Activity of zero-valent sulfur in sulfidic natural waters

BACKGROUND: Ionic and molecular carriers of dissolved (filter-passing) zero-valent sulfur (S(0)) in anaerobic natural waters include polysulfides, S(n)(2−), molecular S(8)(aq), organic macromolecules and certain higher valent thioanions. Because S(0) 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, S(0) is treated collectively as a virtual thermodynamic component, and computational as well as graphical methods for quantifying its activity (a(S)0) in natural waters are presented. From a(S)0, concentrations of the ionic and molecular carriers of S(0) can be calculated easily. RESULTS: Concentration ratios of any two polysulfide ions define a(S)0 (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 (ΣS(0)) and pH provide a more widely applicable approach (Method II). Systematic errors in ΣS(0) 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 ΣS(0)/ΣS(-II) that imply errors in the thermodynamic data for HS(2)(−) and S(2)(−). For samples having low ΣS(0)/ΣS(-II), an interim remedy is recommended: letting pK(a2) = 6.3 for all HS(n)(−) ions. CONCLUSIONS: Newly assembled data for a(S)0 in a selection of anaerobic natural waters indicate that S(0) 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 a(S)0 values in the dataset span conditions favoring Hg-polysulfide complexes vs. Hg-sulfide complexes, implying that a(S)0 could affect Hg-methylation rates in nature. No support is found for the common assumption that a(S)0 = 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 HS(n)(−) species. [Image: see text] ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12932-014-0013-x) contains supplementary material, which is available to authorized users.