Crucial role of the surface hydroxyl groups induced by oxygen vacancies on -Fe_0.9Zn_0.1OOH during interfacial peroxymonosulfate activation for meclofenamic acid degradation

The role of H2O molecule during the PMS activation has been rarely followed, although the interfacial mechanism was inseparable from its adsorption and dissociation on catalysts. An efficient catalyst (alpha-Fe0.9Zn0.1OOH) was synthesized via Zn2+ isomorphous substitution of Fe3+ in alpha-FeOOH to trigger peroxymonosulfate (PMS) for distinguishing the overlooked interfacial process. The alpha-Fe0.9Zn0.1OOH/PMS system showed high performance for meclofenamic acid (MCF) degradation (97.0%) owing to the generation of powerful sulfate radicals (SO4 center dot-) and hydroxyl radicals (center dot OH). Characterization of alpha-Fe0.9Zn0.1OOH via EDS, ESR and XPS spectra confirmed that Zn2+ doping induced the formation of oxygen vacancy (OV) in the structure. The OV-rich structure promoted the adsorption (E-ads = -1.49 eV) and dissociation of H2O to form surface hydroxyl groups (-OHdiss), which was different from the structure -OHlatt. By ATR-FTIR tests in heavy water (D2O), the -OHdiss could be exchanged by PMS, and forming surface complexed equivalent to Fe(III)-(HO)-OSO3-, which enlarged the O-O bond (from 1.317 & Aring; to 1.506 & Aring;). The electron transfer inner equivalent to Fe(III)-(HO)-OSO3- benefited O-O cleavage thus inducing SO4 center dot- generation. Overall, the presence of OV in alpha-Fe1-xZnxOOH greatly promoted this spontaneous process.