以流電極式電容去離子技術回收金屬離子

Abstract

因應經濟發展，金屬在各領域需求量上升，但部分金屬因故導致供給不穩定，且水污染防治法規範排放廢水中金屬含量，因此金屬離子回收技術日趨重要。流電極電容去離子法是近十年發展的新興技術，其電極流動特性展現連續式操作及製程放大的潛力，因此本研究運用此技術分別對含一價鹼金屬鈉、二價重金屬鈷、銅及三價稀土金屬鑭的溶液進行移除及回收，測試其應用的可行性。 首先利用批次法處理含金屬溶液，在原模組施加反向電場進行電極再生。考量流電極中離子的參與而提出被移除離子的遷移機制，一是儲存在活性碳內的電吸附，二是留在電極溶液中與多孔電極所排斥出的同離子維持電中性。研究結果發現，有較小水合半徑及價數的陰離子易以電吸附形式存在活性碳中，也容易作為同離子被排斥；然而，有較大水合半徑及價數的金屬離子一旦進到活性碳孔洞則需要較長時間進行脫附。 連接第二個再生模組，模擬更接近實務操作的電極再生方式，各離子可維持近99%移除及回收率，但以鑭離子有最差的回收速率。接著以連續式進料測試長時間操作，結果顯示各金屬皆可在1440分鐘的操作內維持穩定的移除速率。然而，水解反應在長時間操作下分別對一價鈉的系統與二價鑭、三價鈷的系統有所影響：鈉離子因較快的脫附速率，氫氧根離子和硝酸根離子的競爭使再生溶液鹼化；而鈷和鑭的系統中因較大的脫附阻力，造成氫離子和金屬離子的競爭，氫離子長時間的累積造成再生溶液酸化，且嚴重影響金屬離子的回收。另外，長時間操作下負極鹼化的現象也可能造成氫氧化物沈澱。因此，本研究嘗試降低再生側的施加電壓、控制流電極中pH值，成功抑制水解離，減緩氫離子競爭力，提升鑭離子回收速率。綜合上述，本研究提出各離子可能遷移機制，透過介入其競爭關係，使流電極式電容去離子技術能夠在含金屬廢液的處理有工業應用的潛力。

The demand for metals in industry has increased due to economic development; however, some metal supply is unstable. In addition, the government enact legislation such as effluent standard for metal-containing wastewater. Accordingly, the development of technology for metal ion recovery has become mainstream. Flow-electrode capacitive deionization (FCDI) is an emerging deionization technology in last decade. It shows commercial potential because of continuous operation and scale-up ability due to flowable electrode. The objective of this study is to evaluate the feasibility of using FCDI to recover metal ions of different valence number, such as Na+, Co2+, Cu2+ and La3+. Fundamental studies are conducted under batch-mode deionization and successive regeneration in the same FCDI module. Considering the ion in flow electrode, the migration mechanism of removed ion was proposed. Some acting as counter-ion being electrosorped by the active carbon, while other were distributed in the electrolyte due to charge neutralization of co-ion repelled by carbon electrode. Results showed that anion with smaller hydrated radius and lower charge valence preferentially being adsorbed in the carbon particles but easily been desorbed as the co-ion. However, once metal ions with larger hydrated radius and higher charge valence entered the active carbon pores, they would take longer time to desorb. Connect the second FCDI module with the original one, evaluate the feasibility of simultaneous deionization and regeneration. All ions still maintain almost 99% removal and recovery in batch mode; however, La3+ shows the worst recovery rate during desorption. The feed side is then changed from batch to single-pass mode to evaluate continuous operation in FCDI process. The results revealed that it can maintain steady removal rate for each metal at least 1440 minutes. However, some face regeneration problems due to water splitting under long term operation. The sodium ion has faster desorption rate, which causes the competition between hydroxide ion and nitrate ion alkalizing the regeneration solution. However, hydrogen and metal ions compete due to the desorption resistance for cobalt and lanthanum ion. The hydrogen accumulation leads to acidification of the regeneration solution, and seriously inhibiting the metal ions recovery. In addition, the alkalization in negative flow-electrode may also cause hydroxide precipitation. In this study, we try to reduce the applied voltage for regeneration and also control the pH in flow-electrode. It effectively inhibits water hydrolysis phenomenon, slow down the competitiveness of hydrogen and increase recovery rate of lanthanum. This study proposes a possible migration mechanism of each ion and its competitive relationship based on the results. The flow-electrode capacitive deionization technology shows great potential for metal recovery from wastewater in industrial.