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

• Published in: Chinese chemical letters 2020-10, Vol.31 (10), p.2634-2640
• Main Authors: Wang, Xinhao, Pu, Xueting, Yuan, Yue, Xiang, Yunjie, Zhang, Yuling, Xiong, Zhaokun, Yao, Gang, Lai, Bo
• Format: Article
• Language: English
• Subjects: Iron oxides; p-Nitrophenol; Structural transformation; Successive reactivities; Zero-valent iron
• ISSN: 1001-8417; 1878-5964
• DOI: 10.1016/j.cclet.2020.08.007

The removal rates of p-nitrophenol (PNP) by micron-scale Fe0 (mFe0) were gradient elevated and maintain at a satisfactory level with the successive treatment. The total iron ion concentrations decreased dramatically with the process of continuous reactions. The formation of Fe3O4 could enhance the electron transfer efficiency to reduce the PNP, while the transformation from Fe3O4 to γ-Fe2O3 could weaken the electron transfer efficiency. [Display omitted] It is generally recognized that the formation and accumulation of iron oxides on the surface of zero-valent iron (Fe0) 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 (mFe0) 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 cycle was 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 (γ-Fe2O3), which were in the form of microspheres on the surface of mFe0. 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 γ-Fe2O3. Electrochemical analysis proved that the electron transfer rate of mFe0 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 mFe0 could be enhanced with the addition of scavengers. This study deepened the understanding of the structural transformation and radical production of mFe0 in continuous reactions.