posted on 2024-01-05, 20:51authored byQingpu Wang, Jaehun Chun, Chinmayee V. Subban
Membrane-based systems, such as electrodialysis, play
an important
role in desalination and industrial separation processes. Electrodialysis
uses alternating anion- and cation-exchange membranes with a perpendicular
electric field to generate concentrated and diluate streams from a
feed solution. It is known that under overlimiting current conditions,
reduced charge and mass transfer at the membrane interface leads to
regions of high ion depletion generating instability and vortices
termed electroconvection. While electroconvective mixing is known
to directly impact the separation efficiency of electrodialysis, the
influence of ion concentration gradients across the membrane experienced
in a functional electrodialysis system is not known. Here, we report
the influence of ion concentration gradients across a cation exchange
membrane (Nafion) that is both aligned with and opposed to the applied
electric field. Experiments were conducted by coflowing NaCl solutions
of different concentrations (0.1–100 mM) on each side of the
membrane, and electroconvection was visualized with a fluorescence
dye (Rhodamine 6G). We obtained concentration profiles from fluorescence
image data and systematically measured the thickness of the depletion
boundary layer dBL under different conditions.
We found smaller dBL values at a higher
flow rate both with and without concentration gradients. Our results
show that electroconvection is enhanced when the electric field is
opposite to the direction of the concentration gradient.