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The superpower of membranes: How to deal with concentration polarization and improve separation efficiency?
In the development of membrane science and technology, "concentration polarization" is a crucial concept. This phenomenon refers to the selective transport of certain species through a membrane, resulting in the formation of a concentration gradient at the membrane/solution interface. This not only affects the functionality of the membrane, but also affects the efficiency of various separation processes.
Concentration polarization is an inherent characteristic of all membrane separation processes and will affect the separation effect and membrane life.
In electrochemical processes, concentration polarization also exists. When electric current passes through the electrode/solution interface, the change in electrolyte concentration causes the phenomenon of polarization. "Polarization" here represents the change in potential difference in an electrochemical cell. Compared with the equilibrium value, the concentration gradient formed directly affects the speed of the electrochemical reaction.
In the membrane separation process, the selective transmission ability of the membrane to different substances determines its separation efficiency. For example, during reverse osmosis, the concentration of some substances on the membrane surface increases, while the concentration of other substances decreases because they are selectively delivered. This causes the concentration upstream of the membrane to become rarefied, further exacerbating the phenomenon of concentration polarization.
As the membrane operation process progresses, the flow rate of specific substances will decrease, thereby affecting the rate and effect of separation.
When performing gas separations or other membrane processes, such as nanofiltration, ultrafiltration, and microfiltration, the occurrence of concentration gradients causes the diffusion flux of the solvent to increase and the substance flux in the membrane to decrease. This imbalance will eventually cause the system to reach a steady state, where the flow rate in the membrane corresponds to the flow rate in the fluid, and the efficiency of this process is affected by external pressure and the selectivity of the membrane.
To reduce the effect of concentration polarization, increasing the flow rate of the solution between the membranes and using baffles that promote turbulence have become effective methods. This technology can effectively enhance the mixing degree of the solution, reduce the thickness of the diffusion boundary layer, and thereby improve the separation efficiency.
"Current-induced convection can allow the current density to be significantly higher than the limiting current density in dilute solutions."
Additionally, during electrodialysis, applying higher voltages can further mix the solution, creating a phenomenon known as electroconvection. This phenomenon is due to the volume transport in the charged solution that occurs in the presence of an electric field, effectively improving the overall separation efficiency.
With the rapid development of microfluidic technology, many new ideas have been applied to membrane technology, especially in water desalination and other application fields. These interdisciplinary concepts continue to open up new possibilities for the future of membranes.
How to further improve membrane technology to achieve higher separation efficiency when facing the challenge of concentration polarization? This remains an important issue and challenge in membrane scientific research.
