Effects of Divalent Cations on Electrical Membrane Resistance in Reverse Electrodialysis for Salinity Power Generation

Abstract

Reverse electrodialysis (RED) is an emerging technology that can generate electricity from the mixing of two water streams (i.e., the concentrated and the diluted streams) with salinity gradient. In RED, the higher salinity gradient between water streams yields the higher power production. Therefore, water sources containing a high concentration of salts such as reverse osmosis brine, hypersaline lakes, and produced water from hydraulic fracturing could be considered as feed streams for enhancing energy production in RED. However, these water sources contain not only NaCl but also various multivalent ions, which are likely to increase electrical resistance of ion exchange membranes (IEMs) and potentially decrease power generation. In this study, we investigated the effects of divalent cations in the concentrated stream, including magnesium, calcium, and barium ions on electrical resistance of IEMs in static mode. The electrical resistance of IEMs in static mode was found to be correlated to power production in a bench-scale RED process during continuous operation. As a result, it was found that divalent cation with the smaller hydrated radius showed the higher electrical resistance in the static mode and the increased electrical resistance of cation exchange membrane (CEM) resulted in power reduction during the continuous operation of the bench-scale RED process.

Modeling the influence of divalent ions on membrane resistance and electric power in reverse electrodialysis

“…The results reported in these works clearly prove that the presence of divalent ions in RED water streams leads to a reduction in power output and thus their role in RED stacks requires to be specifically addressed. These ions display a strong influence on RED operation and especially on membrane resistance due to their interactions with the fixed charged groups present in the ion exchange membranes [8,9,[17][18][19]. The divalent ions at the highest concentration in these streams usually are magnesium (Mg 2+ ), calcium (Ca 2+ ) and sulphate (SO 4 2+ ).…”

Design of Monovalent Ion Selective Membranes for Reducing the Impacts of Multivalent Ions in Reverse Electrodialysis

“…Permselectivities of up to 66% were reported by Tedesco et al when using an LCC solution of 0.5 M NaCl and HCC solutions of 5 M NaCl in RED [40]. In fact, the use of highly concentrated solutions also runs the risk of membrane fouling such as scaling [3,38,41], which can be controlled by, for example, periodic air sparging and/or feed water reversal [42]. A high fixed-charge density of an IEM would benefit at high salinity gradients by reducing the permeation of the co-ions, whereas thick membranes would benefit at low salinity gradients by decreasing the osmotic flux.…”

“…In general, further studies focusing on strategies to reduce both uphill transport and resistance are required to mitigate the impact of multivalent ions in RED stack. Developing a new generation of ion In another study, the trend of divalent cations in increasing resistance of RED stack was shown to be Ba 2+ > Ca 2+ > Mg 2+ [38]. The impact of Ba 2+ was observed to be more pronounced due to its lower hydration radius of 4.04 Ȧ compared to Ca 2+ (4.12Ȧ) and Mg 2+ (4.28 Ȧ), thereby leading to a strong electrostatic attraction by the fixed functional groups in the CEM and increasing the stack resistance.…”

Ion Exchange Membrane for Reverse Electrodialysis

“…Thus, high and low salinity solutions need to be supplied alternately across the IEMs for inducing concentration difference. The separation of cation and anion creates a chemical potential in the system so that electrochemical reactions occur to generate electric current (Figure 3) [18][19][20][21]. The larger the concentration difference, the higher the power density can be obtained theoretically, but the power density would be decreased owing to an increase of internal resistance in the real case.…”