Hierarchically Structured All-biomass Air Filters with High Filtration

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A Hierarchically Structured All-biomass Air Filter with High Filtration Efficiency and Low Air Pressure Drop Based on Pickering Emulsion xin Fan, Yu Wang, Wei-Hong Zhong, and Siyi Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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ACS Applied Materials & Interfaces

A Hierarchically Structured All-biomass Air Filter with High Filtration Efficiency and Low Air Pressure Drop Based on Pickering Emulsion Xin Fan†,‡,§, Yu Wang*‡, Wei-Hong Zhong*‡, Siyi Pan*†,§ † College

of Food Science and Technology, Huazhong Agricultural University, No. 1 Shizishan Road,

Wuhan, Hubei 430070, PR China ‡ School

of Mechanical and Materials Engineering, Washington State University, 100 Dairy Road,

Pullman, WA 99164, USA § Key

Laboratory of Environment Correlative Dietology (Huazhong Agricultural University), Ministry

of Education, No. 1 Shizishan Road, Wuhan, Hubei 430070, PR China

Corresponding Authors E-mail: [email protected]

E-mail: [email protected] 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

E-mail: [email protected]

Abstract Although a high efficiency air filter can be achieved from electrospun nanofabrics, it has been challenging to reduce the pressure drop, increase the filtration capacity and improve the production rate of the electrospinning process. Here, we report a hierarchically structured allbiomass air filter with high filtration efficiency and low air pressure drop based on applying pickering emulsions to generate protein-functionalized nanostructures. In specific, the air filter consists of cellulose nanofibers (CNF)-zein nanoparticles as active fillers prepared from pickering emulsions and porous structures of microfibers as the frame from wood pulp. The zein-protein coated nanoparticles, CNF-zein, have multiple ways of contributions to improving removal efficiency for the filters. Firstly, the exposed functional groups of zeinprotein help to trap air pollutants including toxic gasseous molecules via interaction mechanisms. Secondly, the nanoparticles with the high surface area promote the capture capability for small particulate pollutants. Meanwhile, the long-micron wood pulp fibers forming a frame with large pores significantly reduce the pressure drop. Via adjusting the component ratios of in the pickering emulsion, an optimized air filter with the high efficiency for capturing both types of pollutants: particulate matter (PM) and chemical gasses such as HCHO and CO, at extramely low normalized pressure drop, i.e. approximately 1/170 of the zein-based nano air filter by electrospinning. This study initiates a cost-effective strategy for forming a hierarchical nano- and micro-structure enabling high efficiency of capturing particulate pollutants of a wide size range and more species. More significantly, this is the first study of applying pickering emulsion as a critical approach with integration of bio- and nano-technology to make high-performance, green air filters.

1. Introduction Air pollution causes many problems, including the health hazard to human and wildlife, and damage to environment.1-2 Air pollutants are complex and include uneven components, such as particulate matter (PM), chemical gases and biological materials.3 Unknown or unexpected combinations of pollutants make more harmful.4-5 PM is composed by extremely small particles and liquid droplets, including nitrates (NO3-), mineral dusts, sulfates (SO22- and SO42-), black carbon,3 etc., which can be categorized into different levels based on the 2


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particulate size, such as PM2.5 that refers to the particulate matter with size below 2.5 μm and PM2.5−10 with sizes in the range of 2.5−10 μm.4 Long-term exposure in a high level of air pollutant environment, people may have irreversible health damage.2 Therefore, the demand for multi-functional air filters with high-efficiency for simultaneously capturing PM of different sizes and chemical gases of multiple species, as well as low pressure drop is becoming increasingly stronger. To realize the goal, involving nano-structural materials including nanofabrics into air filters have been explored and implies promising for fabricating next generation air filters. In the past several years, nanofabrics made via electrospinning have been employed to further improve the PM removal efficiency due to the high specific surface area and energy. Tremendous efforts on the preparation of nanofibers via electrospinning polymers and natural protein, such as polypropylene (PP), polyacrylonitrile (PAN),4, 6 soy protein,7-8 whey protein,9 silk10-11 and zein.12 Importantly, the protein-based nanofibers fabricated by electrospinning presented the combination of the advantages of the nanofibers with high surface area and proteins that have huge amount of active functional groups enabling efficient capability of capturing gaseous pollutants and small particles from the air3, 7, 13-15. Although the filtration efficiency of the nanofabrics by electrospinning is significantly improved, deterioration of airflow performance is almost inevitable due to the compact nanofabrics. As a result, the thickness or capacity of the nanofabrics must be decreased to ensure low pressure drop of the air filters. To alleviate this deterioration of airflow resistance property, novel ribbon fibers, 13 extremely fine fibers10, 16 and thin filters4, 17 via electrospinning were fabricated. In the studies of ribbon fibers and extremely fine fibers, it is believed that they are promising for preparing air filters with a good balance between removal efficiency and pressure drop. This balance mainly benefits from the unique fabric morphology. For instance, the porous structure of an air filter was controlled by the ribbon-like fibers with self-curving behavior via electrospinning the metastable protein solution;13 the diameter of the extremely fine fibers is lower than 66 nm and the pressure drop of air filers is reduced due to the “slip effect”.18 Although those air filters from the novel ribbon fibers and extremely fine fibers showed highfiltration efficiency and low pressure drop, their fabrication processes are costly and involve complicated processes. Regarding the thin filter, it hardly satisfies the requirement for highcapacity as it has a short life span. Therefore, for air filter fabrics from electrospinning, their practical applications have been limited due to the single nano structures of the materials that have the trade-off between high-efficiency and low pressure drop or high capacity, in addition 3



to the issues relevant to the cost/low production rate and complicated processes. To address these issues, the design of hierarchical structures for advanced air filters via employing active materials has been demonstrated to be greatly promising.19-20 In particular, hierarchical structures that involve both nano- and micro-scaled materials may provide an effective pathway for airflow and a high removal efficiency for capturing air pollutants of multiple species and wide range sizes. Recently, our previous studies have demonstrated that an air filter with hierarchical structures has an extremely low normalized pressure drop.19 However, the removal efficiency for PM, in particular for small particles, is not yet satifactory due to a shortage of proteinfunctionalized nanoparticles.

19

To boost the filtration efficiency without increasing pressure

drop can be realized via the preparation of pickering emulsions for controlling the microstructures, which is proposed and conducted by the authors in this study. Pickering emulsions are fluid systems, in which liquid droplets (oil or water phase) are dispersed into another immiscible liquid (water or oil phase), which are physically stabilized by solid colloidal particles,21-22 such as carbohydrates,23-24 proteins,25-26 lipids27 and flavones et.al.28 Compared with traditional emulsions, particles in pickering emulsions are more stable.29 In addition to the excellent stability of the particles, reported studies had demonstrated that extremely abundant and biodegradable protein, such as SPI,30 WPC,31-32 zein25, 33 et.al, can be successfully applied in pickering emulsions. Therefore, pickering emulsion possesses better sustainability, wildly range of applications and high resistance to coalescence.34 Nowadays, applications of pickering emulsions are primarily limited to such industry areas as food, drugs and cosmetics. Thus far, there has not been any reported study on applying pickering emulsions for preparation of air filters. In this study, we report creating a heirarchical structure air filter based on applying pickering emulsion approach to generate plenty of protein-functionalized nanoparticles. Incorporating the CNF-zein nanoparticles into the porous-structure microfibers from wood pulp as the frame, effectively improve filtration efficiency while maintaining a low level air pressure drop. It is known that zein possesses tremendous functional groups, but most of the functional groups are wrapped inside the protein via intermolecular forces. As illustrated in Figure 1a, the pristine zein protein with four levels of structures bonded by strong interactions, such as hydrophobic interaction, disulfide and hydrogen bonding, etc. Hence, the denaturation is necessary to destroy the three-dimensional entangled structure of the protein, thus resulting 4


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in functional groups exposure. The denaturation procedures of zein are completed via using 80wt% AA/DI solvent.19 The secondary, tertiary and quaternary structures of zein molecules become unstable and unfolded, resulting in plenty of functional groups exposure through the denaturation process. It has been known that the denatured zein can interact with cellulose nanofibrils (CNF) via electrostatic interactions.19 By adding wood pulp to the different E-Z samples, one can find that the uniform white emulsion E-Z15 turns into the solution with EZ15@WP suspended at the top, as shown in Figure 1b. After freeze-drying the mixtures, air filters with a hierarchically combined structure of the CNF-zein nanoparticles and wood pulp micro-fabrics are obtained. The protein-based nanoparticles from pickering emulsions are demonstrated to significantly enhance the filtration efficiency for the hierachical filters. Via adjusting on the component ratios in the pickering emulsion, an optimized air filter with low pressure drop and high PM removal efficiency, in particular, for small particles, was achieved. More significantly, the filter has a good performance of capturing toxic chemicals, such as 88.30% efficiency for HCHO and 60.71% for CO. In addition, the microfibers from wood pulp (WP) employed as the structure frame effectively provide porous structures, leading to the normalized pressure drop for the optimized air filter being approximately 1/170 of the zein-based nano air filters by electrospinning.35 As a consequence, combining the proteinbased pickering emulsion and wood pulp represents an effective method for achieving the unique protein/cellulose air filter with high filtration efficiency, high capacity and low pressure drop. It is noted that these three raw materials are derived from abundant natural resources.36-37

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Figure 1. Schematic illustration of fabrication (a) zein-based pickering emulsion (E-Z) and (b) the air filter (F-Z).

2. Experimental section Materials. Zein protein (95% purity) was obtained from Sigma-Aldrich Co., Ltd. Wood pulp (WP) was obtained from Shanghai Yingjia Industrial Development Co., Ltd. The welldispersed Cellulose nanofibrils (CNF) suspension was purchased from Bioplus Co., Ltd. The diameter and length of CNF are about 35 nm and 100 nm, respectively. Acetic acid was purchased from J.T. Baker. Emulsions preparation and Characterization. Preparation. The zein protein solution was prepared in a mixture solvent of distilled water and acetic acid with a weight ratio of 20:80 at 90 °C for 2 h. A homogeneous yellow denatured zein solution with a solid content of 5% was achieved. 1 g CNF with a solid content of 3% was added into 1 mL, 2 mL, 3 mL and 4 mL 5 wt% zein protein solution and stirred 3 h at room temperature, then, added 9 mL, 8 mL, 7 mL and 6 mL DI water and sonicated, alternating 3 s sonication with a 3 s standby for 20 s. The emulsions with different zein concentration (5 g/L, 10 g/L, 15 g/L and 20 g/L) were obtained, and named as E-Z5, E-Z10, 6


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E-Z15 and E-Z20, respectively. The control sample E-ZS (without CNF) was prepared using the same process as introduced above. Characterization. The zeta potentials and particle sizes of emulsions and wood pulp were determined by dynamic laser light scattering (Zetasizer Nano, Malvern Instruments). The contact angle of emulsion droplets and membrane were determined by the OCA 15 plus instrument and SCA 20 software. The emulsion droplets were investigated by microscope (Leica camera w/Leitz & Fluorescence stereo microscopes). Air filers preparation and Characterization. Preparation. Wood pulp with the mass of 0.9 g added into E-Z5, E-Z10, E-Z15, E-Z20 and E-ZS, respectively. The mixtures were further stirred, stand and dried by freezing drier. Finally, the air filters were obtained, and named as F-Z5, F-Z10, F-Z15, F-Z20 and F-ZS, respectively. Characterization. The interaction between emulsions and wood pulp were investigated by a Fourier transform infrared (FTIR, Thermofisher iS10) spectrophotometer, Energy dispersive X-ray analysis (SEM Tescan Vega3 + EDAX) and TGA (TA SDT Q600). The morphological characteristics of the air filters were investigated by scanning electron microscopy (SEM, FEI SEM142 Quanta 200F). Air filtration testing. The PM with particle sizes ranging from 0.01 to 10 μm and toxic gases (HCHO and CO) were produced by burning joss sticks. The concentrations of PM and toxic chemicals (HCHO and CO) were diluted in a glove box to the level which can be measured by a particle counter (CEM, DT-9881) with PM sensor and chemical sensors for HCHO and CO, please see Supporting Information Figure S1. The airflow speed was fixed to be 4 cm s-1 to test the filtration performance and the pressure drop by a manometer (UEi, EM201-B). Airfiltration testing was performed with a circular air filter sample with a diameter of 37 mm, which was placed in a homemade sample holder.38 The removal efficiency (η) of pollutants (PM and toxic chemicals) was determined via eqn (1): η = (Cp-Cc)/Cp

(1)

where Cp and Cc are the air pollutant concentration before and after filtration testing. The areal density (AD) was determined via eqn (2): 7



AD = m/S

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(2)

Where m and S are the weight and square meter of the air filter. The normalized pressure drop (NPD) as the pressure drop per unit mass and was determined via eqn (3): NPD = ΔP /m

(3)

where ΔP is the pressure drop with units of Pa, and m is the mass with units of g.13, 19 The quality factor (QF) was determined via eqn (4): QF = -ln(1-η)/∆P

(4)

Where ΔP is the pressure drop of the air filter. All measurements were carried out in triplicate. Values given in the figures are the means of triplicates, and error bar indicate the standard deviation. SPSS 17 (SPSS Inc., U.S.A) was used for analysis of variance (ANOVA). Differences among the mean values were determined using Duncan’s multiple range test. Values were considered significant when p< 0.05.

3. Results and Discussion Characterization of the pickering emulsions. The structures of cellulose nanofibrils (CNF) and hydrophobic amino acids of the protein are shown in Figure 2. CNF is hydrophilic due to the richness in hydroxy bonds. Zein is a kind of water-insoluble protein due to its plenty of hydrophobic acetic acids, such as Leucine and Phenylalanine, as shown in Figure 2b. The contact angles of the DI water on a surface of the CNF and CNF-zein films are shown in Figure 2, indicating the surface characteristic of the CNF-zein nanoparticles from pickering emulsions in comparison with that of CNF. The hydrophilic/hydrophobic nature of a surface can be characterized by its wettability.39 Generally, in a pickering emulsion there is a high correlation between the stabilization of the solid particles and the wettability of the particle surface. CNF shows a lower contact angle (around 74°), while CNF-zein presents a higher contact angle (ca.100°). Compared with the CNF, the wettability of the CNF-zein to DI water is decreased. Therefore, the CNF-zein particles in the pickering emulsion have a hydrophobic 8


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surface after emulsification. Therefore, the denatured zein is coated on the surface of CNF. Importantly, the nanoparticles with abundant functional groups on the surface that enables strong interactions with air pollutants, which will be discussed later.

Figure 2. Contact angle of DI water droplets to (a) the CNF film and (b) the E-Z15 film.

The effects of zein concentration on the size of CNF-zein particles are investigated by using an optical microscope and dynamic laser light scattering. The optical images showing the particle sizes for the pickering emulsion samples E-Z5, E-Z10, E-Z15 and E-Z20, are provided in Figure 3a-d; the distributions of their particle sizes are presented in Figure 3e. From Figure 3a and e, the CNF-zein particles in E-Z5 are big and with a broad size distribution from nm to μm. Because of low ratio of zein/CNF in E-Z5, zein cannot be coated on the surface of CNF evenly, reducing the stability of CNF suspension and resulting in precipitation. With increasing concentration of zein protein from 5 g/L (E-Z5) to 10 g/L (EZ10), the zein is more uniformly coated on the surface of CNF. However, the ratio of zein/CNF is not optimized, the CNF-zein particles in E-Z10 aggregate, shown as in Figure 3b. When the concentration of zein protein increased to 15 g/L (E-Z15), the ratio of zein/CNF reached the optimized proportion and the stable CNF-zein nanoparticles were formed, see Figure 3c. The zein protein was more uniformly surrounded the CNF. From Supporting information Figure S2, the zein protein carries plenty of positive charges (ca. 90 mV) at the pH value of 2. Therefore, the CNF-zein particles in E-Z15 are more dispersed due to 9



electrostatic repulsion between the CNF-zein nanoparticles. With continuously increased the concentration of protein to 20 g/L (E-Z20), the CNF-zein particles become unstable due to the high ratio of zein/CNF, as shown in Figure 3d. As a result, the sample E-Z15 implies the smallest particle size among all these samples. Figure 3f shows the phenomena of E-Z15 that is affected by different pH values of the continuous phase. One can find that the emulsion transfers into unstable precipitation phenomena when the pH value is higher than 4. It is because that the coalescence of the denatured zein occurs when the pH value is higher than its isoelectric point (about 4), resulting in the phase separation for the sample with pH values 5 or above (Figure 3f).34 In this work, 80 wt% acetic acid solution provides the experimental condition with a very low pH value of 3. Therefore, the CNF-zein nanoparticles are stably dispersed in the pickering emulsion E-Z15. Therefore, the pickering emulsion E-Z15 is used for the following study for preparation and investigation of the air filter.

10


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Figure 3. Stability of the emulsion. Optical micrographs, photographs and schematics of the emulsions with different concentrations of zein (a) E-Z5, (b) E-Z10, (c) E-Z15 and (d) E-Z20. (e) Particle size of E-Z5, E-Z10, E-Z15 and E-Z20 samples. (f) Phenomena of emulsions (EZ15) by adjusting the pH values of the continuous phase.

Interactions between CNF-zein nanoparticles and wood pulp. As a result of CNF-zein nanoparticles with protein on the surface, the functional groups of zein protein can strongly interact with other materials via electrostatic, hydrophobic and chemical interactions.41 It has been proven that protein can interact with polysaccharides through strong electrostatic interaction.19 The zein protein carries positive charges when the pH value is below its isoelectric point, meanwhile, CNF carries negative charges. After mixing, E-Z15 carries positive charges when the pH value below 5. It is because that the positive charges of zein are far larger than negative charges of CNF, shown as in Supporting information Figure S2. After mixing of E-Z15 and wood pulp, the uniform white emulsion E-Z15 turns into the solution with E-Z15@WP suspended at the top, which indicated that the CNF-zein nanoparticles coated on the surface of wood pulp. This is because there are opposite charges existing in the pickering emulsion E-Z15 (positive) and the wood pulp (negative), as shown in Figure 4a. This can be confirmed by the zeta potential data of them at different pH values, i.e. electrostatic interaction exists between the CNF-zein nanoparticles and wood pulp as displayed in Figure 4b. The protein emulsion carries positive charges (ca. 50 mV) under the low pH value (ca. 2), which is consistent with the statement that when pH value of protein is below its isoelectric point, the protein will be positively charged.19 In contrast, wood pulp carries negative charges (ca. -35mV) at pH value of 8 due to the electron-rich oxygen atoms of polar hydroxyl and ether groups.42 After they are mixed, the negative charges of wood pulp are neutralized by addition of the pickering emulsion E-Z15 with positive charges. Finally, the mixture (E-Z15@WP composites) carries positive charges (ca. 16 mV) due to the numbers of positive charges of emulsion and negative charges of wood pulp. Therefore, the nanoparticles in E-Z15 can be absorbed onto the wood pulp via the opposite charges, as illustrated in Figure 4a. As a result, wood pulp works as a charged frame to absorb the CNFzein nanoparticles from pickering emulsion. This conclusion is confirmed by the FTIR, TGA and EDAX results, which will be discussed in the following.

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Figure 4. Particles of pickering emulsion and wood pulp interaction studies. (a) Illustration and photographs of the interaction between E-Z15 and wood pulp. (b) Zeta potential of E-Z15 and wood pulp before and after mixed.

According to the above-mentioned analysis, it is known that there is a strong chargecharge interaction between CNF-zein nanoparticles and wood pulp. After freeze drying, the air filters (F-Z) with the functionalized nanoparticles and rational porous structure are obtained, which is illustrated in Figure 1a. Figure 5a shows the FTIR spectra of the F-Z5, FZ10, F-Z15, zein and wood pulp samples. For wood pulp, there are obvious peaks for -OH, CH2CH, -CH2 and -CO bonds at 3424, 2889, 1438, and 1047 cm-1, respectively. This result is consistent with the reported cellulose characteristic spectrum.43 The pristine zein protein shows several important absorption characteristic peaks at 3336 cm-1 (-OH bands), 2967 cm-1 (-CH2CH), 1664 cm-1 (-C=O bands) and 1528 cm-1 (amide II -NH bands).44 However, the FZ5, F-Z10 and F-Z15 samples combine characteristic peaks (-C=O bands and -NH bands) of zein and wood pulp (-CO bands), which indicate that the nanoparticles from the pickering emulsions coated on the surface of wood pulp. This conclusion can also be drawn from the thermogravimetric analysis (TGA) cruves, as displayed in Figure 5b. The TGA tests of all the filter samples are conducted at above 500 °C in nitrogen. The 10% weight losses (Td10) of zein, F-Z5, F-Z10 and F-Z15 occurred at 260 °C, however, the wood pulp exhibits a Td10 at ca. 12


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300 °C. It can be seen that these zein protein-based samples have low thermal stability due to the various chemical bonds of amino acid, such as N-N (159 kJ/mol) and C-N (305 kJ/mol) bonds, which possess lower bonds dissociation energies than the -OH (464 kJ/mol), C-C (332 kJ/mol) and C-H (414 kJ/mol) bonds of wood pulp.45 In this case, a lower Td10 results from a zein-based sample, demonstrating existence of the zein protein on the surface of wood pulp. In addition, the SEM-energy dispersive X-ray analysis (SEM-EDAX) mapping of wood pulp and F-Z15 further certifies that there is plenty of CNF-zein nanoparticles on the surface of wood pulp, as shown in Figure 5c-d. Figure 2c displays the EDAX mapping of wood pulp. It shows that C elements owing to wood pulp, few of nitrogen and sulfur elements are distributed on the impurity of wood pulp. Compare with wood pulp, the SEM-EDAX mapping of F-Z15 clearly shows that in addition to C owing to wood pulp, lots of N and S elements that are dominantly contributed from zein protein, which are attributed to the presence of CNF-zein particles on the wood pulp surface. This is consistent with the conclusion obtained from FTIR, which suggests that the zein protein exists on the surface of wood pulp. The above studies show that the CNF-zein nanoparticles from the pickering emulsion can strongly interact with wood pulp, and then form a unique hierarchical composite structure CNF-zein@wood pulp.

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Figure 5. Particles of emulsion and wood pulp interaction studies. (a) FTIR and (b) TGA spectra of wood pulp, zein, F-Z5, F-Z10 and F-Z15. SEM-EDAX mapping of (c) wood pulp, (d) F-Z15.

Morphology and performance of the air filters (CNF-zein@wood pulp, or F-Z). The morphologies of wood pulp, F-ZS (the air filter without emulsification), F-Z10 and F-Z15 are studied by scanning electron microscopy (SEM), as shown in Figure 6. The SEM images in Figure 6a-d indicate that the wood pulp fabrics with porous structures; the pore size is in a range from 10 to 200 μm (more SEM images shown in Supporting Information Figure S3). The long microfibers of wood pulp are built as a porous frame, resulting in reducing the pressure drop. Meanwhile, the wood pulp fabric provides a good mechanical property for the air filter.19 From Figure 6a, the pristine wood pulp fibers have a smooth surface. For the F-ZS sample, the denatured zein protein agglomerates (circled in red) because there is no emulsification process, as shown in Figure 6b and Supporting information Figure S4. From the insert of Figure 6c-d, one can find that the amount of particles on the surface of wood pulp 14


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is significantly increased from F-Z10 to F-Z15, meanwhile, the size of particles is decreased. Based on the SEM images of F-Z10 and F-Z15 (Supporting information Figure S6), the number of CNF-zein particles in F-Z10 and F-Z15 are about 279 ± 62 (means ± standard deviation) and 673 ± 47 in each square micron, respectively. The amounts of CNF-zein particles increased 141% from F-Z10 to F-Z15. Sizes and distribution of the protein particles for F-Z15 and F-Z10 are presented in Figure 6e and f; more particles with ca. 210 nm in FZ15 and ca.530 nm in F-Z10 can be found on the surface wood pulp. Therefore, the size of the particles in the air filter samples is closely related to that of the particles in the pickering emulsion, as shown in Figure 6d-e and Supporting information Figure S5. It is the first time to apply the method of pickering emulsion in fabricating air filters with hierarchically structured composites by incorporating CNF-zein nanoparticles into wood pulp as the frame. Compared with the conventional electrospinning method for making nanofibers, pickering emulsion can realize nanostructured active material (CNF-zein) in cost-effective way for development of multifunctional air filters

Figure 6. Morphological studies of the F-Z air filter. SEM images of (a) wood pulp, (b) F-ZS (without emulsification), (c) F-Z10 and (d) F-Z15. The diameter distribution of the (e) F-Z15 and (f) F-Z10.

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Regarding the filtration performance, the removal efficiency, quality factor, the capacity and the normalized pressure drop are investigated for the air filter samples, F-ZS, F-Z5, FZ10 and F-Z15. Figure 7a and b show the particular matter (PM) removal efficiency. To compare the filtration performance, all of these samples are prepared with same areal density, ca. 525 g m-2, as shown in Supporting information Table 1. Based on Figure 7a, the removal efficiency of PM 2.5-10 stays in the same range of about 95.54%-98.96%, regardless of the zein loading. This result indicates that the large particles are mostly captured because of primary physical filtrataion mechanisms, including sieving, interception, impaction and diffusion.46 It is noted that, even for particulate pollutants, these filtration mechanisms are not effective for removing small particles with size much smaller than the pore size. For small particles, in particular for PM0.3, the removal efficiency lies in the range of 78.49%-93.71%. There are two points worth of discussion here. First, for small pollutants, the removal efficiency of F-Z15 is the highest among these samples. It is because that the F-Z15 presents big amount of protein nanoparticles on the surface of wood pulp, as shown in Figure 6d-e. The small pollutant can be removed by the interaction-based mechanism with the critical contribution from the multiple functional groups in the zein structure and high specific surface area of nanoparticles.7 This explanation is the same reason for the filtration performance of toxic chemicals, as discussed later. Second, for all these three samples, the removal efficiencies are higher than the sample F-ZS without emulsification, especially for that of small air pollutants. It is because that the zein protein agglomerates into clots in F-ZS sample and its specific surface area is low as shown in Figure 6b and Supporting information Figure S4. These results indicate that the CNF-zein nanoparticles play an important role in capturing small air pollutants. It is well known that the filtration of small particles is more challenging as compared with big particles because of the fast diffusion in filters38. The above studies confirm that the nanoparticles in the F-Z15 sample are critical for improving the removal efficiency of small particulate pollutants. According to the significance analysis, the removal efficiency for small particles (PM2.5) has significant difference among the F-Z5, F-Z10, F-Z15 and F-ZS samples at 5 % level. However, the removal efficiency for large pollutants (PM2.510.0)

has no significant difference among the F-Z5, F-Z10, F-Z15 samples, as shown in Table 1.

It is because that the small pollutants (PM2.5) are captured through the high specific surface area and numerous functional groups of CNF-zein nanoparticles; the large particles (PM2.5-10.0) are filtered by physical filtration of wood pulp. The numbers of CNF-zein nanoparticles are significantly bigger than other samples, as shown in Figure 6a-d and Supporting information 16


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Figure S3-5. Hence, there is significant difference among the different samples. It also proved that the CNF-zein nanoparticles can enhance the removal efficiency for small particles. The quality factor (QF), which integrate removal efficiency and pressure drop, can further prove the filtration performance.8, 10 In particular, the QF of F-Z15 is significantly higher than other samples, as shown in Figure 7b. This is highly desired for the high-performance air filter due to the combination of the high removal efficiency and low pressure drop.

Figure 7. Filtration performance. (a) PM removal efficiencies and (b) quality factors of the samples based on pickering emulsion: F-Z5, F-Z10, F-Z15, and zein solution-based sample: F-ZS (without emulsification). (c) Normalized pressure drop of the F-Z5, F-Z10, F-Z15, F-ZS, 17



D-WPC@BC/WP19 and zein/electrospun samples.13,

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(d) Chemical removal efficiency for

formaldehyde (HCHO) and carbon monoxide (CO) by the F-Z5, F-Z10, F-Z15 and F-ZS samples (air filters with different concentration of zein). (e) Schematic of the filtration process of F-Z15. (f) Digital photos of the air filter (F-Z15) before and after filtration.

Table 1. The removal efficiency for PM of F-Z5, F-Z10, F-Z15 and F-ZS samples. 　 F-Z5 F-Z10 F-Z15 F-ZS

PM0.3 2.45a

78.49 ± 72.82 ± 1.31b 93.71 ± 1.03c 29.91 ± 0.47d

PM0.3-0.5 90.25 ±

PM0.5-1.0

1.24a

83.15 ± 1.08b 97.08 ± 1.56c 40.23 ± 1.03d

91.91 ±

3.92a

87.88 ± 0.72b 97.94 ± 0.82c 47.11 ± 0.69d

PM1.0-2.5 94.45 ± 1.73a 88.32 ± 0.77b 97.89 ± 0.43c 60.13 ± 1.11d

PM2.5-5.0 95.54 ±

PM5.0-10.0

2.62a

97.69 ± 1.65a

94.44 ± 0.60a 98.04 ± 2.54a 68.90 ± 0.52b

95.24 ± 1.92a 98.96 ± 3.18a 78.63 ± 1.98b

Each value represents the mean ± standard deviation, (n = 3). Mean values in the same row with different letters are significantly different at 5 % level, as determined by Duncan’s multiple range test. The protein-based air filter can capture not only particulate matter but also chemical gas molecules.13 In this study, we also inspect that the F-Z air filters can removal chemical gas molecules. As shown in Figure 7c, the removal efficiencies for carbon monoxide (CO) and formaldehyde (HCHO) for the F-Z5, F-Z10, F-Z15 and F-ZS are tested. No matter for HCHO or CO, the F-Z15 air filter shows the highest removal efficiency among all samples, ca. 88.3% and 60.7%, respectively. The plenty of denatured CNF-zein nanoparticles with the functional groups have a significant effect on capturing toxic gases. Therefore, the F-Z15 sample is a multi-functional air filter. In addition to particulate and chemical removal efficiencies, the normalized pressure drop (NPD, i.e. pressure drop/mass) is calculated as another important parameter for characterizing filtration performance.19 Usually, a low NPD value means low airflow resistance per gram and well user experience.6 Therefore, an air filter with highperformance should combine a high removal efficiency and low normalized pressure drop. Compared with our previous work of nano fabrics prepared from electrospinning, the NPD values for all the F-Z samples are all very low, as shown in Figure 7d and Supporting Information Table 1. It is because that the F-Z samples are prepared via the freeze-drying method, which can maintain the porous structures, see Figure 6 and Supporting Information Figure S3-5. Therefore, F-Z15 has the best filtration performance, as it has the lowest NPD, 18


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and the highest removal efficiency among these samples. To further understand the unique performance of the F-Z15, Figure 7e illustrates the process of filtration. Conventionally, the removal efficiency of F-Z15 for pollutants is mainly dependent on the three mechanisms: 1) the wood pulp mats can filter big pollutants by four primary sized-based filtration mechanisms; 2) compared with micrometer sized fibers, CNF-zein nanoparticles have lager surface areas and higher surface energies, which can dramatically improve their interactions with PM particles and enhance their efficiencies; 3) as mentioned before, zein contains numerous functional groups that can interact with different types of particles and toxic chemicals in polluted air. The strong interactions between CNF-zein particles and pollutants enhance the capturing capabilities for both toxic chemicals and particulate matter. Hence, the polluted air transferred as clean air by the F-Z15 air filter, see Figure 7e. Figure 7f presents the digital photos of the F-Z15 filter before and after filtration. After filtration test, one can find the color of the air filter transferred from white to yellowish due to the adsorption of air pollutants.

4. Conclusions A hierarchical design of all-biomass composite successfully leads to a high-performance air filter, which has high removal efficiency for capturing both particulate and toxic gaseous pollutants at low pressure drop. Via a combination of CNF and zein from pickering emulsions, tremendous protein functionalized nanoparticles are cost-effectively prepared, resulting in the functions of significantly filtering particulate matter and chemical pollutant (i.e. HCHO and CO). Low pressure drop is realized from the long-micron wood pulp fibers forming a frame with large pores and the method of freeze-drying maintaining the porous structure. Therefore, by controlling the loading of the nanoparticles and wood pulp, it is believed that the hierarchical composite with integration of those multifunctional properties are desired for use in high performance air filters. Moreover, the fabrication processes are viable and the quality is controlled, and all of the raw materials are abundant natural materials. Therefore, this study provides a novel strategy based on protein functionalized nanoparticles for fabrication of high-performance and sustainable air filters.

Acknowledgements 19



This work was partially by USDA NIFA 201567021-22911. The authors appreciate the School of Biological Sciences Franceschi Microscopy & Imaging Center (FMIC) at Washington State University for providing the field emission electron microscope.

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Hierarchically Structured All-biomass Air Filters with High Filtration

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