Phytosynthesis of magnetite (Fe3O4) nanoparticles for arsenite removal from waste water: Characterization, isotherm and kinetic studies

Abstract

Magnetite nanoparticles (Fe₃O₄NPs) was phytosynthesized using aqueous extract of unripe plantain peel and salts of iron. The technique is simple, cost-effective and eco-friendly. Synthesized nanomagnetite was characterized by Fourier-transform Infrared (FTIR) Spectroscopy, X-ray diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive X-ray (SEM/EDX), Transmission Electron Microscopy (TEM), Sear’s surface area and Isoelectric point. The magnetite nanoparticles are well dispersed with spherical surface morphology and mean particle size of 10.2 nm. The nanomagnetite is crystalline in nature with inverse spinel cubic structure and mean crystallite size of 12.4 nm. The specific surface area and isoelectric point of the nanomagnetite are 243.8 m²/g and 7.2 respectively, which make it a potential adsorbent material. Arsenite’s adsorption unto Fe₃O₄NPs was evaluated by batch equilibrium method. The empirical equilibrium data were fitted into Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models to give insight into the pattern of adsorption of arsenite unto Fe₃O₄NPs. The best fit of isotherm model to empirical equilibrium data was obtained using error validity function and regression correlation coefficient value. Based on values of R², the Langmuir model is a better fit between empirical and simulated equilibrium data but D-R model best explains the adsorption process due to its least error validity function (${\chi }^2$ = 41.907). The kinetic data obtained at different contact time were analyzed using pseudo-first-order, pseudo-second-order, Elovic, intraparticles diffusion and Boyd diffusion models. The empirical rate data obey the pseudo-second-order model based on high R² which indicate possibility of two reaction centres on the adsorbent. However, least ${\chi }^2$ value of 3.151 for Elovic kinetic model shows the adsorption process occurred on heterogenous surface by chemisorption. Although the rate-controlling step in arsenite’s adsorption unto Fe₃O₄NPs is multi-stepped but the rate of adsorption is controlled slightly by film diffusion (Boyd diffusion model). Adsorption of arsenite unto Fe₃O₄NPs was optimized and was found to be affected by initial concentration, magnetite dosage, solution pH, and contact time. Maximum adsorption capacity of 483.513 mg/g was achieved at pH 8 over contact time of 60 min at 28 ˚C. The negative value of Gibbs free energy (-17.346 J/mol) indicates the adsorption of As(III) unto Fe₃O₄NPs is spontaneous and occurred by physisorption due to the low value of energy of adsorption (E = 22.3607 J/mol). The effect of co-existing anions on adsorption of arsenite was investigated and results showed marginal negative effect by 3.02–7.26 % with sulphate ion exerting more effect on arsenite’s adsorption unto magnetite nanoparticles. Also, recyclability test indicated good reusability of about 32 % which implies reduction in arsenite’s removal efficiency of Fe₃O₄NP from 46.1 % to 31.74 % over 5 adsorption-desorption cycles. Thus, magnetite nanoparticles is a suitable adsorbent for arsenite removal from aqueous waste water due to its water-stability, high surface area, and good adsorption strength.