594 research outputs found
Overlimiting Current and Shock Electrodialysis in Porous Media
Most electrochemical processes, such as electrodialysis, are limited by
diffusion, but in porous media, surface conduction and electro-osmotic flow
also contribute to ionic fluxes. In this paper, we report experimental evidence
for surface-driven over-limiting current (faster than diffusion) and
deionization shocks (propagating salt removal) in a porous medium. The
apparatus consists of a silica glass frit (1 mm thick with 500 nm mean pore
size) in an aqueous electrolyte (CuSO or AgNO) passing ionic current
from a reservoir to a cation-selective membrane (Nafion). The current-voltage
relation of the whole system is consistent with a proposed theory based on the
electro-osmotic flow mechanism over a broad range of reservoir salt
concentrations (0.1 mM - 1.0 M), after accounting for (Cu) electrode
polarization and pH-regulated silica charge. Above the limiting current,
deionized water ( ) can be continuously extracted from the
frit, which implies the existence of a stable shock propagating against the
flow, bordering a depleted region that extends more than 0.5mm across the
outlet. The results suggest the feasibility of "shock electrodialysis" as a new
approach to water desalination and other electrochemical separations.Comment: 39 pages, 9 fig
An overview of the oil-brine interfacial behavior and a new surface complexation model
Abstract The few existing surface complexation models (SCM) for the brine-oil interface have important limitations: the chemistry of each crude oil is not considered, they cannot capture the water/non-polar hydrocarbons surface charge, the interactions between Na+ and the acid sites are not included, and the equilibrium constants for the adsorption reactions are not validated against experimental data. We address the aforementioned constraints by proposing an improved diffuse-layer SCM for the oil-brine interface. The new model accounts for the chemistry of crude oils by considering surface sites linearly dependent on the TAN (total acid number) and TBN (total base number). We define weak sites to account for the negative surface charge observed for non-polar hydrocarbons in water. We optimize the parameters of our model by fitting the model to reported zeta potential measurements of oil in aqueous solutions. When we validate the optimized model against different experimental data sets, it generally shows a good performance in predicting the surface charge of oil in different brines with different pHs. We show that the acid and base numbers are only useful as a qualitative estimation of the distribution of polar groups at the oil surface, and more sophisticated analysis is necessary to quantify the chemistry of the oil-brine interface
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