摘要:環境中大多數的砷係以無機砷的型態存在,若長期暴露將對人體健康及生態環境直接或間接地造成潛在地威脅,如急性或慢性的毒理效應、烏腳病、皮膚及人體循環系統損傷等,因此砷被國際癌症研究組織列為第一級致癌物,確定具有人體致癌性。無機砷可再區分為三價砷及五價砷,在自然水體中具有帶電性之差異,分別以不帶電荷之亞砷酸鹽及帶電荷之砷酸鹽存在。傳統上針對地下水中砷污染所因應的處理技術包括化學氧化法、植物復育法、薄膜技術、吸附法、離子交換、電動力及化學混凝法等。然而,傳統之砷處理技術,具有能源需求較高、操作成本較高、薄膜表面容易積垢及較無法有效處理含低濃度污染物之水樣,且需要額外添加化學藥劑,有產生二次污染物之疑慮,故在砷處理技術上,持續精進與改善,或發展出節能、經濟且能有效除砷的新穎處理方法有其必要性。電容去離子(capacitive deionization, CDI)係一新穎電化學處理技術,其具備清淨、低耗能、無化學添加劑、無二次污染等副產物、適合處理低濃度之環境水體、具可調控性及可擴充性、電極具再生性等集競爭優勢與應用潛力。其原理是利用外部電場的施加與奈米多孔性活性碳電極的高比表面積,基於電吸附機制,於欲處理之水體中產生電位勢,藉由庫倫靜電力將水中之帶電離子吸附於電極表面上,並於奈米孔洞間形成電雙層,進而產出乾淨水質。值得一提的是,環境水體中五價砷之型態皆屬於帶電離子,故可於電場作用下透過電吸附機制去除,反之,三價砷在水體中並不具有帶電性,故無法直接於電場作用下透過電吸附去除。本研究目的係藉由CDI模組,以電催化材料來修飾活性碳電極材料,針對於較難處理的亞砷酸鹽(三價砷),通過同時的電催化及電吸附反應,將三價砷氧化成五價砷,再以電荷分離的方式從水體中移除,達到移除三價砷之目的,從而增加CDI對於含砷地下水的整體去除效能。本研究擬以過渡金屬之鈷氧化物作為電催化材料,以電沉積方式與多孔性活性碳結合,作為複合電極材料。本研究所製備的複合電極材料,將進行孔洞結構分析、物理化學特性分析、以及電化學反應分析,評估其對於三價砷的電催化活性,最後再以批次式的CDI實驗,進行驗證的工作。本研究的成果將可提升對於地下水體中無機砷之電化學處理技術,在學術發展與實際應用皆有顯著的貢獻。
Abstract: Arsenic (As) contamination of groundwater has been a serious worldwide problem that endangers the health of human beings. Especially, inorganic arsenic compounds have been identified as a form of carcinogen to humans by the International Agency for Research on Cancer. In basic, inorganic arsenic can be classified as arsenate (pentavalent arsenic, As(V)) and arsenite (trivalent arsenic, As(III)). Noteworthy, extreme weather conditions caused by climate change enhance the risk of water shortages. For that reason, the demand for withdrawing groundwater for agriculture could be increased. Negative effects of As in groundwater used for irrigation water in the agricultural production system are of great concern in Taiwan. Many localized groundwater sources have been identified as a potentail problem, in which the arsenic concentration is higher than the irrigation water quality standard of 0.05 mg/L. Therefore, it is essential to develop the effective, energy-saving, and environmentally friendly separation techniques for the removal of arsenic from groundwater. Capacitive deionization (CDI), or referred to electrosorption process, has been regarded as a promosing water-purification technology, which has many advantages such as low operating pressure, low energy consumption, no chemical demand, no secondary waste, and easy regeneration. The mechanism behind CDI to remove ionic species from aqueous solutions is based on the charge separation using nanoporous carbon electrodes for capacitive Elecrical double-layer (EDL) storage. It is worth noting that as comparing with other traditional water treatment processes, CDI is especially suitable for the removal of arsenic at low concentrations in groundwater. Under the assistance of electric filed, arsenic can be electrostatically separated from water to form the EDL within the carbon electrodes. The main purpose of this study is to promote the removal of arsenic (i.e., As(III)) from aqueous solutions via the simultaneous electrocatalysis and electrosorption in CDI. This study will focuse on the modification of the activated carbon (AC) electrode using transition metal cobalt oxide (CoOx) as electrocatayst for CDI applicaiotn. Surface morphology, pore characterization, and chemical properties of the resultant composite electrodes will be characterized in detail. The interaction between the composite electrode and arenic will be investigated by electrochemical measurement (e.g., cyclic voltammetry). Finally, a batch-mode CDI experiment will be carried out to determine the arsenic removal efficiency using the CoOx/AC composite electrode. Herein, As(V) with negative charge can be directly removed by the anode in electrosorption process at 1.2 V. For the removal of As(III), it could be involved with the oxidation of As(III) to As(V), and subsequent electrosorption of the As(V) onto the electrode surface of the anode. It is expectd that the modified CoOx/AC composite electrode could significantly enhance the remval efficiency of As(III) through the electrocatalytic oxidation of As(III) species. The obtained results can improve the current knowledge of CDI technolgy in arsenic removal and provide useful information for the practical filed applications.