TiO2/SiO2/Fe3O4(TSF)光催化降解水中氯胺酮之反應性及機制探討:超氧自由基之貢獻

Abstract

現代藥物使用氾濫，致使環境水體內亦開始有藥物之存在，其難降解之特性對生態環境所帶來之潛在風險使得環境水體內之藥物成為備受關注之議題。在眾多的藥物中氯胺酮(俗稱K他命)由於濫用問題且於環境中相對穩定，更加受到科學家之重視。然而早期所設計之較為傳統的二級污水處理廠無法有效去除這類新興污染物，因此，發展出不同的高級氧化處理程序以去除此類污染物。其中，光催化法被視為淨化水質最有前景的方法，然而所使用之催化劑無法有效回收，限縮其實際應用性。本研究的主要目標是利用一可回收之光催化劑降解水中的氯胺酮物質，並進一步探討其反應機制。此催化劑為一核殼結構的材料，以Fe3O4為核心，並由SiO2包覆，其為中間層，TiO2 參雜於SiO2上(TiO2/SiO2@Fe3O4)，亦簡稱之為TSF。 TSF的材料結構與形狀利用X–射線繞射分析(XRD),掃描電子顯微鏡(SEM),高解像穿透式電子顯微鏡(HRTEM),比表面積分析儀(BET)進行分析。本研究的光催化反應以模擬太陽光反應器做為光源(700 W/m2, 300–800 wavelength)，TSF在最佳反應條件下([ketamine] = 0.3 μM, [TSF] = 100 mg/L, solution pH = 9), 擁有最快的氯胺酮降解速率 (pseudo-first-order rate constant = 0.1917 min–1), 其降解速率略優於TiO2。此外，TSF具磁性之特徵，使其能以磁鐵進行簡易回收再多次利用，批次反應的回收率高達85%，而重複使用之TSF降解氯胺酮的速率並無明顯的減少(rate constant of 1st : 0.1098 min–1 and 2nd : 0.1095 min–1). 系統反應機制則顯示超氧自由基 (•O2–)為氯胺酮降解過程的主要反應物種。TSF材料的價帶與導帶分別是1.89 eV和–1.91 eV，此區間包含了超氧自由基(–0.33 eV)和單氧自由基(+0.65 eV) 在pH7條件下的氧化還原電位，利於此兩種活性物種的生成；然而此區間並不包含氫氧自由基的氧化還原電位(+2.32 eV)，故不利氫氧自由基生成。反應在酸性或鹼性條件下，會消耗超氧自由基，抑制氯胺酮的降解效率；接著根據probe試驗，發現單氧自由基的貢獻可被忽略，超氧自由基與氫氧自由基對降解氯胺酮的貢獻比例為93:7；再透過曝氣實驗探討氧的來源，發現無論曝氮氣或是氧氣，氯胺酮之降解效率皆無明顯的改變，故排除溶氧作為氧的來源之可能性；後利用XPS分析表面氧反應前後之差異，顯示於反應後氧氣有消耗的情況。綜合上述之實驗，證實超氧自由基為本研究中重要的反應物種且其來源為材料表面的晶格氧。最後根據超氧自由基的親核性特性，利用DFT模擬氯胺酮中各原子的電子密度，透過攻擊電子密度最低的位置推測副產物，並利用UHPLC–QTOF–MS分析是否產生該副產物，再次驗證超氧自由基於此反應的重要性。本研究在環境應用上相當具有潛力，本實驗中總有機碳的去除率高達80%，且處理過程中毒性極低(小於0.9毒性單位)，且研究中所發現的副產物亦會在反應後全部去除，為一環境友善之系統。

The existence of pharmaceuticals in aquatic environments has become a critical issue because they are recalcitrant and pose a potential risk to ecosystems. Among pharmaceuticals, ketamine has received increasing attention because of its overuse in daily life and its persistence in aquatic matrices. Because conventional wastewater treatment processes cannot effectively remove these pharmaceuticals, various advanced oxidation processes have been developed and investigated, and photocatalysis has been considered a promising method for water purification. However, photocatalysts are typically difficult to recycle, which hinders their application in water treatment. The aim of this work is to develop a magnetic photocatalyst with a core-shell structure composed of a magnetite core and silica dioxide layer doped with titanium dioxide (TiO2/SiO2@Fe3O4, TSF) for ketamine removal; the associated photocatalytic degradation mechanism is also investigated and elucidated. The structure and morphology of the TSF photocatalyst has been well characterized by utilizing XRD, SEM, HRTEM and BET methods. In the photocatalytic experiment, by utilizing simulated solar irradiation (700 W/m2, 300–800 wavelength) as the light source under optimum experimental conditions ([ketamine] = 0.3 μM, [TSF] = 100 mg/L, solution pH = 9), the TSF photocatalyst possesses higher ketamine degradation efficiency (pseudo-first-order rate constant = 0.1917 min–1) compared with that of commercial TiO2 (P25). Additionally, the magnetic TSF photocatalyst can be collected and reused several times, and the recovery can reach up to 85% in each batch. Furthermore, the efficiency of the catalyst after repeated use shows no significant reduction (rate constant of 1st: 0.1098 min–1 and 2nd: 0.1095 min–1). Superoxide radicals (•O2–) are likely the dominant reactive species for ketamine degradation. The valence band and conduction band of TSF are 1.89 eV and –1.91 eV, respectively. This band gap includes the formation intervals of •O2– (–0.33 eV) and singlet oxygen (1O2) (+0.65 eV), while excluding that of hydroxyl radicals (•OH) (+2.32 eV), at pH 7. In addition, under acidic or basic conditions, •O2– will be consumed less than when under neutral conditions. With the probe test, the contribution of 1O2 can be ignored, and the contribution ratio of •O2– and •OH for ketamine degradation is 93% and 7%, respectively. According to a gas purge experiment and XPS data, purging with both N2 and O2 does not significantly enhance or inhibit ketamine removal efficiency, while the XPS results show the O2 consumption from the material surface during the reaction. Moreover, due to the nucleophilic property of •O2–, the DFT calculation and UHPLC–QTOF–MS byproduct analysis help to determine whether the main byproduct is certainly generated by a •O2– attack. The entire ketamine removal process in this study has potential for use in environmental applications because of its high TOC removal efficiency (up to 80%) and low Microtox® acute toxicity (lower than 0.9 toxicity units). All byproducts are removed (no detection signal) after the reaction, thus showing that the TSF photocatalyst is a promising material to use for water purification.