The Chemodenitrification of Nitrate by Green Rust and Mackinawite and its Geochemical Implications

Soil minerals that can participate in the redox cycling of Fe(II)/Fe(III) have received a significant amount of attention over recent decades in terms of their capability to reduce contaminants. Although evidence has demonstrated that the biogeochemical coupling of Fe and N redox cycles has potentially important capabilities, the relevance of abiotic Fe and N redox transformations remain unclear. Therefore, in this thesis, aspects around abiotic nitrate (NO3-) reduction under environmentally relevant conditions has been undertaken to shed further insight into NO3- chemodenitrification driven by Fe(II)/Fe(III)-bearing minerals. The kinetics of NO3- chemodenitrification in Fe(II)/Fe(III) (oxy)(hydr)oxide heterogeneous systems were investigated at neutral pH (i.e. pH 6.5 to 7.5). Of the suite of minerals examined, NO3- chemodenitrification was only relevant in the presence of green rust (GR), a layered double hydroxide Fe(II)/Fe(III) mineral. High anion (SO42-/Cl-) concentrations could arrest chemodenitrification, most likely through competition for sorption, and hence electron transfer, sites. Stable N and O isotope studies indicated that 15ε and 18ε values produced from NO3- chemodenitrification, principally to ammonium (NH4+), by GR(SO42-) (37.9‰ and 14.4‰) and GR(Cl-) (12.9‰ and 4.5‰) were unique, suggesting the possibility of distinguishing between NO3- chemodenitrification induced by these two minerals as well as from microbial NO3- denitrification. Electron transfer from aqueous Fe(II) to structural Fe(III) in montmorillonite has recently been discovered to be a pathway to GR formation at circumneutral pH. In contrast to the observations described above, the kinetics of NO3- chemodenitrification by these GR minerals were found to be extremely slow. It was demonstrated that the reduction potentials induced by the GR minerals formed through this unique pathway (~ -110 to -210 mV) were much higher than those obtained for GR minerals prepared in the absence of montmorillonite (~ -300 mV). As such, the driving force for electron transfer was decreased to the point that GRs formed through this unique pathway will no longer provide a competitive process to microbial NO3- denitrification reactions. Although iron sulfides are known to facilitate NO3- chemodenitrification, there are still fundamental knowledge gaps on reaction mechanisms and stable N and O isotope dynamics. Here, studies were conducted to examine aspects around NO3- chemodenitrification by mackinawite (FeS). Although it was observed that NO3- chemodenitrification kinetics were faster at acid pH values, in disagreement with thermodynamics, this observation most likely resulted from the higher reactivity of a soluble reductant or possibly FeS oxidation products precipitating at higher pH values passivating redox-active surfaces. These oxidation products included greigite (Fe3S4), elemental sulfur (S0), thiosulfate (S2O32-) and sulfate (SO42-), suggesting that both Fe and S participated in the reduction of NO3-. In this case, the major reduced N species was N2(g), and not NH4+. No N and O stable isotope fractionation was observed for the first electron transfer step in the reduction of NO3- indicating that it is a reversible reaction and, as a result, this reaction will introduce variability to the kinetic isotope effects produced by other concurrent NO3- (chemo)denitrification reactions. Overall, this thesis has significantly advanced knowledge on NO3- chemodenitrification reactions, mechanisms and methods for distinguishing chemical and microbial denitrification pathways