Abiotic Transient Nitrite Occurrences from Nitrate Reduction through Goethite-Mediated Fe(III)/Fe(II) Cycle with Labile Organic Materials and Ammonia

Abstract

The abiotic reduction of NO3− to NO2−—coupled with the oxidation of labile organic materials such as citric acid, syringic acid and natural organic matter (NOM) and NH4+ through the goethite-mediated Fe(III)/Fe(II) cycle under anaerobic condition—was investigated at pH values of 4 and 7. The concentrations of the produced Fe2+ and NO2− were monitored. At a pH of 4, concentrations of Fe2+ increased, except for citric acid; no NO2− was detected. The reason why it was not detected is unclear. A possible reaction was the adsorption of NO2− onto goethite at pH < point of zero charge (pzc) of goethite (6.42) due to electrical attractive force. The maximum production of NO2− at a pH of 7 was in the order of citric acid >> syringic acid > NOM. However, Fe2+ was not detected at this pH even though Fe2+ should be required for NO2− production. To better understand of these phenomena, the adsorptive removal of Fe2+ and NO2− onto goethite was experimentally investigated. More than 90% of the produced Fe2+ and NO2− could be removed rapidly by adsorption onto the surface of goethite at pH 7 and 4, respectively. In addition, the reaction of Fe2+ with NO3− appeared to determine the overall reaction rate of the Fe(III)/Fe(II) cycle because of its relatively slow reaction rate. Using these results, we conclude that NO2− can be produced from NO3− reduction through Fe(III)/Fe(II) cycle with labile organic materials and ammonium at a pH of 7; especially, Fe(III)/Fe(II) cycle with citric acid results the maximum NO2− production higher than 600 μM for a long time (over 200 h) and then disappeared. But, the reasons for its disappearance were not addressed in this study.