Environmental impact, degradation products

When antifreeze becomes unsuitable for use, either because of depletion of inhibitors, presence of corrosion products or corrosive ions, or degradation of the fluid, recycling and reuse of the antifreeze, rather than disposal, may be considered. Although ethylene glycol is readily biodegraded in typical municipal waste treatment faciHties, antifreeze disposal becomes problematic because the coolant may contain hazardous quantities of heavy metals picked up from the cooling system. Recycling may be economically preferred over coolant disposal and reduces the concern for environmental impact. 

Methods are also available to measure degradation products of hexachloroethane in environmental samples, but these products (e.g., tetrachloroethylene) are released to the environment from many other sources and are therefore not useful determinants of the environmental impact of this chemical. 

As an industrial and commercial product, PVA is valued for its solubility and biodegradability, which contributes to its very low environmental impact. Several microorganisms ubiquitous in artificial and natural environments — such as septic systems, landfills, compost and soil — have been identified and they are able to degrade PVA through enzymatic processes. 

One important aspect of any cleanup technique, is the type of degradation products that are produced. These products must be known in order to assess their potential environmental impact and toxicological hazards. One of the major degradation products of diazinon, oxypyrimidine was measured in soil after treatment with parathion hydrolase. Figure 6 shows that oxypyrimidine increases in soil as the diazinon is degraded by the enzyme. 

Apart from the pure process-driven requirements of the disintegration within a defined time span and the proof of ultimate biodegradability, it is necessary to make sure that there is no negative impact of degradation products or intermediates of the degradation process on the environment. Adequate information has to be provided for a comprehensive assessment of environmental and toxicological safety. 

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... 

Oxidation/hydroxylation of aromatic compounds by OH and HOONO is expected to enhance their degradation rate and hence decrease their lifetime on particulate matter, which in the case of pollutants is beneficial from the point of view of human health. Oxidation of PAHs could also lead to the production of photosensitizers such as quinones and aromatic carbonyls [10, 40, 41]. These compounds, if present in the gas phase, are also able to form aggregates and are therefore involved in the formation of secondary organic aerosol [42]. In contrast, nitration induced by OH + N02 or HOONO could lead to highly mutagenic nitro-PAHs [43] or phytotoxic nitrophenols [44, 45], in which case the health and environmental impact of the reaction intermediates is not negligible and is sometimes higher than that of the parent molecules. 

Rook JJ. Possible pathways for the formation of chlorinated degradation products during chlorination of humic acids and resorcinol. In Jolly RL, Cumming RB, Jacobs VA, eds. Water Chlorination Environmental Impacts and Health Effects. Vol. 3 Ann Arbor, MI Ann Arbor Science, 1980. 

There are several areas where further research is needed to better define the atmospheric degradation mechanisms and hence environmental impact of anthropogenic compounds. In general, there is a fairly complete database concerning the kinetics of the reactions which initiate the oxidation of pollutants. Extensive databases, structure activity relationships, and predictive techniques are available for the reaction of most anthropogenic molecules with OH and NO3 radicals and O3. When kinetic data are available for other members of the class, the predictive techniques generally provide reliable (within a factor of 2) estimates of kinetic data for new compounds. However, when the compound is a member of a class of compounds for which no kinetic data exist, the predictive techniques provide estimates which are less reliable (uncertain by typically a factor of 5). Our understanding of the subsequent reaction mechanisms and the identity of the oxidation products is... 

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