Microscale electrodeionization: In situ concentration profiling and flow visualization

Electrodeionization (EDI) is membrane-based desalination utilizing ion exchange membranes and ion exchange resins. By combining Electrodialysis and Ion exchanger, EDI can produce ultrapure water in a continuous-flow manner. Although its theoretical mechanisms are well documented, there is no experimental platform that can provide microscopic details inside of the system. In this paper, we present microscale EDI that can visualize in situ ion concentration, pH, and fluid flows. The platform was fabricated by filling ion exchange resins as a monolayer in a transparent polydimethylsiloxane channel between cation and anion exchange membranes. According to operating voltages (0-15V), distinct behaviors of ion concentration profile, pH shift, and fluid flows were observed in Ohmic, limiting, and overlimiting regimes. It is noteworthy that overlimiting regimes can be sub-categorized as water-splitting and electroconvection regimes. In the early stage (4-8V), water-splitting is dominant with pH change near the membranes and resins; under a higher voltage (8-15V), electroconvection starts to occur even water-splitting tries to suppress the development of the extended space charge layer and corresponding electroconvective instability. Accelerated ionic migration by electroconvection can improve current efficiency up to 80%. This is a clear departure from overlimiting dynamics in electrodialysis (with electroconvection only), ion exchanger (with no distinct regime), and even from that in previous EDI experiments (with water splitting only).