Photocatalysts for water purification.

Abstract

Advanced water purification methods are required to answer the growing demand for clean water throughout the world. Current methods of removing the pollutants rely on moving the pollutants from one place to another rather than breaking them down. The use of advanced oxidative processes (AOPs) presents a highly effective opportunity to achieve the full mineralisation of pollutants without the added cost of regeneration methods. Photocatalysts, such as titanium dioxide and zinc oxide, can be used as AOPs when activated by electromagnetic radiation in the form of ultraviolet and visible light. To facilitate the activation with visible light, titanium dioxide doped with rare earth elements was produced via a sol gel method. Both single doped and co-doped systems were investigated with efficiency determined by the percentage of degraded methylene blue over 48 hours under ultraviolet filtered visible light. The incorporation of rare earth ions restricted the growth of the more active anatase phase and the method produced highly agglomerated, sintered nano particles which exhibited as micron sized particles. The highest methylene blue removal rate achieved in 48 hours for a single doped system was 70% for the 1 mol% yttrium doped titanium dioxide. This was improved further on inclusion of 1 mol% praseodymium which showed an 86% removal of methylene blue over the same time period. The coating of known up-converting phosphors with the successfully developed doped titanium dioxide was investigated. Yttrium silicate doped with praseodymium and lithium, was found to be the most successful known phosphor when used with the commercially available P25 titanium dioxide. When coated with the doped titanium dioxide shell at a 2:1 ratio of phosphor to titanium dioxide, a methylene blue degradation of 94% was reached. Initial tests on the coating of titanium dioxide with the known up-converting phosphor showed that methylene blue was absorbed rather than broken down so was not developed further. An investigation into the incorporation of zinc oxide, both pure and doped with the same successful titanium dioxide system was carried out. Zinc oxide shells were coated onto doped titanium dioxide, the known up-converting phosphor and the doped titanium dioxide coated known phosphor. The crystalline form of zinc oxide was inhibited by the incorporation of rare earth ions, as with the titanium dioxide system, and from the thickness of the zinc oxide shell. The highest degradation achieved was a 91% removal rate for the ZnO-PrY:TiO2-PrY:Y2SiO5-Pr,Li core shell shell structure indicating there was no further improvement on incorporation of zinc oxide, either doped or un-doped.