Tailoring molecular structure in the active layer of thin-film composite membrane for extreme pH condition

A series of thin-film composite membranes with polyurea as the active layer were fabricated by interfacial polymerization of different multi-amine and 1,4-phenylene diisocyanate (PDI) on porous poly(ether sulfone) support. By XPS and SEM analysis, polyethylenepolyamine (PP) was confirmed to be more suitable to react with PDI to form denser and thinner polyurea-based active layer. The best-optimized sample was obtained by varying PP and PDI content during interfacial polymerization process, exhibiting a MgSO4 rejection of 99.0%, with a water flux of 15.4 L m(-2) h(-1) at 2.0 MPa. The acid/alkali-tolerant property of the best-optimized membrane was investigated by both static acid/alkali soaking tests and long-term permeation tests under extremely acidic or alkaline condition. It should be noted that the optimal membrane fabricated by PP can still retain a MgSO4 rejection higher than 93.0% at neural pH testing condition, after exposure to extreme pH condition for 1 year, and the pH stability of the membrane can be comparable to that of the commercial MPS-34 membrane produced by Koch industries. On the other hand, during long-term permeation tests, the optimal membrane fabricated by PP showed stable Na2SO4 rejection higher than 90%, with higher than 90% permeability for H2SO4, implying potential application in the separation of the acid from metal salts.