Eco-friendly synthesis of zinc oxide nanoparticles via Pisum sativum seeds and their application in the adsorptive removal of Congo red dye

Abstract

This study reports the eco-friendly synthesis of zinc oxide nanoparticles (ZnO NPs) using Pisum sativum seed extract as a natural reducing and stabilizing agent. The biosynthesized ZnO NPs were characterized by UV-vis spectroscopy, XRD, FTIR, and FE-SEM, revealing predominantly spherical nanoparticles with uniform distribution, sizes of 30–50 nm, and aggregation driven by polarity and electrostatic interactions. Their adsorption performance was evaluated for the removal of Congo Red (CR) dye from aqueous solutions. Batch experiments demonstrated a maximum removal efficiency of ∼91% under optimal conditions (14 mg L⁻¹ dye concentration, pH 6, 35 °C, 0.015 g adsorbent dose, and 40 min contact time). Adsorption followed pseudo-second-order kinetics, indicating chemisorption, and fit both Langmuir and Freundlich isotherm models, suggesting a combination of monolayer and multilayer adsorption on a heterogeneous surface. Thermodynamic analysis confirmed the process to be spontaneous, endothermic, and associated with decreased randomness at the solid–solution interface, with enhanced efficiency at higher temperatures. These results highlight the potential of Pisum sativum-mediated ZnO NPs as an efficient and promising eco-friendly adsorbent for wastewater treatment and dye removal. Further studies on desorption and regeneration cycles are required to evaluate long-term adsorbent sustainability.