Uphill transport of sulfate and chloride ions under different operational conditions of a reverse electrodialysis (RED) stack

An important and less examined phenomenon reducing power produced by reverse electrodialysis (RED) stacks in the presence of divalent ions is “uphill transport”, where ions diffuse in the opposite direction of concentration gradient driven by chemical equilibrium conditions. Of particular interest is sulfate ion's (SO₄^2–) impact on power generation as it is the second most dominant anion in seawater and many wastewaters. We investigated the impact of operating conditions (feed stream composition, temperature, linear velocity of streams), and reactor characteristics (channel thickness, number of cell pairs) on uphill transport of SO₄^2– versus Cl^–. Stack performance was monitored by recording open circuit voltage (OCV), resistance, and maximum gross power density (P_max). Depending on feed stream composition, uphill transport was observed for Cl^– (≤ 7.76 g) and SO₄^2– (≤ 16.59 g) during OCV and P_max conditions. High concentrations of SO₄^2– in the diluted channels reduced the OCV by 41 % (16.59 g transport of SO₄^2–). Uphill transport for SO₄^2– was twice that of Cl^– in experiments with similar composition ratios. Uphill transport of SO₄^2– increased by increasing temperature, diluted stream velocity relative to the concentrated stream, and decreasing the channel thickness (from 270 to 140 μm), leading to the reduction in OCV. Uphill transport of SO₄^2– under OCV conditions was reduced by increasing cell pair numbers (residence time). These results reinforce how complex water streams containing divalent anions, versus simple NaCl solutions often used in RED stacks, can adversely impact power production due to the uphill transport of anions against concentration gradients.