Green synthesis of N-doped ZnO@Zeolite nanocomposite for the efficient photo-adsorptive removal of toxic heavy metal ions Cr⁶⁺ and Cd²⁺

This research addresses the escalating issue of heavy metal ions contamination in water sources, primarily caused by rapid industrial development. Notably toxic, even at very small quantities of Cr⁶⁺ and Cd²⁺ heavy metal ions can prove fatal to both human and aquatic life. To combat this, a novel N-ZnO@Zeolite nanocomposite was successfully synthesized using Azadirachta indica plant leaf extract through the green method. The nanocomposite exhibited remarkable potential for removing Cr⁶⁺ and Cd²⁺ ions from aqueous solutions. The research primarily revolved around assessing the photocatalytic reduction efficiency of N-ZnO@Zeolite for Cr⁶⁺ and Cd²⁺ heavy metal ions, considering varied factors such as pH, photocatalyst dose, pollutant concentrations, and temperature. The findings demonstrated excellent removal capabilities, achieving 93% for Cr⁶⁺ and 89% for Cd²⁺ ions. Optimal conditions were observed at pH approximately 2 for Cr⁶⁺ and 6 for Cd²⁺, stirring for 60 min in an incubator at 45 °C, with 15 mg catalyst dose. The photocatalytic reduction kinetics were described by the first-order kinetics and Langmuir-Hinshelwood model with R² (0.98982 and 0.9955), K (0.89786 h⁻¹ & 0.87382 h⁻¹), and t_1/2 (0.7718 h or 46 min. & 0.7930 h or 48 min.) for Cr⁶⁺ and Cd²⁺, respectively. The isotherm models Langmuir and Freundlich are in good agreement with the experiment results, according to the equilibrium tests. Using a green synthesized photocatalyst for up to 10 cycles proved its long-term sustainability and excellent efficiency for environmental and industrial applications in the future.