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Transformaion and fate of nanoparticles in aqueous solutions
|關鍵字:||環保工程;基礎研究;nanoscale particle;奈米懸浮液;聚集;酸鹼值;陽離子;腐植酸;suspension, aggregation;pH;cations;humic acid||摘要:||
在所合成之二氧化鈦(TiO2)與氧化鋅ZnO奈米懸浮液中，經電子顯微鏡確認TiO2與ZnO奈米顆粒粒徑大小在奈米等級，其中TiO2奈米懸浮液具有非常良好的穩定性，溶液中之粒子的粒徑可以維持在100 nm以下超過25天以上，而自行合成之ZnO奈米懸浮液中顆粒初始大小為100nm左右。本研究背向光散射技術量測水中奈米顆粒之技術，較傳統之動態光散射技術更能準確偵測高濃度下之粒徑大小。在奈米TiO2顆粒濃度 10 mg/L至 6 g/L 之間，粒徑也未明顯變化，但濃度小於 1 mg/L 則會聚集。而奈米ZnO顆粒濃度 50 mg/L至 500 mg/L 之間，粒徑也未明顯變化，皆維持110nm左右，但濃度小於 1 mg/L 則會使強度不足而影響偵測；高於500 mg/L則會使顆粒間碰撞機率增加而獲得較大顆粒。在15~35℃範圍下，溫度對TiO2與ZnO顆粒粒徑之影響不明顯。穩定性TiO2水溶液顆粒粒徑會隨pH變化，在TiO2之等電點( pH 7.2 )上下一個pH單位可發現明顯的粒徑增加與沉澱現象。穩定性ZnO水溶液顆粒粒徑會隨pH變化，在ZnO之等電點( pH 9.7)上下兩個單位可發現明顯的粒徑增加與沉澱現象。當水溶液中存在塩類時，會因離子電荷可壓縮電雙層，進而促使奈米顆粒聚集沉降，而且提高離子濃度會加快其奈米顆粒絮聚的現象。對此穩定型TiO2而言，隨著塩類濃度的增加，離子強度增加後亦使絮聚現象更趨明顯，Cl-的臨界混凝濃度（critical coagulation concentration, CCC）約為300 meq/L， SO42-的CCC為1.4 meq/L。顯示在相同強度下二價陰離子會較一價陰離子更易中和表面電荷，而使之聚集而且改變水中pH值接近pHZPC時，可使之聚集之鹽類濃度也隨之下降。對於穩定型奈米ZnO，Cl-的CCC為30 meq/L， SO42-的CCC為0.25 meq/L。而奈米顆粒團聚的反應能可以使用DLVO (Derjaguin-Landau-Verwey-Overbeek)理論來解釋。而透過能障(energy barriers)隨著離子強度的增強而降低也符合奈米顆粒大小分析之結果。此外，與奈米粉體結果相比較，此兩穩定性奈米材料之塩類CCC值皆顯著高出許多，顯示欲使之聚集需要較高濃度塩類存在於水環境中。低濃度腐植酸(humic acid, HA)存在會使TiO2之粒徑保持穩定，可能由於腐植酸吸附在顆粒的表面使顆粒穩定，當有含有過高濃度之腐植酸( 60 mg/L )時，會使TiO2奈米懸浮液中之奈米顆粒與腐植酸間產生架橋作用，進而使顆粒團聚，而後沉澱，然而在不同pH條件下腐植酸無法有效的穩定TiO2，在pH 7左右顆粒會快速的聚集。在不同pH 7與11條件下，HA能夠維持ZnO的穩定，在pH 9.4時HA的存在(1mg/L)下ZnO而會較快聚集，當HA的濃度為10mg/L 會較慢聚集，所以當pH接近ZnO的等電點HA的存在能夠減緩ZnO的聚集。黃酸的添加也會使顆粒的表面電位下降，在存在黃酸時，奈米懸浮液中顆粒會隨著黃酸的濃度提高而粒徑增加，但可維持粒徑的穩定性。所以此類穩定性奈米材料仍可能存在潛在環境風險。此二者奈米材料在不同水質環境中之結果，指出當兩種奈米材料存於自然水體中，當受自然水體的pH緩衝作用而使其pH接近中性，顆粒會團聚形成大顆粒而沉澱。另外在在工業廢水或汙水廠之水體中，又存有大量的離子及其強度也都大於本實驗所得到之CCC濃度，此表示奈米顆粒流布於環境水體中，容易團聚而形成沉澱。但這些成果顯示此穩定型TiO2相當穩定，需高濃度鹽類或調整pH才能使之聚集，進而影響其傳輸或者殘留於環境中，造成汙染甚至產生毒害仍需再進一步探討。隨著奈米顆粒應用的增加，需繼續研究商用奈米懸浮液與奈米產品在水環境中之奈米微粒轉換與其宿命，以利評估奈米顆粒對環境的危害。
The fate and transformation of engineered nanoparticles in environmental is of significant interest to ecosystem and human health due to the large application and development of engineered nanoparticles in recent years. Stable TiO2 and ZnO nanoparticles in suspensions were synthesized with benzyl alcohol and diethylene glycol as capping agents, respectively. This research investigated the stability and morphology change of two stable metal oxide nanoparticles in a variety of aqueous conditions. The sizes of these two synthesized particles of TiO2 and ZnO were identified as in nanoscale by a transmission electron microscopy (TEM). Nano-sized particles of TiO2 were stable more than 25 days and ZnO nanoparticles were stable more than 1 day due to their surface modification to create high zeta-potentials. The particle size change with time in the aqueous phase was monitored by a dynamic light scattering (DLS). The morphology and characteristic of these two nanoparticles were also examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and TEM. The particle concentration did not significantly affect the particle size of the stable nanoscale TiO2 suspensions but the particle size of TiO2 nanoparticles increased when the particle concentration less than 10 mg/L. The particle size of zinc oxide maintained approximately 110 nm when the range of particle concentration between 50 mg/L to 500 mg/L. The particle intensity of ZnO was too low to detect when the concentration less than 50 mg/L. For the ZnO concentration higher than 500 mg/L, the particle size increased due to the more collision of particles. The temperature in the range of 15~35 oC also did not significantly affect the stability of TiO2 and ZnO nanoparticles. With the aqueous pH close to the pHzpc of TiO2 nanoparticles (pH 7.2) and ZnO nanoparticles (pH 9.7), the obvious coagulation behaviors were observed. These two stable nanoparticles could suspend when aqueous pH out of 1 and 1.8 units of their pHzpc for TiO2 and ZnO, respectively. The ionic composition and strength can strongly affect the aggregation and sedimentation of these two stable nanoscale materials in the aqueous environment. For the stable TiO2 nanoparticles, the particle size increased more quickly in a high concentration of salts. The critical coagulation concentration (CCC) values for nanoscale TiO2 particles were estimated as 340 meq/L NaCl, 290 meq/L CaCl2, and 1.4 meq/L SO42- respectively. The results indicated that the divalent anion than monovalent is more easily to neutralize the surface charge of particles and increase aggregation in the same ion strength. The second affective factor is the ion strength resulted in compression of the electrical double layer (EDL), and therefore a decrease in the EDL repulsive energy such that the flocculation can be predicted. Multivalent anions could form bridges with nanoscale particles or neutralize their positive surface charges to induce a quick aggregation. Furthermore, these CCC values are higher than those of TiO2 nanoparticle powders in previous study, indicating the strong stability of the synthesized TiO2 nanoparticles. For stable zinc oxide, the CCC values for sodium chloride, calcium chloride and sodium sulfate are 30 meq/L, 30 meq/L and 0.25 meq/L, respectively. The interaction energies of nanoparticles in water can be analyzed using DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. The decreasing energy barriers of nanoparticle with the increase of ion strength increasing are consistent with the observation of the particle size change. Furthermore, the energy barriers decreased to the maximum value as the ion concentration of CCC.In the presence of a low humic acid concentration at low pH, the stability of these TiO2 nanoparticle suspensions can keep several days. When the humic acid concentration was higher than 50 mg/L, TiO2 nanoparticles aggregated because humic acid could bridge these nanoparticles. In different pH conditions, the addition of humic acid cause the pHzpc of TiO2 shift to a low value. The increase of the concentration of humic acid enhanced the pHzpc shift. At pH 7, the quick aggregation of TiO2 nanoparticles was observed even in the presence of humic acid, indicating that humic acid can not maintain the TiO2 nanoparticles stable when aqueous pH close to pHzpc of TiO2. However, humic acid can decrease the aggregation process of ZnO nanoparticles in the presence of humic acid higher than 10 mg/L when the aqueous pH close to pHzpc (pH 9.7) of ZnO. At pH 7 and 10, humic acid can maintain the ZnO nanoparticles stable. In the presence of fulvic aicd, the zeta potentials of TiO2 and ZnO also decreased. With the increase of fulvic acid concentrations, the stability of these two nanoparticles increased. Humic and fulvic acids seem to facilitate the stability of ZnO nanoparticles, which could result from the steric repulsion caused by humic molecule structure of functional groups in humic and fulvic acids.Once these two stable nanomaterials into four environmental water samples, they aggregated because aqueous pH close to the pHzpc of TiO2 nanoparticles and ion strength in water samples enhanced the coagulation process. Hence, the particles are more easily to aggregate in the environment. Especially both effluents from industry wastewater and sewage wastewater contained a lot of ions more than the CCC values of these two stable nanoparticles, they easily aggregated and precipitated in wastewater effluents. These results suggest that these two stable nanoparticles could not cause the nanotoxicity effect in aqueous environment due to the easy aggregation process of them in aqueous phase..The fate of these stable metal oxide nanoparticles in water would depend on pH, ionic strength, ionic composition and humic substance in the aqueous environment. The aqueous pH and pHzpc play key roles in the stability. These findings provide important insights into the ways in which stable nanoparticle change under different aqueous conditions that may be generally relevant to the nanoparticle fate in diverse natural environment.
|Appears in Collections:||土壤環境科學系|
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