Eco-friendly Synthesis and Adsorptive Performance of ZnO/CuO and MnO/CuO Bimetallic Nanoparticles for Heavy Metal Removal from Aqueous Solutions

Abstract

This study reports the green synthesis, characterization, and adsorptive performance of ZnO/CuO and MnO/CuO bimetallic nanoparticles for the removal of heavy metals from aqueous solutions. The nanoparticles were synthesized using Anacardium occidentale (cashew leaf) extract as a natural reducing and stabilizing agent, providing an eco-friendly alternative to conventional chemical synthesis. UV–Vis spectroscopy confirmed nanoparticle formation with surface plasmon resonance peaks at 400 nm for ZnO/CuO and 500 nm for MnO/CuO. The materials were further characterized by FTIR, SEM, XRD, and XRF, which revealed functional groups responsible for reduction, crystalline face-centered cubic structures, and nanosized morphologies with high metal oxide composition. The adsorption efficiency of the nanoparticles toward Cd²⁺, Fe³⁺, and Cr³⁺ ions was systematically investigated under varying parameters such as pH, contact time, dosage, temperature, and concentration. Optimum adsorption occurred at pH 6, 0.4 g dosage, and 45 °C, with removal efficiencies exceeding 95% for all metal ions. Adsorption equilibrium data fitted both Freundlich and Langmuir isotherm models, suggesting the coexistence of multilayer and monolayer adsorption processes. Kinetic analysis indicated that the adsorption followed a pseudo-second-order model, implying chemisorption mechanisms. Thermodynamic parameters (ΔG < 0, ΔH > 0, ΔS > 0) confirmed that the adsorption process was spontaneous, feasible, and endothermic, with increased randomness at the solid–solution interface. These results demonstrate the potential of A. occidentale-derived ZnO/CuO and MnO/CuO nanoparticles as sustainable and highly efficient adsorbents for heavy metal remediation in wastewater treatment.