Abstract
Electrodialysis is utilized for the deionization of saline streams, usually formed by strong electrolytes. Recently, interest in new applications involving the transport of weak electrolytes through ion-exchange membranes has increased. Clear examples of such applications are the recovery of valuable metal ions from industrial effluents, such as electronic wastes or mining industries. Weak electrolytes give rise to a variety of ions with different valence, charge sign and transport properties. Moreover, development of concentration polarization under the application of an electric field promotes changes in the chemical equilibrium, thus making more complex understanding of mass transfer phenomena in such systems. This investigation presents a set of experiments conducted with salts of multivalent metals with the aim to provide better understanding on the involved mass transfer phenomena. Chronopotentiometric experiments and current-voltage characteristics confirm that shifts in chemical equilibria can take place simultaneous to the activation of overlimiting mass transfer mechanisms, that is, electroconvection and water dissociation. Electroconvection has been proven to affect the type of precipitates formed at the membrane surface thus suppressing the simultaneous dissociation of water. For some electrolytes, shifts in the chemical equilibria forced by an imposed electric field generate new charge carriers at specific current regimes, thus reducing the system resistance.
Highlights
Ion transport is relevant in numerous biological and engineering systems [1]
Phenomena Related to Shifts in Chemical Equilibria during Electrodialysis of Multivalent Ions
Chronopotentiometry provides information about different events happening in electrodialysis cells under the influence of an electric field, especially at the diluted diffusion boundary layer
Summary
Ion transport is relevant in numerous biological and engineering systems [1]. Selective ion transport is the key process in desalination of salt solutions by means of electrodialysis as well as in electrochemical energy conversion systems, such as redox flow batteries or reverse electrodialysis [2,3,4,5].In the above-mentioned processes, ion-exchange membranes ensure ionic continuity and selective transport of ions between different compartments. Ion transport is relevant in numerous biological and engineering systems [1]. Selective ion transport is the key process in desalination of salt solutions by means of electrodialysis as well as in electrochemical energy conversion systems, such as redox flow batteries or reverse electrodialysis [2,3,4,5]. In the above-mentioned processes, ion-exchange membranes ensure ionic continuity and selective transport of ions between different compartments. Ion-exchange membranes are polymeric films containing fixed charges in their structure, which make them selective for mobile ions with the opposite charge (counter-ions) and impermeable for mobile ions with the same charge (co-ions). Two main types of ion-exchange membranes exist: anion- and cation-exchange membranes.
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