Biotic and Abiotic Oxidation

Wetland soils and aquatic sediments are uniquely characterized by aerobic and anaerobic interfaces at the soil-floodwater interface or in the root zone of wetland plants (see Chapter 4 for details). Aerobic oxidation of Fe(II) and Mn(ll) is restricted to the thin aerobic layer at the soil-floodwater interface or in the root zone. Thus, the extent of aerobic oxidation of Fe(ll) and Mn(ll) is dependent on the flux of dissolved species from anaerobic soil layers to aerobic zones. At circumneutral pH, concentrations of dissolved Fe(ll) and Mn(II) are very low, thus restricting flux into aerobic portions of the soil. At this pH level, the majority of Fe(II) and Mn(ll) compounds are present as immobile solid phases such as FeCOj, MnCOj, FeS2, Fe(OH)2, and Mn(OH)2. These compounds can be oxidized only when the water table is lowered, thus exposing top portion of the soil profile to aerobic conditions. 

Soil or sediment pH and redox potential Oxygen flux and the thickness of aerobic layer Presence of oxidants with higher reduction potentials Flux of soluble Fe(II) and Mn(ll) 

The Fe(II) oxidation occurs near the outer face of the cytoplasmic membrane, whereas oxygen reduction occurs on the inner face of the membrane. The electrons donated from Fe(II) oxidation are accepted by rusticyanin in the periplasm, followed by a transfer of electrons to cytochrome c 

FIGURE 10,24 Electron flow in Thiobacillus grown in medium with ferrous iron. (Modified from Madigan and Martinko, 2006.) 

Nonenzymatic oxidation of Fe(II) compounds is the most common pathway at circumneutral pH. The overall abiotic oxidation of Fe(II) can be described as follows  

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In waters with high values of Eh, many more biotic and abiotic oxidation reactions become thermodynamically probable (though not necessarily feasible from a kinetic perspective). Similarly, a low oxygen concentration or Eh makes reductive processes more likely. 

A third mechanism by which the structural bonds between Fe atoms in iron oxides may be weakened involves reduction of structural Fe to Fe". In natural environments, reductive dissolution is by far the most important dissolution mechanism. It is mediated both biotically and abiotically. The most important electron donors, particularly in near surface ecosystems result from metabolic oxidation of organic compounds under O2 deficient conditions. In anaerobic systems, therefore, the availability of Fe oxides i. e. the electron sink, may control the degradation of dead biomass and organic pollutants in the ground water zone (see chap. 21). Reductive dissolution is also often applied to the removal of corrosion products from piping in industrial equipment and the bleaching of kaolin. 

Pure culture studies have alluded to some of the factors that may regulate enzymatic Fe(II) oxidation in complex natural environments such as sediments, soils, and the rhizosphere. Neubauer et al. (2002), used microcosms fed with environmentally relevant concentrations of O2 and Fe(II) to investigate the metabolism of an Fe(II)-oxidizing strain isolated from the wetland rhizosphere. They found that both biotic and abiotic Fe(II) oxidation increased linearly (r > 0.90) with the rate of Fe(II) addition (Figure 23). Since the experimental Fe(II) addition rate approximated in situ Fe(III)-diffusion rates in freshwater... 

Under reducing conditions, Tc(IV) is the dominant oxidation state because of biotic and abiotic reduction processes. Technetium(IV) is commonly considered to be essentially immobile, because it readily precipitates as low-solubility hydrous oxides and forms strong surface complexes on iron and aluminum oxides and clays. 

The entry of strongly reduced landfill leachate into a pristine, often oxidized, aquifer, leads to the creation of very complex redox environments. Important processes include organic matter biodegradation, biotic and abiotic redox processes, dissolution/precipitation of minerals, complexa-tion, ion exchange, and sorption. The resulting... 

Trace elements may be present in solution with positive or negative charges and in different redox states. They occur predominantly in cationic form [Pb, Cu, Zn, Ni, Cd, Hg, Cr(III), and Co], but some trace elements are present in anionic form [As, Se, Cr(Vl), Mo, and B]. Redox reactions, both biotic and abiotic, are of paramount importance in controlling the oxidation state, and thus mobility, phytoavailability, and toxicity of many trace elements, including Cr, Se, Co, Pb, As, Ni, and Cu (Huang and Germida, 2002 Sparks, 2003). 

Molecular oxygen regulates both aerobic and anaerobic microbial metabolisms (Bodelier, 2003). Because of intense competition for O2, aerobic rhizosphere bacteria may be adapted to low O2 concentrations and periods of anoxia. Competition for O2 among microbial species, and between biotic and abiotic processes, has received relatively little attention (Laanbroek, 1990) but is likely to influence rhizosphere oxidation rates. Aerobic processes that occur in the wetland rhizosphere are those found in other aerobic-anaerobic interface environments and... 

Explosive compounds may undergo extensive transformation in aquatic systems, by microbial attack or abiotic mechanisms such as hydrolysis, oxidation, phototransformation, and so forth. Therefore, aquatic receptors may be exposed not only to energetic compounds released to the environment but also to their numerous transformation products. The key biotic and abiotic transformations of major explosives are discussed in Chapter 2. 

Based on critical reviews, Lovley (1991, 2004) concluded that there are potential mechanisms for the abiotic reduction of Fe(III) and Mn(IV), but the significance of this process is minimal as compared to biotic reduction catalyzed by microbial activities. Typically, the end products of Fe(II) and Mn(II) are measured as indicators of the biotic and abiotic reduction of Fe(III) and Mn(IV) in anaerobic environments. The reduction of Fe(III) and Mn(IV) as a function of Eh is shown in Figures 10.10 and 10.11. Sodium acetate extractable iron and manganese in anaerobic soils represents Fe(II) and Mn(II), end products of reduction. As expected, extractable Mn(II) and Fe(II) concentrations are low nnder oxidized conditions and increase with a decrease in the Eh of soil. The accumulation of Mn(II) occurs at higher Eh values than the accumulation of Ee(II), suggesting Mn(IV) reduction precedes Fe(III) reduction. Because the reduction of Ee(III) and Mn(IV) occurs... 

Wetlands exhibit distinct redox gradients between the soil and overlying water column and in the root zone (Chapter 4), resulting in aerobic interfaces. For example, the aerobic layer at the soil-floodwater interface is created by a slow diffusion of oxygen and the rapid consumption at the interface. The thin aerobic layer at the soil-floodwater interface and around roots functions as an effective zone for aerobic oxidation of Fe(ll) and Mn(II). Below this aerobic layer there exists the zone of anaerobic oxidation of Fe(ll) and Mn(ll) and reduction of Fe(III) and Mn(IV). The juxtaposition of aerobic and anaerobic zones creates conditions of intense cycling of iron and manganese mediated by both biotic and abiotic reactions. 

Several researchers have also suggested that bacteria mediate mercury reduction [54,55]. SicUiano et al. [56] recently examined the role of microbial reduction and oxidation processes in regulating DGM diel (over a 24h period) patterns in freshwater lakes. The authors demonstrate that photochemi-cally produced hydrogen peroxide regulates microbial oxidation processes and may account for the diel patterns observed in DGM data. Overall, the mechanisms responsible for mercury reduction and the relative contributions of biotic and abiotic processes are still unclear but solar radiation appears to be a common instigator of photo-reduction. 

Solar radiation drives a number of chemical transformations of mercury. These include (i) atmospheric speciation and deposition, (ii) oxidation-reduction in both freshwater and seawater, and (iii) methyl mercury degradation. Both biotic and abiotic redox reactions are influenced. While microbes have been thought to dominate methyl mercury production, abiotic formation cannot be... 

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