EVALUATION OF THE FREE-ENERGY OF FORMATION OF FE(II)-FE(III) HYDROXIDE-SULFATE (GREEN RUST) AND ITS REDUCTION OF NITRITE

The green ''ferrosic hydroxide'', ''ferroso ferric hydroxide'', or ''hydromagnetite'', Fe3(OH)8, claimed to have formed during alkalimetric titrations of acid Fe(II)-Fe(III) sulphate solutions and suggested by many workers to be present in anaerobic soils and sediments, is shown by means of X-ray diffraction to be an Fe(II)-Fe(III) hydroxide-sulphate (sulphate interlayered green rust, GR(SO4)). It has the approximate composition [Fe4IIFe2III(OH)12][SO4.3H2O], and structurally belongs to the pyroaurite-sjogrenite group of layered hydroxides. From solution data, the standard free energy of formation of GR(SO4) is estimated to -4380 +/- 4 kJ . mol-1. Using this value, the reduction of NO2- to N2O, N2, or NH4+ by GR(SO4) are thermodynamically spontaneous processes, both under standard state conditions and conditions which can exist in natural anoxic waters. In aqueous solutions at pH 7 and containing FeSO4, K2SO4, NaNO2, and freshly precipitated poorly ordered ferrihydrite two types of redox reactions between Fe(II) and NO2- are operating: (1) rapid reduction of NO2- when Fe2+ reacts with ferrihydrite to form GR(SO4) and (2) a slower reaction in which NO2- is reduced by Fe(II) in the GR(SO4) lattice. The major oxidation product is goethite (alpha-FeOOH). Thus, it appears that the reduction of NO2- by GR(SO4) is not kinetically hindered. If present in soils and sediments, green rusts may participate in abiotic reduction of NO2-. Further work is in progress to study the kinetics of reduction of NO3- by GR(SO4). This reaction also is thermodynamically spontaneous.