摘要：

Oxidation of pyrite (FeS 2) under anaerobic conditions in marine sediments is experimentally shown for the first time. In slurry experiments with 55FeS 2 and a MnO 2 rich marine sediment an oxidation of 55FeS 2 was detected which decreased with depth and decreasing concentration of MnO 2 in the sediment. FeS 2 and iron sulfide (FeS) were oxidized chemically at pH 8 by MnO 2 but not by nitrate or amorphic Fe(III) oxide. Elemental sulfur and sulfate were the only products of FeS oxidation, whereas FeS 2 was oxidized to a variety of sulfur compounds, mainly sulfate plus intermediates such as thiosulfate, trithionate, tetrathionate, and pentathionate. Thiosulfate was oxidized by MnO 2 to tetrathionate while other intermediates were oxidized to sulfate. The reaction products indicate that FeS 2 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) FeS 2+7.5 MnO 2+11 H +→Fe(OH) 3+2 SO 4 2+7.5 Mn 2++4 H 2O (2) FeS+1.5 MnO 2+3 H +→Fe(OH) 3+S O+1.5 Mn 2+ For FeS 2 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 FeS 2 by Fe(III), and since Fe(III) is a well-known oxidant for FeS 2 even at circumneutral pH, Fe(III) is assumed also to be the oxidant for FeS and FeS 2 in the presence of MnO 2. At the iron sulfide surface, Fe(III) is reduced to Fe(II) which is reoxidized to Fe(III) by MnO 2. Thus, an Fe(II)/Fe(III) shuttle should transport electrons between the surfaces of the two solid compounds.

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