Green synthesis of nickel oxide nanoparticles using Parkia biglobosa flower extracts for the removal of malachite green and acid red 88 dyes from aqueous solution

This study presents the green synthesis of nickel oxide nanoparticles (NiO NPs) using Parkia biglobosa flower extract as reducing and stabilizing agent for the adsorption of malachite green (MG) and acid red 88 (AR88) from aqueous solutions. X-ray diffraction revealed the face-centered cubic structure of the particles with an average crystallite size of 17.14 nm. Fourier transform infrared analysis showed characteristic peaks indicating the presence of NiO, along with absorption peaks corresponding to carboxylic, carbonyl, and hydroxyl groups. Scanning electron microscopy revealed an agglomerated surface morphology, while transmission electron microscopy analysis showed an average grain size of 15.25 nm. Brunauer-Emmett-Teller analysis disclosed that nanoparticles exhibited surface area of 88.56 m²/g. UV-Vis spectroscopy showed a broad absorption peak around 313 nm. Batch adsorption studies showed the optimal conditions for the adsorption of AR88 onto the NPs are 1.0 g of NPs, 60 min duration time, pH 3, 5 mg/L of the dye concentration, and 25 °C, while that of MG onto the NPs are 1.0 g of NPs, duration of 60 min, pH 9, 5 mg/L of the dye concentration, and 55 °C. Non-linear adsorption modeling revealed that Freundlich isotherm and general-order kinetic models best fit the adsorption data. Thermodynamic studies revealed that AR88 adsorption was exothermic, driven by physical forces while MG adsorption was endothermic, driven by chemical forces; both processes were spontaneous and feasible. This study demonstrates the effectiveness of green-synthesized NiO NPs in removing toxic dyes from wastewater and highlights their potential as sustainable adsorbents in environmental remediation applications.