Selective Removal of Barium and Hardness Ions from Brackish Water with Chemically Enhanced Electrodialysis

Abstract

The use of a nonconventional water resource for energy and industrial applications often requires extraction of low-level undesirable ions from matrices of benign dominant ions. In this study, the selective extraction of Ba2+ and Mg2+ from synthetic brine solutions was evaluated using strong acid cation-exchange membranes modified with the surface deposition of macrocyclic molecules including crown ethers and calixarenes. Compared to the bare membrane, vinylbenzo-18-crown-6 (VB18C6) and calix[4]­arene-amended membranes showed increased selectivity for Ba2+ and Mg2+ with respect to the dominant ion (Na+) by up to fourfolds, with the calix[4]­arene-modified membrane achieving more selective separation than VB18C6. Optimal selectivity was achieved at a moderate-to-high current density (3.1–6.3 mA/cm2), which was attributed to the alleviation of transport limitation in the boundary layer by the surface modification. Amendment of a calix[4]­arene derivative with a crown-6 “boot strap” and two carboxylic groups resulted in reduced selectivity due to the formation of strong complexes with divalent ions. These results show that surface deposition of ion sequestrants can be a versatile approach to improve the membrane’s selectivity; however, the performance is sensitive to feed water salinity, current loading, and the macrocycle-ion chemistry, where there is a trade-off between ion affinity and mobility.

Internally Cross-Linked Polysulfone-Based Anion Exchange Membranes with pH Stability for Carboxylate Separation

“…1 H NMR spectrum serves to evaluate the chloromethylation degree of the PSF, with the results visualized in Figure . In the case of the original PSF (Figure a), peaks emerging at about 6.83–7.88 ppm indicate the presence of hydrogens from the benzene ring . Furthermore, a peak noticed at 1.69 ppm corresponds to hydrogens associated with the –CH 3 group.…”

Integrating Divalent-Selective Electrodialysis in Brackish Water Desalination

“…Depending on the membrane characteristics and operational parameters (such as current density), the transport rate of different counterions can differ, enabling separation between counterions. Due to the increased affinity of counterions of higher charge to ion exchange membranes, many commercial cation-exchange membranes have intrinsic selectivity to divalent cations. , Nevertheless, recent studies focused almost exclusively on monovalent selective membranes, which preferentially transport monovalent ions over divalent ions. − Specifically, BW desalination by monovalent-selective ED can maintain essential Ca 2+ and Mg 2+ ions in the desalinated stream ,, and prevent calcium from scaling in the brine. However, due to a permselectivity-conductivity trade-off – inflicting a higher energy demand , – and the high cost of ion-exchange membranes, this process is still economically inferior to BWRO.…”

Comparison of Ion Selectivity in Electrodialysis and Capacitive Deionization

“…Ion selectivity in ED can be achieved via careful design of the membranes, which can affect the population of certain ions within the membrane or the diffusion of ions through the membrane in a selective manner. Proposed designs include tuning the cross-linking of the surface layer, , adding a thin oppositely charged layer to the membrane surface, − using layer-by-layer films, , blending different polymers, , and choosing chemical moieties with affinity and coordination chemistry toward specific or a group of ions. ,− In addition to membrane design, operating conditions, such as the feed flow rate and applied voltage or current, can affect the selectivity achieved …”