壬基酚在多壁奈米碳管上之多層吸附與電化學再生性能評估之研究

Abstract

本研究為解決利用多壁奈米碳管等碳材吸附後之問題，而非僅僅是使用後即丟棄，因此利用電化學氧化方法去再生吸附飽和之多壁奈米碳管為本研究之重點，在電化學氧化反應過程中其反應時間短同時能將污染物降解，使得利用多壁奈米碳管之吸附方式可作為有效之去除水中污染物方法之一，同時電化學氧化方法也可做為環境永續之技術。 首先調查了15個台灣淨水場之原水與處理後之出水中壬基酚和雙酚A的濃度。在被調查的淨水場中，原水中被檢測較高濃度的壬基酚，其可能原因是受廢水排放之污染。同時利用生物測定技術，即E-screen和T47D-KBluc，以評估用於人類健康保護的水質。利用實驗室模組模擬評估淨水場對於壬基酚與雙酚A之去除情形，發現快濾單元比混凝沉澱單元更能有效去除此兩種污染物；而在薄膜試驗中添加腐質酸、鈣離子和改變pH去探討其移除狀況，透由實驗結果可以推論腐植酸可以在薄膜表面上形成凝膠餅，使得分子變大；而添加鈣離子不僅可增加了溶液中之離子強度，同時還形成電雙層，提高了去除效率，藉由此結果可以知道增加離子強度和分子大小都會增加壬基酚和雙酚A去除效率。 利用改質過後之多壁奈米碳管評估從水溶液中去除壬基酚的效果。在實驗條件NP濃度為2.5 mg/L和pH 4時，多壁奈米碳管的最大吸附量為1040 mg NP/g MWCNTs。根據吸附動力學及等溫吸附方程式結果推論Elovich Equation和Elovich Isotherm較符合多壁奈米碳管之特性，是一個多層之吸附；而利用熱力學分析發現，整個反應是一個吸熱反應，和其他研究中吸附酚的結果是相同的；透過熱力學、等溫吸附方程式和動力學等分析，將有助於了解酚類化合物在多壁奈米碳管等碳材吸附劑上的吸附行為和機理。 使用電化學氧化法去再生吸附飽和之多壁奈米碳管，探討其電極之相關基本特性與再生機制。在電化學反應槽中使用三種不同材質之電極碳、鈦及氯化釕，與三種不同之電解質檸檬酸，氫氧化鈉與氯化鈉。多壁奈米碳管再生時在不同之電解質檸檬酸，氫氧化鈉和氯化鈉中之再生效率分別為65.2-72.4、65.8-71.7和64.0-80.5%；透由文獻發現在電解質為氯化鈉時，ClO·和氫氧自由基都是使得多壁奈米碳管再生效果比較好的可能原因之一；而其中氫氧自由基是其他兩種電解質中唯一之氧化物。在再生成本估算中，若多壁奈米碳管再生效果為58.8-69.5%時，每立方米大約為50.67美元。經由再生過後之多壁奈米碳管其在吸附效能可達98%以上，透由結果可以做為多壁奈米碳管等碳材再生時之參考。而氯化釕電極除了處理吸附飽和之碳材外，直接應用在廢水處理中也有相當之可行性。

This study investigates the occurrence and treatment of endocrine disrupting chemicals (EDCs) i.e. nonylphenol (NP) and bisphenol-A (BPA) in 15 Taiwanese water treatment plants. From all the surveyed waters, NP was detected at a high concentration, which could be attributed to contamination by wastewater discharges. The performance of coagulation-sedimentation and rapid filtration in removing a suite of EDCs was evaluated. The rapid filtration process has a better removal of EDCs than the coagulation and sedimentation processes. The humic acid may form a gel cake on the surface of the membrane thereby enlarging action effect; while calcium ions not only increase the ionic strength of the solution but also compress the double layer between the adsorbent and the adsorbate resulted in enhancing the removal efficiency. It was thus concluded that both enhancements of ionic strength and molecular size would increase of EDCs removal efficiency. We evaluated the performance of modified multi-walled carbon nanotubes (MWCNTs) in removing NP from aqueous solutions. The impact of a few experimental factors, i.e., pH, contact-time, MWCNTs concentration, and temperature on the NP removal efficiency of modified MWCNTs were studied. The maximum adsorption capacity of the MWCNTs was 1040 mg NP/g at the initial NP concentration of 2.5 mg/L and pH 4. The adsorption process followed the Elovich kinetics and the Elovich isotherm, indicating it is multilayer adsorption. According to the thermodynamic analysis result, the NP adsorption by the MWCNTs was thermodynamically satisfactory and, for the most part, endothermic like the adsorption of phenol. The result of the current study demonstrated the significance of free binding sites and pore size of MWCNTs in the NP adsorption. This paper will help to better comprehend the adsorption behavior and mechanism of phenolic compounds onto carbon-based adsorbents and establish a regulatory policy to control NP in source waters of Taiwan. The regeneration for reuse of nonylphenol (NP)-laden MWCNTs was studied using electrochemical oxidation to completely mineralize the adsorbed NP to CO2 and water. Factors affecting the regeneration efficiency of MWCNTs and the reusability of the RuO2/Ti electrode were also studied. During MWCNTs regeneration, electrochemical oxidation of the adsorbed NP was carried out using the RuO2/Ti electrode operated at the potential gradient of 2 V/cm for 8 h in 150 mg/L of synthetic solution. Three electrolytes, i.e., 0.03–0.24 M of citric acid, 0.1–0.8 M of NaOH and 0.05–2 M of NaCl, respectively, were used in a glass cylindrical type of electrochemical cell. Three different types of electrodes, i.e., carbon, titanium, and RuO2/Ti (RT) were studied and compared. The NP removal efficiency of MWCNTs in the citric acid, NaOH, and NaCl systems was 65.2–72.4, 65.8–71.7, and 64.0–80.5 %, respectively. It was found that both ClO· and OH· radicals were responsible for NP degradation in the NaCl system; while OH· radical was the only oxidizing species in the other two electrolytes. Additionally, 58.8–69.5% of NP was degraded in the liquid phase at an estimated cost of ca. 50.67 USD per m3 of water. The regenerated MWCNTs could maintain an NP removal performance of 98.6% via adsorption. Bimetallic electrode and type of electrolyte are the key factors affecting the regeneration efficiency of MWCNTs. It is concluded that the RT electrode should be a potential green technology for MWCNTs regeneration in wastewater treatment. In addition, the bioassay techniques i.e. E-screen and T47D-KBluc were also introduced to assess the water quality for human health protection.