Stimulation by lepidocrocite (7-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 (γ-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.