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Langmuir Article Figure 9. Sketch of a scalable shock electrodialysis system, motivated by our experimental results. can consider a different electrodeposition reaction. In the same device, we remove silver ions from 10 mM silver nitrate (AgNO3) through a porous frit and a Nafion membrane by silver deposition at the cathode. The current is sustained by copper dissolution at the anode without allowing sufficient time for ICP to cross the reservoir and reach the frit during a voltage sweep. The results in Figure 8 are similar for both electrolytes and consistent with the theory, thereby showing the generality of the phenomenon. The raw current−voltage data (Figure 8a) indicates a slightly smaller overlimiting conductance and a much smaller limiting current for AgNO3, as suggested by the scaling Ilim ∼ zD. (DAgNO3 = 1.68 × 10−5 cm2/s55 and DCuSO4 = 6.75 × 10−6 cm2/s46 at 10 mM.) At the same voltage, Ṽapp = 1.5 V (bars in Figure 8b), the dimensionless voltage V is also smaller by a factor of 2 (for monovalent vs divalent cations), and the dimensionless current Ĩ = 2.6 for AgNO3 is larger than Ĩ = 1.6 for ̃CuSO4, which implies a wider depletion zone, scaling as (1 − I−1). During water extraction, the dependencies of the deionization factor on flow rate and diffusivity are nicely captured by the simple scaling of eq 12, as shown in Figure 8c. ■ DISCUSSION Our primary finding is that thin double layers in porous media can enable faster ionic transport, leading to new surface-driven mechanisms for OLC based on SC and EOF. In particular, electroconvection driven by EOF can sustain OLC in a heterogeneous porous medium with micrometer-scale pores pressed against an impermeable electrodialysis membrane. The onset of OLC is associated with a macroscopic region of deionization within the pores (outside the double layers), which can propagate against pressure-driven flow like a shock wave. In steady state, the overlimiting conductance is approximately constant (aside from surface charge regulation), in spite of enormous spatial variations in conductivity (up to 3 orders of magnitude). These surprising phenomena are in stark contrast to the constant conductivity of ion-exchange membranes with smaller pores and overlapping (thick) double layers. The nonlinear electrokinetic properties of porous media can be exploited for separations. Our proof-of-concept experiments (Figures 7 and 8) show that deionized water can be continuously extracted from salty water via a porous medium sustaining OLC. It is beyond the scope of this article to build and test a practical desalination system with electrode streams, but a possible design for a scalable shock ED system is shown in Figure 9. A stack of two (or more) separation layers of negatively charged porous media separated by cation exchange membranes sustains an overlimiting current. In each layer, the input solution (e.g., NaCl) undergoes salt enrichment near one membrane and salt depletion with a deionization shock near the other. In pressure-driven cross-flow, these regions are continuously separated into fresh and brine streams upon leaving the porous medium. By varying the position of the fresh/brine stream splitting in each porous layer, high water recovery (wide shock) can be traded against low energy cost (thin shock). As in standard ED, direct current can be sustained at the electrodes by water splitting (or other) reactions whose overpotential becomes negligible compared to the total voltage as the stack size increases. Besides water deionization, such a system may also find applications in brine concentration (e.g., for salt precipitation or forward osmosis) or in nanoparticle separations. Because the separation occurs within the porous medium in cross-flow, the membrane removing ions is electrokinetically shielded and may resist fouling (which is a concern for other desalination methods1,4). Membraneless designs with layered porous media of different pore sizes (analogous to micro/nanochannel junctions20,23,25) may also be possible. Clogging by incoming particles or brine precipitates could be managed by reverse bias, 16175 dx.doi.org/10.1021/la4040547 | Langmuir 2013, 29, 16167−16177PDF Image | Overlimiting Current and Shock Electrodialysis in Porous Media
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