Abiotic reduction of nitrate to ammonium by iron (oxy)(hydr)oxides and its stable isotope (δ15N, δ18O) dynamics
Geochimica et Cosmochimica Acta(2023)
摘要
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.
更多查看译文
关键词
Iron (oxy)(hydr)oxides,Isotopic fractionation,N transformation,Nitrate chemodenitrification,Stable isotope analyses
AI 理解论文
溯源树
样例
![](https://originalfileserver.aminer.cn/sys/aminer/pubs/mrt_preview.jpeg)
生成溯源树,研究论文发展脉络
Chat Paper
正在生成论文摘要