Isotherm and kinetic studies for the adsorption of methylene blue onto a novel Mn₃O₄-Bi₂O₃ composite and their antifungal performance

Metal oxide-based adsorbents are quite in for wastewater treatment because of their selectivity, stable structure and very low solubility in aqueous systems. To explore the adsorption of methylene blue (MB), Mn₃O₄-Bi₂O₃ adsorbents were made using a wet-impregnation technique with various concentrations of Mn₃O₄. The presence of Mn₃O₄ contents on the surface of monoclinic Bi₂O₃ was confirmed through representative scanning electron micrographs. The diffractions pertaining to cubic Mn₃O₄ and monoclinic Bi₂O₃ were noticed in the XRD pattern of 5% Mn₃O₄-Bi₂O₃ which confirm the composite nature of the adsorbent. XPS analysis revealed the existance of Bi 4f, Bi 4d, Bi 4p, Bi4s, and Mn 2p core levels in Mn₃O₄-Bi₂O₃. The adsorption study divulged highest efficiency (∼95% and qe = ∼1.4 mgg^-1) of 5% Mn₃O₄-Bi₂O₃ composite among other contestants in removing 30 ppm MB at 28 (Formula presented.) C, pH 7 and 250 rpm. In addition to the determination of adsorption ability, the effect of preliminary dye concentration (5, 10, 20, and 30 ppm) and contact time (0.5–6 h) on the removal efficiency of prepared adsorbents were also monitored. The adsorption data from the batch experiments were evaluated using the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich-Kaganer (DRK) adsorption isotherms and pseudo 1^st and 2^nd-order kinetic models. The fitting of adsorption isotherms and kinetic models revealed the formation of adsorbate’s monolayer on the surface of adsorbents through the process of chemisorption. Through FTIR measurement, the MB adsorption onto the effective adsorbent (5% Mn₃O₄-Bi₂O₃) was also confirmed. Moreover, TGA analysis showed ∼1.5% weight loss by 5% Mn₃O₄-Bi₂O₃ before MB adsorption whereas ∼2.6% weight loss was noticed after dye adsorption onto the adsorbent. The antifungal activity was evaluated against the fungi A. solani and M. fructicola using the agar well diffusion technique. The 5% Mn₃O₄-Bi₂O₃ composites have exceptional antifungal characteristics compared to Bi₂O₃ and Mn₃O₄, with zone inhibition values of 58.6 and 53.9 mm, respectively.