Structure Design Of Thin-film Composite Nanofiltration Membrane For Desalination

Nanofiltration(NF)membranes,capable for removing multivalent ions and small neutral organics with molecular weight above 150 Da,have been wildly applied to seawater and brackish water desalination,wastewater treatment,food processing and industry of fine chemicals.Commercial NF membranes are based on a thin-film composite(TFC)design,where polyamide(PA)active layers are set atop the ultrafiltration membrane substrates.The active layer plays a crucial role in the separation performance of nanofiltration membranes.Designing and optimizing the morphology structure and molecular network structure of the active layer is the key to the high flux and high selectivity of NF membranes.Increasing filtration area and lowering active layer thickness are two effective approaches to elevate permeation flux,while uniformly-distributed pore sizes can endow the NF membrane with higher selectivity.These ideal structures of active layer must be designed and constructed at the nanoscale or molecular level,which is technically challenging during the interfacial polymerization(IP)process,involving the control of the interface position,monomer diffusion behavior and polymerization degree.In this thesis,crumple-structured PA active layers with enhanced filtration area were constructed through nanoparticle-templated method,ultrathin positively-charged PA active layers were fabricated by confined IP method,PA active layers with narrowly-distributed pore sizes were prepared by using regular-structured monomers in combination with the controlling of diffusing behavior of monomers during the IP process.Three parts are included in this thesis:(1)Design and construction of TFC PA NF membranes with crumpled structure.Effective permeation area of active layer in conventional TFC NF membrane is limited by membrane area and strong binding force between active layer and substrate.Crumpled structure of the active layer was constructed by using sacrificial nanoparticles,aiming to increase the effective filtration area and thus the flux of the membrane.During the IP process,PA layer forms over rugged surface created by sacrificial nanoparticles loaded on the substrate.These nanoparticles can be removed after the formation of PA layers,leaving cavities between the PA layer and substrate,and so forms the crumpled structures.These crumpled structures endow ultrathin PA layers with high permeation flux of 53 L m-2 h-1 bar-1,increased by 50%compared with control groups,while maintaining high rejection of 95%for sodium sulfate.(2)Design and construction of ultrathin positively-charged TFC PA NF membranes.Positively charged NF membranes have great potential in water softening and removal of heavy metal ions.Poly(ethylene imine)(PEI)carrying easily protonated amino groups is commonly engaged to provide positively charged sites in the active layer.During IP process,PEI diffuses much slower than small molecular amines and it is difficult to quickly form an ultrathin and dense PA layer.In this thesis,space-confined IP method was employed by controlling the diffusion distance of PEI to limit the reaction region,thus the thickness of PA can be controlled.PEI molecules were pre-deposited in the carbon nanotube(CNT)network followed by crosslinking reaction to generate active layers inside the network.The CNT network restricts the diffusion of PEI by the interactions between PEI and polydopamine coatings of CNTs and the entanglement of PEI molecules.Meanwhile,it provides a skeleton to promote the formation of a high-quality PA layer.Thickness of the active layer can be tuned by the thickness of CNT film,CONCENTRATION OF ACYL chloride and molecular weight of PEI.A 34-nm-thick active layer shows a flux of 27 L m-2 h-1 bar-1,2-5 times as high as those reported in other literatures,meanwhile its rejection rate for magnesium chloride is 97%.(3)Design and construction of dendrimer derived TFC PA NF membranes with narrowly-distributed pore sizes.According to the separation mechanisms of NF membranes such as pore size sieving and electrostatic repulsion effect,uniform pore sizes with a specific charge are required for high selectivity.In this thesis,dendrimer polyamidoamine(PAMAM)was chosen as monomers.The asymmetric hyperbranched building blocks bring the regular structure into the active layer.With the help of surfactant facilitating faster and more homogeneous diffusion of PAMAM during the interfacial polymerization process,polyamide networks with narrowly-distributed pore sizes were prepared.These dendrimer-derived nanofiltration membranes achieved high Li+/Mg2+selectivity.For mixed salt solution(Li+20 ppm,Mg2+480 ppm),Li+/Mg2+separation factors of the membrane reached 93,much higher than other NF membranes.