Challenges Relating to the Quantification of Ferryl(IV) Ion and Hydroxyl Radical Generation Rates Using Methyl Phenyl Sulfoxide (PMSO), Phthalhydrazide, and Benzoic Acid as Probe Compounds in the Homogeneous Fenton Reaction

Ferryl ion ([(FeO)-O-IV](2+)) has often been suggested to play a role in iron-based advanced oxidation processes (AOPs) with its presence commonly determined using the unique oxidation pathway from methyl phenyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO2). However, we show here that the oxidation products of PMSO, formed on reaction with hydroxyl radical, enhance PMSO2 formation as a result of their complexation with Fe(III) leading to the changes in the reactivity of Fe(III) species in the homogeneous Fenton reaction. As such, PMSO should be used with caution to investigate the role of [(FeO)-O-IV](2+) in iron-based AOPs with these insights suggesting the need to reassess the findings of many previous studies in which this reagent was used. The other common target compounds, phthalhydrazide and hydroxybenzoic acids, were also found to modify the rate and extent of iron cycling as a result of complexation and/or redox reactions, either by the probe compound itself and/or oxidation products formed. Overall, this study highlights that these confounding effects of the aromatic probe compounds on the reactivity of iron species should be recognized if reliable mechanistic insights into iron-based AOPs are to be obtained.

Automated summary

In this study, it was found that the oxidation products of methyl phenyl sulfoxide (PMSO) enhance the formation of methyl phenyl sulfone (PMSO2) by complexing with Fe(III) species. This finding suggests caution in using PMSO to investigate the role of ferryl ion ([(FeO)-O-IV](2+)) in iron-based advanced oxidation processes (AOPs). Additionally, other target compounds were found to modify the reactivity of iron species through complexation and/or redox reactions. Overall, this study highlights the confounding effects of aromatic probe compounds on the reactivity of iron species in AOPs.