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The secret of membrane technology: How to change the flow of water through selective transport?
With the increasing global demand for clean water resources, membrane technology has become an important method to solve water resources problems. Among them, concentration polarization is a key concept, especially in the fields of electrochemistry and membrane science. This article will provide an in-depth look at how concentration polarization affects the membrane separation process and how it changes water flow.
The importance of concentration polarization in electrochemistry
In electrochemistry, concentration polarization refers to the change in electrolyte concentration caused by the passage of electric current through the electrode/solution interface. This phenomenon causes polarization of the electrochemical cell and shifts its electrochemical potential difference from equilibrium. The effect of this shift is caused by the difference between the electrode reaction and the migration speed of the ions in the solution.
When there is a shortage of a chemical involved in an electrode reaction, the concentration of that substance drops, as diffusion occurs to maintain a balance between consumption and supply.
Concentration polarization in membrane science
In membrane science and technology, concentration polarization refers to the concentration gradient produced at the membrane/solution interface due to the selective transfer of some species through the membrane. This phenomenon is inherent to the membrane separation process, mainly due to the membrane's ability to selectively transport certain substances.
In separation processes such as gas separation, membrane distillation, reverse osmosis, nanofiltration and microfiltration, the concentration profile shows that the solute concentration is higher on the upstream surface of the membrane, while the concentration is relatively lower downstream of the membrane.
For example, when an external driving force acts on an initial equilibrium system, the selective permeability of the membrane will cause the concentration of substances upstream of the membrane to decrease, while it will increase relatively downstream. The emergence of this concentration gradient will trigger an additional diffusion flux, eventually causing the system to reach a stable state.
The impact and challenges of concentration polarization
Concentration polarization has a strong impact on the performance of separation processes. Concentration changes in the solution reduce the driving force inside the membrane, thereby reducing the effective separation rate. During pressure driving, this causes an increase in the osmotic pressure gradient, further reducing the net driving pressure gradient.
In addition, concentration polarization can lead to increased salt leakage and increased risk of scaling/fouling, compromising membrane selectivity and longevity.
Strategies to deal with concentration polarization
To reduce the effects of concentration polarization, it is common to increase the flow rate of the solution between the membranes and to use spacers that promote turbulence. These techniques can improve the mixing state of the solution and reduce the thickness of the diffusion boundary layer, a region where the concentration near the electrode or membrane differs from its value in the liquid.
Further mixing may be obtained by applying higher voltages during electrodeionization, since current-induced convection phenomena occur in the flow. This phenomenon, called electroconvection, involves volume transport induced in a charged solution when an electric field is applied.
The bridge between membrane technology and microfluidic devices
There are close connections between membrane science and microfluidics. Inspired by microfluidic devices, many novel electromembrane device designs have emerged to desalinize water in an ultra-limited current range. This promotes innovation in technological applications and provides new ideas for solving water resource problems.
To successfully address global water resources challenges, how will membrane technology further evolve and innovate?
