Biowaste-to-nanoadsorbent conversion: ultrafast cationic dye capture by manganese oxide nanostructures derived from Chrysanthemum morifolium leaves

The urgent need to eliminate hazardous organic dyes from industrial wastewater has driven extensive research into environmentally benign and highly efficient adsorption technologies. Recently, nanoparticles produced from biological waste resources have emerged as attractive candidates because of their sustainability, low production cost, and strong pollutant-removal performance. In this work, manganese oxide nanoparticles (MnO2 NPs) were synthesized through a biowaste-derived route employing extract obtained from discarded biomass of Chrysanthemum morifolium leaf. Plant extracts, rich in diverse phytochemical compounds, acted both as a natural capping agent and reducing agent during the formation of nanoparticles. The environmentally friendly synthesis route produced highly pure MnO2 nanostructures with predominantly quasi-spherical morphology and particle dimensions of approximately 30–60 nm, accompanied by a microporous architecture. Textural analysis revealed a specific surface area of 72.9 m2 g−1, a total pore volume of 0.1036 cm³ g−1, and a mean pore width of 14.85 nm. These Mn NPs displayed remarkable efficiency toward the adsorption of Methylene Blue, reaching 99.9% dye elimination from an initial MB concentration of 7 mg L−1 within only 10 min at 293 K with an adsorbent loading of 0.012 g. Adsorption equilibrium data aligned strongly with the Langmuir model (KL = 3.364 L mg−1, R2 = 0.9951), suggesting uniform surface sites and single-layer dye coverage. Furthermore, adsorption kinetics closely matched the pseudo-second-order equation, as evidenced by the excellent fitting coefficient (R2 = 0.9997). Thermodynamic parameters demonstrated that the adsorption proceeded spontaneously and released heat, supported by negative ΔG° values and an enthalpy change of ΔH° = −18.366 kJ mol−1. The modest enthalpy magnitude further implies that dye uptake was dominated primarily by physisorption mechanisms instead of strong chemisorptive interactions. The adsorption process was mainly controlled by electrostatic attraction and other weak physical interactions on the porous MnO2 surface, resulting in rapid and efficient monolayer adsorption of the cationic dye. The MnO2 nanoparticles exhibited fast adsorption behavior and high efficiency in dye elimination, while their green synthesis route avoided the use of hazardous chemicals and dopants. These characteristics highlight their promise as a sustainable, low-energy, and scalable solution for wastewater remediation.