Sulfate from biological oxidation

Sulfate and Organic Sulfates. Inorganic sulfate ion (SO L-) occurs widely in nature. Thus, it is not surprising that this ion can be used in a number of ways in biological systems. These uses can be divided primarily into two categories (1) formation of sulfate esters and the reduction of sulfate to a form that will serve as a precursor of the amino acids cysteine and methionine and (2) certain specialized bacteria use sulfate to oxidize carbon compounds and thus reduce sulfate to sulfide, while other specialized bacterial species derive energy from the oxidation of inorganic sullur compounds to sulfate. 

The combination of radiolabeled sulfide and the bimane-HPLC method is particularly powerful because one of the main obstacles to the use of labeled sulfide is, that aside from radioactive decay, the compound is subject to rapid oxidation in the presence of air. The breakdown products of chemical sulfide oxidation are the same as those of biological oxidation. Previously it has been impossible to check routinely the purity of the purchased isotope and its subsequent purity during a series of experiments. It is our experience that newly purchased sodium sulfide sometimes contains up to 10% thiosulfate as well as traces of sulfite and sulfate (Figure 2), and that the sulfide once hydrated readily oxidizes if stored in a normal refrigerator. 

Reduced sulfur compounds are ubiquitous in aqueous and atmospheric systems (10,11). Natural sources of reduced sulfur species in aqueous environment result from biological reduction of sulfate, anaerobic microbial processes in sewage systems, putrefaction of sulfur-containing amino-acids (12), oxidative decomposition of pyrite (13), and activities of marine organisms in the upper layers of the ocean (14,15). The build-up of sulfides in areas such as the Black Sea is also giving cause for concern (8). 

For this reason Pb has been used as a tracer of the precipitation fate of S04. Turekian et al. (1989) used the S04 / Pb ratio in aerosols and the flux of °Pb measured in bucket collections to determine the SO flux across the Pacific Ocean. Further, they showed that the S04 / Pb in aerosols from regions of high biological productivity was higher than for normal relatively unpolluted air (Table 3) indicating a sulfate source from the oxidation of dimethyl sulfide (DMS). The measured flux of DMS from the oceans at the equator matched the biogenic flux determined from the °Pb calculation (Table 4). (Actually, as we shall see below, this concordance is probably due to an underestimate of sulfate flux and an overestimate of the fraction of DMS oxidized to sulfate.)... 

The liberation of H2S is controlled not only by the rate of its production by SO -reducing bacteria, but also by its pH-dependent speciation, its tendency to rapidly precipitate as metal sulfides, and its rapid chemical and biological oxidation. Only the protonated species (H2S) is volatile, and at neutral pH, most inorganic sulfide is present as bisulfide ion (HS ), whereas sulfide (S ) dominates under alkaline conditions. These three species are known collectively as SH2S. Hence, the escape of sulfide should be enhanced at low pH. Sulfate reduction is most dominant in marine sediments and this is where the highest emissions of gaseous H2S occur (Hines, 1996). However, DeLaune et al. (2002) reported higher emissions of H2S from brackish... 

The chemotrophic (colorless) sulfur bacteria obtain energy from the chemical aerobic oxidation of reduced sulfur compounds. The overall reactions occurring, concerning the biological oxidation of sulfide, are the formation of sulfur (at low oxygen concentrations) and the formation of sulfate (when there is an excess of oxygen) ... 

Similar heterogeneous reactions also can occur, but somewhat less efticientiy, in the lower stratosphere on global sulfate clouds (ie, aerosols of sulfuric acid), which are formed by oxidation of SO2 and COS from volcanic and biological activity, respectively (80). The effect is most pronounced in the colder regions of the stratosphere at high latitudes. Indeed, the sulfate aerosols resulting from emptions of El Chicon in 1982 and Mt. Pinatubo in 1991 have been impHcated in subsequent reduced ozone concentrations (85). 

Biological activity can be used in two ways for the bioremediation of metal-contaminated soils to immobilize the contaminants in situ or to remove them permanently from the soil matrix, depending on the properties of the reduced elements. Chromium and uranium are typical candidates for in situ immobilization processes. The bioreduction of Cr(VI) and Ur(VI) transforms highly soluble ions such as CrO and UO + to insoluble solid compounds, such as Cr(OH)3 and U02. The selenate anions SeO are also reduced to insoluble elemental selenium Se°. Bioprecipitation of heavy metals, such as Pb, Cd, and Zn, in the form of sulfides, is another in situ immobilization option that exploits the metabolic activity of sulfate-reducing bacteria without altering the valence state of metals. The removal of contaminants from the soil matrix is the most appropriate remediation strategy when bioreduction results in species that are more soluble compared to the initial oxidized element. This is the case for As(V) and Pu(IV), which are transformed to the more soluble As(III) and Pu(III) forms. This treatment option presupposes an installation for the efficient recovery and treatment of the aqueous phase containing the solubilized contaminants. 

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