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

Abstract

Reverse electrodialysis (RED) represents one of the most promising membrane-based technologies for clean and renewable energy production from mixing water solutions. However, the presence of multivalent ions in natural water drastically reduces system performance, in particular, the open-circuit voltage (OCV) and the output power. This effect is largely described by the “uphill transport” phenomenon, in which multivalent ions are transported against the concentration gradient. In this work, recent advances in the investigation of the impact of multivalent ions on power generation by RED are systematically reviewed along with possible strategies to overcome this challenge. In particular, the use of monovalent ion-selective membranes represents a promising alternative to reduce the negative impact of multivalent ions given the availability of low-cost materials and an easy route of membrane synthesis. A thorough assessment of the materials and methodologies used to prepare monovalent selective ion exchange membranes (both cation and anion exchange membranes) for applications in (reverse) electrodialysis is performed. Moreover, transport mechanisms under conditions of extreme salinity gradient are analyzed and compared for a better understanding of the design criteria. The ultimate goal of the present work is to propose a prospective research direction on the development of new membrane materials for effective implementation of RED under natural feed conditions.

Selectivity of Transport Processes in Ion-Exchange Membranes: Relationship with the Structure and Methods for Its Improvement

“…For the development of such technologies the selectivity between ions with different valences of the same sign (mono- and multivalent) is no less important [ 51 , 52 , 53 , 54 ]. The processes of their separation have recently been extensively reviewed [ 55 , 56 , 57 , 58 ]. However, this topic cannot be ignored, and in order to avoid overlapping, in this review, we will consider it briefly, focusing on newer studies in this area.…”

Surface Modifications of Anion Exchange Membranes for an Improved Reverse Electrodialysis Process Performance: A Review

“…Research on utilization of cheaper, non-toxic, sustainable and environment-friendly polymers is still required to design tailor-made AEMs using greener preparation methods. Last but not the least, the electro-membrane resistance must be controlled, owing to the high number of layers often involved in LbL strategies, in which 7.5 bilayers seems to be the optimal value, in order to keep the monovalent membrane permselectivity unaffected [18]. Na2SO4 aqueous solutions, in the presence of humic acid as a model foulant, highlighting an improved net power density up to 17% and an enhanced energy conversion efficiency after LbL modification.…”

“…Moreover, the presence of multivalent ions in natural streams also represents a major challenge that must be overcome because the uphill transport of these ions against their concentration gradients leads to a reduced obtainable power output from RED, according to the Nernst equation [16][17][18]. Therefore, this unfavorable effect must be addressed in not only experimental studies, where a relatively high multivalent ions concentration in the LC compartments of the RED system leads to a decreased process efficiency in terms of stack voltage [19], but should also take into consideration robust modeling tools for the continuous design and optimization of the RED technology [20].…”

“…In this respect, strategies followed so far for developing monovalent permselective and hydrophilic AEMs for RED applications could be easily incorporated into the AAEMFC technology, in order to move forward in seeking improvements of the AEM structure, surface properties, etc. [18].…”

“…Numerous review articles of a good standard can be found in the literature, which consider different aspects of the RED systems, like improvements of electrodes and feed solutions [2], critical operating conditions [6], current problems and challenges [24], potential of ion exchange membranes (IEMs) [7,[25][26][27][28], synthesis and characterization of AEMs [29], impact of multivalent ions [18], permselectivity of IEMs [18,30,31], electro-conductive membranes [32], fouling studies [9,33], etc. Nevertheless, this paper provides an additional insight that focuses on surface AEM modification methods for RED performance improvements.…”