Characterization and Temperature-Dependent Adsorption Potential of Hydrothermally Synthesized Manganese Oxide Nanoparticles

Water pollution usually results from discharge of untreated or partially treated human and industrial waste into water bodies, and nanomaterials are increasingly being recognized as a sustainable alternative remediation approach. The aim of the present study is to determine the structural properties and possible adsorption property of hydrothermally synthesized MnO2 nanoparticles. The synthesized nanoparticles were characterized using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Brunauer-Emmett-Teller surface area analysis, Fourier-transform infrared spectroscopy and UV-Visible spectroscopy. The nanoparticles primarily exhibited small crystallite sizes, structural disorder, and mostly amorphous material. Manganese was dominant in the nanoparticles as evidenced by surface shape and chemical composition but diminished with rising temperature. The surface area peaked at 110oC and decreased at higher temperatures possibly due to pore collapse and sintering effect, suggesting that annealing temperature had a significant impact on the mesoporous structure. FTIR and UV-Visible data suggest the presence of surface hydroxyl groups evidenced by moisture absorption, an O-Mn-O stretching that promotes hydrogen bonding and bandgap energies that allow for possible visible-light photocatalysis. Incorporating these results with existing research, the adsorption mechanism is expected to include electrostatic interactions controlled by surface charge, chemisorption via redox-active Mn sites, cation exchange regulated by potassium intercalation, and synergistic physisorption. The study also suggests that controlled thermal treatment enhanced adsorption performance as nanoparticles annealed at 90-110°C are expected to exhibit optimal adsorption qualities. Overall, findings from this study offer a starting point for producing efficient MnO2-based adsorbents capable of removing organic pollutants and heavy metals in aquatic environment.