以流電極電容去離子技術回收無機酸

Abstract

流電極電容去離子技術（Flow-electrode capacitive deionization，FCDI）是一種基於電化學反應的新興水中離子回收技術。它被認為擁有高能源表現、對環境友善以及擁有實現長時間連續回收的潛力等優點。無機酸作為一種常用的工業原料，被廣泛應用在多種製程之中，因此每年會產生大量的酸性廢液。本研究的目的在於發展流電極電容去離子技術，並且測試其是否可以應用於無機強酸的回收。 首先，為了防止作為流電極的活性碳懸浮液堵塞流道，本研究使用的是一種仿流體化床的流電極流道設計，這個設計導致了流電極流道與進料溶液流道較寬，分別為6mm與3.5mm，因此電阻較高，在施加電壓為1.2V的條件下回收表現較差。為此，通過調整施加電壓、改變碳材中活性碳與導電碳黑濃度的方式來提升流電極電容去離子法的回收表現。在施加3V電壓且流電極中含有10wt%活性碳、1wt%導電碳黑的條件下，對於0.01M鹽酸最大回收速率達到了2.64mmol m-2 min-1，並且在長時間運作之下可以達到99.5%的離子回收率。 接著對於流電極電容去離子技術進行長時間連續回收鹽酸測試，結果可以看出流電極電容去離子技術可以在24小時內持續回收鹽酸，但是在長時間運作之後依然會發生回收表現下降的情況。在3V的施加電壓下，在運作了24小時之後回收速率從最大值的4.04 mmol m-2 min-1下降到了2.99 mmol m-2 min-1，下降了25.99%，電流效率則從75.76 %下降到47.85 %，下降了36.84 %。這是因為除了流電極本身電吸附能力限制之外，還會由於大量發生法拉第反應而使得正極流電極溶液中鹽酸濃度升高，在前述的實驗中，運作了24小時後正極溶液中的氯離子濃度比當時進料溶液中的氯離子濃度高出了4.49g/L，是進料溶液的1870%。當正極流電極溶液中的鹽酸濃度大幅度升高並且遠高於進料溶液中鹽酸濃度時，由於陰離子交換膜無法徹底阻擋氫離子的遷移，因此會導致正極流電極溶液中的高濃度鹽酸擴散到進料溶液之中，使得回收的鹽酸不再只是進料溶液中原有的鹽酸，因此導致了電流效率與回收速率的下降。而在結合了流電極再生程序之後，正極流電極中鹽酸濃度的上升被抑制，在10小時運作之後正極流電極溶液中離子濃度僅從0.14 g/L上升到0.19 g/L。因此不再產生因為濃度梯度導致的鹽酸擴散回進料溶液中之現象，從而使得其在連續運作中表現穩定，不會發生回收速率與電流效率下降的情況。 最後測試了以流電極電容去離子技術回收不同無機強酸，結果可以看出流電極電容去離子技術回收鹽酸與硝酸的表現較好，回收速率分別為4.04 mmol m-2 min-1與2.85 mmol m-2 min-1，但是回收硫酸的表現較差，回收速率僅為0.62 mmol m-2 min-1。這可能是因為離子本身性質與離子交換膜性質所導致的結果。 因此，流電極電容去離子技術有回收無機酸的能力，但是要進行大規模應用還需要對模組與整個程序的參數設計進行更進一步的改良與探討。

Flow-electrode capacitive deionization (FCDI) is a novel technology of ion recovery in solution whose fundamental is based on electrochemical reaction. FCDI has benefits including high energy efficiency, environmental friendliness and continuous operation for ion recovery with simultaneous regeneration of the flow-electrode. Mineral acids, which are basic chemicals for a variety of processes are widely used in industry. A large amount of acidic waste is generated every year. The objective of this study is to develop FCDI and to test if it can recover acid from acidic waste. Firstly, to avoid clogging of the carbon in channel, the design of the flow-electrode channel in this study is similar to a fluidized bed, which leads to a wider flow-electrode channel and feed solution channel. The widths are 6mm and 3.5mm respectively. This results in the increasing resistance and poor performance under 1.2V. To enhance the performance, the contributions of the applied voltage, the concentration of activated carbon and that of conductive carbon black for acid recovery were being studied. The results show that the maximum recover rate is 2.64 mmol m-2 min-1 with 99.5 % hydrochloric acid being recovered from feed solution after long term operation when we apply 3V voltage on the system with 10 wt% activated carbon and 1 wt% conductive carbon flow electrodes. In continuous test, it is showed that FCDI can recover acid continuously. However, performance will decrease after long time recovery without flow-electrode regeneration. When applying voltage under 3V and during 24 hours of operation, the recovery rate decreased from 4.04 mmol m-2 min-1 to 2.99 mmol m-2 min-1 and the charge efficiency decreased from 75.76 % to 47.85 %, which are reduced by 25.99 % and 36.84 %, respectively. The reason is not only the adsorption capacity limitation, but also the increasing concentration in the anode solution due to intense faradic reaction. In the experiment described above, the anode concentration is 4.49 g/L, which is 18.7 times more concentrated than the feed solution. When the concentration in the anode electrode solution is much higher than that in the feed solution, the ions will diffuse from the anode solution to the feed solution because of the concentration gradient and the inability for anion exchange membrane to block the hydrogen ions and chloride ions. In this case, FCDI recovers not only the original ions in feed solution, but also the diffused ones, which leads to the decreasing of performance. After combining the ion removal process and the flow-electrode regeneration process by using two FCDI models, the recovered ions are released from flow-electrode solution. As a result, the concentration of anode solution is no longer higher than that of feed solution. After 10 hours’ operation, the anode concentration only increased from 0.14 g/L to 0.19 g/L, so that FCDI can recover ion from feed solution continuously with the performance remaining stable. Finally, we tested if FCDI can recover different acid by changing feed solution. The performance for the recovery of hydrochloric acid, nitric acid and sulfuric acid had been compared via FCDI. The results show that the recovery rates of chloride acid and nitric acid are 4.04 mmol m-2 min-1 and 2.85 mmol m-2 min-1, respectively, which are better than 0.62 mmol m-2 min-1 of sulfuric acid using FCDI. This is because of the essential properties of ion and ion exchange membrane. This study has proved that FCDI has ability to recover mineral acids. However, in order to apply it in industry, the model as well as the parameters of the process should be further studied and improved.