Visible light driven doped CeO₂ for the treatment of pharmaceuticals in wastewater: A review

The high-level contamination of pharmaceuticals in aquatic environment, and their toxicities is a serious issue. This review highlights the use of ceria photocatalyst for treatment of pharmaceuticals. Cerium oxide (CeO₂) with high oxygen storage, ecofriendly properties, reusability, and photostability contrary to other metal oxides photocatalysts is reportedly a better choice. However, ceria with high band gap energy show photoactivity mainly under UV light. This review highlights pharmaceuticals contamination in water, their contamination level, and toxicities and properties of CeO₂ and different approaches used for extending photoactivity of CeO₂ under visible irradiation. Metals and non-metals doping is found to promote greatly photoactivity of CeO₂ under visible irradiation by narrowing band gap, shift in absorption edge to visible region, crystal defects and yield of oxygen vacancy, lower recombination of conduction band electrons and valence band holes and increasing surface area. The visible irradiation of CeO₂ is found to produce hydroxyl radical ([rad]OH) and superoxide radical (O₂[rad]^–) which contribute in pharmaceuticals degradation. The electron paramagnetic resonance spectroscopy and radical scavenger studies confirmed the formation of reactive oxygen species from CeO₂ photoactivation. Doping was found to incorporate into the lattice of CeO₂ and improve reusability and stability of CeO₂ photocatalyst. The suggested mechanisms involved in the treatment of pharmaceuticals through [rad]OH and O₂[rad]^– is discussed. Furthermore, the outlook and future challenges in the use CeO₂ for photocatalytic degradation of pharmaceuticals and other organic pollutants are evaluated.