Abstract
Electrodialysis is an electric-field-mediated process separating ions exploiting selective properties of ion-exchange membranes. The ion-exchange membranes create an ion-depleted zone in an electrolyte solution adjacent to the membrane under DC polarization. We constructed a microfluidic system that uses the ion-depleted zone to separate ions from the processed water solution. We tested the separation performance by desalting a model KCl solution spiked with fluorescein for direct observation. We showed both visually and by measuring the conductivity of the output solutions that the system can work in three modes of operation referred to as continuous desalination, desalination by accumulation, and unsuccessful desalination. The mode of operation can easily be set by changing the control parameters. The desalination factors for the model KCl solution reached values from 80 to 100%, depending on the mode of operation. The concentration factor, given as a ratio of concentrate-to-feed concentrations, reached zero for desalination by accumulation when only diluate was produced. The water recovery, therefore, was infinite at these conditions. Independent control of the diluate and concentrate flow rates and the DC voltage turned our system into a versatile platform, enabling us to set proper conditions to process various samples.
Highlights
Ion-exchange membranes (IEMs) [1,2] are widely used inmembrane separation processes [3] for the removal of small solutes possessing charge under given conditions
We showed both visually and by measuring the conductivity of the output solutions that the system can work in three modes of operation referred to as continuous desalination, desalination by accumulation, and unsuccessful desalination
We showed that tuning the strength of the electric field and the flow rate in the channel predefines the length of the ion-depleted zone and the position of the ion-preconcentrated band
Summary
Ion-exchange membranes (IEMs) [1,2] are widely used in (electro-)membrane separation processes [3] for the removal of small solutes possessing charge under given conditions. These separation processes include electrodialysis (ED) [4], electrodeionization (EDI) [5], bipolar membrane electrodialysis (BMED) [6], capacitive deionization (CDI) [7], or diffusion dialysis (DD) [8]. ICP has a rather broad meaning and, among others, includes the behavior of ion-selective systems in the DC electric field in association with preconcentration or removal of charged entities [14]
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