吸附現象對奈米薄膜去除水中新興污染物之影響

Abstract

吸附為奈米薄膜去除水中新興污染物之重要機制之一，新興污染物在奈米薄膜過濾過程中，可與薄膜或水中固體物質(如天然有機物)發生吸附反應，進而影響其本身之去除效率。此外，水體中新興污染物並不能被傳統淨水程序或單一高級處理程序所去除。因此，本研究除了以商用奈米薄膜(NF270)對10種(8類)國內外水體中常見且具有不同物理化學特性(MW,logkow and pka)之新興污染物(選定Erythromycin-H2O, Chlorpyrifos, Trimethoprim, Iopromide, Carbamazepine, Sulfamethoxazole, Caffeine, Acetaminophen, Bisphenol A and Estrone)進行奈米薄膜過濾實驗外，亦針對薄膜搭配粒狀活性碳管柱以及薄膜表面改質實驗進行新興污染物去除效能評估。新興污染物多以微量濃度存在於水體中，本研究利用LC-MS-MS分析水中新興污染物濃度，水樣濃度為ng/L等級。 實驗結果發現以NF270奈米薄膜分別在純水以及模擬天然原水中處理八類目標污染物，各類新興污染物之去除效率差異甚多，約介於4%至99%，環境賀爾蒙類之新興污染物被去除率較個人及衛生環境用藥類佳;在奈米薄膜結合活性碳管柱實驗中發現，搭配兩種以上高級處理程序，新興污染物最後之去除率均可達99%以上，然活性碳管柱平均性能表現優於奈米薄膜;以表面改質後之薄膜處理新興污染物，對於新興污染物之去除率亦有提升，唯改質過程耗費額外能源，經膜面鑑定後亦發現會造成膜面增厚，此一現象於過濾時易產生濾阻增加及通量急速下降，故較適合低污水處理量之應用。 吸附現象在本研究結果中扮演重要的角色，八類新興污染物中，logkow高於 2之新興污染物族群，其在過濾或吸附處理過程中，吸附機制表現明顯，由等溫吸附曲線發現，環境賀爾蒙類的新興污染物對於活性碳或是薄膜表面，均有較易被吸附的傾向。此外，由個別吸附相關實驗結果指出，新興污染物會與水中腐植酸、薄膜產生物理性吸附，吸附程度以環境賀爾蒙類較為顯著。 本研究亦針對不同物理性質的新興污染物進行機制探討，由於實驗水樣之pH值接近中性，部分之目標新興污染物在中性下會解離帶負電，靜電排除的機制相對於其他污染物則變得重要，薄膜實驗結果指出在不同水質背景下，主要去除機制將會改變，在純水(水質乾淨或經前處理)條件下，物理篩除仍是薄膜去除新興污染物主要的去除機制;當水質背景含有其他有機物(腐質酸)時，吸附機制則影響整個去除效率。此外，在純水背景下，薄膜去除新興污染物之平均效果較活性碳管柱差，但當水中存在其他有機物質時，因競爭吸附效應，薄膜平均去除效率則較活性碳管柱顯著。 本研究利用3E(經濟面、工程面及環境面)面向就(1)奈米薄膜、(2)活性碳管柱、(3)薄膜/活性碳管柱以及(4)改質厚薄膜等四種處理程序進行成本效益評估，結果顯示後三者之評比均較原始奈米薄膜效益較高，建議後續可針對此三種處理程序於實廠應用之可行性進行評估，以符合全世界日益嚴苛之飲用水標準。

Adsorption is one of the main mechanisms of compounds of emerging concerns (CECs) rejection by a membrane process. CECs could be adsorbed not only by membrane but also by suspended solid during membrane filtration, i.e. NOM. Besides, CECs could not be removed by a conventional water treatment or a single advanced process, even a single activated carbon column or a commercial NF membrane. In this study, the NF270 membrane was coupled with granular activated carbon in fixed-bed column system (GACC) to investigate the performance on CECs removal. The role of Humic acid (HA) in Nano-filtration (NF) processes and adsorption processes was also examined as well. The NF270 membranes were modified further by monomer grafting method to improve the rejection of the selected CECs. Ten CECs, Erythromycin-H2O, Chlorpyrifos, Trimethoprim, Iopromide, Carbamazepine, Sulfamethoxazole, Caffeine, Acetaminophen, Bisphenol A and Estrone, which are commonly found in the aquatic environment, were chosen as model pollutants and grouped in this study. CECs are an important group of organic environmental contaminants that have the potential to cause health risks for humans as well as biota. Concentrations of CECs were analyzed by LC-MS-MS to ng/L level. The results indicated that the eight grouped CECs had different rejections in the range of 4% to 99% by a NF270 membrane both in raw or simulated natural water samples. The adsorption experiments revealed that the hydrophobic CECs can be adsorbed onto granular activated carbons (GAC) easily than the hydrophilic ones, and GACC generally had better removal efficiencies of CECs than membrane process. Adequate existence of HA showed helpful for CECs removal which means CECs can be adsorbed onto HA became a larger molecular then removed by a NF270 membrane easier. Characterization of the modified NF270 membrane using SEM, ellipsometry and ATR-FTIR analysis revealed that NF270 membranes grafting by SPM monomer increased its hydrophilic and thickness of surface layer, and the grafting extent was proportional to the system pressure, monomer concentration and duration time, especially system pressure. Both of the combination system of NF/GACC and the modified membrane studies can remove the targeted CECs to above 90%; For the engineering application, however, the modification method was limited by the quality control that the combination system was better for applying in water treatment in engineering consideration. The contrast of the sequence of NF/GACC or GACC/NF for CECs removal was not obvious in this study, and GACC had better rejections than a raw NF270. The above results suggest that the existence of HA can increase the rejection of CEC, and the modified membrane also work for CECs rejection. The performance on cost-benefit analysis of GACC, the combination system and the modified membrane showed good feasibility on a real scale application.