Abiotic transient nitrite occurrences from nitrate reduction through goethite-mediated Fe(III)/Fe(II) cycle with labile organic materials and ammonia

The abiotic reduction of NO^-₃ to NO^-₂-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 NO^-₂ were monitored. At a pH of 4, concentrations of Fe²⁺ increased, except for citric acid; no NO^-₂ was detected. The reason why it was not detected is unclear. A possible reaction was the adsorption of NO^-₂ onto goethite at pH < point of zero charge (pzc) of goethite (6.42) due to electrical attractive force. The maximum production of NO^-₂ 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 NO^-₂ production. To better understand of these phenomena, the adsorptive removal of Fe²⁺ and NO^-₂ onto goethite was experimentally investigated. More than 90% of the produced Fe²⁺ and NO^-₂ could be removed rapidly by adsorption onto the surface of goethite at pH 7 and 4, respectively. In addition, the reaction of Fe²⁺ with NO^-₃ 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 NO^-₂ can be produced from NO^-₃ 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 NO^-₂ 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.