Oxidative breakdown of acid orange 7 by a manganese oxide containing mine waste: Insight into sorption, kinetics and reaction dynamics

The reaction rates and orders of azo dye, acid orange 7 (AO 7), oxidation by a Mn oxide containing mine tailings were determined. The reaction is pseudo-first order with respect to the tailings surface area concentration [SA] and pseudo-fractional (0.6) order with respect to AO 7 concentration. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy was used to probe surface sorption and oxidation of AO 7 on a synthetic manganite surface. Sorption is rapid, pH dependent and predominantly outer-sphere. At pH 2.7 a distinct lag phase of 8 min has been observed between dye sorption and the initiation of oxidation. This lag phase suggests that either transfer of the first electron is rate limiting or there is a time consuming re-orientation and inner-sphere sorption step prior to electron transfer.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights▶ The reaction rate for the oxidation of azo dye AO 7 by a Mn oxide containing mine tailings is first order with respect to tailings surface area and pseudo-fractional order with respect to dye concentration. ▶ ATR-FTIR investigation revealed an outer-sphere, pH dependent sorption mechanism between the dye and a synthetic manganite surface. ▶ A distinct lag phase exists between the sorption of the dye to a manganite surface and the initiation of oxidation. ▶ This lag phase demonstrates that either inner-sphere coordination or transfer of the first electron is the rate limiting process.