pH, ferrous ions get easily converted to ferric ions, which have a tendency to produce ferric hydroxo complexes with H2O2. While at lower pH, the HO • radical scavenges by H + ions and therefor may inhibit the radical forming activity of iron. It is observed that the highest percentage of degradation was at 3.5µl. It can be explained that more of H2O2 molecules, increases the number of HO • radicals. Therefore, the rate of reaction increases but on further increasing of H2O2, while the rate of reaction decreases because most of the HO • radicals are consumed by the H2O2 and there are less HO • radicals available which will slow down the reaction but there is less degradation compared with photo-Fenton process. (25) Fenton experiments were performed in dark by covering the aqueous solution of Dazomet container with aluminum foil to make it away from light and magnetic stirrer was used so that proper mixing takes place. Fig.9 shows that the optimum value of FeSO4 .7H2O is at irradiation time 90 min and temperature 25 0 C with fixed amount of H2O2 (3.5µl) . It has been noticed that the percentage degradation of dazomet with thermal Fenton reaction at optimum value of FeSO4 .7H2O (70%, 66.8% at 249 and 281 nm respectively) was less than percentage removal of dazomet with photo Fenton reaction where the percentages of degradation were 87%, 96% at 249 and 281 respectively. The reason is when there is no light during the reaction, Fe +3 ions are accumulated in the system and after Fe +2 are consumed, the reaction partially stops. (26)

Effect of pH
The pH effect on thermal Fenton reaction for Dazomet degradation in aqueous solution was studied as shown in Fig.10. The degradation was investigated in the pH range 3 -9 at fixed amounts of FeSO4 .7H2O (0.07g) and H2O2 3.5 µl), the best value was at pH 7 where the percentages of degradation reach to 44.7% and 51.5% at 249 and 281 nm respectively. When the other parameters were kept constant. In the dark, the reaction has been explained to slow down relative to the photo Fenton reaction after depletion of H2O2 because the reconversion of Fe +3 to Fe +2 necessary for the subsequent production of HO • radicals is stopped. (26)

Figure 10. Percentage Degradation of Dazomet on Thermal Fenton Reaction with Varying pH
Solutions at a) 249 nm and b) 281 nm.

Sunlight As Light Source in Photo Fenton Process (Solar Photo Fenton Process)
The degradation experiment of 25 ppm Dazomet in aqueous solution was carried out using sunlight as light source through 15 July 2017 at 12 pm at optimum values of FeSO4. 7H2O, H2O2 and pH (0.07 g, 3.5µl and pH 7 respectively), at irradiation time 90 min. Fig. 11 shows the results of degradation relation with time. According to the results, the solar photo-Fenton process is more efficient for wastewater treatment than others photocatalytic process in neutral conditions

Dazomet Degradation Results Using Photo Fenton, Thermal Fenton, and Solar Photo Fenton Processes
It has been found that the highest degradation of aqueous solution of Dazomet has been obtained from solar photo-Fenton reaction as shown in Table 3. The reasons for these results were explained from the mechanism described early by the equations (1)(2)(3)(4)(5). Irradiation of the Fenton reaction not only regenerates Fe +2 , the crucial catalytic species in the Fenton reaction, but also produces an additional hydroxyl radical, the species responsible for the degradation of organic material. As a consequence of these two effects, the photo-Fenton process is faster than the conventional thermal Fenton process. Moreover, since Fe +2 is regenerated by light with decomposition of water rather than H2O2, the photo-Fenton process consumes less H2O2and requires only catalytic amounts of Fe +2 . The photo-Fenton reaction has several operational and environmental advantages. Therefore the positive effect of irradiation under UV-light or solar light on the rate of degradation is regenerated Fe +2 , which leads to increase the • OH radicals compared to Fenton process. This means photo-reduction of Fe +3 to Fe +2 ions produce secondary HO • radicals with H2O2 according to the mechanism of the photo Fenton reaction (eq. 6-8). (27,28) Effect of Temperature on Reactions Fig.12 shows the results from series experiments performed by using the optimum values of FeSO4. 7H2O and H2O2 (0.07g FeSO4 .7H2O, 3.5µl H2O2) at pH 7 at different temperatures in the range (25-45) °C. It has been found that the Dazomet percentage degradation in aqueous solution increases with increasing temperature. These results could arise from increasing the generation of ·OH, which would enhance the Dazomet degradation. The Arrhenius expression, showing the relationship between the reaction temperature and rate constant k is expressed as follows: where: A is the pre-exponential (or frequency) factor, E is the activation energy (cal•mol -1 ), R is the gas constant (1.987 cal mol -1 K -1 ), T is the reaction absolute temperature (K) In summary, the temperature has a positive influence on the Dazomet degradation in the photo Fenton reaction and thermal Fenton, but the required activation energy for the photoreaction less the the activation energy for the thermal as we belive that the existence of light in photoreaction enchane the reaction more than in thermal reaction as shown in Table 4. (29, 30, 31)   Figure 14 shows the HPLC chromatogram of Dazomet using photo Fenton processes to degrade the Dazomet, it can be observed that the absorbency peaks, at retention time in minutes before irradiation, and after irradiation are different. After 120 min.

Mechanism of Pathway Degradation of Dazomet
From a previous study, it can be suggested that the mechanism of photo degradation of Dazomet, after formation the OH • radicals, can undergo series of reactions with Dazomet. Scheme 1 can describe the products. (1) Dazomet is rapidly degraded in aqueous media to carbon disulfide, formaldehyde, and methylamine. However, in moist soil it degrades mainly to methyl isothiocyanate. It is unstable and decomposes to methylamine in water, probably via thiocarbamic acid, methyl isocyanide in turn degrades to methyl isocyanate, this product was summarized in the scheme illustrated in the scheme 1.

Thermodynamics Quantities Calculations
The thermodynamic parameters of activation that must be considered to determine the process are changes in Gibbs free energy (ΔGº), standard enthalpy (ΔHº), and standard entropy (ΔSº). ΔG° was calculated by ΔG° = ΔH -TΔS The ΔH * , ΔS * , and ΔG° were calculated by the Eyring equation: Where; k is the degradation rate constant, kB is the Boltzman constant (1.3807×10 −23 J K −1 ), h is the plank constant (6.6261×10 -34 Js), R is the gas constant (1.989 cal mol -1 K -1 ), T is temperature (K). Table 4 shows the calculated quantities of Gibb's free energy (ΔG*), enthalpy (ΔH*) and entropy (ΔS°). It can be seen from these results that the enthalpy and Gibb's free energy values in photodegradation are less than the values in thermal degradation, due to the existence of light which plays a role in enhanced the degradation of Dazonet. Additionally the Table shows that the values of entropy in photodegradation are negative which are attributed to stimulated process, where the thermal degradation can be happened spontaneously, so the entropy values are positive.

Conclusion:
It can be concluded from this work that the pesticide Dazomet has been degraded using Fenton reactions (photo-Fenton under UV-light and sun light and thermal Fenton). The optimum values of all used catalyst have been figured out, in addition, pH of the reaction media has been found out. the effect of temperature on all the reactions has been studied in the range (25 -45)°C, it has been noticed that rate constant of the degradation (k) has increased with increasing temperature, and the best percentage degradation of Dazomet was at 45°C in all processes. The thermodynamic functions of activation ΔG * , ΔH * , ΔS * have been calculated.