An old story with new insight into the structural transformation and radical production of micron-scale zero-valent iron on successive reactivities

It is generally recognized that the formation and accumulation of iron oxides on the surface of zero-valent iron (Fe-0) resulting in significant decrease of contaminant degradation rates during the long-term reactions. However, in this study, we found that the removal efficiencies of p-nitrophenol (PNP) by micro zero-valent iron (mFe(0)) could maintain at the satisfactory level in the process of continuous reactions (20 cycles). The removal rate constant (0.1779 min(-1)) of the 5th cyclewas 6.74 times higher than that of the 1st reaction (0.0264 min(-1)), even the 20th cycle (0.0371 min(-1)) was higher than that of the 1st reaction. Interestingly, almost no dissolved iron was detected in the solution, and the total iron concentrations decreased dramatically with the process of continuous reactions. The results of scanning electron microscope and energy dispersive spectrometry (SEM-EDS) and X-ray diffraction (XRD) revealed that the structure and composition of corrosion products change from amorphous to highly crystal with the increase of the number of cycles. The corrosion products were mainly magnetite (Fe3O4) and a small part of maghemite (gamma-Fe2O3), which were in the form of microspheres on the surface of mFe(0). The formation of surface oxidation shell hindered the release of Fe2+. X-ray photoelectron spectroscopy (XPS) results illustrated that partial Fe3O4 could be converted into gamma-Fe2O3. Electrochemical analysis proved that the electron transfer rate of mFe(0) increased with the formation of the oxides shell. However, the consumption of iron core and thicker oxide film weakened the electron transfer rate. Besides, the quenching experiments indicated that the reaction activity of mFe(0) could be enhanced with the addition of scavengers. This study deepened the understanding of the structural transformation and radical production of mFe(0) in continuous reactions. (C) 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.