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Most electrochemical processes, such as electrodialysis, are limited bydiffusion, but in porous media, surface conduction and electro-osmotic flowalso contribute to ionic fluxes. In this paper, we report experimental evidencefor surface-driven over-limiting current (faster than diffusion) anddeionization shocks (propagating salt removal) in a porous medium. Theapparatus consists of a silica glass frit (1 mm thick with 500 nm mean poresize) in an aqueous electrolyte (CuSO$_4$ or AgNO$_3$) passing ionic currentfrom a reservoir to a cation-selective membrane (Nafion). The current-voltagerelation of the whole system is consistent with a proposed theory based on theelectro-osmotic flow mechanism over a broad range of reservoir saltconcentrations (0.1 mM - 1.0 M), after accounting for (Cu) electrodepolarization and pH-regulated silica charge. Above the limiting current,deionized water ($\approx 10 \mu$ $M$) can be continuously extracted from thefrit, which implies the existence of a stable shock propagating against theflow, bordering a depleted region that extends more than 0.5mm across theoutlet. The results suggest the feasibility of "shock electrodialysis" as a newapproach to water desalination and other electrochemical separations.

Overlimiting Current and Shock Electrodialysis in Porous Media