Sulfide isotopic effects

The biochemical reduction of sulfate to sulfide by bacteria of the genus Desulfovibrio in anoxic waters is a significant process in terms of the chemistry of natural waters since sulfide participates in precipitation and redox reactions with other elements. Examples of these reactions are discussed later in this paper. It is appropriate now, however, to mention the enrichment of heavy isotopes of sulfur in lakes. Deevey and Nakai (13) observed a dramatic demonstration of the isotope effect in Green Lake, a meromictic lake near Syracuse, N. Y. Because the sulfur cycle in such a lake cannot be completed, depletion of 32S04, with respect to 34S04, continues without interruption, and 32S sulfide is never returned to the sulfate reservoir in the monimolimnion. Deevey and Nakai compared the lake to a reflux system. H2S-enriched 32S diffuses to the surface waters and is washed out of the lake, leaving a sulfur reservoir depleted in 32S. The result is an 34S value of +57.5% in the monimolimnion. 

Mass Spectral Techniques. Samples for isotope ratio analysis are typically converted to sulfates or sulfides, then to S02(g) for analysis on a mass spectrometer (MS). The precision of the S02 measurement is commonly reported as 0.1 to 0.2 0/00 (16.241. yet systematic errors of 1 0/00 or larger may result from 1) memory effects due to adsorption of S02 on the walls of the MS, and 2) secondary isotope effects due to the existence of two stable isotopes of oxygen, 160 and lsO (251. Both of these errors can be eliminated by using SF6 rather than S02 as the analyte in the MS (25.261. However, existing sulfur fluorination procedures are relatively dangerous and tedious, making the SF6 method less desirable as a routine environmental technique (261. 

Another procedure for sulfur isotope measurements has been developed where samples are converted to solid arsenic sulfide, AS0S3 (s), and measured by thermal ionization mass spectrometry (TIMS) (22). This technique offers several advantages over the gaseous methods in that both memory and isotope effects are eliminated, and the chemical procedure is simpler. A precision of 1 0/00, and the capability of making measurements on small samples, makes the TIMS technique competitive with gas phase MS techniques. 

Mechanisms have been suggested for the N-bromosuccinimide (NBS) oxidation of cyclopentanol and cyclohexanol, catalysed by iridium(III) chloride,120 of ethanolamine, diethanolamine, and triethanolamine in alkaline medium,121 and for ruthenium(III)-catalysed and uncatalysed oxidation of ethylamine and benzylamine.122 A suitable mechanism has been suggested to explain the break in the Hammett plot observed in the oxidation of substituted acetophenone oximes by NBS in acidic solution.123 Oxidation of substituted benhydrols with NBS showed a C-H/C-D primary kinetic isotope effect and a linear correlation with er+ values with p = —0.69. A cyclic transition state in the absence of mineral acid and a non-cyclic transition state in the presence of the acid are proposed.124 Sulfides are selectively oxidized to sulfoxides with NBS, catalysed by ft-cyclodextrin, in water. This reaction proceeds without over-oxidation to sulfones under mild conditions.125... 

The mechanism and kinetic aspects of the retro-ene reaction of the allyl n-propyl sulfide and its deuterated derivatives have been studied using four different types of DFT methods with eight different levels of the basis sets.7 The mechanistic studies revealed that the reaction proceeds through an asynchronous concerted mechanism. Theoretical calculations have indicated that the reaction displays a kinetic isotope effect of 2.86 at 550.65 K. 

Similar reaction pathways have also been found for the oxidation of dimethyl sulfide to dimethyl sulfoxide and dimethyl sulfoxide to dimethyl sulfone by [Ru(bpy)2(py)(0)]2+ with respective rate constants of 17.1 and 0.13 M l s"1 in MeCN at 298 K (48). The complex [Ru(bpy)2 (py)(0)]2+ has also been used electrocatalytically for the oxidation of alcohols, aldehydes, alkenes, and aromatics (23, 49). The kinetics of oxidation of formic acid/formate ion by [Ru(bpy)2(py)(0)]2 +, with a large kinetic isotope effect [ HC02-/ADCo2- = 19 (25°C, /r = 1.0 M)], has been reported (50). A two-electron hydride transfer has been suggested for the oxidation of HC02 by [Ru(bpy)2(py)(0)]2+. A similar mechanism has also been suggested for the oxidation of alcohols (51) and aromatics (52) by [Ru(bpy)2(py)(0)]2+ and other related Ru(IV) oxo complexes (28,... 

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... 

There now exist numerous observations of mass-independent isotopic compositions in nature. Most of these have recently been reviewed and will not be repeated here. When the first laboratory measurements of the mass-independent isotope effect were reported by Thiemens and Heidenreich (1983), their occurrence in nature was not expected, except possibly for the early solar system to produce the observed meteoritic CAI data. It is significant to note that, at present, all oxygen-bearing molecules in the atmosphere (except water) possess mass-independent isotopic compositions. These molecules include O2, O3, CO2, CO, N2O, H2O2, and aerosol nitrate and sulfate. Mass-independent sulfur isotopic compositions are also observed in aerosol (solid) sulfates and nitrates and sulfide and sulfate minerals from the Precambrian, Miocene volcanic sulfates, Antarctica dry valley sulfates, Namibian Gypretes, and Chilean nitrates. In addition, martian (SNC meteorites) carbonates and sulfates possess both mass-independent sulfur and oxygen isotopic compositions. These studies have been reviewed recently (Thiemens et al., 2001 Thiemens, 1999). 

Fry B., Ruf W., Gest H., and Hayes J. M. (1988) Sulfur isotope effects associated with oxidation of sulfide by O2 in aqueous solution. Chem. Geol. 73, 205-210. 

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... 

Metastable Iron Sulfides Organic Sulfur Elemental Sulfur MECHANISM OF PYRITE FORMATION. 4.1 Evidence from Experimental Studies. 4.2 Isotope Effects during Experimental Pyrite Formation. 4.3 Origin of Morphological Variations in Pyrite SULFUR DIAGENETIC PROCESSES IN MARINE SEDIMENTS. 5.1 Depth Distribution of Diagenetic Sulfur Products. 5.2 Rates of Sulfate Reduction... 

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 ... 

Since transport and a considerable time span were involved, the isotopic composition of the sulfide is consistent with a summation of the kinetic isotope effects in both reductions. The initial sulfates, intermediate sulfates and product-sulfide ions had 6 S values typically near +20, 0 to 13, and —25 to —40%o, respectively. itborite spring had an unusually low 5 S value of —62%o for dissolved sulfide. 

Mojzsis, S.J., Coath, C.D., Greenwood, J.P., McKeegan, K.D., and Harrison, T.M., 2003. Mass-independent isotope effects in Archaean (2.5 to 3.8 Ga) sedimentary sulfides determined by ion microprobe anlysis. Geochim. Cosmochim. Acta, 67, 1635-58. 

---