Nitrate Reduction by Surface-Bound Fe(II) on Solid Surfaces at Near-Neutral pH and Ambient Temperature
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2016 American Society of Civil Engineers. Nitrate reduction by Fe(II) species was previously known to occur only in Cu2+-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 Fe(OH)2 gel. In the presence of magnetite (Fe3O4) particles, however, a significant amount of Fe2+ could be adsorbed onto an Fe3O4 surface at pH=7.3 and became surface-bound Fe2+(S.B. Fe2+), which could react with nitrate via the reaction: 12 S.B. Fe2++NO3-+13 H2O 4 Fe3O4 +NH4++22 H+. The reaction stopped when pH decreased to <6.8. Introducing trace amount of O2 or Fe3+(aq) into the nitrate-Fe2+ reactant system was as effective as seeding magnetite particles in triggering the nitrate-Fe(II) reaction, suggesting that lepidocrocite (-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 S.B. Fe2+. 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.
