Abiotic reduction of nitrate to ammonium by iron (oxy)(hydr)oxides and its stable isotope (815N,818O) dynamics

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 degrees 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 x 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 spe-cies 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 (815N-NO3-) isotopic frac-tionation (156) for GR(Cl-) and GR(SO42-) were were calculated to be 12.9 (CI: 8.2, 16)%degrees and 37.9 (CI: 34, 41)%degrees, respectively. Similarly, oxygen (818O-NO3-) isotopic fractionation (186) differed between GR(Cl-) and GR(SO42-): 4.53 (CI: 2.5, 5.8)%degrees and 14.4 (CI: 12, 16)%degrees. However, the ratio of186:156 for both minerals could be fitted to a linear regression of slope 0.369 (CI: 0.361, 0.377). These findings reaffirm the impor-tance of GR minerals to NO3- reduction, especially in iron-rich systems hosting dynamic redox oscilla-tions, 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 186:156 can provide a means to distinguish between this abiotic anoxic reaction from anaerobic bacterial NO3- reduction processes.(c) 2023 Elsevier Ltd. All rights reserved.

Automated summary

The kinetics and stable isotopic fractionation of nitrate (NO3-) reduction in Fe(II)/Fe(III) homogeneous/heterogeneous systems were investigated. The reduction of NO3- was only significant in the presence of layered double hydroxide green rust (GR) minerals. The study provides insight into the importance of GR minerals in NO3- reduction and the potential use of stable isotopic fractionation analyses in distinguishing abiotic anoxic reactions from anaerobic bacterial NO3- reduction processes.