Air ionization as a control technology for off-gas emissions of volatile organic compounds

Environmental Pollution xxx (2017) 1e15

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Air ionization as a control technology for off-gas emissions of volatile organic compounds* Ki-Hyun Kim a, *, Jan E. Szulejko a, Pawan Kumar b, Eilhann E. Kwon c, Adedeji A. Adelodun d, Police Anil Kumar Reddy a a

Department of Civil and Environmental Engineering, Hanyang University, 222 Wangsimni-Ro, Seoul, 04763 South Korea Department of Nano Science and Materials, Central University of Jammu, Jammu, 180011 India Department of Environment and Energy, Sejong University, Seoul 143-747, 05006 South Korea d Department of Marine Science and Technology, School of Earth and Mineral Science, The Federal University of Technology, P.M.B. 704, Akure, Nigeria b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 January 2017 Received in revised form 11 March 2017 Accepted 11 March 2017 Available online xxx

High energy electron-impact ionizers have found applications mainly in industry to reduce off-gas emissions from waste gas streams at low cost and high efficiency because of their ability to oxidize many airborne organic pollutants (e.g., volatile organic compounds (VOCs)) to CO2 and H2O. Applications of air ionizers in indoor air quality management are limited due to poor removal efficiency and production of noxious side products, e.g., ozone (O3). In this paper, we provide a critical evaluation of the pollutant removal performance of air ionizing system through comprehensive review of the literature. In particular, we focus on removal of VOCs and odorants. We also discuss the generation of unwanted air ionization byproducts such as O3, NOx, and VOC oxidation intermediates that limit the use of air-ionizers in indoor air quality management. © 2017 Published by Elsevier Ltd.

Keywords: Air pollution control Ionization Radical VOCs Odors Oxidative destruction

1. Introduction VOCs and odorants pose nuisance and health risks in urban environments (Kim and Park, 2008). Thus, the chronic presence of those pollutants in outdoor environments requires effective emissions control strategies on large emission sources (e.g., from industry). Much research effort has been devoted to the development of various strategies to eliminate or reduce such pollutants (Luengas et al., 2015). Recent developments in air cleaning techniques have resulted in significant advances in indoor air quality (IAQ) control, enabling efficient treatment of diverse chemical (odorants, VOCs, and PMs) and biological (microbes) pollutants (Zhang et al., 2011, 2013; Luengas et al., 2015). Airborne organic matter, such as VOCs, has human health impacts and is the prime cause of poor air quality index (AQI). To meet AQI standards, concentrations of VOCs can be controlled by either destructive or non-destructive methods. For the latter, various porous media with diverse physico-chemical properties can be *

This paper has been recommended for acceptance by Charles Wong. * Corresponding author. E-mail address: [email protected] (K.-H. Kim).

employed for adsorption, absorption, and condensation through which VOCs can be captured, either for recovery or for subsequent thermal destruction. However, these techniques have both advantages and disadvantages. For example, a broad array of VOCs has been treated using sorbents such as activated carbon or zeolites (Zhang et al., 2013). However, to adequately maintain their treatment efficiency, sorbents need to be replaced or regenerated at regular intervals taking into consideration the specific breakthrough time of each VOC. Liquid-phase solvents (such as water and organic solvents) have also been employed as sorption media to capture VOCs depending on their solubility. The VOCs can then be recovered and/or regenerated by solvent distillation, but this is an energy-intensive process. Condensation technology can be used to treat highly concentrated VOC streams. The major drawbacks of these non-destructive methods are the need for post-treatment, and the requirement to clean-up spent materials such as the solid adsorbent (zeolite or activated carbon) or liquid solvent waste. Destructive methods for the removal of VOCs commonly involve thermal oxidation with or without a catalyst. Oxidative methods can efficiently convert VOCs into CO2 and H2O. Despite the good performance of thermal destruction methods, they tend to consume a large amount of energy. Although catalytic oxidation is

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feasible economically, catalyst poisoning can occur due to the production of undesirable byproducts (e.g., sulfur, phosphorus, and halogenated compounds). Biofiltration is a cost effective and ecofriendly alternative relative to conventional techniques. Nonetheless, it has also been found to suffer from several drawbacks (e.g., poor mass transfer from gas to biofilm, inability to separate soluble VOCs, and the release of dust/microorganisms in the process) (Luengas et al., 2015). In recent years, air ionization methods have been proposed and investigated intensively for the treatment of diverse VOCs and odorants. In this review, we provide insights into fundamental aspects of this new, rapid, green, low-cost, synthetic, and potentially effective strategy for air quality remediation. To assess the role of air ionization methods for the air quality control, we evaluate the performance of these methods relative to conventional techniques. Last, we discuss future opportunities for air ionization techniques as reliable tools for controlling VOCs and odors to meet air quality standards. In addition, limitations of air ionization techniques, particularly the release of unwanted byproducts (e.g., NOx and O3) during the treatment of VOCs, are discussed. 2. Conventional methods for VOC removal In this review, we place great emphasis on air ionization methods for removal of VOCs. However, there are diverse approaches that can be taken. As summarized in Table 1, conventional approaches to remove VOCs fall into three categories: (a) physical (sorbent adsorption, solvent adsorption, membrane separation and condensation), (b) chemical (chemical scrubbers, thermal incineration, and catalytic oxidation), and (c) biological (bioscrubbers and biofiltration) methods. The performance of these conventional techniques with regard to VOC removal is described briefly below. To assess whether commonly-used fan-driven cleaning technologies improve air quality, a panel of experts investigated a total of 59 of 26,000 screened articles (Zhang et al., 2011). The efficiency of air cleaning devices was assessed in terms of clean air delivery rate (CADR), which is defined as the product of single-pass removal efficiency and space velocity through the device. The most effective technologies were particle filtration and sorption of gaseous pollutants. However, none of the reviewed technologies (catalytic oxidation, filtration, ozone oxidation, plasma, sorption, or UV destruction of microorganisms) was effective at removing most pollutants, while many of them were found to generate undesirable products. The cost of various conventional VOC control techniques was estimated with reference to high-energy electron beams (Son et al., 2010). The ranges of annual costs (per cubic foot per minute (cfm)) for various methods were 25e120 USD (absorption; product recovery can offset operating costs, but requires rigorous maintenance), 10-35 USD (adsorption, activated carbon, product recovery may offset operating costs, moisture sensitive, some compounds can clog pores, e.g., aldehydes, ketones, and esters), 15-75 USD (biofiltration, less initial investment, less secondary waste, slow, selective microbial VOC, removal), 20-120 USD (condensation, product recovery may offset operating costs, requires rigorous maintenance), 15-90 USD (catalytic oxidation, possible energy recovery (up to 70%), sensitive to operating conditions), 15-150 USD (flaring, possible energy recovery (up to 85%), halogenated compounds may need additional equipment), 15-40 USD (zeolite, effective up to 90% relative humidity, product recovery offsets operating costs, high zeolite cost and limited availability), and 1530 USD (membrane separation, no further treatment, solvent recovery may offset operating costs, membranes are rare and costly). The removal efficiencies of these approaches are generally in the range of 60e99%.

2.1. Physical methods The physical removal of airborne VOCs is based on VOC partitioning between the gas-phase (air) and some other phase (e.g., sorbents (either liquid or solid), transport through membranes, or the VOC condensed phase). For sorbent-based removal, it is essential to consider and evaluate the following factors: partitioning coefficient, sorbent capacity, breakthrough time and volume, and sorbent regeneration (Wells, 2003). Using adsorption techniques, VOCs can be removed from the air stream by physical adsorption onto porous medium like silica gel, alumina, activated carbon, zeolites, and MOF, among other materials (Vellingiri et al., 2016; Kabalan et al., 2016; Zhu et al., 2016; Yang et al., 2013). However, adsorption techniques are limited by the need for post treatment to dispose of spent adsorbents and/or to treat the adsorbed VOCs, which increases the cost of treatment. The use of liquid solvents (usually water, mineral oils, or petroleum oils) to strip VOCs from air streams through contact with the contaminated air results in removal efficiencies of 95e98% (Heymes et al., 2006; Ozturk and Yilmaz, 2006; Tatin et al., 2015). Despite the efficiency of VOC treatment, however, absorption has some disadvantages like the presence of insoluble VOCs and/or need for post-treatment of the absorbent liquid. Condensation based on non-destructive separation allows recovery of VOCs from the air stream in a liquid state with efficiencies as high as 99% (Shi and Huang, 2014). Nonetheless, a very low (cryogenic) condensation temperature is often required to accomplish the capture of targets (in the condensed phase) from the air stream. In addition, this method is only efficient when the concentration of VOCs in the air stream is very high (>1% v/v or > 10,000 ppm) (Gupta and Verma, 2002). 2.2. Biological methods Using biological processes, a wide range of pollutants can be digested into less toxic and odorless compounds. In general, biological processes can be divided into biofiltering processes and bioscrubber processes. The latter involve absorption of contaminants in aqueous media followed by biological treatment (Koutinas et al., 2007; Potivichayanon et al., 2006; Nielsen et al., 2007). Bioscrubbing methods are easily controllable and allow removal of products by washing out to avoid inhibitory effects. However, when attempting to remove less soluble or hydrophobic VOCs (e.g., methane, hexane, toluene, and benzene), limited mass transfer may ~ oz et al., 2007). This be problematic (Nikiema et al., 2005; Mun might be one of the reasons why bioscrubbing is less commonly used than biofltration. Biofilters are another type of short-contact bioreactor. Here, moist, polluted air is flowed over a porous bed of immobilized microorganisms to remove VOCs (Hwang et al., 2008; Torretta et al., 2015; Wang et al., 2009; Steinberg et al., 2005). The removal efficiency of VOCs by biofiltration is limited by mass transfer of target pollutants and the microbial metabolisms specific for target pollutants. Thus, the packing materials need to be changed frequently to enhance the performance of the system, and there may be incomplete removal of VOCs. 2.3. Chemical methods Processes based on chemical scrubbing and thermal oxidation (with/without catalyst) are commonly categorized as chemical treatment methods. Chemical scrubbing is the most common method used to remove airborne pollutants, especially odors and VOCs. Acidic or alkaline solutions (e.g., sulfuric acid and sodium hydroxide (caustic soda)) are used as scrubbing media. To oxidize

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Table 1 Comparison of the performance of conventional methods employed for the treatment of airborne VOCs. Type of treatment

Target VOCs

Removal efficiency (%)

Remarks

References

Toluene, benzene, 1, 2-dichloroethane, and acetone

90e98

   

Product recovery can offset operating costs Careful maintenance is required Requires pre-treatment of VOCs Most VOCs are water insoluble, thus have reduced absorption efficiency  Solution after absorption needs to be disposed of

Jeon et al., 2008; Bay et al., 2006;

Benzene, toluene, dichloromethane/ trichloromethane, n-hexane

80e90

Zhu et al., 2016; Tian et al., 2016 Kabalan et al., 2016

Toluene, octane, acetone, propane, ethane, ethylene, naphthalene

70e85

 Recovery of compounds may offset annual operating costs  Vulnerable to moisture; some compounds like ketones, aldehydes, and esters clog the pores and decrease efficiency  Secondary pollution may arise during the process of desorption  Product recovery can offset operating costs  Requires careful maintenance

Limonene, chlorobenzene, toluene

60e95

Steinberg et al., 2005; Wang et al., 2009; Matteau and Ramsay, 1999

Benzene, hydrogen sulphide, 1, 2-dichloroethane and fluorobenzene

90e99

 Requires less initial investment, less non-harmful secondary waste, and non-hazardous  Slow, and selective microbes decompose selective organics, thus requires mixed cultures of microbes  Requires a large area and a long start-up time  Biological scrubbers are low maintenance and require little operator attention  Gaseous pollutants have to be dissolved in aqueous Phase, which may result in gas transfer problems  These systems produce musty or earthy odors, which depending on the proximity of the receptors, may require additional refining

Benzene, ethylacetate, toluene

90e98

 Efficiency is sensitive to operating conditions  Susceptible to impurities

Thermal oxidation

Benzene, toluene and xylene

95e99

Chemical scrubbers (NaOCl, NaOH, NaHCO3, aqueous NH3 etc.)

Resorcinol, dimethyl sulfoxide, dimethyl sulfide

90e99

 Halogenated and other compounds may require additional control equipment  Noxious gases are released  Widely used to treat odors in wastewater treatment facilities  Efficient removal of various contaminant types and oxidation of H2S  Requires significant level of supervision and maintenance

A. Physical Methods Absorption

Adsorption (activated carbon, activated alumina, silica gels, zeolites, etc.) Condensation

B. Biological methods Biofiltration

Biological scrubbers

C. Chemical Methods Catalytic oxidation

hydrogen sulfide and other reduced sulfur compounds, chemicals like sodium hypochlorite (bleach) are typically used in liquid solutions. Thermal oxidation results in conversion of VOCs into complete oxidation products such as CO2, H2O, NOx, and SO2, depending on their chemical compositions (Salvador et al., 2006; Choi and Yi, 2000). To address the energy-intensive nature of thermal oxidation, catalytic oxidation has been proposed to enhance the destruction efficiency of VOCs at moderate temperatures. For the catalytic oxidation of various VOCs, a number of catalysts (supported noble metals, binary oxides, solid solutions of metal oxides, doped metal oxides, etc.) have been fabricated and commercialized (Bastos et al., 2009; Papaefthimiou et al., 1998; Saqer et al., 2011; Azalim et al., 2011). The main disadvantage of catalytic oxidation is catalyst poisoning (i.e., deactivation) caused by the release of hazardous components (e.g., sulfur and halogens) into the air stream. 2.4. Photocatalytic oxidation (PCO) for VOC removal in indoor environments In the past decades, the well-established and widely-studied (by both academia and industry) photocatalytic oxidation (PCO) has been applied for indoor air quality management (Mo et al., 2009; Mamaghani et al., 2017; Demeestere et al., 2007). Detailed discussion on PCO is beyond scope.

Belaissaoui et al., 2016; Uria-Tellaetxe et al., 2016

Nielsen et al., 2007; Potivichayanon et al., 2006; Koutinas et al., 2007

Papaefthimiou et al., 1998; Azalim et al., 2011; Saqer et al., 2011 Choi and Yi, 2000

Biard et al., 2011; Wu and Lee, 2004

3. Mechanisms of engineered air ionization Air ionization systems are operated by flooding the atmosphere with positive and negative ions. Electrostatic charge represents the accumulation of ions of a particular charge on a non-conducting surface until a steady-state is attained through neutralization (Stalker and Richard, 1988). Natural ionization sources include electromagnetic radiation (e.g., cosmic rays, gamma rays, lightening, and solar ultraviolet radiation) and alpha/gamma radiation from radioactive materials in the Earth's crust (Jokl, 2002). Gas molecules are electrically neutral under STP conditions. The minimum amount of energy needed to ionize gas-phase molecules and atoms is known as the first ionization energy (IE) (Middleton, 1989; Sawant and Jadhav, 2012). If the ionizing energy is enough, then one or more bound electrons can be stripped from the molecule (or atom) into the gas phase as free electrons to yield molecular cations with single and/or multiple charges. These primary positive ion species can undergo sequential ion-molecule reactions with polar neutral water molecules to yield protonated water cluster ions (e.g., Hþ(H2O)4, m/z 73), as observed in high pressure mass spectrometry (Szulejko et al., 1992). Both the natural and anthropogenic pathways for ion formation  are explained schematically in Fig. 1 (Cernecky and Valentova, 2012; Honer, 2010). Excited air molecules (N*2 and O*2), generated during the ionization process can propagate O3 by chain reaction

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with NOx products (Dvorak and Koryta, 1983; Fischer et al., 1983; Sitar, 2006). O3 is a strong oxidizing reagent, which provides a favorable environment for the destruction of VOCs, even at ambient temperature (Obulana and Hostin, 2012; Sitar, 2006; Weschler, 2000). 3.1. Mechanisms of air ionization Basically, an intense electrostatic field around a sharp point will initiate the electrical discharge of high-energy electrons (Steinman, 2006). These energetic electrons will interact with neutral air

molecules to generate either excited neutral air molecules or primary positive ions (and additional (slow) secondary electrons). These slow secondary electrons are quickly thermalized and undergo termolecular electron attachment to O2 to yield the superoxide anion (O 2 ) in a parallel plate free-air ionization chamber (Takata, 2010) (Fig. 2). Positive and negative ions are continuously generated by an intense electrostatic field, and quickly reach a steady-state density (e.g., typically 105 to 106 ions cm3) via ion-ion and ion-electron recombination reactions. Both electrostatic and ionizing radiation can be used to produce both positive and negative ions. There are

 Fig. 1. Schematics of both (a) natural and (b) anthropogenic pathways for ion particle formation (Cernecky and Valentova, 2012; Honer, 2010).

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numerous methods to create weak bipolar air plasma. No single ionization method is yet regarded as being the most suitable option  for all situations (Cernecky et al., 2015). Because ion migration is governed by electric fields and bulk air mass flow, the effectiveness of an ionization system is highly contingent on various parameters such as environmental conditions, the problem to be solved, and the nature of the work (Levit and Beyer, 2011). In addition, another critical factor to consider is the ionization source for the system, e.g., high-energy electron impact, AC, DC, pulsed DC, alpha particles, or X-rays. However, electric field ion sources are not capable of producing sufficient ions for air quality control purposes. Thus, stronger ionization sources such as ionizing radiation (e.g., alpha particles) and corona ionization are used preferentially to generate higher steady-state ion densities. 3.2. Alpha particle-based and corona ionizers Air ionization can be achieved by many routes: heavy particle impact, electron impact, thermal ionization, photon impact, and electric field ionization (Loeb, 1965; Aleksandrov et al., 2001; Anikin et al., 2001; Chernikov et al., 2001). The operational mechanisms of the two most widely used types of ionizers (alpha particle-based and corona ionizers) are further appraised as representative case studies. First, alpha (a) particles (helium nuclei) are considerably heavier than other radioactive decay particles (e.g., b or g particles) (Levit and Beyer, 2011). For that reason, a particles transfer much more slowly than b-particles of the same energy. At such reduced speeds, their electric fields can interact for a comparatively extended time with a molecule's electron cloud to affect energy transfer for an ionization event. Thus, air has greater stopping power (shorter mean free path) for a particles than b-particles. Helium nuclei emitted from Po210 with an estimated translational energy of 5.4 MeV have a mean free path of only ~4 cm in air at 1 atm (Levit and Beyer, 2011). Each a particle creates approximately 250,000 ion-electron pairs in 4 cm. Because b particle ionization results in substantially lower ion density due to its much lower stopping power, it should be less effective at maintaining static charge. Each ionization event creates an equivalent number of positive and negative charges (e.g., N2 / Nþ 2 þ e; 1 positive molecular ion and

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one free electron). Therefore, a particle-based ionizers generate an almost charge-balanced population of positively and negatively charged species without the need for additional ion or electron sources to maintain charge neutrality. The merits of a particlebased ionizers include low flux of a particles, high temperature plasma generation, and reduced concentrations of free radicals/ cluster ions (Shiue et al., 2011). Although air ionization is conventionally generated by coronas from needle electrodes, unwanted particles can also be generated (Shale et al., 1964; Wilson and Brewer, 1973; Vinogradov et al., 2001). To circumvent this shortcoming (i.e., develop clean ionizers), a number of conditions/parameters (e.g., choice of electrode material, sharpness, voltage levels, and wave forms of high voltage signals) have been investigated extensively (Aleksandrov et al., 2001; Levit et al., 2000; ION, 2010; Shiue et al., 2011). To generate air ions, needle electrodes are operated at high voltages (usually 10 ~ 20 kV DC or AC) (Yost, 1989). Among such alternative options proposed or developed, corona ionizers create both neutral and charged species via two mechanisms: (i) erosion of the emitter tip and (ii) plasma formation at the needle tip. In the case of the latter, the needle tip acts as a chemical reactor to convert airborne molecular contaminants (AMCs) to small aerosols which, in turn, agglomerate into larger ones (the second effect) (Levit and Beyer, 2011). Generally, the cleanest ionizers (most erosion free) are the ones that run at the lowest power output. Optimum configurations are those that avoid recombination of positive and negative ions so that sufficient ions are received at the charged object while minimizing the power dissipated at the emitter tip (Wilson, 1987; Gefter, 2008). 4. Types of air ionization approaches for VOC removal 4.1. Air ionization by thermal plasma To generate plasma, various types of discharges are employed: (a) electron-beam (Son et al., 2010; Sun and Chmielewski, 2012), (b) dielectric barrier (Liang et al., 2015; Jin et al., 2016), (c) pulsed corona (Grabarczyk, 2001; Nagato et al., 2006), (d) microwave (Cha and Carlisle, 2001; Wang and Wang, 2016), (e) gliding arc (Bo et al., 2008), and (f) radio frequency (Yuan et al., 2011; Eliasson and

Fig. 2. A parallel plate free-air ionization chamber (Takata, 2010).

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Kongelschatz, 1991) discharge. Air ionization by thermal plasma involves striking a plasma flame at atmospheric pressure using high discharge power (P z 1 kW to 50 MW) at high voltage. High voltage discharge excites gaseous molecules to generate highly reactive species such as ions and radicals (Hammer, 1999). The plasma temperature varies from a low of 1000 K to peak temperatures of 10,000 to 20,000 K depending on such variables as the electric voltage, flow rate of the gas, and plasma source (Fridman and Kennedy, 2011). High electrical voltage together with an elevated temperature expedites the production of high fluxes of reactive ions or radical species, resulting in the destruction of VOCs in a short response time (Bahri and Haghighat, 2014). As such, the advantages of thermal plasma include high specific activation and total fragmentation of molecules. Nonetheless, practical use of thermal plasma is restricted by its high operational cost (Heberlein and Murphy, 2008). Furthermore, because particles in thermal plasma (electrons, ions, atoms, and molecules in the background gas) are in thermal equilibrium, overheating of the reaction media is unavoidable. Although thermally-induced plasma is effective at treating high concentration of VOCs, it is not economically viable for treatment of low concentrations of VOCs. Therefore, this approach is not economically feasible for treating indoor air, which generally has low pollution levels. 4.2. Air ionization by non-thermal plasma Non-thermal (non-equilibrium) plasma (NTP) generated at ambient pressure is another approach for the generation of air ions. NTP generates electrons with high-energy and short residence time capable of initiating destruction of VOCs at ambient temperature (Xiao et al., 2014; Hołub et al., 2014; Stasiulaitiene et al., 2016). The destruction of low concentrations of VOCs in high volume streams by NTP is an important advantage of this approach. However, there is insufficient number of studies to definitively determine the mechanism and removal efficiency according to compound class. A variety of NTP reactors have been developed to treat VOCs. The different non-thermal plasma techniques are shown in Fig. 3. Air ionization by non-thermal plasma has been employed to remove VOCs. However, high power consumption and the production of undesired by-products (e.g., aldehydes, alcohols, acids, aerosols, particulate matter, and non-volatile C-containing compounds) limit the practical application of this technique. To improve the efficiency of non-thermal plasmas, the residence time of VOCs in the plasma zone can be increased. A combination of NTP and heterogeneous catalyst (BaTiO3, TiO2, Fe, Co, Cu, Ni, Mn oxides, Ag, Au, Pt noble metal supported porous silica, alumina or zeolites) thus appears to be a feasible alternative. An increase in the residence time of adsorbate molecules in the plasma zone can lead to considerable improvement in conversion rates (>99%) and superior selectivity (>60%) towards total oxidation products, e.g., CO2 and H2O (Ghaida et al., 2016; Kim et al., 2010; Rousseau et al., 2005). 4.2.1. Reactors without catalyst beds The decomposition characteristics of BTX (prepared at 300 ppm in synthetic air) were investigated after DC corona discharge at atmospheric pressure (Satoh et al., 2012); the experimental setup and results were very similar to those described below in this paragraph (Sakai et al., 2016). Major decomposition products were CO2, CO, HCO2H, and (CHO)2O while C2H2, HCN, and (CHO)2 were found as minor products. It was thus concluded that the BTX was converted to CO2 through diverse intermediates, e.g., CO, HCO2H, and others. Likewise, the decomposition of 300 ppm BTX (in 20/80 O2/N2 synthetic air) was also measured after DC corona discharge in a 9.4 L reactor (10 cm radius, 30 cm height) under static conditions

at atmospheric pressure (Sakai et al., 2016). Decay rates of benzene and toluene were ~1.7 and 2.7 h1, respectively, at a power of 5.5 W and discharge voltage of 27.5 kV. Using a mass change tracking (MCT) approach, the carbon budget was estimated after 1 h of reaction against 300 ppm (or 2100 ppmC) toluene. Toluene was significantly reduced to 75 ppmC (96% removal) and several reaction products such as CO2 (510 ppmC), CO (370 ppmC), and formic acid (340 ppmC) were identified, along with other identified products (210 ppmC) and unaccounted-for products (600 ppmC, largely non-volatile deposits inside the reactor). The removal efficiency for benzene, toluene, and xylene was 0.8, 1.5, and 2.2 g kWh1, respectively. The large amount (370 ppm) of CO generated, however, was disturbing. The destruction efficiency of toluene was also tested by irradiating the model toluene-air gas-mixtures with an ILU-6 accelerator at the Institute of Nuclear Chemistry and Technology (INCT), Poland (Sun et al., 2009). Accordingly, the decomposition efficiency of toluene increased with absorbed dose while decreasing with its initial concentration. At the initial 32 ppm toluene level, the decomposition efficiencies were greatly distinguished between 50% (14.5 kGy dose) and 93.8% (58.0 kGy dose) with benzaldehyde as one of the major by-products. The removal of toluene (14.7 ppm), 1.5-hexadiene (13.3 ppm), and chloroform (19.5 ppm) by negative air ions (mainly O 2 ) was explored using a lab-scale negative air ion (NAI) reaction chamber under static conditions (Wu and Lee, 2004). The stainless steel NAI chamber (id ¼ 39 cm, l ¼ 78 cm) had three discharge electrodes, each consisting of an array of 20 needles with a tip of 0.1 mm in diameter with discharge voltages of up to 30 kV. The concentrations of reaction products (O3, NO, and NO2) were voltage dependent, and were therefore not detected below 17 kV. Hence, removal experiments were carried out at 15 kV. The NAI concentration was about 1.3Eþ06 ions per cm3, while no positive ions were detected. Removal efficiencies after a 12 h reaction (0% RH air) were 13% for toluene, 8.1% for chloroform, and 98% for 1,5-hexadiene. No reaction products were detected by GC-FID for toluene and chloroform; 1,5-hexadiene gave an intermediate product (4-pentenal) that reached a maximum concentration (~2 ppm) at 7e10 h and decayed to ~0 ppm by 16 h (Fig. 1S). The decay of benzene (initially 106 ppm) and toluene (initially 112 ppm) inside a closed 0.71 m3 chamber (with a 4.6 m3 h1 fan) was studied under two conditions: (a) natural decay and (b) with an air ionizer that generated ~175,000 ions cm3 inside the chamber (Cho et al., 2012). The natural (kn) and air ionizer decay (ka) constants were virtually identical, showing that air ionization had a minimal impact. The average kn and ka values (three measurements) for benzene were 1.29 and 1.40 hr1, respectively. The corresponding toluene kn and ka values (two measurements) were 1.30 and 1.39 hr1, respectively. 4.2.2. Catalyst packed beds In a companion article, Van Durme et al. (2007b) installed a catalytic packed-bed reactor (either post plasma {PPC} or in-plasma {IPC}) in their lab-built non-thermal plasma reactor to study the removal of 0.5 ppm toluene (Fig. 4). Two catalysts were used: the commercial catalyst Aerolyst® 7706 TiO2 with 15% Al2O3, and the O3-degrading catalyst CuOMnO2/TiO2. The residence time in the reactor was 1.12 s at a flow rate of 10.8 L min1. Catalyst (10 or 15 g) was loaded on either the IPC or PPC bed for evaluation. Nearly 80% of toluene was removed when 10 g of CuOMnO2/TiO2 (PPC) was used vs. ~0% in the absence of catalyst at an energy dose of ~2 J L1 (1.7 kGy) and flow rate of 10.8 L min1 (Fig. 2S). Performance of the Aerolyst® 7706 catalyst was inferior to that of CuOMnO2/TiO2; an energy dose of 15 J L1 (12.9 kGy) was needed to achieve 80% removal (IPC). No plasma degradation products of toluene were detected by SPME/GC/FID. Outlet emission levels of ozone and NO2

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Fig. 3. Schematic of different non-thermal plasma reactors for generating air ions (Vandenbroucke et al., 2011; Kim, 2004; Urashima and Chang, 2000; Vitale et al., 1996; Yardimci et al., 2000; Zoschke et al., 2012).

were much reduced with catalyst, e.g., O3 (14 ppb) and NO2 (2520 ppb (no catalyst) vs. 550 ppb (PPC, 13 kGy)). No catalytic deactivation was observed over 48 h of continuous operation. If scaled up to 30 m3 room, the plasma catalytic hybrid reactor needs an air processing rate (APR) of about 500 L min1 to treat a room in a reasonable amount of time (~1 h for 80% removal of 0.5 ppm toluene); plasma power input would need to be ~150 W (at 15 J L1 dose) or 0.75 g kWh1 at 45 kV discharge. 4.2.3. Relative humidity Use of non-thermal plasma (DC positive corona discharge) to remove toluene from indoor air has also been explored (Van Durme et al., 2007a). An inlet air stream containing toluene (0.5 ppm) at 1 atm pressure was flowed through a lab-scale non-thermal plasma reactor at 10 L min1. Toluene removal, when examined under three RH conditions (0, 26, and 50%), was about 80% for an energy dose of 60 J L1 (or ~50 kGy). In contrast, ozone concentrations at the outlet were dependent on the energy dose and RH, with levels of ~300 ppm in dry air (0% RH) relative to 150 ppm (60% RH)) at a dose of 75 J L1. A large number of toluene-derived products (e.g., formic acid, benzyl alcohol, and 4-methyl-2-propylfuran) were observed by GC-MS after treatment of air containing 150 ppm toluene (<1% RH, 10.8 L min1 flow, and 54 J L1 dose); these byproducts may be even more hazardous than toluene (Fig. 5). A degradation pathway for toluene was proposed (see Fig. 6, bottom panel). Oxidation products of organic compounds (e.g., toluene, i-octane, and halogenated methanes) in atmospheric air corona plasmas have also been examined (Marotta et al., 2010). A corona discharge reactor operated with DC (±25 kV) or pulsed current was

built as a grounded stainless cylinder (38.5 mm id and 600 mm long) with a 1 mm wire electrode along the axis (Schiorlin et al., 2009). Removal efficiencies (kE) of this reactor were reported in L kJ1 with the flow rate through the reactor at 0.45 L atm min1. The analyte concentration [A] can be expressed as [A] ¼ [A]0.exp(kE.SIE), where [A]0 is the initial concentration and SIE is the specific input power (SIE) in kJ L1. The kE value was higher in humid air (40% RH) than dry air (eDC discharge). For example, the kE values for 500 ppm toluene were -DC 0.42 (dry air) and 0.81 (44% RH) and þDC 0.14 (dry air) and 0.13 (40% RH). A positive ion APCI mass spectrum of blank air showed various hydrated species, e.g., H3Oþ.(H2O)2 (m/z 55, base peak) and NOþ.(H2O)2 (m/z 66); for hexane and i-octane, various CnHþ2nþ1 species were noted whereas for toluene, protonated toluene (m/z 93) was dominant. A number of products (e.g., H2O, CO2 (major), N2O, CO, HCO2H, HNO3, and O3) were detected in the reactor outlet by FTIR after -19 kV DC corona treatment of air containing 500 ppm toluene. 4.3. Air ionization by dielectric barrier discharge (DBD) Dielectric barrier discharge (DBD) is an electrical discharge system that creates non-thermal plasma between two electrodes separated by an insulating dielectric barrier operated at high voltage. The conversion of 2-heptanone by the DBD plasma method was explored under different pulsed modes of excitation, energies, and carrier gas compositions (Chiper et al., 2010). These authors observed that the removal efficiency was 30% higher when 2e3% of oxygen was added to nitrogen than in pure nitrogen. PMs were observed in the plasma due to nucleation of dehydrogenated VOCs (e.g., polyaromatic hydrocarbons and hydroxides) (Borra, 2006;

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4.4. Pulsed corona discharge Pulsed corona discharge is another means to produce NTP to treat VOCs in air streams. In corona discharge, a current flow from an electrode at high potential can ionize a neutral fluid such as air (surrounded by electrode) so as to create a region of plasma around the electrode. The ions generated in bulk gas can thus effectively participate in degradation of VOCs in the air stream. There are two types of corona discharges - positive and negative corona discharge - that result in the production of positive and negative air ions, respectively. The degradation of i-octane and hexane was investigated using positive and negative coronas (Marotta et al., 2008). These authors reported that a radical mechanism was dominant in negative DC corona ionization, whereas ionic reactions prevailed in positive DC corona ionization. The oxidative degradation of toluene and limonene (ppm level at large air flows) was studied using a hybrid pulsed corona reactor (Hoeben et al., 2012). The conversion efficiencies of those two compounds to biocompatible carboxylic acids (e.g., acetic and formic acids) were 74% and 81%, respectively. A simplified mechanism for the degradation of toluene and limonene using ozone as an oxidant is illustrated in Fig. 6 (upper panel) (Hoeben et al., 2012). Microwave-induced plasma is another method capable of generating electrode-less discharges to produce air ions. In this approach, high frequency (GHz) radiation is used to create gas discharges. The technical advantages are low power consumption as well as production of high ion concentrations in stable plasma, even under humid conditions (Cha and Carlisle, 2001). Cha and Carlisle (2001) investigated the removal and destruction of chlorinated and non-chlorinated solvents extracted from soil vapor. They demonstrated that microwave ionization performed better than conventional thermal/catalytic processes used for VOC control. Kim et al. (2014) developed an atmospheric-pressure microwave plasma in a reverse vortex reactor to remove VOCs from polluted air streams. These authors focused on the destruction of VOCs in high flow rate polluted streams as generated in industry. A destruction removal efficiency (DRE) of 98% was achieved for gases containing 550 ppm of VOC at an air flow rate of 5 m3 atm min1. 4.5. The electron impact method

Fig. 4. Schematic of hybrid plasma catalytic system for efficient toluene removal (reproduced from Van Durme et al., 2007b).

Zhang et al., 2013). As the exposure to this particulate matter can pose human health problems, concurrent removal of VOCs along with the generated particulate material is a matter of great concern (Donaldson et al., 1998). Jin et al. (2016) investigated the decomposition of n-hexane in a DBD reactor filled with different kinds of dielectric balls (quartz, less porous, and porous alumina balls); the DBD reactor filled with porous alumina balls was efficient at reducing particle emission by adsorbing liquid particles. Further, the DBD method, when combined with photocatalysis, considerably improved VOC removal efficiencies (Palau et al., 2015; Ghaida et al., 2016).

We consider the electron impact method one of the most sophisticated and novel technologies for treatment of flue gas emitted from diverse sources (e.g., coal-, lignite-, or heavy fuel oil-fired). However, the electron guns used to produce high-energy electrons are very large (several m3), therefore special radiation shielding is required. Hence, this method is not suitable for smallscale indoor air applications, as the electron gun is operated only under vacuum; the high-energy electron beam needs to pass through a window interface into the air stream. When used to treat VOCs at low concentration and high volumetric flow, electron beam irradiation can produce free radicals and ions in a short time span (i.e., 108e101 s) in the reaction medium (Son et al., 2010). As such, the EB process has been shown to be most economical for low VOC concentrations (diluted into high volume flows) with consumption of about 0.2 kWh of electricity per 100 m3 atm (Hirota et al., 1995). Energetic electron impact (EI) of molecules can result in the formation of excited (Rydberg) neutral molecules, leading to pollutant fragmentation and ionization. Thus, highly reactive free radicals produced by the EI system initiate chain reactions, resulting in the cracking of the larger molecules into smaller species (Alfi et al., 2015). EI of surrounding air results in the generation of various primary ions, secondary ions (via by ion-molecule reþ þ þ þ actions), and excited neutrals (such as e, Nþ 2 , N , O2 , O , H2O , * * þ OHþ, Hþ, COþ 2 , CO , N2, O2, N, O, H, OH, and CO) (Son et al., 2010).

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Fig. 5. GC/MS chromatogram of toluene degradation products produced in a positive DC corona discharge: dry air (<1% RH), reactor flow rate ¼ 10.8 L.min1, [Toluene] ¼ 150 ± 1 ppm, dose ¼ 54 J.L1 (or ~47 kGy), and reactor inlet pressure ¼ 101.3 kPa (Van Durme et al., 2007a).

The decomposition characteristics of trimethylamine after electron beam processing were investigated (Son et al., 2013). Addition of water vapor resulted in a 5e30% increase in the overall removal efficiency of trimethylamine. This was attributed to the generation of strong oxidizing -OH radicals by the electron beam that resulted in the degradation of various air pollutants. Chmielewski and Ostapczuk (2010) conducted lab-scale experiments using an 800 keV electron beam irradiation system for the cleanup of the exhaust stream from the burning of the heavy fuel oil, mazut (partly refined crude oil: ~3% sulfur content). However, these authors did not provide any details about the total power consumption or capitalization costs, therefore the industrial cost effectiveness of this technique cannot be evaluated. The removal of SO2 and NOx (as (NH4)2SO4 and NH4NO3) was achieved simultaneously at 98 and 80% efficiency, respectively, under the following optimal conditions: 5 m3 atm hr1 stream flow rate, 20 KW electron beam power (pulse width and frequency not reported), 800 keV electron energy, 12.4 kGy dose (corresponding to ~20 W irradiating power and a temperature increase of 17 K at constant volume), and ammonia/water injection. (NH4)2SO4 and NH4NO3 can be sold as bulk commodity fertilizer to partially offset costs. In addition, removal efficiencies of polycyclic aromatic hydrocarbon (PAHs) and BTX were 42 and 86%, respectively at an electron irradiation dose of 5.3 kGy. This technology can therefore be used for simultaneous removal of acid gases (SO2 and NOx) and organic (PAH and BTX) pollutants. Note that 1 Gy corresponds to a dose of 1 J of energy per kg of matter. The removal efficiency and products formed during electron beam (550 keV) treatment of 38 ppm butylacetate and 22.5 ppm xylene (all isomers) in air (flow rate ¼ 278 L.s1) were determined (Hirota et al., 1995). Butyl acetate removal efficiency was 65% at a 10 kGy dose and formic, acetic, propionic, and butyric acid products were formed. The removal efficiencies (at 10 kGy dose) of xylene (o), xylene (m, p), and ethybenzene were ~85, ~90, and ~70%, respectively. Xylenes (97 mg C m3 input) yielded a number of reaction products (as mg C m3): formic (2.5), acetic (6.3), propionic (2.6), and butyric (1.8) acids, products on filter pad (40.6, largest fraction), CO (5.3), CO2 (6.3), and unreacted xylene (14). Note that CO2 in air (400 ppm ¼ 196 mg C m3) appeared to be removed, as the detected amount in the effluent was only 6.3 mg C m3 (or 13 ppm).

4.6. Hybrid air ionization methods The main drawback of the electron impact beam method is the formation of undesirable byproducts (ozone, CO, CO2, aerosols, and other trace compounds such as benzene, aromatic/aliphatic aldehydes, ketones, acids, and acetates) (Kim, 2002; Sun et al., 2009). However, the extent of such problems can be mitigated by the use of heterogeneous catalysts (Cu, Mn, Pd, Pt metals, V2O5, Mn2O3 oxides, and Pt/Pd supported zeolites/g-Al2O3). Thus, the concurrent use of electron impact beams and catalysts can minimize the formation of byproducts; such hybrid technology can also improve the decomposition efficiencies of VOCs (Kim et al., 2010; Jeon et al., 2008). A hybrid approach using an electron beam 1% Pt/Al2O3 was used for VOC decomposition in a hybrid reactor (Kim et al., 2004). Toluene and styrene were destroyed more effectively using a hybrid system (with a catalyst bed) than by electron beam irradiation only. A similar treatment system consisting of a compact-sized electron accelerator and an ozone decomposition catalyst made of MnO2 was developed and tested for the removal of toluene and xylene (Hakoda et al., 2010). The performance of this electron accelerator was then compared before and after the concurrent use of a catalytic bed. Removal of toluene and xylene increased from 60 to 91% and from 81 to 91%, respectively, as mineralization (i.e., to CO2 and CO) increased from 42 to 100%. Ighigeanu et al. (2008) developed a new hybrid technique by combining electron beams, microwaves, and catalysts. This hybrid system exploited the advantages of the three component techniques, namely (1) very high efficiency of conversion of VOCs into intermediate products by -OH radical reactions after electron beam use, (2) the ability of microwaves to produce non-thermal plasma in electrode-less reaction vessels, and (3) complete oxidation of intermediate products to CO2 and H2O by the catalysts. These authors compared the performance of this system as a whole and each individual component. The removal efficiencies of toluene were superior for the combined system (92.8%) than individual components of the system (Ighigeanu et al., 2008). A number of studies were carried out to evaluate the performance of electron beam processing relative to other VOC treatment approaches. Electron beam processing for many VOCs has been reported to be highly energy efficient relative to pulsed corona

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K.-H. Kim et al. / Environmental Pollution xxx (2017) 1e15

Fig. 6. Simplified oxidative degradation pathways of VOCs: upper ((A) toluene and (B) limonene (Hoeben et al., 2012)) and lower (toluene (Van Durme et al., 2007a)).

(Penetrante et al., 1996). These authors found the net electron beam energy input into a reactor containing carbon tetrachloride (at 100 ppm) was 9 J L1 compared to 555 J L1 for the pulsed corona method (a similar trend was observed for benzene and methylene chloride treatment). In a theoretical industrial setting, treatment of a 1 m3.s1 gas stream would require the excessive power input of 555 kW if the pulsed corona method were used. Relative performance of the electron beam and plasma method was also assessed in terms of removal efficiencies of ammonia (Son et al., 2013). Gas residence time was 10 times less for the electron beam process than plasma. Additionally, energy dose for the electron beam process was much lower (15 kGy) than the plasma (106 kGy) method. As such, the former is much more energy efficient than the latter. Despite such advantages, it is not economically feasible to use an electron beam process to treat indoor air pollutants. A serious limitation is that electron beam accelerators involve large setups/ installations (several m long) and require a shielding system from X-rays (from bremsstrahlung) (Chmielewski, 2007; Chmielewski and Haji-Saeid, 2004). In addition, accelerators self-shielded up to 300 keV have been proposed for VOC treatments. Thus, while these processes are suitable for industry (>m3 s1 flow rate), they are not practical for treating VOCs in indoor environments.

5. Basic characteristics and performance of air ionization approaches To date, current technologies for VOC treatment have a number of disadvantages, e.g., unwanted oxidation by-products, high costs, and limited processing capacity. Thus, it is essential to develop more cost-effective and environmental-friendly technologies to meet the ever-increasing demand for air quality control and management. Air ionization in the foreseeable future may be an emerging effective alternative for treating airborne VOCs. In this section, air ionization is critically reviewed in terms of removal efficiency, throughput, limitations, and cost. Recent developments in the design and operation of air ionization devices have made these devices more reliable and efficient alternatives to treat VOC and odors than conventional methods (Bohlen and Bozoo, 2009; Karim, 2015). Engineered air ionization devices are capable of producing specific ions, including reactive oxygen species (e.g., O3 and O*2), in a controlled manner with minimal byproducts. Air ionization has been adapted to remove airborne pollutants including dust particles, secondary organic aerosols, microorganisms, and allergens from air streams in indoor environments (Liang et al., 2012). The use of ionization has

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Fig. 6. (continued).

advantages over other removal techniques due to its lower energy costs, reduced formation of dangerous byproducts, and several potential health advantages (Daniels, 2001). A number of strategies that have been developed to generate air ions to treat VOCs and odors are presented in Table 2. Note that each method has advantages and disadvantages, and most studies have been conducted under different experimental setups and conditions. Hence, it is a challenging task to assess the relative performance of the different methods. Nonetheless, the performance of hybrid technologies appears to be superior to that of individual methods. In summary, air ionization generates low m/z air ions, including superoxide anions (O2 -), that can react readily with airborne VOCs and odors. Key parameters of air ionizers and other techniques in air quality management are compared in Tables 1 and 2. We discussed the significance of air ionization chemistry and its potential for the advancement of air quality management based on a review of the current literature (e.g., Daniels, 2001). However, researchers around the world are continuing to investigate new air ionization

applications for both industrial processes and biological systems. There are several challenges for air ionization techniques that have to be overcome (Table 3). Further studies are needed to optimize air ionization techniques to increase process efficiency and reduce energy requirements to make them cost-effective. We recommend implementing the following approaches to improve performance: (i) improve the discharge mode, including the structure of the reactor and the frequency/voltage of the power supply and (ii) combine hybrid systems with other approaches (Oda et al., 2002). 6. Conclusions In this review, we described the fundamental characteristics of air ionization methods used to improve air quality with respect to removal efficiency of odors and VOCs. Air ionization technology is gaining ground as a viable technology for the treatment of specific VOCs and odors. Air ionization techniques facilitate destruction of hazardous air/odor contaminates through control of electrostatic discharges.

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Table 2 Comparison of performance of different air ionization methods for the treatment of airborne VOCs. Type of treatment

Target VOCs

Removal efficiency (%)

Remarks

Reference

Electron impact beam

Trimethylamine

90e98

Son et al., 2013; Chmielewski and Ostapczuk, 2010

Dielectric barrier discharge

2-Heptanone, hexane, isovaleraldehyde

75e99

Pulsed corona discharge

Propene, toluene, benzene, n-hexane

77e99

Microwave discharge Thermal plasma

3-Pentanone

98

Mixture of VOCs

100

 Formation of by-products  Applicable to source high emission rate and/or low concentration at ambient air temperature and requires relatively low dose compared to plasma discharge  Applicable only for pilot plant or industrial scale treatment of pollutant air streams  Low energy efficiency, poor selectivity for CO2 even when high conversion rate is reached, and undesirable byproduct formation  Suitable for industrial production, simple operation, and configuration  Higher electron energies and short pulses result in efficient synthesis of reactive oxygen species for efficient oxidative degradation  Low power consumption, production of high levels of ions, and stable plasma under humid conditions  High specific activation and total fragmentation of pollutants to CO2 and CO

Wang et al., 2009; Kim and Park, 2008

Malik et al., 2005; Jarrige and Vervisch, 2006 Kim et al., 2014 Han et al., 1993; Murphy 2001

Table 3 Comparison of various principles and approaches employed in air ionization techniques. Item

Method

Mechanism

Advantages

Disadvantages

Reference

1.

Electron impact (EI)

Easily regulated and quite reproducible

Thermal plasma

Very low energy efficiency. Only a few electrons penetrate into the air stream from the electron beam source. Extremely cost-ineffective and subject to matrix contamination.

Son et al., 2010; Sun and Chmielewski, 2012

2.

3.

Photon impact (PI)

Electron impact ionization of neutral molecules Gas is heated to T > 10,000 K to generate a plasma. Photon ionization (UV, x-rays, g-rays)

4.

Fast ion bombardment

5.

Electric field

Hydrogen fusion reactions emitting fast Hþ and Heþ ions at high MeV translational energies to ionize neutral molecules Involves the passing of a gas stream between highly charged electrodes

Highly efficient and reproducible Alternative ionization source and good efficiency at low cost

Fission of heavier atoms releases high energy ions that can cause secondary ionization, hence, highly productive Considerably high electric field density (up to a few kV/m) is required. Metallic electrodes should also have a sharp geometry for optimum efficiency. Easy to generate and controllable

Air ionization technologies based on monopolar and static electricity discharge have many benefits for air cleaning purposes, including destruction, transformation, and removal of VOCs and odors. Namely, they produce fewer hazardous reactants/byproducts than conventional methods, there is minimal bulk deposition of odor on room surfaces, and energy costs are low. Air ionization technologies also have potential health benefits compared to conventional technologies (e.g., filtration and adsorption). Static electricity discharges (a form of air ionization), although they can pose severe explosion risks, occur during many industrial processes (e.g., paint spraying, bag filling, and surface coating). A monopolar electrostatic precipitator is sufficient to generate ions of one polarity that bind to particulate matter. These particles are then attracted by electrostatic forces to oppositely charged or grounded collection plates. This process is commonly used in industrial clean room, office, and home indoor air applications. For proper management of air quality, air ionization techniques are emerging as promising alternatives to conventional methods.

Fridman and Kennedy, 2011; Bahri and Haghighat, 2014

Relatively low efficiency and exhibits adaptability shortcomings when applied to various conditions. PI ionization cross section about 2e3 orders lower than that required for EI ionization. Cannot be adapted safely for application in many scenarios.

Aleksandrov et al., 2001; Anikin et al., 2001; Chernikov et al., 2001

Extremely energy intensive.

Levit and Beyer, 2011; Luengas  et al., 2015; Cernecky et al., 2015

Luengas et al., 2015; Son et al., 2010; Sun and Chmielewski, 2012

To date, air ionization techniques have been used extensively to remove odors and VOCs from air flows, and their feasibility has been demonstrated in sensitive manufacturing operations. The future of air ionization will be brightened by continuing efforts to improve their cost-effectiveness and minimize the production of by-products. We anticipate that advances in nanotechnology, biotechnology, and other related science sectors will result in air ionization techniques with excellent performance for removing VOCs and odorants from indoor air spaces. Acknowledgements This study was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (No. 2016R1E1A1A01940995). E. E. Kwon also acknowledges the support of an NRF grant funded by the Korean Government (MSIP) (No. 2014RA1A004893). This research was also supported partially by the R&D Center for Green Patrol

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