用流动电位法表征纳滤膜的结构及性能

利用测量流动电位的方法考察了纳滤膜的表面电学性能对纳滤膜的截留性能的影响.首先,采用不同功能层材料制备了复合纳滤(NF)膜,考察功能层的交联时间、单体结构等对表面电性能的影响,研究纳滤膜对不同无机盐的选择截留性能与表面电性能的关系.通过流动电位法测定纳滤膜的表面电学参数,如流动电位(ΔE)、zeta电位(ζ)和表面电荷密度(σd).实验表明,这些电学参数的变化与功能层交联时间和纳滤膜截留率的变化一致,在交联时间为45 s时,3种电学参数的绝对值均最大,而纳滤膜对无机盐的截留率也最大.复合纳滤膜zeta电位的绝对值(|ζ|)按照Na2SO4>MgSO4>MgCl2变化,同截留率的变化相同.带侧基单体交联后得到的纳滤膜的表面电性能参数的绝对值小于不带侧基单体的.因此,流动电位法可用于研究复合纳滤膜的截留机理和功能层结构.

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

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(poly2-(N,N-dimethyl) amino)ethyl methacrylate](PDM) and poly2-(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_4>MgCl_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.