Assisted-Reverse Electrodialysis for permeate remineralization from surface water RO brines: Experimental investigation with multi-ionic solutions

Assisted-Reverse Electrodialysis (A-RED) has been recently proposed as a remineralization strategy for river water Reverse Osmosis (RO) permeate through direct recovery of minerals from its retentate brine. Although the proof of concept was presented, assessing the conceptual feasibility of the application, the actual process performance can be dominated by the poorly known specific transport phenomena of different ions through ion exchange membranes. The present work addresses such uncovered aspects, analyzing the behaviour of an A-RED unit fed by multi-ion solutions including sodium, chlorides, calcium, magnesium, potassium, sulphates and bicarbonates, constituting the major ions in RO retentate produced from surface waters. The effect of brine and permeate conductivities and ionic compositions was investigated using a laboratory-scale stack operating at different applied voltages, obtaining information on apparent transport numbers and membrane selectivity for the different ions investigated. Results highlighted that the use of calcium, sodium and chlorides generally have a faster transport, ensuring higher currents, while magnesium, sulphates and bicarbonates presented lower membrane fluxes. Apparent transport numbers were found independent from feed conductivity and applied voltage, while brine composition strongly affected transport numbers and selectivity, which decreased for CEM with the concentration ratio between the generic cation and sodium. In contrast, AEM selectivity remained constant at ~0.6 and ~1.4 for bicarbonates and sulphates, respectively, despite concentration ratio variation. These results provided deeper insight into ion exchange membrane ion transport phenomena and ions “competition”, providing valuable information for the development and widespread application of river RO permeate remineralization using A-RED technology.