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.

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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, Fe₂O_3, 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.

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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 18^th 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.

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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.

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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

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