Nano- and micro-scale zerovalent iron-activated peroxydisulfate for methyl phenyl sulfoxide probe transformation in aerobic water: Quantifying the relative roles of SO4•-, Fe(IV), and •OH

Multiple reactive intermediates have been proposed to be involved in peroxydisulfate (PDS) activation by zerovalent iron (ZVI), including sulfate radical (SO4 center dot-) produced via iron-oxide shell mediated electron transfer, ferryl ion species (Fe(IV)) formed from Fe(II)-PDS interaction, and hydroxyl radical (center dot OH) generated by ZVI aerobic oxygenation. In this study, evolution of the relative role of these intermediates in microscale and nanoscale ZVI (mZVI vs. nZVI) activated PDS processes is comparatively investigated by using a methyl phenyl sulfoxide (PMSO) probe that selectively reacts with Fe(IV) to produce methyl phenyl sulfone (PMSO2). Interestingly, during PMSO transformation by mZVI/PDS process, yields of PMSO2 (eta(PMSO2)) exhibit three-stage behavior that they first increase to a maximum (similar to 80% but lower than 100%) (Stage I) and then plateau for a period (Stage II) followed by a decrease phase (Stage III). Accordingly, the relative role of Fe(IV) in PMSO transformation is unceasingly improved in Stage I and subsequently reaches equilibrium with that of free radicals in Stage II, while it finally decreases in Stage III. Similar eta(PMSO2) evolution trends are obtained in nZVI/PDS process, except that the eta(PMSO2) increase in Stage I is negligible, possibly due to the exceptional fast nZVI dissolution. It was further clarified by tert-butyl alcohol scavenging assay that, in addition to Fe(IV), the free radical involved in Stages I and II is SO4 center dot-, while (OH)-O-center dot was dominant in Stage III. Moreover, studies on O-2 effect reveal that ZVI aerobic oxygenation participates in mZVI corrosion during the entire process, while it is only involved in nZVI corrosion when PDS content is reduced to a low concentration, indicating that the reactivities of PDS and O-2 are similar in mZVI corrosion, but differ greatly in nZVI corrosion. Additionally, effects of reactant dose and pH on eta(PMSO2) evolution are also explored. Dynamics of the relative role of different reactive oxidants should be taken into account in further applications of ZVI/PDS in situ chemical remediation technology considering their different chemistries.

Automated summary

This study investigates the relative role of different reactive intermediates in microscale and nanoscale zerovalent iron (ZVI) activated peroxydisulfate (PDS) processes. The results show that the relative role of Fe(IV) in PDS transformation varies in different stages, and the fast dissolution of nanoscale ZVI leads to a relatively lower relative role of Fe(IV) in PDS transformation.