Sulfate-reduction reaction

Free sulfur is rarely present in crude oils, but it can be found in suspension or dissolved in the liquid. The crude from Goldsmith (Texas, USA.) is richest in free sulfur (1% by weight for a total sulfur content of 2.17%). It could be produced by compounds in the reservoir rock by sulfate reduction (reaction 8.2). 

No matter which sulfate reduction reaction is used, E°cs (= kathode + -Eanode) < 0, where kathode is the standard reduction potential and fibnode is the standard oxidation potential. Therefore, no, S042 ions cannot oxidize H3As03 to H3As04. 

Copper does not dissolve in 1 mol L 1 sulfuric acid, since E° for the halfreaction 16.16 is too positive relative to sulfate reduction (reaction 16.15) or hydrogen evolution. It will, however, dissolve in hot, concentrated (18 mol L 1) sulfuric acid, in which the activities of H+ and HSO4- are much higher and the activity of water (now a minor constituent) much lower than in the standard-state conditions for which the E° of half-reaction 16.15 applies ... 

This sulfate reduction reaction in anoxic carbonate sediments has potential importance for carbonate dissolution in shallow-water, marine environments, but its global significance remains a question. An observation of interest is that even complete sulfate reduction returns the saturation state of the water to only about half its original value. Thus the sulfate reduction reaction by itself may not promote carbonate precipitation and partial sulfate reduction may result in carbonate dissolution. 

The combination of the sulfate reduction reaction, and reactions primarily with iron and manganese oxide minerals can lead to significant calcium carbonate precipitation. This type of net process can be represented schematically as ... 

In the Floridan aquifer, traces of gypsum are present in the carbonate rock. The high calcium concentration from gypsum dissolution exceeds its value at saturation with calcite, leading to precipitation of the carbonate and the production of additional CO2 (reaction 8). Concurrently, anaerobic decay of buried organic matter (reaction 3) and sulfate reduction (reaction 6) take place. The combination of these processes has caused an increase in the CO2 pressure of the groundwater from 10" bar in the recharge zone to 10" bar downdip as the pH decreases from 8.0 to 7.4 over a map distance of 115 km (Fig. 5.3) (cf. Back and Hanshaw 1970 Plummer et al. 1983). 

In semiarid climates, sulfate deposits in the soil are sometimes reduced under conditions of poor drainage. The conditions necessary for reduction are created in natural basins where water accumulates due to the very low water permeability of sodic soils. The sulfate reduction reaction ... 

Bohlke and Shanks (1994) have used incremental reaction models to show that the high values and large range of values are probably due to sulfate reduction reactions... 

When domestic wastewater is treated by chemical precipitation followed by carbon adsorption, one of the problems encountered is that the carbon columns become highly odorous. The odor derives from the reduction of S04 to HS by microorganisms growing within the carbon column on biodegradable adsorbed organic matter. This is the sulfate-reduction reaction. Attempts to alleviate the situation by injecting air into the columns have met with little success because the oxidation of sulfide by oxygen is not always a rapid process, especially if the sulfides... 

In order to highlight the effect of the cathodic sulfate reduction reaction, the model does not include the role of pH. 

Scheme 14.6 Sulfate reduction reaction carried out by sulfate reducing bacteria.

Lactate is also involved in overall sulfate reduction reactions such as these employed by the Desulfovibrio bacteria. One such reaction is given by (Decker et al. 1970)... 

Fig. 22. The standard partial molal Gibbs free energy of reaction (39) as a function of temperature at Psat- This overall sulfate reduction reaction is used at low temperatures by Desulfovibrio organisms, and it can be seen that more energy is available from this reaction at elevated temperatures...

Nitrates. Iron(II) nitrate hexahydrate [14013-86-6], Fe(N03)2 6H20, is a green crystalline material prepared by dissolving iron in cold nitric acid that has a specific gravity of less than 1.034 g/cm. Use of denser, more concentrated acid leads to oxidation to iron(III). An alternative method of preparation is the reaction of iron(II) sulfate and barium or lead nitrate. The compound is very soluble in water. Crystallisation at temperatures below — 12°C affords an nonahydrate. Iron(II) nitrate is a useful reagent for the synthesis of other iron-containing compounds and is used as a catalyst for reduction reactions. 

Reactions other than those of the nucleophilic reactivity of alkyl sulfates iavolve reactions with hydrocarbons, thermal degradation, sulfonation, halogenation of the alkyl groups, and reduction of the sulfate groups. Aromatic hydrocarbons, eg, benzene and naphthalene, react with alkyl sulfates when cataly2ed by aluminum chloride to give Fhedel-Crafts-type alkylation product mixtures (59). Isobutane is readily alkylated by a dipropyl sulfate mixture from the reaction of propylene ia propane with sulfuric acid (60). 

Bromide ndIodide. The spectrophotometric determination of trace bromide concentration is based on the bromide catalysis of iodine oxidation to iodate by permanganate in acidic solution. Iodide can also be measured spectrophotometricaHy by selective oxidation to iodine by potassium peroxymonosulfate (KHSO ). The iodine reacts with colorless leucocrystal violet to produce the highly colored leucocrystal violet dye. Greater than 200 mg/L of chloride interferes with the color development. Trace concentrations of iodide are determined by its abiUty to cataly2e ceric ion reduction by arsenous acid. The reduction reaction is stopped at a specific time by the addition of ferrous ammonium sulfate. The ferrous ion is oxidi2ed to ferric ion, which then reacts with thiocyanate to produce a deep red complex. 

Sulfite reductase catalyzes the six-electron reduction of sulfite to sulfide, m essential enzymatic reaction in the dissimilatory sulfate reduction process. Several different types of dissimilatory sulfite reductases were already isolated from sulfate reducers, namely desul-foviridin (148-150), desulforubidin (151, 152), P-582 (153, 154), and desulfofuscidin (155). In addition to these four enzymes, an assimila-tory-type sulfite reductase was also isolated from D. vulgaris. Although all these enzymes have significantly different subunit composition and amino acid sequences, it is interesting to note that, as will be discussed later, all of them share a unique type of cofactor. 

Although the foregoing reactions involve dehalogenation by reduction or elimination, nucleophilic displacement of chloride may also be important. This has been examined with dihalomethanes using HS at concentrations that might be encountered in environments where active anaerobic sulfate reduction is taking place. The rates of reaction with HS exceeded those for hydrolysis and at pH values above 7 in systems that are in equilibrium with elementary sulfur, the rates with polysulfide exceeded those with HS. The principal product from dihalomethanes was the polythio-methylene HS (CH2-S) H (Roberts et al. 1992). 

Truly field experiments on microbial reactions are extremely difficult to carry out, but a series of microcosm experiments on the substrates that may support anaerobic sulfate reduction quite closely... 

The base was being prepared by distilling a mixture of hydroxylamine hydrochloride and sodium hydroxide in methanol under reduced pressure, and a violent explosion occurred towards the end of distillation [1], probably owing to an increase in pressure above 53 mbar. It explodes when heated under atmospheric pressure [2], Traces of hydroxylamine remaining after reaction with acetonitrile to form acetamide oxime caused an explosion during evaporation of solvent. Traces can be removed by treatment with diacetyl monoxime and ammoniacal nickel sulfate, forming nickel dimethylglyoxime [3], An account of an extremely violent explosion towards the end of vacuum distillation had been published previously [4], Anhydrous hydroxylamine is usually stored at 10°C to prevent internal oxidation-reduction reactions which occur at ambient temperature [5], See other REDOX REACTIONS... 

Hydrogen sulfide is a well known general metabolite produced on sulfate reduction by certain bacteria. Moreover, organic forms of sulfur can give rise to HS , hence H2S in certain bacteria. Thus, cysteine desulfhydrase (EC 4.4.1.1, cystathionine y-lyase) converts L-cysteine to H2S, pyruvate, and NH3. This enzyme shows a requirement for pyridoxal phosphate and the unstable ami-noacrylic acid is an intermediate (Equation 1) in the reaction ... 

The Monod equation is the relation most commonly applied to describe the rate at which a microbe metabolizes its substrate (e.g., Panikov, 1995). Taking ace-totrophic sulfate reduction as an example, the redox reaction,... 

Reaction in the simulation begins slowly, but proceeds more rapidly as biomass accumulates, reflecting the reaction s autocatalytic nature. The reaction rate continues to increase until most of the acetate is consumed, at which point it slows abruptly to a near stop. In the simulation, dissolved ferrous iron is present in excess amount. The bisulfide produced as a result of bacterial sulfate reduction reacts with the iron,... 

Fig. 18.2. Results of modeling at 25 °C bacterial sulfate reduction using acetate as the electron donor, according to a thermodynamically consistent form of the Monod equation. Labels identify values and line slopes after seven days of reaction.

Fig. 22.7. Thermodynamic driving forces for various anaerobic (top) and aerobic (bottom) microbial metabolisms during mixing of a subsea hydrothermal fluid with seawater, as a function of temperature. Since the driving force is the negative free energy change of reaction, metabolisms with positive drives are favored thermodynamically those with negative drives cannot proceed. The drive for sulfide oxidation is the mirror image of that for hydrogentrophic sulfate reduction, since in the calculation 02(aq) and H2(aq) are in equilibrium.

To establish the stoichiometry of the sulfide formation, Equation (6.3) must be combined with the oxidation process for the organic matter that is the actual electron donor for the heterotrophic sulfate-reducing bacteria. The procedure for the combination of the oxidation and the reduction process steps is the same as outlined in Section 2.1.3. If organic matter is considered simply as CH20, the combination of the oxidation process as depicted in Example 2.2 and the reduction reaction for sulfate shown in Equation (6.3) result in the following redox process ... 

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