環丙沙星抗生素於高嶺土之光降解機制

Abstract

環丙沙星(ciprofloxacin)為奎諾酮類抗生素之一，其廣泛應用於人類與動物等細菌感染之醫療，因而易由醫院廢水及生活污水等排放管道進入表面水體中。自然光解為環丙沙星於環境之主要降解機制之一，其於自然水體中之半衰期為1~3分鐘，然其與固相介質(如礦物、有機物質、氧化鐵等)有極佳親和性，容易積存於底泥或土壤等環境中，若環丙沙星進入固相介質之環境，其自然光解之機制可能受到介質中物質之作用而與水相光解有所不同，故本研究以高嶺土為固相介質，進行環丙沙星於固相環境之光化學反應研究並探討其與水相光解之差異。 本研究建立超音波萃取方法結合吹氮濃縮之技術，分析高嶺土中之環丙沙星偵測極限可至10 μg g-1，回收率為96.6%~113.4%。而研究發現環丙沙星與高嶺土間的作用，可能使環丙沙星的光化學特性有所不同；其於高嶺土之莫耳吸收常數以Kubelka-Munk公式推算為5.8×106 mole-1 L cm-1，遠高於其於水中之莫耳吸收常數(3×104 mole-1 L cm-1)。 研究發現環丙沙星存於高嶺土表面時較為穩定；其於不同高嶺土厚度之光降解試驗，以擬一階反應計算其於第一小時內的光反應速率。當高嶺土厚度(Z)由14 μm增厚至199 μm，環丙沙星之光反應速率由0.0154 min-1降至0.0016 min-1；kp×Z於厚度範圍14 μm至199 μm為一定值(0.30 μm×min-1)，表面光降解反應速率(kp0)藉由Balmer理論推得為0.034 min-1，其半衰期約為水相環境中之20倍，故環丙沙星於固相環境之潛在危害風險可能因而提高。 研究亦發現環丙沙星與高嶺土的交互作用抑制環丙沙星之直接光解機制，而非促進間接光解的反應，故其降解途徑相似於水相直接光解之降解途徑；其中哌嗪環(piperazine ring)之去除為環丙沙星於高嶺土之主要光降解途徑，其副產物(P2, MW=262)以擬定量法(semi-quantification)定量，產率於90分鐘光照後高達38.6%；其他降解途徑分別為哌嗪環之氧化(P3, MW=290)、丙環基之破壞(諾佛沙星以及P4, MW=293)；而諾佛沙星(norfloxacin)之產率於水中及高嶺土中分別為17.6%以及0.95%。水相中之脫氟降解途徑則因高嶺土中缺乏水分未於固相光解中發現相關副產物(P5, MW=329)。 實際環境中底泥與土壤之質地組成區域變異性大且均質性不一，抗生素於其中的宿命亦將更為複雜。本研究已針對抗生素於固相介質之光降解宿命進行初探，其他環境因子對固相光解之影響宜進一步深入研究探討。

Ciprofloxacin is a fluoroquinolone antibiotic that is widely used in the treatment of serious bacterial infections, and it is often released into surface water through hospital wastewater and wastewater treatment plant effluent. The literature has demonstrated its photodegradation potential in natural surface water, but there is limited information about its photochemical behavior in the sorbed state. Ciprofloxacin interacts strongly with solid matrices (e.g., clay minerals, humic substances, and hematite) with potentially different photolysis mechanisms. Therefore, the objective of this study is to investigate the photochemistry of ciprofloxacin in a solid-phase system compared to a water-phase system. Kaolinite was used as the model matrix in this study, and methods were established for the pretreatment, extraction, purification and analysis of ciprofloxacin in kaolinite at the μg g-1 level. The recovery of the extraction method ranged from 96.6% to 113.4%, and the limit of detection was 10 μg g-1. The interaction between ciprofloxacin and kaolinite surface might affect photochemical property of ciprofloxacin. The molar extinction coefficient of ciprofloxacin in kaolinite was 5.8×106 mole-1 L cm-1 at 330 nm based on the Kubelka-Munk function, which was greater than in the water phase (maximum value of 3×104 mole-1 L cm-1 at 271 nm at pH 7). Moreover, ciprofloxacin is more persistent when it is adsorbed onto kaolinite under irradiation. Photolysis of ciprofloxacin in kaolinite with various depths were conducted and photolysis rate constants (kp) were determined by fitting pseudo-first order in the first hour. kp dramatically decreased from 0.0154 min-1 to 0.0016 min-1 as the layer thickness (Z) increased from 14 μm to 199 μm. kp×Z was constant (0.30 μm×min-1), and kp0 (pseudo-first order constant at the surface of the kaolinite layer, where Z=0 μm) was 0.034 min-1 according to Balmer’s model. The half-life of ciprofloxacin in kaolinite was 20 times longer than in the water phase under simulated sunlight, thus increasing the potential environmental risk. Furthermore, the interaction between ciprofloxacin and kaolinite did not result in other photosensitization reactions, and it reduced the direct photolysis; therefore, similar photodegradation pathways were identified from direct photolysis of water phase. The amounts of byproducts were semi-quantified by the calibration curve of standard ciprofloxacin and cleavage of the piperazine ring is the main degradation pathway of ciprofloxacin in kaolinite (38.6% yield of P2, MW=262); other initial degradation pathways include oxidation of the piperazine ring (P3, MW=290) and cleavage of the cyclopropyl group (norfloxacin and P4, MW=293). Norfloxacin was produced with a 17.6% yield in the water phase and a 0.95% yield in kaolinite. Defluorination caused by the addition of OH- resulted in P5 (MW=329) and was only observed in the water phase, likely due to the absence of water in kaolinite. This study indicated that ciprofloxacin is more environmentally persistent in kaolinite than in water. However, the fate of antibiotics in natural sediments and soils is more complicated since the matrix is highly heterogeneous; our knowledge is far from complete in this field. Future work should focus on additional factors, such as humic substances and metal ions, in the solid phase to investigate the photochemical reaction of ciprofloxacin in sediment and in the water/solid phase.