Study on Charged Nanofiltration Membrane Performance and Rejection Mechanism to Salt Solution
|School||Ocean University of China|
|Keywords||Nanofiltration membrane Streaming potential Zeta potential DSPM model Interception mechanism|
Nanofiltration membranes have been widely used in the field of water treatment. The film introduces a charged group, the hydrophilic nature of the film has been strengthened, thereby increasing the permeation rate of the film, and the stain resistance of the film is significantly enhanced. Therefore, to carry out the research and characterization of the performance of nanofiltration membrane surface charge, has a very important significance to promote the wide application of nanofiltration membranes for in-depth understanding of the mechanism of nanofiltration membrane interception. In this paper, nanofiltration membrane surface charge performance as a starting point, and streaming potential experiments to study the various test conditions on the performance of nanofiltration membrane charge, from the aspects of membrane structure parameters, separation performance of the Dutch the nanofiltration membrane transport phenomena on the basis of the salt solution rejection mechanism. According to the formation mechanism of the electric double layer, the surface potential analysis formula gives the film surface Zeta potential. Streaming potential test system using the theoretical calculation approach convection Road to the correct height, to get the proper value of the height of the flow channel. Led him to study the impact of a variety of operating conditions (the choice of electrode, temperature factors, diaphragm soaking time, differential pressure, flow channel height, electrolyte concentration, etc.) flow potential. The results show that: (1) in an alkaline environment, should adopt the Pt electrode, Ag-AgCl electrode (electrolyte solution containing chloride ions); (2) should be adopted before the streaming potential experiments, nanofiltration membrane in the acidic environment should be soaked for 4 hours or more in order to ensure the diaphragm surface charge dissociation adsorbed homeostasis; (3) For the experimental device, the flow channel height control between 60 μm to 300 μm, operation pressure is controlled at 0.4MPa or less to meet the flow channel The laminar flow conditions of the feed solution, the electrolyte concentration is controlled at between 0.0005 to 0.05M Zeta potential obtained more reliable. The effects of different concentrations and different pH values, different components of the electrolyte solution on the streaming potential, zeta potential, surface charge density. The results show that: (1) total conductance of the system including the solution conductance, membrane conductance (conductance and membrane body conductance of the membrane surface), the conductance of the outer loop, Zeta potential calculation should be calculated to total conductance value. With the increase of the concentration of the solution, the proportion of the solution conductivity is gradually increased, the proportion of the membrane conductance gradually decreased, and the proportion of the conductance of the outer loop is substantially constant; (2) With the increase of the concentration of the solution, the absolute value of the Zeta potential of the film surface Save small, however, the surface charge density absolute increase; Freundlich isotherm adsorption at low concentrations, between the surface charge density and concentration of the electrolyte; (3) divalent same ion through the van der Waals force characteristic adsorption into the inner Helmholtz hereby layer, enhances the charge density of the membrane surface, and a divalent counter ion by electrostatic effect to attract to the outer Helmholtz layer, reducing the charge density of the membrane surface; (4) As the pH of the solution decreases, the membrane surface Zeta potential decreases when the pH value is less than the isoelectric point, the film surface is an electrically negative changed; (5) the aqueous solution, the smaller the radius of hydration of the counter ion, the adsorption capacity, the stronger, resulting in film (6) When there is a divalent counter ion was added to the 1-1 electrolyte system, the adsorption ability of the divalent ion, reduce the zeta potential value of the film surface, thereby reducing the surface of the membrane surface Zeta potential is smaller; charge density; (7) When an electrolyte solution through the diaphragm, the more electrolyte solution into the diaphragm supporting layer, thus increasing the conductance value, resulting in the absolute value of the Zeta potential of the film surface increase. In the the DSPM model at the same time simplify the analysis, the film passing parameters experimental characterization. The experimental results show that: the interception of the the neutral solute molecular system (VB12) experiment, draw the reflection coefficient of 0.986, according to the pore model, the calculated membrane pore radius of 0.78 nm; pure water flux experiments, according to Hagen-Poiseuille equation , calculate the film thickness of 5.1μm; different concentrations of NaCl electrolyte system, the calculation and comparison of the two methods to obtain the volume of the charge density value Xd, will be substituted into the DSPM model, prediction of the electrolyte solution retention rate. In low concentrations, the volume charge density should be selected without the value obtained by the electrolyte through the diaphragm when the movable electrical method; when higher concentrations should be selected volume charge density values obtained by the calculation of the fixed charge model.