key: cord-0837678-aokpuanl
authors: de Almeida, Daniela Sanches; Martins, Leila Droprinchinski; Muniz, Edvani Curti; Rudke, Anderson Paulo; Squizzato, Rafaela; Beal, Alexandra; de Souza, Paulo Ricardo; Freire Bonfim, Daniela Patrícia; Aguiar, Mônica Lopes; Gimenes, Marcelino Luiz
title: Biodegradable CA/CPB electrospun nanofibers for efficient retention of airborne nanoparticles
date: 2020-07-17
journal: Process Saf Environ Prot
DOI: 10.1016/j.psep.2020.07.024
sha: 7d915f74773edf41ecef150282e90bc3c8f6c4b5
doc_id: 837678
cord_uid: aokpuanl

The increase of the industrialization process brought the growth of pollutant emissions into the atmosphere. At the same time, the demand for advances in aerosol filtration is evolving towards more sustainable technologies. Electrospinning is gaining notoriety, once it enables to produce polymeric nanofibers with different additives and also the obtaining of small pore sizes and fiber diameters; desirable features for air filtration materials. Therefore, this work aims to evaluate the filtration performance of cellulose acetate (CA) nanofibers and cationic surfactant cetylpyridinium bromide (CPB) produced by electrospinning technique for retention of aerosol nanoparticles. The pressure drop and collection efficiency measurements of sodium chloride (NaCl) aerosol particles (diameters from 7 to 300 nm) were performed using Scanning Mobility Particle Sizer (SMPS). The average diameter of the electrospun nanofibers used was 239 nm, ranging from 113 to 398 nm. Experimental results indicated that the nanofibers showed good permeability (10(-9) cm(2)) and high-efficiency filtration for aerosol nanoparticles (about 100%), which can include black carbon (BC) and the new coronavirus. The pressure drop was 1.8 kPa at 1.6 cm s(-1), which is similar to reported for some high-efficiency nanofiber filters. In addition, it also retains BC particles present in air, which was about 90 % for 375 nm and about 60% for the 880 nm wavelength. Finally, this research provided information for future designs of indoor air filters and filter media for facial masks with renewable, non-toxic biodegradable, and potential antibacterial characteristics.

et al., 2019) and circulatory diseases (Ardiles et al., 2017; Maté et al., 2010) , as well as mental disorders (Almendra et al., 2019; Peng et al., 2017; Song et al., 2018) .

During breathing, the human respiratory system is capable to prevent the entrance of most particulate matter. However, for the small ones, especially PM2.5, around 96% can penetrate the respiratory tract (Löndahl et al., 2007; Xing et al., 2016) . Furthermore, studies have reported that small particles have larger surface areas, being capable of carrying various toxic compounds (Dieme et al., 2012; Kendall et al., 2004; Senlin et al., 2008; Xing et al., 2016) . Therefore, controlling the concentration of these particles is important, mainly in indoor environments (Kelly and Fussell, 2019; Li et al., 2017) , since it causes millions of premature deaths worldwide (Chowdhury and Dey, 2016; Fang et al., 2016; Giannadaki et al., 2016) .

Black Carbon (BC) is an atmospheric pollutant and a type of carbonaceous atmospheric particulate material formed in the combustion process of carbon-containing materials, mainly by burning fossil fuels (coal, diesel, gasoline) and biomass, whose fresh BC particulate ranged from 50 -80 nm of diameter (Bond et al., 2013; Colbeck, 2008; Wallace and Hobbs, 2005) . It is also a short-lived climate pollutant that affects the thermal balance of the planet's surface (Bond et al., 2013) and also human health, impacting the circulatory and respiratory systems (Galdos et al., 2013; Janssen et al., 2011; Segalin et al., 2016) and, as recently verified, the nervous system (Maher et al., 2016; Pun et al., 2019) .

The recent pandemic due to COVID-19, causing more than 550 thouhsand deaths in the world, is due to the transmission of droplets expelled during the sneeze, cough or even conversation of a person contamined by the virus (virus size ranging from 50 to 200 nm, according to Chen et al., 2020; Xu et al., 2020) . Thus, efficient filters for retaining not only the droplets but also the virus are in fact necessary because the virus can remain suspended in the air for 3 hours or more (Doremalen et al., 2020) . The High-Efficiency Particulate Air (HEPA) filters are widely used in indoor air pollution control by air conditioning system and are generally composed of glass fibers (Thomas et al., 1999) . Its main features are the large pressure drop, which occurs during the operation, making the energy consumption increase. (Thomas et al., 1999; Zaatari et al., 2014) . The most common air filters are composed of porous materials in a solid substrate, with very small pore sizes but also low porosity (<30%) (Hinds, 1999; Liu et al., 2017) . Recent studies denote the application of nanofibrous mats (Lv et al., 2018; Zhao et al., 2016; J o u r n a l P r e -p r o o f 4 2017), as they usually have high porosity (>70%), large surface-to-volume ratio, are thin and have higher air filtration efficiency .

Electrospinning is a very efficient and low cost technique to produce nanofibers for several applications. It allows the use of different polymeric solutions and additives to improve and/or assign any characteristic to the mats (Huang et al., 2003) . For example, the fiber diameter of the material obtained by this technique could be controlled (ranging from 10-1000 nanometers), once the process parameters (flow rate, voltage, polymer concentration, collector distance) can be regulated (Zhao et al., 2016) . Currently, the use of nanofibers obtained by this technique has been intensively studied in indoor air filtration (Bian et al., 2020; Robert and Nallathambi, 2020) , and the authors suggest that reducing the fiber diameter contributes to improve filtration performance (Xia et al., 2018) . Future perspectives imply the use of nonprejudicial solvents, as well as bio-based polymers to develop clean and safe technologies (Bortolassi et al., 2019; Matulevicius et al., 2016; Min et al., 2018; Zhu et al., 2019) .

Cellulose acetate (CA) is a semi-synthetic and biodegradable polymer (Buchanan et al., 1993; Puls et al., 2011) widely used to produce nanofibers by electrospinning (Anitha et al., 2013; Nicosia et al., 2016; Sultana and Zainal, 2016; Wutticharoenmongkol et al., 2019) . Due to the common presence of beads in their fibers, thermal and mechanical resistance properties are compromised (Kendouli et al., 2014) . Thus, the use of cationic surfactants as an additive proves to be an effective strategy for reducing beads, improving those properties and also reducing fiber diameters (Abutaleb et al., 2017; de Almeida et al., 2020; Lin et al., 2004; Wang et al., 2010) . Besides, it can attribute biocide characteristics to the nanofibers, such us, the cationic surfactant cetylpyridinium bromide (CPB) that is an antibacterial agent widely used in the pharmaceutical industry (Cole et al., 2011; Lukáč et al., 2013; Malek and Ramli, 2015; Wu et al., 2020) .

Thus, this work aims to produce biodegradable and non-toxic nanofibers of natural-based cellulose acetate (CA) polymer modified with cationic cetylpyridinium bromide (CPB) surfactant by electrospinning technique, and evaluate their performance in filtering airborne nanoparticles. Additionally, nanofiber retention efficiencies for atmospheric PM2.5 and BC were analyzed.

The nanofibers were produced using cellulose acetate (CA) as polymer (average molecular weight 30 kDa) and cationic surfactant cetylpyridinium bromide (CPB), both purchased from Sigma-Aldrich, USA. The solvents for preparing the polymeric solution were a mixture of acetic acid (Synth, São Paulo, Brazil) and distilled water (3:1 v/v). Pure aqueous solution of 1 g L -1 NaCl (CHEMIS) was used to generate nanoparticles (density 2.17 g cm -3 ) necessary for filter performance tests.

The nanofibers were made using 21% (w/v) of CA and 0.5 % (w/v) of the surfactant CPB (de Almeida et al., 2020) in a home-made electrospinning apparatus, composed of a high voltage power supply, a syringe pump (WPI -SP100I, Sarasota-FL, USA), and static aluminum collector in circular format. The optimal concentrations were achieved based in findings from a previous work made in our research group (de Almeida et al., 2020) .

The flow rate was maintained at 0.7 mL h -1 , the voltage at 18 kV and the distance between the needle and the collector was 10 cm. The electrospinning process was performed for 3 h to produce thin films to be used as filter media. Subsequently, the nanofibers were immediately conditioned in an exhaust system to evaporate the solvent traces remaining in the mats.

The morphology of the nanofibers produced was characterized by scanning electronic microscopy (SEM), using a Quanta 250 (Philip-FEI), equipped with energy dispersive spectroscopy (EDS); the samples were dispersed on double-sided tape coated with a thin layer of gold (30 nm). The average diameter was measured using the Software Size Meter  1.1. The filtration tests were performed in circular specimens with 47 mm of diameter and about 500 nm of thickness at constant surface speed of 1.59 cm s -1 , a flow rate of 500 mL min -1 , and 5.3 cm 2 of filtration area. Since the sodium chloride (NaCl) nanoparticles were generated from an aqueous solution (concentration of 1 g L -1 ), they passed downstream and upstream and the particle number size distribution was measured, in order to obtain the nanofiber efficiency using particle analyzer by electric mobility. This process was repeated two times to obtain an average efficiency and the standard deviation. The filtration efficiency was calculated based on particles concentration before and after passing through the filter media.

The permeability experiments were carried out in duplicate, varying the flow rate from 100 to 900 mL min -1 . The pressure drop was measured at each flow rate interval with a digital manometer (VelociCalc Model 3A-181WP09, TSI), which was coupled to the test system, as shown in the scheme of Figure 1 . The permeability constant (k1) of the filter was determined theoretically, in order to evaluate the resistance of the filter media to air flow using Equation 1 (Innocentini et al., 2006) :

where L represents the thickness of the filter, μ is the viscosity of the fluid (air), k1 and k2

are permeability constants of the filter, ρg represents the density of the gas (air) and vs is the surface velocity. ΔP is the pressure drop: the difference between the input and output pressures during the passage of the air containing particles through the filter.

The experiments were conducted at low filtration velocity; therefore, the second term of Equation 1 can be neglected; and the Equation 2 (Darcy's Law) can be applied:

The empirical porosity () of the filter media was calculated as proposed by Ergum (1952), using Equation 3:

where dp is the average diameter of the NaCl particles.

In order to enhance the application of nanofibers as an air filter for particle retention, environmental tests were performed using the mats as a filter media to collect the PM2. 

The nanofibers produced by electrospinning presented a normal diameter distribution (200 to 300 nm), as shown in Figure 3 , and according to the results found in de Almeida et al. (2020). Figure 3 shows the SEM image of mats and their frequency diameter distribution of nanofibers. A regular and uniform morphology was obtained. Studies regarding air filtration materials indicate that small fiber diameters are useful to improve filtration performance (Hosseini and Tafreshi, 2010; Shou et al., 2014; Thomas et al., 1999) , especially smaller aerosol particles, such as BC and virus. The nonalignment of fibers is also a desirable characteristic for air filters as obtained in the CA/CPB nanofibers (Figure 3 a) . Table 1 . Barhate et al. (2006) produced PAN nanofibers by electrospinning and tested them for airborne particle filtration. In their work, the Darcy and Ergun equations were used for calculating the permeability and porosity of nanofibers, respectively. The value obtained for permeability was of the same order of magnitude as that obtained in this work for CA/CPB nanofibers (10 -11 m²), and the porosity (96%) was slightly lower than those found in this work (98%).

Another research, conducted by Bortolassi et al. (2019) , used the same module for air filtration tests for developing electrospun PAN nanofibers, but with the addition of silver nanoparticles, in order to give bactericidal characteristics to this particulate filter. They J o u r n a l P r e -p r o o f used these same equations, and found permeability and porosity values in the same order of magnitude as those found in the present work, ranging from 95 to 97%, depending on the amount of silver nanoparticles added. The permeability constant are higher than those in glass fiber HEPA and ULPA (Ultra Low Penetration Air Filter) filters, which are on the order of 10 -12 m 2 (Yun et al., 2007) . The operation life of the CA/CPB nanofibers should be about 6 months to 1 year, like a HEPA filters composed of glass fiber and borosilicate microfibers which presents similar permeability (Bortolassi et al., 2017) . Figure 5 shows the particle number size distribution of NaCl that was generated by the atomizer, in order to determine the filtration efficiency. A narrow distribution was obtained with an average diameter of approximately 80 nm and a median of 45 nm.

Figure 5 -Particle number size distribution of NaCl generated by the atomizer.

An important advantage of such mats is the high-efficiency filtration for nano size particles, which is almost 100%, considering the particle number size distribution generated of NaCl (7 to 300 nm). Besides, the CA modification performed by the addition of the surfactant improved the thermal and mechanical resistances (Abutaleb et al., 2017) J o u r n a l P r e -p r o o f without loosing their biodegradability and water resistance properties. Further, the CPB addition should assign biocide characteristics to the nanofibers. Malek and Ramli (2015), for example, modified the kaolinite with CPB, and they found an antibacterial activity for CPB concentrations ≥ 1.49 mmol/L, which is a lower value than it used in this work.

Moreover, CA is a naturally sourced (renewable) polymer (Goetz et al., 2016) . Other relevant aspect is that this nanofiber is also capable of retaining the 2019 coronavirus, once the virus is about 50 to 200 nm of size Xu et al., 2020) and has around 3 h of stability in aerosol (Doremalen et al., 2020) .

High-efficiency filtration was obtained even for smaller particles (99.99 %) with particle size diameter lower than 10 nm. Xia et al. (2018) presented particle retention efficiency data, with sizes between 200 and 500 nm for several electrospun nanofibers and most of them were above 95%. The efficiency results obtained by Zhang et al. (2009) using electrospun Nylon 6 nanofibers ranged from 95 to 98% for particles smaller than 50 nm, however, it is a non-degradable synthetic polymer. Liu et al. (2015b) found efficiency of 99.92% for electrospun PAN nanofibers (P = 208 Pa), but they used NaCl larger nanoparticles, ranging from 300 to 500 nm. In the work of Yin et al. (2013) electrospun polyamide 6 membranes were produced, and an efficiency of about 80% was obtained for particles with 300 nm in size and using the membranes with a substrate. As pointed out, other works have achieved high-efficiency filtration in mats produced through electrospinning technique. However, it was made using larger particle sizes and mainly non-natural and biodegradable mats. This is an important issue considering the use of nanofiber as filter media, e.g. in air conditioning systems and particularly in facial masks, since it can prevent environmental impacts associated with the disposal of these materials (Das et al., 2020) .

As mentioned, the performance of CA/CPB nanofiber for atmospheric PM2.5 and BC retention was evaluated by comparing the mass retained in the nanofiber with that of the commercial quartz filter, as well as measuring atmospheric BC simultaneously with and without the CA/CPB filter. From the environmental tests performed, it was possible to obtain the mass of PM2.5, collected in the CA/CPB filter and commercial quartz filters, as shown in Table 2 .

J o u r n a l P r e -p r o o f The variability observed in the mass of the filters (Table 2) is due to the change of the meteorological conditions and emission sources between the sampling days of each filter.

This influence was pronounced for filter 4, which presented the lowest mass in both filters associated with the occurrence of rainfall in the sampling period for this filter. In addition, humidity probably exerted a higher influence on the mass of the quartz filter compared to the CA/CPB filter. The CA/CPB nanofiber filters collected up to twice as much particulate mass as quartz filters, which had 6 kPa pressure drop in 0.4 m/s face velocity (Zíková et al., 2015) . This behavior is possibly due to the reduced pore size of CA/CPB nanofiber (about 10 nm), when compared to the commercial filter, which presents 2.2 µm according to the manufacturer's technical information. A PM2.5 impactor was used which probably collected larger particles than these pore sizes across the filter cake, and justifies the mass found in the quartz filters. Moreover, the SEM images of CA/CPB nanofibers after PM2.5 sampling have shown very small particles across the nanofibers, as shown in Although, the contribution of ultrafine particles (<0.1 µm) in relation to the total mass retained is negligible, this constitutes an important fraction of the particle number, which is also recognized for crossing the physical barriers of human body and for generating various health damages.

Concerning BC measurements, the concentration data measured by both equipments Table 3 . decreased. Furthermore, as already known, filters membrane are not exactly equal among themselves, bringing a significant degree of heterogeneity, even with those commercially produced, which may partially justify the variability found in the filtration efficiency.

Further tests under controlled conditions should be performed. However, it was possible to observe that the CA/CPB nanofiber filter was able to retain BC particles and, more efficiently, those carbonaceous particles generated by biomass burning.

The products obtained by electrospinning technique, specially for air filter market, is growing quickly and various companies already supply it. However, some issues still need to be improved when scaling up the production of these nanofibers, such as low productivity and quality control (Persano et al., 2013; Thenmozhi et al., 2017; Vass et al., 2019) . On the raw material, CA is a commercial product as well as the acetic acid, and are abundant raw materials to produce the nanofibers. The estimated cost to produce the filter (1 m 2 ) is around 2 (two) dollars (for laboratory-scale) and should be reduced by increasing to industrial-scale. Therefore, further studies are needed to best transfer these technologies into industrial applications.

The use of this material as an air filter media is described in a Brazilian patent, in processes of registration at the National Institute of Industrial Property (INPI), Brazil.

In this article, electrospun CA/CPB nanofibers were produced and their application as filtration media was investigated. Good filtration performance was obtained for NaCl nanoparticles with 7 to 299 nm of size, with almost 100% efficiency and 1.8 kPa pressure drop.

The permeability and porosity were similar to those obtained by the same technique and were about 10 -9 cm² and 98%, respectively.

The environmental measurements tests showed that CA/CPB nanofiber was also able to retain atmospheric PM2.5, and with higher efficiency when compared to commercial quartz fiber-based filters. In addition, the CA/CPB nanofiber filter also retained BC particles with high efficiencies (about 90 % for 375 nm and about 60% for the 880 nm wavelength), and higher for carbonaceous particles originating from biomass burning, which represents the smallest fraction of the PM2.5.

From the results obtained of this research, it is possible to conclude that CA/CPB could be applied for capturing particles smaller than 300 nm with high-efficiency filtration including the 2019 novel coronavirus. Finally, this research provided information for future designs of indoor air filter materials in air conditioning equipment, particularly in hospitals, once the pressure drop could be decreased as the CA/CPB nanofibers could be used as a thin layer associated with a thin substrate. Another possible use of it is as a filter media for facial masks that has a short-therm use, especially due to its potential antibacterial properties. Thus, the use of biodegradable material in its composition could reduce the environmental impacts by the disposal of a large amount of these waste.

The authors declare no conflict of interest.

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The authors would like to acknowledge the National Council for Scientific and Technological Development (Conselho Nacional de Desenvolvimento Científico e Tecnológico -CNPq), process No. 306862/2018-2. This study was financed in part by the Coordination for the Improvement of Higher Education Personnel (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior -CAPES), finance Code 001.

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study (thesis).