Biologically produced sulfur particles and polysulfide ions (2005)

_____________________________________________________________________ Sulfur compounds in the environmentSulfur compounds are abundant throughout the world and, as a matter of fact, also outside this world.Our soils contain sulfur compounds in large quantities in, for instance, sulfide minerals (pyrite, FeS 2 ) or sulfate minerals (gypsum, CaSO 4 2H 2 O).Our atmosphere contains gases like sulfur dioxide (SO 2 ) and hydrogen sulfide (H 2 S), emitted by volcanic activities, evaporation from oceans, or fossil fuel combustion, and the oceans are full of sulfate (about 28 mM, the second most abundant anion after chloride) [1].The element sulfur is an essential component in all living organisms, as it is needed in several amino acids.Furthermore, sulfur compounds have been found in meteorites and, for instance, on Jupiter's volcanic moon Io.On earth, sulfur components are cycled between soil, oceans, atmosphere, and living matter in the so-called geochemical and biological sulfur cycles.The geochemical sulfur cycle involves processes such as evaporation from surface waters, sedimentation, and rain [1].The biological sulfur cycle, which is described in more detail in section 2.1 of this thesis, involves the continuous oxidation and reduction of sulfur compounds by microorganisms, plants, and animals [2].A substantial part of today's environmental problems, such as 'acid rain' and acidified lakes, are related to sulfur compounds and disturbances of the sulfur cycles.Acidification of lakes can result from acid mine drainage.Many metals, such as copper, lead, zinc, and iron (pyrite) are present in ores as sulfide minerals.In these ores, sulfide minerals are stable at conditions without oxygen and water.Upon extraction of the metal by mining, however, the conditions change to moist and aerobic, and sulfide can be oxidized to sulfate (sulfuric acid).Wastewaters discharged from mining sites (acid mine drainage) are therefore often rich in toxic metals and have a low pH [3].Acid rain is mainly related to the emission of SO 2 , which can be oxidized in the atmosphere to sulfuric acid.Growing industrialization in North America and Europe resulted in a fast increase of SO 2 emission after the Second World War, mainly because of combustion of fossil fuels containing H 2 S or organic S-compounds.Since the 1970s a number of emission control strategies were introduced to prevent SO 2 emission, such as coal desulfurization, waste gas treatment, and selection of fossil fuels with lower sulfur content.Fortunately, the introduction of emission controls has resulted in a considerable decrease of SO 2 emissions since the 1970s.Over the period 1990-1998, SO 2 emissions decreased by 52% in the EU and by 17% in the United States and Canada.In Asia, however, an overall increase of 16% of SO 2 emissions occurred in this period [4].Polysulfide ions are unbranched chains of sulfur atoms, related to the polysulfanes, H 2 S n .The acidity constants of the polysulfanes are so low that polysulfide ions in aqueous solution can be assumed to be unprotonated [17, 19].Because of difficulties in distinguishing between polysulfide ions of different chain length, the exact distribution of polysulfide ions of different chain length in aqueous solution is not completely clear.At neutral to alkaline pH values S 6 2-, S 5 2-, and S 4 2-are believed to dominate the solution and the shorter chains S 3 2-and S 2 2-were only observed at very high alkalinity (pH > 14) [20].Recently, this was confirmed in a study where polysulfide ions were methylated and subsequently separated by chromatography [21].Chromatograms showed the largest peaks for (CH 3 ) 2 S n for n = 4, 5, and 6, with trace peaks at n = 2 and 3, and also small peaks at n = 7, 8, and 9.At mildly alkaline conditions and when in equilibrium with excess amounts of sulfur, the reported average chain length, x, varies from x = 4.6 [20], x = 4.8 [22], x = 5.4 [23], x = 5.5 [17].Under acidic conditions, polysulfide ions are not stable.Measurement of polysulfide concentrations in aqueous solutions aims at the amount of zero valent sulfur atoms in polysulfide molecules.This concentration, which is termed the polysulfide excess sulfur concentration, [S 0 in S x 2-], can be measured withUV spectrophotometry [24, 25] and voltammetry [26].Both methods cannot distinguish between polysulfide ions of different chain lengths.In this thesis UV spectrophotometry was used for the analysis of polysulfide excess sulfur concentrations.Polysulfide ions are believed to play an important role in biotechnological H 2 S removal processes.First, polysulfide ions are considered to be intermediates in the oxidation of HS -to elemental sulfur, both in the chemical oxidation catalyzed by metal ions and in the biological oxidation by sulfide-oxidizing bacteria [18, 27] (see also section 2.3.3).Furthermore, formation and oxidation of polysulfide ions may affect the rate of H 2 S absorption or the process selectivity for sulfur production.Chapters 3, 4, and 5 of this thesis are particularly concerned with polysulfide chemistry.From pilot plant studies on the removal of H 2 S from pressurized natural gas [28] it was observed that more H 2 S was absorbed in the gas absorber than could be expected _____________________________________________________________________Chapter 2Biologically produced sulfur