Biologically Produced Sulfur

Abstract

Sulfur compound oxidizing bacteria produce sulfur as an intermediate in the oxidation of hydrogen sulfide to sulfate. Sulfur produced by these microorganisms can be stored in sulfur globules, located either inside or outside the cell. Excreted sulfur globules are colloidal particles which are stabilized against aggregation by electrostatic repulsion or steric stabilization. The formed elemental sulfur has some distinctly different properties as compared to normal inorganic sulfur. The density of the particles is for instance lower than the density of orthorhombic sulfur, and the biologically produced sulfur particles have hydrophilic properties whereas orthorhombic sulfur is known to be hydrophobic. The nature of the sulfur and the surface properties of the globules are however not the same for sulfur produced by different bacteria. The globules produced by phototrophic bacteria appear to consist of long sulfur chains terminated with organic groups, whereas chemotrophic bacteria produce globules consisting of sulfur rings (S8). Adsorbed organic polymers such as proteins cause the hydrophilic properties of sulfur produced by a mixed culture of Thiobacilli. The hydrophilicity of extracellularly stored sulfur globules produced by Acidithiobacillus ferrooxidans can probably be explained by the vesicle structure consisting mainly of polythionates (–O3S-Sn-SO3 –). Sulfur compound oxidizing bacteria, especially Thiobacilli, can be applied in biotechnological sulfide oxidation installations for the removal of hydrogen sulfide from gas streams and the subsequent oxidation of sulfide to sulfur. Due to the small particle size and hydrophilic surface, biologically produced sulfur has advantages over sulfur flower in bioleaching and fertilizer applications.