鹵素離子對光催化劑降解磺胺甲噁唑之影響與機制

Abstract

氧化鎢(tungsten trioxide, WO3)、二氧化鈦(titanium dioxide, TiO2)以及氧化鋅(zinc oxide, ZnO)奈米顆粒(nanoparticles, NPs)等均具無毒、高穩定性以及高反應性等優點，本篇以此三種顆粒作為光催化材料。磺胺甲噁唑(Sulfamethoxazole, SMZ)為一種臨床上常見的磺胺類抗生素，然而其廣泛使用導致SMZ流布至環境水體當中，且當前廢水處理與微生物降解SMZ均無法有效移除之，光催化降解被視為最快速有效移除SMZ的方法之一。水體中的常見鹵素離子對光降解可能有正向或負向的影響，本研究則關注於其與不同光催化劑的互動。隨著氯與溴離子濃度的提高，WO3、商用TiO2 (commercial TiO2, CTiO2)以及ZnO NPs聚集與沉降的現象逐步明顯。無鹽類情況下，三者降解SMZ之速率常數分別為0.0081、0.0065以及0.022 min-1。當系統中存在氯離子，其可能於過程中轉為自由基捕捉者，導致WO3與ZnO NPs對SMZ的降解能力減弱。然而CTiO2提供的酸性環境 (pH 4.4 to 4.6 )，可將其轉為活性鹵素物種，可於500 mM NaCl下提高降解速率至0.023 min-1。溴離子同樣使WO3對SMZ的降解能力減弱。然而在500 mM NaBr下，CTiO2與ZnO的降解速率明顯提高至0.21 min-1與0.056 min-1。此些促進的原因可能來自於活性鹵素物種的生成，其生成又與環境的酸鹼值、顆粒表面電荷以及光催化劑的量子產率等有關，故本研究目的為證明鹵素鹽類促進SMZ降解的原因與其機制。光致激發光譜中，高濃度NaBr下可明顯降低CTiO2與ZnO電子電洞對的重組，因而增加催化能力。以coumarin與courmain-3-carboxylic acid (CCA)分別測定溶液中與顆粒表面的•OH，在無鹽類的情況下均可測定到此二者。然而系統中含NaBr的情況下，則無法測得，顯示溴離子會與•OH發生反應。另外藉自由基捕捉的實驗顯示，在含有NaBr的情況下，不論系統是否有•OH清除劑 (methanol)，SMZ的降解均被提升，顯示•OH對其降解影響較小。但系統中具電洞清除劑 (sodium oxalate, Na2C2O4)的情況下，則均會明顯抑制SMZ的降解，顯示活性鹵素物種的形成與電洞息息相關。此外allyl alcohol (AA)作為顆粒表面活性物種(•OH、•Br等)的捕捉者，tert-butyl alcohol (t-BuOH)作為溶液中活性物種的捕捉者的實驗中，CTiO2與ZnO的光催化反應均會被抑制。然而當系統中含有NaBr時，加入AA組具有明顯的抑制，而t-BuOH組則無，證明活性物種應大量存在於顆粒表面。本篇中也藉由N,N-diethyl-p-phenylenediamine (DPD)實驗，證實CTiO2與ZnO於紫外光下可將溴離子反應為活性鹵素物種。而cyclohexene會與Br2反應形成dibromocyclohexane，同時也證實Br2會於系統中生成。在上述兩實驗中，若添加甲醇則會抑制活性溴物種的產量，顯示出其生成亦受到•OH的影響。最後以HPLC-MS分析降解副產物，發現兩種SMZ的溴化產物。分別為在苯環或胺基上接上一個溴，與在苯環位置接上兩個溴。此結果亦直接證明系統中的溴離子將被反應成為一高活性的物種，並具有攻擊SMZ的能力。

Tungsten trioxide (WO3), titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles (NPs) are chosen as photocatalysts in this study. The widespread detection of antibiotics in aquatic environments is raising public health concerns. Sulfamethoxazole (SMZ) was chosen as a target compound because it is one antibiotic extensively applied in human and animals. Photocatalysis is regarded as a green technology to treat the wastewater with SMZ. The presence of halide anions which are common in the water body has both positive and negative influence on the photocatalytic performance. This study aims to investigate the photocatalytic degradation mechanism of SMZ by different photocatalysts with the presence of halide salts. With the increase of chloride or bromide concentration, the aggregation and sedimentation of WO3, commercial TiO2 (CTiO2) and ZnO NPs become more significantly. In the condition without electrolytes, the rate constants of SMZ photodegradation were 0.0081, 0.0065 and 0.022 min-1, respectively. Chloride ions in the systems might scavenge active radicals during the photocatalytic reactions, hence the photodegradation efficiency of SMZ by WO3 and ZnO decreased. However, UV/CTiO2 system provided an acid condition (pH 4.4 to 4.6) to transform chloride ions to reactive halogen species (RHS), thus increased the removal rate constant to 0.023 min-1 in the presence of 500 mM NaCl. When there were bromide ions, the degradation rate of SMZ by WO3 decreased, while the rate constants in UV/CTiO2 and UV/ZnO systems enhanced to 0.21 and 0.056 min-1, respectively. The enhancement of degradation could be attributed to the generation of RHS which were relative to the pH in the systems, particle surface charges and quantum yields of photocatalyst. This study studied why the photodegradation of SMZ enhanced with halide ions. First, with a high NaBr concentration (≥100 mM), the decrease in the intensity of photoluminescence (PL) peaks was observed, which depicted less charge recombination in the presences of NaBr. Second, dissolved and surface hydroxyl radicals (•OH) measured by coumarin and coumarin-3-carboxylic acid, respectively, were both increased gradually during photocatalytic process, but were both totally suppressed with the addition of NaBr, elucidating that bromide ion could react with •OH. Third, after adding different oxidants scavengers, methanol (•OH scavenger), allyl alcohol (AA) (surface-bound oxidants scavenger) and tert-butyl alcohol (t-BuOH) (dissolved oxidants scavenger), the photodegradation of SMZ was inhibited in absence of NaBr; however, SMZ could be removed after NaBr addition, implying the existence of RHS that was generated more on the surface of CTiO2 or ZnO because the inhibition of degradation by AA was much more significant than t-BuOH. Besides, it was further confirmed that the creation of these RHS should be more relative to photogenerated hole than •OH because the degradation rate of SMZ significantly reduced in the presence of Na2C2O4, hole scavenger, whether there was NaBr or not. Furthermore, the results of N,N-diethyl-p-phenylenediamine tests depicted RHS generated in both UV/ CTiO2 and UV/ZnO systems with NaBr. By the cyclohexene test, the production of bromide on the surface of CTiO2 and ZnO with NaBr was confirmed by dibromocyclohexane. Moreover, RHS generation was influenced by the amount of •OH, hence RHS yields decreased in the presence of methanol. RHS can react with unsaturated bonds and electron-rich moieties such as aromatic rings to form halogenated products. There were two halogenated byproducts, mono- and di-brominated derivatives of SMZ in the photodegradation process of SMZ by CTiO2 and ZnO in the presence of NaBr, confirmed by HPLC-MS.