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Biologically Produced Sulfur 179 Fig. 7 Hexadecane-water partition test. Biologically produced sulfur remains in the lower water phase (1) whereas crystalline sulfur remains in the upper hexadecane-phase (2) [40] us with changing salt concentration, indicating the presence of relatively thick adsorbed layers on the sulfur particles (see Fig. 4). This, as well as sur- face charge density measurements showing values comparable to surface charge densities of bacterial cell walls and humic acids, support the sugges- tion of proteins adsorbed on the particles. In addition, electrophoretic mo- bility experiments showed an iso-electric point comparable to the pKa-value of carboxylic acid groups in proteins (pKa=2.3) [45]. As stated above, the presence of proteins on the surface of sulfur globules stored intracellularly has been demonstrated [8, 37]. These well-defined pro- teins act as a membrane between the cytoplasm and the intracellular sulfur particle. It is not known whether the proteins associated with the sulfur par- ticles excreted by Thiobacillus bacteria are well-defined proteins synthesized by the bacterium or if they are originating from organic compounds already present in the liquid reactor system. 3.3 Formation of Biologically Produced Sulfur Biologically produced sulfur is often written as S0, which only indicates that a large part of the sulfur in the sulfur globules is present in oxidation state zero. It should not be mistaken for atomic sulfur, which has a very high enthalpy of formation and therefore cannot exist at ambient temperature [33]. In the previous sections it has been shown that the sulfur in globules can be present as sulfur rings, long-chain polythionates and long sulfur chains terminated by organic groups. The mechanisms in which these forms of sulfur are formed are not known but the formation of sulfur rings is as- sumed to proceed through the formation of polysulfides [3]. Steudel [38] proposed a reaction mechanism for the chemical oxidation of sulfide to sulfur rings (S8), which should take place in a similar way inPDF Image | Topics in Current Chemistry
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