Revealing the heterogeneous activation mechanism of peroxydisulfate by CuO: the critical role of surface-binding organic substrates.

Heterogeneous catalytic activation mechanisms of peroxydisulfate (PDS) by transition metal oxides are generally attributed to the interactions between catalysts and PDS, however, the role of the co-existed organic substrate was largely overlooked in the past studies. In this work, phenol was selected as the target organic pollutant in a CuO/PDS system to evaluate its deep-seated role in participating in the effective activation of PDS. First, optimized reaction conditions as pH of 6.0, CuO of 5.96 g·L and PDS of 2.5 mM were obtained by the response surface methodology (RSM) with a phenol degradation efficiency of 84.0%. It was further found that pre-adsorption of phenol or PDS led to obviously different performances in the phenol degradation with/without the radical scavengers. Two different activation pathways of PDS, i.e., the non-radical pathway mediated by surface deprotonated phenol to generate O and the radical pathway mediated by structural Cu(I)/Cu(II) to produce SO, were therefore proposed, and the former was predominant in the CuO/PDS/phenol system. In addition, HCO and HPO could strongly inhibit the phenol degradation while Cl and NO only performed negligible effects. NaOH washing could regenerate the surface hydroxyl groups and recover the catalytic ability of CuO. The result of this study integrated the interactions among the catalyst, oxidant and substrate, providing new insights into environmental-friendly PDS activation processes.