REACTIVITY OF POLYSULFIDES AND ITS CONTRIBUTION TO CRYPTIC SULFUR CYCLING IN ANOXIC/EUXINIC BASINS

Sulfur cycling, in particular reactions involving elemental sulfur, sulfide, and polysulfide, is important in anoxic/euxinic systems as a primary control of pyritization, sulfurization of organic matter, and the isotopic fractionation of sulfur preserved in minerals. Elemental sulfur is involved in reactions that cycle S, both biotically and abiotically, and transitions in active systems from dissolved elemental sulfur (S_8aq, that is sparsely soluble) to metastable elemental sulfur nanoparticles (S_8nano) to the thermodynamically most stable form α-S₈. We have found that the coarsening process and the solubility of S_8aqare directly influenced by temperature and surfactants (sourced from microbial exudates and/or other sources of natural organic matter). Elemental sulfur is also reactive with other sulfur species, especially sulfide, to undergo nucleophilic dissolution and form polysulfide ions, according to the following reaction.

2HS^- + S₈ <--> 2S₅^2- + H⁺(1)

We show that the kinetics of this forward reaction (1) are controlled by the surface area and character (as affected by the degree of hydrophobicity controlled by precipitation mechanisms and surfactants) of elemental sulfur particles. The kinetics of the reverse reaction, governing the stability of polysulfides, is very fast at pH lower than 10, leading to the formation of S₈-rings that quickly coarsen to S_8nano. However, this fast reverse reaction maintains a small but potentially reactive pool of polysulfides, that may be very difficult to detect directly, creating a ‘cryptic’ cycling of sulfur. The presence of both elemental sulfur and sulfide generates polysulfides; the kinetics of these reactions can maintain a low but rapidly replenished pool of polysulfide available for reaction.

This cryptic S cycling involving polysulfides is fundamental to the chemistry, mineralogy, and isotopic fractionation of sulfur in anoxic and euxinic basins, including: pyritization via polysulfides formation (Rickard and Luther, 2007); coupling of sulfate reduction with anaerobic methane oxidation, linking S and C cycles (Milucka et al., 2012) especially through disulphide; S₂^2- formation; and the sulfurization of organic matter (Raven et al., 2015).