Mitigation of the effects of multivalent ion transport in reverse electrodialysis

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

“…Under the OCV conditions, a higher voltage of Na + forces Mg 2+ to be transported from the river water to the seawater to maintain the electroneutrality that is disrupted due to the initial high transport rate of Na + from HCC to LCC. This process is termed as uphill transport, in which two Na + ions are transported in the opposite direction to the Mg 2+ ions [5][6][7][8][24][25][26][27]. The Na + and Mg 2+ start moving until the overall E i is balanced, reaching an equilibrium (E Na+ = E Mg2+ ); at this point, the uphill transport stops, defined as During uphill transport, the resulting voltage over the membrane is a value somewhere between the theoretical ENa + and EMg 2+ , i.e., less than the expected OCV for a pure solution of NaCl.…”

“…The development of a new generation of monovalent ion-selective membranes represents one of the key strategies to overcoming the problem of uphill transport in the RED process [24,26,48,49]. monovalent ion-selective membranes allow the passage of monovalent ions, but block the transport of multivalent ions (Figure 7a) [24,26,39,48,49]. Several factors, such as the differences in hydrated ionic radii, the differences in migration rate within the membrane phase, and the affinity of the ions with the membrane, affect the permselectivity between ions of the same charge [3].…”

“…The transport number ratio between mono-and divalent ions is used to measure the monovalent ion selectivity of the membrane [24,48,49]. After modification, the improvement of the monovalent ion selectivity of the membrane can be calculated as follows [48,49]:…”

“…where t mon and t div are the transport numbers of the mono-and the divalent ions, respectively, and C mon and C div are the average concentrations of the mono-and the divalent ions in dilute solution, respectively. A higher transport number ratio implies better monovalent selectivity, and vice versa [24,39,48,49,51,53]. Only a few studies focusing on the use of monovalent selective IEMs for RED applications are available [24,26,39,48,49].…”

“…Gao et al found that monovalent-ion selective poly(styrene sulfonate) (PSS)and poly(ethyleneimine) (PEI)-modified AEMs (CJMA-2) showed an increase in power density of up to 17% more than the standard AEMs when using feed solutions containing Cl − , SO 4 2− , and natural organic matter (NOM) [48]. However, Moreno et al revealed that the power densities of RED were not significantly improved by the use of highly cross-linked monovalent Neosepta CMS [24]. Uphill transport was mitigated, but the monovalent selective membranes experienced high resistance due to low ionic mobility of Mg 2+ .…”