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Reduced sulfur and iron species in anoxic water column of Lake Pavin

The vertical distribution of reduced sulfur species (RSS including H2S/HS-, S0, electroactive FeS) and dissolved Fe (II) were studied in the anoxic water column of meromictic Lake Pavin. Sulfide concentrations were determined by two different analytical techniques, i.e. spectophotometry (methylene blue technique) and voltammetry (HMDE electrode). Total sulfide concentrations determined with methylene blue method (∑ H2SMBRS) were in the range from 0.6 µ ; M to 16.7 µ ; M and were substantially higher than total reduced sulfur species (RSSV) concentrations determined by voltammetry, which ranged from 0.1 to 5.6 μ M. The observed difference in the sulfide concentrations between the two methods can be assigned to the presence of electroactive FeS colloidal species. Dissolved Fe was high (>1000 µ ; M), whereas dissolved Mn was only 25 µ ; M, in the anoxic water column. This indicates that Fe is the dominant metal involved in sulfur redox cycling and precipitation. Consequently, in the anoxic deep layer of Lake Pavin, “ free” sulfide, H2S/HS-, was low ; and about 80 % of total sulfide detected was in the electroactive FeS colloidal form. IAP calculations showed that the Lake Pavin water column is saturated with respect to FeSam phase. The upper part of monimolimnion layer is characterized by higher concentrations of S(0) (up to 3.4 µ ; M) in comparison to the bottom of the lake. This behavior is probably influenced by sulfide oxidation with Fe(III) oxyhydroxide species and pyrite formation at the lake bottom.

reduced sulfur ; iron species ; voltammetry ; Pavin Lake

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