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

Hyeoksun Choi,Sanghwa Oh

doi:10.3390/w12041202

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.

Highlights

Nitrogen has many oxidation states and is important for biomolecules such as enzymes and nucleic acids
The concentration of syringic and citric acids in samples were measured using capillary electrophoresis (CE) (P/ACE MDQ, Beckman Coulter, Brea, CA, USA) with diode-array UV-visible detector with buffer solutions consisting of 25-mM phosphate and 0.5-mM tetradecyltrimethylammonium bromide (TTAB, Sigma-Aldrich, >99%, pH 7) for syringic acid and 25-mM ortho-P plus pyro-P and
At a pH of 7 (Figure 3b), a peak NO2 − concentration of approximately 4 μM was observed at about 120 h of reaction time before the concentration decreased but no Fe2+ was detected

Summary

Introduction

Nitrogen has many oxidation states and is important for biomolecules such as enzymes and nucleic acids. Ammonium (NH4 + ) and nitrate (NO3 − ) are two major nitrogen sources in surface and subsurface water systems because they are the final reduced and oxidized nitrogen under anaerobic and aerobic conditions, respectively and nitrite (NO2 − ) is frequently found under specific anaerobic conditions. Nitrogen contamination in surface water and groundwater has mostly been caused by ammonia and nitrate worldwide [1]. Among these nitrogen compounds, NO3 − contamination is generally derived from the oxidation of NH4 + through nitrification by autotrophic bacteria. Ammonia has mainly come from the discharge of agricultural fertilizer and the mineralization of organic nitrogen under aerobic and anaerobic conditions [2,3,4].

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