Sulfate reduction kinetics

Following the calculations in Section 18.5, we take a rate constant k+ for sulfate reduction of 10-9 mol mg-1 s-1, a half-saturation constant for acetate of 70 p, molal, and a growth yield of 4300 mg mol-1 from a study of the kinetics of Desulfobacter postgatei by Ingvorsen el al. (1984). We set a value for KA, the half-saturation constant for sulfate, of 200 p molal, as suggested by Ingvorsen el al. (1984) and Pallud and Van Cappellen (2006). 

Fig. 33.4. Factors controlling rates of microbial activity in the simulation depicted in Figure 33.3, for acetotrophic sulfate reduction (top) and acetoclastic methanogenesis (bottom). Factors include the thermodynamic potential factor Ft, kinetic factors FD = wac/C ac + Kq) and FA = mso4/(mso4 + K A), and biomass concentration [A],...

Pallud, C. and P. Van Cappellen, 2006, Kinetics of microbial sulfate reduction in estuarine sediments. Geochimica et Cosmochimica Acta 70, 1148-1162. 

Nielsen, P.H. (1987), Biofilm dynamics and kinetics during high-rate sulfate reduction under anaerobic conditions, Appl. Environ. Microbiol., 53(1), 27—32. 

Startup effects. Startup effects must also be considered in the interpretation of laboratory experiments. For example, during sulfate reduction, the first small amormt of sulfur to pass through the chain of reaction steps would be subject to the kinetic isotope effects of all of the reaction steps. This is because it takes some time for the isotopic compositions of the pools of intermediates to become enriched in heavier isotopes as described above for the steady-state case. Accordingly, the first HjS produced would be more strongly enriched in the lighter isotopes than that produced after a steady state has been approached. This principle was modeled by Rashid and Krouse (1985) to interpret kinetic isotope effects occurring during abiotic reduction of Se(IV) to Se(0) (see below). Startup effects may be particularly relevant in laboratory experiments where Se or Cr concentrations are very small, as is the case in some of the studies reviewed below. 

Kinetic isotope effects during microbial processes. Micro-organisms have long been known to fractionate isotopes during their sulfur metabolism, particularly during dissimilatory sulfate reduction, which produces the largest fractionations in the sulfur cycle... 

This phenomenon forms the basis for the formulations of Urey (1947) and Bigeleisen and Mayer (1947) for the temperature dependence of isotopic exchange between two molecules. With the nearly simultaneous development of the isotope-ratio mass spectrometer by Nier et al. (1947), the potential for application of stable isotopes was created. Other isotopic fractionation processes are observed in kinetics, diffusion, evaporation-condensation, crystallization, and biology (e.g., photosynthesis, respiration, nitrogen fixation, sulfate reduction, and transpiration). The concomitant isotopic fractionations can also be used to provide details of the relevant process. 

Studies of are equally few. They have been used to show that sulfide is the main source of S04 in metamorphic and sedimentary catchments. There may be some kinetic effects on isotope fractionation if only the surfaces of the sulfides are being oxidized, and the S04 may become enriched in if sulfate reduction occurs (Bottrell et al., submitted). 

A comparison of the equilibrium (Eh) and kinetic (TEAPs) approaches to describe redox processes in a petroleum hydrocarbon-contaminated aquifer was given by Chapelle et al. (1996). In this study. Eh measurements were made with a platinum electrode, and the results plotted on a standard Eh-pH diagram (SUlen, 1952). The results of this analysis are shown in Eigure 11. Based on this analysis, it can be concluded that Fe(III) reduction is the predominant redox process, as none of the measured Eh values are sufficiently negative to indicate sulfate reduction... 

Details of sulfur isotope geochemistry are presented elsewhere in this volume (see Chapter 7.10) and are only highlighted here as related to paleo-environmental interpretations of finegrained siliciclastic sequences. Formation of sedimentary pyrite initiates with bacterial sulfate reduction (BSR) under conditions of anoxia within the water column or sediment pore fluids. The kinetic isotope effect associated with bacterial sulfate reduction results in hydrogen sulfide (and ultimately pyrite) that is depleted in relative to the ratios of residual sulfate (Goldhaber... 

A characteristic feature of sedimentary sulfide in recent marine sediments is enrichment in the light isotope of sulfur. This enrichment is largely a result of the isotopic fractionation introduced during sulfate reduction by SRB. However, the isotopic composition of sedimentary sulfide is frequently found to be lighter than predicted based on the documented fractionation by SRB discussed above. Thus, additional processes beyond single step kinetic reduction of SO to H2S are required to explain the data. 

Rees (1973 see p. 330) demonstrated that the overall isotope fractionation during bacterial sulfate reduction can be influenced by a change in the kinetics of sulfate uptake from first- to second-order due to saturation of enzymic activation, or permeation sites. The sulfate concentration at which the change in kinetics occurs is uncertain and indeed may not be the same for all organisms and all physiological conditions. Postgate (1951) found that the specific rate of Hj-linked sulfate reduction by washed cells of D. desul-furicans was independent of sulfate concentration between 1 and 100 mM. On the other hand, Harrison (1957) reported that with lactate as the electron donor, reduction was first-order with respect to sulfate below 10 mM sulfate but became independent of sulfate concentration above 10 mM. 

Since first-order kinetics with respect to sulfate ion is indicated under some conditions for bacterial sulfate reduction, it is useful to consider the isotopic behaviour of a simple one-step first-order conversion (Fig. 6.2.4a). The term kinetic isotope effect describes the competing reactions (1) and (2),... 

Nakai and Jensen (1964) studied sulfate reduction in a natural mud culture over a period of 65 days. They assumed the reaction to be first-order and found a rate constant of around 0.02 d" and a fe32/fe34 value of 1.02. However, Sakai (cited in Goldhaber and Kaplan, 1974) suggested that this system may have exhibited zero-order kinetics in which case the rate was about 0.5 mg S d" The question of reaction kinetics is probably unresolv-able since it is difficult to reproduce data in laboratory experiments with pure cultures and defined media, let edone evaluate the unknowns arising... 

As previously discussed, Harrison and Thode (1958) invoked a two-step model to account for the range of isotopic fractionation encountered during sulfate reduction by D. desulfuricans. Rees (1973) developed a steady-state multi-step model for isotope fractionation during bacterial reduction. His approach differed from previous attempts in that he included the possibility of zero-order kinetics for describing the uptake of sulfate. His reaction scheme is basically of the form... 

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