Relative contribution of ferryl ion species (Fe(IV)) and sulfate radical formed in nanoscale zero valent iron activated peroxydisulfate and peroxymonosulfate processes

Activation of persulfates (i.e., peroxydisulfate (PDS) and peroxymonosulfate (PMS)) by nanoscale zero-valent iron (nZVI) is reported to be effective in oxidative treatment of environmental contaminants. It has been widely accepted in numerous literature that sulfate radical (SO4•-) formed from the decomposition of persulfates activated by aqueous Fe(II) released from nZVI corrosion is responsible for the oxidative performance in nZVI/persulfates systems. In this work, by employing methyl phenyl sulfoxide (PMSO) as a probe, we demonstrated that the activation of persulfates by nZVI through electron transfer led to SO4•- formation, while the homogeneous activation of persulfate by the released Fe(II) resulted in ferryl ion species (Fe(IV)) generation in nZVI/persulfates systems. Similarly, nanoscale zero-valent aluminum (nZVAl) and zinc (nZVZn) were also demonstrated to be able to donate electron to persulfates leading to SO4•- formation. However, the insulative aluminum oxide shell hindered the electron transfer leading to the poor persulfates decomposition, while the conductive iron and zinc oxide shell enabled the electron transfer process resulting in a continuous generation of SO4•-. Further, it was obtained that the relative contribution of SO4•- and Fe(IV) in nZVI/persulfates systems was independent of the initial concentration of nZVI and PDS, but was positively correlated with PMS concentration. In addition, the increase of pH from 3 to 7 led to the decrease of the relative contribution of Fe(IV), which was rationalized by the decrease of availability of aqueous Fe(II) at higher pH. Our findings not only shed lights on the nature of the reactive intermediate formed in the nZVI/persulfates systems, but also unprecedentedly distinguished the surface activation of persulfates from the homogeneous catalysis process.