Green Synthesis and Characterization of ZnO Nanoparticles Using Brunfelsia americana Leaf Extract: Isotherm, Kinetics, Thermodynamic Insights and Reusability into Malachite Green Dye Removal from Aqueous Solutions

This study presents the eco-friendly approach for synthesizing zinc oxide nanoparticles (ZnO NPs) using Brunfelsia americana leaf extract as a novel bioreductant, serving dual roles as a reducing and stabilizing agent. The influence of solvent type, calcination conditions, and extract concentration on nanoparticle formation was systematically explored to optimize synthesis efficiency. A distinct color change and a UV–visible absorption peak near 360 nm confirmed the formation of ZnO NPs. Structural and morphological analyses using FTIR, XRD, SEM, and EDX revealed the presence of surface functional groups, a predominantly spherical shape, particle sizes ranging from 35 to 50 nm (average ∼45 nm), elemental composition, and a crystalline hexagonal wurtzite phase with a crystallite size of 36.09 nm and the specific surface area was calculated to be 29.63 m2/g (XRD / crystallite-based estimate). The prepared ZnO NPs were evaluated for their ability to adsorb malachite green (MG) dye from aqueous solutions. Optimal adsorption occurred under the conditions: pHZPC of 7.48, solution pH of 8.0, initial dye concentration of 30 mg/L, nanoparticle dosage of 0.3 g/L, contact time of 210 min and agitation speed of 300 rpm. Under these parameters, a maximum dye removal efficiency of 93.89% and an adsorption capacity of 9.39 mg/g were achieved. Reusability assessments showed efficiencies of 85.59% and 78.33% in the first and second cycles, respectively. Adsorption data aligned well with the Freundlich isotherm (R² = 0.9998) and Ho’s pseudo-second-order kinetic model (R² = 0.9958). Thermodynamic findings indicated physisorption as the dominant mechanism. Overall, the green-synthesized ZnO NPs demonstrate significant potential as a low-cost, sustainable adsorbent for dye removal, particularly in resource-limited regions such as Bangladesh.