利用表面改質奈米零價鐵微粒處理受鉻/硝酸鹽/亞硝酸鹽污染水體之研究

DecDecontamination of Chromium/Nitrate/Nitrite Species by Surface-Modified Zero-Valent Iron Nanoparticles

莊添登(Tien-Teng Chuang)

指導教授 ： 林錕松

元智大學/工程學院/化學工程與材料科學學系/碩士(2005年)

摘要

近年來，國內受高污染硝酸鹽/亞硝酸鹽類之養殖業及重金屬鉻污染之水體十分嚴重，造成人體危害及環境之衝擊。因此，本研究之主要目的是探討利用化學還原法製備奈米零價鐵微粒，藉由精密光譜分析儀器檢測奈米零價鐵微粒吸附含氮鹽類及含鉻溶液後之變化與結構分析，並利用聚乙二醇(PEG)保護膜來增加零價鐵之還原污染物之效率。
實驗中，由穿透式電子顯微鏡(TEM)分析可明顯發現，在奈米零價鐵微粒表面上有5~10 nm之PEG高分子膜。經由同步輻射XANES分析奈米零價鐵粉對於處理硝酸鹽/亞硝酸鹽是將其還原為氮氣，零價鐵經氧化反應由零價轉為三價；奈米零價鐵粉對於六價鉻是進行還原反應將其還原成三價鉻；表示表面吸附及氧化還原反應，只在奈米零價鐵粉表面上同時進行。吸附動力結果顯示奈米零價鐵對於六價鉻是為擬一階反應(pseudo-first-order)，處理污染水體濃度越低，則吸附方程式越趨近線性，而奈米零價鐵對於處理硝酸鹽/亞硝酸鹽則以二階反應使其線性迴歸。奈米零價鐵還原硝酸鹽及亞硝酸鹽之污染水溶液；硝酸鹽水溶液系統之pH值將會維持在8~9左右；亞硝酸鹽水溶液則維持在8.5~9.5左右。硝酸鹽類降解反應過程中，pH值為其重要之控制因子，低pH值環境下有助於奈米零價鐵粉對硝酸鹽降解速率之提昇。經由陰離子層析儀(IC)數據結果顯示每克零價鐵粉可分別還原100~300 ppm硝酸鹽及150~400 ppm亞硝酸鹽污染水體。由原子吸收光譜(AAS)數據結果顯示零價鐵可以有效地完全去除六價鉻污染水體。掃描式電子顯微鏡(FE-SEM)及X光粉末繞射儀(XRPD)結果顯示吸附鉻之奈米零價鐵粉微粒表面可能為紅褐色氧化鐵，結構成鬆散狀。經由X光吸收近邊緣結構(XANES)及延伸X光吸收精細結構(EXAFS)光譜顯示，吸附六價鉻之奈米零價鐵粉微粒上之鉻還原成Cr2O3；相反地，還原硝酸鹽及亞硝酸鹽後之奈米零價鐵粉微粒則氧化成Fe2O3。此外，亦利用共振非彈性X光散射光譜(RIXS)分析，可以更精確瞭解反應前後零價鐵微粒中Fe元素之價數介於2~3間。由於奈米零價鐵粉對於受鉻及含氮鹽類污染物之去除具有高活性、效果佳及且已有初步成效，故未來應用於重金屬污染場址之現址復育（in-situ remediation）技術。本研究亦是典型應用同步輻射光源及原子級分析光譜，研究奈米零價鐵微粒吸附受毒性鉻金屬及硝酸鹽/亞硝酸鹽污染水體，在其表面氧化還原反應之實例。

關鍵字

奈米零價鐵微粒 ； 鉻污染 ； 硝酸鹽/亞硝酸鹽 ； 吸附動力學 ； X光吸收近邊緣結構 ； 延伸X光吸收精細結構

並列摘要

In recent years, chromium, nitrate, and nitrite contaminants are very serious problems in Taiwan. Chromium, nitrate or nitrite species are known to be toxic and carcinogenic, site remediation is necessarily required in order to reduce the risk to human races and ecosystems. Therefore, the main objectives of the present study were to investigate chemical reduction of chromium, nitrate or nitrite species by nanoscale zero-valent iron (ZVI) in aqueous solution and related reaction kinetics or pathways. Nanophase zero-valent iron powders could be enhanced the efficiency of reduction by adding polyethylene glycol (PEG ) nanofilms.
Experimentally, ZVI nanoparticles of this study were prepared by borohydride reduction method at room temperature and ambient pressure. At the surface of ZVI nanoparticles there coated about 5~10 nm film of polyethylene glycol measured by TEM. The existence of Cr species on the Fe(0) nanoparticles was also confirmed by XANES. It was also found that mainly Cr(III) with a small amount of Cr(0) was adsorbed on the Fe(0) nanoparticles. Kinetics analysis from batch studies revealed that the removal of Cr(VI) from aqueous reaction with nanoscale Fe(0) appeared to be a pseudo first-order with respect to contaminant substrates. The kinetic model of nitrate/nitrite reduction by nanoscale Fe(0) powder is proposed as second-order kinetic equation. The results showed that the degradation reaction of nitrate/nitrite was significantly effected by pH values. The faster rates of nitrate/nitrite reduction were shown at lower pH. The Cr-adsorbed Fe(0) nanoparticles measured by FE-SEM and XRD were abnormally incompact, it was possible that Fe(0) nanoparticles were, to some extent, oxidized in the adsorption process. This work exemplifies the utilization of XANES, XRD, and XPS to reveal the speciation and possible reaction pathway in a very complex adsorption and redox reaction process. EXAFS spectra showed Cr(VI) reduce to Cr2O3 and nitrate/nitrite reduce to N2 while oxidizing the Fe(0) to Fe2O3 electrochemically. In addition, by using resonant inelastic X-ray scattering (RIXS) technique, the fine valent of iron between 2 and 3 may be further investigated. It is also very clear that decontamination of chromium, nitrate or nitrite species in groundwater via the in-situ remediation with nanophase Fe(0) permeable reactive barriers would be environmentally attractive in the future.

並列關鍵字

Zero-valent iron nanoparticles ； Chromium contamination ； Nitrate/nitrite species ； Adsorption kinetics ； XANES ； EXAFS

參考文獻

94. 李志甫，X光吸收光譜術在觸媒特性分析上的應用，The Chinese Chem. Soc., 53(3), 280-293(1995).

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