電容去離子技術中二價金屬離子與奈米孔洞活性碳電極電化學行為之研究

Electrochemical Behaviors of Divalent Metal Ions and Nanoporous Activated Carbon Electrodes in Capacitive Deionization Process

李儂(Nung Lee)

指導教授 ： 侯嘉洪

國立臺灣大學/工學院/環境工程學研究所/碩士(2016年)

https://doi.org/10.6342/NTU201602162

摘要

電容去離子技術(Capacitive deionization, CDI)為低耗能、無化學藥劑使用、無二次汙染物產生、且電極易再生之新穎水處理技術，其操作原理結合了奈米孔洞碳材與電化學處理程序之特性，利用外加電場於多孔碳電極，使水中帶電汙染物與離子因電荷間的庫倫靜電力而被吸引，離子由於電吸附效應而被儲存於電雙層中，當外部電場被移除後，離子便會由電雙層中釋出，產生高濃度的濃縮溶液並使電極再生。然而，將電容去離子技術應用於金屬離子之移除程序時，系統之電極表面被觀察到有沉積物之產生(Huang et al. 2016, Oda and Nakagawa 2003)，然而卻鮮少有文獻對電極表面之沉積物做進一步探究。因此，金屬離子在電容去離子系統中產生沉積物之反應機制以及去除機制仍有進行研究與釐清的必要性，以掌握電容去離子技術應用於金屬離子移除作業之學理基礎，並提昇實際應用的可行性。
本研究目的為利用奈米孔洞碳材作為電容去離子技術之電極去除水中二價金屬離子機制之基礎性研究，研究中則選用標準氧化還原電位不同的銅(Cu2+ /Cu, E0= +0.34 V)、鋅(Zn2+ /Zn, E0= −0.76 V)與鈣(Ca2+ /Ca, E0= –2.87 V)等二價金屬離子進行CDI處理程序，並針對實驗後的奈米孔洞碳電極進行掃描式電子顯微鏡（Scanning Electron Microscope）、能量散佈分析(Energy Dispersive Spectrometer)、X光能譜分析(X-ray Photoelectron Spectroscopy)等物理與表面分析，以進一步觀察電極表面是否有沉積現象之發生並探討二價金屬離子之去除機制。
而研究結果指出，使用電容去離子技術去除單一二價金屬離子時，在施加固定電壓的情況下，擁有較高標準氧化還原電位之金屬其去除效率會較標準氧化還原電位低的金屬更易從水中分離出來，但在脫附過程中效率卻較低，且在電極表面分析中較容易觀察到沉積之現象，且由X射線光譜分析儀之結果可知，其沉澱物之生成涉及不可逆之法拉第反應，因此在電容去離子系統去除二價金屬離子之過程中，除了有電吸附效應之產生，亦可觀察到電沉積反應。
而在電容去離子技術去除混合二價金屬離子之試驗中，結果顯示具有高氧化還原電位之金屬較容易利用電容去離子技術將其從水中分離出來，擁有較高的去除效率，因此具有高氧化還原電位之金屬被認為有較好的去除競爭性。
綜上所述，本研究結果指出二價金屬離子在電容去離子系統中之去除機制除了電雙層電吸附以外亦包含了電沉積效應，且除了文獻所述之離子價數以及離子水合半徑(Hou and Huang 2013)，標準氧化還原電位亦是電容去離子系統選擇性去除之影響因素之一。

關鍵字

電容去離子技術 ； 氧化還原電位 ； 重金屬 ； 電吸附 ； 電沉積

並列摘要

Capacitive deionization (CDI) is a novel water purification technology for the removal of ions with the features of low-energy consumption, no membrane component, no secondary waste, and easy regeneration of electrodes (Anderson et al. 2010). The working principle of the CDI system is to apply an external electric field on a pair of porous carbon electrodes, and thereby ions can be electrostatically captured onto the oppositely charged electrode surface. Notably, our previous study reported that copper ions can be effectively removed from solutions by electrosorption and electrodeposition(Huang et al. 2014). The removal mechanism has a dependency on the amplitude of applying electric voltage. However, researches related to heavy metals removal with CDI technology are still limited.
The main object of this study is to give a fundamental aspect of heavy metal removal in CDI process using activated carbon electrodes. Divalent heavy metals with different reduction potentials are selected, including Cu2+, Zn2+, and Ca2+, for the following experiments: electrochemical characterization and CDI tests. Cyclic voltammetry and electrochemical impedance spectra measurements were carried out to clarify the physicochemical properties of divalent metal ions in aqueous solution. A batch-mode CDI cell was performed at an applied voltage of 1.2 V to remove divalent heavy metal ions at 1 mM. After the CDI process, the electrode surfaces are further analyzed by Scanning Electron Microscope, Energy Dispersive Spectromete, X-ray Photoelectron Spectroscopy, for determining the removal characteristics of divalent heavy metals onto the carbon electrodes. According to the results, the divalent heavy metals can be successfully removed by CDI process. The removal mechanism is not only by electrosorption of electrical double-layer charging, but also by the electrodeposition of reduction of metals onto the electrode surface. More specifically, the reduction potential of divalent heavy metals plays an important role to determine their electrochemical behaviours during the CDI process. For example, Zn2+ ions with a lower reduction potential (Zn2+ /Zn, E0= −0.76 V) are mainly removed by electrosorption, which is further confirmed by depolarizing the electrode. In contrast, Cu2+ ions, possessing a high reduction potential (Cu2+ /Cu, E0= +0.34 V), have a tendency to be reduced to be cuprous oxide in CDI. The electrodeposition of Cu2+ ions on the electrode surface are also observed by Scanning Electron Microscope, Energy Dispersive Spectromete, X-ray Photoelectron Spectroscopy. Furthermore, the removal selectivity is studied by a mixture of Zn2+ and Cu2+ ions to investigate the competition between different divalent ions. As evidenced, Cu2+ ions are preferred to be separated from water than Zn2+ ions. The research findings can provide useful information to understand the removal characteristics of different heavy metals with CDI technology for water purification and wastewater treatment.

並列關鍵字

capacitive deionization ； divalent metal ions ； standard redox potential ； electrosorption ； electrodeposition

參考文獻

Anderson, M.A., Cudero, A.L. and Palma, J. (2010) Capacitive deionization as an electrochemical means of saving energy and delivering clean water. Comparison to present desalination practices: Will it compete? Electrochimica Acta 55(12), 3845-3856.

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