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

•Evolution of Fe(IV), SO4·−, and ·OH in mZVI/PDS process shows three-stage behavior.•Role of Fe(IV) relative to SO4·− increases in Stage I and plateaus in Stage II.•Relative role of ·OH formed by ZVI aerobic oxygenation takes the lead in Stage III.•Initial promotion of the relative role of Fe(IV) in nZVI/PDS process is negligible.•Effects of pH and reactant dose on reactive oxidant evolution are explored.