Application of the Rd/w framework to assess Donnan dialysis performance

Donnan dialysis exploits electrochemical potential gradients across ion-exchange membranes to separate ions between feed and draw solutions. This technique has been applied for treatment and recovery of chemicals in water and wastewater. Previous studies have arbitrarily selected the draw solution chemistry, making it difficult to fairly compare experimental outcomes. A universal framework is needed to standardize design and interpretation of Donnan dialysis systems. We calculated the Rd/w parameter, which is related to the draw ion concentrations in the feed and draw solutions at Donnan equilibrium, for previous studies. Rd/w values were used to determine theoretical recoveries and compare them to experimental outcomes. Of the literature data, 57% matched the theoretical recovery, 37% underperformed due to operating time constraints or transport limitations, and 6% outperformed Donnan equilibrium due to use of integrated processes. Ultimately, this work highlights the benefits of the Rd/w framework for standardizing interpretation of Donnan dialysis systems.