A review of solar and visible light active TiO₂ photocatalysis for treating bacteria, cyanotoxins and contaminants of emerging concern

Research into the development of solar and visible light active photocatalysts has been significantly increased in recent years due to its wide range of applications in treating contaminants of emerging concern (CECs), endocrine disrupting compounds (EDCs), bacteria and cyanotoxins. Solar photocatalysis is found to be highly effective in treating a wide range of CECs from sources such as pharmaceuticals, steroids, antibiotics, phthalates, disinfectants, pesticides, fragrances (musk), preservatives and additives. Similarly, a number of EDCs including polycyclic aromatic hydrocarbons (PAHs), alkylphenols (APs), bisphenol A (BPA), organotins (OTs), volatile organic compounds (VOCs), natural and synthetic estrogenic and androgenic chemicals, pesticides, and heavy metals can be removed from contaminated water by using solar photocatalysis. Photocatalysis was also found effective in treating a wide range of bacteria such as Staphylococcus aureus, Bacillus subtilis, Escherichia coli, Salmonella typhi and Micrococcus lylae. The current review also compares the effectiveness of various visible light activated TiO₂ photocatalysts for treating these pollutants. Doping or co-doping of TiO₂ using nitrogen, nitrogen-silver, sulphur, carbon, copper and also incorporation of graphene nano-sheets are discussed. The use of immobilised TiO₂ for improving the photocatalytic activity is also presented. Decorating titania photocatalyst with graphene oxide (GO) is of particular interest due to GO's ability to increase the photocatalytic activity of TiO₂. The use GO to increase the photocatalytic activity of TiO₂ against microcystin-LR (MC-LR) under UV-A and solar irciation is discussed. The enhanced photocatalytic activity of GO-TiO₂ compared to the control material is attributed to the effective inhibition of the electron-hole recombination by controlling the interfacial charge transfer process. It is concluded that there is a critical need for further improvement of the efficiency of these materials if they are to be considered for bulk industrial use.