Abiotic reduction of nitrate to ammonium by iron (oxy)(hydr)oxides and its stable isotope (δ¹⁵N, δ¹⁸O) dynamics

Abstract

In this study, the kinetics and stable isotopic fractionation of nitrate (NO₃‾) reduction in Fe(II)/Fe(III) homogeneous/heterogeneous systems were investigated at circumneutral pH (i.e., pH 6.5 to 7.5) and ambient temperature (24 °C). Of the suite of iron (oxy)(hydr)oxides examined, NO₃‾ reduction by Fe(II) was only significant in the presence of layered double hydroxide green rust (GR) minerals. Comparison of type 1 and type 2 GRs demonstrated that type 1 (GR(Cl‾)) had the fastest NO₃‾ reducing ability, with an Fe(II)-normalized pseudo first-order kinetic rate constant of kobs (NO₃‾) = 16.2 × 10¹ M‾¹ d‾¹. Anion concentrations (SO₄²− or Cl‾) and the ratio of [Fe(II)] to [Fe(III)] also influenced reduction kinetics. Ammonium (NH₄⁺) was the predominant reaction end-product (>50%) with the remaining reduced N species concluded to be comprised of N₂(g) as other intermediates (nitrite (NO₂‾), nitric oxide (NO) and nitrous oxide (N₂O) were not detected during NO₃‾ reduction. Apparent nitrogen (δ¹⁵N-NO₃‾) isotopic fractionation (¹⁵ε) for GR(Cl‾) and GR(SO₄²−) were were calculated to be 12.9 (CI: 8.2, 16)‰ and 37.9 (CI: 34, 41)‰, respectively. Similarly, oxygen (δ¹⁸O-NO₃‾) isotopic fractionation (¹⁸ε) differed between GR(Cl‾) and GR(SO₄²−): 4.53 (CI: 2.5, 5.8)‰ and 14.4 (CI: 12, 16)‰. However, the ratio of ¹⁸ε:¹⁵ε for both minerals could be fitted to a linear regression of slope 0.369 (CI: 0.361, 0.377). These findings reaffirm the importance of GR minerals to NO₃‾ reduction, especially in iron-rich systems hosting dynamic redox oscillations, including hyporheic zones, estuarine sediments and groundwater aquifers. These results also demonstrate that N and O stable isotope kinetic fractionation analyses as well as the ratio of ¹⁸ε:¹⁵ε can provide a means to distinguish between this abiotic anoxic reaction from anaerobic bacterial NO₃‾ reduction processes.