電解質對二氧化鈦奈米顆粒的聚集及其降解雙酚A之影響

Abstract

二氧化鈦奈米顆粒(titanium dioxide nanoparticles，TiO2 NPs)是一種常見的奈米材料，並應用於許多消費產品當中。它們於環境中的宿命如聚集與沉降，及對生態系統和人類健康的影響日益受到關注。雙酚A(4,4'-(propane-2,2-diyl)diphenol，BPA)已長時間被應用，然而，暴露於雙酚A可能會干擾內分泌系統的運作。本研究的目的是了解TiO2 NP之聚集與沉降，並研究聚集之TiO2光降解模式汙染物BPA之機制。 商用TiO2奈米顆粒(commercial TiO2 NPs，CTiO2 NPs)的顆粒濃度並不會影響其穩定性，CTiO2 NPs在溶液pH值接近其等電點(pHpzc = 6.0)時會發生聚集及沉降。CTiO2的顆粒大小隨著NaCl及Na2SO4濃度的提高而增加，而其臨界凝聚濃度(critical coagulation concentration，CCC)分別為100及1.5 mM，並且CTiO2於Na2SO4存在下的聚集及沉降幅度明顯較於NaCl存在下高。以過濾及離心除去溶液中的NaCl後，已聚集的CTiO2顆粒大小降低至85~100 nm。不同腐植酸濃度下，只在含20 mg/L SRHA時，CTiO2才會發生聚集及沉降現象。在綜合pH、NaCl及SRHA等因子之實驗中，除了在pH 7，200 mM NaCl及10 mg/L SRHA的狀況下，CTiO2的聚集及沉降都會發生。 BPA的降解速率隨著NaCl及NaBr濃度的提高與TiO2粒徑增加而增加，然而，在高濃度NaNO3的存在下，BPA的降解速率不但沒有增加，甚至會降低，這表明鹵素離子會影響BPA的降解速率，並且NaBr增加幅度大於NaCl。再以預先已聚集的粉體TiO2奈米顆粒（ATiO2），即粒徑不隨鹽類有大變化，進行BPA的降解實驗時，觀察到BPA降解速率及NaCl濃度之間存在良好的相關性。在加入500 mM NaCl之前及之後，BPA分別在中性及酸性條件下，有最高的降解速率，這可能是由於pH會影響高濃度NaCl所衍生之次氯酸（hypochlorous acid，HOCl）及次氯酸根離子(hypochlorite ion，OCl－)之間的轉換。次氯酸的反應性遠高於次氯酸根離子，而在酸性條件下次氯酸的含量較豐富，因此可提高與BPA的反應性。添加0.2 M甲醇會降低CTiO2對BPA的降解，但同時含有NaCl時，則仍較只添加甲醇為高，所以氫氧自由基不是降解BPA的唯一因子。在日光照射之BPA降解實驗中，高NaCl/NaBr濃度也會增加CTiO2光降解BPA，而且LC-MS的分析顯示，在高濃度NaBr會產生單溴BPA（monobromo-bisphenol A）。BPA降解速率的增加是由於Cl－或Br－濃度的提高，而不是TiO2顆粒大小的增加。氯或溴離子於TiO2表面接受光照時，由鹵素離子衍生的鹵素自由基能夠與BPA反應，進而增加BPA的降解速率。比對不同苯環有機化合物於含NaCl之光降解，有機化合物的結構可能導致自由基攻擊的特異性。

Titanium dioxide nanoparticle (TiO2 NP) is one of the most common nanomaterials and used in several consumer goods. As a result, their fate such as aggregation and sedimentation and effects on the ecosystem and human health are of growing concern. Bisphenol A (4,4'-(propane-2,2-diyl)diphenol, BPA) has been used for a long period of time, however, its exposure could result in endocrine disruption. This study aims to conduct the aggregation and sedimentation of TiO2 NPs and to understand photodegradation mechanism by using TiO2 NP aggregates as a catalyst and BPA as a model molecule. The particle concentrations of commercial TiO2 (CTiO2) NPs did not affect their stability. The CTiO2 NPs aggregated and settled down at pH that closed to the point of zero charge around 6.0. The size of CTiO2 increased with the increasing concentrations of NaCl and Na2SO4, and their critical coagulation concentrations were 100 and 1.5 mM, respectively. The magnitude in aggregation and sedimentation of CTiO2 in the presence of Na2SO4 was higher than that in NaCl. After removing NaCl, the sizes of aggregated CTiO2 were reduced to 85~100 nm. Among all SRHA concentrations, CTiO2 NPs aggregated and settled down at 20 mg/L SRHA. When mixing these above factors together, aggregation and sedimentation occurred except for the condition of pH 7, 200 m M NaCl and 10 mg/L SRHA. The degradation rate of BPA increased with increasing NaCl and NaBr concentrations. However, with 500 mM NaNO3, the degradation kinetics of BPA decreased, which indicates that only halide ions eventually affect BPA degradation rate. The enhanced effect by NaBr is higher than NaCl. In the BPA degradation experiments conducted by pre-aggregated TiO2 NPs, whose size does not change a lot with electrolytes, a good correlation between BPA degradation rate and NaCl concentration was observed. BPA had higher degradation rate under neutral and acidic conditions before and after adding 500 mM NaCl compared to basic conditions. This may be because pH affects the transformation between hypochlorous acid (HOCl) and hypochlorite ion (OCl－) generated from high concentration of NaCl. The reactivity of HOCl is much higher than that of OCl－. HOCl is more abundant in acidic conditions than in basic conditions, thereby increasing the degradation of BPA. The decrease of BPA degradation with 0.2 M methanol was higher than that with NaCl and 0.2 M methanol, indicated that hydroxyl radical was not the only factor for BPA degradation. Under sunlight, the degradation of BPA by CTiO2 NPs with NaCl/NaBr also increased. Furthermore, LC-MS analysis showed the formation of monobromo-bisphenol A. Halogen radicals produced from photocatalytic reaction of halide ions on TiO2 NPs were capable of reacting with BPA, leading to the increase of BPA degradation. Compared to different aromatic compounds, the specificity of radical attack may be due to the configuration of organic compounds. The increase in the degradation rate of BPA is due to the increasing Cl or Br ion concentration instead of TiO2 particle size.