Synthetic magnetite, maghemite, and haematite activation of persulphate for orange G degradation

Abstract

Due to the widespread application of persulphate (PS) for in-situ chemical oxidation (ISCO), the PS activating role of naturally occurring minerals, such as iron oxides, has been the subject of a number of studies. However, major discrepancies remain as to the effectiveness, mode, and factors that influence iron oxides activation of PS. In this study, an attempt has been made to bridge this important knowledge gaps by a systematic study of PS activation, measured by orange G degradation, using commercial and self-synthesised magnetite, maghemite, and haematite particles. The results showed that the activation of PS by iron oxides does not depend on mineralogy, surface area or concentration of surface OH groups, but on crystalline inhomogeneities or structural irregularities. Significant dissolution of iron oxides accompanied PS activation, in a mainly homogeneous process, requiring a low pH environment to be effective. The activation of PS by iron oxides at neutral pH was found to be no better than dissolved iron activation contrary to some earlier publications. The results also suggest that under alkaline conditions, PS alone was more effective in degrading orange G than with iron oxides or dissolved iron activation. Phosphate buffer significantly retarded orange G degradation by iron-activated or unactivated PS with negative implication for ISCO in non-acidic, buffering environments. The results of this study contribute to enhancing the fundamental understanding of ISCO processes.