Fe reduction

Sulfur exists naturally in several oxidation states, and its participation in oxidation/reduc-tion reactions has important geochemical consequences. For example, when an extremely insoluble material, FeS2, is precipitated from seawater under conditions of bacterial reduction, Fe and S may be sequestered in sediments for periods of hundreds of millions of years. Sulfur can be liberated biologically or volcanically with the release of H2S or SO2 as gases. 

Aerobic respiration Denitrification Mn(IV) reduction Fe(III) reduction Sulfate reduction Methane fermentation Nitrogen fixation... 

H2 as a reductant (Fe, Ni) in anaerobic archaea Energy capture related to ATP formation (Fe, NADH, flavin, quinones)... 

Figure 2-36 shows the occupied electron level (donor level) of reductant Fe ,, ) and the vacant electron level (acceptor level) of oxidant FeJ,) referred to the standard gaseous electron level at the outer potential of aqueous solution. 

Redox reactions involving the nickel(IV) complex are also subject to divalent metal ion catalysis (170, 171). Oxidations of the two-electron reductant ascorbate (40) and the one-electron reductant [Fe(CN)6]4-(172) have been examined in some detail. Both reactions have as the rate-determining step the transfer of one electron from the reductant to nickel(IV) in an outer-sphere process to give an undetected nickel(III) transient. Spectroscopic properties of the nickel(III) species have been determined by pulse radiolysis (41). 

Senesi et al. (1977), using the methods of electron spin resonance and Mossbauer spectroscopy in conjunction with chemical methods, established that at least two and possibly three forms of binding of Fe occur in humic materials. Ferric iron is firmly bound and protected in tetrahedral or octahedral coordination this form of binding of iron is resistant to chemical complexing and reduction. Fe adsorbed on the outer surfaces of humic materials is less firmly bound. The iron-fulvic acid complexes studied contain from 5.5 to 50.1% Fe, but a large part of the iron is bound to the surficial octahedral position. 

The factors that influence the flux of energy through aerobic-anaerobic interface ecosystems have been the focus of this review chapter. Here we consider the net effect of these influences on carbon metabolism in marine and freshwater ecosystems. Thamdrup (2000) recently compiled studies that reported the relative contributions of O2 reduction, Fe(III) reduction, and SO4 reduction to carbon metabolism in marine ecosystems (n = 16). On average, the dominant pathway was SO4 reduction (62 17%, X SD). Aerobic respiration and Fe(III) respiration contributed equally to carbon metabolism (18 10% and 17 15%, respectively). Compared to previous compilations, —50% of the amount... 

The initial or rate limiting step for anion breakdown in metal oxalate decompositions has been identified as either the rupture of the C - C bond [4], or electron transfer at a M - O bond [5], This may be an oversimplification, because different controls may operate for different constituent cations. The decomposition of nickel oxalate is probably promoted by the metallic product [68] (the activity of which may be decreased by deposited carbon, compare with nickel malonate mentioned above [65]). No catalytically-active metal product is formed on breakdown of oxalates of the more electropositive elements. Instead, they yield oxide or carbonate and reactions may include secondary processes [27]. There is, however, evidence that the decompositions of transition metal oxalates may be accompanied by electron transfers. The decomposition of copper(II) oxalate [69] (Cu - Cu - Cu°) was not catalytically promoted by the metal and the acceleratory behaviour was ascribed to progressive melting. Similarly, iron(III) oxalate decomposition [61,70] was accompanied by cation reduction (Fe " - Fe ). In contrast, evidence was obtained that the reaction of MnC204 was accompanied by the intervention of Mn believed to be active in anion breakdown [71]. These observations confirm the participation of electron transfer steps in breakdown of the oxalate ion, but other controls influence the overall behaviour. Dollimore has discussed [72] the literature concerned with oxalate pyrolyses, including possible bond rupture steps involved in the decomposition mechanisms... 

In oxidized surface waters and sediments, dissolved iron is mobile below about pH 3 to 4 as Fe and Fe(lII) inorganic complexes. Fe(III) is also mobile in many soils, and in surface and ground-waters as ferric-organic (humic-fulvic) complexes up to about pH 5 to 6 and as colloidal ferric oxyhydroxides between about pH 3 to 8. Under reducing conditions iron is soluble and mobile as Fe(II) below about pH 7 to 8, when it occurs, usually as uncomplexed Fe ion. However, where sulfur is present and conditions are sufficiently anaerobic to cause sulfate reduction, Fe(H) precipitates almost quantitatively as sulfides. Discussion and explanation of these observations is given below. Thermodynamic data for iron aqueous species and solids at 25°C considered in this chapter are given in Table A12.1. Stability constants and A//° values computed from these data are considered more reliable than their values in the MINTEQA2 data base for the same species and solids. 

In the absence of O2, competition among anaerobic microbes for electron donors sets up a series of alternative terminal electron-accepting processes in the order NO reduction, Mn(lV) reduction, Fe(III) reduction, SO -- reduction, and methanogenesis (Figure 9.2 Ponnamperuma, 1972 Megonigal et al., 2004). To a lirsi approximation, a single terminal electron-accepting process dominates... 

The discrepancy between the relative rates of sulfate reduction, FeS inventories, and the resulting quantity of pyrite formed at each station may be due to several processes acting alone or in concert (1) the rate of conversion of FeS to FeSj may differ, although this was largely discounted previously (2) FeS may be reoxidized prior to pyrite formation or (3) FeSz may be oxidized after formation. 

Oxide Reduction. Fe and Mn oxides can be solubilized as Fe " and Mn " under reducing conditions by both biogenic and abiogenic reactions (Krauskopf, 1957). Because Fe, Mn oxides may act as terminal electron acceptors for some microbial metabolic pathways or as oxidants of reduced products of microbial metabolism, the reduction of each of these oxides in surficial sediments is often depicted as a generalized metabolic reaction (Bostrom, 1967 Stumm and Morgan, 1970 Froelich et ai., 1979 Berner, 1980) ... 

Table 12-1 Reductive Fe release from recombinant ferritins reconstituted to constant mineral size (480 Fe/molecule) the effect of proline substitution for the highly conserved leucine 134.

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