Preparation And Application Of Porous Janus Membrane To Forward Osmosis Process

Forward osmosis(FO)is an emerging "engineered osmosis technology" for various desalination and water treatment applications.The driving force of FO is the osmotic pressure difference between draw solution(DS)and feed solution(FS),separated by a semipermeable membrane.Considerable interests in FO are inspired by its nature of spontaneous diffusion,leading to low energy consumption.However,FO still faces several challenges,such as membrane fouling,internal concentration polarization(ICP),and lack of high performance membranesDeveloping high performance osmotic membranes has been a research focuing in FO,as it can minimize both membrane fouling and ICP.Desirable FO membranes should possess high water permeability,ion selectivity,and fouling resistance,but low structural parameter to mitigate ICP.Most high performance FO membranes consist of dense active layers and porous support layers.Numerous nanomaterials,such as graphene oxide,carbon nanotubes,aquaporins,metal organic frameworks,and metal/metal oxide nanoparticles have been incorporated either into the active layer or the support layer or between these two layers.The traditional wisdom believes that both the support layer and the active layer are preferable to be highly hydrophilic to maximize membrane permeability and minimize membrane fouling.More recently,a hydrophilic support-free selective layer was used for FO with zero ICP.However,the mechanical stability and the difficulty to scale up of the single-layer membrane prevent its widespread applications.Janus membranes with opposing properties at the interface have become emerging materials for various separations.Based on these problems,this thesis prepared Janus membranes and applied them to FO process to achieve excellent FO performance.Firstly,we prepared superhydrophilic PVDF microporous membrane by phase inversion method while superhydrophobic fluorinated silica nanoparticles were fixed to micro-nano structure of superhydrophilic PVDF membrane to prepare Janus membrane and applied this Janus membrane for FO process,the asymmetric infiltration structure of Janus membrane can effectively reduce concentration polarization,achieve unidirectional mass transfer of water molecules and prevent salt ions penetrating reversely.Since nano-sized hydrophilic channels stretch through the membrane,the opposite side of the Janus membrane owns different wettability and possess unidirectional transfer characteristic.The permeability coefficient can reach 2.2 L m-2 h-1 bar-1,when reverse salt flux is 0.007 g L-1.Secondly,asymmetric Janus microporous membrane was prepared by electrospinning PVDF on superhydrophilic cellulose acetate membrane.The air layer trapped in polyvinylidene fluoride nanofiber membrane can effectively prevent reverse diffusion of salt solute in draw solution to improve the selectivity,while the cellulose acetate hydrophilic membrane on the other side can protect the air layer and act as anti-pollution and support layer.The thickness of the hydrophobic nanofiber layer(1.7 to 9.1 μm)can be controlled by electrospinning time.Because of osmotic pressure,water molecules on the feed solution will transport from the hydrophilic side to the hydrophobic side of the Janus membrane undirectionally,and the salt ions in the draw solution are blocked by the air layer effectively.The thickness of hydrophobic membrane is critical to water flux and reverse salt flux,with thickness reducing and the concentration of draw solution increasing,water flux and reverse salt flux will increase.When hydrophobic air layer of the Janus membrane is wetted by infiltration(eg,for 2 hours),the water flux drops sharply,at the same time,reverse salt flux rises sharply.After further drying the membrane,water flux and reverse salt flux are fully recovered,which prove multiple cycle stability of Janus membrane.