Electrospun Magnetic Nanoparticle-Decorated Nanofiber Filter and Its

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Electrospun magnetic nanoparticle-decorated nanofiber filter and its applications to high-efficiency air filtration Juyoung Kim, Seung Chan Hong, Gwi-Nam Bae, and Jaehee Jung Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02884 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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Environmental Science & Technology

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Electrospun magnetic nanoparticle-decorated

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nanofiber filter and its applications to high-

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efficiency air filtration

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Juyoung Kim,1,2,† Seung Chan Hong,3,† Gwi Nam Bae,1 and Jae Hee Jung1,4,5,*

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1

Center for Environment, Health, and Welfare Research, Korea Institute of Science and

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Technology (KIST), Seoul 02792, Republic of Korea

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2

Department of Chemical Engineering, University of Seoul, Seoul 02504, Republic of Korea

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3

Department of Mechanical and Aerospace Engineering, Seoul National University, Seoul

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08826, Republic of Korea 4

Division of Energy & Environment Technology, KIST School, Korea University of Science and

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Technology, Seoul 02792, Republic of Korea 5

Green School, Korea University, Seoul 02841, Republic of Korea

13 14 15

†

These authors equally contributed to this work.

*Correspondence should be addressed to: [email protected]; Tel.: 82-2-958-5718

16 17 18 19 20 21 22

Keywords : magnetic nanoparticle, nanofiber, air filtration, electrospinning, indoor air Manuscript information: 16 text pages, 3 tables, 5 figures Supplementary information: 5 tables, 8 figures

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1 ACS Paragon Plus Environment


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Abstract

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Filtration technology has been widely studied due to concerns about exposure to airborne dust,

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including metal oxide nanoparticles, which cause serious health problems. The aim of these

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studies has been to develop mechanisms for the continuous and efficient removal of metal oxide

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dusts. In this study, we introduce a novel air filtration system based on the magnetic attraction

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force. The filtration system is composed of a magnetic nanoparticle (MNP)-decorated nanofiber

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(MNP-NF) filter. Using a simple electrospinning system, we fabricated continuous and smooth

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electrospun nanofibers with evenly distributed Fe3O4 MNPs. Our electrospun MNP-NF filter

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exhibited high particle collection efficiency (~97% at 300 nm particle size) compared to the

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control filter (w/o MNPs, ~68%), with a ~64% lower pressure drop (~17 Pa) than the control filter

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(~27 Pa). Finally, the filter quality factors of the MNP-NF filter were 4.7 and 11.9 times larger

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than those of the control filter and the conventional high-efficiency particulate air filters (>99%

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and ~269 Pa), respectively. Furthermore, we successfully performed a field test of our MNP-NF

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filter using dust from a subway station tunnel. This work suggests that our novel MNP-NF filter

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can be used to facilitate effective protection against hazardous metal oxide dust in real

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environments.

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TOC Art

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Introduction

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Recently, concerns about airborne dust, including nanosized metallic particulates, have greatly

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increased due to their adverse effects on the human body.1, 2 Metallic dust, which exists in

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oxidized forms in the atmosphere, contributes to the production of reactive oxygen species,

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which damage cells and tissues and cause inflammation in vivo.3 Moreover, these species can

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lead to genotoxic effects4, cell membrane disruption, and lung and systemic cardiovascular

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diseases.5 Additionally, Maher et al. reported a possible association between Alzheimer’s

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disease and metal oxide nanoparticles.6

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Unfortunately, people in contemporary society are at high risk of exposure to airborne

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metal oxide nanoparticles in daily life. For example, metal oxides are widely generated in

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industrial plants,7 on roads8 and indoors.9 A previous study, which found 85.2 µg/g lead and

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2,740 µg/g iron, reported a noticeable concentration of metal compounds in house dust.10 In

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particular, there is a significant amount of metal oxide dust in subway stations. This dust is

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mainly generated by the brakes, wheel-rail interface, and vaporization of the metal when sparks

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are created. Aarnio et al. reported the presence of a high proportion of iron-containing

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compounds (29 ± 7.0 µg/m3) at an underground subway station in Helsinki, Finland.11 Kang et

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al. also measured the concentration and composition of airborne particles in a subway in Seoul,

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Republic of Korea, and reported that iron-containing particles were abundant, at 61-79 wt%.12

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Fibrous air filters have been widely used to remove air pollutants. These devices

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perform well at particle collection, are cost-effective, and are simple to fabricate. Airborne

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particles pass through a filter medium and are collected on the filter by physical deposition

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mechanisms such as inertial impaction, interception, and Brownian diffusion.13 As there is a

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trade-off between particle removal efficiency and pressure drop, there have been numerous

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efforts to improve particle removal efficiency of filters without increasing their pressure drop. For

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example, the electrostatic air filter, which consists of dielectric materials with a quasi-permanent

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electrical charge, can enhance particle capture by Coulombic attraction and dielectrophoresis

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without increasing the pressure drop.14, 15 However, the lifetime of electrostatic air filters is

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relatively short because the electrostatic charge on fiber dissipates as airborne dust is deposited

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on the filter.

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Air filtration systems based on magnetic forces have been studied recently as they can

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potentially be used to remove metallic nanoparticles.16 Li et al. and Huang et al. theoretically

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and experimentally demonstrated the effectiveness of magnetic fields for removing metallic

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aerosols using a metal screen mesh.17-19 Son et al. utilized a magnetic filter composed of a



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permanent magnet array to remove airborne particular matter generated in a subway tunnel in

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Seoul, Republic of Korea.16 Despite the efforts to fabricate an efficient magnetic filter, these

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devices did not filter nanosized metal dust sufficiently as the pore size of the metal mesh was

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too large. Therefore, a new type of magnetic filter is required to efficiently remove metallic

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airborne nanoparticles.

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Electrospinning is a powerful method for fabricating multipurpose filters. This simple

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process can be used to produce uniform fibers with diameters smaller than a few microns, into

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which functional materials can be incorporated.20-22 Due to these advantages, electrospun fibers

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have been used widely in diverse applications, such as in filter media for ultrafine aerosols,23

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antimicrobial filters,24 liquid filtration,25 tissue engineering scaffolding,26 and nano-sensors.27

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However, the electrospinning method has never been used for fabricating magnetic filters for

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the filtration of airborne metal oxide particles.

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In this study, we demonstrate a novel polymer-based magnetic nanoparticle-

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incorporated nanofiber (MNP-NF) filter fabricated using a simple electrospinning process. To

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maximize the magnetization of the filter, we incorporated Fe3O4 magnetic nanoparticles (MNPs)

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into the fiber. We characterized and optimized the MNP-NF filter by measuring its

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physicochemical properties, including its chemical components, morphology and size

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distribution, as well as the hysteresis loop of filters fabricated using different concentrations of

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MNPs. To evaluate the performance of the MNP-NF filter, we measured its particle collection

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efficiency and pressure drop using a test metal oxide dust.

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Our MNP-NF filter both effectively removed metal oxide dusts (>97% of filtration

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performance (collection efficiency)) and solved the high pressure drop problem observed in

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existing HEPA filters, resulting in a filter quality 11.9 times higher than that of a HEPA filter.

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Furthermore, the MNP-NF filter demonstrated high applicability in real environments in a field

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test, which was conducted in a subway station in Seoul, Republic of Korea. This study

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demonstrates that the MNP-NF filter has satisfactory filtration performance for nanosized metal

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dusts, with a low pressure drop and a simple manufacturing process.

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Materials and methods

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Solution preparation

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The Polyvinylpyrrolidone (PVP) was dissolved in ethanol (1.00983.2511; Merck KGaA,

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Darmstadt, Germany) with a concentration of 15 wt%. Then, the Fe3O4 MNPs, which had


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diameters in the range 50-100 nm, were added to make Fe3O4/PVP solutions with MNP

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concentrations of 5, 10, and 20 wt%. The prepared solutions were continuously stirred at room

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temperature using an Analog tube rotator (MX-RD-E; Scilogex, London, UK) until a

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homogeneous mixture was obtained. This step was included to prevent the particles from

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agglomerating.

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The viscosity of the solutions was measured in a static DC field using a Physica

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Couette-type rheometer (Physica MCR 301; Anton Paar, Graz, Austria) with a high-voltage

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generator. The surface tension of the solutions was measured using an Automatic Force

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Tensiometer (Sigma 702; KSV Instruments Ltd., Helsinki, Finland) with a measurement range

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of 1-1000 mN/m and resolution of 0.001 mN/m. The conductivity of the solutions was measured

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using a conductivity meter (EC-40N; Istek Inc., Seoul, Republic of Korea). The measured

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physical properties of each solution are shown in Table S1.

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Eletrospinning system for the MNP-NF filter

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The SS tip (U-102; IDEX Health & Science, Oak Harbor, WA, USA) was fixed to a moving holder,

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and a positive 15 kV DC voltage was applied over it using a high-voltage generator (Korea

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switching Co., Seoul, Republic of Korea). The electrically grounded aluminum foil collector was

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located 10 cm under the tip. This was required to stabilize the electrical gradient. The flow rate

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of the Fe3O4/PVP solutions loaded into a 10-mL syringe (i.d. = 15.85 mm; Gastight 81620;

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Hamilton Co., Reno, NV, USA) were precisely controlled using a syringe pump (KD200; KD

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Scientific Inc., Hollison, MA, USA) at a constant flow rate of 3 mL/h. The MNP-NFs were

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electrospun on a metallic screen filter support (o.d. = 25 mm, thickness = 0.3 mm) for 2 minutes.

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Filtration test system for the MNP-NF filter

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A schematic diagram of the experimental setup for the filtration test is shown in Figure S1. To

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generate the test metal oxide dust, ~7.5 mg of Fe2O3 nanoparticles were stirred into 30 mL of

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deionized water and poured into a one-jet Collision nebulizer (BGI Inc., Butler, NJ, USA). The

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solution was nebulized at a rate of 1 L/min. To remove the moisture, the test dust was passed

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through a diffusion dryer. Finally, it was passed through the MNP-NF filter. The filters were

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attached to a reusable magnet with a diameter of 0.47 cm and a height of 1.02 cm. The

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maximum magnetic field strength of the magnet was 500 mT.

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The number concentrations of the test particles before and after passing through the

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filter were measured using a wide-range particle spectrometer (WPS; 1000XP, MSP Corp.,



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Shoreview, USA) which can measure the number concentration of aerosols in the size range

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from 10 nm to 10 μm. The pressure drop between the upstream and downstream areas of the

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filter was measured using a micromanometer (FE012; Furness Control, Ltd, Bexill, UK). The

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pressure drop of each filter was gauged three times at each face velocity. The face velocities

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were 3.4, 5.1, and 6.8 cm/s.

148 149 150

Results and discussion

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Fabrication of the MNP-NF filter

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We used a simple electrospinning system to fabricate a polymer-based nanofiber filter

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containing MNPs, as shown in Figure 1(a). We chose Fe3O4 nanoparticles as the magnetic

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material for the MNP-NF filter. Fe3O4 MNP, which has ferrimagnetic properties, exhibits stronger

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magnetism than other metal oxide nanoparticles.28 Furthermore, Fe3O4 MNPs are cost-effective

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because they are abundant in nature and easy to synthesize.29 A solution of PVP and the Fe3O4

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MNPs was supplied precisely to a stainless steel (SS) tip by using a digital syringe pump. Then,

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a high voltage was applied to the SS tip. To maintain the stability of the electrospinning, we

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placed an electrically grounded aluminum foil collector below the SS tip. The collector prevented

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the accumulation of charge in the electrospun fibers, leading to a stable electrical gradient

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between the SS tip and the collector. We carefully observed the whole electrospinning process

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using a CCD camera.

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Mechanism of airborne metal oxide dust filtration using the MNP-NF filter

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The metal oxide dust filtration mechanism of the MNP-NF filter is briefly explained in Figure 1(b)

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and (c). To remove the dust, the MNP-NF filter uses the magnetic force induced by the

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magnetization of the MNPs in the filter. This leads to effective and selective filtration of the

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nanosized metal oxide dust. The magnetic force, Fm, which pulls the metal oxide dust into the

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filter, can be expressed as

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𝐹𝑚 =

𝑉𝜒 (𝐵 𝜇0

· 𝛻)𝐵

(1)

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where V is the volume of the dust, χ is the magnetic susceptibility of the dust, μ0 is the magnetic

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permeability in a vacuum, and B is the magnetic flux density surrounding the dust.30 The

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magnetic flux density depends on the magnetization of the MNPs in the MNP-NF filter. This

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relationship can be expressed as

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𝐵 = 𝜇0 (𝐻 + 𝑀)

(2)


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where H is the external magnetic field and M is the magnetization of the MNPs by the external

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magnetic field.30, 31 When an external magnetic field, such as a permanent magnet, is applied

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to the MNP-NF filter, the Fe3O4 MNPs in the MNP-NF become magnetized, leading to their spins

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aligning in the same direction along the magnetic field. Then, the magnetic flux density near the

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MNP-NF filter increases in comparison to that near the control filter (w/o MNPs). The increased

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magnetic flux density strengthens the magnetic force, enabling the filter to attract the dust. This

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improves the particle collection performance of the filter without inducing the classical trade-off

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between collection efficiency and the pressure drop of filter.

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Characteristics of the electrospun MNP-NF

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Before fabricating the MNP-NF filter, we investigated the Fe3O4 MNPs in the electrospun MNP-

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NF (20 wt%) using transmission electron microscopy (TEM; TitanTM 80-300; FEI, Houston,

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USA), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analysis

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(Dmax2500/PC; Rigaku), as shown in Figure 2. The TEM images show MNPs were distributed

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evenly in the MNP-NF, whereas no such particles were contained in the control fiber (Fig. 2(a)).

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In Figure 2(b), the EDS spectrum of the control filter contains only 0.2 and 0.5 eV peaks. These

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indicate the presence of C and O. The spectrum of the MNP-NF, however, includes additional

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peaks at 0.7, 6.4 and 7.1 eV, indicating that Fe is present.32 Figure 2(c) shows the XRD patterns

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of the Fe3O4 MNPs, the control fiber, and the MNP-NF. The MNPs had peaks at 2θ = 30.2, 35.6,

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43.2, 57.1, and 62.7 degrees. These values are in agreement with the results from the previous

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study.33 The pattern of the MNP-NF includes both the MNPs and control fiber patterns. Hence,

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the MNP-NF contains both MNPs and the control fiber.

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To identify the morphological characteristics of the MNP-NFs as a function of MNP

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concentration, we analyzed scanning electron microscopy (SEM; SL30 ESEM-EFG; Philips

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Electron optics, Eindhoven, The Netherlands) images of the control fiber and MNP-NFs

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containing 5, 10, and 20 wt% MNPs. Figure 2(d) shows that all the fibers had smooth,

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continuous surfaces with no beads or disconnections. The normalized size distributions of the

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fibers obtained from the SEM images are shown in Figure 2(e). All fibers had monomodal

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distribution curves, although the fiber size increased as the MNP concentration increased. The

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mean fiber diameter was 1.5 ± 0.25 µm in the case of the control fiber and 2.2 ± 0.42 µm in the

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case of the 20 wt% MNP-NF (Table 1). These results can be explained in terms of the

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characteristics of the electrospinning process, which balances the electrical force and the

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viscous force. As shown in Table S1, the viscosity of the solution increased from 0.51 to 0.54



209

Pa·s as the MNP concentration was raised from 0 (control) to 20 wt%. As the viscous forces in

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a fluid suppresses the elongation of the fibers during electrospinning, for the same electrical

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potential, the size of the resulting fibers increases as the viscosity of the solution increases.34

212 213

Morphological characteristics of the MNP-NF filter

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After characterizing the electrospun MNP-NF, we fabricated an MNP-NF filter and investigated

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its morphological characteristics of the filter. Figure S2(a) shows the control filter and the MNP-

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NF filter (20 wt%). As Fe3O4 MNPs are black, the color of the MNP-NF filter is dark grey

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compared to the control filter, which is white. The diameters of the fibres in the MNP-NF filter

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were larger than those in the control filter (Table 1). Hence, the SEM image of the MNP-NF filter

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shows a more porous fiber deposit morphology under the same fiber aerial mass density

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condition, ~2.0 mg/cm2 (Fig. S2(b)). The filters have porosity, 1 - α, which can be expressed as

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1−𝛼 =

𝜌0 −𝜌 𝜌0

(3)

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where α is the packing density or solidity, ρ0 is the density of raw PVP with MNPs, and ρ is the

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density of the fibrous filter. The porosity increased from 0.958 to 0.962 as the concentration of

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MNPs in the filter increased from 0 (control) to 20 wt%.

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We also measured the pore size distributions of the control and MNP-NF filters (5, 10,

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20 wt%) using a porosimeter (AutoPore IV 9520; Micromeritics Instrument Corp., USA). As

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shown in Figure S3, the pore sizes of all filters showed a Boltzmann distribution. The geometric

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mean diameter (GMD) of the pores increased from 6.2 ± 0.04 to 11.1 ± 0.25 µm as the MNP

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concentration increased from 0 (control) to 20 wt% (Table S2).

230 231

Magnetization of the MNP-NF filter

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To evaluate the magnetic properties of the MNP-NF filter, the hysteresis loops of the filters with

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various MNP concentrations were analyzed using a vibrating sample magnetometer (VSM;

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Microsense, EZ9, Lowell, MA, USA). As shown in Figure 3, the saturation magnetization, Ms, is

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defined as the maximum magnetization, which occurs when all of the magnetic domains are

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aligned in the direction of the external magnetic field.35 This depends on the MNP

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concentration.36 The higher the concentration of MNPs in the filter, the more MNP domains

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there are to align along the magnetic field, leading to higher magnetization. Therefore, Ms

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increased significantly, from 0.38 to 10.60 emu/g, as the MNP concentrations increased from 0

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to 20 wt%. Further details regarding the magnetization parameters of the MNP-NF filters are

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shown in Table 2.


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Filtration performance test

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We performed filtration performance tests of the filters with various concentration of MNPs by

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measuring the particle collection efficiency, pressure drop and filter quality factor. A schematic

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diagram showing the experimental setup for these tests is shown in Figure S1. The filter was

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fixed to the center of the filter holder with a cylindrical magnet, as shown in Figure 4(a). Fe2O3

248

(Iron(III) oxide, 44896; Alfa Aesar, Ward Hill, MA, USA) nanoparticles were used as the test

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metal oxide dust because Fe2O3 nanoparticles are known to be some of the most abundant

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indoor metallic airborne particles.10, 11, 37-40 Figure 4(b) shows the particle size distribution and a

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TEM image of the test metal oxide dust. The mode diameter of the dust was 50.7 ± 1.21 nm.

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The geometric mean diameter (GMD) and geometric standard deviation (GSD) were 42.5 ±

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3.68 nm and 1.7 ± 0.01 (Table S3), respectively.

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First, we measured the filter performance in the absence of an external magnetic field.

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Figure 4(c) shows the collection efficiencies of the MNP-NF filter (20 wt%), control filter, and

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base mesh without any magnet. The collection efficiency, η, of the test dust is defined as

257

𝜂 =1−

𝐶𝑑𝑜𝑤𝑛 𝐶𝑢𝑝

(4)

258

where Cup and Cdown are the number concentrations of particles (particles/cm3air) measured

259

upstream and downstream of the filter, respectively.13 The control and MNP-NF filters exhibited

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similar collection efficiencies over the whole particle size range (>66%). The collection

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efficiencies of particles smaller than 100 nm and larger than 1 µm were greater than 80%;

262

however, the efficiencies of the collection of dust particles with diameters of 300 nm were less

263

than 70%. This is due to the fundamental filtration mechanism of the fibrous filter. Generally,

264

there are three main mechanisms for depositing airborne dust onto a filter: interception, inertial

265

impaction, and diffusion. Large dust particles (>1 µm) with sufficient inertia are deposited onto

266

the filter by interception and inertial impaction. Nanosized dust (95%) for all sizes of dust tested.

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These results demonstrate that the magnetized MNPs greatly enhanced the filtration of metal

286

oxide dust in the MNP-NF filter.

287

To investigate the relationship between the strength of the magnet and the particle

288

collection efficiency of the MNP-NF filter (20 wt%), we also measured the collection efficiency

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at a range of magnet strengths (Fig. S4). The result shows that the collection efficiency of the

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filter increases linearly as the strength of the magnet increases. This result is consistent with

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the theoretical background that the collection efficiency increases with increasing external

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magnetic field strength,30,31 and proves that the efficiency is increased by magnetization of the

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external magnetic field rather than the filter itself.

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To ensure that the dusts were completely trapped inside the MNP-NF filter, we

295

performed an SEM analysis of the filter interior. Figure S5 shows the locations where the SEM

296

analysis was performed on the filter (Fig. S5(a)) and the SEM images at each location (Fig.

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S5(b–f)). The results show that the metal oxide dusts are completely trapped inside the filter

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regardless of the location of the filter.

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As the pressure drop is related to the energy consumption, maintenance, and life cycle

300

of the filter, the pressure drop is an important parameter to consider alongside the filtration

301

efficiency, as it determines the performance of the filter. Figure S6 shows the relationship

302

between the pressure drop and the velocity of the air entering the MNP-NF filters at various

303

MNP concentrations. As the filter face velocity increased from 3.4 to 6.8 cm/s, the pressure drop

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of the 20 wt% MNP-NF filter increased from 17.1 to 36.2 Pa. The pressure drop of the control

305

filter rose more steeply, from 26.8 to 56.8 Pa. Also, as the concentration of MNPs in the MNP-

306

NF filter increased, the filter pressure drop decreased. This is due to the increase in the diameter

307

of the fibers in the filter, which reduces the drag force of air flow. In the case of a fibrous filter,


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the pressure drop, ΔP, can be theoretically calculated as a function of the cumulative drag on

309

the fibers in the filter:

310

𝛥𝑃 =

𝜎𝑡𝑣 [64𝛼 1.5 (1 + 𝑑𝑓2

56𝛼 3 )]

(5)

311

where σ is the air viscosity, t is the filter depth or thickness, v is the face velocity at the filter

312

surface, and df is the fiber diameter.13 This equation (5) implies that the larger the fiber and pore

313

size of the filter, the lower the pressure drop.

314

To quantitatively compare the performance of the control and MNP-NF filters, the filter

315

quality factor, qf, was calculated. This is defined as the ratio between the collection ability and

316

pressured drop of the filter. We calculated this under conditions with 300-nm dust and a face air

317

velocity of 3.4 cm/s. The quality factor is defined as

318

𝑞𝑓 =

𝛾𝑡 𝛥𝑃

=

𝑙𝑛⁡(

1 ) 1−𝜂

𝛥𝑃

(6)

319

where γ is the fractional capture of the airborne particles. Equation (6) implies that a filter with

320

a high collection efficiency and low pressure drop has a high filter quality qf.13 Figure 4(e) shows

321

that the particle collection efficiency increased from 73.3 to 96.9% and the pressure drop

322

decreased from 26.8 to 17.1 Pa for the 0 (control), 5, 10, and 20 wt% MNP-NF filters. Therefore,

323

the filter quality factors increased significantly, from 49 ± 0.1 /kPa for the control filter to 202 ±

324

0.7 /kPa for the 20 wt% MNP-NF filter, as shown in Figure 4(f). Table 3 shows comparisons of

325

the filter qualities of the control filter and a conventional high-efficiency particulate air (HEPA)

326

filter without an external magnetic field with those of the MNP-NF (20wt%) filter with a 500-mT

327

magnet. The collection efficiency and pressure drop of the control filter were 68.3 ± 1.0% and

328

26.8 ± 0.194 Pa, respectively, leading to a filter quality factor of ~43 /kPa. The HEPA filter had

329

a higher collection efficiency, at 98.9 ± 0.58%. However, the pressure drop of the HEPA filter

330

was also much higher than those of the others (268.9 ± 5.52 Pa), which resulted in a quality

331

factor of only ~17 /kPa. Consequently, our MNP-NF filter performed much better than both the

332

control filter (4.7 times higher qf) and the HEPA filter (11.9 times higher qf), filtering metal oxide

333

nano-dust with a low pressure drop.

334 335

Field test using real subway dust

336

We performed a field test of the MNP-NF filter to evaluate its performance in a real environment.

337

As a subway station is a representative indoor environment in which there are many nanosized

338

metal oxide dust particles, we used the airborne dust generated in a subway station

339

(Janghanpyeong – Gunja, Line 5) in Seoul, Republic of Korea. We installed the experimental



Page 12 of 26

340

apparatus in the subway tunnel, as shown in Figure 5(a). The color of the collected subway dust

341

was a deep dark brown (Fig. 5(a)), indicating that the subway dust contained metal oxide dust

342

particles. To identify the chemical components of the subway dust, we analyzed X-ray

343

fluorescence (XRF, ZSX Primus II; Rigaku Corp., Beijing, China) images, as shown in Figure

344

5(b). The most abundant compound in the dust was Fe2O3, which made up 66.2 wt% of the dust

345

collected (46% α-Fe2O3(hematite) and 54% γ-Fe2O3(maghemite) in the Fe2O3). Details are in

346

Table S5 and Figure S7. Additionally, the EDS spectrum contained high peaks for elemental Fe

347

and O, as shown in Figure S8. These results indicate that the subway dust had strong magnetic

348

susceptibility (χ). The TEM image and aerodynamic size distribution of the dust particles are

349

shown in Figure 5(c). Airborne dusts contained particles with a broad range of diameters, from

350

9.3 to 339.8 nm, and with a bimodal size distribution, with the first peak at 16.5 ± 1.08 nm and

351

second at 69.8 ± 0.57 nm. According to the TEM image, the dust particles were spherical and

352

partly agglomerated. The subway dust collection efficiency of the 20 wt% MNP-NF filter is shown

353

in Figure 5(d). For particles 300 nm in diameter, the collection efficiency of the MNP-NF filter

354

was enhanced by more than 15.1% in comparison to that of the control filter. Furthermore, the

355

collection efficiency of the MNP-NF filter was greater than 83% for all particles, compared to 68%

356

for the control filter. These results demonstrate that the MNP-NF filter performs excellently with

357

respect to subway dust filtration. Hence, we think that our MNP-NF filter can be used for

358

controlling the air quality, particularly in relation to metal oxide dust, in real indoor environments.

359

In summary, we developed a novel, easy-to-fabricate MNP-NF filter with high filtration

360

performance and a low pressure drop. This filter efficiently removes airborne dust, including

361

metal oxides. The MNP-NF filters were fabricated using a simple electrospinning process with

362

polymer solutions containing Fe3O4 MNPs. These MNPs exhibit strong magnetism. The

363

collection efficiency of the MNP-NF filter was greater than 97% for the test metal oxide dust,

364

which is an enhancement of ~29% in comparison to the control filter (w/o MNPs). The pressure

365

drop of the MNP-NF filter was much lower (~17 Pa) than those of the control filter (~27 Pa) and

366

a conventional HEPA filter (~269 Pa). Our MNP-NF filter exhibits remarkable filter quality (qf)

367

for removal of metal oxide dust; qf was 4.7 times greater than that of the control filter and 11.9

368

times greater than that of the HEPA filter. Furthermore, we conducted a field test to demonstrate

369

that the MNP-NF filter can be used in a real environment. In conclusion, the electrospun MNP-

370

NF filter shows excellent filtration performance and strong potential for use as an indoor air

371

quality control system to reduce the concentrations of hazardous metallic nanoparticles in the

372

air. Furthermore, if a roll-to-roll process is applied to the production method for this filter, it will


Page 13 of 26


373

be possible to manufacture MNP-NF filters continuously and over a large area, leading to the

374

possibility of commercialization in the near future.

375 376 377

References

378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407

1.

De Boeck, M.; Hoet, P.; Lombaert, N.; Nemery, B.; Kirsch-Volders, M.; Lison, D., In vivo genotoxicity of hard metal dust: induction of micronuclei in rat type II epithelial lung cells. Carcinogenesis 2003, 24, (11), 1793-1800.

2.

Costa, D. L.; Dreher, K. L., Bioavailable transition metals in particulate matter mediate cardiopulmonary injury in healthy and compromised animal models. Environ. Health Perspect. 1997, 105, (Suppl 5), 1053.

3.

Karlsson, H. L.; Nilsson, L.; Möller, L., Subway particles are more genotoxic than street particles and induce oxidative stress in cultured human lung cells. Chem. Res. Toxicol. 2005, 18, (1), 19-23.

4.

Golbamaki, N.; Rasulev, B.; Cassano, A.; Robinson, R. L. M.; Benfenati, E.; Leszczynski, J.; Cronin, M. T., Genotoxicity of metal oxide nanomaterials: review of recent data and discussion of possible mechanisms. Nanoscale 2015, 7, (6), 2154-2198.

5. Szalay, B. Iron oxide nanoparticles and their toxicological effects: in vivo and in vitro studies. University of Szeged, 2012. 6.

Maher, B. A.; Ahmed, I. A.; Karloukovski, V.; MacLaren, D. A.; Foulds, P. G.; Allsop, D.; Mann, D. M.; Torres-Jardón, R.; Calderon-Garciduenas, L., Magnetite pollution nanoparticles in the human brain. P. Natl. Acad. Sci. USA 2016, 113, (39), 10797-10801.

7.

Laforest, G.; Duchesne, J., Characterization and leachability of electric arc furnace dust made from remelting of stainless steel. J. Hazard. Mat. 2006, 135, (1), 156-164.

8.

Adachi, K.; Tainosho, Y., Characterization of heavy metal particles embedded in tire dust. Environ. Int. 2004, 30, (8), 1009-1017.

9.

Tahir, N. M.; Chee, P. S.; Jaafar, M., Determination of heavy metals content in soils and indoor dusts from nurseries in Dungun, Terengganu. Malays. J. Anal. Sci. 2007, 11, (1), 280-286.

10. Chattopadhyay, G.; Lin, K. C.-P.; Feitz, A. J., Household dust metal levels in the Sydney metropolitan area. Environ. Res. 2003, 93, (3), 301-307. 11. Aarnio, P.; Yli-Tuomi, T.; Kousa, A.; Mäkelä, T.; Hirsikko, A.; Hämeri, K.; Räisänen, M.; Hillamo, R.; Koskentalo, T.; Jantunen, M., The concentrations and composition of and exposure to fine particles (PM 2.5) in the Helsinki subway system. Atmos. Environ. 2005, 39, (28), 5059-5066.



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12. Kang, S.; Hwang, H.; Park, Y.; Kim, H.; Ro, C.-U., Chemical compositions of subway particles in Seoul, Korea determined by a quantitative single particle analysis. Environ. Sci. Technol. 2008, 42, (24), 9051-9057. 13. Hinds., W. C., Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles. Wiley-Interscience: New York, 1999. 14. Hwang, G. B.; Park, H.-S.; Bae, G.-N.; Jung, J. H., Effects of Electric Field Strength on an Antimicrobial Air Filter. Aerosol Air Quality Res. 2014, 14, (3), 1028-1037. 15. Sim, K. M.; Park, H.-S.; Bae, G.-N.; Jung, J. H., Antimicrobial nanoparticle-coated electrostatic air filter with high filtration efficiency and low pressure drop. Sci. Total Environ. 2015, 533, 266-274. 16. Son, Y.-S.; Dinh, T.-V.; Chung, S.-G.; Lee, J.-h.; Kim, J.-C., Removal of particulate matter emitted from a subway tunnel using magnetic filters. Environ. Sci. Technol. 2014, 48, (5), 2870-2876. 17. Li, L.; Greenberg, P. S.; Street Jr, K. W.; Chen, D. R., Study of a magnetic filter system for the characterization of particle magnetic property. Aerosol Sci. Technol. 2011, 45, (3), 327335. 18. Huang, S.; Zhang, X.; Tafu, M.; Toshima, T.; Jo, Y., Study on subway particle capture by ferromagnetic mesh filter in nonuniform magnetic field. Sep. Purif. Technol. 2015, 156, 642654. 19. Huang, S.; Park, H.; Park, Y. K.; Jo, Y. M., Dynamic trajectory and capture of fine dust by magnetic mesh filters based on a particle velocity model. Aerosol Sci. Technol. 2015, 49, (8), 633-642. 20. Greiner, A.; Wendorff, J. H., Electrospinning: a fascinating method for the preparation of ultrathin fibers. Angew. Chem. Int. Edit. 2007, 46, (30), 5670-5703. 21. Huang, C.; Soenen, S. J.; Rejman, J.; Trekker, J.; Chengxun, L.; Lagae, L.; Ceelen, W.; Wilhelm, C.; Demeester, J.; De Smedt, S. C., Magnetic electrospun fibers for cancer therapy. Adv. Funct. Mater. 2012, 22, (12), 2479-2486. 22. Li, D.; Xia, Y., Electrospinning of nanofibers: reinventing the wheel? Adv. Mater. 2004, 16, (14), 1151-1170. 23. Liu, C.; Hsu, P.-C.; Lee, H.-W.; Ye, M.; Zheng, G.; Liu, N.; Li, W.; Cui, Y., Transparent air filter for high-efficiency PM2.5 capture. Nat. Commun. 2015, 6, 6205. 24. Choi, J.; Yang, B. J.; Bae, G.-N.; Jung, J. H., Herbal extract incorporated nanofiber fabricated by an electrospinning technique and its application to antimicrobial air filtration. ACS Appl. Mater. Interf. 2015, 7, (45), 25313-25320. 25. Homaeigohar, S. S.; Buhr, K.; Ebert, K., Polyethersulfone electrospun nanofibrous composite membrane for liquid filtration. J. Membrane Sci. 2010, 365, (1), 68-77.


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26. Sill, T. J.; von Recum, H. A., Electrospinning: applications in drug delivery and tissue engineering. Biomaterials 2008, 29, (13), 1989-2006. 27. Lim, S. K.; Hwang, S.-H.; Chang, D.; Kim, S., Preparation of mesoporous In2O3 nanofibers by electrospinning and their application as a CO gas sensor. Sensor. Actuat. B-Chem. 2010, 149, (1), 28-33. 28. Teja, A. S.; Koh, P.-Y., Synthesis, properties, and applications of magnetic iron oxide nanoparticles. Prog. Cryst. Growth Ch. 2009, 55, (1), 22-45. 29. Cornell, R. M.; Schwertmann, U., The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses. Wiley-VCH: Weinheim, Germany, 2003. 30. Gijs, M. A. M., Magnetic bead handling on-chip: new opportunities for analytical applications. Microfluid. Nanofluid. 2004, 1, (1), 22-40. 31. Zborowski, M.; Sun, L.; Moore, L. R.; Williams, P. S.; Chalmers, J. J., Continuous cell separation using novel magnetic quadrupole flow sorter. J. Magn. Magn. Mater. 1999, 194, (1), 224-230. 32. Schinhammer, M.; Hänzi, A. C.; Löffler, J. F.; Uggowitzer, P. J., Design strategy for biodegradable Fe-based alloys for medical applications. Acta Biomater. 2010, 6, (5), 17051713. 33. Liu,

Q.;

Zhong,

L.-B.;

Zhao,

Q.-B.;

Frear,

C.;

Zheng,

Y.-M.,

Synthesis

of

Fe3O4/polyacrylonitrile composite electrospun nanofiber mat for effective adsorption of tetracycline. ACS Appl. Mater. Interf. 2015, 7, (27), 14573-14583. 34. Huang, Z.-M.; Zhang, Y.-Z.; Kotaki, M.; Ramakrishna, S., A review on polymer nanofibers by electrospinning and their applications in nanocomposites. Compos. Sci. Technol. 2003, 63, (15), 2223-2253. 35. Chikazumi, S., Physics of Ferromagnetism. Oxford Univ. Press: New York, 1997. 36. Majetich, S.; Scott, J.; Kirkpatrick, E.; Chowdary, K.; Gallagher, K.; McHenry, M., Magnetic nanoparticles and magnetocrystalline anisotropy. Nanostruc. Mat. 1997, 9, (1), 291-300. 37. Jung, H.-J.; Kim, B.; Ryu, J.; Maskey, S.; Kim, J.-C.; Sohn, J.; Ro, C.-U., Source identification of particulate matter collected at underground subway stations in Seoul, Korea using quantitative single-particle analysis. Atmos. Environ. 2010, 44, (19), 2287-2293. 38. Jung, H.-J.; Kim, B.; Malek, M. A.; Koo, Y. S.; Jung, J. H.; Son, Y.-S.; Kim, J.-C.; Kim, H.; Ro, C.-U., Chemical speciation of size-segregated floor dusts and airborne magnetic particles collected at underground subway stations in Seoul, Korea. J. Hazard. Mat. 2012, 213, 331-340. 39. Chillrud, S. N.; Epstein, D.; Ross, J. M.; Sax, S. N.; Pederson, D.; Spengler, J. D.; Kinney, P. L., Elevated airborne exposures of teenagers to manganese, chromium, and iron from steel dust and New York City's subway system. Environ. Sci. Technol. 2004, 38, (3), 732737.



481 482 483 484 485

40. Maskey, S.; Kang, T.; Jung, H. J.; Ro, C. U., Single‐particle characterization of indoor aerosol particles collected at an underground shopping area in Seoul, Korea. Indoor Air 2011, 21, (1), 12-24. 41. Kulkarni, P.; Baron, P. A.; Willeke, K., Aerosol measurement: principles, techniques, and applications. John Wiley & Sons: 2011.

486 487 488

ASSOCIATED CONTENT

489

Supporting Information

490

The Supporting Information is available, free of charge, on the ACS Publications website

491

at DOI:

492

Physical properties of the PVP solution with various MNP concentrations, pore size distributions

493

of the control and MNP-NF filters, size characteristics of test airborne metal oxide dust, particle

494

collection efficiency at specific dust sizes, SEM analysis of the dust-trapped MNP-NF filter,

495

filtration performance test of the MNP-NF filter with a range of magnet strengths, filter pressure

496

drop data, XRF data, EDS spectrum, XRD and RIR quantitative analysis of subway dust (PDF)

497 498

AUTHOR INFORMATION

499

Corresponding author

500

*E-mail: [email protected]. Telephone: +82-2-958-5718. Fax: +82-2-958-5805.

501

Author Contributions

502

†J.K.

503

Notes

504

The authors declare no competing financial interest.

505 506

ACKNOWLEDGMENTS

507

This research was supported by the KIST Institutional Program. This research was also

508

supported in part by the Ministry of Land, Infrastructure and Transport (17RTRP-B082486-04),

509

and the Ministry of Environment, Republic of Korea, as the “Public Technology Program based

510

on Environmental Policy (2016000160008)”.

and S.C.H. contributed equally to this work.

511 512 16 ACS Paragon Plus Environment

Page 16 of 26

Page 17 of 26


513 514

Table 1. Size characteristics of the control fiber and MNP-NFs.

515

a

516

b

517

c

518

d

Sample

Mean ± SDa (µm)

CVb

d50c (µm)

Control (nd= 128)

1.5± 0.25

0.167

1.47

5 wt% (n = 115)

1.6 ±.0.24

0.151

1.57

10 wt% (n = 123)

1.8±.0.22

0.122

1.78

20 wt% (n = 101)

2.2 ±.0.42

0.185

2.33

Mean diameter ± standard deviation of the fibers.

Coefficient of variation.

Cut-off fiber diameter at a cumulative number concentration of 50%. Number of samples.

519



520 521

Table 2. Magnetic properties of the control and MNP-NF filters.

522

a

523

b

524

c

Sample

Msa (emu/g)

Mrb (emu/g)

Hcc (Oe)

Control

0.38

0.02

140

5 wt%

3.76

0.29

200

10 wt%

5.84

0.34

200

20 wt%

10.60

0.80

200

Saturation magnetization. Remnant magnetization.

Coercivity.

525


Page 18 of 26

Page 19 of 26


526 527

Table 3. Filtration characteristics of the control and HEPA filters without a magnet and the MNP-

528

NF (20 wt%) filter with a 500-mT magnet. Sample

ηa (%)

Δp b (Pa)

qf c (/kPa)

Control (w/o magnet)

68.3 ± 1.01

26.8 ± 0.19

43 ± 0.1

MNP-NF filter (20 wt%, w/ magnet)

96.9 ± 3.57

17.1 ± 0.23

202 ± 0.7

98.9 ± 0.58

268.9 ± 5.52

17 ± 0.3

HEPAd

529

a

530

b

531

c

532

d

Particle collection efficiency on 300-nm particle size. Filter pressure drop at 3.4 cm/s of face air velocity.

Filter quality factor. 0.5 μm of thickness.

533 534



535

Figure Legends

536

FIGURE 1. Electrospinning system and filtration mechanism for removing metal oxide

537

dust using the MNP-NF filter. (a) Schematic diagram of the electrospinning

538

system for fabricating the MNP-NF filter. (b) Schematic diagram of the MNP-NF

539

filter, which can efficiently remove nano-sized metal oxide dust (red sphere) from

540

the atmosphere due to the magnetic force (yellow light) generated by the

541

magnetization of the MNPs in the filter. (c) The MNP-NF filter filtration mechanism

542

of the metal oxide dust.

543

FIGURE 2. Characteristics of the electrospun MNP-NFs. (a) TEM images, (b) EDS spectra

544

of the control fiber and MNP-NF with 20 wt% concentration, confirming the

545

existence of Fe3O4 in the MNP-NF. (c) XRD patterns of the Fe3O4 MNPs, control

546

fiber and MNP-NF (20 wt%). (d) SEM images and (e) size distributions of the control

547

fiber and MNP-NFs (5, 10, 20 wt%), with normalized cumulative curves.

548

FIGURE 3. Hysteresis loops of the control filter and MNP-NF filters (5, 10, 20 wt%).

549

FIGURE 4. Performance test of the MNP-NF filter using metal oxide test particles. (a)

550

Photographs of the filtration test system. (b) Size distribution and TEM image of the

551

test Fe2O3 airborne dust. (c) Collection efficiencies of the base mesh, control filter,

552

and MNP-NF filter (20 wt%) without magnets on the filters. (d) Collection efficiencies

553

of the control filter and MNP-NF filters (5, 10, 20 wt%) with a 500-mT magnet

554

attached to the filters. (e) Collection efficiency and pressure drop of the MNP-NF

555

filters (0(control), 5, 10, 20 wt%) under conditions using 300-nm diameter test dust

556

and a face velocity of 3.4 cm/s. (f) Filter quality factor (qf) of the filters.

557

FIGURE 5. Field test of the MNP-NF filter in a subway station in Seoul, Republic of Korea.

558

(a) Photographs of the subway tunnel and airborne dust generated in the tunnel. (b)

559

Composition of the chemical compounds found in the dust, analyzed by XRF. (c)

560

Size distribution and TEM image of the subway dust. (d) Subway dust collection

561

efficiency using the control filter and 20 wt% MNP-NF filter.

562


Page 20 of 26

Page 21 of 26

563


Figure 1 Stage

(a)

(b)

Magnetic nanoparticle incorporated nanofiber filter

Syringe pump

(c) Air streamline Fm

`

SS tip (+)

Camera High-voltage generator

MNP-NF

Magnetic flux density

Collector

564 565

Fig. 1. Electrospinning system and filtration mechanism for removing metal oxide dust

566

using the MNP-NF filter. (a) Schematic diagram of the electrospinning system for

567

fabricating the MNP-NF filter. (b) Schematic diagram of the MNP-NF filter, which can

568

efficiently remove nano-sized metal oxide dust (red sphere) from the atmosphere due to

569

the magnetic force (yellow light) generated by the magnetization of the MNPs in the filter.

570

(c) The MNP-NF filter filtration mechanism of the metal oxide dust.

571



Figure 2 (b)

(c)

Arbitrary unit (a.u.)

C

1 μm

5 μm

MNP-NF (20 w%)

Fe

O

Fe3O4

MNP-NF (20 w%)

Fe Fe C

Control

MNP-NF (20 w%)

Control

O

(d)

0

1 μm

5 μm

Control

2

MNP-NF (5 w%)

i

4

6

10 20

8

40

5 μm

iv

5 μm

5 μm

80

MNP-NF (20 w%)

MNP-NF (10 w%) iii

ii

60

2Theta (deg)

Energy (keV)

5 μm

(e) Normalized concen. (%)

25

Control 20 15

10 w%

5 w% Cumul. Normalized Concen.

100

20 w% 75

d50

50

10 25

5 0 0.5 1.0 1.5 2.0 2.5 3.0 0.5 1.0 1.5 2.0 2.5 3.0 0.5 1.0 1.5 2.0 2.5 3.0

Fiber diameter (µm)

Cumul. normalized concen. (%)

(a) Control

Arbitary unit (a.u.)

572

Page 22 of 26

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

573 574

Fig. 2. Characteristics of the electrospun MNP-NFs. (a) TEM images, (b) EDS spectra of the

575

control fiber and MNP-NF with 20 wt% concentration, confirming the existence of Fe3O4

576

in the MNP-NF. (c) XRD patterns of the Fe3O4 MNPs, control fiber and MNP-NF (20

577

wt%). (d) SEM images and (e) size distributions of the control fiber and MNP-NFs (5, 10,

578

20 wt%), with normalized cumulative curves.

579


Page 23 of 26

580


Figure 3 (a)

Magnetization (emu/g)

10

5

MNP-NF filter 20 wt% 10 wt% 5 wt% Control

0 2 1

(b)

-5 0 -1

-10 -20000

581 582

-10000

0

-2 -300 -200 -100 0

100 200 300

10000

20000

Magnetic field (Oe)

Fig. 3. Hysteresis loops of the control filter and MNP-NF filters (5, 10, 20 wt%).

583



Figure 4 (b)

Aerosol Outlet

ii

iii Filter

1.0

100

40 nm

0.8 0.6

Metal oxide dust (Fe2O3)

0.4 0.2

Magnet

0.0 10

Back side Increase of η as increasing concentration of MNPs

(d) Collection efficiency,  (%)

100 80 60

MNP-NF filter 20 wt% 10 wt% 5 wt% Control

40 20

Control w/o magnet Base only

0 10

100

Particle diameter (nm)

100

1000

60 MNP-NF filter (20 wt%) Control filter Base only

40 20 0 10


1000

Collection efficiency of 300 nm (%)

Front side

80

(e)

100

1000


(f)250

3.5

100 3.0

90 80

2.5

70 2.0 60 50

1.5 0

5

10

20

Concentration (wt%)

Filter quality (kPa-1)

Aerosol Inlet

Normalized concentration

MNP-NF filter

(c) Collection efficiency (%)

(a) i

Pressure drop (mmH2O)

584

Page 24 of 26

202±0.7

200 150

120±0.3

100 50

84±0.5 49±0.1

0 0

5

10

20

Concentration (wt%)

585 586

Fig. 4. Performance test of the MNP-NF filter using metal oxide test particles. (a)

587

Photographs of the filtration test system. (b) Size distribution and TEM image of the test

588

Fe2O3 airborne dust. (c) Collection efficiencies of the base mesh, control filter, and MNP-

589

NF filter (20 wt%) without magnets on the filters. (d) Collection efficiencies of the control

590

filter and MNP-NF filters (5, 10, 20 wt%) with a 500-mT magnet attached to the filters.

591

(e) Collection efficiency and pressure drop of the MNP-NF filters (0(control), 5, 10, 20

592

wt%) under conditions using 300-nm diameter test dust and a face velocity of 3.4 cm s-

593

1

. (f) Filter quality factor (qf) of the filters.

594


Page 25 of 26

Figure 5 (a)

Subway dust

Experimental system

(b)

10 mm

Carbon comp. (10.0%)

Others (8.6%)

SO3 (1.3%)

CaO (3.9%) Al2O3 (4.0%) SiO2 (7.3%)

(d)

12

Fe2O3 (66.2%)

100

Collection efficiency (%)

Subway tunnel

(c) Particle concentration, dN/dLogDp (x103 #/cm3)

595


9 100 nm

6

3

0 1

10

100

1000

Particle diameter, Dp (nm)

+Δ15.1% @ 300 nm

80 60

Increase of particle collection efficiency

40 20 0 10

MNP-NF filter (20 wt%) Control filter

100

1000


596 597

Fig. 5. Field test of the MNP-NF filter in a subway station in Seoul, Republic of Korea. (a)

598

Photographs of the subway tunnel and airborne dust generated in the tunnel. (b)

599

Composition of the chemical compounds found in the dust, analyzed by XRF. (c) Size

600

distribution and TEM image of the subway dust. (d) Subway dust collection efficiency

601

using the control filter and 20 wt% MNP-NF filter.

602



51x46mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 26 of 26

Electrospun Magnetic Nanoparticle-Decorated Nanofiber Filter and Its

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