利用本土異化金屬還原菌Shewanella sp. KR12降解四種除草劑之研究

Abstract

Shewanella菌屬為異化金屬還原菌中具兼性厭氧特性之一大類，可利用包括含氧鹽類及多種金屬離子等物質作為厭氧呼吸最終電子接受者，特定菌株甚至可以還原金屬形成胞外生物性金屬奈米顆粒，另在厭氧條件下偶氮染劑的脫色或含氯有機汙染物脫氯亦有優良表現，於生物整治領域極具發展潛力與商業價值。其中，結合生物機能開發低耗能、低成本、高催化活性之生物性金屬奈米顆粒，正是目前環境奈米科技產業逐漸走向綠色產業的研究重點之一。 本研究使用本土異化金屬還原菌Shewanella sp. KR12在厭氧條件下，提供甲酸鈉作為電子提供者，利用菌株的硫還原能力與鋅離子反應，合成生物性硫化鋅奈米顆粒，並透過高解析度穿透式電子顯微鏡 (High Resolution-Transmission Electron Microscopy)、能量散射光譜儀 (Energy Dispersive Spectrometer)、冷場發射掃描式電子顯微鏡 (Cold Field Emission Scanning Electron Microscopy)、X光粉末繞射儀 (X-Ray Powder Diffractometer) 等分析奈米顆粒之型態與結構；再以近年台灣河川水體檢出頻度最高的四種除草劑草殺淨 (ametryn)、達有龍 (diuron)、樂滅草 (oxadiazon) 及殺丹 (thiobencarb) 為目標降解物，分別評估Shewanella sp. KR12與生物性硫化鋅奈米顆粒應用於厭氧條件下生物復育或水汙染光催化處理之潛能。 試驗結果顯示，Shewanella sp. KR12可能由降解半胱胺酸產生硫化氫與鋅離子發生沉澱，形成平均直徑約10 nm球形硫化鋅奈米顆粒分布於菌體表面、周質空間和液體培養基中，且無絮聚 (aggregation) 之情形；其最強光吸收峰出現在波長290~300 nm，於水體中達有龍與樂滅草降解展現良好光催化效果。另由生物降解試驗，推測Shewanella sp. KR12可在厭氧條件下以樂滅草和殺丹作為碳源，進而將兩者降解。故本研究結果提供一可同時結合重金屬移除、生物性奈米顆粒合成、水污染光催化處理技術之環境復育潛力材料。

The genus Shewanella is a group of metal reducing facultative anaerobes which are noted for their outstanding ability to utilize a wide range of materials including various metal ions as electron acceptors in anaerobic respiration processes, which allow them to couple the degradation of organic pollutants to the removal of heavy metals in environment. Hence, Shewanella bacteria are considered rising stars in the bioremediation field. In particular, several strains have been found to form metallic bio-nanoparticles extracellularly by bioreduction of metal ions. Recently, the concept of “green chemistry” is becoming more significant to environmental nanotechnology development. In the present study, a local dissimilatory metal reducing bacterium Shewanella sp. KR12 was used to biosynthesis zinc sulfide (ZnS) nanoparticles, whereby the immobilization of zinc ions from zinc sulfate solution by bacterial sulfur reduction. Characterization of nanoparticles were analyzed using transmission electron microscopy (TEM), energy dispersive spectrometer (EDS), cold field emission scanning electron microscopy (FESEM), X-ray powfer diffractometer (XRD) and ultraviolet-visible spectroscopy (UV-Vis). Furthermore, photocatalytic properties of ZnS nanoparticles and organic compounds biodegrading ability of Shewanella sp. KR12 were both investigated for application as remediation materials for degradation of four most frequently detected herbicides in river water in Taiwan, such as ametryn, diuron, oxadiazon and thiobencarb. Our findings revealed that the hydrogen sulfide (H2S) possibly produced through cysteine degradation by Shewanella sp. KR12 then dissociated into S2- to form precipitates with Zn2+ as extracellular ZnS nanoparticles. The nanoparticles were observed to have well dispersion in bacterial surface, periplasm and liquid medium, with a spherical shape and an average diameter of 10 nm. UV-Vis spectrum indicated ZnS bio-nanoparticles had a strong absorption between 290 and 300 nm. Further studies demonstrated biogenic ZnS nanoparticles formed by Shewanella sp. KR12 showed good catalytic performance toward the degradation of diuron and oxadiazon in aqueous solution under UV irradiation. In addition, strain KR12 exhibits degrading ability toward oxadiazon and thiobencarb in direct biodegradation test of herbicides under anaerobic condition. Conclusively, this approach of coupling biosynthesis of nanoparticles with heavy metal removal may offer a potential material for photocatalytic water treatment.