REACTIVITY OF ELEMENTAL SULFUR NANOPARTICLES AND THEIR ROLE ON THE BIOGEOCHEMICAL CYCLING OF SULFUR

Sulfur cycling incorporates an array of electron exchange reactions that transform the element through its 8-electron step valence states, from S(-II) up to S(VI), in a variety of abiotic and biotic reactions. Elemental sulfur formation and dissolution is an important aspect of sulfur metabolisms, but the mechanisms governing these reactions are often not well characterized in microbial environments. The formation of elemental sulfur involves polymerization (S-S bonding) and aggregation of S₈ rings into nanoparticulate elemental sulfur (S_8nano). The nucleophilic reaction with sulfide (as H₂S or HS^-, reaction 1), is a key process in the formation and dissolution of elemental sulfur:

HS^- + (n-1)/8 S₈ = S_n^2- + H⁺ (1)

This reaction can lead to formation of S₈ rings via chain elongation reactions of polysulfide (forward direction), or coarsening process of S₈-rings to form S_8nano and α-S₈ (reverse direction). Elemental sulfur is also influenced by the presence of surfactant molecule coatings, similar to microbial organic exudates, that impacts elemental sulfur nanoparticle size and reactivity.

We employ an array of molecular methods in order to investigate S_8nano coarsening processes and nucleophilic dissolution (reaction 1), incorporating electrochemical, chromatographic, spectrophotometric and spectroscopic techniques. The experimental data show that the kinetics of the reaction are influenced significantly by the surface area of the nanoparticles, the amount of the electron nucleophile (HS^-), as well as the surface character of elemental sulfur (organic surfactant coating).

Observed sulfur speciation in Yellowstone National Park thermal areas indicates nucleophilic dissolution of elemental sulfur (reaction 1), is a key control on the (bio)availability of sulfur species in solution, and is an important link to the carbon cycle in these systems via sulfurization of organic matter.