Efficient oxidation by sono-photo-electrocatalysis of rhodamine B using MgFe2O4 as photoanode

Abstract

The present work describes the colour removal of Rhodamine B (Rh B), a cationic dye by photo-electrocatalysis and sono-photo-electrocatalysis on MgFe2O4 as an anode. The spinel MgFe2O4 synthesized by sol–gel route was characterized by physical and electrochemical methods, a preamble of Rh B oxidation. The XRD pattern shows the formation of the single phase, which crystallizes in a face-centred cubic lattice (space group, Fd − 3 m), with spherical crystallites (0.42 nm). The Zeta-sizer analysis gives an average grain size of 0.46 µm and a zeta potential of − 30 mV. The SEM analysis revealed the porosity of the oxide and the Mg-O and Fe–O bonds were confirmed by the FT-IR analysis. The direct optical gap (2.16 eV) assigned to d − d internal transition comes from the crystal field splitting of Fe3+ octahedrally coordinated. The low electron mobility is assigned to a narrow conduction band of Fe3+—3d parentage with activation energy (0.12 eV) in conformity with a conduction mechanism by small lattice polaron hopping. The intensity potential J(E) profile in Na2SO4 (10−2 M) exhibits a small hysteresis similar to a chemical diode. The semi-logarithmic plot (logJ − E) indicates the chemical stability of MgFe2O4 in the working solution (Na2SO4). Curiously and unlike most spinels, the capacitance plot exhibits n-type conduction confirmed by chrono-amperometry, plotted at the free potential (+ 0.5 V) with a flat band potential (Efb) of 0.29 V, due to Fe3+ insertion. As an application, Rh B (20 mg L−1) was successfully oxidized by photo-electrocatalysis on MgFe2O4 with an abatement of 75% under solar irradiation and a direct current of 150 mA which has a bactericidal effect. An enhancement up to 97% has been reached by sono-photo-electrocatalysis at a frequency of 60 kHz; almost complete discoloration occurred within 90 min in the “US-Electric Current-Sunlight-MgFe2O4”. The Rh B elimination follows a pseudo-first-order kinetic with a rate constant of 3.9 × 10−2 mn−1 (t1/2 = 18 min), and a reaction mechanism is suggested.