Hexavalent chromium reduction by ZnO, SnO2 and ZnO-SnO2 synthesized using biosurfactants from extract of Solanum macrocarpon

Abstract In this study, ZnO, SnO2 and their composite (ZnO-SnO2) were synthesized by green route using aqueous extract of Solanum macrocarpon fruit and were used for the photo-reduction of hexavalent chromium. The synthetic route involved a two-step procedure, induced by temperature via calcination at 350 and 600 ºC. The composite was prepared by the treatment of a mixture of the precursor compounds to a temperature up to 800 ºC, and the extension of the temperature to 1000 ºC, resulted in the emergence of ZnO-SnO2-ZTO. The nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV–vis spectroscopy. The XRD studies of the binary oxides confirmed a hexagonal wurtzite structure for the ZnO and a cubic structure for the SnO2, without any change in the diffraction patterns or supplementary diffraction peaks. The morphology of the nanoparticles indicated fairly spherical shapes for the ZnO, that tend to agglomerate with increase in temperature. The SnO2 showed rectangular shapes at both temperatures of reaction, while the ZnO-SnO2 composite showed the presence of both morphologies of the component binary oxides. In the photo-enhanced degradation study, under ultra-violet light, the effect of pH (2–8), concentration of chromium(VI) (2–8 ppm), and photocatalyst dosage (25–150 mg/L) on the reduction of Cr(VI) to Cr(III) were investigated. The reduction showed higher efficiency in acidic environment than in the alkaline environment, and also with increase in photocatalyst dosage. The composite exhibited the highest photoreduction efficiency, above 90%, at the optimum condition of pH 2, 150 mg/L photocatalyst, 2 ppm chromium solution after 90 min. These low-cost and non-toxic metal oxide and their green synthesized composite have great potentials for Cr(VI) pollution clean-up from waste water.