Natural abiotic reactions

The refractory compounds in the HMW DOM pool seems to be generated through abiotic reactions that act to link degradation products into macromolecules. These new chemical bonds create molecular structures that enhance the overall refractory nature of the DOM. The chemical changes lead to increased crosslinking, aromaticity, cyclization, esterification, and nitrogen depletion. The general types of chemical reactions responsible are oxidations, polymerizations, and condensations. Considerable debate exists as to whether these reactions are wholly abiotic or whether they are, at least in part, microbially mediated. 

In addition to the environmental pH and type of hydrolysis, contaminant hydrolysis in sediments is controlled by the properties of the contaminant molecule and the sediment constituents. In a natural sediment, it is difficult to determine if hydrolysis is a biologically mediated or an abiotic reaction. A simple test consists of measuring the contaminant disappearance rate in the sediment and in distilled water, with a pH adjusted to that of the sediment. If the two disappearance rates are similar, the process can be assumed to be abiotic, while if the rate is greater in the sediment system, the hydrolysis is biologically mediated. 

Figure 2.1 summarizes various abiotic reactions that TNT, a typical nitroaromatic compound (NAC), may encounter in natural media [23-26],... 

Environmental Fate. Further investigation would resolve the discrepancies in the data for anaerobic degradation of 1,1,2-trichloroethane. Additional studies are needed to characterize the nature of the transformation and to clarify whether biotic, abiotic, or catalyzed abiotic reactions are involved. Will these reactions generally occur under environmental conditions A determination of the half-life in representative groundwater and sediment-water systems would be useful. From the available evidence, biodegradation in aerobic systems appears unlikely, although additional studies, particulary in soil, are desireable and would clarify this point. 

The reduction of NACs to the corresponding amino compounds is a widely observed reaction in sediments, soils, and aquifers. As has been illustrated in this paper, there are numerous potential electron donors present in natural systems (e.g., reduced iron species, reduced sulfur species, reduced NOM constituents) that may reduce NACs in abiotic reactions. Because all these species are involved continuously in biogeochemical processes, and are thus intimately coupled to microbial activity, it is not very meaningful to separate NAC reduction in a given environment into strictly abiotic and strictly biological processes (Figure 13). 

Elements such as C, N, O, S, and Cl that are components of many organic compounds exist naturally as mixtures of stable isotopes. The ratios of these in a compound reflect the different rates of reaction at isotopically labeled positions, and therefore reflect the fractionation—biotic or abiotic—by which it was synthesized or to which the compound has been subjected. Techniques have been developed whereby the ratios C/ C (5 C), (5 N), (5 0),... 

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). 

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