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標題: 光誘導不同分子大小及化學組成之腐植酸還原Cr(VI)
The photo-induced reduction of Cr(VI) as influenced by humic acids with various molecular weights and chemical compositions
作者: 陳思穎
Chen, Ssu-Ying
關鍵字: humic acid;腐植酸;molecular weight;chromium;分子量;六價鉻
出版社: 土壤環境科學系所
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在本研究中,利用超濾膜的技術,將來自於台灣北部陽明山山區所萃取出的腐植酸,分級成四種不同分子大小的物質(>100 kD、50~100 kD、10~50 kD和<10 kD),並且將分級後之各分子大小腐植酸,利用元素分析及傅立葉紅外線光譜測定其特性,並用以解釋在UV光照與否,對於不同分級大小之腐植酸移除和吸附無機污染物Cr(VI)之影響。元素分析結果顯示,碳、氫和氮均隨著腐植酸分子量變小而減少,而氧含量則呈現相反之趨勢。另外,以13C-NMR分析含碳種類,結果顯示,芳香性碳含量與羧基官能基均隨著分子量的減少而增加,而脂肪性碳則隨分子量的增大而增加。
在避光系統中,不同分級之腐植酸移除Cr(VI)之速率極緩慢,且隨著溶液中pH值的提升,Cr(VI)移除量減少;而在光照系統中,由於UV光激發腐植酸上之官能基,使其成不穩定態,因而釋放出電子,以增進Cr(VI)之還原。實驗結果顯示在本研究之實驗條件下,均以HA(<10 kD)之反應速率最快,主要可能為HA(<10 kD)具有較高的芳香族官能基及羧基含量,且有較多的含氧官能基。此結論是基於光譜分析的結果,由光譜分析腐植酸分子與Cr(VI)反應前後結構上之改變,發現經Cr(VI)反應後,腐植酸結構中的羧基、含氧官能基以及芳香性碳含量均明顯減少,由於分子越小之腐植酸具有較多之芳香性碳含量、羧基和含氧官能基,因此與Cr(VI)反應後,能較其他大分子還原較多Cr(VI)。

Humic substances (HS) strongly affect the fate of trace metals in soils and aquatic environments. Because HS is a huge organic molecules with molecular weights ranging from several hundred to several hundred thousands Daltons, the elucidation of which components of HS possess photo-induced properties responsible for photo-reduction of toxic Cr(VI) to Cr(III) is less possible. Therefore, in this study, an ultrafiltration technique was used to fractionate HS, originated from a Taiwan peat soil, into four fractions of different nominal molecular weight (>100 kD, 50~100 kD, 10~50 kD and <10 kD). The specific domains or functional groups of HA involving in Cr(VI) adsorption/reduction were then systemically evaluated. Each fraction of HS was first characterized by elemental analysis and spectroscopic techniques followed by examining the sorption and transformation of Cr(VI) on each fraction of HS under illumination. The results of elemental analysis indicated that the contents of C, H, and N decreased with a decrease in molecular weight of HAs, but the content of O exhibited an opposite tendency. 13C -NMR studies showed that the aromatic C and carboxylic C increased with decreasing the molecular weight of HAs; however, aliphatic C decreased with a smaller molecular weight of HAs.
In a dark system, Cr(VI) removal by each fraction of HAs proceeded slowly, particularly at a higher pH. The application of UV light promoted Cr(VI) reduction probably due to the enhancement of electron-transfer between reactants upon the absorption of light energy by specific functional groups on HAs. The rate of Cr(VI) reduction by HAs with a molecular weight of <10kD exhibited the highest which may be attributed to the presence of high contents of aromatic C, carboxylic groups and oxygen containing groups in this fraction of HAs. This conclusion is based on the changes in the structures of HAs before and after reacting with Cr(VI). The results of spectroscopic analysis of HAs found that Cr(VI) treatment would lead to a significant decrease in carboxyl groups, oxygen containing groups and aromatic C in HAs. Because these specific functional groups are concentrated in HAs with a molecular weight of <10kD, the rapid reduction of Cr(VI) by this fraction of HA is expected.
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