四環素(tetracycline)為一種常見的聚酮類廣譜抗生素，用於治療人類疾病與添加在動物飼料中促進生長，四環素具有較低的Kow值(辛醇-水分配係數)和高溶解度，因此傾向存在於水相之中，由於其廣效的抗菌能力且成本低廉，在醫療以及畜牧業的大量使用已使得四環素能在環境水體中被偵測到，若四環素長期地暴露在環境中，將可能對生態造成威脅或產生生物抗藥性等問題。先前有文獻指出，四環素能夠被水中溶氧經由螯合的Mn(II)或Cu(II)離子傳遞電子而氧化降解；天然有機質(NOM)為環境中普遍存在的物質，其結構上的官能基容易與金屬產生配位或螯合，因此，我們懷疑環境中溶解性有機質之存在很可能會影響四環素與金屬離子之結合並改變降解效果。本研究選擇數種有機質模型化合物(pyromellitic acid、alginic acid、Aldrich humic acid)、國際腐植質學會(IHSS)有機質和經採樣取得之環境有機質(鹽水溪河川水、鹽水溪底泥孔隙水、沙崙農場畜牧廢水)，探討這些有機物如何影響Mn(II)與Cu(II)催化降解四環素。
結果顯示，有機質在Mn與Cu之系統中都能略微抑制四環素的降解，由於有機質能與金屬螯合，降低了四環素受到金屬氧化降解的機會，且隨著有機質分子越大、疏水性越高與濃度越高，其抑制情形就越明顯 (抑制能力由高至低：humic acid > alginic acid > pyromellitic acid)；競爭離子影響實驗中發現，在Mn的反應系統下，四環素之降解會明顯受到鈣、鎂離子抑制，而在Cu系統下則無明顯抑制效果，此是由於Mn與Cu在四環素結構上之螯合點位不同所致；此外，因為環境水樣中含有鹼度，會使Cu系統中的反應pH值上升，進而加速四環素之轉化速率；研究結果也顯示，溫度上升亦能夠增快四環素之降解(40℃ > 25℃ > 15℃)。整體而言，pH值、競爭離子與溫度對於降解四環素中扮演著重要角色，相較之下，溶解性有機質之影響則較不顯著。

Tetracycline is a widely used polyketide broad-spectrum antibiotic on human and livestock to prevent diseases and added in animal feed to promote animals’ growth rate. Due to its wide antibacterial spectrum and low cost, the large consumption of tetracycline increases its release into the environment. Many literatures have reported that long-term and low-dose tetracycline exposure may cause biological resistance and result in adverse effect to the ecosystem. Previous study has indicated that tetracycline could be oxidized under aerobic condition when mediated by Mn(II) or Cu(II). Natural organic matters (NOM) are ubiquitous substances with complicated chemical structures in the environment. They possess various functional groups which can coordinate or chelate with metal ions. Therefore, it was suspected that the presence of organic matters may influence the degradation of tetracycline by metal ions. This study selected three model organic compounds (pyromellitic acid, alginic acid and Aldrich humic acid), standard organic matters from International Humic Substance Society (IHSS), and local organic matters collected from surface water and a pig farm to illustrate their impacts on tetracycline oxidation mediated by Mn(II) and Cu(II).
The results show that organic matters slightly inhibited the degradation of tetracycline because organic matters can chelate with metal ions and reduce the tetracycline-metal complex formation. Organic matters with larger molecular size, higher hydrophobicity and concentration caused stronger inhibition to the reactions (suppression from high to low: humic acid > alginic acid > pyromellitic acid). The presence of competitive ions (e.g. Ca(II) and Mg(II)) significantly decreased the reaction rate in the Mn(II) system, while they caused minor effect on the Cu(II) system, which is because the coordinated site of Cu(II) on tetracycline is different from that of Mn(II), Ca(II) and Mg(II). The pH value of reactions in the Cu(II) system conducted in local water samples increased due to the alkalinity and subsequently promoted the transformation of tetracycline. Temperature was also found as a factor which significantly enhanced the reactivity (40℃ > 25℃ > 15℃). To sum up, the experimental results suggested that organic matters could actually suppress tetracycline degradation by Mn(II) and Cu(II) but in a minor way. Other environmental factors including pH, competitive ions and temperature, played more important roles in the reactions.

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