Preparation and properties of polyvinyl alcohol (PVA) / mesoporous silica supported phosphotungstic acid (MS-HPW) hybrid membranes for alkali recovery

Diffusion dialysis (DD) separation process is shackled by the ion flux and selectivity. To solve this problem, the development of membrane materials is very important. In this study, mesoporous silica has been synthesized first and then phosphotungstic acid was adsorbed in the mesopores under vacuum condition. The obtained functionalized mesoporous silica materials (MS-HPW) have been then blended with pre-crosslinked polyvinyl alcohol (PVA) solution to prepare a series of hybrid membranes. The as-prepared membranes possess ion exchange capacities (IECs) of 0.015–0.030 (mmol/g) and water uptake (WR) of 98.3%–196.6%, as well as good thermal and mechanical stability. Alkali resistance of the hybrid membranes has been tested in 2 M NaOH solution at 25 °C and the results are acceptable: swelling degree is in the range of 134.3%–160.2% and mass loss is in the range of 3.2%–12.2%. The as-prepared membranes have been used for alkaline diffusion dialysis (DD) process of NaOH/Na2WO4 system and the results are encouraging: PVA/MS-HPW hybrid membranes exhibit higher ion flux and selectivity than that of PVA and PVA/MS membranes. The mesoporous structure and OH− adsorption ability of MS-HPW material are considered to play important roles during the ion transport.