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184 Wilfred E. Kleinjan et al. The sulfur particles are fed to the gas absorber by the liquid recycle due to an incomplete separation of solid sulfur in the separation step of the pro- cess. In the bioreactor, the polysulfide anions either decompose into sulfide and sulfur (Eq. 28), or are oxidized to thiosulfate (Eq. 29). Sulfate can also be formed by oxidation of HS (Eq. 30): S2 þHþ !HS þðx1ÞS0 ð28Þ x S2 þ 3=2O2 ! S2O2 þ ðx 2ÞS0 ð29Þ x3 HS þ2O2 !SO2 þHþ ð30Þ 4 In principal, oxygen from air is the only reagent needed for this process. However, due to the occurrence of sulfate and thiosulfate formation, extra sodium hydroxide has to be dosed to the bioreactor and a bleed stream is necessary to prevent accumulation of sodium salts. The amount of NaOH that has to be added to the bioreactor can be minimized if the occurrence of (thio)sulfate formation is minimized. Previous research showed that sulfide oxidation to sulfate can be selectively prevented by applying a low oxygen concentration [56]. The oxidation of anionic polysulfides, however, cannot easily be prevented by controlling of the oxygen concentration because the oxidation rate of polysulfides is higher than the oxidation rate of sulfide, as was suggested by Chen and Morris [59]. In order to determine the best way to prevent the thiosulfate formation, more should be known about the role of the polysulfide anions and their specific interaction with the biologically produced sulfur particles. 5 Applications of Biologically Produced Sulfur Sulfur produced by microorganisms in H2S removal plants such as described in the previous section, can be handled in a number of ways. Dried sulfur solids can be used in sulfuric acid production (99% sulfur purity needed) or the formed sulfur sludge can be directed to a smelter where it is converted into high purity sulfur (>99.9%). Unfortunately, currently more sulfur is produced worldwide than is needed as pure chemical and therefore sulfur is also stored in landfill (95–98% sulfur purity needed). Although solid sulfur is considered as a non-hazardous refinery waste, landfill is an undesirable option, partly because acidification by oxidation has to be prevented. To avoid the landfill, possible applications of the biologically produced sulfur have been investigated. In bioleaching and in agriculture, it was found that the specific properties of biologically produced sulfur (small particle size, hydrophilic surface) have clear advantages over the use of inorganic sulfur. Bioleaching is used in mining to dissolve metals from sulfide-ores. The aim of bioleaching is to achieve pH values that are low enough to solubilize a maximum of metals. In some cases additional elemental sulfur is added asPDF Image | Topics in Current Chemistry
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