Investigation of Membrane Charge on Nanofiltration Performance

• Main Authors: Sheng-Ru Gu, 顧聲茹
• Other Authors: Ching-Jung Chuang
• Format: Others
• Language: zh-TW
• Online Access: http://ndltd.ncl.edu.tw/handle/88101953190997996862

碩士 === 中原大學 === 化學工程研究所 === 95 === ABSTRACT The electrostatic effect (i.e. Donnan effect) is one of the main factors which influence separation efficiency in nanofiltrations. The magnitude of Donnan potential depends on the bulk charge density in membrane, however how to determine the charge density by measurement is still not well developed. In order to investigate the effect of membrane charge on the separation behavior of nanofiltration, a method combining the hydraulic permeation test and streaming potential measurement to determine the bulk charge density of the membrane was developed. Then the effects of membrane surface zeta potential and bulk charge density on the salt rejection using four different electrolytes were analyzed under various ionic concentrations and pHs. Two commercial nanofiltration membranes, DL and NF-270, were used for the experiments. The salt rejection was also predicted using a theoretical relationship based on Linearized Transport model and compared with the experimental values. Experiment results indicated that in the pH range from 3.8 to 8.0 used in the study the zeta potentials of DL and NF-270 membranes are negative and decrease with cationic strength of electrolyte solution. The bulk charge density of the NF membranes increases with the increase of the pH value and ionic concentration. Salt rejection sequences of DL and NF-270 membranes measured are the same as R(MgSO4) > R(Na2SO4) > R(NaCl) > R(CaCl2). Such a behavior agrees qualitatively with the theoretical prediction based on the sieving effect and the magnitude of Donnan potential; R (MgSO4) > R(Na2SO4) > R(NaCl) is dominately determined by sieving effect, while R(NaCl) > R(CaCl2) is due to Donnan effect. Based on the bulk charge density, the salt rejection was predicted and compared with the measured values. In some cases both have a well agreement, but for some electrolytes there is a great difference. One of the reason for such a discrepancy may be due to the commercial membranes used are composites in which the net charge density in the skin layer can not determined from the measured global (bulk) charge density.