Unraveling charge transport in carbon flow-electrodes: performance prediction for desalination applications

Abstract

We report the design and analysis of a salt metathesis process using Flow-Electrode Capacitive Deionization (FCDI) for the generation of a concentrated valuable magnesium sulfate solution from dilute MgCl2 and Na2SO4 (or K2SO4) solutions. First, a batch mode decomposition and recombination of the MgCl2, Na2SO4, and K2SO4 solutions was studied with varying initial concentrations. Current efficiencies of ∼100% were observed for each cycle. In a so-called decomposition step, two different salt solutions are decomposed into electrically charged slurries having the counterions adsorbed. Swapping the slurries with the stored counterions during the recombination step results into new salt solutions upon discharge including the desired product. Both, purity of products and overall conversion of ions into the products, depend on operational parameters, while maximal achieved MgSO4 purity was as high as 93% with a concentration factor of 6.3 and a discharge current efficiency of ∼85%. Finally, a semicontinuous FCDI metathesis system was investigated. Performing the recombination step at appropriate process conditions also allows the concentration of the resulting product solutions by a factor as high as 81.5 with MgSO4 purity of ∼80% and current efficiencies of 96%. Future improvements in process configurations and membrane ion selectivity will render the process even more selective.