engleski

Sulfur speciation in two contrasting anoxic environments

Using different electrochemical methods it is possible to measure a variety of soluble sulfur compounds (1-2). In this work we used HMDE electrode to measure different dissolved and particulate sulfur species, including ∑ S2-, ∑ S0 and metal sulfide species in two contrasting anoxic environments: seawater lake, Rogoznica Lake (Croatia) and crater lake, Pavin Lake (France). In the Pavin Lake samples, in parallel sulfide concentrations were determined by using second analytical technique: spectrophotometric methylene blue and obtained results were compared to ones obtained by electrochemical measurements. Obtained ∑ S2-and ∑ S0 in electrochemical measurements were used for mathematical calculations of sulfur speciation (S8, polysulfides) in the water column of both lakes. Calculations were made on the basis of voltammetric measurements for ∑ S2- and ∑ S0 and by using equilibrium constants for polysulfide disproportionation [(n-1)/8S8(s) + HS- = Sn2- + H+] reported by Giggenbach and Kamyshny (3-4). Anoxic layer in Rogoznica Lake (usually situated from 10 to the 15 m depth) is characterized by relative high free S2- concentrations (up to the 10-2 M) which completely binds and precipitates almost completely metals from the water phase (5). Our calculations showed that during stratification period anoxic part of Rogoznica Lake water column is saturated with respect to the rhombic and/or colloidal form of S8 and in the same time S8 concentrations in the oxic part are low (up to the 4, 12 x 10-9 M, Fig. 1A). Major polysulfide species concentration (S52-, S42-, HS4-) in anoxic water column are relative high (up to the 2 x 10-4) implying that polysulfides are contributing the most to the ∑ S0 (up to the 1.40 x 10-4 M) found and measured by voltammetric methods. Electrochemical sulfur speciation in anoxic part of Pavin Lake water column (usually situated from 62 to the 92 m depth) is completely different than in Rogoznica Lake. ∑ S0 concentrations determined by voltammetry in Pavin Lake were lower (up to the 3.40 x 10-6 M). ∑ S2- concentrations determined with methylen-blue technique were in the range from 0.6 µ ; M to 16.7 µ M and are substantial higher than ∑ S2- concentrations determined by voltammetric measurements which ranged from 0.1 to 3.7 μ M. Observed difference in the RSS concentrations between the two methods can be assigned to the presence of FeS dissolved and/or colloidal species which presence is confirmed electrochemicaly with the appearance of voltammetric peaks typical for FeS species, situated at -1.1 V (Fig. 1B ) in the anoxic part of the Pavin Lake samples (6). Consequently, in anoxic deep layer of Lake Pavin free S2- was relatively low (up to the 3.7 x 10-6 M) ; and about 80 % of total sulfide detected (<0.2 μ m) was in the FeSaq and/or FeSam colloidal form. Obtained results are supported as well by high concentrations (>1000 µ M) of dissolved Fe (<0.2 µ M) determined in the anoxic water column of Pavin Lake, which indicate that Fe is dominant metal involved in sulfur redox cycling and precipitation.

sulfur ; anoxic ; FeS

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