Wastewater treatment as well as biological activities require efficient and cost-effective alternative materials which motivates the interest of scientists for different types of nanomaterials. Nanomaterials are the best class of materials which can be used as adsorbents to treat wastewater and equally can be utilized as antioxidant, antimicrobial agents. The present study involves the synthesis of metal oxide nanoparticles (Co3O4, CuO and Mn2O3 NPs) by hydrothermal approach followed by thermal decomposition of their respective metal carbonates at their scrupulous decomposition temperature. The prepared NPs were characterized using scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and X-ray diffraction (XRD). The porous, spherical/irregular texture of prepared NPs was confirmed by SEM. Further, XRD analysis validated the formation of smaller size crystals of pure Co3O4, CuO and Mn2O3 NPs. Thermal stability of as prepared Co3O4, CuO and Mn2O3 NPs was investigated by TGA. The adsorptive behaviour of all the three NPs was investigated against the removal of an azo dye tartrazine (TZ). The optimum conditions for TZ removal were investigated using well established adsorption parameter like pH, shaking speed, temperature, initial TZ concentration and contact time. The maximum Langmuir adsorption capacity of TZ onto Co3O4, CuO and Mn2O3 NPs was found to be 204.3, 184.7 and 177.6 mg/g respectively. Langmuir, Freundlich and Temkin adsorption isotherms were applied to TZ adsorption data and results indicated that the Freundlich model was best fitted having higher correlation factor (r2) indicating multilayer adsorption. Kinetic studies revealed that TZ adsorption onto all three NPs was better descried by pseudo-second-order (PSO) kinetic model. Antimicrobial nature of Co3O4, CuO and Mn2O3 NPs was studied against fungal and bacterial strains via disk diffusion method and CuO NPs were found with highest inhibition (31 mm and 20 mm) against Staphylococcus aureus and Candida albicans. Antioxidant behaviour of Co3O4, CuO and Mn2O3 NPs was observed against DPPH (2, 2-diphenyl-1-picrylhydrazyl) radical and highest inhibition (66%) of Co3O4 was observed as compared to Mn2O3, CuO. In short, these multifunctional NPs can be used for pollutant removal and equally can be utilised for biological activities. Display omitted