Metal oxide nano-cluster (polyoxometalates) as a versatile material for treatment of wastewater from textile dyes: A review

Abstract Textile dye-containing wastewater is a major challenge today due to its high toxicity and chemical stability. Polyoxometalates (POMs) are anionic polynuclear metal-oxygen complexes made of early transition metals in high oxidation states, typically built from corner and edge-sharing MO6 octahedral units. These metal oxide polyanions are widely recognized for traits such as strong redox behaviour, structural tunability and catalytic activity under light and electrochemical conditions. This review provides a detailed overview of current studies on the application of POMs and POM-based materials, such as MOF-composites, for the degradation of textile dyes from wastewater. Dyes are classified based on their charge, i.e., cationic dyes - positively charged, anionic dyes - negatively charged and non-ionic dyes - neutral dyes, with literature-reported studies focusing on Methylene Blue, Malachite Green, Rhodamine B, and Crystal Violet as cationic dyes, and Congo Red and Methyl Orange as anionic dyes. Azo dyes are discussed separately in detail due to their large-scale usage in the present-day textile industries. The mechanisms involved in dye removal - adsorption, photocatalytic, electrochemical process, and Fenton-like reaction are discussed. Studies on electro-Fenton and photoelectron-Fenton processes, incorporating POMs have shown extremely high efficiencies in the degradation of industrial dyes because of their enhanced reactive oxygen species generation and enhanced catalytic stability. Factors affecting dye degradation, initial dye concentration and adsorbent concentrations, the effect of pH change and temperature dependence are also explained briefly. Additionally, adsorption isotherms, adsorption kinetics, and desorption studies are discussed to test dye-POM interactions and material reusability. The review highlights the potential of POM-based systems for efficient dye degradation and outlines the direction for future research.