# Application of Gaseous ClO2 on Disinfection and Air Pollution Control: A Mini Review

Special Issue on COVID-19 Aerosol Drivers, Impacts and Mitigation (VII)

Tse-Lun Chen1, Yi-Hung Chen2, Yu-Lin Zhao3Pen-Chi Chiang This email address is being protected from spambots. You need JavaScript enabled to view it.1

1 Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 10673, Taiwan
2 Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan
3 Department of Mechanical Engineering, National Taiwan University, Taipei 10673, Taiwan

Revised: September 4, 2020
Accepted: September 5, 2020

Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

Chen, T.L., Chen, Y.H., Zhao, Y.L. and Chiang, P.C. (2020). Application of Gaseous ClO2 on Disinfection and Air Pollution Control: A Mini Review. Aerosol Air Qual. Res. 20: 2289–2298. https://doi.org/10.4209/aaqr.2020.06.0330

## HIGHLIGHTS

• Chemical principles of gaseous ClO2 production was reviewed.
• Case studies of using gaseous ClO2 in hospitals and food industries were discussed.
• Process chemistry and applications of ClO2 gas works as an oxidizer were provided.

## ABSTRACT

During the sever pandemic of coronavirus, the development and deployment of efficient disinfection technology have attracted hospitals’ attention. Chlorine dioxide (ClO2) gas has been validated as an efficient disinfector and air pollution control due its high oxidation ability. This article reviewed the principles and application of ClO2 gas on disinfection, sterilization and air pollutants abatement. The principles of ClO2 gas production, chemistry and related generator issues were discussed. We also review some case studies of the application of ClO2 gas in the medical field and food industry as a sterilizer. Oxidation of nitrogen oxide (NOx), sulfur oxide (SOx), mercury (Hg), and volatile organic compounds (VOCs) using ClO2 gas has been investigated. The process chemistry and demonstration of applying ClO2 gas for air pollutants oxidation and absorption have also been provided. In conclusion, we suggest the future priority research direction of ClO2 gas application are included the development of smart and robust ClO2 gas release system, the integration of an innovative robotic technology in ClO2 sterilization for epidemic prevention, and the evaluation of ClO2 emissions impact on indoor air quality in hospitals.

Keywords: ClO2; Disinfection; Sterilization; Oxidation; COVID-19.

## INTRODUCTION

The international emergency of the coronavirus disease 2019 (COVID-19) pandemic has raised health and sanitation concerns for human society. Precautionary principles should be taken into account to overcome sever outbreak (Hsiao et al., 2020). To efficiently prevent infection from human to human, physical capture and biological disinfection items have been utilized (Chen et al., 2017). Significant capture of over 97% by the physical filters such as the N95FFR has been observed in a wide particle size range (Ratnesar-Shumate et al., 2008). In hospital care, the Centers for Disease Control and Prevention (CDC) estimated that hospital-associated infection (HAI) reaches approximately 1.7 million patients per year in the U.S., resulting in the additional healthcare cost of $4.5 to$6.5 billion each year (Klevens et al., 2007). Disease outbreak and cleaning assignment related to the reduction of bacterial and viral organism transmission have received considerable attention (Aygun et al., 2002). Alternatives for disinfection in hospital rooms including fumigation agents and ultraviolet (UV) light technologies have been deployed. Typical fumigation agents such as ozone (O3) and hydrogen peroxide (H2O2) vapor and UV technologies have been utilized in clinics (Umezawa et al., 2012). In addition, three fumigants for room decontamination in the intensive care units (ICU) are formaldehyde gas, hydrogen peroxide vapor, and chlorine dioxide (ClO2) gas. In the U.S., large numbers of animal and neonatal ICUs have been contaminated when using fumigants to avoid the infection of multiple patients.

ClO2 gas is a U.S. EPA recognized sterilizer that has been applied for decontamination in the federal buildings. ClO2 gas has a green-yellowish color and irritating odor and is a volatile and strong molecule. This type of gas is unstable and when photo-oxidized by sunlight it generates the chloride (Cl), chlorite (ClO) and chlorate (ClO3). In 1814, ClO2 was discovered by Sir Humphrey Davy, who produced the gas from the reaction between sulfuric acid (H2SO4) and potassium chlorate (KClO3), followed by replacing the H2SO4 with hypochlorous acid (HOCl) and the KClO3 with sodium chlorate (NaClO3) (Kastner and Das, 2005). ClO2 gas has been used for small spaces in patient care rooms due to its penetrating ability. However, some research has argued that gaseous or aqueous ClO2 performed non-efficient decontamination in the healthcare environment because ClO2 would break down under ambient conditions to limit its penetrating ability (Canter et al., 2005). ClO2 can also be applied for the disinfection of drinking water by controlling the chemical reaction. The concerning organisms in drinking water are disease causing viruses and pathogens. Bacterial spores (e.g., Bacillus) in drinking water treatment processes have been investigated using ClO2 for disinfection (Young, 2016). The generated ClO2 gas is dissolved into the drinking water for further treatment with the desired dose of chemical agent by adjusting the gas flow rate. Haas (2011) found that the application of ClO2 shows superior inactivation of microorganisms as compared to chlorine (Cl2), especially in the disinfection of drinking water. The target waterborne pathogens could be eliminated via oxidation of ClO2.

This article reviewed the chemistry, principles and application of ClO2. We firstly scrutinized the principles of gaseous ClO2 generation and its generators. Application of ClO2 gas for disinfection and sterilization in terms of medical biocide, the food industry and industrial odor control were further depicted. We also reviewed the recent literature of application of gaseous ClO2 for air pollution control, especially nitrogen oxide (NOx), sulfur oxide (SOx), mercury (Hg), and volatile organic compounds (VOCs) oxidation and its mechanisms.

### Gaseous Chlorine Dioxide Generation

#### Principles

ClO2 gas is difficult to store and transport. Due to its explosive and unstable characteristics then, it needs to be manufactured on site. ClO2 gas or Cl2 gas are usually produced from sodium chlorite (NaClO2), or sodium chlorate (NaClO3) with peroxydisulfate (H2S2O8) or hydrochloric acid (HCl). The series of chemical reactions is presented in Eqs. (1) to (3). The reactions between NaClO2 and Cl2 gas, or HCl or H2S2O8 can generate the desired ClO2 gas with sodium chloride or sodium sulfate. In some case, ClO2 gas can also be made from the reaction of sodium hypochlorite (NaOCl) with NaClO2 and HCl (Eq. (4)).

ClO2 gas as an alternative for chemical sterilization or as an oxidative gas works offers bactericidal, virucidal and sporicidal properties like chlorine and processes efficiently at temperatures from 25°C to 30°C, which do not lead to the formation of trihalomethanes or chloramines and is not mutagenic or carcinogenic for humans (Kowalski and Morrissey, 2004). Stevens (1982) demonstrated the organic halogen formation with ClO2 dosages of 5, 10, and 20 mg L–1 with the temperatures of 4 to 36°C. He concluded the non-significant effect of temperatures on organic halogen formation when applying the ClO2 disinfection. ClO2 gas is commonly used for decontaminating spaces, surfaces and equipment with concentrations between 10 and 30 mg L–1. Lerouge (2012) applied ClO2 gas for the sterilization of medical products through rapid spread for a duration of 1.5–3 hr. without post-sterilization.

However, the problem is that ClO2 gas is explosive. The solution containing ClO2 can escape from the liquid when people use it for disinfection. Jin et al. (2008) investigated that limiting the concentration to lower than 9.5% to 10% in the air is suggested to avoid explosion. Exposure to chlorine from ClO2 could cause irritation and burns. The chain reaction related to ClO2 gas explosion were listed in Eqs. (5) and (6). The content of Cl2O3 product could determine the explosive level.

Eye exposure can also result in irritation, watering eyes and blurry sight. They concluded that the induction time (i.e., the time interval from sparking to explosion) was 2195 ms and 8 ms at 10% and 90% of ClO2 gas, respectively. The maximum explosion pressure was increased from 0.024 MPa to 0.641 MPa as ClO2 concentration was enhanced from 10% to 90%. When ClO2 gas is decomposed due to explosion, the stable intermediate (Cl2O3) is formed. ClO2 gas can spontaneously decay depending on the temperature, humidity and light intensity. Although no previous research has mentioned that ClO2 can react with building materials or contents, it has been reported that O3 gas can react with indoor carpet. Therefore, to ensure the adequate and safe application of ClO2 gas for space decontamination, the appropriate dose, sufficient diffusion time and non-explosive concentration should be provided. In addition, ClO2 can be absorbed by the skin, which can damage tissue and blood cells. When people inhale ClO2 it can cause many symptoms including coughing, sore throat, severe headaches, lung oedema and bronchio spasma and lung exposure of ClO2 can cause bronchitis. Thus, the health standard of 0.1 ppm for ClO2 exposure has been suggested (WHO, 2002).

#### ClO2 Gas Generators

The main challenge of ClO2 gas application is the difficulty of storage due to its unstable characteristics. As a result, ClO2 gas needs to be manufactured on-site. Development of ClO2 generators has been investigated since the early 1990s. Jeng and Woodworth (1990) found that 3% to 5% gaseous ClO2 was generated with air mixture by vaporization of a chlorine solution using 500 g of sodium chlorite chips in a ta column container. Eylath et al. (2003) proposed a prototype ClO2 gas generator that produces 2% ClO2 gas with 98% nitrogen in small plastic cartridges carrying solid NaCl. When they set the target of ClO2 gas concentration to 5 mg L1 for use in spatial sterilization, the exposure time was 20 minutes. This prototype generator had a maximal volume of 8,000 L of ClO2 gas with 100 mg L1 concentration under an atmospheric pressure of 50 Pascals. In addition, Cl2 gas formation should be avoided during production of ClO2 gas. Huang and Chen (2017) used the iodometric method to analyze the exhausted gas from a ClO2 generator which contained Cl2 gas and ClO2 gas. The absorbance value of ClO2 occurred at the wavelength of about 280-470 nm. They developed a large-scale ClO2 generator which is also a gas flow spectrophotometer to real-time analyze ClO2 gas concentration. The ClO2 gas stripped from the reacted solution can achieve the yield ratio of 91.21% and the concentration of 3.5%. Some researchers proposed that ClO2 gas can be stripped out from the high purity of ClO2 solution using an advanced gas-liquid contactor (Huang and Chen, 2017). By using nitrogen as the stripping gas and passing it through a contactor, ClO2 gas could be released. The gas flow rate and temperature are the key operating parameters for ClO2 gas stripping from an aqueous ClO2 solution with a known concentration. The temperature of 30°C to 40°C was maintained by a water bath and a constant concentration of ClO2 solution was consecutively circulated to strip the ClO2 gas (Hultén et al., 2017).

Currently, several commercialized ClO2 gas generators have been deployed in different fields. OxyChem corporation invented a ClO2 gas chemical feeder. Three basic designs include a vacuum feed system that pulls stream into the generator, and a pressure feed system that pushes the steam back into the generator. The liquid chemicals (e.g., HCl and NaOCl) are mixed with the precursor chemical with a required residence time of 15 minutes to generate the ClO2 gas. A CA-300 type ClO2 gas generator was developed by PurgoFarm, Korea. The design uses aqueous NaClO2 which flows through the multi-porous membrane electrode assembly under an acid condition to strip out highly pure ClO2 gas via an electrochemical driving force (Gates, 1988). Recent developments of ClO2 generators focus on robust control and sustained release technology to produce smaller, more precise amounts of doses, thereby increasing the practical feasibility.

### Chlorine Dioxide Gas Used in Disinfection and Sterilization

#### Medical Biocide

To control indoor bio-aerosols and viruses in hospitals and for medical equipment, many aerobiological technologies (i.e., ventilation, filtration, ultraviolet germicidal irradiation, photocatalytic oxidation, ionization and thermal sterilization) and medical biocides such as disinfectors have been developed (Yoosook et al., 2009). O3 gas has been used for disinfection due to its high oxidization and germicidal efficacy. Huang et al. (2012) demonstrated 90% germicidal efficacy for E. coli, C. famata, and spores of P. citrinum using gaseous O3 that has substantially higher activity than ClO2. Apart from ozonation, gaseous ClO2 is another option that is used as an oxidizer or disinfectant in the medical area. ClO2 can be used in liquid or gas form and is applied effectively against pathogenic microorganisms such as fungi, bacteria, viruses, spore-forming bacteria and bio film. As follows, we review some case studies of the application of ClO2 gas decontamination in hospitals.

Luftman et al. (2006) evaluated the ClO2 gas used to decontaminate a 4800 m3 facility with a total ClO2 dosage of 400 ppm per hr in a single evening. This trial test proved ClO2 gas to provide effective disinfection in a normal hospital room. Liou et al. (2007) investigated the sterilization effect of indoor air of laboratories, hospitals and conference rooms using 200–1000 ppm ClO2 gas. They mixed 3.35% NaClO2 solution and 85% H2PO4 at a 10:1 volume ratio to generate the ClO2 gas. At the end concentration of 625 ppm, the disinfection efficacy of 90% was achieved in 30 minutes. Lorcheim (2011) confirmed that ClO2 gas can be used to treat beta-lactam manufacturing in a pharmaceutical plant. With various concentrations of ClO2 gas and exposure times, the pharmaceutical manufacturer asked for 3-log (99.9%) reduction of eight different beta-lactams on various surfaces. Lowe et al. (2013) exposed ClO2 gas at 10 sites in a hospital room to evaluate the germicidal efficacy of 385 ppm ClO2 gas for health-associated diseases and pathogens such as Acinetobacter baumannii, E. coli, Enterococcus faecalis, Mycobacterium smegmatis, and Staphylococcus aureus. They verified that 7 to 10-log reductions of viable organisms could be achieved. Sealing of the doors of each room was implemented to prevent gas leakage. Shirasaki et al. (2016) applied ClO2 gas as a sterilizer in a hospital room (87 m3) with concentrations from 0.8 ppm to 40.8 ppm for 2 h to 3 h. They were no differences in concentration between 0.1 m to 2.5 m above the floor. Under 80% relative humidity, the colony formation of both Staphylococcus aureus and E. coli was inhibited with 3 ppm exposure of ClO2 gas. In addition, in situ generated ClO2 gas has been considered for halogen-based biocides. ClO2 is 4–7 times more powerful than Cl2, even though it is less stable under acid condition. ClO2 gas is easy decomposed under sun light so there is little tendency to form organochlorine by-products (Karsa, 2007).

#### Food Industry

Foodborne disease, which originates from contaminated food or drinking water, is a major concern of food safety. The presences of foodborne disease outbreaks due to global warming-related pathogenic bacteria has increased (Hall et al., 2002; Choi et al., 2015). The disinfection of drinking water is mainly used in the food and beverage industry. Gaseous ClO2 has been shown to be an eﬀective disinfector for decreasing microbial loads on various fruits and vegetables (Gómez-López et al., 2009). Research of application of ClO2 gas for the reduction of microbial populations in water has been less common. Although aqueous ClO2 use in drinking or washing water for fruits and vegetables has been approved by the U.S. Food and Drug Administration (FDA), gaseous ClO2 has not be authorized for use in food processing (Smith et al., 2014). This is because ClO2 gas can generate chlorine residues on food products during decontamination. However, recent researchers have argued that low concentrations of Cl residues on the food are not toxic. ClO2 gas has higher penetration capacity than in the aqueous form and is more effective for oxidizing contamination without the formation of trihalomethane. ClO2 gas has been commonly used to reduce the microbiological problem on green peppers, roma tomatoes, tomatoes, and frozen blueberries (Ahmed et al., 2017). Thus, case studies of ClO2 gas treatment for food processing are reviewed in the following paragraph.

Golden et al. (2019) demonstrated that ClO2 gas releasing sachets can be utilized in small food processors of spices as a post-lethality treatment alternative. In the spice industry, Salmonella enterica is a major concern for food security. The application of slow-releasing ClO2 gas of 100, 200, or 500 mg ClO2 per kg-spice from sachets has been employed and the performance of Salmonella decontamination of black pepper, cumin, and sesame seed after 1, 10, and 30 days post-treatment has been evaluated. The trial tests indicated that the Salmonella of black pepper, cumin, and sesame seed decreased from 0.81 to 2.74 log CFU g–1 via ClO2 disinfection. Kim and Song (2017) integrated ClO2 gas with ultraviolet-C (UV-C) light, and fumaric acid to inactivate Listeria monocytogenes and E. coli O157:H7 on plums. With ClO2 gas of 30 ppm for 20 min, fumaric acid of 0.5%, and UV-C of 10 kJ m–2, the populations of L. monocytogenes and E. coli O157:H7 were decreased by 6.26 and 5.48 log CFU g1, respectively. Kim et al. (2016) evaluated the antimicrobial effect of Salmonella inoculation onto eggshells using ClO2 gas under various concentrations, contact times, humidity levels, and percentages of organic matter. The inactivation of Salmonella was more effective with ClO2 gas treatment under high moisture conditions. A 4 log reduction in the population of Salmonella was achieved after 30 min of ClO2 exposure at 20 ppm, 40 ppm, and 80 ppm. They suggested using ClO2 gas as a sterilizer for controlling Salmonella in the egg industry. In addition, ClO2 gas can also be applied for disinfection in cafeterias. Hsu and Huang (2013) tested two different approaches of gaseous ClO2 releasing by an aerosol device in disinfecting a student cafeteria. One was a one-off treatment and the other was a twice-daily treatment. The bioaerosol levels of bacteria and fungi were measured. The air quality before and after disinfection was evaluated. They measured that the average levels of bacteria and fungi decreased 65% and 30% from 972.5 ± 623.6 and 1534.1 ± 631.8 colony-forming units (CFU) m–3, respectively, in the one-off treatment. The twice-daily ClO2 treatment enhanced the reduction levels of bacterial and fungal concentrations by 74% and 38%, respectively.

### Chlorine Dioxide Gas Used as an Oxidizer for Gaseous Pollutants

#### Principles

The research related to chlorine oxide chemistry has been investigated in a wide variety of ﬁelds. Atmospheric interactions of chlorine species were the main focus of early research. The application of ClO2 as an oxidizer agent also drawn attention for air pollution control, especially NOx control. The simultaneous removal of NOx and SOx using ClO2 oxidation has gained interest in ﬂue gas cleaning. Traditional NOx and SOx control faces the challenges of high operation cost and energy demand, thus the simultaneous removal of multi-air pollutants (i.e., including gas and particulates matters) has been investigated in recent years (Chen et al., 2020). This idea involves the treatment of air pollutants in the same unit process, thereby reducing the process units and land area. The oxidation combining absorption process for NOx and SOx reduction is a practical approach because the chemical interactions of sulfur and nitrogen species in the liquid phase promote wet scrubbing as an available option (Ajdari et al., 2015). NO can be converted to NO2 with O2 under atmospheric conditions, though the reaction rate is too slow to affect the NO oxidation from boiler to stack. Therefore, the introduction of oxidative agents such as O3, H2O2, and ClO2 can significantly improve the NO oxidation (Jin et al., 2006).

Apart from using the O3 and H2O2 as an oxidizer for NOx, the application of ClO2 gas to NOx removal has been investigated in recent years. It is cheaper than O3 only if the ClO2 generator can be successfully on-line operation. The use of H2O2 has the higher explosive risk. The chemical interactions between NO, SO2, and ClO2 have been investigated. Although the SOx species have high water solubility that can be absorbed easier than NO, the chemistry of ClO2 on SO2 oxidation can affect the NOx species oxidation. Fig. 1 shows an overview of the process chemistry of NOx-SOx-ClO2 in the gas and liquid phase. The ClO decomposed from ClO2 and NO oxidation can further oxidize the SO2 to create SO3 and Cl. The absorption of SO2 and SO3 on the liquid side can contribute to the formation of sulfuric acid (H2SO4) and sulfurous acid (H2SO3). Some nitrogen-sulfur complexes can be formed as well. The dissociation of H2SO4 and H2SO3 can lead to acid pH in the solution. These chain reactions are highly dependent on pH. When the pH of solution is below 2, the formation of N2O, a kind of greenhouse gas, happens. When the pH value is around 4, the predominant reaction pathway is the formation of hydroxylamine disulfonate acid (HADS) (Haunstetter and Weinhart, 2015). Therefore, the pH control should be considered in the oxidation-absorption of NO-SO2-ClO2 treatment, which could not only increase the de-NOx and de-SOx but also avoid strong greenhouse gas emissions (i.e., N2O).

Fig. 1.
Process chemistry of NOx and SOx oxidized by ClO2. Adapted from (Haunstetter and Weinhart, 2015).

The reaction between H2O2 and NOx usually occurs in the aqueous condition, where the high gas-liquid mass transfer barrier should be overcame. The reaction of ClO2 gas with NO has been reported. Table 1 shows the reaction kinetic between NO and ClO2 in the gas phase and its related dissociation in the liquid phase. It can be seen that all the reaction constants in the gas phase are larger up to 7 to 15 power of 10. The oxidation of NO to NO2 in reactions 1 and 2 occurs under ambient conditions. When the vapors and liquid of ClO2 exist, the hydrolysis of chlorine occurs to form HCl, as shown in reactions 5 and 6. The strong acid formation can contribute to a low pH condition. The stoichiometric ratio for NO to ClO2 is 0.5. When the Cl is consumed to generate the HCl and ClO (reaction 3), the molar ratio decreases to approximately 0.4. The overall oxidation-absorpton reaction of NO and ClO2 is 5NO + 3ClO2 + 4H2O →5HNO3 + 3HCl. In reactions 7 to 9, the oxidation of NO and NO2 convert to higher valence NOx species such as N2O3 and N2O4, which can easily dissolve into water (Johansson et al., 2018). Although some studies have debated whether gaseous HNO3 can be formed from NO2 and vapors (England and Corcoran, 1974), gaseous nitric (HNO3) and nitrous aci (HNO2) are quickly transferred in aqueous condition when the vapors present. Table 2 shows the reaction kinetic between NO and SO2. Reactions 1 to 3 are the main reaction of ClO2 oxidizing SO2. The other chain reactions would be occurred in the same time based on the temperature and pressure condition. In addition, Table 3 presents the reaction kinetic of Cl species in NOx-SOx-ClO2 system. As the Cl atom generated from ClO2 decomposition, it can be synthesized to Cl2 followed by dissolving into the vapor to form HCl. In reactions 4 and 5, NO2 and HNO3 can also react with Cl or ClO to form the high active chlorine nitrate (ClNO3) that could be decomposed into NO2 and ClO with high activity energy (reaction 6).

#### NOx-SOx-Hg Control

ClO2 gas application oﬀers additional possibilities other than the use of aqueous ClO2 solution, which depends on the production method. Gaseous ClO2 can convert the NO to NO2, thereby increasing the feasibility of the downstream scrubbing process for simultaneous removal of NOx and SOx. Even though gaseous ClO2 has hazardous and unstable characteristics, the proper management of manufacturing of ClO2 production can decrease the accident risk. The research related to the simultaneous removal of NOx and SOx via enhanced oxidation by ClO2 from ﬂue gas based on technical-scale experiments and simulations was initiated five years ago.

Li et al. (2015) used ClO2 gas to oxidize NO with the oxidation efficiency of 82% at the ClO2/NO molar ratio of 0.8. They found the oxidation selectivity of NO with ClO2 was higher than that of SO2 oxidation, which was observed until NO became completely oxidized. The other critical factor influencing NO oxidation efficiency was temperature. When the temperature ranged from 70℃ to 290℃, NO oxidation efficiency decreased with the rising temperature. However, they also found that the NO oxidation efficiency slightly decreases with rising SO2 concentration, which represents minor competition between NO and SO2. In addition, the mercury (Hg) gas can be efficiently oxidized to Hg2+ by ClO2 with an oxidation efficiency of 86% at the ClO2/NO molar ratio of 0.3. Hultén et al. (2017) found that higher oxidation efficiency of NO was gained when more ClO2 gas was added. The presence of SO2 with NO can lead to nitrogen-sulphur interactions in the liquid phase. A ClO2/NO molar ratio of 0.6 can contribute to the NO removal efficiency of 79%, and it could be further enhanced to 94% with an absorption solution of Na2CO3 and Na2SO4. They also balance the nitrogen in the system, playing a major part as NO is converted to HNO3 in the condensate liquor and HNO2 in the absorption solution. Johansson et al. (2018) focused on NOx and SOx removal chemistry from combustion-derived ﬂue gases using gaseous ClO2. The simulated NO and SO2 ﬂue gas concentrations were 250 ppm and 1000 ppm, respectively. Under the temperature range of 100°C to 180°C, vapors in the range of 0% to 25%, and ClO2/NO molar ratios in the range of 0.2 to 0.6, they observed that the water concentration has no effect on ClO2 oxidation. The temperature should be limited to under 200°C to avoid lower oxidation of NO with ClO2. Gaseous HNO3 formed with the ClO2/NO molar ratio of 0.6. Furthermore, Johansson et al. (2019) used ClO2 gas treatment for ﬂue gases from a 100 kW gas-ﬁred furnace. The integration of a wet-scrubber and ClO2 oxidation in the gas phase achieved NO and SO2 removal efficiency of up to 90% and 99%, respectively. Afterward, the absorption ratio of NO2 in the liquid phase using Na2SO3 and Na2CO3 reached up to an estimated 82%.

The U.S. EPA proposed Mercury and Air Toxics Standards to regulate the mercury emission from power plants due to 91% coal-combustion basis mercury emission (U.S. EPA, 2011). During the coal combustion, mercury would be released major as Hg0 form. When mercury reacts with different acid gas, the three different forms including Hg0, Hg2+ (e.g., HgCl2), and particulate Hg (Hgp) could be presence (Hutson, 2007). Because of the low water solubility of mercury, the use of oxidizer for mercury either in gas phase or in liquid phase can significantly enhance the absorption efficiency of mercury. Therefore, the application of ClO2 gas or solution in the existing scrubber technology can decrease the mercury emissions (Krzyzynska and Hutson, 2012). The chemical reaction between ClO2 and Hg0 in the acid condition is shown in Eq. (7):

4ClO2 + Hg0 + 14H+ ⇋ HgCl2 + 2HOCl + 6H2O           (7)

Zhao et al. (2008) reported finding mercury(II)-chlorine ion (HgClO2+) could be reacted with ClO2 under acidic conditions and form Hg2+ and ClO3 in an acidic solution of sodium chlorite as shown in Eq. (8):

HgClO2+ + ClO2 ⇋ Hg2+ + ClO3 + Cl                        (8)

#### Volatile Organic Compounds (VOCs) Control

The necessitates of a wide range of volatile organic compound (VOC) emissions remediation in industries is also a kind of odor resource (Sironi et al., 2007). VOC treatment and odor control can be conducted using ClO2 in wet scrubbers. The “Odor Control Rules” promulgated by the U.S. EPA have raised public concerns about non-attainment zones, which are areas with poor air quality that cannot meet the national standard. A few studies of the kinetics of ClO2 reaction with VOCs have discussed process design and optimization. Table 4 shows the current cases of VOCs treatment using ClO2 gas. Kastner et al. (2003a) provided kinetic analysis showing that ClO2 could not be reacted with aldehyde types of VOCs such as hexanal and 2-methylbutanal, considering the pH and temperature effects. The main removal pathway of aldehyde removal was via mass transfer in the wet scrubber (Kastner and Das, 2005). Methanethiol, Ethanethiol, dimethyl sulfide (DMS), and dimethyl disulfide (DMDS) can efficiently react with ClO2. The overall reaction of ClO2 on ethanethiol, DMS or DMDS is a second and third order reaction, thus the reaction rate depends on the pH. For example, the reaction rate of ethanethiol and DMDS could be increased from 25 to 4200 L mol–1 s–1 and 1.4 × 106 L mol–1 s–1 from pH of 3.6 to 5.1 and pH of 9.

These odor-causing compounds can be controlled in a wet scrubber in a small range of pH. Moreover, Kastner et al. (2003b) also suggested that incorporating oxidation kinetics into a wet scrubber system can appropriately predict the removal efficiency and reaction rate with increasing pH, but practical trials are still needed for industrial scale scrubbers. Wu et al. (2015) applied ClO2 gas for 1.0 mg m–3 formaldehyde (CH2O) decomposition under different reaction of 5, 10, 20, 30, 45, 60 min and the dosages of ClO2 were set as 5, 25, 50 and 100 mg m–3. They found that the removal rate of CH2O was increased by ClO2 dosage and contacting time. The favorable ClO2 gas concentration and reaction time for 1.0 mg m–3 CH2O treatment were concluded to 30 mg m–3 and 30 min, respectively. The removal ratio of CH2O was increased from 10% to 60% at exposure time of 60 min and ClO2 dosage of 25 mg m–3. The highest removal ratio of CH2O was up to 90% with dosage of ClO2 of 100 mg m–3. When ClO2 gas dissolved into aqueous solution, the reactivity of ClO2 with organic compounds is predominant to in the gas phase because the existence of organic compounds in water generally in low concentration. Ganiev et al. (2005) summarized the reaction kinetic reactivity on ClO2 with organic compounds, in terms of hydrocarbons, alcohol, ketones, aldehydes, carboxylic acids, amines, thiols and phenols.

## CONCLUSIONS

Gaseous ClO2 has been implemented as a disinfector, sterilizer and oxidizer in different fields. The principle of ClO2 gas production is the reaction between sodium chlorite and hydrochloric acid. Due to the unstable and easy to decompose characteristics of ClO2 gas, in-situ production technology needs to be developed. ClO2 gas can be used for antimicrobial decontamination in the medical area, food processing and odor mitigation because of its high penetration and oxidation ability. In addition, the oxidation of air pollutants such as NOx, SOx, Hg, and VOCs using ClO2 gas has been successfully demonstrated in recent research. Consequently, this mini review provided an overall introduction of ClO2 gas application. To enhance the ClO2 processing and treatment efficiency, future priority research directions include development of substantial and reliable ClO2 gas release systems, combination of for innovative robotic technology for ClO2 sterilization of epidemic prevention, and evaluation of the effects of ClO2 dosages on indoor air quality in hospital.

## ACKNOWLEDGMENTS

High appreciation goes to the Ministry of Science and Technology (MOST) of Taiwan (Grant No. MOST 107-2221-E-002-009-MY3 and MOST 108-2911-I-002-549) for the financial support.

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