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 (Fe0), 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 Fe0 (i.e., a direct reduction mechanism) through an iron corrosion product layer (magnetite), rather than indirectly via H2 gas, which was produced by the reaction between Fe0 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 Fe0 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.

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