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Oxidation of pyrite (FeS2) under anaerobic conditions in marine sediments is experimentally shown for the first time. In slurry experiments with 55FeS2 and a MnO2 rich marine sediment an oxidation of 55FeS2 was detected which decreased with depth and decreasing concentration of MnO2 in the sediment.
FeS2 and iron sulfide (FeS) were oxidized chemically at pH 8 by MnO2 but not by nitrate or amorphic Fe(III) oxide. Elemental sulfur and sulfate were the only products of FeS oxidation, whereas FeS2 was oxidized to a variety of sulfur compounds, mainly sulfate plus intermediates such as thiosulfate, trithionate, tetrathionate, and pentathionate. Thiosulfate was oxidized by MnO2 to tetrathionate while other intermediates were oxidized to sulfate. The reaction products indicate that FeS2 was oxidized via the “Thiosulfate-mechanism” and FeS via the “Polysulfide-mechanism” (Schippers and Sand, 1999). The processes are summarized by the overall equations: (1) FeS2+7.5 MnO2+11 H+→Fe(OH)3+2 SO42−+7.5 Mn2++4 H2O (2) FeS+1.5 MnO2+3 H+→Fe(OH)3+SO+1.5 Mn2+ For FeS2 oxidation the reaction rates related to the mineral surface area were 1.02 and 1.12 nmol m−2 s−1 for total dissolved S and total dissolved Fe, respectively. Since these values are in the same range as previously published rates for the oxidation of FeS2 by Fe(III), and since Fe(III) is a well-known oxidant for FeS2 even at circumneutral pH, Fe(III) is assumed also to be the oxidant for FeS and FeS2 in the presence of MnO2. At the iron sulfide surface, Fe(III) is reduced to Fe(II) which is reoxidized to Fe(III) by MnO2. Thus, an Fe(II)/Fe(III) shuttle should transport electrons between the surfaces of the two solid compounds.
