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
