Quantitative evaluation of relative contribution of high-valent iron species and sulfate radical in Fe(VI) enhanced oxidation processes via sulfur reducing agents activation

Recently, it has been reported that sulfur reducing agents (e.g., sulfite and thiosulfate) can effectively activate Fe(VI), leading to a fast degradation of various contaminants. However, the reactive intermediates (i.e., high-valent iron species versus free radicals) involved in the Fe(VI)/sulfite and Fe(VI)/thiosulfate systems remain far from clear. In this work, by using methyl phenyl sulfoxide (PMSO) as a mechanistic probe, we quantitatively assessed the relative contribution of high-valent iron species versus sulfate radical in these two systems. High-valent iron species oxidized PMSO to methyl phenyl sulfone product (PMSO2), while sulfate radical oxidized PMSO to hydroxylated and/or polymeric products. The yield of PMSO2 (mole of PMSO2 formed per mole of PMSO consumed) was quantified to be in the range of 30–95% under various conditions. This finding suggested the contribution of both high-valent iron species and sulfate radical in the Fe(VI)/sulfite system, which was dependent on solution chemistry (i.e., solution pH and [Fe(VI)]:[sulfite] ratio). Comparatively, the yield of PMSO2 in the Fe(VI)/thiosulfate system remained as high as 90–100% under various conditions. This finding indicated that only high-valent iron species was involved in the Fe(VI)/thiosulfate system and the contribution of sulfate radical was negligible. The discrepancy in the effects of oxygen, alcohol scavenger and co-existing components (e.g., Cl−, HCO3−, and humic acid) on the Fe(VI)/sulfite versus Fe(VI)/thiosulfate systems also indicated the involvement of different oxidants in these two systems.