奈米薄膜去除消毒副產物有機前質之機制研究

Abstract

本研究以以兩種奈米薄膜（NF70、NF270），對具有不同官能基與解離常數之消毒副產物有機前質目標有機物（resorcinol、phloroglucinol、m-hydroxybenzoic acid、tannic acid）進行NF薄膜過濾實驗，配合薄膜表面一系列物化特性分析實驗，包含截留分子量、薄膜孔徑、親疏水性（接觸角）、膜面形貌觀測（SEM、AFM）、膜面電性（流線電位）、膜面元素組成（XPS、SEM之X光能量散射光譜儀）、膜面粗糙度等，以對薄膜去除消毒副產物有機前質之機制進行深入探討。 實驗結果發現兩種奈米薄膜對所選定之目標污染物均有不錯的去除率。藉由改變溶液pH值來驗證薄膜之去除機制，發現兩種薄膜在高pH值的環境中對目標有機物均有非常良好的去除率，目標有機物表面解離之帶負電官能基可與薄膜表面所帶之負電進行靜電排斥；對於不帶電之小分子有機物則以空間阻礙去除機制及吸附為主。水中存在的鈣離子並不會明顯影響目標有機物之去除率，但薄膜對鈣離子的去除率會隨pH上升與目標有機物的存在而增加，因為鈣離子可與目標有機物形成錯合物，故可提高薄膜之截留率。 SEM與AFM表面觀測結果均發現NF270比NF70薄膜表面來的平滑，表面波峰至波谷（peak-to-valley）的距離比NF70少一半；且薄膜粗糙度越高，通量衰減越嚴重，因目標污染物越溶液吸附卡在薄膜凹陷的孔隙中而造成阻塞，故NF70薄膜的通量衰減比NF270薄膜嚴重，且清水通量也比NF270薄膜少了將近50％。此外，在AFM膜面粗糙度的計算中，發現膜面粗糙度會隨掃瞄面積之增加而增加，故在比較不同薄膜之粗糙度時，需基於相同之掃瞄面積，以得到合理之比較結果。 本研究對所使用膜材建立之基本特性資料及研究成果，可作為其他應用研究之基礎；研究成果並可供實際淨水工程應用時之參考，對於淨水工程小分子天然有機物之去除，提供一深具潛力之淨水法，對國內自來水工業淨水技術之提升大有助益，亦能提供自來水公司未來更換傳統處理程序後之參考。

In this study, two types of commercial nanofiltration (NF) membranes (NF70 and NF270) were chosen to remove four model disinfection by-product (DBP) precursors (resorcinol, phloroglucinol, 3-hydroxybenzoic acid, and tannic acid) with different functional groups. The clean NF membranes were characterized by physico-chemical properties including molecular weight cutoff, membrane pore radius, hydrophobicity (contact angle measurement), membrane surface morphology (SEM and AFM), membrane surface charge (streaming potential measurement), membrane elemental composition (XPS and energy dispersive X-ray spectrometer, EDX) and surface roughness (AFM) to validate the NF rejection mechanisms of DBP precursors. The filtration experiments of the model compounds were assessed under various pH values (3-10) in which the removal efficiencies for both membranes were reasonably good at high pH values. Electrostatic repulsion is the prevailing mechanism for the model compounds with negatively ionizable functional groups rejected by the negatively charged NF membranes at high pH values, while steric hindrance exclusion and adsorption are controlling factors for the rejection of unionized small organic molecules. The presence of calcium does not significantly affect model compounds retentions. The calcium rejection rises with the presence of model compounds as well as an increase of pH due to its formation of complex ion between calcium and model compounds. For the examination of membrane surface roughness, it is essential to use the same scan area when comparing the surface roughness of different membranes. In general, membranes with rougher surface posses a higher fouling potential, which makes the flux decline more rapidly.