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182 Wilfred E. Kleinjan et al. In other methods H2S is absorbed in a solution after which the dissolved H2S is converted to elemental sulfur without stripping the gas from the solu- tion. Differences between different methods basically consist of a different absorption solution or a different oxidation catalyst. Disadvantages are the high operational costs due to high pressures and high temperatures and the need of special chemicals (catalysts). 4.2 Biotechnological Removal of Hydrogen Sulfide from Gas Streams To overcome the disadvantages of physico-chemical processes for H2S re- moval, the use of microorganisms can be an interesting alternative. Several microorganisms are capable of the oxidation of H2S at ambient temperatures and pressures, and both phototrophic and chemotrophic organisms have been studied for their industrial application. The phototrophic sulfur oxidiz- ing bacteria Chlorobium limicola forma thiosulfatophilum has been studied [48, 49] but phototrophic bacteria have the disadvantage of the requirement of light and therefore the need of a transparent reactor surface and reaction solutions. Of the chemotrophic bacteria, heterotrophs as well as autotrophs have been studied. The heterotrophic Xanthomonas bacterium has been studied for H2S removal [11] but showed lower removal rates than other au- totrophic bacteria and, in addition, heterotrophic organisms have the disad- vantage of the requirement of organic compounds. Chemoautotrophic bacte- ria (especially Thiobacilli) are the types of sulfur oxidizing bacteria that have been studied and used mostly in H2S removal processes. The use of Thiobacillus species has been studied quite extensively. Sub- lette and Sylvester especially focused on the use of Thiobacillus denitrificans [50–52] for aerobic or anaerobic oxidation of sulfide to sulfate. In the anaer- obic oxidation NO3 was used as an oxidant instead of oxygen (confirm Table 2). Buisman used a mixed culture of Thiobacilli for the aerobic oxida- tion of sulfide to elemental sulfur and studied technological applications [53–55]. Visser showed the dominant organism in this mixed culture to be a new organism named Thiobacillus sp. W5 [6]. The production of sulfur from sulfide has some distinct advantages over the production of sulfate. First, elemental sulfur is seen as a less harmful form of sulfur than sulfate. Secondly, the separation of the insoluble sulfur from aqueous streams is easier than separation of sulfate and thirdly less oxygen is needed for the oxidation, which saves energy costs for aeration: 2H2S þ O2 ! 2S0 þ 2H2O ð22Þ H2S þ 2O2 ! H2SO4 ð23Þ According to Kuenen, the formation of sulfate yields more energy than the formation of sulfur [2]. To stimulate formation of sulfur, the oxygen con- centration should be limited [56, 57]. Several reactor systems possible for the biotechnological removal of H2S from a gas exist. Currently, three types of biotechnological process systemsPDF Image | Topics in Current Chemistry
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