Abstract
BackgroundIonic and molecular carriers of dissolved (filter-passing) zero-valent sulfur (S0) in anaerobic natural waters include polysulfides, Sn2−, molecular S8(aq), organic macromolecules and certain higher valent thioanions. Because S0 is rapidly transferred among these various carriers, its biogeochemical roles in such processes as dehalogenation of organic compounds, chelation of trace metals, and anaerobic microbial metabolism are not determined solely by one ionic or molecular species. Here, S0 is treated collectively as a virtual thermodynamic component, and computational as well as graphical methods for quantifying its activity (aS0) in natural waters are presented. From aS0, concentrations of the ionic and molecular carriers of S0 can be calculated easily.ResultsConcentration ratios of any two polysulfide ions define aS0 (Method I). Unfortunately these concentrations are often too low in nature for accurate quantification with current methods. Measurements of total divalent sulfur (ΣS-II), zero-valent sulfur (ΣS0) and pH provide a more widely applicable approach (Method II). Systematic errors in ΣS0 measurements are the main limit to accuracy of this method at the present time. Alternative methods based on greigite solubility and potentiometry are discussed. A critical comparison of Methods I and II reveals inconsistencies at low ΣS0/ΣS-II that imply errors in the thermodynamic data for HS2− and S2−. For samples having low ΣS0/ΣS-II, an interim remedy is recommended: letting pKa2 = 6.3 for all HSn− ions.ConclusionsNewly assembled data for aS0 in a selection of anaerobic natural waters indicate that S0 is always metastable in the surveyed samples with respect to disproportionation to sulfide and sulfate. In all the surveyed environments, sulfur-rich minerals, such as greigite, covellite and orpiment, are stable in preference to their sulfur-poor cohorts, mackinawite, chalcocite and realgar. The aS0 values in the dataset span conditions favoring Hg-polysulfide complexes vs. Hg-sulfide complexes, implying that aS0 could affect Hg-methylation rates in nature. No support is found for the common assumption that aS0 = 1 in reducing natural waters. This paper calls attention to an urgent need for improved measurement methods, especially for total zero-valent sulfur, as well as new determinations of ionization constants for all HSn− species.Electronic supplementary materialThe online version of this article (doi:10.1186/s12932-014-0013-x) contains supplementary material, which is available to authorized users.
Highlights
Ionic and molecular carriers of dissolved zero-valent sulfur (S0) in anaerobic natural waters include polysulfides, Sn2−, molecular S8(aq), organic macromolecules and certain higher valent thioanions
Zero-valent S can occur in polythionates (O3SSnSO32−, n ≥ 0) and in organic polysulfides or polysulfanes (RSn−, RSnR’, n ≥ 2), but in most sulfidic natural waters these contribute negligibly to total dissolved zero-valent sulfur (ΣS0) [6,10]
If S8 and all polysulfides are in equilibrium with one another owing to rapid S-atom transfer reactions, equation 5 expresses the relationship between dissolved ΣS0, aHS, and pH
Summary
Ionic and molecular carriers of dissolved (filter-passing) zero-valent sulfur (S0) in anaerobic natural waters include polysulfides, Sn2−, molecular S8(aq), organic macromolecules and certain higher valent thioanions. The most commonly reported intermediates are zero-valent sulfur, thiosulfate (S2O32−), and sulfite (SO32−) [5,6,7,8]. In aqueous solutions, dissolved molecular zerovalent sulfur occurs almost entirely as S8 [9]. If (and only if) free sulfide is present, dissolved zero-valent sulfur will be found in polysulfides, HxSnx-2 (x = 0 to 2, n = 2 to >8). Zero-valent S can occur in polythionates (O3SSnSO32−, n ≥ 0) and in organic polysulfides or polysulfanes (RSn−, RSnR’, n ≥ 2), but in most sulfidic natural waters these contribute negligibly to total dissolved zero-valent sulfur (ΣS0) [6,10]
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