Kinetics of nitrate reduction by green rusts—effects of interlayer anion and Fe(II):Fe(III) ratio

The reaction between the synthetic chloride form of green rust (GR) with an Fe(II):Fe(III) ratio of 3:1 (GRCl, Fe4.5IIFe1.5III(OH)12Cl1.5xH2O) and nitrate has been studied at GRCl concentrations up to 2.5 mM and nitrate concentrations from 0.7 to 14.2 mM in non-buffered aqueous suspensions under an argon atmosphere. Nitrate is reduced stoichiometrically to ammonium while GRCl is oxidized to magnetite with concurrent decrease of pH from 8.2 to 6.1. No nitrate reduction was observed in homogenous Fe(II) solutions at neutral pH. The rates of ammonium formation and consumption of Fe(II) in GRCl (Fe(II)GR) are well described by first-order reaction kinetics. No clear rate dependence with respect to nitrate was found, but it appears that above a nitrate concentration of approximately 7 mM and at fixed concentrations of Fe(II)GR, the rate of nitrate reduction is almost constant and independent of the nitrate concentration. Under these conditions, the rate law may be given by:d[NH4+]dt=k[Fe(II)]GRwhere k=2.0±0.5×10−5 s−1. Interlayer chloride in GRCl could be exchanged with sulphate without change in the Fe(II):Fe(III) ratio of the GR. The resulting sulphate GR reduced nitrate six times slower than the original GRCl but the reaction was five times faster when compared with reduction by sulphate GRs having Fe(II):Fe(III) ratios of 2:1. Thus, the high rate of nitrate reduction by GRCl can be attributed partly to ready exchange of chloride with nitrate in the GR interlayer and partly to a high proportion of Fe(II) in the octahedral layers of GR.