| Summary: | This work used a variety of experimental studies to explore the elimination of cadmium and lead ions from aqueous solutions using a novel method for the biosynthesis of nanoparticles of zinc oxide sorbents (ZnO-NPs) from mangrove leaf extract. The influences of important factors affecting the adsorption technique were determined, including the pH value, contact duration, the initial concentration of metal ions, nano-adsorbent dose, different temperatures, and interfering ions. To confirm the formation of synthesized ZnO NPs and validate the properties of green-synthesized sorbents, a variety of analytical methods were used, including UV–vis spectroscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). The results showed that the average diameter of the ZnO-NPs was approximately 29.1 nm (spherical at the nano-size regime). The adsorption reaction rate was examined by comparing pseudo-second order against pseudo-first order templates. From the observed records, the adsorption reaction of Cd<sup>2+</sup> and Pb<sup>2+</sup> on the ZnO-NPs fitted well with the pseudo-second order template. Freundlich, Langmuir, Dubinin–Radushkevich, and Tempkin equilibrium isotherm models were used to evaluate the sorption of Cd<sup>2+</sup> and Pb<sup>2+</sup> onto the sorbent material. Based on the parameters extracted from each model, as well as the model-fitting values, the preferential isotherms for Pb<sup>2+</sup> and Cd<sup>2+</sup> ion adsorption on ZnO-NPs were the Dubinin–Radushkevich and Langmuir models, respectively. ZnO-NPs have the potential to be used as an effective and promising adsorbent material for eliminating metal ions from water solutions.
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