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Tunable Filtration Media Employing Alternating Current Electrokinetics Shahnawaz Molla and Subir Bhattacharjee* Department of Mechanical Engineering, UniVersity of Alberta, Edmonton, AB T6G 2G8, Canada ReceiVed February 4, 2008. ReVised Manuscript ReceiVed March 28, 2008 Separation of colloidal particles from aqueous media by barrier filtration is typically dictated by sieving mechanisms. Here, we demonstrate that colloid filtration by porous membranes can be considerably augmented by suitably superimposing an alternating current (AC) electric field on the membrane. The combined steric-dielectrophoretic filtration can result in very high rejection of the particles compared to solely steric rejection.
1. Introduction Filtration is perhaps the most ubiquitous physical method for separation of the components of solid-liquid mixtures, such as colloidal dispersions or macromolecular suspensions. Selection of filter media is typically dictated by the size of the solid entities in the mixture. Nominally, the pore size of the filter medium should be smaller than the solid entity, such that they are prevented from passing through the medium while the solvent passes readily through the pores. As the solid particles in the mixture become smaller, narrower pores are necessary for the filter media to retain them. Consequently, the driving force (typically pressure) required for solvent permeation through these media increases substantially. Furthermore, in these operations, the retained solids tend to accumulate on the filter media, leading to a reduction in permeability or their fouling.1,2 Finally, most conventional filter media act as passive barriers and do not provide tunable selectivity for different components of a complex colloidal fluid. The filtration of particles is not only achieved by size exclusion effects but by additional repulsive forces. In particular, it is well-known that ions are retained by nanofiltration membranes that have physically much larger pores than the ionic dimensions, predominantly due to electrostatic interactions.3,4 The commonly adopted techniques for retaining particles by larger pores involve utilization of repulsive intermolecular and colloidal forces, such as electrostatic and hydration forces,2,5–7 between the particles and the pore walls. These forces act selectively on the particles, hindering their entry into, and transport through, the pores compared to the solvent.9 However, the ranges of these forces are extremely small, and * To whom correspondence should be addresssed. Telephone:(780) 492 6712. Fax: (780) 492 2200. E-mail:
[email protected]. (1) Mulder, M. Basic principles of membrane technology, 2nd ed.; Springer: New York, 1996. (2) Kang, J. S.; Shim, J. K.; Huh, H.; Lee, Y. M. Langmuir 2001, 17, 4352– 4359. (3) Bhattacharjee, S.; Chen, J. C.; Elimelech, M. AIChE J. 2001, 47, 2733– 2745. (4) Szymczyk, A.; Sbay, M.; Fievet, P. Langmuir 2005, 21, 1818–1826. (5) Wang, X. L.; Tsuru, T.; Togoh, M.; Nakao, S. I.; Kimura, S. J. Chem. Eng. Jpn. 1995, 28, 372–380. (6) Childress, A. E.; Elimelech, M. EnViron. Sci. Technol. 2000, 34, 3710– 3716. (7) Ma, H. M.; Bowman, C. N.; Davis, R. H. J. Membr. Sci. 2000, 173, 191– 200. (8) Carroll, T.; Booker, N. A.; Meier-Haack, J. J. Membr. Sci. 2002, 203, 3–13. (9) Bhattacharjee, S.; Sharma, A.; Bhattacharya, P. K. Ind. Eng. Chem. Res. 1996, 35, 3108–3120.
they are dictated by the chemistry of the membrane, the particles, and the solvent. Furthermore, the short range of these colloidal forces render them more effective in particle retention during nanofiltration or ultrafiltration, with marginal influence on the retention of larger colloidal entities during microfiltration. Thus, the so-called molecular weight cutoff (MWCO) or the size of particles that can be separated by a filter medium or membrane is usually preordained at the fabrication stage of such filter media. There is no means of tuning the retention of different sizes of particles through filter media or membranes during a filtration operation. In this communication, we propose a new mechanism for achieving tunable particle retention by filters based on alternating current (AC) electrokinetics in a nonuniform electric field. While the concept is applicable to particle suspensions formed in any solvent, we will focus on aqueous colloidal suspensions in this report. Particles suspended in water, when subjected to a highly nonuniform electric field at a high frequency, experience a dielectrophoretic (DEP) force.10,11 Depending on the sign of the real component of the ClausiusMossotti factor of the suspension, the force on the particles either attracts (positive DEP) or repels (negative DEP) them from regions of high field nonuniformity. This phenomenon has been extensively studied as a mechanism of colloid or cell manipulation over the past several decades.12–19 In our earlier studies, we used numerical simulations and experiments employing microfabricated electrode arrays to assess the efficacy of negative DEP in reducing fouling of channel walls.20–22 It was observed that suitably designed electrode arrays, when actuated with a high electric field nonuniformity at a high frequency, can exert (10) Pohl, H. A. J. Appl. Phys. 1951, 22, 869–871. (11) Jones, T. B. Electromechanics of Particles; Cambridge University Press: Cambridge, 1995. (12) Pethig, R.; Markx, G. H. Trends Biotechnol. 1997, 15, 426–432. (13) Markx, G. H.; Pethig, R.; Rousselet, J. J. Phys. D: Appl. Phys. 1997, 30, 2470–2477. (14) Gascoyne, P. R. C.; Wang, X. B.; Huang, Y.; Becker, F. F. IEEE Trans. Ind. Appl. 1997, 33, 670–678. (15) Green, N. G.; Morgan, H. J. Phys. D: Appl. Phys. 1997, 30, L41–L44. (16) Durr, M.; Kentsch, J.; Mu¨ller, T.; Schnelle, T.; Stelzle, M. Electrophoresis 2003, 24, 722–731. (17) Holzel, R.; Calander, N.; Chiragwandi, Z.; Willander, M.; Bier, F. F. Phys. ReV. Lett. 2005, 95, 128102. (18) Docoslis, A.; Tercero Espinoza, L. A.; Zhang, B.; Cheng, L.; Barbara, A.; Israel, B. A.; Alexandridis, P.; Abbott, N. L. Langmuir 2007, 23, 3840–3848. (19) Suzuki, M.; Yasukawa, T.; Shiku, H.; Matsue, T. Langmuir 2007, 23, 4088–4094. (20) Molla, S. H.; Bhattacharjee, S. J. Membr. Sci. 2005, 255, 187–199. (21) Molla, S. H.; Masliyah, J.; Bhattacharjee, S. J. Colloid Interface Sci. 2005, 287, 338–350. (22) Molla, S. H.; Bhattacharjee, S. Langmuir 2007, 23, 10618–10627.
10.1021/la800363w CCC: $40.75 2008 American Chemical Society Published on Web 05/06/2008
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of multiple repeat units of the sandwich structure is shown in Figure 1b. Figure 1c and d shows the electric potential and field norm distribution in such a pore, obtained by solving the Laplace equation20 in an axisymmetric cylindrical coordinate system, when the consecutive conducting layers are actuated with a 180° phase shifted AC signal. In Figure 1c, the potential varies between +1 (white) and -1 (black), whereas in Figure 1d, the electric field intensity varies by 5 orders of magnitude, with the highest intensity regions shown in white. In our previous studies employing planar electrode arrays,20,22 we obtained a strong repulsive DEP force on colloidal particles due to a spatially inhomogeneous field at high frequencies of the applied potential. The time averaged DEP force is expressed as
FDEP ) 2πεma3Re[K(p/, m/)] ∇ (E · E)
(1)
where a is the radius of the suspended particle, E is the electric field, and Re[K(p/, m/)] is the real component of the frequencydependent Clausius-Mossotti factor Figure 1. (a) Schematic of the conductor/dielectric sandwich porous medium, (b) cross-sectional view of a pore, and (c) potential and (d) electric field distribution in the axisymmetric pore. The simulated potential and field distributions were obtained by solving the Laplace equation with constant potentials of +1 and -1 applied on the light and dark electrode strips, respectively. The dielectric material has no charge.
repulsive DEP forces of a much longer range on the particles (in the order of micrometers) compared to other colloidal forces. Here, we show how such a nonuniform electric field can be created across a multilayered porous medium containing alternate layers of conducting and nonconducting materials. Such nonuniform fields can hinder the movement of particles across the pores of the filter medium. We demonstrate here how such a mechanism can dramatically modify the particle retention of filter media during a filtration process in a tunable manner, with the tuning achieved by modifying the amplitude and frequency of the applied AC signal.
2. Theoretical Background The repulsive DEP forces can be generated across a filter medium constructed as a sandwich of conducting and dielectric layers as depicted in the conceptual model in Figure 1a. Here, four layers constituting a single repeat unit of the sandwich structure are depicted. The textured layers are nonconducting, while the light and dark shaded layers are conducting. The medium contains several circular cylindrical pores. A single pore consisting
K(p/, m/) )
p/ - m/ p/ + 2m/
(2)
The Clausius-Mossotti factor is determined from p/ and m/, the complex permittivities of the particle and the solvent, respectively. The complex permittivity is given by / ) j(σ/ω), where j ) -1, is the permittivity, σ is the conductivity, and ω is the angular frequency of the applied AC potential. Depending on the relative difference in dielectric properties of the particles and the solvent, the real component of the Clausius-Mossotti factor ranges between +1.0 and -0.5. According to eq 1, a negative value of the factor indicates repulsion from the high field gradient regions, or negative DEP. For the nonuniform electric field depicted in Figure 1d, and at high frequencies of the applied AC potential, a repulsion of particles from the pore walls is expected, which in turn will restrict the radial positions in the cylindrical capillary that the particles can sample. Consequently, it is expected that, with pores of much larger dimensions compared to the colloidal particles, one can observe a focusing effect on the particles, whereby the net transport of these particles through the pore will be hindered compared to the solvent.3,9 The theory of such hindered transport is extremely well-developed, particularly for conventional colloidal interactions, and we do not reiterate it here. Instead, we demonstrate experimentally that such hindered transport in the
Figure 2. (a) Schematic of the dead-end filtration setup. (b) Stainless steel wire mesh and porous material assembly. (c) Microscope images of three porous materials with pore size of 10 µm, namely, nylon mesh, polycarbonate track etch (PCTE) membrane, and glass capillary array (GCA).
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Table 1. Properties of the Porous Materials Used in the Experiments pore size thickness structure open area material
GCA
nylon filter
PCTE membrane
10 µm 1 mm cylindrical 50% lead glass
10 µm 50 µm woven 4% nylon 6/6
10 µm 10-30 µm cylindrical 16% polycarbonate
presence of a high-frequency electric field can dramatically enhance the particle retention by a filter medium.
3. Materials and Methods A dead-end filtration cell as shown in Figure 2a was employed for the experiments. The upper part of the filtration cell is a column (height 80 mm and ID 43 mm) to contain the feed suspension. The lower part serves as a holder for the porous materials with a liquid exit port to allow permeate flow. The permeate flow rate was controlled using a valve (P445, Upchurch Scientific, Oak Harbor, WA). Discs of porous materials (47 mm diameter) were placed between two circular stainless steel wire meshes (47 mm diameter and 55 µm mesh opening, McMaster Carr, Robbinsville, NJ) as shown in Figure 2b. This assembly represents a single repeat unit of two conducting layers acting as electrodes on two sides of a dielectric material, as depicted in Figure 1a. The assembly was mounted on the base of the filtration cell, and an O-ring (Nitrile) was placed on top to create a seal between the feed and permeate sides. The wire meshes were connected to a dual channel function generator (AFG320, Tektronix) which applied a high frequency sinusoidal AC signal (106 Hz and amplitude 10 Vpp) with a phase difference of 180° to the two electrodes. A linear voltage amplifier (F20AD, FLC Electronics AB, Sippedalsvagen, Sweden) was used when potentials higher than 10 V were necessary. An oscilloscope (TDS3014B, Tektronix) was also connected to the two wire meshes to monitor the electrical signals. Three porous materials were used, namely, nylon mesh filter (Spectrum Laboratories, CA), polycarbonate track etched membrane (PCTE, Sterlitech Corp., WA), and a glass capillary array (GCA, Burle Industries, PA). Figure 2c shows the microscopic (10× magnification) images of these porous materials. The relevant properties of these materials are listed in Table 1. The nylon filter consists of individual strands woven into a mesh screen, whereas the PCTE membrane is made from a thin, microporous polycarbonate film with a smooth, flat surface. The GCA consists of millions of precision glass capillary tubes fused together. A colloidal suspension containing 2 µm diameter polystyrene sulfate particles (IDC Corp., OR; relative permittivity r ) 2-3, bulk conductivity σ ) 10-7 S/m) in an aqueous suspension at a concentration of 107 mL-1 was used
in all experiments. The particles readily pass through the media in the absence of the applied electric field. The assembled filtration cell containing the wire mesh/filter medium sandwich was filled with DI water, and the permeate flow rate was regulated at 0.1 mL/min. The water column was then replaced with the aqueous feed suspension. The height of the feed suspension was 70 mm throughout each experiment to keep the hydrostatic pressure on top of the porous layer constant. Each experiment lasted for 150 min, of which the filtration was conducted in the absence of any applied electric field for the first 60 min. During the rest of the time, the AC potential was applied to the wire meshes. The feed and permeate suspensions were collected at 15 min intervals. The particle concentrations in the feed and permeate were recorded using a UV-vis spectrophotometer (GENESYS 10 UV, Thermo Scientific). Both the feed and permeate samples were returned to the feed column after each concentration measurement to maintain the total particle concentration and feed volume constant.
4. Results and Discussion Figure 3 shows the variation of the rejection of polystyrene sulfate particles with time in the experiments with different porous materials. The percent rejection is expressed by
[ ]
Rejection % ) 100 1 -
cp cf
(3)
where cf and cp are the feed and permeate concentrations, respectively, measured during the experiments. The experiments with the PCTE membranes were conducted by placing a single sheet (upright triangle), two sheets (inverted triangle), and three sheets (tilted triangle) of the PCTE membrane between the wire meshes. The rejection during the initial 60 min of the experiment was nearly zero for the nylon (filled square) and PCTE membranes (open triangles). For the glass capillary array (filled circles), this initial rejection was slightly higher (about 13%). After application of the AC signal, the rejection started to increase, which clearly indicates that the applied electric field gradient imparts forces on the suspended particles to hinder their transport through the pores. The rejection of particles by the GCA was 99.9% after 120 min. Notably, the 1 mm thick GCA required application of a high potential difference of 200 Vpp to obtain the high rejection. For the nylon mesh filter and PCTE membranes, which ranged between 10 and 50 µm in thickness, rejections of about 80% were achieved by applying approximately 10 Vpp. From these experiments, it appears that the application of high frequency AC potentials across porous media can create a strong force field to manipulate particle transport through the pores. The repulsive DEP forces on the particles hinder their transport through the pores. It is also apparent from the experiments that with the particle rejection being primarily electrodynamic in origin, the filtration can be performed using highly porous substrates, thereby requiring very small applied pressure differentials across these media to drive the solvent through the pores. In the experiments reported, the permeation was solely driven by the hydrostatic head of the feed column. Furthermore, the rejection can be tuned by adjusting the applied potential difference.
5. Concluding Remarks Figure 3. Comparison of AC electric field enhanced rejection of 2 µm polystyrene sulfate particles by different porous media. The solid line for the GCA serves as an eye guide. The different thicknesses of the PCTE membranes were obtained by stacking sheets of the membrane. The AC voltage (Vpp ) 10 V for nylon and PCTE and 200 V for GCA) was applied at 60 min. The frequency was 106 Hz.
Using dielectric permittivity differences between the suspended particles and the suspending medium, energy efficient filtration mechanisms can be developed that obviate the requirement of high pressure, small pore size filter media or membranes. Notably, the experimental demonstration reported here employs only a single layer of porous medium
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sandwiched between two electrodes. The efficiency of the system will be considerably enhanced if a multilayer structure as shown in Figure 1b can be employed. Although multilayer architectures as indicated here are quite common in the field of high energy density capacitors, to our knowledge, application of such materials as filter media has not been reported. Based on our findings here, it appears that such materials might have
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application in the development of energy efficient tunable filtration processes. Acknowledgment. Financial support from Natural Sciences and Engineering Research Council (NSERC), Canada, Canada Research Chairs (CRC) program, and Alberta Ingenuity Fund is gratefully acknowledged. LA800363W
