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標題: TiO2與ZnO奈米微粒在水體環境介質中之聚集研究
The aggregation of TiO2 and ZnO nanoparticles in aqueous environment
作者: 曾勇銘
Tseng, Young-Ming
關鍵字: TiO2
出版社: 土壤環境科學系所
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摘要: 在本研究中所使用之奈米顆粒(nanoparticles, NPs)包含了商用粉體二氧化鈦(TiO2)與氧化鋅(ZnO)NPs、自製穩定性TiO2與ZnO奈米顆粒懸浮液及商用穩定性奈米TiO2懸浮液,經電子顯微鏡確認這些TiO2與ZnO奈米材料顆粒粒徑大小在奈米等級。粉體NP一進入水溶液則迅速地團聚,形成NP之團粒。以超音波粉碎機分散粉體NP,方可取得懸浮性良好之奈米顆粒懸浮液。自製之二氧化鈦(TiO2)與氧化鋅ZnO奈米懸浮液,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100 nm左右。而商用TiO2奈米懸浮液具有良好的穩定性,溶液中之顆粒粒徑能夠維持約50~70 nm左右超過30天以上。 在一般台灣水體環境溫度範圍15~35℃下,溫度對此三種奈米材料之顆粒粒徑影響不明顯。此三種奈米懸浮液中顆粒粒徑皆會隨pH變化,在TiO2之等電點(粉體pH 7.8、自製pH 7.2、商用pH 6.47 )上下一個pH單位可發現明顯的粒徑增加與沉澱現象。水中pH值在ZnO之等電點(粉體 pH 9、自製 pH 9.7)上下兩個單位時,皆可發現明顯的粒徑增加與沉澱現象。 介面活性劑可分散商用粉體NP,但依其濃度及性質亦可使奈米顆粒絮聚。對商用粉體TiO2而言,十二烷基硫酸鈉(sodium dodecyl sulfate, SDS)可以有效維持TiO2大小,十六烷基三甲基溴化銨(hexadecyltrimethylammonium bromide, CTAB)在其0.1、1、10 臨界微胞濃度(critical micelle concentration, CMC)濃度之下,也皆可有效地分散TiO2在原有大小,TritonX-100 (TX-100)只在CMC濃度下可以分散TiO2而使NP粒徑未明顯增加。對商用粉體ZnO而言,相較於未有界面活性劑存在之NP變化,除了SDS在10 CMC時有絮聚現象,三種介面活性劑皆可使ZnO NP增加之速率減緩。存在SDS 10 CMC時其pH為3.8,在此pH值大部分之ZnO以解離為Zn2+,因此造成誤判。 在腐植酸(humic acid, HA)存在的情況下,對粉體NP而言添加HA有助於維持水溶液中TiO2 NP之穩定,可能因為HA可產生空間位阻或是因吸附而使顆粒表面帶有負電荷,產生靜電排斥力而懸浮;對ZnO而言,存在30 mg/L以下之HA有助於ZnO NP維持穩定,但HA濃度提高至50 mg/L則會因ZnO與HA產生架橋作用而快速的聚集。對自製NP懸浮液而言,低濃度HA存在會使TiO2之粒徑保持穩定,可能由於HA吸附在NP的表面使顆粒穩定,當有含有過高濃度之HA ( 60 mg/L )時,會使TiO2奈米懸浮液中之奈米顆粒與HA間產生架橋作用,進而使顆粒團聚,而後沉澱。雖然在可使NPs穩定之HA濃度(30 mg/L)之下,在pH 7左右TiO2顆粒會快速的聚集。在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(1 mg/L)下ZnO而會較快聚集,所以當pH接近NPs的等電點HA的存在能夠減緩NPs的聚集。商用TiO2奈米懸浮液中存在10 mg/L之HA時對粒徑的影響不大,當腐植酸濃度提高至30 mg/L時NP則會快速的聚集,這可能是由於較高濃度的HA會與TiO2奈米顆粒產生架橋作用,也可能是因為由於奈米懸浮液中NP表面所帶之正電荷因存在HA而中和其表面電荷,造成顆粒的聚集沉澱。而HA的存在會使所試驗之NP表面帶負電荷而使等電點轉移,而存在的HA濃度越高轉移的程度也越大。 在離子強度對商用TiO2 NP懸浮液穩定性測試中,當水溶液中存在塩類時會因離子電荷可中和顆粒表面電荷,進而促使奈米顆粒因斥力降低而聚集沉降,而且提高離子濃度會加快其奈米顆粒絮聚的現象。對此商用奈米TiO2 NP懸浮液而言,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290 meq/L, SO42-的CCC為2.3 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。而奈米顆粒團聚的反應能可以使用DLVO (Derjaguin-Landau-Verwey-Overbeek)理論來解釋。在商用TiO2奈米懸浮液中存在HA時,會因為HA降低了顆粒的表面電位而使得Cl-與SO42-讓商用TiO2顆粒聚集的臨界聚集濃度降低。而存在HA時Cl-的CCC濃度為100 meq/L,SO42-的CCC為1.8 meq/L。此外,此商用穩定性TiO2 NP之塩類CCC值,較粉體奈米材料高出許多,而與自製穩定性奈米材料相近,顯示欲使穩定性NP聚集需要較高濃度塩類存在於水環境中。 所試驗之NP在不同水質環境中之結果,指出當這些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,TiO2與ZnO NP會團聚形成大顆粒而沉澱。HA能夠維持粉體奈米材料之穩定性,而存在低濃度之HA也能夠維持自製TiO2 (pH 2.5)之穩定性,此結果顯示當環境中存在HA時可能使顆粒維持穩定而懸浮。但當水環境中pH接近期pHzpc,或環境中若存有大量的離子及其強度超過其CCC值,NPs會聚集。一般水體pH值接近TiO2 NP等電點而容易團聚,而ZnO NP則具有溶解作用,但於本實驗對商用TiO2奈米懸浮液之試驗及先前研究中所得到離子之CCC濃度高於一般水體中塩濃度,所以所試驗之奈米顆粒流布於環境水體中仍可能存在威脅。
The success of nanotechnology resulted in the widespread application of nanomaterials in commercial products. The increasing use of nanomaterials in consumer products has led to the increased concerns about their potential environmental and health impacts. To better understand the transport, fate, and behavior of nanoparticles (NPs) in aquatic systems, it is essential to understand their behaviors in different aqueous conditions. This research investigated commercial TiO2 and ZnO NPs, synthetic TiO2 and ZnO NP suspensions and commercial TiO2 NP suspension in various aqueous conditions. We use the direct ultrasonic probe to disperse the NP powder and get the NP suspension. Synthetic TiO2 and ZnO NP suspensions were synthesized with benzyl alcohol and diethylene glycol as capping agents, respectively. The sizes of commercial TiO2 and ZnO NPs powder, synthesized TiO2 and ZnO NPs and commercial TiO2 NP were identified as in nanoscale by a transmission electron microscopy (TEM). The temperature in the range of 15~35 oC did not significantly affect those nanomaterials. With the aqueous pH close to the pHzpc of commercial TiO2 NP powder (pH 5.9) and ZnO NP powder (pH 8.98), synthetic TiO2 NPs (pH 7.2) and ZnO NP (pH 9.7), and commercial TiO2 NP (pH 6.47) the obvious coagulation behaviors were observed. These nanomaterials could suspend when aqueous pH out of 2 units of their pHzpc . Surfactants can disperse and aggregate the commercial NPs powder, which depends on the properties of surfactant and the nature of NPs. For TiO2 NPs, TiO2 NPs maintain stable for several days in the presence of sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium bromide (CTAB) surfactants although the particle size of TiO2 NPs increased a little. For TritonX-100 (TX-100), a neutral surfactant, the well dispersion of TiO2 NPs only occurred when its concentration in its critical micelle concentration (CMC). As compared to the aggregation of ZnO in absence of surfactants, these three surfactants can maintain the stability of ZnO NP except the 10CMC SDS. The low pH value of 3.83 was observed for the rapid aggregation of ZnO NPs in the presence of 10CMC SDS, which could result from the neutralization of SDS with many dissolved zinc ion species. Nano-TiO2 NP powder can maintain stable for more than ten days in the presence of humic acid (HA) due to the steric hindrance and negative surface charge of HA. For nano-ZnO NP powder, ZnO NPs maintained stable as HA concentration below 30 mg/L. When HA concentration increased higher than 50 mg/L, TiO2 NPs powder aggregated quickly. It could result from the cross-link of humic acid molecules with NPs aggregates. In the presence of a low HA concentration at low pH, the stability of these synthetic TiO2 NPs can keep several days. When the HA concentration was higher than 60 mg/L, TiO2 NPs aggregated because HA could bridge these NPs. In different pH conditions, the addition of HA caused the pHzpc of synthetic TiO2 shift to a low value. The increase of the concentration of HA enhanced the pHzpc shift. At pH 7, the quick aggregation of TiO2 NPs was observed even in the presence of humic acid, indicating that HA can not maintain the TiO2 NPs stable when aqueous pH close to pHzpc of TiO2. However, HA can also decrease the aggregation process of synthetic ZnO NPs 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, HA can maintain the synthetic ZnO NPs stable. The ionic composition and strength can strongly affect the aggregation and sedimentation of NP in the aqueous environment. For commercial TiO2 NP, the particle size increased more quickly in a high concentration of salts. The critical coagulation concentration (CCC) values for commercial TiO2 particles were estimated as 290 meq/L Cl- and 2.3 meq/L SO42-, respectively. In the presence of HA (10 mg/L), the CCC values of Cl- and SO42- decreased. The CCC values for commercial TiO2 particles in the presence of HA were estimated as 100 meq/L Cl- and 1.8 meq/L SO42-. The fate of these nanomaterials in water would depend on pH, ionic strength, ionic composition, surfactants and humic substance in the aqueous environment. The aqueous pH and pHzpc play key roles in the stability of NPs. 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.
其他識別: U0005-1508201023252000
Appears in Collections:土壤環境科學系



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