| A large number of studies show that the charged characteristic of nanofiltration membrane influenced significantly the membrane performance such as the flux, permselectivity, the antifouling ability and so on. Now, it is becoming a hot subject in the field of membrane science.In this work, a tangential streaming potential measurement system was successfully fabricated and integrated on the basis of our previous work. Experimental investigations on membrane/solution interfacial electrical phenomena of NTR-7450 nanofiltration membrane which had dissociated functional groups were performed by electrokinetic method. In order to obtain the accurate zeta potential, the corresponding measures were taken in the streaming potential measurement, for example, the determination of the total conductance of the system including the surface conductance, the membrane body conductance and the electrolyte solution conductance by means of the electrochemical workstation, using the special Ag/AgCl electrode to eliminate the liquid junction potential, alteration of the channel heights and so on. Furthermore, the surface charge density could be estimated by the Gouy-Chapman double-electric layers theory and evaluated the influence of the ionic strength, types of anions (Cl-, SO42- and PO43-) and types of cations (Na+, Mg+, and La3+) on the membrane surface charge. The experimental results were as follows:In the case of KCl, K2SO4 and K3PO4 electrolyte solution, the mechanism of charge formation of NTR-7450 membrane was explored. That is, its charge formation was attributed to the dissociation of the functional group at relatively low concentration (for example,0.1～0.5mol·m-3). However, at the relatively high concentration (for example, 1.0～10mol·m-3), the charge formation of NTR-7450 nanofiltration membrane should be ascribed to the specific adsorption. And, the relationship between the volume charge densities, X, and the feed solution concentration, C, agreed well with Freundlich absorption isotherms. That was, for KCl solution, ln|X|(mol·m-3)=2.337+0.722 ln C (mol·m-3); for K2SO4 solution, ln|X|(mol·m-3)= 3.584+1.119 ln C(mol·m-3); for K3PO4 solution, ln|X|(mol·m-3)= 2.988+1.067 ln C(mol·m-3).With respect to NaCl, MgCl2, and LaCl3 electrolyte solution, the charge formation could be attributed to the combination of the dissociation of the functional groups and the specific adsorption. And, the former played a dominant role only in NaCl and MgCl2 salt solutions with a relatively low concentration (0.1～0.5mol·m-3). However, with reference to LaCl3 salt solution, the charge effect should be resulted from the specific adsorption to a larger extent. And, the relationship between the volume charge densities, X, and the feed solution concentration, C, agreed well with Freundlich absorption isotherms. That was, for NaCl solution, ln|X|(mol·m-3)= 0.796+1.298 ln C(mol·m-3); for MgCl2 solution, In|X|(mol·m-3)= 0.644+0.501 ln C(mol·m-3); for LaCl3 solution, ln|X|(mol·m-3)= 2.190+1.102 ln C(mol·m-3).On the basis of the exploration on the electrical properties of the NTR-7450 nanofiltration membrane, the research object of the adsorption-amphoteric model was extended from the polyamide nanofiltration membranes to the nanofiltration membranes with dissociated functional groups. And the simplified expression of volume charge density of membrane surface was acquired. According to the volume charge density deduced from electro-kinetic method, membrane surface dissociation and adsorption parameters in different electrolyte solutions were calculated by the simplified expression. Furthermore, the charged mechanism of sulfonated polyethersulfone nanofiltration membrane in different electrolyte solutions was attempted to be evaluated by means of adsorption-amphoteric model.At last, according to the experimental results, combined the membrane surface charge density determined using the Donnan steric pore model & dielectric exclusion (DSPM-DE) model with the one which deduced from the streaming potential measurement by means of electrokinetic method and the Gouy-Chapman double-electric layers theory, the membrane surface charge density could be correlated with the feed concentration in terms of a Freundlich isotherm. According to the relationship, the membrane surface charge density was calculated in the whole concentrations of electrolyte solution. Obviously, it was helpful to acquire the retention of salt solution in the whole concentrations range according to the DSPM-DE model. The model evaluated results agreed quite well with the experimental data in the case of NaCl electrolyte solution. Consequently, the agreement between the model evaluated results and the experimental data indicated that the combination of DSPM-DE model with the electrokinetic method was feasible for characterizing the charged properties and predicting separation properties of sulfonated polyethersulfone nanofiltration membrane in the whole concentrations of electrolyte solution.
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