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178 Wilfred E. Kleinjan et al. Fig. 6 Vesicle model proposed by Steudel for sulfur globules excreted by Ac. ferrooxi- dans. Vesicles are composed of long chain polythionates (O3S-Sn-SO3) in which small amounts of sulfur rings can be present (after [24]) long-chain polythionates present on the surface. Indeed HPLC [39] and XANES [28] analysis showed the presence of polythionates in the cultures of Ac. ferrooxidans excreting sulfur globules. XANES analysis did however also show that S8 rings were not present. Another model proposed by Steudel [24] consists of vesicles composed of a polythionate membrane. The inside of the vesicle may contain water, and sulfur rings are only present in low concentrations in the membrane part of the vesicle. This model is in accor- dance with the observed experimental data obtained by XANES analysis and therefore seems suitable for globules produced by Ac. ferrooxidans. It should be noted that polythionates are only stable at low pH [33] and it is therefore unlikely that the polythionate vesicle model is applicable to sul- fur globules produced by bacteria growing at another pH than the acidic conditions at which Ac. ferrooxidans grows [5] (see Fig. 6). Janssen et al. have studied the properties of sulfur produced by bacteria of the genus Thiobacillus grown at neutral to alkaline pH [19, 40, 41]. The sulfur was produced in a bioreactor in which a mixed culture of Thiobacilli was present. X-ray diffraction of sulfur particles excreted by Thiobacillus sp. showed the presence of orthorhombic sulfur crystals. The solubility of crystalline or- thorhombic sulfur in water is known to be only 5 mg l1 [42]. In the solubili- ty test shown in Fig. 7 it was seen that the biologically produced sulfur par- ticles can be dispersed in water but not in hexadecane, whereas crystalline orthorhombic sulfur is soluble in hexadecane but not in water. The reason for the observed hydrophilicity of the biologically produced sulfur particles has to be attributed to the hydrophilic properties of the surface of the sulfur particles. Because of the relatively high stability of the biologically produced sulfur particles at high salt concentrations, it is concluded that the colloidal stability is not merely based on electrostatic repulsion. It is known that hy- drophobic sulfur can be wetted by Thiobacillus thiooxidans bacteria due to formation of organic surface-active substances [43, 44]. It was concluded that the particles are stabilized by long chain polymers, most probably proteins. Indeed, measurements with dynamic light scatter- ing showed a relatively high variation of the determined hydrodynamic radi-PDF Image | Topics in Current Chemistry
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