Performance of three anion-exchange membranes in fluoride ions removal by electrodialysis.

The performance of three anion-exchange membranes (AEMs) in the fluoride ions reduction by electrodialysis (ED) is performed on real and synthetic water. The electric potential method measures the potential difference (PD) between two synthetic anion solutions separated by ACS, AFN and AXE membranes. The selectivity of these three AEMs coupled with the membrane CMX, is a cation-exchange membrane (CEM) towards different ions. The removal rate is influenced by the thickness of the polarization layer (PL) which reduces the material transfer and provides an additional barrier. The greater the thickness δ of the PL, the longer the passage time and consequently the removal rate of anions is small. Using the unstirred layer model, δ for each ion will be determined. According to the potential measurement method, none of the tested AEMs are selective to fluoride ions and the order of selectivity is as follows: AFN> AXE> ACS. Best membrane couple selected for fluoride ion removal is ACS/CMX and ion selectivity follows the order: Cl^-> NO^-₃>F^-> HCO^-₃> SO₄^2-. For ACS membrane, both the demineralization rate (DR) and δ of fluoride ions are influenced by the initial concentration of the co-ion according to the following order: NO^-₃> Cl^-> HCO^-₃> SO^2-₄.