Integrated response surface methodology for the optimization of green-synthesized MnO2 nanoparticles and their application in phenolic pollutant removal

The sustainable green synthesis of MnO 2 nanoparticles (NPs) was optimized using response surface methodology (RSM) based on a central composite design (CCD). The developed quadratic model was highly significant (F = 21.28, p < 0.0001) with strong predictability (R 2 = 0.9551). The optimized synthesis conditions for the reproducible formation of MnO 2 NPs were identified as 20 g precursor, 500 mL extract, 75 °C, and pH 6. The synthesized NPs displayed characteristic absorption maxima at 266 nm and were phase-pure α-MnO 2 with a tetragonal structure, nanoscale size (7–8 nm), and defect-rich crystalline nature. Surface characterization proved good colloidal stability (− 28.8 mV) along with the presence of phytochemical-derived functional groups, indicating effective bio-reduction and capping. Under the optimized conditions, the synthesized MnO 2 NPs displayed excellent efficiency for the removal of 4-Chlorophenol (98.47%) and Bisphenol A (97.61%). RSM analysis proved that the adsorbent dosage and contact time were the dominant factors for the removal of pollutants. Kinetic studies proved that the removal of pollutants followed a pseudo-second-order model and proved the involvement of chemisorption in the removal process. Isotherm and thermodynamic analyses revealed favorable, spontaneous, and endothermic adsorption, with a higher affinity toward 4CP. The high qmax values of 16.26 mg g -1 for 4CP and 11.93 mg g -1 for BPA indicate the superior adsorption capacity of MnO 2 NPs. Mechanistic investigations using HPLC, LC–MS/MS, and FT-IR proved a synergistic adsorption–oxidation pathway with the involvement of reactive oxygen species (ROS), leads to progressive degradation into low-molecular-weight intermediates. The nanoparticles retain > 90% removal efficiency after five reuse cycles, indicates good stability and reusability. Overall, this study demonstrates a green and scalable synthesis approach for MnO₂ NPs with dual adsorption–catalytic functionality, offering an effective strategy for the removal of phenolic contaminants from aqueous systems.