Biotic and Abiotic Reduction

Peijnenburg, W.J., Hart, M.J., den Hollander, H., de Meent, D., Verboom, H., and Wolfe, N., QSARs for predicting biotic and abiotic reductive transformation rate constants of halogenated hydrocarbons in anoxic sediment systems, Sci. Total Environ., 109/110, 283-300, 1991. 

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. 

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... 

As mentioned previously, treatment of chlorinated solvents in groundwater can occur via biotic and abiotic reductive processes. In the past, it was assumed that biotic and abiotic reductive pathways are independent. As a result, the research in these two areas was... 

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. 

The C02 flux at the atmosphere vegetation cover boundary is determined in many respects by the soil processes involved in organic matter transformation. To better understand the biotic and abiotic mechanisms that control C02 emission from the soil, Jassal et al. (2005) compared measured C02 fluxes in a forest with their distribution profile in the soil of a 54-year-old coniferous forest on the eastern coast of Vancouver. It was established that C02 concentration grows at all depths of the soil layer with rising temperature and humidity. This is explained by soil diffusion reduction and changes in soil ecosystem functioning. It was noted that more than 75% of C02 emitted from the soil was generated at a depth of 20 cm, and almost total C02 flux forms from the 0 cm-50 cm layer. 

Profiles of dissolved iron in seawater show the influence of both biotic and abiotic processes. At stations in the Northeast Pacific, dissolved iron concentrations are low in surface waters, reflecting biological uptake. Iron concentrations also show peak values at depth, corresponding to the oxygen minimum zone (Martin and Gordon, 1988), suggesting the abiotic reduction of Fe(III) back to the soluble Fe(II). 

Denitrification can be supported by electron donors other than organic carbon such as Fe(II), Mn(ll) and H2S, and they can proceed by both abiotic and biotic pathways. Abiotic reduction of no " to N2 coupled to Fe(II) oxidation may occur at low rates in the pH range 2-7 (Postma, 1990) ... 

Vinylidene chloride is a human-made chemical and is not naturally found in the environment. It can be found from the breakdown of polyvinylidene (PVDC) products, and from the biotic and abiotic breakdown of 1,1,1-trichloroethane, tetrachloroe-thene, 1,1,2-trichloroethene, and 1,2-dichloroethane. Biotransformation of the chemical in groundwater can form vinyl chloride through reductive dechlorination, which is subsequently mineralized to carbon dioxide. The major transport process from water, soil and sediment is volatilization. Half-lives of... 

Similar to the possibility of concurrent reduction of sulfate and ferric iron by a culture of a single bacteria (Coleman et al. 1993 see section 7.4.3.4) other iron reducing bacteria were found to additionally maintain dissimilation with more than one electron acceptors under suboxic conditions (Lovley and Phillips 1988) or even under oxic conditions (Myers and Nealson 1988a). In the presence of Fe(III) and Mn(IV) strain MR-1 was found to reduce both but additional manganese reduction occurred due to the immediate abiotic reaction with released Fe (Myers and Nealson 1988b). The interactions of biotic and abiotic reactions are shown in Fig. 7.17. 

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. 

Explain the difference between biotic and abiotic iron and manganese reduction. Which is the dominant process ... 

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. 

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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