Reductive transformation and mineralization of an azo dye by hydroxysulphate green rust preceding oxidation using H2O2 at neutral pH

In this study, the reactivity of hydroxysulphate green rust (GR(SO42-)) toward reductive transformation, oxidative degradation and mineralization of organic compounds was evaluated using Methyl Red (MR) as model pollutant. The GR(SO42-) was synthesized by co-precipitation method and characterized by X-ray diffraction (XRD), Mössbauer spectroscopy and Fourier Transform Infrared (FTIR) analyses. Reductive decolourization of MR solution occurred in the presence of GR(SO42-), while no total organic carbon (TOC) decay was observed during the equilibration time. Significant TOC removal (87%) was noted when H2O2 was added to the GR(SO42-)/MR mixture after the preliminary reduction step. UV–Vis analysis, dissolved iron and H2O2 concentration measurement, and batch sorption test showed that the heterogeneous Fenton-like reaction is the main mechanism by which the pollutant was mineralized. Increasing of H2O2/Fe(II) ratio did not affect significantly the mineralization rate of MR. However, slight decolourization of MR and absence of TOC abatement were noted when both MR and H2O2 were simultaneously mixed with the GR(SO42-). XRD analysis, Mössbauer spectroscopy and FTIR spectroscopy revealed that the oxidation end-products of GR(SO42-) were mainly a poorly crystallized goethite when GR was oxidized after equilibrating with MR in solution. However, a badly crystallized iron oxide was formed when GR was immediately oxidized. In all cases, the interlayer anion (SO42-) was ejected from GR structure to aqueous solution. These results suggest that the GR(SO42-)/H2O2 system could be used to promote the reduction/oxidation reaction of organic pollutants.