Enhanced Oxidation of Reactive Black 5 Using a Ferrous Oxidation–Based Treatment Process in the Presence of Hydrous Ferric Oxide

The major limitations of the Fenton process are the working pH range (2–5) and the high cost of ${\mathrm{H}}_2{\mathrm{O}}_2$. These limitations were unraveled using the ${\mathrm{Fe}}^{2+}$-air oxidation process, in which ferrous ions were used with continuous aeration without ${\mathrm{H}}_2{\mathrm{O}}_2$ at a higher pH (pH = 10). This paper studies the effect of the initial hydrous ferric oxide (HFO) concentration on the ${\mathrm{Fe}}^{2+}$-air process. The enhancement in degradation of the dye [Reactive Black 5 (RB5)] was observed using the ${\mathrm{Fe}}^{2+}\text{-}\mathrm{air}$ process in the presence of initial HFO, as compared to the ${\mathrm{Fe}}^{2+}$-air process in the absence of initial HFO. The enhancement is possibly attributable to the adsorption of ferrous ions, dye, and oxygen onto the HFO surface and leading to effective utilization of reactive oxidizing species. The oxidizing entities appear to be generated during oxidation of adsorbed ferrous ions under oxic conditions—probably on the surface of HFO. Furthermore, the ${\mathrm{Fe}}^{2+}$-air process in the presence of HFO reduces the concentration of ferrous ions required for decolorization of RB5 compared to the ${\mathrm{Fe}}^{2+}$-air process without HFO and hence resulting in an overall-economical process for the pretreatment or degradation of recalcitrant compounds.