1. INTRODUCTION
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2. MATERIAL AND METHODS 2.1 Materials Pyridine standard solution was supplied by Fluka with a purity better than 98.0%. It was used to prepare a synthetic wastewater specimen. Aqueous solutions were made using deionized water, which was prepared by an Elga B114 Deionizer using C114 cartridges, (EC = 5 μS.cm@25 oC and TDS=3.5 ppm). All other reagents were of reagent grade obtained from Fluka and used as received. 2.2 Ultrasonic Reactor Setup The degradation experiments were carried out in an ultrasonic cleaner bath (Honda Electronics PS-60, Capacity 15 L). The bath operates at 360 W and 40 KHz. An Erlenmeyer flask was used as the reaction vessel. The volume of the solution was 100 mL. The bath temperatures were maintained by proper recirculation of water. The solution temperature was also monitored regularly. The efficiency of a reaction occurring in the vessel under ultrasonic conditions depends strongly on the distance of the bottom of the reaction vessel to the bottom of water bath. The distance was carefully measured through preliminary experiments, so that the ultrasonic intensity reached the maximum at thebottom of the flask. For an ultrasonic frequency of 40 KHz, this distance was measured to be 1 cm. The reactor was sealed with a silicone stopper wrapped with an aluminium foil to ensure the minimum loss due to the evaporation of the volatile compounds. The syringe needle was pierced through the septum of the stopper for sampling purposes. All sonochemical experiments were conducted in duplicate. The averages of the parallel experimental data were calculated and taken into account in the analyses of sonochemical kinetics. The error of all parallel experiments was under 5%. 2.3 Photochemical Reactor Setup The experiments were carried out in a 250 mL glass immersion-based photochemical reactor, charged with 100 mL of aqueous solution, where the solution was illuminated by means of a low-pressure, 10 W mercury lamp with 90% emittance at 254 nm, located axially and held in a quartz immersion tube. The source was turned on and the time count was initiated. Samples (ca. 5 mL) were withdrawn at regular times for UV-Vis analyses. 2.4 Quantitative analyses Samples periodically drawn from the vessel were quantitatively analyzed by measuring their absorbances using a Shimadzu UV-Visible spectrophotometer. Initially, tests were carried out by UV scans from a wavelength of 200 to 500 nm to determine the absorption maxima of the pyridine molecule. 2.5 Qualitative analysis of the degradation products by GC-MS GC–MS analyses were performed on a Shimadzu QP 2000 instrument, equipped with an Equity-5 column (Supelco) (30 m × 0.25 mm × 0.25 μm), coated with 5% phenyl/95% methyl polysiloxane. Separation of the by-products was conducted under the following chromatographic conditions: Injector temperature, 240 0C; oven temperature program, 50 0C ramped at 5 0C min-1 – 250 0C followed by another ramp of 10 0C min-1 – 290 0C held for 2 min. High-purity helium was used as the carrier gas at a flow of 1 mL min-1. The temperatures of the ion source and the interface were set at 240 0C and 290 0C, respectively. The MS was operated in electron ionization mode with a potential of 70 eV and the spectra were obtained at a scan range from m/z 50–450 (full scan mode). The scan time was 46 min and 1.0-μL injections were introduced, using a split ratio varying from 1/2 to 1/20.

3. RESULTS AND DISCUSSION 3.1. Optimization of the Operating Conditions 3.1.1 Effect of Initial Pyridine Concentration Since the industrial wastewater contains pollutants in varying concentrations, the effect of the initial concentrations on the reaction rate was tested for pyridine. The effect of solute concentration on the degradation of pyridine was investigated at initial concentrations of 10 and 100 mg/L for both photochemical and sonochemical experiments. As shown in Figure 1, the increase in irradiation time causes a decrease of the concentration of pyridine in the solution and thus, the removal increases. It follows from the data obtained that degradation of pyridine depends upon the time of irradiation. For the sonochemical irradiation experiments, the removal rates of pyridine decreased from 43% to 13% with an increase of the initial concentration from 10 to 100 mg/L, respectively, as shown in Figure 2. This suggests that increasing the initial concentration of the solution would decrease the removal rates of pyridine. This is because the increase of initial concentration of the volatiles results in the weakening effect of cavitation [12]. However, the total amount of pyridine degraded after 60 min at 100 mg/L which was as much as three times larger than that degraded at 10 mg/L. On the other hand, the photochemical experiments show a better removal of pyridine. The removal rates of pyridine decreased from 90% to 60% with increasing the initial concentration from 10 to 100 mg/L, respectively. These indicate that ultraviolet irradiation is far more effective than ultrasound irradiation in the degradation of pyridine in aqueous solutions. The results show that the removal efficiency using photochemical process is approximately 90% at pyridine's initial concentration at 10 ppm, so it can be said that we achieved a complete degradation of pyridine. The sonochemical process achieves an efficiency of 43% for the same initial concentration with an irradiation time of 60 min.

The degradation curves of pyridine by UV and US radiation are well fitted by a mono-exponential curve, suggesting that a first-order homogeneous reaction model can be taken in consideration for describing the kinetic behavior. Considering that the UV and US radiation are solely responsible for pyridine removal from the solution, according to Eqs. (1) – (3), and considering that the limiting step is the scission of the starting material, the kinetic equation that describes the process is −푑퐶푑푡=퐾퐶 (4) In Equation (4), k is the first-order rate constant and C is the concentration of pyridine at each moment when t > 0. Integration of Eq. (4), with the usual restriction of C = C0 at t = 0, will lead to a linear plot of ln(C0/C) versus t with a slope of k being the first-order rate constant in Figure 3. The photochemical and sonochemical experiments, under the conditions of constant irradiating flux for different initial concentrations of pyridine, show a variation in the k values (Table 1). The apparent first order rate constants decreased with an increasing initial concentration of the pyridine, indicating a non-elementary nature of the sonochemical and photochemical reactions. This dependence of reaction rate constants on the initial concentration are compared well with the existing literature [13-15].
For sonochemical experiments, a 10-fold increase in the initial concentration of pyridine leads to a 5-fold decrease on the rate constant of the process. Meanwhile, for the photochemical experiment, a 10-fold increase in the initial concentration of pyridine leads to a 4-fold decrease on the rate constant. The observed decrease on k as the initial concentration of pyridine increases can be explained in terms of the less availability of oxidizing species, such as OH radicals, through the direct photolysis or sonolysis of H2O as the concentration of the solution gets more intense. An efficiency comparison of the processes of photo-degradation and sono-degradation showed that the photo-degradation of pyridine was more efficient than sono-degradation. It can also be observed from the results that UV had a potential to degrade pyridine. More than 90% degradation was achieved after about 60 min at 10 ppm concentration of pyridine. The poor effects of US alone on the degradation efficiency may be attributed to the fact that low ultrasound frequencies, hindering the development of hydroxyl radicals [16]. Hence, for pyridine, being a volatile and highly soluble compound, reactions inside or in the vicinity of the bubble, where fast thermal decomposition and increased concentrations of radicals exist, are unlikely to occur to an appreciable extent and, therefore, its degradation will be driven by hydroxyl radical-mediated secondary activity in the liquid bulk. Thus, US irradiation process generally demands a high contact time for significant degradation efficiency [17]. On the other hand, UV irradiation had a high potential to produce the highly reactive hydroxyl radical. This would explain the discrepancies in the reactivity of pyridine between sonochemical and photolytic reactions since the latter involves the participation of a more diverse range of reactive species (i.e., radicals and electrons) than the former [18, 19]. Generally, the photolytic degradation can be defined as a cyclic photo-process in which pyridine undergoes photodegradation, but the catalyst is regenerated spontaneously to allow the sequence to continue indefinitely until all the substrate is destroyed [20].

3.1.2 The effect of UV/H2O2 and US/H2O2 irradiation processes Hydroxyl radicals generated in water by ultrasonication and photolysis can produce hydrogen peroxide in the system according to equations (5)-(17). In these reactions “irr” denotes the UV or US irradiation. Whether additional hydrogen peroxide has a synergistic effect on the overall degradation of pollutants [21, 22], for this purpose some experiments were conducted at various concentrations of added H2O2. H2O + irr →•OH + •H (5) O2 (dissolved) + irr →2•O (6) •OH + •O→•OOH (7) •O + H2O→2•OH (8) •H + O2→•OOH (9) •OH + •H→H2O (10) 2•OH→H2O + •O (11) •OOH + •OH→O2 + H2O (12) 2•OH→H2O2 (13) 2•OOH→H2O2 + O2 (14) •H + H2O2→•OH + H2O (15) •OH + H2O2→•OOH + H2O (16) 2•H→H2 (17) In this part of study, sonicative and photoloytic experiments were repeated with pyridine solutions including H2O2 to study the effect of H2O2 addition for the degradation of pyridine. H2O2 was added to 100 mL of pyridine solution with an initial concentration of 100 ppm in such an amount that its concentration was 100 ppm, 300 ppm or 500 ppm in the solution. Figure 4 displays the effect of H2O2 amount added in the sonochemical degradation of pyridine, while the inset shows the reaction kinetics for the degradation, in which for all reactions, the kinetics followed first-order rate laws (R2 93). As seen from Figure 4, in the concentration range studied, the amount of H2O2 added has a positive contribution on the degradation of pyridine when compared with the absence of H2O2. Increasing the amount of H2O2 increases the degradation of pyridine. According to the hot spot theory, the temperature and pressure of localized hot spots formed can excessively reach 5000 K and 1000 atm, respectively, in ultrasonic cavitation. Under these conditions, hydrogen peroxide readily decomposes into hydroxyl radicals, according to the following equations, causing a high degradation rate [23]. H2O2 + us → 2 ∙OH (18) .H + H2O2 → .OH + H2O (19) The amount of H2O2 that can be produced by ultrasound itself is too small to dissociate into large amounts of •OH. Thus, an additional amount of H2O2 is generally needed to significantly accelerate the degradation process. H2O2 will increase the formation of •OH in two ways. It could either through the self-decomposition as a result of ultrasound irritation or as the reduction of H2O2 at the conduction band as shown in reactions (18) and (19), respectively [24]. Figure 5 shows the effect of different H2O2 concentrations on the degradation of pyridine with respect to the irradiation time. It can be seen that photodegradation increases with an increase in the amount of H2O2 concentration, up to the optimum value and then decreases when the H2O2 concentration is increased. This trend can be explained by the fact that H2O2 itself acts as an effective hydroxyl radical scavenger at concentrations that are specific for the pollutant in question.
Figure 6 displays the effect of H2O2 amount added on the rate of photochemical degradation of pyridine in aqueous solution. As it can be seen from the result, the addition of 100 ppm and 300 ppm H2O2 can enhance the degradation when compared with the absence of H2O2. However, 500 ppm H2O2 has a negative contribution on the decomposition of pyridine. So there may be an optimum amount of H2O2 to increase the degradation rate.

During this process, ultraviolet radiation is used to cleave the O-O bond in hydrogen peroxide and generate the hydroxyl radical. The reactions describing UV/H2O2 process are presented below [25]. H2O2 + uv → 2 HO• (19) H2O2 + HO• → ΗΟ2• + Η2Ο (20) H2O2 + ΗΟ2• → HO• + Η2Ο + Ο2 (21) 2 HO• → H2O2 (22) 2 ΗΟ2• → H2O2 + Ο2 (23) HO• + ΗΟ2• → H2O + Ο2 (24) In the previous equations, Eq. 19 is the rate-limiting reaction because the rates of the other reactions are much higher than that of Eq. 19. Theoretically, in the UV/H2O2 process, a higher initial hydrogen peroxide concentration produces a higher concentration of hydroxyl radicals, which causes even more degradation of pyridine. However, an optimal hydrogen peroxide concentration exists because overdosing of hydrogen peroxide would lead to a reaction with hydroxyl radical and the formation of HO2• (Eq. 20). Moreover, there is an optimum concentration for H2O2. Beyond this limit, the presence of H2O2 is detrimental to the degradation reaction due to a scavenging action. Therefore, the combination of UV with H2O2 was necessary for the production of hydroxyl radicals to initiate the degradation of pyridine at a reasonable time scale even for the higher pyridine concentrations. These results are in good agreement with other findings in the literature, such as the one by Aleboyeh et al. [26], who showed that a combination of UV plus H2O2 in comparison with UV alone increases the removal rates of Acid Orange 8 and Methyl Orange, for 172 and 137 times respectively. In this view, the UV radiation is combined with a powerful oxidant, H2O2; the degradation efficiency of pyridine is significantly enhanced due to hydroxyl radical production caused by the photolysis of H2O2, as reported by other researchers [27]. In conclusion, results with UV/H2O2 and US/H2O2 processes indicate that the oxidation was exclusively due to the attack of hydroxyl radical when hydrogen peroxide was in excess. Pyridine degradation rate by US and US/H2O2 was strongly dependent on initial concentration of hydrogen peroxide, as shown in Figure 4. Figure 5 shows that UV/H2O2 is more efficient than UV light alone for the degradation of pyridine. A comparison of photo-degradation and sono-degradation rate constant in Table 2, showing that the photo-degradation of pyridine was more efficient than sono-degradation. The results indicated that in the cases of UV/H2O2 and US/H2O2, a sufficient amount of H2O2 was necessary, but a very high H2O2 concentration would inhibit the reaction rate. The optimum H2O2 concentration was achieved in the range of 100 - 300 ppm at a pyridine concentration of 100 mg/L. A degradation of 99% was obtained with UV/H2O2 within 5 minutes while degradation efficiency by using UV (< 20%), US (< 2%), and US/H2O2(<10%) processes were negligible for this kind of pollutant at these operating conditions specified.

3.2. GC-MS analyses of the degradation products Figure 7(a) shows the GC-MS response of an injected pyridine sample (10 mg/L) which was previously subjected to ultrasonic irradiation for 10 min. The retention time for pyridine is at 4.43 min. On the other hand, Figure 7(b) shows the response of an injected pyridine sample (10 mg/L) which was previously subjected to ultraviolet irradiation for 10 min. Pyridine appears at the same retention time at 4.43 min. The chromatogram shows two other peaks which could be attributed to the degradation products of pyridine . The results show that the ultrasonic and photolytic irradiations cause a cleavage of the pyridine ring to produce acetylene and diacetylene at 26 and 52 m/z, respectively, as degradation products. These are the only barely detectable degradation products found. In addition, comparing the abundance of pyridine (concentration) in cases of UV and US irradiations indicate that the UV process is far more effective for pyridine degradation than US does.
Having a look at the literature regarding the degradation of pyridine and pyridine derivatives, one sees that there are inconsistencies about the reaction mechanism through holes or hydroxyl radicals. Therefore, Agrios and Pichat [28] suggest that pyridine reacts over TiO2 predominantly via the formation of a radical centered on the pyridine ring. Some researchers reported that free radicals would be generated by applying ultraviolet irradiation simultaneously with oxidants [15, 20]. Figure 8 shows the tentative degradation pathway proposed for UV/US degradation of pyridine.
In a previous work, Zechmeister et al. reported the ultrasonic cleavage of the pyridine ring in aqueous silver nitrate [29]. Roughly 5% of the ring atoms are precipitated as a mixture of silver acetylide, silver diacetylide, and silver cyanide. It has been shown that similar cleavage reactions can also be realized in the absence of silver, with pyridine and pyrrole. Under these conditions, free acetylene and hydrogen cyanide were reported to evolve [30]. Very volatile products are directly formed through the degradation of pyridine. C4H4 and C4H2 are the most common hydrocarbon products formed [21, 22, 32]. It has been previously proven that acetylene, diacetylene, and hydrogen cyanide can be formed during aquasonolyses of pyrrole, N-methylpyrrole, and pyridine [29-31]. In the future, products from sonolyses and/or photolysis of pyridine should be further investigated.

4. CONCLUSION In this experimental study, the degradation of pyridine was investigated by using several advanced oxidation processes: US, US plus H2O2, UV, and UV plus H2O2. The optimal operating condition for each process was established. The conclusions drawn from this study can be summarized as follows: The degradation efficiency proceeded very slowly when US, US plus H2O2, and UV processes were used. UV plus H2O2 process was found to be a suitable treatment method for complete degradation for pyridine within very short time. The reaction rate constant was increased with increasing H2O2 concentration; however, the marginal benefit became decreasing with further increasing of H2O2 due to the scavenging effect of excess H2O2. Pseudo-first order kinetics with respect to pyridine concentrations were found to fit all the experimental data. The results show that the ultrasonic and photolytic irradiations cause the cleavage of the pyridine ring to produce acetylene and diacetylene at 26 and 52 m/z, respectively, as main degradation products barely detected by GC-MS.

Even though the biological methods are highly practical and advantageous, the toxic and hazardous compounds encountered in the waste inhibit the process [10, 11].

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