Abiotic degradation processes

Hexachloroethane may biodegrade in soil, but abiotic degradation processes are not expected to be significant. Hexachloroethane is biotransformed in soil under both aerobic and anaerobic conditions, but proceeds more rapidly in anaerobic soils (Spanggord et al. 1985). Loss of 99% of hexachloroethane was reported after 4 days of incubation anaerobically and after 4 weeks under aerobic conditions. Volatilization contributed to aerobic losses. 

A plethora of environmental forces compromise the structural integrity of chemicals in the environment. Many prominent abiotic degradative processes occur due to the influences of light (photolysis) and water (hydrolysis). 

Despite the wider availability of experimental data in these nations, risk assessments for new substances often demand estimation of environmentally important parameters. For example, degradation and partitioning processes must be considered in environmental exposure assessments, but neither Henry s Law constant nor abiotic degradation processes such as hydrolysis and photolysis are included in the MPD. Only if a substance is not readily biodegradable may requirements for abiotic degradation testing be imposed. 

Abiotic degradation transforms organic compounds by chemical reactions such as oxidation, reduction, hydrolysis, and photodegradation. Abiotic degradation processes do not usually achieve a complete breakdown of the chemical (mineralization). 

Temperature can also directly affect the polymer. For example, the rate of abiotic degradation processes such as hydrolysis, is controlled by temperature [15]. Furthermore, the mobility of the polymeric chains is related to the environmental temperature. This in turn affects the bio-availability of the polymer because an higher mobility will facilitate the contact between the susceptible chemical bonds and the enzymic active sites [24]. 

Moreover, a-, p- and y-HBCD diastereoisomers are chiral. Thus HBCD have three pairs of enantiomers (+)a, (—)a, (+)p, (—)p, (+)y and (—)y. The enantiomers have identical physicochemical properties and abiotic degradation rates, but may have different biological and toxicological properties and therefore different biotransformation rates. These transformations may result in nonracemic mixtures of the enantiomers that were industrially synthesized as racemates [16, 19]. The rates of metabolisation process of the enantiomers of chiral environmental pollutants may be significantly different [20],... 

The ultimate fate of higher alkyl silanols such as those produced in trisiloxane surfactant degradation, for example CH3-Si(OH)2-CH2 CH2CH2OH, has not been described, and is an area requiring further investigation. The mechanisms described above for the degradation of the methyl siloxanes may or may not be applied to higher alkylated versions. The Si-C bond is not susceptible to hydrolysis [7], and as such the abiotic elimination processes are not likely to occur. 

The results indicate that persistence of organosilicone surfactants in the parent molecule form will be limited on typical soil media and in aqueous environments. Reduced recovery was considered to be a result of abiotic degradation and/or strong sorption processes. Losses were most significant on solid media exhibiting extreme pH values and were also enhanced in the presence of clay substrates. Studies on clays indicated that pH, potential for intercalation and surface charges are important factors in the removal process. 

No information could be found in the available literature on the transformation of cyanogen or cyanogen chloride in soil or sediment however, because these compounds are highly volatile gases, biotic or abiotic degradation would not be expected to be significant fate processes compared to volatilization. 

For air, the major degradation process involves reactions with free radicals such as hydroxyl groups (Atkinson 1985). For other media, it is clear that chloroform can be mineralized through both abiotic and biotic processes. Information in the available literature (Bouer and McCarty 1983 Rhee and Speece 1992) documents the disappearance of chloroform in water and soil media under both aerobic and anaerobic conditions as well as identification of the end products. 

Diazinon is subject to a variety of abiotic and biotic degradation processes in all environmental compartments. 

Although diazinon has been detected in groundwater samples in both the United States and Canada (Cohen 1986 EPA 1989a Frank et al. 1987, 1990b HazDat 1996), no studies were identified concerning diazinon transformation and degradation processes within aquifers. Based on theoretical considerations, abiotic hydrolysis mechanisms would be expected to degrade diazinon within a few months (Chapman and Cole 1982 Cowart etal. 1971). 

Since the environmental degradation of polyethylene starts with abiotic oxidation, the determination of abiotic oxidation products is an important step towards establishing the environmental degradation mechanisms and environmental impact of the material. In a secondary process, microorganisms may utilise these abiotic degradation products and the low molecular weight... 

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