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Biologically Produced Sulfur 181 stored extracellularly or intracellularly. Van Gemerden however suggested that the location of the electron acceptor in the HS oxidation step deter- mines whether sulfur is stored extracellularly or intracellularly [46] as is ex- plained in Fig. 8. This is supported by research of Then and Trüper [8] on sulfide oxidation in Halorhodospira abdelmalekii, excreting sulfur globules extracellularly. They showed the cytochrome c-551 to have a catalytic effect on the oxidation of sulfide and to be located on the outside of the cell mem- brane. In Al. vinosum cytochrome c-551 is located in the periplasmic space, the space between the outer cell wall and the cytoplasmic membrane, which is also the location of storage of sulfur globules [47]. 4 Sulfur Compound Oxidizing Bacteria in Industrial Applications 4.1 Removal of Hydrogen Sulfide From Gas Streams A number of environmental problems are strongly related to the emission of sulfur compounds, such as SO2, in the atmosphere. SO2 emission is mainly due to combustion of fossil fuels containing H2S or organic S-compounds. In the atmosphere, SO2 is oxidized forming sulfuric acid resulting in acid rain. Fortunately, the emission of SO2 has decreased considerably since the 1970s due to selection of low sulfur-content fuels, waste gas treatment and specialized combustion processes. In order to prevent SO2 emission, H2S has to be removed from gas streams prior to combustion. Apart from environmental reasons, removal of H2S from waste gas streams is also required for health reasons (H2S is a toxic gas, lethal at concentrations exceeding 600 ppm) and to prevent corrosion of equipment. Gases that can contain H2S and need treatment are, for instance, natural gas, syngas and biogas (formed in anaerobic wastewater treatment). Removal of H2S from gas streams is generally done in two steps. In the first step, H2S is separated from the gas stream and in the second step, the removed H2S (dissolved in liquid or as concentrated gas) is converted into elemental sulfur. Several processes exist that purify gases according to these two steps. The method most commonly used is based on absorption of H2S in an amine solution and the subsequent stripping of the dissolved H2S from this solution forming a concentrated gas (Claus gas) which is then converted into elemental sulfur in the Claus process. In the Claus process, a third of the H2S gas is first burned to SO2 after which the remaining H2S reacts with the formed SO2 to elemental sulfur: H2S þ 1=2O2 ! 2=3H2S þ 1=3SO2 þ 1=3H2O ð19Þ 2=3H2Sþ1=3SO2 ÐS0 þ2=3H2O ð20Þ H2Sþ1=2O2 ÐS0 þH2O ð21ÞPDF Image | Topics in Current Chemistry
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