Nanofiltration membrane embedded with hydroxyapatite nanowires as interlayer towards enhanced separation performance

Abstract

The triple-layered composite nanofiltration (NF) membrane consisting of thin polyamide (PA) top layer covered on polymeric porous membrane with interlayer has been popularly accepted, even commercially applied, to solve the “trade-off” issue. The nanofiltration performance was determined by the physicochemical properties of all layers. In the current work, hydroxyapatite (HAP) nanowires were adopted as a new type of interlayer material. Its superiority of ultra-hydrophilicity and electronegativity made it a container of aqueous monomers. As a result, ultra-thin and defect-free PA layer with a thickness of ~28 nm was generated owing to the low piperazine (PIP) concentration and restricted diffusion. In addition, the ultra-hydrophilic polysulfone (PSf) support layer was also helpful to reduce mass transfer resistance. The prepared PA/HAP/PSf composite membrane presented high salt aqueous solution permeation (177 L m −2 h −1 under 6 bar), excellent divalent salt rejection (98.2% for Na 2 SO 4 ) and high mono/bivalent salt selectivity (19 for NaCl/Na 2 SO 4 mixed solutions). This work proposed a useful strategy to construct highly permeable and selective NF membrane. PA/HAP/PSf triple-layered electronegative nanofiltration membrane showing high selectivity to monovalent/bivalent ions • HAP nanowires were adopted as interlayer of composite NF membrane for the first time. • The prepared PA/HAP/PSf NF membrane possessed ultra-thin and robust electronegative PA layer. • High rejection of 98.2% and salt permeation of 177 L m −2 h −1 under 6 bar for Na 2 SO 4 were achieved.