探討鹵素離子對二氧化鈦奈米顆粒聚集和紫外光照下光催化降解磺胺甲噁唑之效應

Abstract

磺胺甲噁唑(Sulfamethoxazole，SMZ)是一種於人類及動物醫療常用的抗生素。SMZ流佈於水體中造成污染，是目前最需進行詳細風險評估及發展處理方法的污染物之一。光催化降解被視為最有效快速移除SMZ的方法之一，其中二氧化鈦(titanium dioxide, TiO2)奈米顆粒為典型且具有發展潛力的光催化奈米材料．其具有很好的穩定性，但在含有鹽類的環境下，TiO2奈米顆粒易發生聚集與沉降現象，進而減少比表面積與表面活性位置，可能會降低其光催化效率。然而本研究發現當系統中含有NaBr時，會使P25 (一種商用TiO2)奈米顆粒聚集，但其光降解SMZ之速率不但沒有降低，甚至隨著鹽類濃度上升而升高。紫外光照2小時後，P25可以降解75%的SMZ，而當系統中有100 mM NaBr時，光照45分鐘即可降解95%的SMZ，其降解反應速率常數為0.0636 min-1，比P25的速率常數0.0104 min-1高6倍。因此本研究的目的是瞭解TiO2在不同條件下降解SMZ的速率變化，與探討在含有鹵素鹽類的系統中TiO2光催化降解SMZ的機制，並找出可能使之促進反應的原因。從以草酸根離子作為光生電洞的捕捉劑實驗中可發現，P25 + 100 mM NaBr的組別其反應比P25被抑制地更多，顯示NaBr促進反應的原因跟光生電洞有關。以coumarin和coumarin-3-carboxylic acid (CCA) 分別測定溶液中和近TiO2表面的•OH，在P25的組別皆可測到此兩種•OH ，但系統中含有NaBr時，則無法測到•OH，表示Br-也會與•OH 發生反應。N,N-diethyl-p-phenyldiamine (DPD) 實驗結果證實了P25 + NaBr經紫外光照的系統中產生了活性溴物種。另外，系統中可能也有Br2 的產生。由於Br2 會與cyclohexene反應產生dibromocyclohexane，也會與Br-反應產生Br3-，而系統中可以測得以上兩種產物，證實反應過程中有產生Br2。這很可能是因為在有NaBr的情況下，Br-與光生電洞和•OH反應產生•Br，進而產生其他活性溴物種，包含Br2，來促進TiO2降解SMZ，且此反應可能發生在TiO2表面。本研究中有製備一種材料將Br修飾在P25表面，而此材料對於SMZ的光催化降解也有促進的效應，並且以DPD法也可以測得活性溴物種，顯示促進反應的關鍵在表面。在以allyl alcohol (AA) 作為表面活性氧物種(•OH、•Br等)的捕捉者，tertiary butanol (t-buOH)為溶液中活性氧物種捕捉者的實驗中，P25的光催化反應皆會被抑制，且抑制程度相近，但在系統中含有NaBr時，加入AA的反應會有非常明顯的抑制情形，而加入t-buOH則只會稍微抑制反應，證明表面活性氧物種作用的重要性。最後以HPLC-MS分析副產物，發現系統中會存在SMZ的溴化產物，分別是在苯環或胺基上接上一個Br，以及在苯環位置接上2個溴，間接證實了系統中有產生活性溴物種去攻擊SMZ。

Since the widespread detection of synthetic antibiotics in aquatic environments is raising public health concerns, there is growing interest in the development of technologies to efficiently remove these antibiotics. Photocatalysis is a green technology to treat the wastewater with antibiotics such as sulfamethoxazole (SMZ), one of the broad-spectrum antibiotics. Titanium dioxide (TiO2) is the most well-known and promising photocatalyst. However, in natural water systems, TiO2 may be subject to the deactivation by the presence of inorganic ions, including halide anions. The presence of halide anions can have both positive and negative influence on the photocatalytic performance. This study aims to investigate the photocatalytic degradation mechanism of SMZ in aqueous suspensions of P25, one commercial TiO2, nanoparticles (NPs) with the addition of halide salts. When there were halide salts in the solution, P25 aggregated and became larger particle agglomerates, resulting in the decrease of surface area and active site on the surface, which reduced the photocatalytic performance of TiO2. Therefore, the addition of NaCl slowed down the photodegradation rate. However, the photodegradation rate of SMZ by TiO2 with the addition of NaBr increased instead of decreasing. With the addition of 100 mM NaBr, the photodegradation efficiency was 95% after 45 min illumination, and reached equilibrium of 99% removal efficiency at 75 min. The photodegradation rate constant of P25 was 0.0104 min-1, increasing to 0.0636 min-1 with the addition of 100 mM NaBr. Since the increase of degradation kinetics enlarged with the increase of NaBr concentration, the enhancement of SMZ degradation could be related to the presence of bromide ions. The degradation of SMZ by P25 was significantly inhibited by allyl alcohol (AA) and tertiary butanol (t-buOH), two oxidant scavengers. The photogenerated holes on the surface of TiO2 were consumed by C2O42-, resulting in the inhibition of SMZ degradation. Under UV irradiation, gradual increases in the fluorescence of TiO2 with coumarin and coumarin-3-carboxylic acid (CCA) were observed with the UV irradiation time, indicating that the dissolved and surface •OH increased. However, with the addition of NaBr, the generation of both surface and dissolved •OH was totally inhibited. Furthermore, the results of N,N-diethyl-p-phenyldiamine (DPD) test depicted that there were reactive bromine species generated in the system of P25 with NaBr. By the cyclohexene test, the production of bromine on the surface of P25 with NaBr was confirmed by dibromocyclohexane. Some Br- even reacted with bromine to form Br3-. Under UV light irradiation, Br- was proposed to react with the photogenerated holes of TiO2 or •OH to produce •Br and reactive bromine species, accelerating the photodegradation of SMZ. Compared to P25, surface brominated P25 (HBr-P25) had an enhanced photocatalytic performance. Reactive bromine species generated on HBr-P25 were also detected by DPD method and the cyclohexene test. On the other hand, in the system of P25 with 100 mM NaBr, the degradation rate of SMZ was significantly decreased by AA (surface-bound oxidants scavenger), but was slightly decreased by t-buOH (dissolved oxidants scavenger), thus confirming the importance of surface-bound oxidants. This enhanced reaction by bromide ions occurred on the surface of TiO2. Reactive bromine species can react with unsaturated bonds and electron-rich moieties such as aromatic rings to form halogenated products. HPLC-MS results showed that there were two halogenated byproducts, mono and di-brominated derivatives of SMZ, in the photodegradation process of SMZ by P25 in the presence of NaBr and by HBr-P25.