Mechanically combined persulfate on zerovalent iron: Mechanistic insights into reduction and oxidation processes

[Display omitted] •Mechanically combined Na2S2O8 and ZVI (PSZVIbm) was developed using ball milling.•Rapid destruction of the passive layer was found in PSZVIbm at neutral pH.•Surface-combined Na2S2O8 showed different effects on oxidation and reduction.•Main radical species were different depending on the persulfate injection method. Persulfate (S2O82−; PS) has been used with zerovalent iron (ZVI) for the remediation of various contaminants. However, the passive layer of commercial ZVI limitates the corrosion of ZVI and reaction with PS. In our initial attempt, we mechanically combined Na2S2O8 and micro-sized ZVI by simple ball milling to improve the reaction of PS and ZVI. The synthesized composite (PSZVIbm) was compared to the system in which Na2S2O8 was separately injected with ball-milled ZVI (ZVIbm/PS). The reaction in ZVIbm/PS depended on surface-bound Fe(II) to initiate PS consumption. Conversely, the PS immediately destroyed the passive layer and enhanced the corrosion of ZVI in PSZVIbm. Therefore, PSZVIbm reduced nitrobenzene rapidly with higher efficiency in deionized water (94.2%) compared to that of ZVIbm/PS (73.1%). Furthermore, PSZVIbm was less affected by the real groundwater matrix and removed nitrobenzene with higher efficiency (64.7%) than ZVIbm/PS (10.3%). Due to the different PS consumption rates and radical species, PSZVIbm removed phenol with lower efficiency (22.7%) than ZVIbm/PS (61.5%), showing the different effects of surface-bound Na2S2O8 on the reduction/oxidation capacities of ZVI. This study suggests a new scalable method to control the reactivity of ZVI without the use of toxic chemicals and wastewater generation.