Article pubs.acs.org/Langmuir
Particle Deposition on Microporous Membranes Can Be Enhanced or Reduced by Salt Gradients Abhishek Kar, Rajarshi Guha, Nishant Dani, Darrell Velegol,* and Manish Kumar* Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: Colloidal particle deposition on membranes is a continuing scientific and technological challenge. In this paper we examine the role of a previously unexplored phenomenon diffusiophoretic particle transport toward a membranein relation to fouling. Diffusiophoresis is an electrokinetic transport mechanism that arises in salt gradients, especially when the ions have different diffusion coefficients. Through experiments conducted with salt diffusing across microdialysis membranes, with no advection, we show experimentally that diffusiophoresis induces colloidal deposition on the surface of microporous surfaces. We used transient salt (NaCl, KCl, LiCl) gradients and fundamental electrokinetic modeling to assess the role of diffusiophoresis in colloidal fouling. Based on (i) difference in diffusion coefficients of ions, (ii) zeta potential on the particles, and (iii) ionic gradient applied across the walls of the membrane, colloidal fouling could be both quantitatively and qualitatively predicted. Our understanding enabled us to stop particle deposition by adding calcium carbonate outside the membrane, which generates a stronger electric field in a direction opposite to that created by salt diffusing from the membrane. We propose that accounting for this diffusiophoretic mode of particle deposition is important in understanding membrane fouling. different diffusion coefficients in the solution. A finite difference in diffusion coefficients of the constituent ions can give rise to a spontaneous electric field (E) in solutions,4 which then causes electrophoresis of particles. Furthermore, the particles can also migrate due to a “chemiphoretic” mechanism.6−8 The origin of this mechanism is somewhat subtle but well-known. In essence, within the electrical double layer (EDL), the fluid pressure is higher than in the bulk, and this pressure increases with ionic strength (smaller Debye length). Thus, when a gradient of ionic strength exists, a pressure gradient exists across the surface. For a symmetric Z:Z electrolyte, the combination of these two effects results in particle motion with a speed (Udp) given by
1. INTRODUCTION Colloidal particle deposition onto membranes is frequently cited as the primary cause of fouling in many industrial and scientific processes. The primary mechanism of the deposition for decades has been related to filtration; that is, as fluid is transported toward the membrane, it carries entrained particles that can penetrate into membrane pores or accumulate on membrane surfaces. Particle−membrane interactions (primarily electrostatic1 and hydrophobic interactions2) then lead to immobilization and flux decline. Here we explore the importance of an additional process, called diffusiophoresis, in causing particle transport toward or away from microdialysis membranes. Despite the fact that the transport process of diffusiophoresis has been known for over 65 years,3−5 this mechanism has not been examined so far in membrane science to explain particle deposition on membrane surfaces. Diffusiophoresis is a process of particle transport in a salt gradient.6−9 The process has been shown experimentally to cause transport of particles in steady-state salt gradients,10 and the experimental results are well-explained using electrokinetic modeling.11 More recently, dissolving calcium carbonate particles, and other geologic and biologic systems, have been used to create salt gradients that drive particle motion through this mechanism.12 Here we provide a brief description of the mechanism of diffusiophoresis caused by salt gradients (∇n) present in a system. When a salt gradient exists, it often happens that the ions from the salt diffuse at different rates based on their © XXXX American Chemical Society
Udp =
⎧ ⎛ Zeζp ⎞⎤⎫ ⎪ ∇n ε ⎪ kT D+ − D− 2k 2T 2 ⎡ ⎨ ζp − 2 2 ln⎢1 − tanh2⎜ ⎟⎥⎬ ⎪ ⎥ ⎢ η⎪ Ze D + D 4 kT n Z e ⎠⎦⎭ ⎝ ⎣ + − ⎩
(1)
where e is the proton charge, ε is the permittivity of the medium, η is the solution viscosity, D+ and D− are the diffusivities of cation and anion, respectively, ζp is the particle zeta potential, k is the Boltzmann constant, T is temperature, and n is the concentration of the salt. Typically, a charged particle with a zeta potential of order of 2kT/e (∼50 mV) will move due to electrolyte gradient of 1 M/cm with speeds of several μm/s. Received: November 14, 2013 Revised: December 20, 2013
A
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(HFM) which had a 280 μm outside diameter (OD) and 40 μm wall thickness for each of our experiments. HFM fibers were obtained from Spectrum Laboratories, Rancho Dominguez, CA. Each HFM was washed with ethanol using 1 mL syringes (BD Biosciences) fitted with a 21G precision needle before use. HFM was then washed again with 10 mM NaCl (or 10 mM KCl, 10 mM LiCl), loaded with 10 mM NaCl, and both ends of the hollow fiber were sealed with wax. The sealed HFM was inserted into either a 0.9 mm or 1 mm square glass capillary (Vitrotubes). The capillary surrounding the HFM was filled with DI water containing suspended sPSL particles ( βNaCl, a stronger electric field is generated (∼0.8 V/m for LiCl and ∼0.45 V/m for NaCl). On the other hand, K+ ions have comparable ion diffusivity as that of Cl−, which leads to very weak electric field of the order of ∼0.035 V/m and subsequent sPSL tracer movement mainly through chemiphoretic mechanism with half the speed of the NaCl case. Experimental measurements are corroborated by system modeling of diffusiophoretic velocity profiles, both spatiotemporally (Figure 5a) and with different salts (Figure 5b). The particle speeds in our system were not affected by any fluid flow mechanism such as diffusioosmotic flows that can arise on charged surfaces due to salt gradients. These fluid flows were negligible in our case since the salt gradients were orthogonal to the surface of the membrane. Further, the contribution from fluid flows inside the pores of the membrane scaled only as far as 1 μm away from the membrane surface for a 100 nm pore.31 Presence of CaCO3 Micropumps. The behavior of particle transport was seen to be altered by changing the positioning of salt in the system. However, this effect was most evident when we used CaCO3 micropumps outside HFM, in the bulk region of capillary which completely reversed the motion of tracers in a NaCl gradient (Figure 6). The difference between effects of calcium carbonate and sodium chloride results in particle motion toward the dominant species in the system, i.e., CaCO3 micropumps. Since the gradient of NaCl dies down rapidly to a more stable value, the system functions similar to a gradient emanating from a single source. Reversing the electric field created by a salt gradient using dissolving CaCO3 in the bulk solution can prevent or reverse particle deposition. McDermott et al. showed how CaCO3 microparticles are able to generate strong diffusiophoretic transport during the dissolution process in DI water on glass substrates.12 The dissolution generates OH−, HCO3−, and Ca2+ ions with considerable diffusivity differences (DOH− = 5.27 × 10−9 m2/s, DHCO3− = 1.19 × 10−9 m2/s, and DCa2+ = 0.792 × 10−9 m2/s), sufficient to generate a stronger electric field than NaCl owing to the high diffusion coefficient of the hydroxyl ion. When CaCO3 microparticles are kept on the outside surface of the HFM, an electric field toward the membrane and opposite to that generated by the NaCl is developed. This reverse electric field in conjunction with lower ζp (−67.9 mV) in the presence of CaCO3 reduces tracer speeds and eventually removes particles from HFM surface creating an exclusion zone (Figures 6a,b).
Figure 5. (a) Diffusiophoretic particle velocities (Udp, μm/s) decrease with time and change with distance as measured and modeled for 10 mM NaCl inside HFM/3 μm sPSL particles in DI outside at t ∼ 120 s and t ∼ 180 s in 1 mm i.d. capillary, and (b) Udp depends on nature of salts as measured and modeled for 10 mM NaCl, 10 mM KCl, or 10 mM LiCl inside HFM/3 μm sPSL particles in DI outside at t ∼ 180 s in 1 mm i.d. capillary (for NaCl and KCl) and 0.9 mm i.d. capillary (for LiCl). The symbols represent experimental velocities, and the lines represent modeled velocities. The asymmetric velocity profiles show a dependence on physical placement of the HFM within the capillary and the time scales of measurements. The maximum velocity of the particles corresponds to the position where maximum ∇n/n occurs. E
dx.doi.org/10.1021/la4044107 | Langmuir XXXX, XXX, XXX−XXX
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ASSOCIATED CONTENT
S Supporting Information *
Figures with iso-osmolar electrolyte concentrations showing significantly less particle deposition on the membrane when the salt with higher β was taken inside the HFM compared to the salt with lower β outside; the COMSOL model for simulating the transient salt gradients in our HFM studies. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*Phone (814) 865-8739; Fax (814) 865-7846; e-mail manish.
[email protected] (M.K.). *Phone (814) 865-8739; Fax (814) 865-7846; e-mail velegol@ psu.edu (D.V.). Notes
The authors declare no competing financial interest. Figure 6. CaCO3 can be used to prevent particle deposition on the membrane surface. (a) Schematic of aggregation prevention and exclusion zone creation by CaCO3 microparticles as seen experimentally at 4 min in (b). (c) shows the same after t = 1 min, and (d) shows the system after approximately 15 min.
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ACKNOWLEDGMENTS
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REFERENCES
We thank the National Science Foundation (CBET IDR 1014673) for funding this project. We also convey our gratefulness to Prof. Andrew Zydney (Pennsylvania State University) for insightful discussions and suggesting a way forward in this area of research.
The CaCO3 particles (Figure 6a) create an exclusion region of tracer particles (Figure 6b) next to the membrane surface. This observation supports our idea that we can reduce or reverse particle deposition by controlling the direction of the diffusiophoresis in the system (Supporting Information Movie 4). Figures 6c,d are a series of time lapse images showing how particle aggregation near to HFM is affected with time in the presence of 0.1 mM CaCO3. The resultant electric field in the NaCl−CaCO3 system does not allow particle deposition on the membrane. The particles remain stable or relax at a certain distance from the membrane wall (exclusion zone in Figure 6b) and slowly move out toward capillary wall (Figure 6d), possibly because of resultant opposite electric field.
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5. CONCLUSION Particle deposition onto membranes causes fouling and is thus a bottleneck in many membrane processes. In this paper we have explored the hypothesis that particle deposition occurs not only due to a filtration effect but also due to a diffusiophoretic transport effect. We investigated a system with induced salt gradients across the wall of a microporous hollow fiber membrane and observed that particle deposition is enhanced by the proposed mechanism of diffusiophoresis. By measuring and modeling all the required parameters (zeta potential, concentration gradient, and diffusion coefficients) in the system, we observe overall agreement between our modeled speeds and experimental results to within 17%. With an understanding of the role of this mechanism, we were successful in reducing particle deposition using CaCO 3 micropumps. The experiments and analyses presented in this paper may further our understanding of particle deposition in membrane systems in the presence of salt gradients and provide possible ways of mitigating excessive particle deposition and accompanying flux decline. F
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