Sustaining reactivity of Fe0 for nitrate reduction via electron transfer between dissolved Fe2+ and surface iron oxides

Abstract

The mechanism of the effects of Fe2+aq on the reduction of NO3− by Fe0 was investigated. The effects of initial pH on the rate of NO3− reduction and the Fe0 surface characteristics revealed Fe2+aq and the characteristics of minerals on the surface of Fe0 played an important role in NO3− reduction. Both NO3− reduction and the decrease of Fe2+aq exhibited similar kinetics and were promoted by each other. This promotion was associated with the types of the surface iron oxides of Fe0. Additionally, further reduction of NO3− produced more surface iron oxides, supplying more active sites for Fe2+aq, resulting in more electron transfer between Fe2+ and surface iron oxides and a higher reaction rate. Using the isotope specificity of 57Fe Mossbauer spectroscopy, it was verified that the Fe2+aq was continuously converted into Fe3+ oxides on the surface of Fe0 and then converted into Fe3O4 via electron transfer between Fe2+ and the pre-existing surface Fe3+ oxides. Electrochemistry measurements confirmed that the spontaneous electron transfer between the Fe2+ and structural Fe3+ species accelerated the interfacial electron transfer between the Fe species and NO3−. This study provides a new insight into the interaction between Fe species and contaminants and interface electron transfer.