Computer Science
Scientific paper
Jun 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.2599b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 12, pp.2599-2608
Computer Science
6
Scientific paper
The green "ferrosic hydroxide", "ferroso ferric hydroxide", or "hydromagnetite", Fe 3 (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 [Fe 4 III Fe 2 III (OH) 12 ][SO 4 · 3H 2 O], and structurally belongs to the pyroaurite-sjögrenite 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 NO 2 - to N 2 O, N 2 , or NH 4 + 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 FeSO 4 , K 2 SO 4 , NaNO 2 , and freshly precipitated poorly ordered ferrihydrite two types of redox reactions between Fe(II) and NO 2 - are operating: (1) rapid reduction of NO 2 - when Fe 2+ reacts with ferrihydrite to form GR so4 and (2) a slower reaction in which NO 2 - is reduced by Fe(II) in the GR so4 lattice. The major oxidation product is goethite ( -FeOOH). Thus, it appears that the reduction of NO 2 - by GR so4 is not kinetically hindered. If present in soils and sediments, green rusts may participate in abiotic reduction of NO 2 - . Further work is in progress to study the kinetics of reduction of NO 3 - by GR so4 . This reaction also is thermodynamically spontaneous.
Bruun Hansen Hans Christian
Kragholm Borggaard Ole
Sørensen Jan
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