Antimicrobial nanoparticle-coated electrostatic air filter with high filtration efficiency and low pressure drop

Science of the Total Environment 533 (2015) 266–274

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Short communication

Antimicrobial nanoparticle-coated electrostatic air filter with high filtration efficiency and low pressure drop Kyoung Mi Sim a, Hyun-Seol Park b, Gwi-Nam Bae c, Jae Hee Jung c,⁎ a

Department of Integrated Biomedical and Life Science, Korea University, Anam-ro 145, Seongbuk-gu, Seoul 136-701, Republic of Korea High Efficiency and Clean Energy Research Division, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea Center for Environment, Health and Welfare Research, Department of Energy and Environmental Engineering, Korea University of Science and Technology (UST), Korea Institute of Science and Technology (KIST), Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea

b c

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• An antimicrobial S. flavescens nanoparticle-coated electrostatic (ES) filter was prepared. • A corona discharge electrification process enhanced the filtration efficiency of the air filter. • The antimicrobial ES filter had high filtration efficiency and a low pressure drop. • This study provides useful information about the development of a hybrid air purification system.

a r t i c l e

i n f o

Article history: Received 19 April 2015 Received in revised form 10 June 2015 Accepted 1 July 2015 Available online xxxx Editor: D. Barcelo Keywords: Electrostatic air filter Antimicrobial filter Antimicrobial natural product Antimicrobial nanoparticle Air filtration Corona discharge

⁎ Corresponding author. E-mail address: [email protected] (J.H. Jung).

http://dx.doi.org/10.1016/j.scitotenv.2015.07.003 0048-9697/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t In this study, we demonstrated an antimicrobial nanoparticle-coated electrostatic (ES) air filter. Antimicrobial natural-product Sophora flavescens nanoparticles were produced using an aerosol process, and were continuously deposited onto the surface of air filter media. For the electrostatic activation of the filter medium, a corona discharge electrification system was used before and after antimicrobial treatment of the filter. In the antimicrobial treatment process, the deposition efficiency of S. flavescens nanoparticles on the ES filter was ~12% higher than that on the pristine (Non-ES) filter. In the evaluation of filtration performance using test particles (a nanosized KCl aerosol and submicron-sized Staphylococcus epidermidis bioaerosol), the ES filter showed better filtration efficiency than the Non-ES filter. However, antimicrobial treatment with S. flavescens nanoparticles affected the filtration efficiency of the filter differently depending on the size of the test particles. While the filtration efficiency of the KCl nanoparticles was reduced on the ES filter after the antimicrobial treatment, the filtration efficiency was improved after the recharging process. In summary, we prepared an antimicrobial ES air filter with N99% antimicrobial activity, ~92.5% filtration efficiency (for a 300-nm KCl aerosol), and a ~0.8 mmAq pressure drop (at 13 cm/s). This study provides valuable information for the development of a hybrid air purification system that can serve various functions and be used in an indoor environment. © 2015 Elsevier B.V. All rights reserved.

K.M. Sim et al. / Science of the Total Environment 533 (2015) 266–274

1. Introduction Microorganisms are ubiquitous in the environment. They are present in water, soil, air, plants, animals, and humans. In particular, as indoor air quality (IAQ) management has become an important issue for modern society, interest in microorganisms has mainly focused on their presence in the air, where they are referred to as bioaerosols (Goyer, 2001; Nazaroff, 2014). Bioaerosols are airborne particulate matter with a biological origin, and include viruses, bacteria, fungi, and a variety of living materials. They can travel freely with airflow movement and can spread over a wide area in a short period of time (An et al., 2004; Smith et al., 2009). Exposure to high concentrations of these airborne pollutants can have harmful effects in humans, including contagious infectious diseases, acute toxicity, allergies, and cancer (Larsson et al., 2004; Morawska and Zhang, 2002; Pöyhönen et al., 2004). Therefore, controlling exposure to bioaerosols is important for disease control and prevention, and there is a growing research interest in microbiological indoor pollutants. Aerosol filtration is the most widely used technique for the control and removal of hazardous bioaerosols, and is applied in a variety of residential and industrial air conditioning systems for indoor air cleaning (Fisk, 2013; Hinds, 2012; Li and Hopke, 1992). Fibrous filters have been widely used to separate solid matter from particulate laden airflow streams because of their simple structure and low material costs. There are four physical mechanisms of particle filtration by which an aerosol particle can be deposited onto a fiber in a filter: inertial impaction, gravitational sedimentation, interception, and diffusion (Podgórski et al., 2006). Fiber filters can be classified as pre filters, medium filters, highefficiency particulate air (HEPA) filters, or ultra-low particulate air (ULPA) filters, according to their particle filtration efficiency (Ahn et al., 2006; Schroth, 1996). The filtration efficiency of a medium filter is 60–90% and the corresponding pressure drop is 15–30 mmAq. A HEPA filter can eliminate 99.97% of particles with a particle diameter of 0.3 μm with a pressure drop of 25–50 mmAq. A ULPA filter removes more than 99.999% of 0.12–0.17-μm particles, with a similar pressure drop to a HEPA filter (Chuaybamroong et al., 2010; Hanley et al., 1994; Jamriska et al., 1997). HEPA and ULPA filters are often used in cleanrooms, electronic semiconductors, or as indoor air purifiers to remove unwanted particles from the air. The filtration efficiency of a general fiber filter is increased as the solidity of the filter increases, which is directly proportional to the air pressure drop. Therefore, for a general air filter, a high pressure drop is unavoidable, which in turn requires a large loss of energy to achieve a high filtration efficiency (Fisk et al., 2002). When filter fibers have been charged, the electrostatic force between particles and fibers can significantly augment the filtration efficiency without increasing the filter pressure drop. This is particularly useful for improving the filtration of particles in the size range of 0.15–0.5 μm (Aussawasathien et al., 2008). A charged fiber creates an electric field in its vicinity that exerts a force on a charged particle. The field created by a charged fiber can also polarize a particle, and the force on a polarized particle has an important role in increasing the filtration efficiency of small particles. Normally, electret (or electrostatic) filters are made of dielectric polymer fibers that have a quasipermanent electrical charge. Also, the fibers gain an electric charge from their surroundings, depending on various electrical charging processes such as corona charging, triboelectric charging, or induction charging (Gu and Schill, 1997; Romay et al., 1998; Tsai et al., 2002). Because of its advantages, electrostatic (ES) filters have been widely used in residential and industrial air conditioning systems (Boelter and Davidson, 1997; Grass et al., 2004). Although air filtration systems can be used to improve IAQ, they can become a source of contamination by microorganisms harmful to human health. Antimicrobial filters can provide significant benefits, because they can rapidly inactivate captured microorganisms and minimize the number of live/viable particles resuspended from the filter by passing air (Pyankov et al., 2008). Various techniques have been

267

investigated to impart antimicrobial activity on filter media. Filter coating techniques, using an antimicrobial material such as silver (Ag) and copper (Cu) nanoparticles, carbon nanotubes (CNT), and biocidal chemicals are considered to be promising methods for imparting antimicrobial ability with relatively little cost (Ji et al., 2007; Lee et al., 2010). Recently, extracts of natural products with antimicrobial activity have been considered as novel, efficient, and cost-effective materials for the development of antimicrobial filter media (Dixon, 2001). Plant extracts, such as Melaleuca alternifolia (tea tree), Eucalyptus, and Sophora flavescens, in particular, can be used as a coating for filters to inactivate fungal spores, bacteria, and influenza viruses (Huang et al., 2010; Hwang et al., 2015a, 2015b; Pyankov et al., 2008; Pyankov et al., 2012). The treatment of filter surfaces with nanosized particles of a natural product is an effective method for enhancing their antimicrobial activity, because the nanosized natural products provide the maximum possible specific surface area to contact surrounding agents. In this study, we developed an antimicrobial natural-product nanoparticle-coated ES air filter. Antimicrobial natural-product S. flavescens nanoparticles were produced using an aerosol process consisting of nebulization–thermal drying, and were continuously deposited onto the surface of air filter media. For the electrostatic activation of the surface of the filter medium, a corona discharge electrification system was used before and after the antimicrobial treatment of the filter media. We evaluated the antimicrobial ES air filter in terms of the deposition efficiency of antimicrobial nanoparticles, the filtration efficiencies of nanosized KCl aerosols and submicron bacterial bioaerosols, the filter pressure drop, and its antimicrobial activity against bacterial bioaerosol. Additionally, we compared the filtration performance before and after the additional charge re-activation process. 2. Materials and methods 2.1. Preparation of the antimicrobial nanoparticle-deposited ES filter Dried S. flavescens roots were purchased from the Kyung-dong Oriental Herbal Market, Seoul, Korea (Jung et al., 2013; Sim et al., 2014a, 2014b). A voucher specimen is on record at the Functional Food Center, Korea Institute of Science and Technology (KIST), Gangneung Institute, Korea. Dried S. flavescens roots (600 g) were extracted three times with pure ethanol (1 L) by refluxing for 3 h. After filtration, the ethanol extract was evaporated in vacuo and freeze-dried. A 0.25-g sample of S. flavescens powder was dissolved in 40 mL of ethanol and sonicated for 10 min. The solution was then filtered through a cellulose acetate membrane filter with a pore size of 0.24 μm to eliminate any insoluble residue. To prepare an antimicrobial ES air filter, S. flavescens nanoparticles generated using the nebulization–thermal drying process were deposited onto the fiber surfaces of an ES filter (Fig. 1(a)). Twenty milliliters of the S. flavescens solution were poured into a one-jet Collison nebulizer (BGI Inc., Waltham, MA, USA). The nebulizer was supplied with 1 L/min of HEPA-filtered clean air. The resulting S. flavescens aerosol was passed through both an activated carbon absorber tube and a thermal glass quartz tube heater (75°C temperature and ~3 s residence time) to remove the ethanol. Activated carbon is a form of carbon processed to have small, low-volume pores that increase the surface area available for adsorption or chemical reactions. The size and number concentration of the nanoparticles generated were measured using a scanning mobility particle size (SMPS) system consisting of a differential mobility analyzer (DMA 3081; TSI Inc., Shoreview, MN, USA) (Chen et al., 1998; Siefert, 1984) and a condensation particle counter (CPC 3010; TSI Inc.) (Gamero-Castano and de la Mora, 2000; Kaufman, 1998) based on the electrical mobility of the particles in the range of 14–673 nm. The S. flavescens nanoparticles were deposited on ES filters (polyurethane fiber filter; fiber diameter = 2 μm; thickness = 0.6 mm). The quantity of deposited S. flavescens nanoparticles on filters was

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HEPA filter

(a)

Clean air tank Mass flow controller

Antimicrobial filter preparation

2.3. Evaluation of filtration performance

Controller Filter holder

Active carbon absorber

Thermal dryer SMPS 210Po

Collison nebulizer (S. flavescens)

Filtration test P

Antimicrobial filter

Diffusion dryer

210Po

Collison nebulizer (S. epidermidis and KCl particles)

APS

SMPS

(b) High voltage power supply Discharging needle

(-)

(i.e., an ES filter) and for the recharging of the antimicrobial-treated filter.

(-)

Filter

Rotational drum Fig. 1. Schematic of the experimental setup. (a) The nebulization–thermal drying process for the preparation of antimicrobial S. flavescens nanoparticles (solid line), and the filtration performance tests (dotted line), (b) the electrostatic activation system using a corona discharge for the ES filter.

determined by weighing filters using a microbalance (Mettler MT5; Mettler-Toledo International Inc., Seoul, Republic of Korea) before and after the particle deposition process. The nanoparticle concentration deposited on the filter per unit cross-sectional filter surface area was ~4.44 μg/mm2filter (Jung et al., 2011). All filters were stored in a diffusion dryer box packed with silica beads for 24 h at room temperature.

2.2. Electrostatic activation of the filter medium Fig. 1(b) is a schematic of the corona discharge electrification system for the electrostatic activation of the filter medium (or for the preparation of the ES filter). The corona discharger creates negative (−) unipolar ions between needle-like tungsten electrodes, with very sharp ends, and a cylindrical steel ground electrode, which was attached to a rotatable drum. The needles were connected to a high-voltage DC power supply. The filter medium was wrapped onto the surface of the cylindrical electrode. The distance between the discharge and ground parts was ~15 mm. The applied voltage of the corona discharger was −10 kV and the period of electrification of the filter was 5 min. The rotational velocity of the drum was 15 rpm. The electrostatic potential of filter before and after the corona discharge electrification was checked at 1 cm from the filter surface with an electrostatic voltage monitor (EVM102; Sunje Hi-Tek, Busan, Republic of Korea). The corona discharge electrification system was used for the electrification of a pristine filter

A filtration test was conducted using two types of test particle with different sizes. First, as a submicron test particle, Staphylococcus epidermidis (KCTC 1917) was chosen. This Gram-positive bacterium has been commonly used in bioaerosol research because staphylococci are associated with serious health risks to humans and animals (Jung et al., 2011; Leppänen et al., 2014). Staphylococcal toxins are a common cause of food poisoning, as they can be produced in improperly stored food. Also, S. epidermidis is found indoors and on human skin and mucous membranes. Second, as a nanosized test particle, KCl nanoparticles were used in the filtration study. The size range of KCl nanoparticles was ~ 20–600 nm and the peak diameter and NMAD were ~ 90 nm and ~83.6 nm, respectively. The KCl nanoparticle has commonly been used as a test particle in filtration tests and particle collection research (ASHRAE52.2, 1999). S. epidermidis was cultured in nutrient broth (NB; Becton Dickinson, Franklin Lake, NJ, USA) at 37 °C for 24 h. In log-phase growth, cells were harvested by centrifugation (5000 ×g, 10 min). For bioaerosol generation, test bacteria pellets were carefully washed three times with sterilized deionized water (SDW) and subsequently diluted to obtain the final bacterial suspension. A 20-mL aliquot containing ~5 × 107 colony forming units (CFU)/mL was placed in a six-jet Collison nebulizer operating at a pressure of 1 psig (Jung and Lee, 2013). The KCl solution (3% (w/v)) was prepared by combining 0.6 g of KCl with 20 mL of SDW. Using a nebulizer, test bacteria and KCl nanoparticles were aerosolized at 5 L/min air (~ 33.4 cm/s) in a dry and filtered airstream. Dispersed bioaerosols and KCl nanoparticles were passed through the diffusion dryer to remove moisture and through a 210Po neutralizer to reduce the electrical charge on the test particles. The bacteria and KCl nanoparticles were then introduced into the filter holder in which an antimicrobial ES filter was installed (Fig. 1(a)). To investigate the filtration efficiency of ES filters, the concentrations of test aerosols at the ES filter inlet (Cinlet) and outlet (Coutlet) were measured using an SMPS and aerodynamic particle sizer (APS; 3321, TSI Inc.). APS sizes airborne particles in the range 0.5–20 μm using a timeof-flight technique by measuring the aerodynamic diameter. The filtration efficiency according to the particle diameter (η) was defined as: ηð%Þ ¼ ð1−Coutlet ⁄Cinlet Þ  100%:

ð1Þ

To determine the pressure drop of antimicrobial ES filters, the filter pressure drop was measured across pressure taps installed on the upstream and downstream sides of the filter holder. The pressure drop between the electric filter inlet and outlet was analyzed under various airflow conditions (13.3–40 cm/s) using an electro manometer (Model 332, Furness Controls, UK) with a measuring range of 10 mmAq. The air face velocity is defined as the average velocity of the air across the face area of filter, calculated by dividing the air volume by the total area. Before the measurement of filter pressure drop, we measured the offset pressure drop of the filter screen support without installation of a filter in the filter holder. Then, the pressure drop of the antimicrobial ES air filter was measured at various face air velocities. 2.4. Antimicrobial filter test Bioaerosols were deposited continuously onto filters during a period of ~5 min in all experiments to obtain identical loading on all filters. Approximately 106 cells were inoculated onto the test filter and incubated for a contact time of 30 min at room temperature (Jung et al., 2011). After the contact time had elapsed, each filter was removed from the filter holder and placed into 5 mL of phosphate-buffered saline (PBS; pH 7.4) with 0.05% Tween 80 and sonicated for 10 min to ensure the

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extraction of the bacteria. The bacterial suspension was serially diluted and plated onto nutrient agar (NA; Becton Dickinson), followed by incubation at 37 °C for 24 h. Colonies were then counted. The inactivation rate was calculated using the following equation:  Inactivation rate ð%Þ ¼

1−

CFU experiment CFUcontrol

  100%;

ð2Þ

where CFUexperiment and CFUcontrol are the number of bacterial colonies derived from the antimicrobial and pristine (control) ES filters, respectively. 2.5. Electron microscopy analysis The morphology of airborne S. flavescens nanoparticles, S. epidermidis bioaerosols, and filter fibers were examined using an analytical transmission electron microscope (TEM; CM30, Philips Electron Optics, Eindhoven, The Netherlands) and a scanning electron microscope (SEM; XL30 ESEMFEG; Philips Electron Optics). Airborne S. flavescens nanoparticles were sampled on a copper TEM grid (type A coated with a carbon film, Ted Pella Inc., Redding, CA, USA) using a nanometer aerosol sampler (3089, TSI Inc.) downstream of the nebulization–thermal drying system. The filter samples were coated with Pt–Pd using an ion-sputtering coater (model IB-3; Eiko Co., Ltd., Ibaraki, Japan) and visualized under high vacuum conditions (10−5 mbar) using the SEM. Bacterial bioaerosols were sampled on to a 13-mm polycarbonate membrane (PCM) filters with a 0.4-mm pore size (Isopore Membrane Filters HTTP01300; Millipore, Billerica, MA, USA). The PCM filters were coated with osmium using a chemical vapor deposition method (HPC-1SW, Vacuum Device Inc., Ibaraki, Japan) and then analyzed using the SEM. 2.6. Statistical analysis The data were analyzed statistically by an analysis of variance, Mann–Whitney U-test, and linear regression using the SPSS software (ver. 21; SPSS Inc., Chicago, IL, USA). A p value b 0.05 was considered to indicate statistical significance. 3. Results and Discussion 3.1. Antimicrobial treatment of the ES filter using S. flavescens natural-product nanoparticles

ranging from 20 to 660 nm. The particle size distribution of S. flavescens nanoparticles was a unimodal, lognormal distribution with a mode diameter (peak) of 88.2 nm, a geometric mean diameter (GMD) of 139.4 nm, and a geometric standard deviation (GSD) of ~ 1.7 (high polydispersity). The number median aerodynamic diameter (NMAD) was ~ 135.8 nm. The TEM image of the nanoparticles showed that the morphology of the nanoparticles was mostly nonagglomerated and spherical (Jung et al., 2011).

3.1.2. Deposition characteristics of S. flavescens nanoparticles on the filter During the antimicrobial treatment on the filter with S. flavescens nanoparticles, we investigated the adhesion characteristics of antimicrobial nanoparticles on the ES air filter. Fig. 3 shows the deposition efficiencies of S. flavescens nanoparticles on the pristine (Non-ES) and ES filters. The particle deposition efficiency was determined by the ratio of the total particle number concentration of S. flavescens nanoparticles between the inlet and outlet of the filter. The deposition efficiency of the Non-ES filter (56 ± 3.0%) was ~ 12% lower than that of the ES filter (68 ± 6.0%; p = 0.049). This demonstrates that an electric charge applied across the fibers of the ES filter improves the S. flavescens nanoparticle deposition efficiency of the filter. Therefore, the electrostatic activation process for the ES filter can reduce the wastage of antimicrobial materials during aerosol-based antimicrobial treatment due to an increase in the particle deposition efficiency of the filter. Finally, in this study, S. flavescens nanoparticles were deposited on a filter with a density of 4.4 ± 0.20 μg/mm2filter to produce an antimicrobial activity of N99% (18- and 15-min particle deposition times were used for Non-ES and ES filters, respectively). Fig. 4 shows the surface morphology of the fibers before and after antimicrobial treatment with the same quantity of S. flavescens nanoparticle deposition on Non-ES and ES filters. Although S. flavescens nanoparticles resulted in spatial dendritic growth on both filters, the SEM image showed no clear difference in particle deposition pattern between Non-ES and ES filters. A previous study showed that deposits of solid particles had a tendency to take the shape of irregular chains and bend in the absence of electrostatic forces on fiber surfaces (Billings, 1966), but the dendrites appeared quite straight when external electric fields were present (Walsh and Stenhouse, 1997; Wang et al., 1980). While these particle dendrites generally increase the collection efficiency and the pressure drop across a Non-ES filter, the particle loading reduces the collection efficiency of ES filters in the early stages of filtration because deposited particles diminish the electrostatic effects (Wang, 2001). Additionally, the deposition efficiency of particles on the filter depends on the air flow rate of the filter (Hofmann et al., 2003). The deposition efficiency of S. flavescens nanoparticles, as shown in Fig. 3, would increase with the decrease of air flow rate because diffusion is

20 S. flavescens nanoparticle

15

10 1 µm

5 GMD : 139.4 nm GSD : 1.70

0 1

10

Face velocity : 73.7 cm/s

100

100

1000

Particle diameter, Dp (nm) Fig. 2. Size distribution and TEM image of S. flavescens nanoparticles.

Deposition efficiency of natural-product nanoparticles (%)

Particle concentration, dN/dLog Dp (x106 particles/cm 3 )

3.1.1. Size distribution of S. flavescens nanoparticles Fig. 2 shows the size distribution and a TEM image of S. flavescens nanoparticles. From the particle size measurement by SMPS, the nebulization process produced nanoparticles with a broad size distribution,

269

80

60

40

56± 3.0%

68± 6.0%

Non-ES filter

ES filter

20

0

Fig. 3. The deposition efficiency of S. flavescens nanoparticles. Star indicates p b 0.05 (n = 3).

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Fig. 4. SEM images of antimicrobial-treated Non-ES and ES filters.

the more dominant physical filtration mechanism for nanosized S. flavescens particles (Hinds, 2012). 3.2. Filtration characteristics of the antimicrobial ES filter We evaluated the filtration efficiency of the antimicrobial ES filter using nanosized KCl aerosols and submicron bacterial bioaerosols. We also investigated how the antimicrobial treatment using S. flavescens nanoparticles and the recharging process of the filter affected the filtration efficiency. 3.2.1. Filtration efficiency of the nanosized KCl aerosol Fig. 5 shows the fractional filtration efficiency of the filter for nanosized KCl aerosol measured with SMPS. The filtration efficiency (η) of the Non-ES filter decreased with particle size to ~300 nm regardless of the antimicrobial treatment (Fig. 5(a)). However, after the recharging process, the filtration efficiency improved significantly (p = 0.049). Compared to the Non-ES filter in Fig. 5(a), the ES filter had an enhanced filtration efficiency for the KCl aerosol that was almost constant regardless of particle size (Fig. 5(b)). Although the filtration

efficiency curve decreased slightly after the antimicrobial treatment was applied to the ES filter, the recharging process restored and enhanced the filtration efficiency. Fig. 6 and Table 1 show the total particle filtration efficiency for KCl aerosol, ranging from 20 to 660 nm. Although the antimicrobial treatment of the Non-ES filter did not result in a considerable change to the filtration efficiency, the recharging process of the antimicrobial Non-ES filter increased the filtration efficiency to ~ 77.2% (+ Δ29.0%). The filtration efficiency of the antimicrobial-treated ES filter (~74.3%) was ~6% lower than that of the initial ES filter (~80.9%). The recharging process of the antimicrobial ES filter increased its filtration efficiency to ~88.4% (+Δ14.1%). The difference in filtration efficiency between the initial Non-ES and ES filters (without antimicrobial treatment) was ~30% for the nanosized KCl aerosol. This indicates that the electrostatic charges on the fibers of the ES filter produce a relatively high filtration efficiency for KCl aerosol. It has previously been shown that filtration efficiency increases with the surrounding electric field strength (Hwang et al., 2014). As shown in Figs. 5 and 6 and Table 1, the antimicrobial treatment of the filter resulted in a greater reduction of the filtration efficiency on the ES filter

K.M. Sim et al. / Science of the Total Environment 533 (2015) 266–274

Re-charging process of antimicrobial filter

Antimicrobial treatment

No treatment (Pristine filter)

271

Non-ES Filter

(b) ES Filter

6

100

Efficiency

Inlet Outlet

80 60

4 40 2

20 0

0 10

100

10

100

10

100

1000

100

8

80

6

60

Filtration efficiency (%)

(a)

Particle concentration, dN/dLogDp (x104 #/cm3)

8

4 40 2

20 0

0 10

100

10

100

10

100

1000

Particle diameter, Dp (nm) Fig. 5. Fractional filtration efficiency of nanosized KCl aerosols on filters.

(−Δ6.6%) compared to the Non-ES filter (−Δ2.9%). This result shows that the loss of electrostatic charge on the filter by the deposition of antimicrobial S. flavescens nanoparticles could lead to a large reduction in the filtration efficiency of the ES filter. Surprisingly, the recharging process of the filter increased its filtration efficiency significantly by electrostatically reactivating the filter surface. Thus, use of a recharging process could prevent decreases in filtration efficiency by antimicrobial treatment and so improve the filtration efficiency of the filter. Although electrostatic charges on the filter after antimicrobial treatment were not measured in this study, previous experimental studies on electret filters have shown that the collection efficiency generally decreases with particle loading in the early stages of filtration due to charge reduction on the filter (Baumgartner and Loffler, 1986; Baumgartner et al., 1986). Walsh and Stenhouse (1997, 1998) suggested that the degradation of filtration efficiency is not only a result of charge neutralization, but also of charge screening (Brown et al., 1988).

Filtration efficiency of KCl nanoparticles (%)

100

Non - ES filter

ES filter

80

60

3.2.2. Filtration efficiency of submicron S. epidermidis bioaerosol Fig. 7(a) shows the size distribution of S. epidermidis bioaerosols measured by an APS. The size distribution was unimodal, and the peak size was clearly recognizable with an aerodynamic mode diameter of ~ 0.89 μm. The GMD and GSD of S. epidermidis bioaerosols were ~ 0.84 ± 0.1 μm and 1.32 ± 0.1, respectively. As shown in the SEM image of the S. epidermidis bioaerosol (Fig. 7(b)), their morphology was mostly uniform, non-agglomerated, and spherical. Fig. 8 and Table 2 show the total filtration efficiency of S. epidermidis bioaerosols measured by APS. The filtration efficiency of an antimicrobialtreated Non-ES filter (~85.9%) was ~23% higher than that of a pristine (Non-ES) filter (~62.7%). Although the recharging process of antimicrobial Non-ES filters further increased the filtration efficiency of S. epidermidis bioaerosol (to ~ 89.2% (+Δ3.3%)), the difference was not statistically significant (p = 0.127). For the ES filter, the antimicrobial treatment did not result in a considerable change in the filtration efficiency of S. epidermidis bioaerosol (initial ES filter: ~84.4%; antimicrobial-treated ES filter: ~89.0%). However, the recharging process of the antimicrobial ES filter increased the filtration efficiency to ~95.0% (+Δ6%, p = 0.049). The difference in the filtration efficiency between the initial Non-ES and ES filters (without antimicrobial treatment) was ~22% for the submicron S. epidermidis bioaerosol (Fig. 8 and Table 2). While the antimicrobial treatment of the ES filter decreased the filtration efficiency of nanosized KCl aerosol (− Δ6.6%, p = 0.049), there was no statistically significant difference in the filtration efficiency of submicron S. epidermidis bioaerosol (+Δ4.6%, p = 0.127). This result demonstrates that the electric force due to the electrostatic charge of

40 Table 1 Total particle filtration efficiency (%) of nanosized KCl nanoparticles.

20

Type

No treatment (Pristine filter)

Antimicrobial treatment

Recharging process of antimicrobial filter

Non-ES filter ES filter

51.1 ± 0.71a 80.9 ± 0.81

48.2 ± 0.73 74.3 ± 0.86

77.2 ± 1.14 88.4 ± 0.93

0

No treatment (Pristine filter)

Antimicrobial treatment

Re-charging process of antimicrobial filter

Fig. 6. Total particle filtration efficiency of nanosized KCl aerosols on filters.

a

Average ± standard deviation.

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Particle concentration, 3 3 dN/dLogDp (x10 particles/cm )

2.0

S. epidermidis

(a)

(b)

5 µm

1.5

1.0

1 µm 0.5

0.0 0.1

GMD : 0.84.µm GSD : 1.32 1

10

Particle diameter, Dp (µ m) Fig. 7. (a) Size distribution and (b) SEM images of S. epidermidis bioaerosols.

the ES filter enhanced the filtration efficiency of both test particles, despite their different sizes. The antimicrobial treatment; i.e., the loading of antimicrobial S. flavescens nanoparticles onto the filter medium, decreases the filter porosity and leads to an increase in the filtration efficiency of submicron particles because the interception and impaction of particles is a more dominant physical mechanism of filtration of submicron particles, such as S. epidermidis (Hinds, 2012; Thomas et al., 2001). Based on the classical theory of filtration (Hinds, 2012; Kulkarni et al., 2011), the single-fiber efficiencies for diffusion, interception and impaction were calculated as 0.048, 0.004, and 0.001 for 0.1-μm particles, and 0.006, 0.332, and 1.053 for 1.0-μm particles, respectively (filter thickness = 0.6 mm, filter solidity = 0.1, fiber diameter = 2 μm, and face velocity = 0.33 cm/s). For this filtration condition, interception and impaction are negligible for small particles (i.e., 0.1 μm of KCl or S. flavescens aerosol), but increase rapidly for particles larger than 0.3 μm. Diffusion is the only important mechanism for particles below 0.2 μm, but is of decreasing importance for particles above that size (Hinds, 2012). Although the recharging process increased the filtration efficiencies of antimicrobial-treated ES and non-ES filters (Tables 1, 2), the filtration 120

Filtration efficiency of S. epidermidis bioaerosols (%)

Non-ES filter

ES filter

100 80 60

efficiency of the antimicrobial-treated non-ES filter did not reach that of the antimicrobial-treated ES filter despite the recharging process. The difference in particle deposition pattern between antimicrobialtreated ES and non-ES filters is likely to affect the difference in filtration efficiency after the recharging process because the shape and morphology of agglomerative particle deposition could affect the surrounding electric field that exerts a force on particles (Wang, 2001). Additional studies are required to evaluate the effects of the particle deposition pattern and electrification of the filter on the filtration efficiency. 3.3. Antimicrobial efficacy and pressure drop of the antimicrobial ES filter In this study, S. flavescens nanoparticles were deposited on a filter with a density of ~ 4.44 μg/mm2filter to produce an antimicrobial activity of N99% against S. epidermidis bioaerosols. In Fig. 10, the antimicrobial ES filter showed antimicrobial efficiencies of ~75.5% at 1.49 μg/mm2, ~90.9% at 2.98 μg/mm2, ~99.2% at 4.44 μg/mm2, and ~99.5% at 5.92 μg/mm2. The antimicrobial effect was sustained regardless of the filter recharging process. Previous studies have indicated that the inactivation efficiency of an antimicrobial filter increases with an increase in the antimicrobial particle concentration (Hwang et al., 2014; Jung et al., 2011). The S. flavescens nanoparticle concentration deposited on the filter per unit cross-sectional filter surface area was approximately 1.36 × 1010 particles/cm2filter. Fig. 9 shows the pressure drop across the S. flavescens-coated filters at various face airflow velocities. As the face airflow velocity increased from 13.3 to 40 cm/s, the pressure drop across the antimicrobialtreated filter increased from 0.75 to 2.57 mmAq, while for the pristine filter, the increase was from 0.48 to 1.40 mmAq. The pressure drop increases linearly with increasing face flow velocity (Davies, 1973; Kuwabara, 1959). Thus, if the linear relationship between pressure drop and face velocity could be obtained in the proper range of face

40 Table 2 Total particle filtration efficiency (%) of submicron S. epidermidis bioaerosols.

20 0

No treatment (Pristine filter)

Antimicrobial treatment

Re-charging process of antimicrobial filter

Fig. 8. Total particle filtration efficiency of submicron S. epidermidis bioaerosol on filters.

Type

No treatment (Pristine filter)

Antimicrobial treatment

Recharging process of antimicrobial filter

Non-ES filter ES filter

62.7 ± 3.89a 84.4 ± 3.41

85.9 ± 3.17 89.0 ± 1.34

89.2 ± 1.12 95.0 ± 1.07

a

Average ± standard deviation.

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serves various functions and can be used in indoor environments. Additional studies are required to optimize the system to reach N 95% filtration efficiency and a low pressure drop. Acknowledgments This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2013K000386), the Railway Technology Research Project, funded by the Ministry of Land, Infrastructure and Transport (14RTRP-B08124901), Republic of Korea, and was partially supported by the KIST Institutional Program. References

Fig. 9. Variation in the pressure drop across the antimicrobial ES filter.

velocity, the pressure drop at different ranges of face velocity could be predicted as long as the solidity remains constant (Kulkarni et al., 2011). Fig. 9 shows that there was no significant difference in the pressure drop before and after recharging of the filter, and the presence of the S. flavescens coating was the only variable that determined the filter pressure drop. According to previous studies, significant changes in the pressure drop across fibrous filters caused by the deposition of solid aerosol particles normally occur in the order of several grams per square meter of particle deposition (Japuntich et al., 1997; Thomas et al., 2001). However, the pressure drop across the antimicrobial ES filter in this study (~0.8 mmAq at 13.3 cm/s) was markedly lower than has been reported previously for the same level of antimicrobial activity (~7.5 mmAq at 13.3 cm/s) (Jung et al., 2011). 4. Conclusions In this study, we successfully prepared an ES air filter coated with antimicrobial nanoparticles of S. flavescens using aerosol-based technology. This study showed that an antimicrobial ES air filter (~99% of antimicrobial activity) with a recharging process had ~ 88% and ~ 95% of filtration efficiency for nanosized and submicron test particles, respectively, and a low pressure drop (~ 0.8 mmAq at 13.3 cm/s) regardless of the antimicrobial treatment of filter. This study provides useful information about the development of a hybrid air purification system that

120

Inactivation rate (%)

100 80 60 40

S. epidermidis 20 0 0

1

2

3

4

5

6 2

Weight of deposited particles (µg/mm ) Fig. 10. Variation in the inactivation rate of S. epidermidis bioaerosol on the S. flavescens nanoparticle-deposited ES filter.

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