Persistent toxic substances degradation

Combustion in an incinerator is the only practical way to deal with many waste streams.This is particularly true of solid and concentrated wastes and toxic wastes such as those containing halogenated hydrocarbons, pesticides, herbicides, etc. Many of the toxic substances encountered resist biological degradation and persist in the natural environment for a long period of time. Unless they are in dilute aqueous solution, the most effective treatment is usually incineration. 

All pesticides that can come into contact with the environment are subject to a risk assessment. The basis for this risk assessment is provided by data from environmental fate and environmental toxicity studies, which are carried out in the laboratory or under field conditions. The fate (adsorption, degradation, and mobility) of the active substance must be studied in soil, air, water, and sediments. The laboratory studies are frequently performed with C-labeled substances to make the mass balance easier. It is important to know how a substance degrades in the environment, because sometimes the degradation products are more persistent than the parent substance. DDT, for instance, is converted to metabolites by stepwise dechlorination (Eq. 11.9). The metabolites (e.g., DDD or DDA) can be found in soil for many years after the DDT itself is degraded. 

HCHs persist in the environment. In air, the different forms of HCH can be present as a vapor or adsorbed to small particles. HCH can remain in the air for long periods of time and residues can travel great distances, depending on environmental conditions. HCH is degraded to less toxic substances by algae, fungi, and bacteria. 

The estimated world production in the period 1930 to 1974 is about 1.2 X 10 tons," of this about one third has been released into the environment without any precautions regarding toxic effects on biota and any care to prevent environmental pollution. This has led to the widespread occurrence of PCBs aU over the world, even in remote areas. The U.S. Environmental Protection Agency (EPA), under the provisions of the Toxic Substances Control Act, specifically banned most of the uses of PCBs in 1997. Current releases of PCBs are mainly as a result of the cycle of these persistent contaminants from soil to air and back to soil again. Other possible sources of contamination, such as leaching, occurs. Moreover, PCBs can be unintentionally produced as by-products in a wide variety of chemical processes which contain chlorine and hydrocarbon sources, during water chlorination, and by thermal degradation of other chlorinated organics. ... 

One advantage of organophosphate insecticides lies in their fairly rapid biological and chemical degradation in plants and animals to non-toxic substances. They are also not accumulated in nutritional chains. The persistence of chlorinated hydrocarbons, organophosphates and of the other commonly used pesticides in water and fish is presented in Table 9.5 [28]. 

Biodegradation, hydrolysis, and sorption influence the environmental fate of LAS, AS, and AES. Primary degradation of surfactants is important because this process usually results in loss of surfactancy and reduced toxicity (5, 6). Complete mineralization ensures that persistent intermediates will not be formed and that biodegradation will be an effective mass-removal mechanism in the environment. Sorption and association of surfactants with particles or dissolved organic substances are processes that decrease bioavailability and can be correlated with decreased surfactant toxicity (7). 

Listed" waste is any waste that contains a substance that is "listed" by the USEPA as hazardous. This type of waste has been listed based on the waste s "toxicity, persistence, and degradability in nature, potential for accumulation in tissue, and other related factors such as flammability, corrosiveness, and other hazard characteristic." Rules have been developed by the USEPA to ensure proper disposal of these types of hazardous waste. The mixture rule states that any substance mixed with a "listed" hazardous waste becomes a hazardous waste. If it is not a "listed" waste, but instead a characteristic waste, and the mixture does not exhibit any of the characteristics, the mixture is not considered hazardous. The "derived from" rule states that any waste derived from the treatment of a "listed" hazardous waste remains a hazardous waste. Similar to the mixture rule, if the by-product of a characteristic waste does not exhibit any of the hazardous characteristics, it is not considered hazardous. 

Phenols of enviromnental interest are derived from a wide variety of industrial sources, or present as biodegradation products of humic substances, tannins, and lignins, and as degradation products of many chlorinated phenoxyacid herbicides and organophosphorous pesticides. Phenols, especially chlorophenols, are persistent, and toxic at a few pg/1. Therefore, phenols are hsted at the US-EPA hst of priority pollutants and the EU Directive 76/464/EEC as dangerous substances. The samples to be analysed can be surface waters or industrial effluents. 

Environmental effects can be examined using studies on the toxicity, persistence and bioaccumulation for the substance in representative studies in individual species, in microcosms and in observations during field trials. Modelling of the transport and fate of the substance is also helpful. Surveys assist in providing baseline data on habitats and communities present. One aim of this exercise will be to determine how tolerant the environment in question will be at accepting the substance before some form of environmental degradation occurs. 

Microbial metabolism, or the lack thereof, determines whether chemicals persist or leave the environment quickly. When organic compounds are completely metabolized to carbon dioxide, then any detrimental environmental effect the chemical might manifest is alleviated. In some cases, for example with the PCBs described above, incomplete metabolism occurs. For the majority of substances, the resulting degradation products will be less toxic to humans and the environment than the parent compound (see chapters by Sinclair and Boxall and Escher et al.). For example, it is beneficial for PCBs fo undergo even partial reductive dechlorination less chlorinated PCB congeners are generally less toxic and persistent than their more heavily chlorinated counterparts. 

If a substance biodegrades in an aquatic environment, further information is needed on whether the substance has degraded to innocuous molecules or to relatively persistent and toxic derivatives. 

Persistent Organic Pollutants (POPs) and Persistent, Bioaccumulative and Toxic (PBT) substances are carbon-based chemicals that resist degradation in the environment and accumulate in the tissues of living organisms, where they can produce undesirable effects on human health or the environment at certain exposure levels (Pavan Worth, 2006). 

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