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'''Concentration polarization''' is a term used in the scientific fields of [[electrochemistry]] and [[Membrane technology|membrane science]].

==In electrochemistry==
In [[electrochemistry]], '''concentration polarization''' denotes the part of the [[Electrical polarity|polarization]] of an [[electrolytic cell]] resulting from changes in the electrolyte concentration due to the passage of current through the electrode/solution interface.<ref>S.P. Parker, McGraw-Hill Dictionary of Scientific & Technical Terms 6E, 2003.</ref> Here ''polarization'' is understood as the shift of the [[electrochemical potential]] difference across the cell from its equilibrium value. When the term is used in this sense, it is equivalent to “[[Overpotential#Concentration overpotential|concentration overpotential]]”.<ref>A.J. Bard, G.R. Inzelt, F. Scholz (Eds.), Electrochemical Dictionary, Springer, Berlin, 2012.</ref><ref>J. Manzanares, K. Kontturi, In: Bard A.J., Stratmann M., Calvo E.J., editors. In Encyclopedia of Electrochemistry, Interfacial Kineticsand Mass Transport, VCH-Wiley, Weinheim; 2003.</ref> 
the changes in concentration (emergence of concentration gradients in the solution adjacent to the electrode surface) is the difference in the rate of electrochemical reaction at the electrode and the rate of ion migration in the solution from/to the surface. When a chemical species participating in an electrochemical electrode reaction is in short supply, the concentration of this species at the surface decreases causing diffusion, which is added to the migration transport towards the surface in order to maintain the balance of consumption and delivery of that species.{{Vague|date={{CURRENTMONTHNAME}} {{CURRENTYEAR}}}}

[[File:Fig1 Concentration polarization.jpg|thumb|Fig. 1. Fluxes and concentration profiles in a membrane and the surrounding solutions. In Fig. '''a''', a driving force is applied to a system initially at equilibrium: the flux of a selectively permeating species in the membrane, <math>J_1^m</math>, is higher than its flux in solution, <math>J_1^s</math>. Higher flux in the membrane causes decreasing concentration at the upstream membrane/solution interface, and increasing concentration at the downstream interface ('''b'''). Concentration gradients gives rise to diffusion transport, which increases the total flux in solution and decreases the flux in the membrane. In steady state, <math>J_1^s=J_1^m</math>.]]

==In membrane science and technology==
In [[Membrane technology|membrane science and technology]], concentration polarization refers to the emergence of concentration gradients at a membrane/solution interface resulted from selective transfer of some species through the membrane under the effect of transmembrane driving forces.<ref>E.M.V. Hoek, M. Guiver, V. Nikonenko, V.V. Tarabara, A.L. Zydney, Membrane Terminology, in: E.M.V. Hoek, V.V. Tarabara (Eds.), Encyclopedia of Membrane Science and Technology, Wiley, Hoboken, NJ, 2013, Vol. 3, pp. 2219–2228.</ref> 
Generally, the cause of concentration polarization is the ability of a membrane to transport some species more readily than the other(s) (which is the [[membrane permselectivity]]): the retained species are concentrated at the upstream membrane surface while the concentration of transported species decreases. Thus, concentration polarization phenomenon is inherent to all types of membrane separation processes. In the cases of [[gas separation]]s, [[pervaporation]], [[membrane distillation]], [[reverse osmosis]], [[nanofiltration]], [[ultrafiltration]], and [[microfiltration]] separations, the concentration profile has a higher level of solute nearest to the upstream membrane surface compared with the more or less well mixed bulk fluid far from the membrane surface. In the case of [[Kidney dialysis|dialysis]] and [[electrodialysis]], the concentrations of selectively transported dissolved species are reduced at the upstream membrane surface compared to the bulk solution. 
The emergence of concentration gradients is illustrated in Figs. 1a and 1b. Fig. 1a shows the concentration profile near and within a membrane when an external driving force is just applied to an initially equilibrium system. Concentration gradients have not yet formed. If the membrane is selectively permeable to species 1, its flux (<math>J_1^m</math>) within the membrane is higher than that in the solution (<math>J_1^s</math>). Higher flux in the membrane causes a decrease in the concentration at the upstream membrane surface (<math>c_1'</math>) and an increase at the downstream surface (<math>c_1''</math>), Fig. 1b. Thus, the upstream solution becomes depleted and the downstream solution becomes enriched in regard to species 1. The concentration gradients cause additional diffusion fluxes, which contribute to an increase of the total flux in the solutions and to a decrease of the flux in the membrane. As a result, the system reaches a steady state where <math>J_1^s=J_1^m</math>. The greater the external force applied, the lower <math>c_1^\prime</math>. In electrodialysis, when <math>c_1'</math> becomes much lower than the bulk concentration, the resistance of the depleted solution becomes quite elevated. The current density related to this state is known as the [[limiting current density]].<ref>H. Strathmann, Ion-Exchange Membrane Separation Processes, Elsevier, Amsterdam, 2004 p. 166</ref>

Concentration polarization strongly affects the performance of the separation process. First, concentration changes in the solution reduce the driving force within the membrane, hence, the useful flux/rate of separation. In the case of pressure driven processes, this phenomenon causes an increase of the [[osmotic pressure]] gradient in the membrane, which reduces the net driving pressure gradient. In the case of dialysis, the driving concentration gradient in the membrane is reduced.<ref>R.W. Baker, Membrane Technology and Applications, John Wiley & Sons, 2012.</ref> In the case of electromembrane processes, the potential drop in the diffusion boundary layers reduces the gradient of electric potential in the membrane. Lower rate of separation under the same external driving force means increased power consumption.

Moreover, concentration polarization leads to:
* Increased salt leakage through the membrane
* Increased probability of [[Fouling|scale/fouling]] development

Thus, the selectivity of separation and the membrane lifetime are deteriorated.

Generally, to reduce the concentration polarization, increased flow rates of the solutions between the membranes as well as spacers promoting turbulence are applied [5, 6]. This technique results in better mixing of the solution and in reducing the thickness of the diffusion boundary layer, which is defined as the region in the vicinity of an electrode or a membrane where the concentrations are different from their value in the bulk solution.<ref>{{GoldBookRef |title=diffusion layer (concentration boundary layer) |file=D01725 |accessdate=27 March 2019 }}</ref> In electrodialysis, additional mixing of the solution may be obtained by applying an elevated voltage where current-induced convection occurs as gravitational convection or electroconvection. Electroconvection is defined <ref>R.F. Probstein, Physicochemical Hydrodynamics, Wiley, NY, 1994.</ref> as current-induced volume transport when an electric field is imposed through the charged solution. Several mechanisms of electroconvection are discussed.<ref>I. Rubinstein, B. Zaltzman, Electro-osmotically induced convection at a permselective membrane, Physical Review E 62 (2000) 2238.</ref><ref>N.A. Mishchuk, Concentration polarization of interface and non-linear [[electrokinetic phenomena]], Advances in Colloid and Interface Science 160 (2010) 16.</ref><ref name=victor11>V.V. Nikonenko, N.D. Pismenskaya, E.I. Belova, P. Sistat, P. Huguet, G. Pourcelly, C. Larchet, Intensive current transfer in membrane systems: modeling, mechanisms and application in [[electrodialysis]], Advances in Colloid and Interface Science 160 (2010) 101.</ref><ref>Y. Tanaka, Ion Exchange Membranes: Fundamentals and Applications, Elsevier, Amsterdam, 2007.</ref> In dilute solutions, electroconvection allows increasing current density several times higher than the limiting current density.<ref name=victor11/> Electroconvection refers to [[electrokinetic phenomena]], which are important in [[microfluidic]] devices. Thus, there is a bridge between membrane science and micro/nanofluidics.<ref>J. De Jong, R.G.H. Lammertink, M. Wessling, Membranes and microfluidics: a review, Lab on a Chip—Miniaturisation for Chemistry and Biology 6 (9) (2006) 1125.</ref> Fruitful ideas are transferred from [[microfluidics]]: novel conceptions of electro-membrane devices for water desalination in overlimiting current range have been proposed.<ref>S.-J. Kim, S.-H. Ko, K.H. Kang, J. Han, Direct seawater [[desalination]] by ion concentration polarization, Nature Nanotechnology 5 (2010) 297.</ref><ref>M.Z. Bazant, E.V. Dydek, D. Deng, A. Mani, Method and apparatus for desalination and purification, US Patent 2011/0308953 A1.</ref>

==References==
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[[Category:Electrochemistry]]
[[Category:Membrane technology]]