How Chemical Nature of Fixed Groups of Anion-Exchange Membranes Affects the Performance of Electrodialysis of Phosphate-Containing Solutions?

Abstract

Innovative ion exchange membranes have become commercially available in recent years. However, information about their structural and transport characteristics is often extremely insufficient. To address this issue, homogeneous anion exchange membranes with the trade names ASE, CJMA-3 and CJMA-6 have been investigated in NaxH(3−x)PO4 solutions with pH 4.4 ± 0.1, 6.6 and 10.0 ± 0.2, as well as NaCl solutions with pH 5.5 ± 0.1. Using IR spectroscopy and processing the concentration dependences of the electrical conductivity of these membranes in NaCl solutions, it was shown that ASE has a highly cross-linked aromatic matrix and mainly contains quaternary ammonium groups. Other membranes have a less cross-linked aliphatic matrix based on polyvinylidene fluoride (CJMA-3) or polyolefin (CJMA-6) and contain quaternary amines (CJMA-3) or a mixture of strongly basic (quaternary) and weakly basic (secondary) amines (CJMA-6). As expected, in dilute solutions of NaCl, the conductivity of membranes increases with an increase in their ion-exchange capacity: CJMA-6 < CJMA-3 << ASE. Weakly basic amines appear to form bound species with proton-containing phosphoric acid anions. This phenomenon causes a decrease in the electrical conductivity of CJMA-6 membranes compared to other studied membranes in phosphate-containing solutions. In addition, the formation of the neutral and negatively charged bound species suppresses the generation of protons by the “acid dissociation” mechanism. Moreover, when the membrane is operated in overlimiting current modes and/or in alkaline solutions, a bipolar junction is formed at the CJMA- 6/depleted solution interface. The CJMA-6 current-voltage curve becomes similar to the well-known curves for bipolar membranes, and water splitting intensifies in underlimiting and overlimiting modes. As a result, energy consumption for electrodialysis recovery of phosphates from aqueous solutions almost doubles when using the CJMA-6 membrane compared to the CJMA-3 membrane.

Phosphates Transfer in Pristine and Modified CJMA-2 Membrane during Electrodialysis Processing of NaxH(3−x)PO4 Solutions with pH from 4.5 to 9.9

“…The ion-exchange capacity, water content of the CJMA-2 membrane, as well as conductivity and effective counterion (Cl − ) transport number in NaCl solution are comparable to those of other AEMs [ 82 ] manufactured by the Hefei Chemjoy Polymer Materials Co. Ltd. A high water content with a relatively low ion-exchange capacity is indicative of weaker cross-linking of the CJMA-2 ion-exchange matrix compared to, for example, an ASE anion-exchange membrane (Astom, Shunan, Japan) [ 83 ]. The relatively weak cross-linking of the main chains of the CJMA-2 matrix should reduce steric hindrance in the transport of large hydrated phosphates in this membrane.…”

“…Development of the unsteady Rubinstein–Zaltzman electroconvection [ 86 ] and water splitting affect the slope of the overlimiting section III. Indeed, most of the weakly basic groups on the surface of the CJMA-2 are protonated in an acidified solution [ 83 ]. Thus, most of them take part in the transfer of counterions.…”

“…Similar curves were obtained earlier in Na x H (3−x) PO 4 solutions with pH ≥ 9 for membranes (AX, ASE, CJMA-3, etc.) containing mainly quaternary ammonium groups [ 31 , 83 ]. Indeed, the solution contains over 99% Na 2 HPO 4 ( Table 1 ).…”

“…CVC acquires a similar form during long-term operation of the CJMA-2 membrane in 0.02 M Na x H (3−x) PO 4 solution with pH 6.9 ± 0.1 ( Figure 12 b). Note that, under similar conditions, the CVCs of AEMs with strong basic groups have two plateaus [ 31 , 32 , 83 ]. These CVCs are similar to those described in [ 83 ] for the CJMA-6 membrane (Hefei Chemjoy Polymer Materials Co., Ltd., Hefei, China), which contains a mixture of strongly and weakly basic fixed groups and studied alkaline solutions.…”

Anion-Exchange Membrane “Polikon A” Based on Polyester Fiber Fabric (Functionalized by Low-Temperature High-Frequency Plasma) with Oxidized Metal Nanoparticles

“…The new AEM with a net structure was formed by quaternizing BPPO/PECH with N,N,N,N-tetramethyl-1,6-hexanediamine [ 17 ]. It was shown in [ 18 ] that, in addition to steric hindrances, the transfer of phosphates in AEMs are affected by the ion-exchange capacity and the composition of fixed groups (strongly basic, weakly basic), as well as the degree of crosslinking of the polymer matrix. In spite of the wide variety of commercial AEMs, their characteristics—in particular their electrical conductivity and counterion permselectivity—are unsatisfactory for some applications, such as electrolyte solution concentration [ 19 ].…”