GREEN-SYNTHESIZED METAL OXIDE NANOPARTICLES FOR PHOTOCATALYTIC DYE DEGRADATION: A COMPREHENSIVE REVIEW OF PLANT-MEDIATED ROUTES, DEEP EUTECTIC SOLVENTS, AND SURFACE ENGINEERING STRATEGIES FOR SUSTAINABLE WASTEWATER TREATMENT

Abstract

The classical treatment techniques used for conventional dyes such as adsorption, coagulation and membrane filtration require high energy and cost, and are only able to move the dye to a secondary waste stream; synthetic dyes produced by the textile, pharmaceutical and cosmetic industries pose a risk to aquatic ecosystems as well as to human health. The mineralization of organic dyes to end products that are harmless has led to the development of advanced oxidation technologies (AOT) based on heterogeneous photocatalysis using metal oxide nanoparticles (ZnO, TiO2, CuO and Fe2O3). The synthetic routes that use plant extracts, microorganisms and biomolecules as reducing and capping agents eliminating hazardous chemical reagents, are referred as green synthesis routes and are cost effective and eco-friendly. This review summarizes the recent publications on green synthesis methods of metal oxide nanoparticles, characterization techniques employed to confirm their structural and optical properties and the photocatalytic performance of nanoparticles on the degradation of some representative dyes, methylene blue, malachite green, Rhodamine B, Congo red, malachite green, and Reactive blue. It also examines the production of reactive oxygen species (ROS) in the reaction/process, the operating factors, which affect the degradation efficiency, the role of the capping organic layer on the degradation efficiency and the emerging application of Deep Eutectic Solvents (DES) as green reaction media. The conclusion of the review indicates that the three major challenges for the practical use of green photocatalysts for wastewater treatment technology are yet to be met by reproducibility, scale-up and limited visible light activity.