STIMULATION BY LEPIDOCROCITE (GAMMA-FEOOH) OF FE(II)-DEPENDENT NITRITE REDUCTION

Fe(II)-dependent nitrite reduction was studied in oxygen-free reaction flasks at pH values from 6.0 to 8.5 and in the presence and absence of the Fe(III) oxyhydroxide lepidocrocite (gamma-FeOOH). When ionic Fe2+ was added to flasks containing lepidocrocite, rapid Fe2+ consumption and release of H+ took place. The solid phase turned dark and contained magnetite (Fe3O4). This reaction was independent of the presence of NO2-. However, rapid NO2- reduction primarily to N2O was observed only after the Fe2+ binding to lepidocrocite was initiated; insignificant NO2- reduction was observed in control flasks without lepidocrocite. Both Fe2+ binding and associated H+ release and NO2- reduction were negligible at pH values of 7.0 and below, but increased dramatically as pH was raised to 8.5. The Fe(II) associated with a reactive complex formed during Fe2+ binding to lepidocrocite, and not the ionic Fe2+, seemed responsible for the reduction of NO2-. The catalytic effect of Fe(III) oxyhydroxide may stimulate Fe(II)-dependent formation of N2O from NO2- (chemodenitrification) in geochemical systems such as sediments and subsoils.