Synthesis of Silica Supported Iron Oxide Nanoparticles for Hexavalent Chromium Removal from Aqueous Solutions

Abstract

Iron oxide nanoparticles (NPs) were synthesized by the chemical precipitation method using 1:2 ferric chloride hexahydrate (FeCl₃.6H₂O) and ferrous sulphate heptahydrate (FeSO₄.7H₂O) precursors and ammonium hydroxide (NH₄OH) precipitating agent. These NPs were evaluated for their performance as adsorbents for hexavalent chromium ions (Cr (VI)) in aqueous solutions. Silicon dioxide gel (SiO₂) was also incorporated during synthesis to improve the adsorption characteristics of the NPs. The NPs (with out and with SiO₂ gel (iron oxide NPs-SiO₂) were characterized by XRD and FTIR spectroscopy. By XRD, it was evidenced that the iron oxide NPs and iron oxide NPs-SiO₂ adsorbents were mixed oxides of magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) with small crystallite size of 13.5 nm and 12.8 nm respectively. The mixed oxides were formed as a result of the synthesis parameters and drying temperature at 150⁰C in air. The change of the black color of the just synthesized NPs to orange brown also inferred the transformation of Fe₃O₄ to γ-Fe₂O₃. Further evidence of mixed oxides is provided by the FTIR analysis wherein the characteristic bands of γ-Fe₂O₃ around 582 cm^-1 and the distinct high-frequency shoulder in the proximity of 635 cm^{- 1} to 642 cm^-1 are evidenced besides those of Fe₃O₄.The effect of different parameters such as pH, Cr (VI) ions concentration, adsorbent dose and contact time on the efficiency of adsorption showed that the iron oxide NPs-SiO₂ adsorbent showed better performance for the removal of Cr (VI) ions under acidic pH at room temperature compared to the iron oxide NPs adsorbent. The maximum uptake of Cr (VI) ions by the iron oxide NPs-SiO₂ adsorbent increased from 80% to 96.5% by decreasing its initial concentration from 60 mg l^-1 to 20 mg l^-1 at optimum conditions. Analysis of the adsorption isotherms of the iron oxide NPs-SiO₂ adsorbent by the Langmuir and Freundlich models showed that the experimental data fitted well to the Langmuir model suggesting monolayer coverage of Cr (VI) ions mainly on the outer surface of the adsorbent.