Reactivity of elemental sulfur nanoparticles and their role on the formation of sulfur intermediates

Abstract:

Sulfur biogeochemical cycling involves a vast network of chemical reactions including oxidation, reduction and disproportionation, polymerization (S-S bonding), and aggregation that leads to nanoparticulate elemental sulfur (S_8(nano)) and eventually the most stable bulk mineral form, a-S₈. These reactions can occur in aqueous systems inorganically, intracellularly as assimilatory or dissimilatory reactions in microbial cells, or in presence of microbial organic exudates (e.g. surfactants).

Elemental sulfur participates in nucleophilic and hydrolysis reactions to form sulfur intermediates, such as polysulfide ions. The nucleophilic reaction with sulfide (as H2S or HS-, reaction 1), is a key process in the dissolution of elemental sulfur:

xS₈ + H₂S = S_x^2- + 2 H⁺ (1)

Polysulfides are inherently unstable in acidic conditions, experiencing a series of chain elongation reactions to form the S₈ ring form of molecular elemental sulfur (overall this is the reverse of reaction 1).

S₈ rings are also inherently insoluble in water and undergo a very rapid coarsening process to form S_8nano that then coarsens more slowly via an Ostwald ripening process to form colloidal particles of a-S₈. Coarsening kinetics are influenced by temperature, and the presence/absence and type of surfactants [1]. The amount of polysulfide at any point is thus a balance between the forward and reverse overall reactions represented in reaction 1, and influenced by microbial generation of reduced sulfide and/or microbial production of organic surfactants, and the size of the elemental sulfur particles.

Here we present some preliminary data on the kinetics of elemental sulfur consumption (reaction 1) that have been tested in various sizes and surface properties of the sulfur nanoparticles. The surface area and presence (or absence) of surfactant molecules influences the kinetics of polysulfide formation. The cycling of that reaction may play a significant role to the element’s bioavailability to microorganisms, which incorporate the nanoparticulate (rather than the bulk a-S8) form of elemental sulfur or polysulfides in their specific metabolisms [2].

1. Garcia and Druschel (2014) Submitted Manuscript

2. Boyd and Druschel (2013) Appl. Environ. Microbiol. 79 (6) 2061-2068