Abiotic Nitrate Reduction to Ammonium:  Key Role of Green Rust

Abstract

Leaching of nitrate from soils and sediments can be reduced in anoxic environments due to denitrification to N 2 O/N 2 or reduction of nitrate to ammonium. While microbial dissimilatory reduction of nitrate to ammonia is well known, it is shown here that this conversion can also proceed at appreciable rates in abiotic systems in the presence of green rust compounds [Fe II 4 Fe III 2 (OH) 12 SO 4 ‚yH 2 O]. In the reaction nitrate is stoichiometrically reduced to ammonium, and magnetite (Fe 3 O 4 ) is the sole Fe-containing product. At a constant pH of approximately 8.25 and 25 °C, the rate expression is given as: dwhere k ) 4.93 × 10 -5 ( 0.39 × 10 -5 L mol -1 s -1 . In anoxic soils and sediments, this reaction may also lead to a nitrate to ammonium reduction, at rates of similar magnitude or even higher than microbial reduction rates. Hence green rust should be considered a possible important reductant for nitrate reduction to ammonium in subsoils, sediments, or aquifers where microbially mediated reduction rates are small.

Marine redox variations and nitrogen cycle of the early Cambrian southern margin of the Yangtze Platform, South China: Evidence from nitrogen and organic carbon isotopes

“…However, the ␦ 15 N bulk values (0-2‰) are lower than the average ␦ 15 N nitrate value of modern seawater (+5‰) (Sigman et al, 2009). One possible explanation for this is that the nitrate reservoir of the local basin was small, probably because of intensive loss of nitrate through denitrification, anammox, and reduction by Fe 2+ at the redox boundary (Busigny et al, 2013;Hansen et al, 1996;Shen et al, 2013;Straub et al, 1996;Weber et al, 2006a,b) (Fig. 6c), driving N 2 -fixation as compensation to lower the ␦ 15 N bulk values toward 0‰ (Ader et al, 2014).…”

“…6a). In this case, the nitrate reservoir is likely to have been small, and was either reduced to ammonia by mixed Fe(II)/(III) oxy(hydr)oxide species through abiotic processes (Hansen et al, 1996), or converted to NO 2 -/N 2 /NH 4 + mediated by anaerobic nitrate-dependent Fe(II)-oxidizing microorganisims under dominantly ferruginous conditions (Busigny et al, 2013;Shen et al, 2013;Straub et al, 1996;Weber et al, 2006a,b) (Fig. 6a), although nitrogen isotopic fractionation effects associated with these processes are still unclear.…”

Formation of single domain magnetite by green rust oxidation promoted by microbial anaerobic nitrate-dependent iron oxidation

“…The very slow transformation of GR to magnetite in the abioticcontrol might be related to the slow diffusion of an amorphous Fe(III) (hydr)oxide phase 1 reacting with green rust (Fig. 4, Table 1) and/or to nitrate reduction (Hansen et al, 1996), 2 whereas nitrate might be much more rapidly reduced through bacterial activity in BoFeN1 3 cultures and thus unavailable for GR oxidation. Moreover, in the present study, periplasmic lepidocrocite crystals are strongly anisotropic, 17 elongated parallel to the cell wall (Fig.…”

“…Abiotic green 22 rust oxidation is kinetically controlled and leads to either lepidocrocite, goethite, magnetite or 23 ferric hydroxycarbonate depending on pH conditions and the nature and concentration of the 24 oxidant (e.g. Hansen et al, 1996;Ruby et al, 2010 nitrate (10 mM sodium nitrate) were provided as a carbon source and as a terminal electron 20 acceptor, respectively. After 2 days, bacteria were harvested by centrifugation (5000 g, 15 21 min) and rinsed twice in 0.6 g.L -1 NaCl.…”

Abiotic nitrate incorporation, anaerobic microsites, and the ferrous wheel

“…First, while all of these possible catalysts promote NO 3 ¡ reduction, none have NO 2 ¡ as their end product. Copper and green rusts yield NH 3 (Ottley et al 1997;Hansen et al 1996), while adsorbed Fe 2+ yields N 2 (Postma 1990). Second, it appears improbable that this catalysis would happen in the acid forest soils where abiotic NO 3 ¡ incorporation has been hypothesized to occur.…”