Mechanism of nitrate reduction by zero-valent iron: Equilibrium and kinetics studies

The first objective of this study was to deepen our understanding of the mechanism of nitrate (NO3-) reduction by zero-valent iron (Fe-0), especially the electron transfer process from electron donor(s) to NO3- (i.e., an electron acceptor) under the presence of iron corrosion products. To achieve this objective, batch experiments were performed to investigate the influence of several variables, including aqueous pH, the solid-liquid ratio, the concentration of augmented ferrous ion (Fe2+), and the reaction time. The experimental results showed that the NO3- reduction efficiency was enhanced by either decreasing the aqueous pH or increasing the solid-liquid ratio. Additionally, the NO3- reduction efficiency at near neutral pH was both stoichiometrically and kinetically enhanced by augmenting the Fe2+ in the aqueous phase. These experimental data consistently indicated that NO3- received electrons directly from Fe-0 (i.e., a direct reduction mechanism) through an iron corrosion product layer (magnetite), rather than indirectly via H-2 gas, which was produced by the reaction between Fe-0 and an acid (i.e., an indirect reduction mechanism). Based on the observation that the NO3- reduction efficiency at near neutral pH was enhanced by augmenting Fe2+, the second objective of this study was to investigate the stoichiometric relationship between the amount of augmented Fe2+ and the amount of NO3- additionally reduced by augmenting Fe2+ with the final goal of effectively removing NO3- at near neutral pH without leaving Fe2+ in the treated water. The experimental results demonstrated that Fe-0 coated with an iron corrosion product (magnetite) repeatedly reduced NO3- to ammonium ion at near neutral pH as long as the Fe2+ was augmented in the aqueous phase, and the concentrations of NO3- and Fe2+ in the treated water were both reduced to near zero if the proper amount of Fe2+ was augmented based on the stoichiometric relationship derived in this study. (C) 2011 Elsevier B.V. All rights reserved.