A novel MgCr2O4/WO3 hetero-junction photocatalyst for solar photo reduction of hexavalent chromium Cr(VI)

[Display omitted] •New hetero-system MgCr2O4/WO3 was used for photoreduction of the hexavalent chromate.•MgCr2O4 and WO3 were prepared by the sol–gel and chemical precipitation methods.•Hexavalent chromium (30 mg/L) is totally photo-reduced within 120 min at (pH 4) under sunlight.•The kinetics of the photocatalytic reaction of Cr (VI) reduction to Cr (III) followed a first order model. MgCr2O4 produced by the sol–gel route is active under the visible range. The spinel is characterized structurally, physically and photo electrochemically. From the diffuse reflectance measurement, its optical gap (1.69 eV) is suitable for the solar spectrum. Using the Mott Schottky method, the flat band potential is −0.15 VSCE. A BET area value of 30 m2/g was found. In a heterogeneous MgCr2O4/WO3/Cr (VI) system, the photoreduction of the hexavalent chromate is successfully achieved; WO3 is synthesized by chemical precipitation. The electrons of the conduction band (CB, −1.66 VSCE) of MgCr2O4 excited by solar light are injected into CB-WO3 (-0.21 VSCE). After 120 min, a rate of 100% conversion of 30 mg/L of Cr (VI) into Cr (III) was obtained under optimal conditions (pH ∼ 4, 25 °C, [C7H6O3] = 10−5 mol/L, MgCr2O4/WO3 (1/1) and a dose of 1 mg/mL). The C7H6O3 scavenger used in the chromate solution Cr(VI) is advantage for avoiding the photocorrosion of MgCr2O4/WO3 hetero-system and improving the photoactivity. The kinetic of the photocatalytic reaction of Cr (VI) reduction to Cr (III) follows a first order model with an apparent constant of 0.02 min−1 (t1/2 = 35 min).