Study On Preparation And Separation Properties Of Nanofiltration Membranes Doped With Boron Nitride Nanosheets

In today’s society with increasingly prominent environmental problems,water treatment is particularly important.Nanofiltration has been widely concerned as a kind of efficent water treatment technology because of its high separation efficiency and low energy consumption.The traditional nanofiltration membrane has a"trade-off"effect due to the limitation of membrane material.The improvement of rejection ability usually leads toa degradation of permeation flux.In recent years,with the discovery of graphene,two-dimensional materials have received extensive attentions.More and more two-dimensional materials have been used in seawater desalination,sewage treatment and other water treatment fields due to their unique atomic thickness and micron size.Therefore,it is of great significance to prepare composite nanofiltration membrane with good separation performance and stability by doping two-dimensional materials.In this study,boron nitride nanosheets（BNNS）were firstly functionalized,polyethyleneimine（PEI）-aqueous phase incorporated with BNNS or functionalized BNNS（FBNNS）was then employed to perform interfacial polymerization with trimethyl chloride（TMC）-organic phase on polysulfone（PSf）ultrafiltration membrane to fabricate PEI/BNNS（FBNNS）/TMC composite nanofiltration membranes.The functionalized boron nitride nanosheets and the obtained membranes were characterized by IR,ATR-FTIR,XRD,XPS,zeta surface potential measurement,SEM,TEM,contact angle measurement and other instruments.The separation performance and stability of composite nanofiltration membrane were tested by cross-flow permeation tests.The results obtained from experiments are as follows:（1）Functionalization of BNNS:BNNS could be amino-functionalized by hydrothermal method with 3-aminopropyltriethoxysilane and magnetic functionalized by hydrothermal method with Fe（NO₃）₃·9H₂O.Both NH₂-BNNS and NH₂-BNNS-Fe₃O₄ exhibit good dispersibility in water-based reagents,showing the absorbance of 3.9846 A and 2.1706 A,respectively;TEM shows that the nanosheets display a stacking structure;IR,XRD and XPS analyses confirm the existences of N-H and Fe-O bonds in NH₂-BNNS-Fe₃O₄.These bonds can not only improve the dispersibility of nanosheets in aqueous solutions,but also endow the nanosheets with magnetic response.In addition,XPS analysis shows that the step of magnetic functionalization consume partial amino groups.（2）Fabrication and properties of composite membrane:BNNS-doped composite nanofiltration membrane was successfully prepared by interfacial polymerization of PEI aqueous solution doped with NH₂-BNNS or NH₂-BNNS-Fe₃O₄ and TMC organic solution.XPS analysis demonstrated that the amino groups of NH₂-BNNS and NH₂-BNNS-Fe₃O₄ participated in the interfacial polymerization.TEM confirmed the order distribution of nanosheets in membrane PEI/NH₂-BNNS-Fe₃O₄/TMC.Static water contact angle showed that the composite membrane doped with FBNNS exhibited better hydrophilicity.Zeta potential analysis demonstrated that the surface of fabricated membranes were positively charged at pH less than 7.0.（3）Separation performance of composite nanofiltration membrane:The modified membrane PEI/NH₂-BNNS-Fe₃O₄/TMC fabricated with a doping nanosheet concentration of 0.010 wt.%exhibited the best separation performance in terms of water flux and salt rejection,showing a rejection of 88%to 500 ppm MgSO₄ solution and a pure water flux of 19 L/（m²·h）under 0.6 MPa.Compared with the composite membrane with no magnetic field,membrane PEI/NH₂-BNNS-Fe₃O₄/TMC fabricated magnetic field exhibited a higher salt rejection and a flux higher by about 10%.The interlayer nanochannels constructed by nanosheets in the composite improved membrane permselectivity.Membrane PEI/NH₂-BNNS-Fe₃O₄/TMC possessed good performance stability in 120-h continuous filtration with 500 ppm MgCl₂ aqueous solution under 0.6MPa,showing a stable rejection of 85%and flux of 19 L/（m²·h）.