| Summary: | 博士 === 國立臺灣大學 === 環境工程學研究所 === 99 === Titanate nanotubes are nano-materials with special properties, such as high specific area, adsorption capacity and strong photocatalytic capability; and they have been widely studied for applications in various fields recently. Titanate nanotubes can be synthesized within three hours by a microwave hydrothermal method, which is simpler and more time and energy efficient than conventional hydrothermal methods. This study aimed at evaluating the adsorption capacity and photocatalytic capability of titanate nanotubes, made by the microwave hydrothermal method, in removing some common pollutants from aqueous media, and exploring the reaction mechanisms.
Titanate nanotubes synthesized by a microwave hydrothermal method are mainly consist of sodium, titanium, oxygen, and hydrogen atoms, and their chemical structure can be represented as NaxH2-xTi3O7. The sodium content on the surfaces of titanate nanotubes was increased with irradiation power; this phenomenon could be attributed to the intercalation of sodium atoms in the TNT nanostructure which enlarges the space between inter-layers. The sodium ions on the surfaces can exchange with target ions, as well as be adsorbed onto the inner and outer surfaces of titanate nanotubes. However, the titanium ions on the surfaces of titanate nanotubes had minor effects on their adsorption and photocatalytic reactions.
Titanate tubes used in this study were synthesized from commercial titanium dioxide (Degussa P25). Lead, ammonia, and perfluorooctanoic acid (PFOA) were chosen as the target polluants. The research firstly evaluated the removal of lead ions from water by titanate nanotubes through adsorption. It then evaluated their photocatalyic capability in reducing concentrations of ammonia nitrogen and perfluorooctanoic acid in aqueous solutions. For the case of ammonia removal, the nanotubes were combined with cadmium sulfide to enhance their photocatalytic capacity. Effects were also made to discuss plausible mechanisms for the adsorption and photocatalyitc reactions.
The experimental results indicate that these titanate nanotubes are good adsorbents. Titanate nanotubes synthesized by the microwave hydrothermal method have specific surface area three times larger than that of Degussa P25 (50 m2/g). Due to their high specific surface areas, they could remove almost 100% of lead ions from water through adsorption and the reaction was fast and reached equilibrium within 30 minutes. The maximum adsorption capacity was found to be 2,000 mg Pb(II)/g of titanate nanotubes. The pHZPC of titanate nanotubes is around 3, thus they are more favorable in adsorbing cations under higher pH values and in adsorbing anions under acidic conditions. Although titanate nanotubes could be catalysts in photocatalytic removal of pollutants, they were not effective as expected. It might be caused by the fact that these reactions were heterogeneous in nature and the shielding effects caused by the porous structure of the nanotubes. About half of ammonia nitrogen and perfluorooctanoic acids in aqueous solutions could be removed by titanate nanotubes under UV irradiation, and the presence of the nanotubes as photocatalysts could enhance the completeness of the reactions.
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