Thin-film composite nanofiltration membrane based on polyurea for extreme pH condition

Abstract

A series of thin-film composite membranes based on polyurea has been fabricated by interfacial polymerization with branched polyethyleneimine and piperazine in the aqueous phase, and multi-isocyanate in the organic phase on porous polyethersulfone support for extreme pH nanofiltration. It is found that 1,4-phenylene diisocyanate and toluene diisocyanate are more suitable to react with multi-amine to form the dense crosslinking layer, thereby achieving improved permeation and separation properties. By optimizing the ratio of polyethyleneimine to piperazine in the aqueous phase and 1,4-phenylene diisocyanate content in the organic phase, the best performance membrane with a MgSO4 rejection higher than 97.0% has been obtained. The surface properties of the polyurea-based membranes, such as surface charge, hydrophilicity, roughness, and morphology structure have been characterized in detail. After exposure to 20% (wt/v) HCl, 20% (wt/v) H2SO4, as well as 20% (wt/v) NaOH aqueous solution for one year, PEI-0.4/PDI-0.25 membrane can still retain MgSO4 rejection higher than 90% at neutral pH testing condition, with more than a two-fold increase of water flux, due to the swelling of the polyurea separation layer. The relationship between the molecular structure of polyurea and acid/alkali stability of membrane is also investigated. More importantly, the optimal polyurea-based nanofiltration membrane exhibits comparable acid/alkali stability to commercial MPS-34 membrane produced by Koch industries, displaying a broad field of application with good prospects.