Transformation processes pesticides

Water leaves the field either as surface mnoff, carrying pesticides dissolved in the water or sorbed to soil particles suspended in water, or as water draining through the soil profile, carrying dissolved pesticides to deeper depths. The distribution of water between drainage and mnoff is dependent on the amount of water appHed to the field, the physical and chemical properties of the soil, and the cultural practices imposed on the field. These factors also impact the retention and transformation processes affecting the pesticide. 

Bollag J-M, Liu S-Y (1990) Biological transformation processes of pesticides. In Cheng HH (ed) Pesticides in the soil environment processes, Impacts and Modeling. Soil Science Society of America, Madison, Wisconsin, pp 169-211. 

Bollag, J.-M. and S.-Y. Liu. Biological transformation processes of pesticides in Pesticides in the Soil Environment Processes, Impacts, and Modeling, SSSA Book Series 2, Cheng, H.H., Ed. (Madison, WI Soil Science of America, Inc., 1990), pp. 169-211. 

Metal ions in aerobic, natural waters, such as Cu % Fe, Mn % Mg +, and Ca, may catalyze hydrolysis of organic contaminants. Blanchet and St. George (1982), for example, showed that interaction of organophosphate esters with Cu and Mn led to the hydrolysis of pesticides. However, similar studies with Mg and Ca did not induce any transformation process. 

The main environmental factors that control transformation processes are temperature and redox status. In the subsurface, water temperature may range from 0°C to about 50°C, as a function of climatic conditions and water depth. Generally speaking, contaminant transformations increase with increases in temperature. Wolfe et al. (1990) examined temperature dependence for pesticide transformation in water, for reactions with activation energy as low as lOkcal/mol, in a temperature range of 0 to 50°C. The results corresponded to a 12-fold difference in the half-life. For reactions with an activation energy of 30kcal/mol, a similar temperature increase corresponded to a 2,500-fold difference in the half-life. The Arrhenius equation can be used to describe the temperature effect on the rate of contaminant transformation, k ... 

The microbial metabolic process is the major mechanism for the transformation of toxic organic chemicals in the subsurface environment. The transformation process may be the result of a primary metabolic reaction, when the organic molecule is degraded by a direct microbial metabolism. Alternatively, the transformation process may be an indirect, secondary effect of the microbial population on the chemical and physical properties of the subsurface constituents. Bollag and Liu (1990), considering behavior of pesticides, defined five basic processes involved in microbially mediated transformation of toxic organic molecules in the soil upper layer environment. These processes are described next. 

Chlorinated Pesticides. The major metabolic degradation pathways for toxaphene in all organisms are probably reductive dechlorination and reductive dehydrochlorination. In some cases, oxidative dechlorination has been observed to result in hydroxy derivatives, acids and ketones.76 Aldrin is transformed into dieldrin in biotic as well as abiotic transformation processes. 

As may be apparent from many of the examples presented above, models of processes that affect the fate and transport in groundwater of petroleum hydrocarbons and chlorinated aliphatic hydrocarbons are much more prevalent than models of other contaminants. Much research has yet to be accomplished to improve our understanding and ability to model transformation processes, particularly for contaminants like polynuclear aromatic hydrocarbons, nitro-aromatics, and pesticides. 

While many redox transformations of pesticide compounds can occur abioticaUy, virtually all such reactions in namral systems are facilitated, either directly or indirectly, by biological processes (Wolfe and Macalady, 1992). Some pesticide compounds may be taken up by living organisms and directly oxidized or reduced through the involvement of a variety of redox-active biomolecules (Bollag, 1982). Enzymes that have been found to be responsible for the biological oxidation of pesticide compounds... 

Two major types of processes that can affect the amount of pesticides present and available for transport through the soil profile are retention and transformation. The retention processes do not affect the total amount of pesticide present in the soil but can decrease or eliminate the amount available for transport. On the other hand, the transformation processes actually reduce or totally eliminate the amount of pesticide present and available for transport. 

Abiotic mechanisms for degradation of herbicides in soil, discussion, 15 Abiotic transformation processes of pesticide dissipation in soil hydrolytic reactions, 5 role in pesticide degradation, 4-5... 

Considerable evidence exists that microbial activities contribute to the formation of reactive products and also to the alteration of environmental parameters such as pH, redox potential, or other factors that are conducive to the secondary or nonenzymatic transformation of pesticidal molecules. Incorporation of pesticide molecules or their intermediates into soil humus often takes place by interaction between enzymatic and nonenzymatic processes. 

The second case refers to hormones, pharmaceuticals, and other compounds that are ingested, metabolized, and excreted by mammals (Table 1). Usually a hormone or pharmaceutical is extensively metabohzed in the body and is excreted by mammals as a mixture of different metabolites. Although the general belief is that metabolism renders a drug more water soluble and consequently less hazardous for the aquatic environment, there are exceptions for pro-drugs and specifically acting metabolites. The third case refers to environmental transformation products of pesticides and other environmental pollutants (Table 1), which are formed both by abiotic and biotic transformation processes. 

Human and veterinary pharmaceuticals are often highly metabohzed before they are excreted into wastewater or the environment. Thus, we have to consider both the metabohsm during the pharmacokinetic phase in the target organism and the environmental transformation processes. For simphcity, we focus in the following on the metabohtes formed in organisms, but environmental transformation products could be treated in an analogous way as the pesticides discussed above. Also veterinary and human pharmaceuticals can be approached in a similar fashion but the examples below refer to human pharmaceuticals. 

A well-documented example of this approach is a study of the fate of the pesticide, chlorpyrifos, in a pond in Missouri (87 reviewed by Branson, 75). The conceptual model includes rate constants for each of the transport and transformation processes affecting the final concentrations of chlorpyrifos in the water, the sediment, and the fish. The rate constant for evaporation from water is estimated from data on vapor pressure, solubility, and molecular weight (for techniques, see 33, 68, 71). The remaining rate constants for the pond model are derived from laboratory studies. When validated by sampling in a pond, the predicted and environmentally measured concentrations of chlorpyrifos showed close agreement (75). 

Once in the air, pesticides can exist as vapor, liquid aerosols or be sorbed/partitioned on dust and particulates. Even though in-depth information exists for particle phase distributions of organics in the atmosphere (24, 25, 26, 27, 28), few studies have appeared in the literature that assesses the vapor/aerosol distribution of pesticides under actual tropospheric conditions (29). Further research to determine how pesticides are distributed among atmospheric phases will be needed to better gauge the overall significance of wet/d deposition versus gas and particle-phase transformation processes occurring in air. 

Organophosphorus insecticides are applied to plants and soils using a variety of methods and formulations. Because formulation and initial placement affect exposure of these compounds to transformation processes and their availability for transport in surface runoff, the influence of these factors must be understood. Formulation in particular may exert an important influence on organophosphorus insecticide loads in surface runoff. Organophosphorus insecticides are rarely applied alone, but are mixed with other substances to enhance their performance and safety. These formulation ingredients can make up to 99.5% of the applied pesticide product and include organic solvents, surfactants and polymers. 

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