| Summary: | 碩士 === 中原大學 === 化學工程研究所 === 98 === Membrane charge density is one of the main factors which influence separation efficiency in nanofiltration, however very few pay attention to how to measure the charge density in membrane. In this study, a method combining the hydraulic permeation test and streaming potential measurement was developed to determine the membrane charge density. Two commercial nanofiltration membranes, DK and NF-270 were used to investigate the effect of pH and electrolyte concentration on the charge density and salt rejection. In addition, the salt rejection was also predicted using a theoretical relationship based on Linearized Transport model and compared with the experimental values.
Experimental results indicated that in the pH range from 5 to 6.5 used in the study the zeta potentials of DK and NF-270 membranes are negative and decrease with electrolytic concentration. However, charge densities of both membranes increase with the increase of the pH and ionic concentration.
Donnan potential between solution and membrane was evaluated using the measured bulk charge density of membrane. Results showed that the two membranes used have higher Donnan potential in NaCl solution than in CaCl2 solution. Results from DK membrane showed that Ca2+ rejection increases with the CaCl2 solution concentration, while NF-270 gives opposite tendency.
Based on the membrane charge density, the salt rejection was predicted and compared with the measured values. The differences between both are about 5-10 % and 10-30% for filtering NaCl and CaCl2 solution, respectively. This discrepancy may be due to that commercial membranes used are composites in which the charge density in the skin layer is quite different from that in the supported layer. The method developed in the study can only provide the global charge density of membrane, how to determine the charge density in the skin layer remains a challenge.
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