Adsorption of Pb²⁺ using biosynthesized ZnO nanoparticles derived using Azadirachta indica (neem) leaf extract

In the present manuscript, zinc oxide nanoparticles (ZnO NPs) were synthesized using a cost-effective and environmentally benign way utilizing Azadirachta indica (neem) leaf extract (AILE). Rietveld refined X-ray diffractogram of biosynthesized ZnO NPs revealed wurtzite-type hexagonal crystallinity with space group P6(3)mc. The optical spectrum of ZnO NPs carried out using ultraviolet visible-diffuse reflectance spectroscopy (UV-DRS) showed an energy band gap of 3.35 eV. FTIR spectrum of the ZnO NPs fabricated via green synthesis route showed a broad anomaly at 3451 cm(-1) (hydroxyl group) belonging to phenolic content present in AILE. Microstructural properties were obtained using transmission electron microscopy which showed pseudo-spherical formation of ZnO NPs with an average diameter of particle as 8.5 nm. The TGA-DTA curve of ZnO NPs depicted the thermal stability of synthesized NPs below 405 degree celsius. The study investigated the impact of different parameters, specifically contact time and pH, on the adsorption process on adsorption of Pb2+ using ZnO NPs. It was observed that the adsorption process exhibited a strong dependence on pH. To analyze the kinetics of the process, experimental data was assessed using both the pseudo-first-order and pseudo-second-order kinetic equations. The pseudo-first-order model fit of the data at 100 ppm of Pb2+ concentration gives the R-2 = 0.863 with k ' = 2 x 10(-2) min(-1), respectively, whereas the pseudo-second-order model gives the better fit with R-2 = 0.976 which is higher as compared to pseudo-first-order (R-2 = 0.863). Additionally, the equilibrium data was modeled using Langmuir and Freundlich isotherm models. The value of the equilibrium constant (K) and R-2 value with the Langmuir model were found to be 0.99 and 0.209, respectively, and for the Freundlich model were 3.176 and 0.97, respectively. The experimental results showed good agreement with the Langmuir isotherm model.