環丙沙星抗生素於高嶺土懸浮水體中之光降解機制

Abstract

環丙沙星 (Ciprofloxacin) 為奎諾酮類抗生素之一，此類抗生素被廣泛運用於人類以及動物之細菌性疾病治療，因此普遍在環境水體中檢出微量濃度。在自然水體中，光解為環丙沙星主要的降解途徑；然而，環境中之固體基質(例如：懸浮固體物與泥土)容易吸附環丙沙星，進而影響環丙沙星在自然水體中的光降解機制以及其在環境中的宿命。故本研究以高嶺土為實驗中之懸浮固體基質，進行環丙沙星於高嶺土懸浮水體中之光化學反應機制探討，並釐清其中各種反應機制進而推導出適當之模擬方程式以建立環丙沙星於高嶺土懸浮水體中之光降解模型。 本研究發現環丙沙星在高嶺土存在之水體中，其吸脫附平衡在30分鐘內即達到穩定，並且有 94-96% 之環丙沙星被吸附於高嶺土上；當光解機制參與作用時，環丙沙星在高嶺土懸浮液中相比在純水中更具持久性。在懸浮液中之光解速率遠低於在純水中之光解速率，並且其速率常數(ktotal)隨著高嶺土濃度(KAO)增加而降低(於 KAO = 2.5, 5.0 and 10.0 g L-1分別為ktotal = 0.014, 0.009 and 0.005 min-1 )；在高嶺土懸浮液水相中，環丙沙星的濃度降低主要受懸浮液水相之光解速率(kW)以及脫附機制作用影響，另一方面在懸浮液固相上環丙沙星的濃度降低則是主要因脫附機制造成。光解模擬研究顯示推估之模擬模式對於環丙沙星之懸浮水體光解有極佳之模擬效果(平均相對誤差：± 5-20%)，並指出吸附比例為關鍵的影響因素，其中分配係數(K(Ctotal))與高嶺土濃度(KAO)為此比例之重要影響參數；在此模型中，假設 K(Ctotal) 在光照的過程中維持恆定，不論是否有高嶺土存在kW皆保持為定值，且懸浮液固相之光解速率(kS)極小而可忽略。 相對地，在不同水體基質中高嶺土懸浮液之光解研究顯示，在景美溪水以及模擬河水中之環丙沙星(Ctotal)光降解較快，而較之於純水之光降解速率仍然較緩慢；其結果顯示K(Ctotal) 在不同水體基質之高嶺土懸浮液光解機制中仍為一重要影響因素；然而在模擬結果與實驗結果間之部分差異顯示，除了 K(Ctotal) 以外，其他水質參數如 pH 值和陽離子的存在可能會對吸脫附機制造成影響。此外。自然環境中的光敏性物質如硝酸根、碳酸根以及溶解性有機物可能利於懸浮水體中固相之光解，促使環丙沙星在自然水體中之光降解量略為提升。整體而言，本研究顯示當環境中高濃度固體懸物存在時，環丙沙星光降解半衰期延長將可能提高其環境風險故需更進一步調查與研究。

Ciprofloxacin is a type of fluoroquinolone antibiotic that is widely used to treat the bacterial diseases of humans and has been frequently found in aquatic environments. The photolysis of ciprofloxacin is found to be one of the main natural attenuation process in the environment. However, it is also likely to be adsorbed onto solid matrices (e.g., suspended solids and soil), which may lead to the different photolysis mechanisms of ciprofloxacin in natural waters and its different fates in the environment. Therefore, the objectives of this study are to study the photochemical behavior of ciprofloxacin in a kaolinite suspension system and to clarify its mechanisms in this system to derive a suitable model simulating photodegradation in a suspension system. In the presence of kaolinite, the sorption of ciprofloxacin quickly reached equilibrium within 30 min, and 94-96% of ciprofloxacin was adsorbed on kaolinite. When photolysis was involved, the ciprofloxacin in the kaolinite suspension system was more persistent under irradiation. The photodegradation rate of the total ciprofloxacin in kaolinite suspension (ktotal) was much slower than that in DI water, and ktotal decreased as the kaolinite concentration increased (ktotal = 0.014, 0.009 and 0.005 min-1 for KAO = 2.5, 5.0 and 10.0 g L-1, respectively). The disappearance of ciprofloxacin in liquid within the suspension was dominated by both photolysis (kW) and desorption processes. On the other hand, the disappearance of ciprofloxacin on the kaolinite within the suspension was controlled by the desorption process alone. A simulation fitted the results well (relative error: ± 5-20%), and the ratio between ciprofloxacin adsorbed on the kaolinite and in the water was the most important factor. The ratio depends on the partition coefficient (K(Ctotal)) and concentration of kaolinite (KAO). In the model, it was assumed that K(Ctotal) remains constant during irradiation, kW remains the same with or without kaolinite suspension, and photolysis on kaolinite within the suspension (kS) was ignored. In different aqueous matrices, the photodegradations of ciprofloxacin (Ctotal) in kaolinite suspension within synthetic river water and Jingmei River water were faster than that within DI water, but they were still slower than that in an aqueous system without suspension. K(Ctotal) was also considered an important factor affecting photolysis in suspension within different aqueous matrices. However, the difference between experimental data and simulation results indicates that in addition to the partition coefficient, factors such as the pH value and the presence of cations could affect the sorption equilibrium, and the presence of photosensitizers such as nitrate, bicarbonate and DOM may accelerate the photolysis on kaolinite within suspension. The results of this work indicated that previous works on photolysis in pure water phase may have underestimated the environmental risk of ciprofloxacin, as high concentrations of suspended solids may cause ciprofloxacin to have a longer half-life in the environment.