Double cross-linking PVA-SiO2 hybrid membranes for alkali recovery

Abstract Non-charged PVA-SiO2 hybrid membranes are prepared through sol-gel process among polyvinyl alcohol (PVA) and double cross-linking agents, which are selected from four types of alkoxysilanes including tetraethoxysilane (TEOS), tetramethoxysilane (TMOS), γ-aminopropyl triethoxysilane (APTEOS) and methacryloxypropyl trimethoxy silane (MPS). The double cross-linking agent can combine their advantages including different hydrolysis rates of OC2H5 and OCH3 groups, high compatibility of NH2, and polymerizable C C bonds. The NH2 group from APTEOS can enhance membrane homogeneity, thermal stability and flexibility due to its compatibility with PVA–OH groups. The membranes have high selectivity (62–101) but low permeability in diffusion dialysis process for separating NaOH/Na2WO4 solution. Single type of Si(OCH3) groups produces large silica particles (1.5–5μm), which reduce membrane stability but enhance permeability. The membrane has low strength and flexibility, and high swelling degrees in 65 °C water (373–408%) and weight loss percentage in 65 °C NaOH solution (25.8–28.7%). The permeability can reach up to 0.0076–0.0092 m/h but the selectivity is only 23.7–25.0 at 20–40 °C. The combination of Si(OCH3) and Si(OC2H5) groups (TEOS and MPS) can obtain high strength and flexibility (20.3 MPa, 571%), acceptable swelling degrees (232–279%) and balanced permeability and selectivity (0.0043–0.0071 m/h, 45–110). Hence, double cross-linking agents can be combined to prepare optimized neutral membranes for diffusion dialysis.