Layer-by-layer modified anion-exchange membranes for phosphate recovery

Abstract

Ion exchange membranes (IEMs) are used to selectively transport ions between two solutions. Most membranes are not selective to certain ion species. This study investigates the possibility of increasing phosphate selectivity (in the form of H2PO4 – ) of anion exchange membranes (AEM) by modifying commercial AEMs with a Layer-by-Layer (LbL) assembly of a few promising candidate polyelectrolytes (PE): Poly(diallyldimethylammonium chloride) (PDADMAC), poly(allylamine hydrochloride) (PAH), and poly(allylamine hydrochloride)-Guanidinium (PAH-Gu). The anionic layer was composed of poly(sodium 4-styrene sulfonate) (PSS). Membranes were modified with 4 layers. This was done on only one side, the asymmetric modification ((PE/PSS)4), or both sides of the membrane, the symmetric modification ((PE/PSS)2). The membranes were then characterized using low-frequency impedance spectroscopy to obtain, among other parameters, the resistance of the membrane. Asymmetrically modified membranes showed non-reproducible impedance data, while their symmetric counterpart was in good agreement with a theoretical model. Compared to the bare membrane, a decrease in the membrane’s resistivity was found for all the modified membranes in the studied salt concentrations (0.1 to 0.4 M). In the low concentration region, the PAH-Gu modified membrane had the lowest resistance. This result is most likely caused by a high affinity between Guanidinium and phosphate. The electric properties of other parameters, such as diffusive boundary layer and electrical double layer resistance, were found to be independent from the membrane modification. We then performed permselectivity measurements with a competing anion, sulfate. To this end, electrodialysis was performed in the underlimiting current regime. Modified membranes showed no increase in discriminatory function for phosphate. This could be due to several reasons. For example, there might be a pH-change near the surface of the membrane, changing the charge of the phosphate anion. Additionally, the PAH-Gu membrane did show an increase in overall anion flux compared to the bare membrane. These results are promising and warrant further research into phosphate selective receptors in combination with IEMs.