Insights into persulfate activation by FeO for phenol removal: The production and effect of Fe(IV)

[Display omitted] •Persulfate is synergistically activated by hybrid homogeneous-heterogeneous process.•SO4•−, •OH and Fe(IV) made concerted efforts to phenol degradation.•A new formation path of aqueous Fe(IV) in Fe2+/persulfate system was proposed.•The effect of Fe(IV) on phenol degradation was disclosed from a DFT perspective.•Various phenolic compounds were effectively degraded in FeO/persulfate system. This study presents new findings on persulfate (PDS) activation by iron(II) oxide (FeO). PDS was activated through both homogeneous and heterogeneous approaches, while the latter played a dominant role. The species responsible for phenol oxidation were identified as SO4−, OH, and high-valent Fe(IV). A new possible formation pathway of aqueous Fe(IV) is proposed, involving electron transfer between SO4−/OH and Fe(III)–OH complex. Density functional theory calculations support that Fe(IV) is apt to attack phenol by electrophilic addition on the benzene ring and hydrogen atom abstraction on the phenolic hydroxyl group. Phenol removal was accelerated at a higher catalyst dosage or PDS concentration but inhibited at higher pH. The coexistence of inorganic anions (e.g., PO43−, SO42−, NO3−, and Cl−) and fulvic acid have different effects on this system. Phenol degradation mainly occurred through hydroxylation and eventual ring-opening. In addition, the FeO/PDS system has not only achieved a high removal effect on other phenolic compounds but has also exhibited high performance in actual water. Thus, this system is anticipated to be a promising treatment technique for phenolic-polluted water.