奈米鐵顆粒分散性與有機酸對含鹵素有機汙染物反應性影響之研究

Abstract

奈米級零價鐵 (Nanoscale zerovalent iron, NZVI)及雙金屬奈米鐵顆粒因具高比表面積與還原勢能，能有效移除許多含鹵素有機汙染物，而廣泛應用於污染場址整治中。然而奈米鐵顆粒 (Fe nanoparticles, Fe NPs) 因受顆粒特性與環境因子影響而易聚集並沉澱，降低移動性和反應性。此外，奈米鐵顆粒受環境因子如離子組成和離子強度等生成鈍化層改變顆粒表面特性，降低奈米鐵顆粒的反應性與長效性。故如何提升並延長奈米鐵顆粒的有效性為當前一重要課題。除了奈米鐵顆粒的作用外，其反應後生成之鐵離子與鐵氧化物在環境中與天然有機物之作用亦不可忽視。因此本研究透過使用穩定分散的奈米鐵顆粒，探討不同反應條件下其之反應性與反應機制，並探討後續於環境整治中的應用性。奈米鐵顆粒分別以物理性分散法與化學穩定劑修飾法分散懸浮奈米顆粒，添加羧酸以移除鐵顆粒表面的鈍化層以增加並延長有效性，更進一步探討腐植酸 (humic acid, HA) 與鐵 (銅) 離子生成的錯合物和共沉澱物的反應性，用以評估奈米鐵顆粒後續處理之可行性。 相較於奈米鐵團粒，分散之奈米鐵顆粒具較多的比表面積，因而增加對汙染物如五氯酚之吸附量。常見陰離子 (Cl−, NO3− 和HCO3−) 對穩定分散之奈米鐵顆粒的影響指出，因NO3− 被鐵還原後於鐵表面生成氫氧化鐵沉澱，進而促進對汙染物的吸附/絮聚現象，增加移除效率。而HCO3− 因於鐵表面生成碳酸鐵沉澱而明顯抑制反應速率，Cl− 則沒有明顯促進作用。使用羧甲基纖維素 (Carboxymethyl cellulose, CMC) 作為分散劑包覆於鐵表面可有效分散並懸浮奈米顆粒。鹼性環境下由於CMC的解離使顆粒更為分散而增加CMC-Fe的反應性；CMC層促進汙染物如十溴二苯醚擴散至鐵表面，但亦會抑制電子傳輸，降低降解效率。相較於未修飾的奈米鐵顆粒，CMC層可保護鐵表面受到陰離子的侵蝕作用，故其反應性並未受陰離子之影響。由於CMC可延長奈米鐵顆粒的有效性，使CMC修飾之奈米鐵顆粒具有現地處理之潛力。 有機羧酸如甲酸 (formic acid, FA)、草酸 (oxalic acid, OA)、檸檬酸 (citric acid, CA) 可透過移除鈍化層改變表面特性，促進鐵顆粒的反應性。在相同酸當量濃度下，鐵顆粒對汙染物如三氯乙烯的反應速率為FA > OA > pure water ≅ CA。FA對Fe2+無明顯錯合能力，透過提供H+移除鐵表面鈍化層並生成磁鐵礦而提高吸附率；OA與CA對Fe2+具高錯合能力，可藉由形成可溶性錯合物移除鈍化層，OA和CA與鐵的錯合物依其氧化還原電位亦可進一步移除汙染物或降解副產物。然而高濃度的OA生成草酸鐵沉澱於鐵顆粒表面，反而降低反應性。鐵離子 (Fe2+) 與HA的錯合反應亦可能改變其形態與反應性。Fe2+的反應性受pH影響，於pH 9環境中反應性較高。HA 與Fe2+錯合物 (HA-Fe) 亦有相同趨勢。還原性HA與Fe2+的錯合物 (RHA-Fe) 較未處理HA與Fe2+錯合物 (UHA-Fe) 有較高的反應性；但HA與Fe的錯合反應可能因與污染物競爭電子而降低Fe2+的反應性。 此外，RHA抑制Fe2+於鹼性環境下生成沉澱物，卻形成更多的HA-Fe3+。而高濃度Fe2+和Cu2+在HA存在下可共沉澱生成膠體顆粒。HA-Fe的膠體顆粒相比較，HA-Cu有較高的反應性，並且有較明顯的顆粒外型與結構。RHA-Cu顆粒的穩定性高甚至有小部分Cu2+還原為Cu0，其反應性也較UHA-Cu高。 此研究針對穩定分散的奈米鐵顆粒與汙染物之間的反應，得其反應機制與不同環境條件下之反應性；由CMC修飾奈米鐵顆粒探討現地應用之可行性；透過表面特性的改變，增加鐵顆粒的有效性，並發現腐植酸與鐵 (銅) 離子之間的錯合反應可生成具反應性之錯合物甚至膠體顆粒。對於奈米鐵顆粒應用於現地整治方面，可提供進一步資訊評估其可行性。鐵 (銅) 離子於厭氧環境中可生成具反應性的錯合物與膠體顆粒，可進一步建立與天然腐植物質相關之綠色整治技術。

Nanoscale zerovalent iron (NZVI) and bimetallic Fe nanoparticles (NPs) have significant potential for the remediation of a wide array of priority pollutants. Their properties of a large surface area and high reduction potential generated significant interest in their application for in-situ remediation. However, Fe NPs aggregate immediately that significantly reduce their mobility and reactivity. Furthermore, corrosion processes form precipitates on the Fe surface, whose passive layers resulted in a rapid decrease in activity and longevity. Therefore, methods to enhance/extend the colloidal stability and reactivity of Fe NPs are needed. On the other hand, it is important to understand the reactivity of Fe ions and Fe oxides after the application of Fe NPs. Fe NPs were dispersed successfully via physical (ultrasonication (US)) and chemical (CMC stabilizer) dispersion methods under different environmental conditions. Carboxylic acids including formic acid (FA), oxalic acid (OA) and citric acid (CA), were applied to prolong the Fe particles reactivity by removing passive layers on the Fe surface. Furthermore, the activity of Fe (Cu) complexes/precipitates with humic acid (HA) to mimic natural environments was assessed for subsequent treatments. Well-dispersed bare Fe NPs enhanced the adsorption of contaminants such as pentachlorophenol onto the Fe surface, as compared to Fe aggregates. In the presence of common anions (Cl−, NO3− and HCO3−), NO3− was reduced by Fe NPs and generated more Fe (hydro)oxides that responded to increase the adsorption/coagulation of the target compound. The inhibition of Fe reactivity by HCO3− may be due to the formation of precipitates on the Fe surface while Cl− only caused a small increasing in Fe reactivity. The presence of CMC suspended Fe NPs very well and dispersed them into individual particles instead of nanoparticle aggregates. Due to the properties of CMC, the reactivity of CMC-Fe NPs toward decabromodiphenyl ether was enhanced under alkaline conditions. The reaction rate was considered as a surface chemical reaction because the CMC layers induced diffusion for the target compound to the Fe surface. But CMC layers may also hinder the electron transfer. Anions did not influence the reactivity of CMC-Fe NPs compared with bare Fe NPs, indicating that the CMC layers may inhibit surface corrosions and thus prolong the reactivity of Fe NPs in the environment. Common carboxylic ligands (FA, OA and CA) induced the reactivity of Fe particles toward trichloroethylene following an order of FA > OA > pure water ≅ CA by dissolving Fe oxides from the Fe surface. FA provided protons to promote the dissolution of passive layers and to convert iron oxides to form magnetite which increased the adsorption of the target compound onto the Fe surface. With the strong complexing ability, OA and CA could form dissoluble complexes to remove passive layers. But a high concentration of OA resulted in reprecipitated of Fe oxalate back onto the Fe surface which then inhibited its reactivity. Moreover, these Fe-ligand complexes could further degrade the target compound depending on their redox properties. The activity of Fe2+ and HA-Fe complexes depended on the pH which had strong interactions toward the target compound at pH 9. RHA-Fe complexes had higher reactivity than UHA-Fe, but these complex forms potentially reduced the Fe2+ reactivity because of electron competition with the target compound. Moreover, RHA prevented Fe2+ from precipitating but also caused higher Fe2+ oxidation. Furthermore, HA co-precipitated Fe and Cu colloids showed ability to remove the organic contaminants such as Reactive black 5. HA-Cu colloids had higher reactivity than HA-Fe colloids, which could result from the nature of metal, the shape and the morphology of particles. RHA-Cu colloids stably dispersed in aqueous solutions. Among them, a small amount of Cu0 was generatead by RHA. RHA-Cu colloids also had a stronger reactivity than UHA-Cu colloids for breaking azo bonds in the target compound. To conclude, this study presents the reaction characteristics and removal mechanisms of bare Fe NPs that were well suspended under different conditions and the potential for CMC-Fe NPs for in-situ treatments. The characteristics and reactivity of metal complexes/colloids with ligands and HA in reducing environments have pointed out the potential of these Fe (Cu) complexes/colloids for sustainable/green remediation.