Green synthesis of ZnO nanoparticles using Nauclea latifolia fruit extract for adsorption of Congo red

Abstract

This study demonstrates the synthesis of ZnO nanoparticles (NPs) by a green approach using an aqueous extract of Nauclea latifolia fruit as a reducing and stabilizing agent. The synthesized nanoparticles were subsequently engaged for the removal of Congo red (CR) dye from the aqueous solution. X-ray diffraction (XRD) revealed the hexagonal wurtzite structure of the particles with an average crystallite size of 10.05 nm. Fourier transform infrared (FTIR) analysis showed characteristic peaks indicating the presence of ZnO, along with functional groups corresponding to aldehyde, carboxylic, and hydroxyl groups. Scanning electron microscopy (SEM) revealed an agglomerated surface morphology, while transmission electron microscopy (TEM) analysis showed grain sizes ranging from 9.22 to 12.54 nm. Brunauer-Emmett-Teller (BET) analysis disclosed a high specific surface area of 268.22 m2/g for the nanoparticles. UV–Vis spectroscopy showed a broad absorption peak around 365 nm. Batch adsorption studies demonstrated that the optimal conditions for the adsorption of CR onto the NPs are 0.05 g of ZnO NPs, duration of 60 min, pH 3, 5 mg/g of CR dye concentration, and 25 °C. Non-linear adsorption modelling revealed that Freundlich isotherm and general-order kinetic models best fit the adsorption data. The thermodynamic investigation confirmed the exothermic, spontaneous, and feasible nature of the adsorption process, primarily driven by physical forces. Overall, this study establishes the efficiency of green-synthesized ZnO NPs for CR dye removal from aqueous solutions, presenting a sustainable, cost-effective, and eco-friendly alternative for wastewater treatment.