CHARACTERIZATION OF STRUCTURE AND PROPERTIES OF NANOFILTRATION MEMBRANES BY USING STREAMING POTENTIAL METHOD

Abstract

Study,streaming potential measurements were use to investigate the effect of surface electric properties of composite nanofiltration(NF) membranes on their separation performance.NF membranes have been prepared by different functional materials.The different charged functional groups will give different hydrolyzed properties in the same electrolyte solution,and the charged properties will be different too.The streaming potential measurements can be used to characterize the surface charged properties of membranes.The experiments were carried out with plate NF membranes.When an electrolyte solution is forced to flow through a membrane pore by means of a hydraulic pressure,a streaming potential is generated across the flow path.The membranes were prepared using(poly[2-(N,N-dimethyl) amino)ethyl methacrylate](PDM) and poly[2-(N,N-dimetbyl amino) ethyl acrylate](PDA) as functional layers,respectively.Several electrolyte solutions were used as measured media such as 0.005 mol/L MgSO_4,MgCl_2 and Na_2SO_4 solutions.Effects of cross-linking time of the functional layer of the composite NF membrane and molecular structure of monomers on the surface electric performance of NF membranes were studied. Relations of selective rejection performance with surface electric performances of NF membrane to different inorganic salts were investigated.Streaming potential(ΔE),zeta potential(ζ) and the surface charge density(σ~d) at the exterior surface of NF membrane were obtained by the streaming potential measurement.The results show that the variations in these three eletric parameters are in accordance with the variations in the cross-linking time of the functional layer and with the variations in the rejection of the NF membrane to inorganic salts.When the cross-linking time is 45 s,all of the absolute values of these electric parameters take their maximum values.The electric parameters correlated well with the rejection of NF membrane to inorganic salts. The absolute value of zeta potential(|ζ|) of the composite NF membrane increased as Na_2SO_4 MgSO_4MgCl_2,and the flux and rejection show the same increasing.The absolute values of the surface electric data of the NF membrane obtained by PDM with methyl groups on the main chains are smaller than the NF membrane obtained by PDA without methyl groups.The atomic force microscopy(AFM) was chosen for morphological characterization of the surface of NF membranes.AFM measurements reveal that the chemical structures of the monomers strongly affect the surface morphology.The surface roughness of the PDM NF membrane is higher than that of the PDA NF membrane.The increase in the number of methyl groups on the main chains of monomers results in the membrane morphology,which affects the surface eclectic properties of the membrane.The AFM measurements conform the streaming potential data.Therefore,the streaming potential method is a useful tool to be used in studies of rejection mechanism and studies of functional structure of composite NF membranes.