Sulfur: abiotic oxidation

Oxidation of reduced sulfur species. Oxidation of reduced sulfur species in the presence of oxygen can occur spontaneously, without bacterial mediation. Bacteria of the family Thiobacteriaceae are probably the most important bacteria involved in sulfur oxidation. Of these, bacteria of the genus Thiobacillus have been most studied (Goldhaber and Kaplan 1974 Cullimore 1991). The first product of sulfide oxidation abiotically or by Thiobaccillus is thought to be elemental sulfur according to... 

Abiotic oxidation kinetics of sulfur and Fe(ll). The abiotic oxidation rates of pyrite and ferrous iron have been studied extensively for more than 30 years (cf. Garrels and Thompson 1960 McKibben and Barnes 1986 Nicholson et al. 1988 Moses and Herman 1991). The work on pyrite-oxidation kinetics has been updated and critiqued by Williamson and Rimstidt (1994), while Wehrli (1990) has synthesized the published data on Fe oxidation. (See also King et al, 1995.)... 

In the presence of oxygen, abiotic oxidation of hydrogen sulfide can occur. However, this reaction can be catalyzed by sulfur-oxidizing bacteria (Figures 11.15 and 11.16). Oxidation of reduced sulfur occurs at interfaces the oxic-anoxic interface for abiotic and chemolithotrophic oxidation and, in the anoxic zone, the light-dark interface for phototrophic oxidation. 

We cover each of these types of examples in separate chapters of this book, but there is a clear connection as well. In all of these examples, the main factor that maintains thermodynamic disequilibrium is the living biosphere. Without the biosphere, some abiotic photochemical reactions would proceed, as would reactions associated with volcanism. But without the continuous production of oxygen in photosynthesis, various oxidation processes (e.g., with reduced organic matter at the Earth s surface, reduced sulfur or iron compounds in rocks and sediments) would consume free O2 and move the atmosphere towards thermodynamic equilibrium. The present-day chemical functioning of the planet is thus intimately tied to the biosphere. 

Isotope effects also play an important role in the distribution of sulfur isotopes. The common state of sulfur in the oceans is sulfate and the most prevalent sulfur isotopes are (95.0%) and (4.2%). Sulfur is involved in a wide range of biologically driven and abiotic processes that include at least three oxidation states, S(VI), S(0), and S(—II). Although sulfur isotope distributions are complex, it is possible to learn something of the processes that form sulfur compounds and the environment in which the compounds are formed by examining the isotopic ratios in sulfur compounds. 

Note that some reactions are reversible (indicated by . "), whereas others are irreversible under environmental conditions. The dotted arrow indicates that, in principle, a reaction is possible, but no clear evidence exists showing that the reaction proceeds abiotically in the dark. b For oxidation states of nitrogen in various functional groups see Table 2.5. c For oxidation states of sulfur in various functional groups see Table 2.6. 

Figure 8.20. Simplified scheme for the oxidation of H2S by O2 mediated by a variety of bacteria. The gradient zone between O2 and H2S is the environment of many colorless sulfur bacteria, among which the type Beggiatoa often reach high population densities and form white mats on the mud or sediment-water interface. If light penetrates at the zonation between O2 and H2S, phototrophic, often colorful, sulfur bacteria grow. Reduced sulfur can also be oxidized abiotically, for example, by Fe(III)(hydr)oxides or even by O2 in the presence of metal-ion catalysts.

The role of sulfur- and iron-oxidizing bacteria. As already noted, the rates of FeS2 and Fe(II) oxidation in environmental systems often differ substantially from the abiotic rates. Usually natural rates are much faster than laboratory abiotic rates. The reasons include inorganic catalysis and especially enzymatic oxidation by microorganisms. Oxidation of Fe(ll), for example, is catalyzed by some clays and metals, including Al, Fe, Co +, Cu, and Mn, and also HPO (Stumm and Morgan 1981). 

Sulfur and oxygen isotopes. Stable sulfur and oxygen isotopes can provide clues regarding the relative importances of O2 versus Fe(III) oxidation of pyrite, and whether the oxidation is abiotic (sterile) or involves bacteria (Taylor et al. 1984a, 1984b). The isotopic data are reported in S (per mil or %o) units, where... 

The high iron concentrations are difficult to explain. The stoichiometry of the 02/FeS2 oxidation reaction (Eq. 12.39a) indicates that 3.5 moles of O2 are consumed to produce a mole of Fe. Given the atmospheric O2 solubility of 8.4 mg/kg at 25°C, for example, oxidation of pyrite by O2 can produce a Fe concentration of only 4.2 mg/kg. The low Eh values in the pools make it doubtful that measureable DO is present in any case. Based on the foregoing discussion of sulfur and oxygen isotopes and bacterial oxidation, pyrite oxidation in the pools is probably largely abiotic with ferric iron as the oxidant. Oxidation rates must be slow, given the fact that Fe(III) Fe(II) at these Eh values. 

Ga ago is the discovery of sulfate minerals in deposits of that time (Walter et ah, 1980). Although small amounts of oxygen from abiotic photolysis of water could have resulted in the oxidation of reduced sulfur compounds to form sulfates, it is also possible that part or most of the sulfate was derived from anaerobic photosynthesis according to reaction (2) above. 

In soils and sediments rich in sulfides, abiotic reduction of Fe(III) and Mn(IV) is possible. For example, sulfides produced during sulfafe reduction can reduce Fe(III) to Fe(II) and Mn(IV) to Mn(ll). Manganese oxides are known to be more reactive with reduced sulfur compounds than with Fe(lll) oxides. 

As for nitrogen, biogenic redox processes are essential to convert sulfur from its largest oxidation state +VI (sulfate) to its lowest -II (sulfide). Sulfate is the most stable compound in the atmosphere once it has been produced there is no abiotic reduction possible in the climate system. Organisms need sulfur in the form of thiols RSH (simple thiols are called mercaptanes) and sulfides R2S. Thiols are (similar to alcohol ROH) weak acids (but much stronger). Thiols are easy oxidized into sulfides (this reaction is the main function in biological chemistry Fig. 5.25), thereby thiols provide hydrogen for the reduction of other molecules (the dissociated form RS" acts as an electron donor) ... 

Most LCAs are performed only xmtil Step 2, since impact assessment and interpretation involve many more qualitative assumptions. In this case, LCA are called life cycle inventories (LCIs). This latter is a tool required to estimate the direct and indirect inputs of each step of a biofuel pathway. The results are the use of resources (eg, energy consumption) and the environmental emissions (eg, CO2, sulfur oxides, nitrogen oxides). LCIs permit the assessment of impact categories, such as climate change, photooxidant formation, acidification, eutrophication, ecotoxicity and human toxicity, and the depletion of biotic and abiotic resources. These factors of the LCI will be converted into environmental damages. Various indicators can be derived from these mechanisms at intermediate levels (midpoints) or damage levels (endpoints) after normalization, often weighting approaches. 

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