Abstract
Industrial production, urban growth, and agricultural intensification have been a source of toxic and persistent pollutants such as synthetic dyes, heavy metals, pharmaceutical compounds, pesticides, and pathogens to aqueous environments, challenging the conventional treatment processes for wastewater. Metal oxide nanoparticles (MONPs) such as TiO₂, ZnO, CuO, Cu₂O, Fe₃O₄, γ-Fe₂O₃, MnO₂, and SnO₂ have been considered as attractive materials for environmental applications due to their high surface area to volume ratios, adjustable bandgaps, abundance of surface active sites, superparamagnetic behavior, and high catalytic efficiency. The present review summarizes the synthesis methods of MONPs covering both top-down and bottom- up approaches with particular focus on sol-gel process, hydrothermal synthetic route, chemical precipitation, and green plant mediated route. The effects of fundamental operating parameters such as the pH of reaction, temperature, precursor concentration, and duration of contact on particle size, morphology, crystallinity and surface chemistry are discussed in detail. Various advanced characterization techniques such as UV-Vis spectroscopy, FTIR, XRD, SEM/TEM, BET analysis, DLS and zeta potential analysis are discussed to correlate structure-function properties. The review also describes in greater detail the two main postulated detoxification processes of interfacial adsorption (physisorption and chemisorption) and photocatalytic degradation through the generation of reactive oxygen species. The evaluation of the performances of the modi- fied MONP and the mixed-oxide heterostructure indicates a great efficiency in pollutant removal, with CuO obtaining fluoride adsorption capacity higher than 3000 mg/g and TiO2 degrading more than 98% of dyes. Nonetheless, ecotoxicity, photocorrosion, particle agglomeration, catalyst recovery, and reproducibility of synthesis constitute severe obstacles for large-scale application. In conclusion, prospective research directions that focus on greener synthesis, improved reusability, and reduced environmental impact are suggested to further progress MONP-based wastewater treatment technologies.