Carboxylic acid type PVA-based hybrid membranes for alkali recovery using diffusion dialysis

One multisilicon copolymer, i.e. a copolymer containing pendent siloxane and anhydride groups is prepared from the copolymerization of maleic anhydride (MA) and methacryloxypropyl trimethoxy silane (γ-MPS). Thereafter, it is used for the preparation of hybrid membranes through in situ sol–gel process in poly(vinyl alcohol) (PVA) solution. The anhydride groups in MA are less hydrophilic as compared with acidic carboxylic acid (–COOH) groups. Hence the copolymerization process can be more easily controlled without formation of gel. During the sol–gel process, the anhydride groups hydrolyze to generate geminal –COOH groups, which provide the membranes with relatively high cation exchange capacities (CECs) that can be applied for the diffusion dialysis process to recover NaOH from the mixture of NaOH/Na2WO4.The membranes have the initial decomposition temperature (IDT) of 227–243 °C, tensile strength (TS) of 14.2–28.3 MPa and elongation at break (Eb) of 18.8–67.3%. In addition, water swelling decreases as the content of multisilicon copolymer increases despite the increasing of the IEC values, which is attributed to the gradually strengthened crosslinking between PVA and the copolymer components. At 20 °C, UOH (the dialysis coefficients of OH− ions) are in the range of 0.0095–0.0123 m/h, and the separation factor (S) values are in the range of 28.4–54.4. The UOH values are significantly higher than the values of the commercial membrane (0.00137 m/h) and other polymer based hybrid membranes (0.0014–0.0022 m/h). This is due to the unique membrane structure and the presence of different functional groups. PVA provides hydrophilic matrix and hence the membranes cannot be classified into the common “three-phase” structure membranes. The presence of –OH groups assists and enhances the transport of OH− ions.

► Multisilicon copolymers poly(MA-co-γ-MPS) with bionic structure are prepared.
► Cation exchange hybrid membranes were prepared therefrom.
► Membrane performances for alkali recovery are excellent.
► The –OH and –SiOH groups can enhance the transport of OH− ions.
► Adjacent –COOH groups may increase ionic domain interconnectivity to accelerate Na+ transport.