Hydrogen sulfide redox

Arsenic is another element with different bioavailabiUty in its different redox states. Arsenic is not known to be an essential nutrient for eukaryotes, but arsenate (As(V)) and arsenite (As(III)) are toxic, with the latter being rather more so, at least to mammals. Nevertheless, some microorganisms grow at the expense of reducing arsenate to arsenite (81), while others are able to reduce these species to more reduced forms. In this case it is known that the element can be immobilized as an insoluble polymetallic sulfide by sulfate reducing bacteria, presumably adventitiously due to the production of hydrogen sulfide (82). Indeed many contaminant metal and metalloid ions can be immobilized as metal sulfides by sulfate reducing bacteria. 

NKK s Bio-SR process is another iron-based redox process which instead of chelates, uses Thiobacillusferroidans )2iQ. - 2i to regenerate the solution (9). This process absorbs hydrogen sulfide from a gas stream into a ferric sulfate solution. The solution reacts with the hydrogen sulfide to produce elemental sulfur and ferrous sulfate. The sulfur is separated via mechanical means, such as filtering. The solution is regenerated to the active ferric form by the bacteria. 

Like ammonia, hydrogen sulfide (oxid. no. S = —2) can act only as a reducing agent when it takes part in redox reactions. Most often the H2S is oxidized to elementary sulfur, as in the reaction... 

C04-0149. Surface deposits of elemental sulfur around hot springs and volcanoes are believed to come from a two-step redox process. Combustion of hydrogen sulfide (H2 S) produces sulfur dioxide and water. 

H. Kimura, Y. Nagai, K. Umemure, and Y. Kimura, Physiological roles of hydrogen sulfide synaptic modulation, neuroprotection, and smooth muscle relaxation. Antioxidants Redox Signaling 7, 795-803 (2005). 

The sewer is dominated by heterotrophic microorganisms that degrade and transform wastewater components. These processes proceed under redox conditions determined by the availability of the electron acceptor. The importance of the processes for the sewer and the surroundings is not just caused by the removal and transformation of organic substrates — the electron donor—but is also a result of transformation of the electron acceptors exemplified by the formation of hydrogen sulfide from sulfate. 

The Richard Process - A Claus Alternative The Claus reaction process is by far the most common for recovering elemental sulfur from hydrogen sulfide. This heterogeneous gas phase catalysis over alumina of the redox reaction between H2S and SO2,... 

Note that for describing the EH value of a hydrogen sulfide solution, instead of reaction 10 in Table 14.2, we could also use the redox couple involving S(s) and HS (instead of H2S) ... 

Let us now turn to some kinetic considerations of NAC reduction. As an example, consider the time courses of nitrobenzene (NB) concentration in 5 mM aqueous hydrogen sulfide (H2S) solution in the absence and presence of natural organic matter (Fig. 14.7). As is evident, although reduction of NB by H2S to nitrosobenzene and further to aniline (Eq. 14-31) is very favorable from a thermodynamic point of view (see Fig. 14.4), it seems to be an extremely slow process. However, when DOM is added to the solution, reduction occurs at an appreciable rate (Fig. 14.7). In order to understand these findings, some general kinetic aspects of redox reactions involving NACs should be recognized. 

Hydrogen ligands, 689-711 Hydrogen selenide metal complexes, 663 Hydrogen sulfide metal complexes, 516 Hydrogen telluride metal complexes, 670 Hydroporphyrins, 814-856 basicity, 853 dehydrogenation, 853 demetallation, 854 deuteration, 853 mass spectra, 852 metallation, 854 NMR, 852 non-aromatic, 855 photochemistry, 854 redox chemistry, 855 synthesis, 852... 

Bacterially produced elemental sulfur can also react with hydrogen sulfide form polysulnde ions. Thus, polysulfide ions should constitute a significant fraction of sulfur nudeophiles in reducing sediments especially where sulfide oxidation is incomplete, such as in intertidal and salt marsh sediments (31321. The polysulfide ions should also be important at redox boundaries (anoxic/ suboxic) in the water column of marine anoxic basins, such as the Black Sea. 

The members of the hydrogen sulfide family are both thermodynamically and kinetically unstable in oxic seawater. The thermodynamic bias against them can be appreciated in a simple redox couple to sulfate, the predominant form of oceanic sulfur. 

Hydrogen sulfide formation through dissimilatoiy sulfur reduction has for years been known as a source of environmental sulfur. This compound has invited recent study because of its possible effect on the redox chemistry of sea water. Both the lifetime and the oceanic concentrations of this reactive and highly toxic compound are the focus of the fifth section. 

Density layer oq = 16.10-16.15 kgm 3. This layer constitutes the lower part of the redox zone. The onset of hydrogen sulfide occurs just below the depths of maximum particulate manganese and iron. The reduction of Mn(III) and Mn(IV) by sulfide is very intensive [63,75] and model estimates [88] suggest these reactions can balance the hydrogen sulfide flux from below. A deeper phosphate maximum occurs about 5-10 m below the appearance of hydrogen sulfide. The vertical gradient of hydrogen sulfide increases at this depth (Fig. 2). 

Our studies showed that the biogeochemical system of the redox layer is subjected to temporal variability on a seasonal scale (connected with the seasonality of OM production) and interannual changes. Surface ventilation of dissolved oxygen down to the depth of the CIL (ag = 14.5 kg nr3) occurs in the winter from a combination of the NW shelf and the centers of the gyres. The intensity of ventilation is determined by climate forcing which may be determined by large-scale climate patterns like the NAO. This ventilation sets the upper boundary conditions for the downward transport of O2. Therefore, the position of the hydrogen sulfide boundary in the density field is connected with the climate variability, related to the NAO index. 

Wetlands, at times, may be a poor environment for the formation of metal oxides and/or oxyhydroxides because of the typically low redox potential (Eh). Optimizing the activity of sulfate-reducing bacteria (e.g., desulfovibrio) in the anaerobic zone would be a more effective way of removing metals and sulfates from AD (Kleinmann, 1989). These sulfate-reducing bacteria consume acidity and most of the hydrogen sulfide they produce reacts with heavy metals to create insoluble precipitates. The reactions are shown below ... 

Field measurements of the pH-Eh-E32- relations in reducing environments where hydrogen sulfide, polysulfides and elemental sulfur were found were in good agreement with the redox level estimated from the concentrations of the sulfur species ... 

KDF-55 This filtration option is a copper—zinc formula that relies on a chemical process called oxidation/reduction (redox) to remove pollutants from the water. It is effective on bad tastes and odors, heavy metals, chlorine, hydrogen sulfide, and iron but has little ability to alkalize water. 

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