膜電荷對奈米過濾效能影響之探討

Investigation of Membrane Charge on Nanofiltration Performance

顧聲茹(Sheng-Ru Gu)

指導教授 ： 莊清榮

中原大學/工學院/化學工程研究所/碩士(2007年)

https://doi.org/10.6840/cycu200700631

摘要

摘 要

於奈米過濾程序中，靜電效應為影響其分離效能主要因素之一，常以道南電位做說明，而欲了解該現象，應先得知膜內淨電荷密度，但基於實驗之限制，文獻上大多以數值解配合實驗數據修正其不足之處，再進行合理的估算。有鑑於此，本研究使用流線電位法決定奈米膜之膜面界達電位與整體淨電荷密度，並分析離子種類、濃度以及溶液pH值對其影響，選用型號為DL與NF-270之奈米膜分別對不同離子於不同溶液濃度與pH值之下進行過濾分離，且探討其分離機制，此外，本研究將淨電荷密度量測之結果配合Linearized Transport 模式估算其阻擋率，並與實驗值結果進行比較。
濾膜之膜面界達電位深受溶液性質的影響，隨著溶液pH值增加而增加，另外，於不同鹽溶液之下，其膜面界達電位會隨著陽離子強度增強而降低，然而利用流線電位法決定DL與NF-270膜之淨電荷密度，其隨著溶液pH值增加而負電荷量增加，亦隨著濃度的提高而增加。
至於DL與NF-270膜對於不同鹽類其阻擋率大小依序為R(MgSO4) > R(Na2SO4) > R(NaCl) > R(CaCl2)，藉由膜孔徑與水合離子間的篩分效應以及從淨電荷密度之結果所估算的道南電位，其中R(MgSO4) > R(Na2SO4) > R(NaCl)主要是由篩分效應所決定，而R(NaCl) > R(CaCl2)則是決定於道南效應。
本文最後將量測淨電荷密度值帶入Linearized Transport 模式預估濾液端的濃度，並與實驗得到的鹽阻擋率相比較，其結果與實驗值之平均差距為30%，因奈米濾膜是屬於複合膜，而皮層為帶有電荷且影響分離效能的主要層，但基於實驗上的限制，無法單獨量測其電荷密度，但就整體而言，其趨勢是與實驗值一致的，所以本研究利用流線電位法求得的膜內淨電荷密度，對於奈米過濾之分離效能的預估能建立有效之參考數據。

關鍵字

鹽阻擋率 ； 奈米過濾 ； 淨電荷密度 ； 道南電位

並列摘要

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.

並列關鍵字

salt rejection ； nanofiltration ； Donnan potential ； net charge density

參考文獻

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