奈米過濾膜電荷密度分析及其對過濾效能影響之探討

Analysis of Membrane Charge Density and Its Influence on Nanofiltration

李成豪(Cheng-hao Li)

指導教授 ： 莊清榮

中原大學/工學院/化學工程研究所/碩士(2010年)

https://doi.org/10.6840/cycu201000869

摘要

奈米過濾程序中，濾膜的電性是影響其分離效能的重要因素ㄧ，而在一般文獻中常藉由鹽類阻擋之實驗數據配合數值解的方式反推估膜整體之淨電荷密度。本研究則藉由流線電位法量測薄膜之界達電位以及利用交流電阻法量測膜孔內溶液之電導度，配合理論模式來決定膜整體之靜電荷密度，並分析離子種類、pH值以及離子濃度對膜淨電荷密度的影響。在恆壓過濾實驗中，本研究選用Desal DK以及NF-270兩型號之商業奈米膜材進行過濾，實驗結果也與膜淨電荷密度配合Linearized Transport模式所預估之鹽類阻檔結果相比較。
由於奈米濾膜選擇層多由高分子所組成，因此溶液的性質對膜面界達電位之影響甚大，在DK以及NF-270之中，膜面界達電位與道南電位皆隨著溶液pH值的提高而增加，而隨著離子濃度的增加而下降，薄膜整體淨電荷密度之負電荷則隨著離子濃度與pH值的增加而增加。
在NF-270與DK於NaCl溶液進行過濾時，離子阻擋率隨著濃度的增加而下降，而DK於CaCl2溶液時，Ca2+離子阻擋率會隨著濃度的增加而上升，但在NF-270於CaCl2時，結果則相反。
最後本研究將膜整體之淨電荷密度值配合Linearized Transport模式預估離子阻擋率，在定性上與實驗所作之結果有相同之趨勢，而於定量上誤差皆在30%以內，然而估算結果與實驗結果的誤差原因為，由於奈米濾膜分離效能多由選擇層所主導，但是基於實驗上之限制，只能量測膜整體之淨電荷密度，導致誤差的存在，但整體而言，於定性上在奈米過濾分離效能上之結果能有效的預估。

關鍵字

奈米過濾 ； 淨電荷密度 ； 道南電位 ； 鹽阻擋率

並列摘要

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.

並列關鍵字

Net charge density ； Nanofiltration ； Salt rejection ； Donnan potential

參考文獻

左煒憶，”以電滲透及流線電位決定薄膜之膜孔界達電位”，私立中原大學化學工程研究所，碩士學位論文(2001)。

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