Deciphering Effects of Surface Charge on Particle Removal by TiO2 Polyacrylonitrile Nanofibers

Over the recent years, the continued increase in the number of particles in air has become a public concern. This problem can be addressed by using nanofibers to filter fine particles. However, nanofibers possess complex characteristics. As such, the effects of surface charge require further studies. In this study, the surface voltages of nanofibers were analyzed with an electrometer after these fibers were charged through corona discharge to investigate the mechanism of filtration. Results indicated that the surface voltage of 2.0% TiO2 polyacrylonitrile fibers (TPFs) can reach up to 0.97 kV and then decrease to 0.60 kV after 96 h. Particles were optimally removed by charged polyacrylonitrile fibers (PFs) and TPFs. Particle penetration decreased by 71% of TPF and 36% of PF. Scanning Electron Microscopy and Nitrogen adsorption/desorption isotherms revealed that increasing the surface area and roughness of these materials are more favorable for charge maintenance to promote particle removal. Our research could provide an in-depth understanding of the effects of surface charge on particle removal and show how systems can be optimized for further applications.


INTRODUCTION
As main air pollutants, particles have become a major public health concern since the emergence of air pollution in developing countries (Especially China) (Lu et al., 2016;Wang et al., 2016;Wen et al., 2016l Zhao et al., 2016).Particle pollutants are closely related to humans because these particles are produced through anthropogenic processes, such as combustion or vehicle emissions.These suspended particle emissions also pose a health risk to humans.For instance, particles larger than 2.5 µm can accumulate in the respiratory tract or lungs (Janssens et al., 2003;Li et al., 2013).Likewise, particles smaller than 2.5 µm may build up in the lungs and adversely affect lung functions; consequently, some diseases may develop (Janssens et al., 2003;Li et al., 2013;Zhu et al., 2016).Cai et al. (2015) further reported a linear relationship between cumulative PM 2.5 concentration and outpatient respiratory diseases.Hence, economic development and its effect on the environment should be balanced, and the influence of these developments on human health should be considered.
Traditional methods, including electrostatic collection (Wettervik et al., 2015), cyclone (Wang et al., 2010), wet washing (Vega et al., 2014), and filtration (Tanabe et al., 2011;Chang et al., 2016), have been applied to reduce particulate pollutant emission and address particle pollution.Among these methods, filtration is the most preferred particle removal approach because of its low cost and small land requirement (Ratnesar-shumate et al., 2008).Nanofibers prepared through electrostatic spinning are potential materials for filtration because these nano-sized fibers can efficiently filter fine air particles.Polymeric nanofibers containing metal oxide nanoparticles have also been widely considered because they possess a large surface area, small pore size, and high porosity (Park et al., 2007).Polymeric nanofibers can strongly influence electrostatic interactions between dust particles and nanofibers (Cho et al., 2013).Polymeric fibers are widely used in various fields (Table 1) (Qiu and Yu, 2008;Doh et al., 2008;Kim et al., 2009;Azad et al., 2010;Francis et al., 2011;Zhang et al., 2011;Li et al., 2012;Nirmala et al., 2012;Batool et al., 2013;Choi et al., 2013;Wang et al., 2015;Zhang et al., 2016).Ratnesar-Shumate et al. (2008) found that polymers as poor conductors maintain rather than release the charge generated through electric field stretching on the surface.
The mechanism of particle removal is complex because of multiple factors that work simultaneously.Although some mechanisms, such as interception, inertial impaction, and sieving, involved in filtration with nanofibers have been elucidated, the function of electrostatic attraction requires further studies.The ability of nanofibers to maintain charges is key to understanding charge stability.The charge stabilities of some materials, such as polyacrylonitrile (Schreuder-Gibson, et al., 2005), polystyrene (Schreuder-Gibson, et al., 2005), polycarbonate (Collins et al., 2012), and polyacrylonitrile (Collins et al., 2012), have been analyzed, but the relationship between surface charge and particle deposition has been seldom discussed.Hence, this study investigated the relationship between surface charge and particle removal efficiency by TiO 2 polyacrylonitrile nanofibers.The individual and interactive effects of operating parameters, including TiO 2 loading, face velocity, and surface voltage, on the treatment efficiency of particles were also explored.Nanofiber characteristics were analyzed through Scanning Electron Microscopy and Nitrogen adsorption/desorption isotherms, and electrometry.

Preparation of Polyacrylonitrile Fibers (PFs) and TiO 2 PFs (TPFs)
PFs and TPFs were synthesized through electrospinning by using the following raw materials: polyacrylonitrile powder (Aldrich), dimethylacetamide (DMAC, Sinopharm), and TiO 2 (Digussa, P25).In typical PF preparation, 1 g of polyacrylonitrile powder was first mixed with 20 mL of DMAC solution at 60°C.The solution was then magnetically stirred for 6 h and ultrasonically vibrated for 1 h to obtain a precursor solution.Then, the PFs were prepared through electrospinning with a 10 ml syringe (0.52mm in diameter) and stainless steel needles.The applied voltage between the tips and the collector ranged from 13 kV to 16 kV, and the distance was 15 cm.The feed rate was 1.0 mL h −1 , and a syringe pump with a rotating metal drum (diameter, 8 cm; rotation speed, 100 rpm; rotation rate, 20-30 cm s −1 ) wrapped with spunbondnylon was used to collect the PFs.
In TPF preparation, 1 g of polyacrylonitrile powder and a certain amount of TiO 2 were initially mixed with 20 ml of DMAC solution as a precursor solution at 60°C.Then, the same process as PF preparation was carried out.A schematic of the preparation method is shown in Fig. 1(a).The prepared fibers, due to slightly charged after electrospinning, were named as "Initial charged fibers" to distinguish from the "corona-charged fibers" prepared subsequently.

Corona Charging of PFs and TPFs
In PF and TPF charging, the air was fed into the syringe with stainless steel needles instead of the precursor solution.The syringe was connected to a 16 kV voltage source at a distance of 15 cm, while the PFs and TPFs were used as grounded collectors.The feed rate was 1.0 mL h −1 , and a syringe pump with a rotating metal drum (diameter, 8 cm; rotation speed, 100 rpm; rotation rate, 20-30 cm s −1 ) was  wrapped for 1 h.During charging, the temperature and humidity of the reaction were controlled at 25°C and 45%.

Progress of Filtration
A filtration was carried out in a columnar reactor.The charged PFs/TPFs under different dosages or the initial charged PFs/TPFs were fixed as filter medium in a stainless steel clip.The size of the column used in this research was 60 cm × 2.5 cm × 2.5 cm.The experimental flowchart is presented in Fig. 1(b).During the experiment, the particles were generated through a particle generator that produced KCl particles of 0.3-0.5 µm in mode value of the distribution.The qualitative and quantitative properties of the particles before and after the medium were analyzed by using a scanning mobility particle sizer (SMPS; Model 3934, TSI Inc., St. Paul, MN, USA)

Characterization of the Nanofibers
The surface morphologies of the TPFs were analyzed by using SEM (Model: 5136M).The nitrogen adsorption/ desorption isotherms of the TPFs were obtained at 77 K on a Micromeritics 3H-2000PS1 volumetric adsorption analyzer.The surface voltage of the fiber was analyzed through electrometry (Model: EST103), and voltage was measured every 10 min to evaluatethe charge retention time.

SEM
The surface morphologies of the electrospun PFs and TPFs examined through SEM are displayed in Fig. 2. In Fig. 2(a), the pure PFs exhibit continuous and smooth surfaces.The coarse surfaces of the TPFs are also apparent.Compared with the pure PFs, 2% TPFs were almost completely wrapped by the TiO 2 nanoparticles.
The SEM images of the TPFs supported with spunbond nylon (Fig. 3) illustrate the fiber shape of the spunbond nylon with a diameter of 20 µm.The average diameter of the TPF was approximately 240 nm, with the beads and TPF attached on the spunbond nylon as a support.The TiO 2 nanoparticles were also visible because of their agglomeration to form the rough surface of the TPFs.Compared with the pure PFs, 2% TPFs were almost completely wrapped with TiO 2 nanoparticles.

BET
The characteristics of fibers can be indentified from the adsorption/desorption isotherms by comparing with the standard classification of isotherms (Type I to Type VI) (Sing, 1985;Williams and Reed, 2006).On the other hand, the adsorption/desorption isotherms can also be explained by the typical hysteresis loop (H1-H4) (Sing, 1985).As the result shown in Fig. 4, adsorption/desorption behaviors of PFs and TPFs are closely related to the Type IV which indicate the filling and emptying of mesopores by capillary condensation ( (Williams and Reed, 2006).The sharp uptake under relative pressure of 0.9-1.0related to the existence of the voids between fibers (Jin et al., 2005;Li et al., 2015).In addition, typical H1 hysteresis loops can be observed in Fig. 4. Since hysteresis loop always appeared in the multilayer adsorption region, the H1 hysteresis loop also indicated the capillary condensation in uniform pores (Sing, 1985).
According to the results, the PFs and TPFs have uniform mesopores and the specific surface areas of PFs, 0.5%TPFs, 1%TPFs, 2%TPFs, and TiO 2 are listed in Table 2.These surface areas increased as TiO 2 content increased.This result was possibly due to the relatively larger surface area of TiO 2 (51.2 m 2 g −1 ) than those of the fibers.

Surface Charge Level and Temporal Decay
To understand the ability of the TPFs to maintain charge, we analyzed the surface voltage after charging through corona discharge (Table 3 and Fig. 5).A small amount of voltage was observed on the surface of the pure PF (0.74 kV).In all of the samples, the surface voltage increased with as TiO 2 loading increased.This effect could be attributed to the large and rough surface area of the materials, as confirmed through the BET method (Cho et al., 2013).The smooth surface of PF facilitates the transfer of surface charge, and the large surface area of TPF is favorable for carrying charges (Lamb et al., 1975;Cho et al., 2013;Wan et al., 2014).
Comparing the initial voltage and the voltage after 96 h, we observed that the TPFs maintained charges to a greater extent than the PFs did.This phenomenon might be attributed to the larger and rougher surface of the TPFs than that of the PFs.The surface voltage of 2% TPF decreased by 38%, which was much smaller than 57%, 50%, and 40% of the PF, 0.5%TPF, and 1%TPF, respectively.The decrease in surface charges may stabilize after this parameter decreases sharply (Igratova et al., 2008).The same result was observed in our study (Fig. 5).

Particles Removal Efficiency of TFs and TPFs Effect of Surface Voltage
The effect of surface voltage was analyzed in pure PFs and 2% TPFs with and without further charging by corona discharge.This step was included because charging can critically influence electrostatic attraction.Particle removal was efficiently achieved by the charged PFs and 2% TPFs (Fig. 6).The particle removal efficiency of the initial charged PFs and 2% TPFs was lower than that of the charged fibers.The particle penetration of TPFs was decreased from 1.89% to 0.54%, which was evidently effective than PFs (1.89% to 0.54%).This observation might be attributed to

Effect of TiO 2 Loading
Initial charged TPFs with various TiO 2 loads were analyzed using 1 g m −2 as weight basis at a face velocity of 0.5 cm s −1 .
The filtration efficiency was measured for 15 min.The final particle concentrations under various particle sizes as determined by SMPS are listed in Fig. 7. Combined with the result in Fig. 6, the corona charged PF material has a higher penetration than the corona charged 2% TPF material, however the initial charged PF has a lower penetration than the initial charged 2% TPF (Fig. 7).This result may be due to the PF diameter, which was the smallest among the diameters of the test materials.This parameter may the main effluence  ; face velocity: 0.5 cm s -1 ).
factors for particle filtration under low surface charge (Wilcox et al., 2010).In addition, the removal efficiency of initial charged TPF in this study increased as TiO 2 loading increased from 0.5% to 2%.This result is inconsistent with that of Cho et al. (2013), who demonstrated a considerable increase in filtration efficiency by adding TiO 2 .In our research, the initial charged TPFs most efficiently removed particles at 2% TiO 2 loading, and the particle penetration (20-500 nm) was 1.83%.The increased efficiency may be attributed to the additional TiO 2 content on the TPFs, and TiO 2 potentially enhances the material surface area and provides additional active adsorption sites for particle removal (Hung et al., 2011).Surface roughness also allows the formation of an enlarged nonslip zone and expands the stagnation zone (Lamb et al., 1975).The surface voltage also increased (Table 3) when the TiO 2 loading increased to improve the electrostatic attraction of particles (Fig. 6).

CONCLUSION
This study focused on the removal of particles, a major air pollutant, by TiO 2 -modified nanofibers.BET and SEM results confirmed that PF and TPF charged through corona discharge significantly removed the particles because of the electrical attraction between materials with large and rough surface areas.An increase in the percentage of TiO 2 in the nanofibers correspondingly enhanced the particle removal efficiency because of the high surface charge and large surface area.

Table 1 .
Summary of nanofibers preparation and application.

Table 2 .
Surface areas of various TPFs samples.

Table 3 .
Surface voltage of various TPFs samples.