Concentration polarization is a term used in the scientific fields of
electrochemistry
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...
and
membrane science.
In electrochemistry
In
electrochemistry
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...
, concentration polarization denotes the part of the
polarization of an
electrolytic cell
An electrolytic cell is an electrochemical cell that utilizes an external source of electrical energy to force a chemical reaction that would not otherwise occur. The external energy source is a voltage applied between the cell′s two electrod ...
resulting from changes in the electrolyte concentration due to the passage of current through the electrode/solution interface. Here ''polarization'' is understood as the shift of the
electrochemical potential
In electrochemistry, the electrochemical potential (ECP), ', is a thermodynamic measure of chemical potential that does not omit the energy contribution of electrostatics. Electrochemical potential is expressed in the unit of J/ mol.
Introductio ...
difference across the cell from its equilibrium value. When the term is used in this sense, it is equivalent to “
concentration overpotential”.
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.
In membrane science and technology
In
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.
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
Gas separation can refer to any of a number of techniques used to separate gases, either to give multiple products or to purify a single product.
Swing adsorption techniques
Pressure swing adsorption
Pressure swing adsorption (PSA) pressurizes ...
s,
pervaporation Pervaporation (or pervaporative separation) is a processing method for the separation of mixtures of liquids by partial vaporization through a non-porous or porous membrane.
Theory
The term ''pervaporation'' is a portmanteau of the two steps of the ...
,
membrane distillation
Membrane distillation (MD) is a thermally driven separation process in which separation is driven by phase change. A hydrophobic membrane presents a barrier for the liquid phase, allowing the vapour phase (e.g. water vapour) to pass through the m ...
,
reverse osmosis
Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to separate ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pre ...
,
nanofiltration
Nanofiltration is a membrane filtration process used most often to soften and disinfect water.
Overview
Nanofiltration is a Membrane technology, membrane filtration-based method that uses nanometer sized pores through which particles smaller t ...
,
ultrafiltration
Ultrafiltration (UF) is a variety of membrane filtration in which forces such as pressure or concentration gradients lead to a separation through a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained in the s ...
, and
microfiltration
Microfiltration is a type of physical filtration process where a contaminated fluid is passed through a special pore-sized membrane filter to separate microorganisms and suspended particles from process liquid. It is commonly used in conjunction ...
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
dialysis and
electrodialysis
Electrodialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis c ...
, 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 (
) within the membrane is higher than that in the solution (
). Higher flux in the membrane causes a decrease in the concentration at the upstream membrane surface (
) and an increase at the downstream surface (
), 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
. The greater the external force applied, the lower
. In electrodialysis, when
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.
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
Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane.
It is also defined as the measure of the tendency of a solution to take in a pure ...
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. 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
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
, 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. 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 as current-induced volume transport when an electric field is imposed through the charged solution. Several mechanisms of electroconvection are discussed.
[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
Electrodialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis c ...
, Advances in Colloid and Interface Science 160 (2010) 101. In dilute solutions, electroconvection allows increasing current density several times higher than the limiting current density.
[ Electroconvection refers to ]electrokinetic phenomena Electrokinetic phenomena are a family of several different effects that occur in heterogeneous fluids, or in porous bodies filled with fluid, or in a fast flow over a flat surface. The term heterogeneous here means a fluid containing particles. Part ...
, which are important in microfluidic
Microfluidics refers to the behavior, precise control, and manipulation of fluids that are geometrically constrained to a small scale (typically sub-millimeter) at which surface forces dominate volumetric forces. It is a multidisciplinary field tha ...
devices. Thus, there is a bridge between membrane science and micro/nanofluidics. Fruitful ideas are transferred from microfluidics
Microfluidics refers to the behavior, precise control, and manipulation of fluids that are geometrically constrained to a small scale (typically sub-millimeter) at which surface forces dominate volumetric forces. It is a multidisciplinary field tha ...
: novel conceptions of electro-membrane devices for water desalination in overlimiting current range have been proposed.[M.Z. Bazant, E.V. Dydek, D. Deng, A. Mani, Method and apparatus for desalination and purification, US Patent 2011/0308953 A1.]
References
{{reflist
Electrochemistry
Membrane technology