Environ. Sci. Technol. 1996, 30, 2053-2056
Abiotic Nitrate Reduction to Ammonium: Key Role of Green Rust H A N S C H R . B . H A N S E N , * ,† CHRISTIAN B. KOCH,‡ HANNE NANCKE-KROGH,† OLE K. BORGGAARD,† AND JAN SØRENSEN§ Chemistry Department, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark, Physics Department, Building 307, Technical University of Denmark, DK-2800 Lyngby, Denmark, and Microbiology Section, Department of Ecology and Molecular Biology, Royal Veterinary and Agricultural University, Rolighedsvej 21, DK-1958 Frederiksberg C, Denmark
Leaching of nitrate from soils and sediments can be reduced in anoxic environments due to denitrification to N2O/N2 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 [FeII4FeIII2(OH)12SO4‚yH2O]. In the reaction nitrate is stoichiometrically reduced to ammonium, and magnetite (Fe3O4) is the sole Fe-containing product. At a constant pH of approximately 8.25 and 25 °C, the rate expression is given as: d[NH4+]/dt ) k[Fe(II)]GR[NO3-],where 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.
Introduction Iron(II) reduces nitrate to nitric oxide in hot dilute sulfuric acid or to ammonia in alkaline solutions (1). At neutral pH, nitrate reduction by iron(II) is observed only at elevated temperatures (2) or at room temperature when catalysts such as Cu(II) or Ag(I) are present (3). Under conditions similar to those in anoxic soils and sediments, the participation of greenish iron(II)-iron(III) hydroxides in nitrate * Corresponding author telephone: +45-35-28-24-12; fax: +4535-28-20-89; e-mail address:
[email protected]. † Chemistry Department, Royal Veterinary and Agricultural University. ‡ Technical University of Denmark. § Microbiology Section, Royal Veterinary and Agricultural University.
S0013-936X(95)00844-3 CCC: $12.00
1996 American Chemical Society
reduction has been reported (4). These hydroxides, termed ‘green rusts’ (GR), easily precipitate from partially oxidized Fe(II) solutions with pH at or above neutrality (5-7). GRs may form under moderately reducing conditions in soils and sediments (6, 8), and although their identification is difficult due to rapid oxidation by air, it has been found in a number of environments (9-12). GRs are structurally analogues to pyroaurite (13) consisting of positively charged hydroxide layers of variable composition [(FeII6-xFeIIIxOH12]x+ sandwiching interlayers containing anions (A) and water molecules [A‚yH2O]x-. Nitrous oxide is produced in the reduction of nitrite by GR with A ) SO42- (GRSO42-) (6), and at higher pH ammonia is probably formed (4). Thermodynamic calculations indicate that nitrate reduction by GRSO42- is a spontaneous reaction under earth surface conditions with several possible products (6). Measurements on sediments show that at some depth oxidized compounds, e.g., NO3-, disappear from solution. Below this depth, reduced species, e.g., NH4+ and Fe(II), appear in solution. Considerable interest has been given to this reduction of NO3- as this protects groundwater from excessively high NO3- concentrations (14-17). Both biotic and abiotic reactions may be involved, but virtually no information is available on the kinetics of the abiotic reactions allowing their relative importance to be established. Similar transition zones may temporarily exist in the soil profile indicating that inorganic reactions may compete with the well-established biotic reaction. GRs are likely to form in non-acid, Fe(II)-containing soils and sediments. Detailed insight into the reaction between GRs and nitrate is thus required for assessment of the possible environmental significance of abiotic conversion of nitrate to ammonia. In this study, we demonstrate that GRSO42- reacts with nitrate with the corresponding products being magnetite and ammonium. The kinetics of the reaction at pH = 8.25 and 25 °C is quantified.
Experimental Section GRs are very sensitive to air oxidation; hence all handling was carried out in an argon atmosphere (99.999%), and all solutions were argon saturated before use (50 mL min-1, g2 h). To exclude possible photoredox effects, all reactions were carried out in the dark. GRSO42- was synthesized by air oxidation of an 0.05 M FeSO4 solution using a ‘pH-stat’ apparatus set at pH 7.0 and using NaOH as titrant (6). The precipitate was separated by filtration and washed before resuspension in 0.2 dm3 of water in an 0.3 dm3 gas-tight and thermostatted (25 °C) reaction vessel equipped with a pH combination electrode, inlets for base (0.1 M NaOH) and argon, and valves for withdrawal of liquid and gas samples. Reaction of the GR suspension with nitrate was carried out at constant pH using the ‘pH-stat’ described above. After addition of NaNO3, the concentrations of Fe(II) and NH4+ were monitored. Fe(II) was measured both in the supernatant in samples passed through 0.22-µm Millipore filters [Fe(II)sol] and in suspension samples following digestion of the GRSO42- in 0.025 M HCl for 5 min and then passing the solution through 0.22-µm Millipore filters [Fe(II)GR+sol]. The filter was found to retain all particulates as no increase in the concentration of Fe could be observed on acidification of the filtrate. Fe(II) was determined using
VOL. 30, NO. 6, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2053
B
A
FIGURE 1. Production of NH4+ vs consumption of NO3- for the reaction between GRSO42- and NO3- in two experiments with initial NO3concentrations of 1.77 (b) and 0.71 mM (9), respectively. Initial concentrations of Fe(II)GR were 11 (b) and 12 mM (9), and a constant pH of 8.1 was maintained.
the 1,10-phenanthroline method (18). All solutions mixed with 1,10-phenanthroline were kept in the dark until measurement to minimize the risk of photochemical redox reactions. Ammonium was determined by gas diffusion into buffer/indicator solutions (19) in suspension samples quenched by air oxidation of GR and passed through 0.22µm Millipore filters. In some experiments NO3-, N2O, N2 (experiments using 15N-labeled NO3-), and H2 were also determined. Nitrate was determined by a modified Griess method after reduction to nitrite on a Cd column (20), and gases were determined by gas chromatography. X-ray diffraction (XRD) and Mo¨ssbauer spectroscopy are used to test identity and purity of the starting GRSO42- and solid end products. Samples for XRD were collected on 0.22-µm filters, preserved against oxidation by admixing glycerol (21), and scanned as smears using Co KR radiation. Samples for Mo¨ssbauer spectroscopy were withdrawn as aqeous suspensions, transferred to Perspex sample holders, and quenched by dropping into liquid N2. Mo¨ssbauer spectra were obtained using a constant acceleration spectrometer at temperatures between 16 and 250 K. Isomer shifts are given relative to the centroid of the spectrum of natural R-iron at room temperature. Double deionized, carbon-filtered water and acid washed glassware were used throughout.
FIGURE 2. X-ray diffractograms of solids at start (A) and end (B) of reaction between GRSO42- and NO3-.
Results and Discussion Reactants and Products. In experiments where both NO3and NH4+ were determined, the amount of nitrate consumed equaled the amount of ammonium produced (Figure 1). The concentrations of N2O and N2 were below detection limits (
