DBD-NTP Reactor Test for Degradation of Methylene Blue

DBD-NTP Reactor Test for Degradation of Methylene Blue Abstract: Electrical discharges generated at water-gas interface in a nonthermal plasma (NTP) reactor were utilized for the degradation and mineralization of a model aqueous organic pollutant methylene blue. NTP based advanced oxidation processes (AOPs) have presented a great potential to remove contaminants from wastewater. The degradation of pollutions will greatly depend on the active species generated in NTP process. It was observed that both degradation efficiency and mineralization of the pollutant increased on addition of metal oxide catalyst, hydrogen peroxide and Fe+2 to plasma reactor. It has been observed that methylene blue degradation followed first-order kinetics and degree of mineralization increased as a function of time. 1. Introduction The presence of aqueous organiccompounds in water may have adverse health effects on humans and aquatic organisms[1-4]. Wastewater, especially from paper, textile and pharmaceutical industries may contain highly hazardous and toxic compounds[5, 6]. Typical organic pollutants like pharmaceuticals, dyes, etc are toxic and may contain some non-degradable intermediates that may havea potential carcinogenicity and mutagenicity[7, 8]. One ofthe best practiced methods for remediation of these pollutants, adsorption, at best, may tranfer the pollutant to another phase, whereas, biodegradation may be time consuming [6, 9]. Ingeneral, mineralization of these pollutants is much desired. To achieve mineralization, advacned oxidation processes (AOPs) like photo-Fenton, photocatalytic, ultrasonic degradation and sonolysis combined with ozonolysis have been proposed[7, 10-14]. Yet another addition to AOPs is nonthermal plasmas (NTP) generated by electrical discharges. Non-thermal plasmas (cold plasma) are characterized by high electron temperatures (Te) and clod heavy particle temperature (Th). Due to the high electron temperature, the average gas temperature is much lower than that of the electron temperature. NTP based AOPs are gaining attention for remediation of gas and water bound pollutants and especially electric discharges at the water gas interface offers specific advantages like generation high energy electrons that may initiate the reaction, multiple oxidants for mineralization, mild operating conditions and possibility of scale up, etc.Oxidation of pollutant in AOPs proceeds via generation of one of the powerful oxidants, hydroxyl radical (OH, 2.8 V)that can mineralize a majority of the organic pollutants [15-18]. Plasma technologies have agreat potential and are widely used in a large number of technical applications like abatement of air pollutants, surface modification, lasers, etc[19]. The application of plasmas in environmental application has been growing at an exponential rate. Electrical discharges generated at gas-water interface may induce different physical and chemical effects like high electric fields, UV radiation, overpressure shock waves, and the formation of chemically active species [16, 19-22]. The interaction of the high energy electrons created by the discharge with the water molecules produces various reactive species, namely ions (H+, H3O+, O+, H, O, OH), molecular species (H2, O2, H2O2) and radicals (such as O†¢, H†¢, OH†¢) [6, 23-26].In addition, the hot electrons may have higher energy than the dissociation energy of water (5.16 eV) [6, 25,27]. However, even though the presence of UV light has been confirmed, direct photo oxidation of pollutant in water is very limited and among the active species; hydroxyl radical, atomic oxygen, ozone and hydrogen peroxide are the most important ones [28, 29].Thus electrical discharges may provide a capsule of oxidizing species with varying oxidation potentials. For example, OH†¢ radical, one of the most important oxidants, has a very short life time and is mainly generated from the direct dissociation of water molecules in the plasma region [30-32]. The presence of multiple oxidizing species provides various avenues to combine with catalysts. For example, once the presence of hydrogen peroxide (H2O2) is confirmed, addition of Fe-catalysts may facilitate Fenton type reactions. In a similar manner, in-situ decomposition of ozone on a suitable catalyst may lead to the formation of atomic oxygen, which has still higher oxidation potential than H2O2 and ozone. For the effective utilization of these short lived species, generally, metal oxide catalysts like Al2O3, Fe2O3, SiO2, TiO2, ZnO, etc are often integrated with NTP. These catalysts facilitate the in-situ decomposition of ozone, leading to the formation of atomic oxygen, which is a stronger oxidant to ozone. In general, nonthermal plasma reactors may be classified as the sub-atmospheric discharge reactors that demand a reduced pressure (radio frequency, microwave discharge,etc) and that are capable of operating at atmospheric pressure (Corona, dielectric barrier discharge, glow discharge etc). However, as the formation of these active species may depend on the reactor configuration[6, 33-35] it is worth mentioning the widely tested plasma rector models like corona discharge, dielectric barrier discharge (DBD), glow discharge, plasma jet, and gliding arc, etc. 1.1. Dielectric barrier discharge Dielectric barrier discharge (DBD) configuration is characterized by at least one insulating dielectric layers, which is placed between the electrodes. Its use in environmentalapplications can be tracked back to middle 18th century, when Siemens (1857) used it to generate ozone. The classical DBD configuration is illustrated in Figure 2. The advantage of DBD over the other dischargeslies inhaving the option to workwith NTP atatmosphericpressureandcomparatively straight forward scale-upto large dimensions. 1.2. Corona discharge Corona discharge is featured by relatively non-uniform electric field distribution, when compared to DBD, caused by the sharp edge or sharp point of its electrode. In general, one of the electrodes of corona discharge reactors is a needle or a thin wire that may provide a point to plate type discharge propagation. The electric field near the electrodes would be sufficiently higher than the rest of the discharge volume. The typical electrode configurations of corona discharge are illustrated in Figure 3. 1.3. Gliding Arc discharge The gliding arc (GA) is anunique non-thermal plasma that has relatively high plasma density, power and operating pressure in comparison with other non-equilibrium discharges. It has a dual character of thermal and nonthermal plasma, and can involve relatively high electric powers compared to the corona discharge. It is generated between two metal electrodes with a high velocity gas or gas–liquid fluid flowing between the electrodes. However, for environmental applications like decontamination of air and water pollutants, either corona or DBD is widely tested.DBD configuration has been reported as a promising technique for the removal of air pollutants [33][36-42]. [33][33, 34][33][33, 34]However, majority of the literature deals with treatment of air pollutants. Discharge in water is different to that in air due to differences between the characteristics of water and air[14][14][35]. As stated earlier, plasma generated at air-water interface is known to produce a variety of oxidants that are capable of mineralizing the target organic compounds. Among these oxidants, primary oxidants like ozone, H2O2 are important that may be converted to the secondary oxidants like OH radicals. 1.4. H2O2 production H2O2formation in NTP reactors was reportedwith a variety of feed gases (Ar,O2,air and N2) and interesting observation is that its formation takes place even in the absence of oxygen bubbling[58]. H2O2formation in the water for three model gases followed the order N22.Table 2 summarizes the selected reports that deal with quantitative information on H2O2formation and the corresponding reactions responsible for its formation are summarized below. It may be concluded that the feed gas may affect the formation of oxygen basedreactive species such as OH, O3, O and positive and negative charged ions like †¢O2+,H3O+, †¢O2, †¢O3-.. H2O2 formed by the following reaction (Equations 1-6). 1.5. Ozone production Ozone is yet another oxidant reported that has major applications during NTP abatement of pollutants. The ozone formation and the corresponding reactions of ozone are summarized in Eq. 7-12. Ozone reacts with organic compounds present in wastewater directly via molecular and indirectly through radical type chain reactions. Both reactions may occur simultaneously[35][35]. It is known that ozone reacts with unsaturated functional groups present inorganic molecules, leading the conversion of the pollutant [65]. Sim[35]plified reaction mechanism of ozone in aqueous environment is given in Eq. 8-12. Inaddition, NTP is known toproduce ultra-violet (UV) light due to excited nitrogen molecules present in air [15][33]. Therefore, in aqueous solution, UV light induced dissociation of H2O2 molecules may lead to the formation of hydroxyl radicals(Eq. 13) [66, 67][35]. Eq. 14-17 summarizes the pathway the possibilities in which ozone, H2O2 and UV light may induce the formation of various oxidants that can mineralize the pollutants. Discharges in water may also change the pH of the solution significantly, due to the formation of various inorganic and organic acids[6, 16, 21,68]. The formation of inorganic acids is a result of a series of reactions involving back ground gas like nitrogen, as shown in eq-18 to 23. It has been observed that pH of the solution decreases rapidly due to the formation of water soluble ions. As a result, conductivity of solution also increases significantly. 2. Degradation of a model dye methylene blue A schematic of the reactor used for these experiments are shown in figure X. The electrical discharge was produced in a parallel plane type coaxial NTP-DBD reactor by a high-voltage 0–40 kV AC source transformer (Jayanthi transformers). Quartz is a common material used as a dielectric, due to its excellent dielectric properties and its resistance to ozone. The reactor is a transparent quartz cylinder with an inner diameter of 19 mm and wall thickness of 1.6 mm. Silver paste painted on the outer surface of the quartz tube acts as the outer electrode, whereas a cylindrical stainless steel rod served as the inner electrode. The discharge length was 20 cm and the discharge gap was around 3.5 mm. 2.1. Effect of initial concentration and applied voltage Figure 4 presents the degradation of 100 ppm of a model pollutant methylene blue (MB)as function of time for different voltages at 50 Hz. Increasing voltage favor higher conversion, as conversion of MB improved 91% to 95% on increasing voltage from 14 to 18 kV after 25 min [15]. This may be due to the availability of more energetic electrons at 18 kV that may lead to higher degradation. However, degradation decreases with increasing the initial concentration. At14 kV increasing MB concentration from 50 ppm to 100 ppm decreased the conversion from 97% to 91%.A rapid increase in the degradation was observed during the initial stage of plasma treatment followed by a slow increase, probably due to competition between pollutant molecules and the intermediate products formed during the degradation. 2.2. Effect of discharge gap It is known that performanceof NTP reactors depend on the distance between the electrodes[69]. In order to understand this observation, during MB degradation, the electrode distance was varied between 1.5 to 4.5mm. As seen from the Fig.5, for 100 ppm MB degradation at 18 kV, in 25 min thedegradation reached 86, 89.5, 93.5 and 92%, respectively for 1.5, 2.5, 3.5 and 4.5 mm discharge gap, indicating the optimum discharge gap of 3.5 mm. A similar observation was made by Hao et al. who varied the electrode separation between 10 to 20 mm and after 15 min, the conversion of 4-chlorophenol (4-CP) decreasedwith increasing the inter-electrode separation. Approximately 78% conversion of 4-CP was obtained with 10 mm inter-electrode separation that decreased to only 45% with 20 mm. With a relatively larger inter-electrode separation, more energy is required for plasma channel formation, whereas optimum separation may also provide plasma-photochemical effects and subsequently a faster degradation of 4-CP. 2.3. Effect of feed gas Feed gas may also influence the degradation of the pollutants in NTP reactors. In order to understand this, degradation of 100 ppm of MB was followed at 18 kV by bubbling 200 ml/min of O2, argon and air(Fig. 6). As seen in Fig. 6, the MB degradation was 97.4, 53.2 and 93.4 %, respectively for oxygen, argon and air. The highest degradation of MB with oxygen and air may be due to formation of oxygen based active species like OH†¢, O3, O†¢, H2O2 and positive and negative charged ions like †¢O2+, H3O+, †¢O2, †¢O3.This observation is consistent with report by Du et al., for different feed gases, including air, oxygen, nitrogen and argon during the degradation of a dye AO7. The degradation during the plasma treatment is highest for oxygen and least with nitrogenbubbling [62]. 2.4. Effect ofNa2SO4 As explained earlier, electric discharges produce various reactive species. However, these species may not oxidize the pollutant. For example, the direct interaction of ozone with pollutant is may be ruled out. One of the ways of improving the performance is by adding suitable additives like metal oxides, carbon, ferrous ion and hydrogen peroxide, etc that may facilitate the secondary reactions leading to the formation of strong oxidants.Large amounts of sulphate are generally essential in the dye bath for successful dyeing, which results in high concentrations of sulphate discharge in the effluent. To examine the effect of Na2SO4 concentration on decolorization efficiency, 50 mg/L of SO4-2 in 100 ppm MB was prepared. The solution was treated at 16 kV applied voltage and the observed degradation was 97.5% against 93.5 % with plasma alone. The excess SO4-2 may react with the hydroxyl radicals to generate SO4-†¢which is more reactive than hydroxyl radicals. 2.5. Effect of H2O2 and Fe+2addition Many researchers have reported the formation of hydrogen peroxide (H2O2) and improved performance during the degradation of pollutants in NTP reactors.The prominent reactions with H2O2 may involve the homolytic fission of the O−O bond leading to theformation of reactive hydroxyl radicals with high oxidizing power (2.8 V). These hydroxyl radicals may attacks the organic pollutants to initiate the degradation.Hence presence/addition of H2O2 may increase the concentration of active OH†¢ and thus accelerate the degradation rate. As the present study confirmed the formation of 80 ppm of H2O2, influence of Fe2+was studied by adding 50 mg of ferrous sulphate. The addition of Fe+2 may facilitate the formation of à ¢- OH, à ¢- HO2 ,etc, via the Fenton reaction (Eq. (25)as shown below Addition of Fe2+ shows positive effect on MB degradation (96.5%), where the conversion of 100 ppm MB increased from 96.5% at 16 kV from 93% with plasma approach alone (Fig. 7). 2.6. Effect of metal oxide catalyst Ozone is one of the oxidizing species with high oxidizing power andis one of the important species formed in NTP.However, direct reaction of ozone with pollutant may not be effective and in order to utilize the potentials of ozone, often a catalyst is combined with plasma. In this context, oxygen deficient CeO2 catalysts are beneficial for ozone decomposition due to presence of defect induced vacancies. During the present study, addition of 100 mg of the catalyst to test solution increased the degradation to 99 % from 93.5 % without catalyst at 16 kV. It is reported that the improvement in the degradation efficiency with catalytic plasma approach is due to in situ formation of atomic oxygen that is capable of mineralizing pollutant due to higher oxidation potential (2.42 V) than ozone (2.07 V) [22]. It is known that many of the excited species produced in NTP are short-lived and addition of catalysts may either enhance the life time of short-lived species or/and facilitate the format ion of secondary oxidants. The catalytic decomposition of ozone may be explained as given in the following equations [15, 26]. 2.7. Mineralization of Aqueous Organic Pollutant The degradation of the dyes was followed by TOC (total organic carbon), which is an index of the pollutant concentration in the solution. TOC indicates the degree of mineralization of the target compound. The plasma treatment of dye solutions decreased the TOC with time and Table 1 presents the data on the decrease of TOC under different flow rates and voltages. The decreasing TOC with increasing treatment time indicated that the degraded organic carbon may be converted into CO, CO2 and H2O [26, 51][32]. [32]This accounts for a degradation process of the solute and consequently for the detoxication level of water. The CO, CO2 released during the reaction confirms that some amount of degraded dye was mineralized. However, during the present study, COX analyzer was utilized only for qualitative analysis. Pollutant + plasma (Active species) →CO + CO2 + H2O (35) 2.8. Energy efficiency The degradation efficiency may be better illustrated by the amount of pollutant decomposed per unit of energy (represented as energy yield). The energy yield of the degradation was calculated by using the following relation[15, 17,18], where C is initial pollutant concentration, V is the volume of the solution, degradation (%) is % degradation at time t, P is power and ‘t’ is time. It was observed that as a function of time the energy yield decreases and percent degradation increases. Increasing applied voltage increases the power and decreases the energy yield. Energy Efficiencies reported for plasma discharge processes are given in Table 1. For MB degradation, in presence of CeO2 catalyst and H2O2 the energy yield increased to 46.2 and 51.3 respectively, whereas, for plasma alone the value is 43.1 g/kWh for 100 ppm initial concentration at 16 kV. Feed gases also change the reactor efficiency as well as the energy yield, as with oxygen it is increased to 45.4, whereas, with argon it is only 14.41 g/kWh. 2.9. Degradation Kinetics During the plasma treatment of aqueous organic pollutants it has been observed that concentration of pollutants in solution decreases with time and the degradation followed first order kinetics. The rate constant was calculated based on Eq-7[45, 68, 70,71]. ln(Ct / C0) = –k1t (24) where Ct, C0, and k1, are the concentration of Aqueous Organic Pollutant for a given reaction time, initial concentration, and first-order rate constant (min-1), respectively. 3. Conclusions A DBD-NTP reactor was tested for the degradation of 100 ppm methylene blue. It was observed that the optimum discharge gap was 3.5 mm and addition of hydrogenperoxide improved the performance of the reactor, probably due to the formation of hydroxyl radical via Fenton reactions. Increasing voltage leads to higher conversion, and catalyst addition increased both conversion and mineralization of the dye.