Nitrate Reduction by Surface-Bound Fe(II) on Solid Surfaces at Near-Neutral pH and Ambient Temperature

Nitrate reduction by Fe(II) species was previously known to occur only in ${\mathrm{Cu}}^{2+}$-catalyzed and alkaline conditions or at high temperatures. In this study, a reactant system consisting of nitrate, iron oxide particles, and Fe(II) (in different forms) was used to study possible reactions between nitrate and Fe(II). At near-neutral pH, nitrate could not be reduced by aqueous Fe(II) species or by $\mathrm{Fe}{(\mathrm{OH})}_2$ gel. In the presence of magnetite (${\mathrm{Fe}}_3{\mathrm{O}}_4$) particles, however, a significant amount of ${\mathrm{Fe}}^{2+}$ could be adsorbed onto an ${\mathrm{Fe}}_3{\mathrm{O}}_4$ surface at $\mathrm{pH}=7.3$ and became surface-bound ${\mathrm{Fe}}^{2+}(\mathrm{S.B.}{\mathrm{Fe}}^{2+})$, which could react with nitrate via the reaction: $12\mathrm{S.B.}{\mathrm{Fe}}^{2+}+{\mathrm{NO}}_3^{-}+13{\mathrm{H}}_2\mathrm{O}\to 4{\mathrm{Fe}}_3{\mathrm{O}}_4\downarrow +{\mathrm{NH}}_4^{+}+22{\mathrm{H}}^{+}$. The reaction stopped when pH decreased to $<6.8$. Introducing trace amount of ${\mathrm{O}}_2$ or ${\mathrm{Fe}}^{3+}(\mathrm{aq})$ into the nitrate-${\mathrm{Fe}}^{2+}$ reactant system was as effective as seeding magnetite particles in triggering the nitrate-Fe(II) reaction, suggesting that lepidocrocite ($\gamma \text{-}\mathrm{FeOOH}$) is a precursor for initiating the nitrate-Fe(II) reaction. Hematite and Kaolinite particles could also serve as the reactive sites for the nitrate-Fe(II) reaction upon surface transformation by $\mathrm{S.B.}{\mathrm{Fe}}^{2+}$. The observed abiotic nitrate reduction by Fe(II) could be an important link between the nitrogen cycling and the Fe(II)/Fe(III) redox couple in the biosphere or geosphere.