A review of some metal-oxide based nanocomposites for photocatalytic treatment of wastewater

The photocatalytic activity of nanosized composite materials based on some common metal-oxides has been reviewed in the context of their potential application in the treatment of wastewater. A large volume of published data has been systematically analysed to understand the process of photocatalytic degradation under various combinations of the material, dye and source of excitation. The quantities taken into consideration for the analysis are the average particle size, apparent rate constant ( kobs ), and maximum percent degradation achieved. Semiconducting titanium dioxide (TiO2), zinc oxide (ZnO) and copper oxide (CuO) were identified as the three best photocatalysts that can be used after some meticulous modifications, in the treatment of wastewater under visible light irradiation. It was also concluded that the best performance can be obtained with photocatalyst nanoparticles (NPs) of average size in the range of 20 to 70 nm. Among the photocatalysts reviewed, the best degradation was produced by bismuth-sulphur co-doped TiO2 NPs of around 7 nm average particle size. With a rate constant as high as 6.08 x 10-2 min-1, this material produced nearly 100% degradation of Indigo Carmine within 40 min under visible light. The ZnO NPs of 40-70 nm average size degraded nearly 99% of Malachite green dye under ultraviolet (UV) irradiations in just 40 min with a very high rate constant of 11.10 x 10-2 min-1. CuO NPs, synthesised through green methods, produced nearly 95% degradation of Methylene blue (MB) in 2 h, with a rate constant of 2.62 x 10-2 min-1 under solar irradiation.