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Journal of Aerosol Science

Journal of Aerosol Science 58 (2013) 33–40

0021-85

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/jaerosci

Removal of bacteria and odor gas by an alumina supportcatalyst and negative air ions

Seong Jin Yun, Youngjin Seo n

Environmental Technology Institute, Coway Co., Ltd., Seoul, Republic of Korea

a r t i c l e i n f o

Article history:

Received 19 October 2012

Received in revised form

3 December 2012

Accepted 3 December 2012Available online 3 January 2013

Keywords:

Alumina support catalyst

Negative air ion

Bacteria removal

Gas removal

Restroom cleaning

Indoor air control

02/$ - see front matter & 2013 Elsevier Ltd.

x.doi.org/10.1016/j.jaerosci.2012.12.006

esponding author. Tel.: þ82 2 870 5153; fa

ail address: youngjin.seo@gmail.com (Y. Seo

a b s t r a c t

Installation of high-performance filter systems in restrooms is necessary because of

bacterial and odor gas contamination from feces. Presented herein is a sterilizing

deodorizing filter (SDF) consisting of a negative air ion (NAI) filter and an alumina

support catalyst (ASC) of copper and phosphoric acid. The Escherichia coli removal of the

SDF was evaluated with three DC voltages (�3,–5, and �7 kV) at the space velocities of

1.6�104, 2.8�104, and 5.6�104 h�1. The performance of the SDF peaked at 99.9% at the

high voltage of �7 kV and the high space velocity of 6�104 h�1 for bacteria removal.

In addition, the average bacteria removal efficiency of the SDF was 1.2 times higher than

the NAI filter alone at the space velocity of 5.6�104 h�1. The gas removal performance of

the NAI filter remained the same regardless of the duration of the experiment, but was

very low at 16.7% for ammonia and 12.7% for hydrogen sulfide. The SDF and ASC filter

maintained the removal rate of 100% for the first 15 min of the experiment. However,

after 300 min, the gas removal efficiencies of the former were higher than those of the

latter by 28.8% and 15% for ammonia and hydrogen sulfide, respectively. The reason for

this phenomenon is that the ASC improves performance by increasing contact with the

NAIs as a result of gas adsorption and gas removal.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

These days, most people spend more than 80% of their time indoors; accordingly, a number of studies on indoor airquality are underway. Unfortunately, many studies show that indoor air quality worsens as time passes because ofinsufficient ventilation and the increase of indoor pollutants in many cases. Ventilation is the simplest and most importantway to improve indoor air quality (Daisey et al., 2003; Lee & Chang, 2000; Godish & Spengler, 1996). However, the oil crisisof the 1970s has brought energy conservation to attention, making the strengthening of the air tightness of buildings andchanging building structures for ventilation difficult (Kimura, 1993; Smulski, 1999; Tang & Tang, 2011). In addition, indoorpollution levels are critical owing to the chemicals generated from indoor furniture, wallpaper, paint, etc. (Cho et al., 2007;Xia, 2012). In some cases, indoor air can be contaminated with Legionella bacteria when an air conditioner is turned on(Dondero et al., 1980; Goh et al., 2000; Mui et al., 2008; Rajasekar & Balasubramanian, 2011). Restrooms are especiallysusceptible to the generation of odor as well as bacteria, but the lack of ventilation can easily lead to bacterialcontamination of items such as toothbrushes. Best et al. (2012) reported an experiment in which toilets containing stoolsamples contaminated with Clostridium difficile were flushed without closing the lid. The bacteria were spread to nearby

All rights reserved.

x: þ82 2 870 5124.

).

S.J. Yun, Y. Seo / Journal of Aerosol Science 58 (2013) 33–4034

surfaces, and were detected on the floor around the toilet and in the tank behind the toilet. On the other hand, when the lidwas closed, bacteria were not detected outside of the toilet. In the former case, C. difficile was detected continuously for aslong as 90 min after flushing, with amounts gradually decreasing from 25 cm above the toilet. Best suggested that foodpoisoning and gastroenteritis-causing norovirus may spread in the same manner. To eliminate the odor caused by feces, airfresheners are used in many homes. Unfortunately, while fragrances may hide unpleasant odors, they may also be anothercontaminant that can be converted to harmful secondary pollutants after reacting with other airborne substances(Nazaroff & Weschler, 2004). For instance, when vaporized terpene oil is fed in a 120 ppb ozone atmosphere, the twosubstances may react to form formaldehyde (Singer et al., 2006). In addition, the fragrances in air fresheners may causeasthma and allergies, and the volatile compounds they contain also have the potential to cause other health problems(Bridges, 2002).

For this reason, many studies aimed at removing gas and bacteria from indoor spaces such as restrooms have beenconducted. The most representative tools are negative air ions (NAIs) and catalysts. NAIs act as strong oxidants that candecompose gases and bacteria. They are known to effectively remove hazardous substances by combining oxygen andwater vapor in air to release electrons through corona discharge. Arnold et al. (2004) reported that a negative ionizer7.6 cm above a stainless steel surface killed 99.9% of bacteria including Campylobacter jejuni, Esterichia coli, Salmonella

enteritidis, Listeria monocytogenes, and Staphylococcus on the surface. Furthermore, 99.8% of Bacillus stearothermophilus in a13-m3 room was eliminated in 3 h with NAIs. For bacteria aerosol in air, Tyagi and Malik (2010) conducted a reduction testwith an NAI generator against Pseudomonas fluoresens in a 93.75-L chamber, and found that the bacteria removal efficiencyof the generator was 45.5% in the first 4 h and 58.6% after 12 h. In addition, NAIs have excellent performance in removinggas pollutants from indoor air (Shaughnessy & Sextro, 2006; Sano et al., 1997). In a corona discharge reactor with a supplyof �10 to �15 kV in the presence of a gas mixture consisting of 20% O2 and 80% N2, the formaldehyde removal efficiencywas up to 95.5% for the space velocity of 44 h�1 (Sano et al., 1997). Catalyst technology generally uses a catalyst coated onthe surface of a porous support. In particular, metal catalysts have excellent performance at high temperature in theconversion of harmful substances in air to nitrogen and water vapor (Gang et al., 2000; Xu et al., 2010). They also have theadvantage of longer life compared to ordinary adsorbents.

However, there are problems associated with the use of NAIs and catalysts. NAIs can take as long as a few hours orrequire an atmosphere of high ion concentration in order to remove air contaminants. It is difficult to expect efficientoxidation performance of NAIs if the contaminants are supplied continuously at a low space velocity. Problems associatedwith adsorbent support catalysts include low performance at room temperature, easily saturated pores, and sharpdeterioration of deodorizing performance (Kim et al., 2010). In the present study, a filter is proposed for the removal ofbacteria and odor gas contaminants from air flowing at a low space velocity in a restroom. The proposed filter is composedof a brush ionizer, and an alumina-supported copper and phosphoric acid catalyst. Alumina was chosen to play twoimportant roles in this study: (1) to act as a carrier for the catalyst and (2) to act as an adsorbent. It has been widely used asa carrier because catalysts can be coated on its surface. In addition, the large surface area of its pores results in an increasedamount of catalyst on its surface, thereby decreasing the flow rate of harmful materials such as odor gases and bacteria inair passing through the alumina, and increasing the adsorption of the materials in its pores. NAIs generated by the ionizersupplied with DC voltage in the range of �3 to �7 kV were tested for bacteria removal performance in single-pass air.In addition, an experiment was conducted to assess the rate of conversion of gaseous pollutants ammonia and hydrogensulfide. Copper and phosphoric acid were chosen as catalysts that stimulate the oxidation of ammonia and hydrogensulfide, respectively, by decreasing the threshold energy of gas decomposition. The performance of the filter was comparedwhen either NAIs or the catalyst were present, and when both were present.

2. Preparation

2.1. Alumina support catalyst (ASC)

Nanosilica was attached to the surface of the alumina to increase the surface area of the carrier. Nanosilica sol wasprepared by stirring a solution of 80 g of tetraethyl orthosilicate (TEOS, 78-10-4; Sigma-Aldrich Co., LLC, USA), 50 g ofaqueous ammonia (purity 30%, 1336-21-6; Duksan Pure Chemicals Co., Ltd., Korea), and 1 L of distilled water in 1 kg ofethanol (purity490%, 64-17-5; Duksan Pure Chemicals Co., Ltd., Korea) for 6 h at 30 1C. All distilled water used in theexperiment was purified using a water purification system (aquaMAX

TM

, Basic 360; YL Instrument Co., Ltd., Korea).A copper solution was prepared by stirring 2 g of copper (10125-13-0; Duksan Pure Chemicals Co., Ltd., Korea) and 100 mLof distilled water in an agitator (MS200D; Misung Scientific Co., Ltd., Korea) for 30 min. The aqueous copper solution wasadded to the prepared nanosilica sol and the mixture was stirred for 3 h at 60 1C to yield the copper catalyst.

Nanosilica sol was made in the same manner described above and 2 mL of phosphoric acid (purity485%, 6532-4100;Daejung Chemical & Metals Co., Ltd., Korea) was added. The mixture was stirred for 3 h at 60 1C to yield the phosphoricacid catalyst. The two prepared catalysts were added to 1 kg of purified water and the mixture was stirred for 1 h at200 rpm. Next, 40 g of 2–3 mm spherical alumina (1344-28-1, Sigma-Aldrich Co., LLC, USA) was added and the mixturewas stirred at room temperature for 1 h, then dried in an oven (DO-42/81/150; Hanyang Scientific Equipment Co., Ltd.,Korea) for 24 h at 110 1C. In order to eliminate impurities in the ASC and induce chemical bond formation between the

Fig. 1. The sterilizing deodorizing filter system.

S.J. Yun, Y. Seo / Journal of Aerosol Science 58 (2013) 33–40 35

alumina carrier and the catalysts, the obtained ASC was heat treated in a furnace (MF-27; Hanyang Scientific EquipmentCo., Ltd., Korea) for 5 h while injecting air at 550 1C and a rate of 100 cm3/min.

2.2. Sterilizing deodorizing filter (SDF)

Fig. 1 shows the schematic diagram of the SDF. The brush ionizer (N6; Fupong Technology Co., Ltd., Hong Kong) wasinserted in the front and 12 g of ASC was inserted in the back of a cylindrical polyvinyl chloride (PVC) tube with an internaldiameter of 20 mm and length of 65 mm. The brush ionizer consisted of a bundle of hundreds of carbon fibers and wassupplied with negative DC voltage electricity using a high voltage generator (HVG, DSHF-2010; EST Co., Korea).

2.3. Test bacteria

E. coli was used as a common representative bacterium for this study. E. coli (ATCC 15597) was sampled with a platinumloop and added to 40 mL of tryptic soy broth (TSB, Difco Laboratories, MI, USA). Then, it was shaken in an incubator(SI-600R; Jeio Tech Co., Ltd., Korea) and cultured for 24 h at 37 1C while rotating at a speed of 120 rpm. The cultured strainswere centrifuged (Fleta-5; Hanil Science Industrial Co., Ltd., Korea) for 10 min at 5000 rpm. The radius of the centrifugewas 15 cm. The corresponding g force for the centrifuge at 5000 rpm is 4192.5g. We used the following website for quickcalculation of the g force: http://www.endmemo.com/bio/grpm.php. According to Peterson et al. (2012), a wide variety offorces (ranging from roughly 1000g to 12,000g) are used, but the reasons for particular choices for harvesting were notmentioned. However, we believe that the force at 4192.5g does not significantly affect the bacterial cell surface. Aftercentrifugation, the supernatant was discarded and 0.85% sodium chloride (Daejung Chemical & Metals Co., Ltd., Korea) wasadded and shaken to mix the precipitated germ. A 105–106 CFU/mL E. coli suspension was made in a collision nebulizer(MRE CN24; BGI Inc., Waltham, MA).

3. Experimental setup and testing methodology

3.1. Bacteria removal test

Fig. 2 illustrates the experimental apparatus used to evaluate the sterilization performance of the SDF. The experimental deviceis composed of a compressor (DOA-P704-AC; Gast Manufacturing Inc., MI, USA), a flow meter (Rate Master; Dwyer instruments,Inc., IN, USA), a nebulizer (BGI Inc., MA, USA), an impinger (BioSampler; SKC Inc., PA, USA), a filter, a needle valve (Union Metal Co.,Ltd., Korea), and stainless steel tubes and connectors (Union Metal Co., Ltd., Korea). The experimental apparatus is a closed single-pass system. As the system is rotated by the compressor, the bacteria are collected in the impinger during their pass from thenebulizer to the impinger via the SDF. In this way, no bacteria are detected after the impinger.

First, the empty SDF without the ASC and ionizer was installed in the experimental apparatus to measure the amount ofbacteria present under control conditions. The impinger filled with 40 mL of 0.85% sodium chloride and the nebulizer filledwith 40 mL of the E. coli suspension were connected to the apparatus. Bacteria were captured in the impinger whileoperating the experimental device at the space velocity of 1.6�104 h�1 for 2 min. At this time, the captured number ofbacteria was set to ‘‘# of bacteria of the control’’ (Eq. (1)). Then, the SDF without the ASC was installed in the experimental

Fig. 2. Schematic of the apparatus for the sterilizing test.

S.J. Yun, Y. Seo / Journal of Aerosol Science 58 (2013) 33–4036

device and bacteria were collected in the impinger from the air passing through the NAI filter for 2 min while beingsupplied negative DC high voltage from the HVG to assess the disinfection performance of negative ions in the samemanner as in the control test. These experiments were carried out at –3, –5, and –7 kV. Next, the SDF without the ionizerwas installed in the experimental apparatus and bacteria were collected from the air passing through the ASC for 2 min toevaluate the disinfection performance of the catalyst. After the completion of the comparison test, the SDF containing boththe ionizer and ASC was connected to the experimental device, and the bacteria were collected for 2 min from the airpassing through the SDF to evaluate its disinfection performance. This experiment was carried out at the space velocities of1.6�104 h�1, 2.8�04 h�1, and 5.6�104 h�1 while supplying the voltages of –3, –5, and –7 kV to the ionizer at eachvelocity. The number of bacteria collected in this experiment was set to the number of bacteria in air leaving the filter.The bacteria removal efficiency was determined as follows:

Bacteria Remival Efficiency¼ 1�# of bacteria in air leaving the filter

# of bacteria of the controlð1Þ

3.2. Cultivation and counting of bacteria

A 1-mL impinger sample was added to a Petri dish (10090, 90n15 mm; SPL Life Sciences Inc., Korea) and approximately15 mL of tryptic soy agar (TSA) medium was added. The Petri dish was shaken from side to side to mix the medium withthe sample and was then cultured in an incubator for 24 h at 37 1C. The cultured bacteria colony was counted using acolony counter (CC-560; Suntex Instruments Co., Ltd., Taiwan).

3.3. Gas removal test

The experimental apparatus shown in Fig. 3 was used to evaluate the gas conversion rate. The gas removal efficiency of theASC, NAI filter, and regular alumina were also evaluated for comparison to the SDF. The dilution of malodorous gas and air wasadjusted using mass flow controllers (MFCs, 5850E; Brooks Instrument, PA, USA), and the gas was constantly supplied to the SDFat the experimental concentration of 10 ppm and space velocity of 1.6�104 h�1. Ammonia and hydrogen sulfide standard gaswere used as odor gases, and certain amounts were continuously supplied using MFCs. For the evaluation of odor removalperformance, the concentration difference was measured at the front and rear of the filter using gas detection tubes (3 L forammonia, 4LB for hydrogen sulfide; GASTEC Corp., Japan). The conversion of gas was calculated using

Conversion of Gas %ð Þ ¼ 1�Concentration of gas in air leaving the filter

Concentration of gas in air entering the filterð2Þ

4. Results and discussion

4.1. Bacteria removal efficiency

Fig. 4 shows the disinfection performance at the space velocity of 1.6�104 h�1 (face velocity¼0.75 m/s). The averagebacteria reduction of the SDF over the three different voltages was 93.1%, which was better than the value of only the ASC

Fig. 3. Schematic of the apparatus for the deodorizing test.

Fig. 4. Bacteria reduction of the SDF at different operating modes.

S.J. Yun, Y. Seo / Journal of Aerosol Science 58 (2013) 33–40 37

filter (20%) or the NAI filter (77.4%). In the NAI filter experiment, the bacteria reduction increased from 69.1% to 82.8% as negativevoltage supplied to the ionizer increased from �3 kV to �7 kV. This is because the voltage inequality between the ionizer and theair increases as the DC voltage supplied to the ionizer increases, which results in an increase in the amount of NAIs and the iondensity of the filter. For the SDF, the bacteria reduction increased from 90.9% to 95.4% as the negative voltage to the ionizerincreased from �3 kV to �7 kV, but the differences are very small. This is because the impact of NAI density on the performanceof the SDF is low compared to the NAI filter since NAIs in the SDF have greater contact with the bacteria owing to the adsorptionof bacteria on the ASC carrier (alumina). Furthermore, there is an increased chance of collisions of NAIs and bacteria in the SDFbecause bacteria have to follow the tortuous air stream as they pass through the ASC inside the filter. It is also noted that bothcatalysts are involved in bacteria removal. However, copper is more efficient because it accepts electron(s) or ion(s) from the NAIgenerator more easily owing to its high conductivity.

Fig. 5 shows the results of the bacteria reduction of the SDF with respect to the space velocity. The average bacteriareduction performance increased as the space velocity decreased (93.1% for 1.6�104 h�1, 96.8% for 2.8�104 h�1, and99.8% for 5.6�104 h�1). The rate of reduction of harmful materials such as bacteria and gases in air increases as the airflow rate decreases in the column filter. An increased residence time of harmful materials resulting from decreased air flowvelocity causes an increased probability of contact and contact time between the filter and bacteria; thus, the probabilityof removal by means of the NAIs and ASC in this test is increased. The space velocity is defined as the flow rate of a fluiddivided by the volume of a bed (column) in a cylinder (Marsh, 2001). On the other hand, the space velocity of the streamincreases as its velocity in the same cylinder filter decreases. Therefore, the reduction of bacteria increases as the spacevelocity of the air stream (including bacteria) decreases.

As mentioned above, the performance increased as the voltage increased at the same space velocity. However, thestandard deviation of the performance at the space velocity of 1.6�104 h�1 was less than those at the space velocities of

Fig. 5. Bacteria reduction of the SDF at different space velocities and supplied voltages.

Fig. 6. Conversion of ammonia.

S.J. Yun, Y. Seo / Journal of Aerosol Science 58 (2013) 33–4038

2.8�104 h�1 and 5.6�104 h�1. This result indicates that the influence of the voltage applied to the ionizer onperformance increased as the flow rate increased.

For practical application, the lifetime of the filter must be considered. When the bacteria reduction tests werecontinuously repeated 30 times, the reduction rates were higher than 99.97% for the filter consisting of the ASC and NAIs atthe voltage of �7 kV and the space volume of 5.6�104 h�1. There was no difference between the reduction rates of the1st and 30th tests. This is because the catalyst used in the test is not poisoned by bacteria. The bacteria reduction test withthe ASC filter without NAIs was not conducted because the reduction rate of this filter was lower than 5%.

4.2. Gas removal efficiency

Figs. 6 and 7 show the ammonia and hydrogen sulfide removal efficiency, respectively, after 300 min at the space velocity of1.6�104 h�1 and the voltage of -7 kV. The gas removal efficiency of the alumina support decreased rapidly as time passed until itno longer removed the gas after 90 min and 40 min for ammonia and hydrogen sulfide, respectively. In contrast, the removal rateof the NAI filter for ammonia and hydrogen sulfide remained almost unchanged throughout the experiment, with the values of16.3% and 12.7%, respectively. This is because the gas removal mechanism of alumina relies entirely on gas adsorption of pores,whereas the NAIs oxidize and remove gas as a result of the formation of a constant concentration of ions inside the filter (Wu &Lee, 2004), which provided unchanged deodorizing performance over time.

Fig. 7. Conversion of hydrogen sulfide.

S.J. Yun, Y. Seo / Journal of Aerosol Science 58 (2013) 33–40 39

The best performance for both ammonia and hydrogen sulfide was obtained when the NAI filter and ASC were used together.For 15 min after the start of the experiment, the gas removal rate was 100% for both the SDF and the ASC filter alone. However,the removal efficiency of the former was higher, and after 300 min, the difference increased to 28.8% and 15% for ammonia andhydrogen sulfide, respectively. Interestingly, these values were larger than the gas removal rate of the NAI filter alone. This isbecause in the SDF, some gas is removed as a result of oxidation by NAIs at the front of the ASC, and the remaining gas is adsorbedby the alumina pores where it has a greater chance to contact NAIs, thereby increasing gas oxidation by the NAIs. In addition, theASC increases the odor gas reduction rate by decreasing the threshold energy of gas decomposition. When negative ions collidewith gases adsorbed on the ASC, they decompose the gases more efficiently. Thus, the two catalysts remove bacteria adsorbed inthe pores of the ASC by accepting electron(s) or ion(s) that are produced by the NAIs.

In tests of long term efficiency, the gas reduction rate of the ASC filter decreased from 100% to 0% for a 10 ppm ammoniastream during a 90 min test. On the contrary, the rate of the SDF containing both the ASC and NAI filter decreased steadilyfrom 100% to 68.3% for a 10 ppm ammonia stream during a 900 min test. It can be concluded that the catalysts on the ASCare poisoned by gases, but this phenomenon is delayed by donation of electron(s) or ion(s) from the NAIs to the catalyst.Finally, it should be noted that the gas reduction rate would be expected to increase when an additional oxidant, i.e., anoxidation product, is present. A representative oxidant is ozone. Additional ozone definitely results in increased reductionefficiency, but it can also cause health-related problems such as asthma. Therefore, it is important to characterize theoxidation products of ammonia and hydrogen sulfide in the SDF, which might be a topic for future study.

5. Conclusions

In this study, a new form of SDF was investigated for removing E. coli and odor gases such as ammonia and hydrogensulfide, which mainly exist in restrooms. The SDF was composed of a tube with a brush ionizer containing hundreds ofcarbon fibers in the front, and a copper and phosphoric acid catalyst coated on alumina in the back. At the space velocity1.6�104 h�1, the bacteria removal efficiency of the NAI filter increased rapidly from 69% to 83% as the negative voltageincreased from �3 kV to �7 kV. In contrast, the efficiency of the SDF showed a much smaller increase from 99.7% to 99.9%as the negative voltage increased from �3 kV to �7 kV. This is because the bacteria-containing air follows a meanderingstream pattern through the ASC, but a long trajectory without the ASC; when the bacteria are adsorbed by the ASC pores,they have a better chance of contact with the NAIs. As the space velocity decreased, the average bacteria reduction of theSDF decreased and the impact of the negative DC voltage supplied to the ionizer increased. Degassing experiments wereconducted for 300 min while continuously supplying ammonia and hydrogen sulfide to the SDF. Gas removal performanceof the NAI filter remained constant regardless of the experiment time, but was very low (16.7% for ammonia and 12.7% forhydrogen sulfide). For both the ASC filter and the SDF, the removal performance was maintained at 100% for the initial15 min of the experiment, but the differences increased to 28.8% and 15% for ammonia and hydrogen sulfide, respectively,after 300 min. This phenomenon is because of the increased performance resulting from the increased chance of contact ofNAIs with the gases adsorbed by the ASC, and subsequent oxidation of the gas by the NAIs.

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