- 學位論文
以電容去離子技術移除六價鉻之機制探討
Understanding the Mechanisms behind Cr(VI) Removal in Activated Carbon-based Capacitive Deionization
摘要
近年來，六價鉻化合物被廣泛用於電鍍、金屬防鏽、木材防腐、皮革與紡織染色、水泥與其他產品等製程，造成之環境污染可能對於生物體產生嚴重危害，並已被世界衛生組織(WHO)之國際癌症研究署(IARC)列為第一類(Group 1)致癌物。而於環境中，六價鉻常以鉻酸根或二鉻酸根等重金屬含氧陰離子(heavy metal oxyanions)形式存在，因帶負電荷，較易與氯離子等相互競爭，使其去除格外困難。相較而言，其帶正電之三價鉻物種作為另一常見鉻氧化態，毒性與移動性皆較六價鉻為低。為有效去除水體中之六價鉻，現有吸附、還原、電混凝等方法皆可能將六價鉻轉換為三價鉻，然而這些技術於系統再生與低濃度污染水體之處理較為受限。而電容去離子技術(Capacitive deionization, CDI)作為一電化學水處理技術，可於施加外部電壓下形成電場，使水中離子經電雙層吸附儲存於碳電極孔洞，於處理六價鉻過程中並可能伴隨轉換反應，於六價鉻陰離子去除展現良好潛力。 本研究目的為經由電容去離子系統有效去除水體中之六價鉻，並研析在不同酸鹼條件下六價鉻電吸附與電轉換之機制。本研究以連續式(single-pass mode)電容去離子系統進行實驗；配製初始濃度為50 mg/L之六價鉻溶液，並施加1.2 V之電壓，以討論在不同pH值(pH 3與pH 4.8)下，六價鉻物種之電吸附與脫附效果，以及六價鉻於充電過程中還原為三價鉻之轉換表現。再者，經由連續操作10個循環實驗，驗證此系統的穩定性。結果顯示：於pH 3條件下，六價鉻的整體吸附容量較低(1.1±0.07 mg/g)，但展現較高之轉換效能(59.5±19.9%)，相較而言，於pH 4.8下系統展現較高之吸附容量(5.3±0.4 mg/g)，與較低之轉換效能(6.2±3.3%)。此外，後續掃描電子顯微鏡(SEM)與X射線光電子能譜(XPS)分析結果顯示，於pH 3下操作之電極具有較高之鉻含量，並確認三價鉻物種的存在，可說明於pH 3條件下系統具良好的六價鉻轉換效能。綜上而言，本研究針對電容去離子系統移除水中六價鉻過程的電吸附與電轉換現象進行探討，其研究結果將有助於後續操作程序之優化與電極材料之開發。
並列摘要
Hexavalent chromium (Cr(VI)), referred to as one of the heavy metal oxyanions (HMOAs), is a carcinogen widely found in industrial waste and the environment raising many concerns. To cope with the chromium pollutants, there were various methods (i.e., adsorption and membrane separation) proposed for the removal while the performance could be limited. Capacitive deionization (CDI) is a promising electrochemical technology for removing ions based on the electrosorption process using highly porous carbon materials. In this study, the CDI experiments were operated in a single-pass mode with activated carbon electrodes. The initial concentration of the targeted sodium dichromate (Na2Cr2O7) solution is 50 mg/L as Cr. It is demonstrated that the removal characteristics of Cr(VI) have a dependence on solution pH. At pH 3, the removal of Cr(VI) is mainly ascribed to the electrosorption process at 1.2 V, showing a relatively lower sorption capacity compared with which at pH 4.8. However, the increasing acidity would lead to the higher tendency of Cr(VI) reduction. As the result, experiments at pH 3 showed good transformation efficiency for Cr(VI) into Cr(III), about 59.5±19.9%, while which at pH 4.8 was 6.2±3.3%. Cycling experiments were also performed to show the stability of the system at different pH. Moreover, the results of the scanning electron microscope confirmed the more obvious presence of electrodeposited Cr on the cathode surface at pH 3, compared with which at pH 4.8. For further investigation, electrode surface characterization such as X-ray photoelectron spectroscopy was also conducted, and Cr(III) existence was again proved, with the consistency that larger amounts of Cr(III) were found at pH 3. In this study, the ions are released back to the solution, during the discharging step (short-circuit mode). Note that the Cr(III) ions are observed in the solution, further demonstrating the reduction of Cr(VI) in the charging step. In conclusion, this study provides new insight into Cr(VI) reduction and removal in the CDI process and thus a new option for future operation.