奈米零價銅之合成及其移除六溴環十二烷之研究

Abstract

天然土壤環境在淹水狀態下，因微生物代謝而造成低氧化還原潛勢。在此狀態下，腐植物質如腐植酸也會被還原，且會和一些金屬離子產生交互作用，其中銅離子較容易被還原。為了解還原的環境是否能促使零價銅奈米顆粒 (nanoparticles, NPs)生成，並進一步與持久性有機汙染物反應，新興汙染物六溴環十二烷 (hexabromocyclododecane, HBCD)被選為目標化合物。本篇研究先以硼氫化鈉合成奈米級零價銅 (Cu NPs)，Cu NPs移除HBCD之反應速率隨著Cu NPs劑量升高而增加，當Cu NPs劑量為3.5 g/L時反應速率最高，其質量標準化擬一階速率常數為14.4 min-1g-1。Cu NPs移除HBCD之反應速率隨著初始HBCD濃度升高而下降。Cu NPs移除HBCD之反應速率隨溫度增加而上升，並計算出活化能為32.2 kJ/mole，顯示此反應機制為表面控制。Cu NPs移除HBCD之反應速率隨著初始pH值增加而下降，由於pH 4時Cu NPs表面主要為零價銅，而pH上升至9時，Cu NPs表面部分轉化為氧化銅及氫氧化銅而鈍化。此外，也發現Cu NPs對於HBCD的吸附隨著初始pH值上升而增加。另一方面，Cu NPs等電點為8.1，pH值升高時較易聚集，因此也可能降低了反應速率。 為了解還原性腐植酸和二價銅錯合後銅物種的轉化，以硼氫化鈉還原腐植酸後，再以還原性腐植酸與不同的二價銅劑量作用，孵育不同時間之腐植酸-銅。於0.9 g/L腐植酸及175 mmol/kg二價銅中，以X光近邊緣吸收光譜儀觀察銅物種於孵育12小時後，主要為有機酸-銅形式，傅立葉轉換-紅外光譜儀分析顯示1384及1598 cm-1的波峰，表示腐植酸與銅的主要以羧基-銅的化學鍵錯合。而48小時後，X光近邊緣吸收光譜儀分析主要為零價銅。並以掃描式光電子能譜顯微術觀察到空間上腐植酸中有零價銅的分布。單顆粒感應耦合電漿質譜儀量測腐植酸-銅，銅的顆粒大小約為50 nm。腐植酸-銅和HBCD反應24小時後，移除的六溴環十二烷可達到34.5%，並測得溴離子，為還原脫溴的反應，顯示還原性腐植物質與其所還原之零價銅，可移除六溴環十二烷。

Because of flooding and microbial metabolism, humic acid in some natural reduction environments could be reduced and then interact with metal ions; moreover, the copper ion is easily to be reduced. To understand whether a reduced condition can facilitate the formation of zero valent Cu NPs and then interact with persistent organic pollutant (POP), HBCD, one of POP, is chosen as a target compound. First, Cu NPs were synthesized by sodium borohydride. The removal of HBCD by Cu NPs was increased with the Cu NPs dosage increased but with the initial HBCD concentration decreased. The highest rate constant presented in 3.5 g/L Cu NPs, moreover the mass-normalized rate constant was 14.4 min-1g-1. The removal of HBCD was also increased with the temperature increased. The estimated activation energy was about 32.21 kJ/mol, which is considered as surface controlled reaction. The removal of HBCD was decreased with the initial pH increased. However, with pH increased the passivation of Cu NPs such as copper (hydro)oxide was found and then contributed to the increase of HBCD adsorption efficiency. On the other hand, the pHzpc of Cu NPs is 8.1, the Cu NPs would aggregate with the increased pH and then reduce the rate constant. To understand the transformation of complexes of reduced humic acid and cupric ion, humic acid was chemically reduced by sodium borohydride, and cupric ion was well-mixed with the reduced humic acid (RHA) under different Cu(II) ion dosages and incubation period. The copper species of RHA-Cu were organic acid-copper complexes after the reacting time of 12 hours. Metallic copper was observed for the reacting time of 48 hours in the condition of 0.9 g/L RHA with 175 mmol/kg Cu(II) which were analyzed by X-ray adsorption near edge structure. The spatial distribution of zero valent copper in the humic acid were investigated by scanning photoemission microscopy. There were two peaks at 1384 and 1598 cm-1 owning to the carboxylate vibrations, indicating that the HA complexes with Cu(II) via chemical bonding which were investigated by Fourier transform infrared spectroscopy. The particle size of metallic copper of RHA-Cu was around 50 nm was analyzed by single particle-inductively coupled plasma-mass spectrometry. The removal efficiency of HBCD by RHA-Cu was 34.5% after 24 hours and bromide ions were detected, indicating a reduction and debromination reaction of HBCD. Our results demonstrate the metallic copper can be synthesized by humic substances in a redox condition, which could provide a potential for HBCD removal through this abiotic chemical process.