Sulfur isotopes kinetics

KINETIC ISOTOPE EFFECTS THEORETICAL CONSIDERATIONS AND INSIGHTS FROM THE SULFUR ISOTOPE LITERATURE... 

Sulfur. A kinetic isotopic effect, accompanying the conversion of sulfate... 

In electron impact (El) mass spectra PCDTs show a strong molecular M1 ion and the expected clustering due to chlorine and sulfur isotopes. The major difference in the El mass spectra of the PCDDs and PCDTs is the formation of a strong M+-COCl in the former and a strong M+-2C1 in the latter compounds. PCDDs and PCDFs could be detected with good sensitivity via the reaction M+— (M-COCl)+ by QUAD MS/MS. Mass-analyzed ion kinetic energy (MIKE) MS/MS was found suitable for detecting the PCDTs via the reaction M+ —> (M-2Cl)+, whereas PCDDs and PCDFs were monitored by the reaction M+ —> (M-COCl)+ [31]. 

The isotope ratios of the sulfur isotopes are also affected by kinetic and equilibrium isotope effects. Kinetic isotope effects are marked in the reduction of sulfates to hydrogen sulfide by bacteria (enrichment of the lighter isotopes in H2S). The equilibrium isotope effect in the reaction... 

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... 

Kinetics and the selectivity of sulfur isotopes during microbial reductions under... 

KINETICS AND THE SELECTIVITY OF SULFUR ISOTOPES DURING MICROBIAL REDUCTIONS UNDER LABORATORY CONDITIONS... 

Early work by Thode et al. (1949) established that there are large variations in the isotopic composition of sulfur compounds in nature. Since then sulfur isotope abundance data have been frequently used to elucidate many terrestrial processes including the genesis of sulfide ore bodies. The mechanisms of isotopic fractionation (alteration of relative isotopic abundances) can be broadly categorised under exchange processes or the kinetic isotope effects discussed on pp. 324ff. Isotopic exchange may be represented by the reaction ... 

Chemical reaction pathways for input into the isotopic model have been computed using the EQ3/6 reaction pathway modeling codes (6). Distribution of sulfur isotopes between aqueous species and minerals are calculated using a new computer code (EQPS.S). Isotopic fractionation factors (I) are used by the code to determine the distribution among components as described below. Thus, this approach does not make or apply any assumptions about the chemical mechanism by which isotopic exchange or transfer occurs. The descriptive , rather than mechanistic approach, is due in part to the lack of understanding of such mechanisms and inability of chemical reaction codes to handle kinetics of homogeneous solution reactions. 

Sulfur isotope exchange between sulfate and sulfide. Ohmoto and Lasaga (1982) investigated the kinetics of sulfur isotope exchange between aqueous sulfates and sulfides at hydrothermal conditions ( 200 to 400°C). They concluded that the rates of chemical reactions between sulfates and sulfides are essentially identical to the sulfur isotope exchange rates because both the chemical and isotopic reactions involve simultaneous oxidation of sulfide-sulfur atoms and reduction of sulfate-sulfur. They expressed the rate of reaction as a second order rate law... 

Susnow RG, Dean AM, Green WH, Peezak P, Broadbelt LJ (1997) Rate-based constraction of kinetic models for complex systems. JPhy Chem 101 3731-3740 Tanaka N, Rye DM, Xiao Y, Lasaga AC (1994) Use of stable sulfur isotope systematics for evaluating oxidation reaction pathways and in-cloud-scavenging of sulfur dioxide in the atmosphere. Geophys Res Let 21 1519-1522... 

In eq. (1.14.15), the subscript (t=small) denotes the initial reaction stage (about 2% completion of the reaction, while (r=< ) denotes 100% completion of the reaction. This method gave the sulfur-34 kinetic isotope effect of 1.77%, i.e., 1.0177. The meaning of this result can be evaluated if we know... 

Only within the past few years have serious attempts been made to estimate quantitatively the differences in reactivity between thiophene and benzene and between the 2- and 3-position of thiophene. Careful investigation on the acid-induced exchange of deuterium and tritium have shown that the ratios of the exchange rates in the 2- and 3-positions are 1045 61 for deuterium and 911 60 for tritium in 57% by weight aqueous sulfuric acid at 24.6°C. A kinetic isotope effect in the isotopic exchange has been found to be k-r/kr, = 0.51 0.03 in the 2-position and kr/kjy — 0.59 0.04 in the... 

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... 

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