Chlorination as a simple but effective method to improve the water/salt selectivity of polybenzimidazole for desalination membrane applications

Inspired by the enhancement of the desalination performance of polyamide membranes upon controlled chlorination, chlorination was applied to the polybenzimidazole (PBI) membrane hoping to improve its desalination performance. Chlorination was conducted at three pHs (4,7,10) with chlorination density up to 50,000 ppm*h via a simple soaking treatment. The structure and physicochemical changes caused by the hypochlorite treatment were characterized by SEM, AFM, XPS, FTIR and XRD. After chlorination, the morphologies did not change significantly. But the Cl element was incorporated onto polymer chains, leading to the scission of intermolecular hydrogen bond and denser packing of PBI chains. Simultaneously, the C O group emerged after chlorination, indicating ring opening of imidazole in polymer backbone. More severe damage occurs at pH 4 with more chlorine introduction, which is due to the strong oxidizability of abundant chlorine or hypochlorous acid in acid solution, while chlorination at pH 10 mostly promotes imidazole hydrolysis. Chlorination mostly alter PBI permeation properties by altering its diffusion properties, with significant enhancement of water diffusion but minimal change on salt diffusion. This result should be due to the dense packing of PBI chains. After chlorination at pH 10, the salt permeability even slightly decreased owing to the dense packed polymer chains and the electrostatic repulsion of charged membrane formed by hydrolysis. As a result, chlorination can increase the diffusion selectivity of water/salt increased by an order of magnitude, as well as the water/salt permselectivity without sacrificing the water permeability. Chlorination at basic pH especially has a positive effect on PBI membrane with improved selectivity for desalination process and sustained mechanical strength.