Cation exchange hybrid membranes based on PVA for alkali recovery through diffusion dialysis

Organic–inorganic cation exchange hybrid membranes have been prepared through sol–gel reaction of poly(acrylic acid-co-methacryloxypropyl trimethoxy silane (poly(AA-co-γ-MPS)) in presence of poly(vinyl alcohol) (PVA). The membranes are characterized by FTIR, SEM, thermal stability and mechanical strength. Td values are in the range of 264–270 °C. TS ranges from 8.4 MPa to 32.2 MPa and Eb in the range of 72.1–103.5%. The membranes can remain integrity in 65 °C hot water for more than 12 days and the swelling behavior is better suppressed as poly(AA-co-γ-MPS) content increases. The membranes are tested for recovering sodium hydroxide from the mixture of sodium hydroxide (NaOH) and sodium tungstate (Na2WO4) at different temperatures (35–55 °C) through diffusion dialysis (DD). Both permeability and selectivity are excellent despite the low IEC of the membranes and the low diffusivity of OH− ions. The dialysis coefficients of OH− (UOH) are in the range of 0.010–0.042 m/h, similar to previously reported values for acids (UH: 0.005–0.055 m/h). The hybrid membrane containing 50 wt% poly(AA-co-γ-MPS) has the highest separation factor of 95.7 at 35 °C. The DD performances have been correlated with the membrane physico-chemical structure, especially the presence of large amount of –OH groups.