The effects of dispersion methods and organic acids on the reactivity of iron nanoparticles to remove halogenated organic contaminants

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

Reductive debromination of hexabromocyclododecane by nanosized zerovalent iron

六溴環十二烷（Hexabromocyclododecane, HBCD）為一溴化阻燃劑，已廣泛使用於各種傢具及商業產品中以降低火災發生之風險。因HBCD化合物的物化性質使其具有環境持久性，加上高生產量所致，使HBCD大量分佈於環境中。此研究中使用實驗室合成之奈米級零價鐵（nanoscale zero-valent iron, NZVI）與兩種以羥甲基纖維素鈉（sodium carboxymethyl cellulose, CMC）穩定形奈米零價鐵顆粒 CMC-Fe及CMC-Ni/Fe雙金屬顆粒來探討HBCD移除之動力，並了解不同環境因子如溫度、pH、陰離子、腐植酸等的影響。5 g/L之NZVI與CMC-Ni/Fe皆可於30分鐘內完全移除HBCD，其反應速率常數分別為0.208與0.144 min-1，然而CMC-Fe在經過反應時間1小時後僅移除25.5 %，其可能由於鐵顆粒表面包覆之CMC阻礙了電子轉移所致。對NZVI及CMC-Ni/Fe而言，提高反應溫度則反應速率增加，計算其活化能分別可為18.0 與 7.10 kJ/mol。不同初始pH對NZVI與CMC-Ni/Fe降解影響不明顯。而不同陰離子對NZVI反應性皆有抑制的影響，在實驗條件下，反應速率由快至慢為純水>Cl- NO3->HCO3-，由於NZVI於陰離子溶液中快速聚集為團塊，致使顆粒粒徑超過奈米等級，降低其在含陰離子溶液中之反應性。HCO3-與Cl-陰離子對CMC-Ni/Fe降解HBCD影響不太大，低濃度之NO3-對CMC-Ni/Fe卻有促進的作用，然而高濃度之NO3-則抑制了零價鐵的活性，NO3-可與HBCD競爭零價鐵而轉化成NH4+，其轉換率為NZVI大於CMC-Ni/Fe，而降低零價鐵對HBCD之作用。腐植酸在低濃度下（1-5 mg/L）對NZVI與CMC-Ni/Fe似乎有一些促進作用，當增加腐植酸濃度至5 mg/L以上，NZVI速率則有明顯降低，過量的腐植酸可與零價鐵形成錯合物，減少鐵表面之反應位，然而對CMC-Ni/Fe而言，則沒有太大的促進或抑制作用。以土壤溶液中試驗NZVI及CMC-Ni/Fe對HBCD之移除能力，發現NZVI之反應性明顯降低，CMC-Ni/Fe之移除力則有促進現象，且其顆粒粒徑在土壤溶液中可穩定維持在65 nm左右。HBCD降解副產物以氣相層析儀串聯質譜儀偵測到脫溴產物，推測其降解反應為脫溴還原途徑。 因穩定型CMC-Ni/Fe雙金屬奈米顆粒可有效移除HBCD並維持其奈米等級的顆粒大小，此外其反應性較不受環境因子如溫度、pH、陰離子及腐植酸等影響，故具高潛力可應用於環境整治。Hexabromocyclododecane (HBCD) belongs to the large family of brominated flame retardants (BFRs) and is widely used as additives in many household and commercial products to reduce their flammability. Because of its environmental persistence and high production volume in the past, HBCD has been widely detected in the environment. In this study we conducted to investigate the removal of HBCD by using nanoscale zero-valent iron (NZVI) particles and two stabilized NZVI suspensions, sodium carboxymethyl cellulose (CMC) stabilized NZVI (CMC-Fe) and CMC-Ni/Fe bimetallic nanoparticles. We also evaluate the effects of environmental factors such as temperature, pH, anions, and humic acid on the degradation kinetics. HBCD was almost removed from aqueous solutions by NZVI or CMC-Ni/Fe with 5 g/L iron loading in 30 min. The pseudo-first-order the rate constants were 0.208 min-1 and 0.144 min-1 for NZVI and CMC-Ni/Fe, respectively. However, only 25.5 % removal efficiency of HCBD with CMC-Fe nanoparticles was observed within 1 hr. The CMC could occupy the reactive sites on the iron surface then hinder the mass transfer of HBCD to the reactive sites of Fe. With the increase of temperature, the degradation kinetics of both NZVI and CMC-Ni/Fe increased. The activated energies of NZVI and CMC-Ni/Fe nanoparticles were 18.0 and 7.10 kJ/mol, respectively. The effect of pH was not obvious for these two particles. The removal rates in the presence of these three anions were observed in the order of: pure water>Cl- "≅" NO3->HCO3-, which may result from the quick aggregation of bare NZVI particles in the presence of these electrolytes and the possible complexation of anions with oxidized iron surface. In the contrary, for CMC-Ni/Fe nanoparticles, Cl- and HCO3- slightly affect degradation rates, whereas the high concentration of NO3- could inhibit the reactivity of CMC-Ni/Fe0. The transformation efficiency of NO3- to NH4+ decreased with increasing NO3- concentration, and less NO3- transformation in CMC-Ni/Fe system was observed as compared to bare NZVI. Degradation rates of NZVI or CMC-Ni/Fe nanoparticles slightly increased with humic acid (HA) concentration increased up to 5 mg/L. High concentration of HA (>5 mg/L) could inhibit the degradation kinetics of NZVI and CMC-Ni/Fe, possibly because the complexes of humic acid and dissolved iron species may compete for the reactive sites on the iron surface with HBCD and forming surface passivating layers; subsequently inhibited iron corrosion and reduce the HBCD reduction rate. The removal rate of NZVI particles declined in soil solution; however, for CMC-Ni/Fe nanoparticles, the fast removal efficiency can still be achieved in 5 minutes, and the particle size sustained in about 65 nm through the reaction. The less brominated byproducts of HBCD were identified by gas chromatography-mass spectroscopy to suggest the reductive debromination process. Because the stabilized CMC-Ni/Fe can maintain nanoscale size to transport through subsurface effectively to remove halogenated compounds and to be unaffected by the above discussed common environmental factors, we suggest CMC-Ni/Fe nanoparticles having a high application potential in the environmental treatments.摘要 I Abstract II Tables of contents IV List of Tables VII Chapter 1 Introduction 1 1.1 Background 1 1.2 Objectives 2 Chapter 2 Literature review 4 2.1 Introduction of HBCD 4 2.1.1 The characteristics of HBCD 4 2.1.2 The distribution and toxicology in the environment 5 2.1.2.1 The usage of HBCD 5 2.1.2.2 The possible emission source and distribution 6 2.1.2.3 Bioaccumulation and biomagnifications of HBCD 6 2.1.2.4 The toxicity of HBCD 8 2.1.2.5 The transformation of HBCD 8 2.2 Introduction of nano zero-valent iron (nano-ZVI) particles 9 2.2.1 The characteristics of NZVI 9 2.2.2 The characteristics of zerovalent bimetal particles 12 2.2.3. The characteristics of stabilized NZVI particles 14 2.3 Application of nano-ZVI particles on soil and groundwater environment 17 2.3.1 Surface area 17 2.3.2 Temperature 18 2.3.3 The pH value and buffer solutions 19 2.3.4 Anions 20 2.3.5 Organic matter 21 Chapter 3 Materials and Methods 22 3.1 Chemicals 22 3.2 Synthesis of nanoscale zero-valent iron (NZVI) particles 22 3.3 Synthesis of nanoscale CMC stabilized bimetal (CMC-Ni/Fe) suspensions 23 3.4 Characterization of the synthesized NZVI 24 3.4.1 Transmission electron microscope (TEM) 24 3.4.2 Dynamic light scattering (DLS) 24 3.4.3 Brunauer-Emmett-Teuller surface area 25 3.4.4 X-ray absorption near edge structure, XANES 25 3.4.5 X-ray diffractometry, XRD 26 3.5 Analysis methods of HBCD 26 3.5.1 HBCD stock solution 26 3.5.2 Extraction method of HBCD samples 26 3.5.3 Extraction method of HBCD in solid phase and aqueous phase 27 3.5.4 Analytical methods for HBCD 27 3.5.5 Byproduct analysis and characteristics 27 3.5.6 Analysis methods of ions concentration 28 3.6 Batch experiments 28 3.6.1 Fe dosage, Ni content, and CMC concentration 28 3.6.2 Initial pH and temperature 29 3.6.3 Anions and humic acid 29 3.6.4 Soil solution 29 3.7 Reaction kinetics of HBCD by NZVI or CMC-Ni/Fe 30 3.8 Removal efficiency 31 3.9 Debromination efficiency 31 3.10 Activation energy 32 Chapter 4 Results and Discussion 33 4.1 Characterization 33 4.1.1. TEM 33 4.1.2 XANES analysis 39 4.1.3 Zeta-potential 43 4.2 Degradation of HBCD with NZVI and CMC-Ni/Fe 44 4.2.1 Extraction efficiency 44 4.2.2 The degradation efficiency of HBCD with bare NZVI and stabilized-NZVI 46 4.2.3. NZVI system 48 4.2.3.1 Effect of Fe dosage on the degradation of HBCD by NZVI 48 4.2.4. CMC-Ni/Fe system 50 4.2.4.1. Effect of Fe dosage on the degradation kinetics of HBCD by CMC-Ni/Fe 50 4.2.4.2 Effect of Ni2+ loading on the degradation kinetics of HBCD by CMC-Ni/Fe 52 4.2.4.3 Effect of CMC/Fe molar ratio on the degradation kinetics of HBCD by CMC-Ni/Fe 54 4.3 Effect of temperature on the degradation efficiency of HBCD 55 4.4 Effect of initial pH on the degradation efficiency of HBCD 59 4.5 Effect of anions on the degradation efficiency of HBCD 62 4.5.1 Effect of chloride (Cl-) on the removal of HBCD by NZVI and CMC-Ni/Fe 62 4.5.2 Effect of nitrate (NO3-) on the removal of HBCD by NZVI and CMC-Ni/Fe 68 4.5.3 Effect of bicarbonate (HCO3-) on the removal of HBCD by NZVI and CMC-Ni/Fe 75 4.5.4 Effect of three anions (Cl-, NO3-, and HCO3-) 81 4.6 Effect of humic acid on the degradation efficiency of HBCD 86 4.7 Soil solution 92 4.8 Byproducts production and degradation pathway 95 Chapter 5 Conclusions 99 Reference 101 Appendix 11