New insights into stoichiometric efficiency and synergistic mechanism of persulfate activation by zero-valent bimetal (Iron/Copper) for organic pollutant degradation

Highlights: • DCP oxidation can be notably enhanced by Fe/Cu compared to nZVI. • The rate constant of DCP oxidation is linearly correlated to the Cu ratio in Fe/Cu. • A synergistic effect exists between Cu and Fe in activating persulfate. • The Fe and Cu species in Fe/Cu determine their synergistic effects in DCP oxidation. • Cu atoms on Cu₂O layers are the sites dominantly responsible for PS activation. Extensive studies have been devoting to investigating the catalytic efficiency of zero-valent iron (Fe⁰)-based bimetals with persulfate (PS), while little is known in the stoichiometric efficiency, underlying mechanisms and reaction center of zero-valent bimetallic catalysts in activating PS. Herein, nanoscale zero-valent Fe/Cu catalysts in decomposing 2,4-dichlorophenol (DCP) have been investigated. The results show that the increase of Cu ratio from 0 to 0.75 significantly enhances the DCP degradation with a rate constant of 0.025 min⁻¹ for Fe⁰ to 0.097 min⁻¹ for Fe/Cu(0.75) at pH ∼3.3, indicating Cu is likely the predominate reaction centers over Fe. The PS decomposition is reduced with the increase of Cu ratios, suggesting the stoichiometric efficiency of Fe/Cu in activating PS is notably enhanced from 0.024 for Fe⁰ to 0.11 for Fe/Cu(0.75). Analyses indicate Cu atoms are likely the predominant reaction site for DCP decomposition, and Fe atoms synergistically enhance the activity of Cu as indicated by DFT calculations. Both ${\text{SO}}_4^{\text{⦁}-}$ and ⦁OH radicals are responsible for reactions, and the contribution of ${\text{SO}}_4^{\text{⦁}-}$ is decreased at higher pH conditions. The findings of this work provide insight into the stoichiometric efficiency and the reaction center of Fe/Cu catalysts to activate PS for pollutant removals.