| Nanofiltration has been widely used in water treatment. Due to the charged groups on the membrane, the hydrophilic propertity of the membrane has been strengthened, which leads to an increased permeability. At the same time, membrane fouling resistance has been enhanced significantly. Therefore it is of great importance to carry out the research on NF membrane surface properties and characterization for in-depth understanding of the rejection mechanism, and promote the wider application of nanofiltration.The research of this work focuses on the charge characteristics and the rejection mechanism of the charged nanofiltration membrane. Streaming potential experiments were carried out to investigate a variety of influencing factors on the nanofiltration membrane electrical properties, such as operating conditions and test conditions. The transport and rejection mechanism of the membrane have also been investigated from structure parameters, separation performance, etc.to understand the rejection mechanism better.Zeta potential formula has been derivated from the research on the formation mechanism of the double-layer which provide guidance to the research of nanofiltration membrane electrical properties.Analysis was carried out to investigate the operating conditions of the streaming potential experiments including selection of electrodes, membrane soaking time, operating pressure, temperature, channel height, and electrolyte concentrations.The results showed that Pt electrode should be used in alkaline environment, while Ag-AgCl electrode being used in acidic solution with Cl-. Temperature should be controlled at certain value for comparition of different datas; Time of more than 4 hours is needed for the membrane to be soaked in the test solution to ensure dynamic equilibrium between the dissociation and adsorption;Appreciate channel height should be lied between 60μm and 300μm, and operating pressure below 0.4MPa to satisfy the conditions of laminar flow for the accurate calculation of the Zeta potential; The better concentration range of the testing electrolyte should be lied between 0.0005M and 0.05M.Investigated the impacts of different components, concentrations and pH of electrolyte solutions on streaming potential, Zeta potential and surface charge density of the membrane were investigated. The results showed that the total conductance including solution conductance, membrane conductance and outer channel conductance was the appropriate conductance value for calculation of the zeta potential.As the solution concentration increases, the proportion of the solution conductivity increases, while the proportion of membrane conductance decreases gradually. With the increase of concentration, the absolute value of Zeta potential decreases while the membrane surface charge density increases. At low concentrations, the relationship between the membrane surface charge density and electrolyte concentration is in good agreement with Freundlich-isotherm adsorption; The absolute value of Zeta potential decreases with pH decreases.Due to the adsorption of divalent co-ion, the charge density increases, and the adsorption of divalent co-ion led to the decrease of membrane charge density. The smaller the hydration radiu of the count-ion is, the stronger the adsorption capacity is, which lead to a lower absolute value of Zeta potential. When divalent counter-ions were added to 1-1 electrolyte system, the absolute value of Zeta potential reduces, as well as the surface charge density.Detailed analysis was carried out to simplify the DSPM model, and the transport parameters of the membrane ware experimentally characterized. The reflection coefficient was obtained by retention experiments, and the membrane pore radius was estimated. Also,the thickness of the membrane was calculated according to Hagen-Poiseuille equation. The values of volume charge density obtained by two methods at different NaCl concentrations were substituted into the DSPM model to predict the rejection of electrolyte solution. The results showed that at low concentrations, the prediction of the rejection is excellent with the volume charge density obtained from electrokinetic method predicted is better. While at relatively high concentrations, the prediction of the rejection with the volume charge density obtained from TMS model is better. |
