Abiotical reaction

RDX and its partial reduction product hexahydro-l-nitroso-3,5-dinitro-l,3,5-triazine were degraded by K. pneumoniae to methylenedinitramine, and then to CHjO and methanol, while abiotic reactions produced NjO (Zhao et al. 2002). 

Humic acid and the corresponding fulvic acid are complex polymers whose structures are incompletely resolved. It is accepted that the structure of humic acid contains oxygenated structures, including quinones that can function as electron acceptors, while reduced humic acid may carry out reductions. These have been observed both in the presence of bacteria that provide the electron mediator and in the absence of bacteria in abiotic reactions, for example, reductive dehalogenation of hexachloroethane and tetrachloromethane by anthrahydroquininone-2,6-disulfonate (Curtis and Reinhard 1994). Reductions using sulfide as electron donor have been noted in Chapter 1. Some experimental aspects are worth noting ... 

Methylation of both metals and metalloids has been observed for both fungi and bacteria. These metabolites may, however, be toxic to higher biota as a result of their volatility. The Minamata syndrome represents the classic example of the toxicity of forms of methylated Hg to man, even though the formation of Hg(CH3)2 was probably the result of both biotic and abiotic reactions. 

It was shown, however, that these were formed even in sterile controls by undetermined abiotic reactions. 

Nijenhuis 1,1 Andert, K Beck, M Kastner, G Diekert, H-H Richow (2005) Stable isotope fractionation of tetrachloroethene during reductive dechlorination by Sulfospirillum multivorans and Desulfitobac-terium sp. strain PCE-S and abiotic reactions with cyanocobalamin. Appl Environ Microbiol 71 3413-3419. 

Whereas plausible fungal metabolites from anthracene, acenaphthylene, fluorene, and benz[fl]anthracene—anthracene-9,10-quinone, acenaphthene-9,10-dione, fluorene-9-one, and benz[fl]anthracene-7,12-quinone—were found transiently in compost-amended soil, these were formed even in sterile controls by abiotic reactions (Wischmann and Steinhart 1997). 

As for waste from the production of chemicals, the array of structures represented by agrochemicals is truly enormous. Only some illustrative examples are provided, and it is important to emphasize that not only the original compound, but also potential metabolites should be considered. The pathways for biodegradation of many of the structures have been presented in Chapter 9 and reference should be made to these for details. There is increased interest in the degradation of agrochemicals after application, and abiotic reactions including photochemical degradation that are important on the soil surface are discussed in Chapter E... 

Metabolites may be produced by biochemical transformation of the substrate rather than by degradation, or may result from partial abiotic reactions. These products may be (a) terminal and persistent or (b) toxic to other components of an ecosystem—including the microorganisms that produce them. Both of these represent important considerations that are illustrated by examples in this book. 

Harrison and Thode 1957). Although these abiotic reactions do not occur at less than 40°C and are of limited relevance in low-temperature aqueous environments, they provide a foundation for understanding S isotope systematics. 

The body of research on isotopic fractionation induced by sulfate and nitrate reduction provides insight into selenate, selenite and chromate reduction. For sulfate and nitrate oxyanions, reduction is generally microbially mediated, is irreversible, and involves a fairly large but variable isotopic fractionation. As described below, Se and Cr oxyanion reduction follows suit, though abiotic reactions may have a greater role in some transformations. 

More likely, there are ephemeral intermediate species with short residence times, and the reaction proceeds in several steps with several intermediates. In such a reaction pathway, changes in the relative rates of the reaction steps can result in changes in the fractionation. Furthermore, there may be multiple pathways by which a chemical transformation can occur. For example, transformation of Se(IV) to Se(0) could proceed via simple abiotic reaction, or via uptake of FlSeOj by a plant, reduction to Se(-ll) within the plant, incorporation into amino acids, death and decay of the plant, release of the Se(-II), and oxidation to Se". The overall transformation, from Se(lV) to Se(0), is the same, but because the two reaction pathways differ greatly, the overall isotopic fractionation may be greatly different. 

In the earliest work, Krouse and Thode (1962) found the Se isotope fractionation factor Sse(iv)-se(o) to bc 10%o ( l%o) with hydroxylamine (NH2OH) as the reductant. Rees and Thode (1966) obtained a larger value, 12.8%o, for reduction by ascorbic acid. Webster (1972) later obtained 10%o for NHjOH reduction. Rashid and Krouse (1985) completed a more detailed study, and found that the fractionation factor varied with time over the course of the experiments. They explained the variations observed among the experiments in all four studies using a model in which reduction consists of two steps. With the rate constant of the second step two orders of magnitude smaller than the first, and kinetic isotope effects of 4.8%o and 13.2%o for the hrst and second steps, respectively, all the data (Table 3) were fit. Thus, kinetic isotope effects of apparently simple abiotic reactions can depend on reaction conditions. 

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