Carboxyl-type boronic acid copolymers (CBACs) were synthesized by a radical polymerization method and used for the preparation of polyvinyl alcohol (PVA)-based composite membranes via a solution mixture method. The as-prepared composite membranes exhibited a water uptake (WR) of 122.6-150.0%, an ion exchange capacity (IEC) of 0.0147-0.0518 mmol g-1, and excellent mechanical (elongation at break (Eb) of 103.8-148.4%, tensile strength (TS) of 38.7-58.6 MPa) and thermal stability. The alkali resistances of the as-prepared membranes were tested by immersing the samples into 2 mol L-1 NaOH solutions at 25 °C for 60 h, and the results were encouraging: the mass loss and swelling degree of the as-prepared membranes were in the ranges of 1.9-5.9% and 222.6-241.9%, respectively. The separation performances of the as-prepared membranes were evaluated by the diffusion dialysis (DD) process with an NaOH/Na2WO4 mixture at room temperature. The results demonstrated that the dialysis coefficients of hydroxide (UOH) were in the range of 0.0147-0.0347 m h-1, and the separation factors (S) were in the range of 29.5-62.6. The introduced carboxyl groups from CBACs and the -OH groups from PVA were both deemed to play significant roles in the promotion of ion transport: the -COO- groups formed negatively charged transport channels for Na+ by electrostatic attraction, and the -OH groups promoted the transport of OH- via hydrogen bonding.
Keywords: alkali recovery; carboxyl-type boronic acid copolymers (CBACs); composite membranes; diffusion dialysis (DD).