The amount of organic pollutants in wastewaters discharged by various industries is increasing every year and is causing serious global environmental problems. These pollutants include a wide range of persistent organic chemicals, such as pharmaceuticals and endocrine-disrupting compounds. We are now faced with the challenge of removing these compounds from their effluents before they are discharged. In this regard, photocatalysis is a promising technology for wastewater treatment as it offers many advantages over conventional and other advanced treatment options. Traditional physical techniques (adsorption on activated carbon, ultrafiltration, reverse osmosis, coagulation by chemical agents, ion exchange on synthetic adsorbent resins, etc.) have been used for the removal of these pollutants, but these methods only succeed in transferring organic compounds from water to another phase, thus creating secondary pollution. Microbiological or enzymatic decomposition, biodegradation, ozonation and advanced oxidation processes such as Fenton (Fe²⁺ + H₂O₂), photo-Fenton (Fe²⁺ + H₂O₂ + UV), H₂O₂/UV processes have also been used for organic pollutants removal from wastewaters, but these methods are either ineffective or too expensive. On the other hand, photocatalysis is energy saving and is able to completely oxidize the organic pollutants to water and carbon dioxide. Metal oxide semiconductor nanoparticles have been used as high activity photocatalysts; such as ZnO, TiO₂, CdS, Fe₂O₃ and HNbO₃. These semiconductors, however, can only be excited by UV light due to their large band

　

　

　

gaps. For better utilization of sunlight and indoor illumination, it is desirable to develop photocatalysts that can be excited by visible light. In this project we have developed a series of highly efficient visible light photocatalysts based on K₂Nb₄O₁₁ and related niobium oxides. These semiconductors are doped with various metals and non-metals. Doping with metals can lead to increased photocatalytic efficiencies, while doping with non-metals can shift the optical absorption of the photocatalysts to the visible region. We have successfully prepared a series of doped niobium oxides which show high photocatalytic activity in the degradation of organic pollutants such as Orange G (an organic dye) and Bisphenol A (an endocrine disruptor) under visible light (λ > 400 nm). For example, the photocatalytic activity of N-doped K₂Nb₄O₁₁ is much higher than that of undoped K₂Nb₄O₁₁ and TiO₂ P25, indicating the great positive effect of nitrogen doping. Another advantage of doped K₂Nb₄O₁₁ is that they readily settle into the bottom of the solution after photocatalysis, and can be readily recovered and reused without loss of photoactivity. On the other hand, TiO₂ cannot be separated from the solution after photo-catalysis even by centrifugation and therefore it is difficult to be reused. Our project should lead to the development of a novel and practical nanotechnology for environmental remediation.

Prof Tai-chu LAU
Department of Biology and Chemistry
City University of Hong Kong
bhtclau@cityu.edu.hk