使用金奈米顆粒沉積活性碳纖維布結合電熱再生系統吸附與回收汞蒸氣

Abstract

由於汞的高危害性，近年來，各國逐漸對含汞產品實行了更嚴格的限制。同時由於技術的日新月異，大量的液晶顯示器(LCD)和螢光燈管等含汞產品被汰換及回收。根據台灣環保署資料，在 2019年約有5,000噸廢棄螢光燈管需要被破碎及回收 (Taiwan EPA, 2019)，顯示未來在回收廠內去除及回收低濃度的汞蒸氣有其重要與必要性。目前在螢光燈拆卸過程中用於除汞的吸附劑主要是含硫(SAC)或鹵素官能基的活性碳；儘管SAC對汞蒸氣的去除效果很高，但是由於硫或鹵素官能基與汞形成的鍵結能極強，使得飽和的吸附劑難以再生，因此這些飽和吸附劑被歸類為有害事業廢棄物，須進行固化及最終掩埋。本研究旨在製備一種有效且可再生的汞吸附劑；透過採用同時具有分散劑與還原劑功能之氯化四羥甲基磷(tetrakis(hydroxymethyl)phosphonium chloride, THPC) 使氯金酸 (chloroauric acid)在高度分散的情況下還原成金奈米顆粒，附載在活性碳纖維布上 (Au-ACFC-THPC)，SEM的結果顯示其平均顆粒大小為1-100 nm。汞吸附實驗結果顯示經金改質後Hg0吸附容量有明顯的提升，負載不同重量百分濃度金 (0.1, 0.2, 0.5及0.7 wt.%) 的Au-ACFC-THPC 對於Hg0之90% 飽和吸附容量分別為: 559±41, 1032±249, 1706±153及 1338±329 μg/g。同時為瞭解本材料對水的抗性，於相同吸附條件下添加3% (v/v)的水氣進行競爭吸附，並與先前本實驗室以電沉積合成的含金碳纖維布 (GE-1200s) 進行比較，結果顯示0.5 wt.%-ACFC-THPC及GE-1200s的吸附容量均有下降的趨勢，分別由1706±153 下降至 824 ±279 µg/g 及408±5 下降至 269±74 µg/g，但0.5 wt.%-ACFC-THPC 之吸附容量仍為GE-1200s的3倍左右。為瞭解汞吸附之型態，將吸附汞的活性碳纖維布進行程序升溫脫附(Hg-TPD)，由未處理前材料Raw-ACFC在72.9℃及142.9℃ 出現之特徵峰判斷汞應是以吸附材孔洞物理性吸附與C=O官能基化學吸附為主。由Au-ACFC-THPC於129.7至185.6℃出現的單一且對稱的特徵峰，顯示汞主要以金汞齊的物理性吸附在材料上。通過電熱吸附與再生系統(Electrothermal Swing System, ETS)進行三循環汞吸附和再生實驗，Au-ACFC-THPC在不同Hg0進流濃度下(30, 75, 100±5 μg/m3)，對Hg0的吸附和再生效率皆能穩定保持在95%左右，然而Raw-ACFC的吸附效率則為66.1到94.7%不等，較低的吸附效率可能源自於再生過程中孔洞結構的塌陷降低表面積，或是其較小的吸附容量所導致；此外，TEM結果顯示經12個循環的ETS電熱升溫及降溫後未觀察到金奈米顆粒大小因熱效應產生顯著變化。本研究提出了汞的吸脫附機制，並進一步應用擴散控制的質傳模式模擬Hg0脫附的過程，最後對外部質傳係數(kf)及孔隙間擴散係數(kp)進行探討以利未來工程規模的電熱系統設計。

Owing to the high toxicity of mercury (Hg), tighter limitations on using Hg-containing products in various countries have been gradually executed in recent years. In the meantime, a considerable amount of Hg-containing products such as liquid-crystal displays (LCD) and fluorescent lamps are replaced and recycled due to rapid technological developments. Approximately 5,000 tons of wasted fluorescent lamps need to be crushed and recycled in 2019 (Taiwan EPA, 2019). Consequently, it is necessary to remove and recover the low concentration of Hg in the recycling plants. Currently, activated carbon impregnated with sulfur functional groups (SAC) or halogen groups are frequently used for Hg removal in fluorescent lamps disassembling processes. Although a high removal efficiency can be achieved, it is difficult to regenerate the exhausted adsorbents due to the high bonding energy of Hg and S or halogen groups, thus these adsorbents are classified as hazardous waste and need to be solidified and end up in the landfill. In this study, we aim at developing an effective and renewable adsorbent by adopting the tetrakis(hydroxymethyl)phosphonium chloride (THPC) method to prepare activated carbon fiber cloth (ACFC) loaded with gold (Au) nanoparticles with size ranging from 1 to 100 nm. The synthesized Au-ACFC-THPC with gold loading ratios of 0.1, 0.2, 0.5, and 0.7 wt.% were prepared, and the elemental Hg (Hg0) saturated adsorption capacities obtained at 90% breakthrough were 559±41, 1032±249, 1706±153, and 1338±329 μg/g, respectively. Competitive adsorption experiments with H2O (3% (v/v)) were also carried out to examine the water resistance of 0.5 wt.%-ACFC-THPC and GE-1200s (Au-electrodeposited ACFC with electrodeposition period of 1200s, which was also developed in our laboratory). The Hg0 adsorption capacities of 0.5 wt.%-ACFC-THPC and GE-1200s decreased from 1706±153 to 824 ±279 µg/g and from 408±5 to 269±74 µg/g, respectively, indicating that the Hg0 saturated adsorption capacity of 0.5 wt.%-ACFC-THPC was still appreciated even with the presence of water vapor in the gas stream. To gain a more comprehensive understanding of Hg0 adsorption and desorption pattern on ACFCs, temperature-programmed desorption (Hg-TPD) experiments were carried out. Results of Hg-TPD showed that Hg0 was both physically and chemically adsorbed on Raw-ACFC by its well-developed pore structures and C=O functional groups, with the corresponding desorption temperatures at 72.9℃ and 142.9℃, respectively. In contrast, the corresponding desorption temperatures of Au-ACFC-THPC were all shown with a single, symmetric peak ranging from 129.7 to 185.6℃, indicating that Hg was physically adsorbed on Au-ACFC-THPC in the form of Au-Hg amalgam. Furthermore, the desorption activation energy (Ed) of Raw-ACFC and Au-ACFC-THPC were estimated, with a value in the range from 20.4 to 83.8 kJ/mole, suggesting that physical adsorption dominates the adsorption processes. The results of the 3-cycle electrothermal swing system (ETS) experiments showed that the adsorption efficiency of Au-ACFC-THPC remained at approximately 95% and demonstrated great stability under various inlet Hg0 concentrations (30, 75, and 100±5 μg/m3), while the efficiency of Raw-ACFC varied in the range of 66.1% to 94.7%. The decrease in the adsorption efficiency of Raw-ACFC might be caused by the collapse of pore structure after the electrothermal regeneration procedure or by the extremely low Hg0 adsorption capacity compared with Au-ACFC-THPC. Notably, significant changes in the size and morphology of Au nanoparticles were not observed after 12-cycle of ETS operation. The diffusion-controlled mass transfer models were also applied to explain the Hg0 desorption kinetics. External mass transfer coefficient (kf) and intraparticle diffusivity (kp) were further obtained for the future practical engineering design of ETS for Hg0 adsorption and recovery.