Photocatalytic reduction of Cr(VI) onto the spinel CaFe2O4 nanoparticles

Abstract

The nanoparticles of the spinel CaFe2O4 (CFO) with a crystalline size of 34 nm were synthetized by nitrate route. The phase was identified by X-ray diffraction (XRD) and confirmed by the Fourier transform infrared (FTIR); the lattice constants were refined by the Rietveld method. The optical properties were measured by diffuse reflectance and a direct forbidden band of 1.93 eV was determined. Also, the photo-electrochemical properties were investigated to build the potential diagram of the junction CFO/HCrO4− solution in order to predict the interfacial phenomena. Curiously, CFO is n-type semiconductor with a flat band potential of -0.24 VSCE enough cathodic to be involved in an electronic exchange with the chromate species. Indeed, the thermal variation of the conductivity follows an experimental law with an activation energy of 0.17 eV. The oxide is stable against photo-corrosion in presence of oxalic acid which captures the photoholes and enhances the photoactivity through the separation of electron/hole pairs. The photo reduction of Cr (VI) under visible-light irradiation is improved on CFO nanocomposite. Herein, the effect of pH, CFO dose and Cr(VI) initial concentration (Co) on the photo reduction of Cr(VI) was investigated. An abatement of 86 % of HCrO4− was achieved under optimal conditions (pH ∼ 3, CFO dose 0.6 g/L, Co 30 ppm) upon illumination (16 mW cm-2). The photoactivity reached 100 % under solar light (98 mW cm-2).