Study of adsorption of anionic dyes over biofabricated crystalline α-MnO₂ nanoparticles

The leaf extract of Ficus retusa plant was used for fabrication of α-MnO₂ nanoparticles (NPs). The extract was utilized as a reducing agent for green synthesis of nanomaterial. The synthesis of nanocrystals was confirmed using different analytical techniques such as field emission scanning electron microscopy (FESEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD) spectroscopy and thermogravimetric analysis (TGA). The synthesis of NPs was studied over a wide range of temperatures from 80 to 800 °C. It was found that perfectly crystalline α-MnO₂ NPs were successfully synthesized at 800 °C. The synthesized NPs were applied as an adsorbent for adsorption of azo dyes such as methyl red (MR) and methyl orange (MO) which are released as wastes from industries into water bodies and pollute the water. The removal efficiency was analysed and optimized depending on various parameters like pH, concentration of NPs, and contact time. The experimental data was explained by three isotherm models, viz. Langmuir, Freundlich and Temkin isotherms. Kinetic and thermodynamic studies of adsorption were also carried out, which depicted that the adsorption process of both dyes was exothermic in nature and followed pseudo-second-order kinetics. The results confirmed that NPs are easily fabricated through a green route and prove to be an excellent adsorbent for the removal of MO and MR dyes from their aqueous solutions. The maximum adsorption capacity of NPs synthesized was found to be 116.1 mg g^-1 and 74.02 mg g^-1 for MO and MR dyes, respectively. Graphical abstract.