Sulfides to sulfates

Peroxomonosulfuric acid oxidi2es cyanide to cyanate, chloride to chlorine, and sulfide to sulfate (60). It readily oxidi2es carboxyflc acids, alcohols, alkenes, ketones, aromatic aldehydes, phenols, and hydroquiaone (61). Peroxomonosulfuric acid hydroly2es rapidly at pH <2 to hydrogen peroxide and sulfuric acid. It is usually made and used ia the form of Caro s acid. 

Thiobacillus thiooxidans is an aerobic organism that oxidizes various sulfur-containing compounds to form sulfuric acid. These bacteria are sometimes found near the tops of tubercles (see Chap. 3, Tubercu-lation ). There is a symbiotic relationship between Thiobacillus and sulfate reducers Thiobacillus oxidizes sulfide to sulfate, whereas the sulfate reducers convert sulfide to sulfate. It is unclear to what extent Thiobacillus directly influences corrosion processes inside tubercles. It is more likely that they indirectly increase corrosion by accelerating sulfate-reducer activity deep in the tubercles. 

Organisms also evolved powerful detoxifying mechanisms that remove toxic materials or convert them to non-toxic forms or nutrients. Examples of alterations to non-toxic forms are the conversions of hydrogen sulfide to sulfate and nitrite to nitrate. The prime example of development of the ability to use a toxic substance is the evolution of aerobic metabolism, which converted a serious and widespread toxin, oxygen, into a major resource. This development, as we have seen, greatly increased the productivity of the biosphere and generated the oxygen-rich atmosphere of today s Earth. 

Oxidation of sulfides to sulfates XS + 2 02 —>XS04 (where X represents a metal)... 

The major metabolic pathway for hydrogen sulfide in the body is the oxidation of sulfide to sulfate, which is excreted in the urine (Beauchamp et al. 1984). The major oxidation product of sulfide is thiosulfate, which is then converted to sulfate the primary location for these reactions is in the liver (Bartholomew et al. 1980). 

The major metabolic pathway of hydrogen sulfide is the oxidation of the sulfide to sulfate in the liver (Beauchamp et al. 1984). Methylation also serves as a detoxification route. Hydrogen sulfide is excreted primarily as sulfate (either as free sulfate or as thiosulfate) in the urine. 

Cork [283], Sublette [284], and others have identified a number of chemolithotrophic bacteria which oxidize elemental sulfur and use reduced or partially reduced sulfur compounds as an energy source, in the presence of various carbon sources (such as carbon dioxide or bicarbonate) and reduced nitrogen (e.g., ammonium ion). In the case of Cork et al. s work, the anaerobic photosynthetic bacterium Chlorobium thiosulfatophilum is used to convert sulfides to sulfate. The economics of this process was not favorable due to the requirement of light for the growth of the bacterium. 

Sublette [285] describes a process for desulfurizing sour natural gas using another commonly known chemolithotrophic microorganism, the aerobic bacterium T. denitrifi-cans. This patent describes a process wherein bacteria of the Thiobacillus genus convert sulfides to sulfates under aerobic conditions. Sublette defined the ideal characteristics of a suitable microorganism for the oxidative H2S removal from gaseous streams as ... 

More specifically, the invention involves the use of Thiobacillus denitrificans under anaerobic conditions to oxidize sulfur compounds such as hydrogen sulfide to sulfate. The process may be carried out in various ways such as in a batch or a continuous bioreactor system using a suspended or an immobilized biocatalyst. The method is particularly applicable to treating natural gas containing hydrogen sulfide and producing a biomass byproduct. 

Bioleaching, particularly of sulfidic ores, has received much attention since the early 1980s. The conditions needed and mechanisms for such processes have been reviewed. Microbial processes involve complete oxidation of sulfide to sulfate, e.g.,... 

Aerobic bacteria oxidizing Hjfo water, organic . . . / rnatter and methane to CO2, sulfide to sulfate... 

Disulfides have also removed the activity of certain proteins in bacteria. The bacterium Paracoccus panto-us has a cluster of enzymes responsible for the oxidation of sulfides to sulfate known as the sulfur-oxidizing... 

In basic solution Mn04 oxidizes sulfide to sulfate ... 

Organisms such as Thiobacillus thiooxidans and Clostridium species have been linked to accelerated corrosion of mild steel. Aerobic Thiobacillus oxidizes various sulfur-containing compounds such as sulfides to sulfates. This process promotes a symbiotic relationship between Thiobacillus and sulfate-reducing bacteria. Also, Thiobacillus produces sulfuric acid as a metabolic by-product of sulfide oxidation. 

This is remarkable in view of the small standard Gibbs energy decrease. Some species of the archaeobacteri-um Sulfolobus are able either to live aerobically oxidizing sulfide to sulfate with 02 (Eq. 18-22) or to live anaerobically using reduction of sulfur by Eq. 18-33 as their source of energy.369... 

Oxidation of Reduced S. Indirect evidence suggests that microbial oxidation of sulfide is important in sediments. If it is assumed that loss of organic S from sediments occurs via formation of H2S and subsequent oxidation of sulfide to sulfate (with the exception of pyrite, no intermediate oxidation states accumulate in sediments cf. 120, 121), the stated estimates of organic S mineralization suggest that sulfide production and oxidation rates of 3.6-124 mmol/m2 per year occur in lake sediments. Similar estimates were made by Cook and Schindler (1.5 mmol/m2 per year 122) and Nriagu (11 mmol/m2 per year 25). A comparison of sulfate reduction rates (Table I) and rates of reduced S accumulation in sediments (Table III) indicates that most sulfide produced by sulfate reduction also must be reoxidized but at rates of 716-8700 mmol/m2 per year. Comparison of abiotic and microbial oxidation rates suggests that such high rates of sulfide oxidation are possible only via microbial mediation. 

All measured profiles of sulfate reduction in sediments indicate that much sulfide production and, by inference, oxidation occurs in permanently anaerobic sediments (78, 73, 90,101). The two most likely electron acceptors for anaerobic sulfide oxidation are manganese and iron oxides. Burdige and Nealson (151) demonstrated rapid chemical as well as microbially catalyzed oxidation of sulfide by crystalline manganese oxide (8-Mn02), although elemental S was the inferred end product. Aller and Rude (146) documented microbial oxidation of sulfide to sulfate accompanied by reductive dissolution... 

The CRS AVS ratio reflects the trophic state of a lake (cf. Urban (28), Table V). This observation may be explained by the preceding kinetic considerations. The high organic matter supply in eutrophic lakes leads to an intensive mineralization rate by both iron- and sulfate-reducing bacteria. However, reoxidation of Fe2+ to ferric oxide or of sulfide to sulfate does not take place because an anoxic hypolimnion prevents penetration of oxygen. Therefore FeS can build up, but the sediment becomes depleted with respect to reactive iron. 

Ultrasonic irradiation of 25-mL solutions of bivalent sulfur at pH 10 (borate, I = 0.06 M) resulted in a linear decrease of [S(—II)] with sonication time. A post-irradiation oxidation of sulfide to sulfate was observed and was attributed to the subsequent thermal reaction of HS- with H202, which was... 

Aerobic conditions are not always required to oxidize sulfide to sulfate in pyrite and other sulfide minerals (Bednar et al., 2005 Jonas and Gammons, 2000, 58). As shown in the following reaction from Evangelou, Seta and Holt (1998, 2084), dissolved Fe(III) may be an effective oxidant ... 

Thus, the appearance of free sulfate does not require the advent of free oxygen in the Archean environment. Certainly sufficient free sulfate had appeared in the hydrosphere prior to development of the pathway of dissimilatory sulfate reduction. Schidlowski (1979) argues that the small fractionations observed between sulfide and sulfate 834s values of pre-2.7 billion year rocks (Figure 10.11) are consistent with the hypothesis that the oxidation of sulfide to sulfate by photosynthetic bacteria preceded the bacterial pathway of dissimilatory sulfate reduction and may have been responsible for early free dissolved sulfate concentrations in the hydrosphere. 

Some microbes can produce organic acids, such as formic and succinic, or mineral acids such as sulfuric acid. Some bacteria can oxidize sulfur or sulfide to sulfate or reduce sulfates, very often to hydrogen sulfide as end product. (Dexter)14... 

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