Abiotic processes reactions

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. 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

Labile and refractory DOM undergo abiotic photochemical reactions in the photic zone, especially in the sea surfece microlayer where physical processes concentrate DOM into thin films. Some of these reactions appear to be important in the formation of refractory DOM and others in its degradation. For example, DOM exuded by diatoms during plankton blooms has been observed to be transformed into humic substances within days of release into surfece seawater. Laboratory experiments conducted in seawater have demonstrated that photolysis of labile LMW DOM promotes the chemical reactions involved in humification and produces chemical structures foimd in marine humic substances. 

Voudrias, E. A. Reinhard, M. (1986). Abiotic organic reactions at mineral surfaces. In Geochemical Processes at Mineral Surfaces, ed. J.A. Davis K. F. Hayes, pp. 462-86. Washington, DC American Chemical Society. 

Environmental organic matter is a composite of humic and nonhumic substances, which is formed through operation and interactions of various biotic and abiotic processes. Humic substances are formed through both selected preservation (residue) and catalytic synthesis mechanisms. Both enzymatic and mineral catalyses contribute to the formation of humic substances in the environment. The relative importance of these catalytic reactions would depend on vegetation, microbial population and activity, enzymatic activity, mineralogical composition and surface chemistry of environmental particles, management practices, and environmental conditions. Selective preservation pathways would play a more important role in humification processes in poorly drained soils and lake sediments, compared with more aerated environmental conditions. 

Some versions of metabolism first schemes have drawn criticism in the literature.71,72 The schemes have assumed that a self-sustaining metabolic network must be autotrophic 73 that is, it must obtain its carbon supply entirely from carbon dioxide, rather than using carbon compounds made by abiotic processes (Sections 5.2.1 and 5.2.2). The assumption that chemicals in proposed reactions in the central metabolic cycle will be catalyzed by other compounds that participate in the cycle has also been questioned.74 Such features are attractive but not essential to the idea of life with small molecules. It has been argued that the objections can be met in principle by introducing a small number of assumptions 75... 

We have chosen to follow Watts [24] and discuss chemical and biological transformation processes in the same section. Watts notes that, although this approach is somewhat nontraditional, it is advantageous in that understanding of the abiotic chemical reactions serves as a conceptual basis for understanding the biochemical reactions (which are essentially the same except for the fact that the biochemical reactions are mediated by microorganisms). Where a reaction is predominantly abiotic or biotic, it will be noted in the discussion. In this section, the fundamentals of each chemical or biological reaction will be discussed, and model formulations for the reaction kinetics presented. 

In addition, although most abiotic processes are nonenantioselective, not aU are indeed the case. Nucleophilic 5 jv2-substitution reactions at a chiral center will result in chiral inversion to the antipodal enantiomer. While such processes are often biologically mediated, as for the nonsteroidal anti-inflammatory drugs [328], they can also be abiotic. Appropriate sterile controls should be used for experiments with such compounds, as was done in the demonstration of microbial chiral inversion of ibuprofen in Swiss lake water [329]. Photolysis of a-HCH [114], /3-PCCH [114], and chlordane compounds [116] was demonstrated not to be enantioselective, as expected for an abiotic process. However, this may not be the case for some pyrethroids, known to isomerize photolytically. 

The principal abiotic processes that transform uranium in water are formation of complexes and oxidation-reduction reactions that have been described in Section 5.3.1. In seawater at pH 8.2, it was shown that U(IV) exists as 100% neutral hydroxo complexes, and U02 " and U(VI) exist as 100% carbonate complexes. In freshwater at pH 6, U(IV) was shown to exist as 100% hydroxo... 

Abiotic and biotic reactions may occur that can produce or consume chemical species in porous media, and transport equations must therefore include descriptions of such processes for prediction accuracy. For example, microorganisms may use ethanol as a carbon source, and consume ethanol as it is transported through the porous system. Examples of other reactive processes include radioactive decay and abiotic degradation. Reactions of chemical species in porous media are often expressed using ... 

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