- 學位論文
利用電輔助廢鐵連續式系統還原六價鉻
Electro-assisted Cr(VI) Reduction Using Scrap Iron Packed Column In Continuous System
摘要
為了還原六價鉻(Cr(VI))，設計一種連續式電還原系統，該系統之陰極為不銹鋼反應器，而陽極為將廢鐵填充至鈦網所製成的籠子。本研究的目的是藉由不同的操作參數及廢液之特性，探討使用電還原系統還原Cr(VI)的效果。本研究，將探討不同的水力停留時間(HRT, 10、15、30、60分鐘)、電流供應率(CSR, 0%、25%、50%、75%、90%、100%)、Cr(VI)的初始濃度(450、550、650、750 mg/L)、及插入額外的陰極(0、1、2支)對還原Cr(VI)的影響以及廢鐵溶解的情況。pH變化、Cr(VI)還原率、鎳(Ni)去除率、總鉻(TCr)去除率及電流效率將作為實驗結果討論。最後，將評估在設定不同參數的情況下，電還原系統的能源消耗及運營成本。 研究結果顯示，不同的水力停留時間並不會影響Cr(VI)的還原。基於廢水的Cr(VI)濃度及進流速率，提供理論所需的電流，即可穩定地將Cr(VI)還原；在低CSR的情況下，證實了額外的機制將有助於電還原系統還原/去除Cr(VI)，例如，在陰極表面直接還原、廢鐵表面上的化學還原、以及藉由氫氧化鐵、氫氧化鉻進行吸附去除。因為在假設只有間接還原的反應下，即溶出Fe(II)並藉由氧化所釋出的電子還原Cr(VI)，Cr(VI)的還原率將與CSR值相同。但在提供25%、50%、75%的CSR值，Cr(VI)的還原率都超過預期值的20%以上。因此，藉由額外的機制，提供的CSR值不需固定在100%，即可達到完全還原Cr(VI)的效果；在CSR固定為90%的情況下，理論上能夠還原Cr(VI)的量並不太會受到Cr(VI)初始濃度的影響。但是，由於濃度的增加，將削弱額外的機制輔助Cr(VI)還原；通過插入額外的陰極，電源的電壓輸出功率能有明顯的降低，並代替了加入電解質提高導電度的方法。除了大大的減少耗能，更減少了化學藥品的消耗；去除每莫爾的Cr(VI)所耗的能量為運營成本的主要因素。藉由增加HRT及插入額外的陰極都可以有效地降低能量成本，尤其插入額外的陰極為最有效的方法。將HRT從10分鐘增加到60分鐘將使能量成本降低約70％，但電極成本卻會增加約49.7％。然而，從插入0到2的額外陰極將分別降低能量成本和電極成本約71.4％和39.8％。 此研究證明了連續式電還原系統將有效的還原Cr(VI)，並且通過不同的操作參數，能計算出如何最有效率得還原/去除Cr(VI)，以及實現最節省的操作成本。
並列摘要
A newly designed continuously electro-reduction system comprised of a stainless steel reactor as cathode and a cage made of titanium mesh packed with scrap iron as sacrificial anode was employed for Cr(VI) reduction. The objectives of this study are to investigate the effects of operational parameters and waste liquid characteristics in the continuous system of electrochemical reduction process (ERP). More specifically, the effects of HRT (10, 15, 30, 60 min), current supply ratio (CSR) (0%, 25%, 50%, 75%, 90%, 100%), initial Cr(VI) concentration (450, 550, 650, 750 mg/L), and number of extra cathodes inserted (0, 1, 2 extra cathodes), on the Cr(VI) reduction and dissolution behavior of scrap iron were investigated. The results of Cr(VI) reduction efficiency, Ni removal efficiency, total Cr (TCr) removal efficiency, current efficiency and pH were present interrelatedly with each other. Finally, the energy consumption and operational costs of ERP were assessed. The result shows that HRT is not the influence factor for Cr(VI) reduction complete. By suppling theoretical current needed, based on the Cr(VI) concentration of wastewater and influent flowrate, the Cr(VI) could be reduced smoothly. At low CSR, the occurrence of extra mechanisms, such as direct electro-reduction on the cathode, chemical reduction on the scrap iron surface, and adsorption by ferric hydroxides precipitates, has been confirmed. Because the Cr(VI) reduction efficiencies exceed more than 20% of the expected value, which is the same as the CSR value when indirect reduction, i.e., dissolving and oxidizing Fe(II) for reducing Cr(VI), is the only reaction responsible for the reduction of Cr(VI). Thus, supplied current doesn’t need to be fixed at 100%, because of the occurrence of extra mechanisms. When the CSR of 90% was fixed, the theoretical Cr(VI) removal efficiency, 90%, would not be affected by initial concentration of Cr(VI). However, the extra Cr(VI) removal efficiency would be decreased by weakening extra mechanisms because of increasing initial Cr(VI) concentration. By inserting extra cathodes, the voltage output of the power supply would be decreased effectively, which alternated adding NaCl to increase conductivity. Meanwhile, the energy consumption and chemical consumption could be saved for reducing Cr(VI). Operation cost was affected obviously by energy consumption per mole Cr(VI). Both of increasing HRT and inserting extra cathodes decrease energy cost, and the inserting extra cathodes is the most effectively method. Increasing HRT from 10 to 60 min would decrease energy consumption around 70%, but increase electrode cost around 49.7%. However, inserting extra cathodes from 0 to 2 would decrease both of energy consumption and electrode cost around 71.4% and 39.8%, respectively. Using continuously electro-reduction system comprised of a stainless steel reactor as cathode and a cage made of titanium mesh packed with scrap iron as sacrificial anode is proved that reducing Cr(VI) is effective. By adjusting operation parameter, the most effective Cr(VI) reduction/removal could be determined and the most thrift operation cost could be achieved.