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標題: | 紫外光誘導自由基連鎖反應還原水中硝酸鹽為氮氣 UV-light Induced Radical Chain Reaction to Reduce Nitrate to Nitrogen in Water |
作者: | 陳薇羽 Wei-Yu Chen |
指導教授: | 劉雅瑄 Sofia Ya-Hsuan Liou |
關鍵字: | 硝酸鹽還原,甲酸,紫外光,氫氧自由基,二氧化碳陰離子自由基, Nitrate Reduction,Formic acid,UV irradiation,Hydroxyl radical,Carbon Dioxide Radical Anion, |
出版年 : | 2022 |
學位: | 碩士 |
摘要: | 在現今可利用的淡水資源中以地下水的儲量與可利用程度最高,近年來更因區域降水的分布狀況受大氣熱平衡擾動的影響,突顯水資源循環和維護的重要性,並使得其開發利用與分配狀況備受重視。然而,淺層地下水受人為活動高度影響,導致地下水資源污染問題屢見不鮮。尤其隨著人口增長、精緻農業和畜牧業蓬勃發展,大量氮源污染物進入自然水體中,使硝酸鹽污染成為最大宗的問題之一。又由於硝酸鹽經人體代謝後會轉變為更具毒性的亞硝酸鹽,故尋求妥善處理飲用水與民生用水中的硝酸鹽污染為一不可忽視的挑戰。為此,本研究藉水中硝酸鹽在紫外光照射下可自然光解為亞硝酸鹽的傾向,以不同紫外光波長控制其光解路徑,達成硝酸鹽氮的氧化數初步降低。與此同時,研究善用光解過程中產生之大量強氧化力活性物種—氫氧自由基,透過添加有機小分子甲酸作為犧牲試劑,使兩者經自由基反應鏈形成新的強還原力活性物種—二氧化碳陰離子自由基,又透過控制添加試劑的反應時間區段與試劑濃度,使二氧化碳陰離子自由基(Eo(CO2/CO2•−) = −1.9 V)能適當地還原亞硝酸鹽為最終含氮產物,並進一步有效提升最終產物之氮氣選擇性。研究結果顯示,光波長254 nm、反應起始0 min添加甲酸與甲酸水溶液濃度15 mM之系統有最佳效果,硝酸鹽可在3至4小時間完全去除,過程中其會先轉變為亞硝酸鹽,後生成一氧化氮等含氮氣相中間產物,最終進一步則被還原為一氧化二氮與氮氣,且在反應中期與末期分別有大量二氧化碳與氫氣伴生。此外,結果亦證實系統內各活性物種生成來源且反應各階段可能存在不同活性物種主導反應發生:活性氫原子啟動硝酸鹽降解至亞硝酸鹽的反應階段,而二氧化碳陰離子自由基則接續作用於亞硝酸鹽上,使之進一步還原為含氮氣相產物,且其最大累積濃度與水中氫離子濃度皆與最終產物之氮氣選擇性高度相關。 Among freshwater resources, groundwater has the highest reserves and availability. In recent years, the regional precipitation distribution has been affected by the disturbance of atmospheric heat balance, highlighting the importance of water resources circulation and maintenance, and the development and utilization are highly valued. However, shallow groundwater is greatly affected by human activities, resulting in frequent pollution of groundwater resources. Especially with the growth of population and the vigorous development of agriculture and animal husbandry, a large amount of nitrogen source pollutants are discharged into natural water bodies, making nitrate pollution one of the biggest problems.Therefore, in this study, the trend of natural photolysis of nitrate to nitrite in water under UV light irradiation was used, and the photolysis path was controlled by different UV wavelengths to achieve a preliminary reduction of the oxidation number of nitrate nitrogen. At the same time, the study made good use of a large number of strong oxidative active species— hydroxyl radicals generated during the photolysis process. By adding organic small molecule formic acid as a sacrificial reagent, the two formed a new strong reductive active species— carbon dioxide anion radicals through the free radical reaction chain. In addition, by controlling the reaction time period and the concentration of added reagents, the carbon dioxide anion radical (Eo(CO2/CO2•−) = −1.9 V) can properly reduce nitrite to the final nitrogen-containing product, and further effectively improve "nitrogen selectivity" of the final product. The results show that [254 nm, 0 min, and 15 mM] (light wavelength, formic acid addition time point and formic acid solution concentration) is the optimal system, and nitrate can be completely degraded within 3 to 4 hours. During this process, it is converted to nitrite, then to nitrogen intermediates (gas phase) such as nitric oxide, and finally reduced to nitrous oxide and nitrogen. Moreover, a large amount of carbon dioxide and hydrogen production was associated with the reaction in the middle and end stages of the reaction, respectively. As for the mechanism and the nitrogen selectivity of final products, the results also confirmed the source of each active species in the system and different active species may dominate the reaction at each stage of the reaction:Active hydrogen atoms initiate the reaction stage of nitrate degradation to nitrite, and carbon dioxide anion radicals continue to act on nitrite, which is further reduced to nitrogen-containing phase products, and the maximum cumulative concentration of nitrite and hydrogen ion concentration in water was highly correlated with the nitrogen selectivity of the final product. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/87618 |
DOI: | 10.6342/NTU202210075 |
全文授權: | 未授權 |
顯示於系所單位: | 地質科學系 |
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