Endowing the nanostructured ion channels of anion conductive membranes with negative charge to boost the Cl−/SO42− ion separation via electrodialysis

Construction of homogeneous polymer ion-exchange membranes (IEMs) with suitable ion transport channels and stable structures is crucial to the separation of mono-/multi-valent anions. Amphoteric ion-exchange membrane (AIEM) is a special type of IEM that possesses unique properties due to the presence of both anion exchange groups and cation exchange groups. In this work, we have grafted 1-bromoheptane and 3-bromopropanesulfonate onto the poly(aryl ether sulfone) chain through nucleophilic substitution reactions. Unlike the IEM attaching different ion-change functional groups onto one side-chain, this kind of AIEM bears two kinds of ion-change functional groups on dual side-chains. The results show that the as-prepared AIEMs could maintain low water absorption (<20 ​%) and swelling ratios (<11 ​%). The optimal surface area resistance is 6.31 ​Ω∙cm². The perm-selectivity (Cl⁻/SO₄²⁻) of the optimized PAES-TA/BS-0.85 AIEMs in a binary mixed solution system at 2.5 ​mA∙cm² could be achieved as high as 67.46, which is much higher than commercial ACS IEMs under the same conditions. Small Angle X-ray scattering reveals a distinct phase separation pattern within the AIEM matrix, which is due to the introduction of sulfonates to increase the hydrophilicity of the side-chains. The combined effects of pore size screening and electrostatic repulsion render the high selectivity of as-prepared AIEMs.