Kinetics, pesticide degradation

For those pesticides that are cometabolized, ie, not utilized as a growth substrate, the assumption of first-order kinetics is appropriate. The more accurate kinetic expression is actually pseudo-first-order kinetics, where the rate is dependent on both the pesticide concentration and the numbers of pesticide-degrading microorganisms. However, because of the difficulties in enumerating pesticide-transforming microorganisms, first-order rate constants, or half-hves, are typically reported. Based on kinetic constants, it is possible to rank the relative persistence of pesticides. Pesticides with half-hves of <10 days are considered to be relatively nonpersistent pesticides with half-hves of >100 days are considered to be relatively persistent. 

Lartiges SB, Garrigues PP. 1995. Degradation kinetics of organophosphorus and organonitrogen pesticides in different waters under various environmental conditions. Environ Sci Technol 29 1246-1254. 

What materials are likely to contain pesticide residues following application What are the known degradation kinetics of the crop protection product being evaluated ... 

Garcia-Repetto R, Martinez D, Repetto M. 1994. The influence of pH on the degradation kinetics of some organophosphorous pesticides in aqueous solutions. Vet Hum Toxicol 36(3) 202-204. 

Lartiges, S. B. Garrigues, P. P. (1995). Degradation Kinetics of Organophosphorus and Organonitrogen Pesticides in Different Waters under Various Environmental Conditions. Environmental Science Technology, Vol. 29, No. 5, (May 1995), pp. 1246-1254, ISSN 0013-936X... 

Scow, K. M. (1993)- Effect of sorption-desorption and diffusion processes on the kinetics of biodegradation of organic chemicals in soil. In Sorption and Degradation of Pesticides and Organic Chemicals in Soil, ed. D. M. Linn, T. H. Carski, M. L. Brusseau F-H. Chang, pp. 73-114. Madison, WI Soil Science Society of America, American Society of Agronomy. 

One advantage of HPLC is that the analysis of unstable pesticides may be performed directly in aqueous medium without the extraction step or following extraction and concentration. Although the direct approach is quite useful for formulations or for kinetic studies to monitor the parent compounds in the presence of degradation products, its usefulness is limited in the case of environmental samples, where the concentration is usually in the parts-per-billion range (31). 

The UV/ozone process can treat pesticides without little generation of refractory products. One of the most common pesticides found in water supplies is Propoxur. The oxidation kinetics was developed in terms of the reaction orders and apparent kinetic constants (Benitez et al., 1994). The amount of chemical removed (Xp) is inversely proportional to the amount of initial Propoxur concentration (Cp), ozone partial pressure (kPa), temperature, and pH. When Propoxur is degraded by UV /ozone, the reactions can be represented by the following general reactions ... 

Many of the early studies on kinetics of soil chemical processes were obviously concerned with diffusion-controlled exchange phenomena that had half-lives (r1/2) of 1 s or greater. However, we know that time scales for soil chemical processes range from days to years for some weathering processes, to milliseconds for degradation, sorption, and desorption of certain pesticides and organic pollutants, and to microseconds for surface-catalyzed like reactions. Examples of the latter include metal sorption-desorption reactions on oxides. 

The Michaelis-Menten equation is often employed in soil-water systems to describe kinetics of ion uptake by plant roots and microbial cells, as well as microbial degradation-transformation of organics (e.g., pesticides, industrial organics, nitrogen, sulfur, and natural organics) and oxidation or reduction of metals or metalloids. Derivation of the Michaelis-Menten equation(s) is demonstrated below. 

The complex nature of soil and soil processes means that many factors besides the type of reaction kinetics can aifect the course of the disappearance of a pesticide. An example would be a case of a volatile pesticide for which a significant part of the loss was caused by vaporization from the soil. A complete mathematical analysis would take into account chemical degradation, vaporization, and diffusion. It would yield a mathematical relation different from and more complicated than one expressing the chemical kinetics alone. An example is simultaneous diffusion and degradation of soil fumigants as described by Hemwall (3,4). 

Carbodiimides can be used as stabilizers in thiophosphate based pesticides to prevent hydrolytic degradation. Some carbodiimides show insecticidal and acaricidal properties. Diafenthiuron [l-t-butyl-3-(2,6-diisopropyl-4-phenoxyphenyl)thiourea], an effective insecticide and acaricide, may act via its derived carbodiimide. This transformation is accomplished by sunlight degradation in aqueous solution. The carbodiimide causes inhibition of ATP phosphorylation. An H-labeled derivative of diafenthiuron, [phenoxy-4- H]diafenthiuron, has been prepared to study its photochemical and metabolic degradation. " The biological activity of N-(pyrid-3-yl)thioureas toward spider mites is sensitive to the kinetics of the formation of the carbodiimides and their photochemical stability. ... 

A number of experimental studies have established that both microbial and chemical degradation can be approximately described by first-order kinetics (24). Most pesticide models employ such an approach. As with linear sorption, this relatively naive representation of a fundamentally more complicated process is a simplifying assumption to make mathematical solutions possible and data requirements reasonable. Implicit in the assumption is the belief that the accuracy of simulation of pesticide fate is more dependent upon other factors than a very precise representation of the degradation process. These factors include spatial and temporal variability of the degradation process itself as affected by water, temperature, substrate, and pH, and variability in the transport of pesticide through the soil profile. There is little information to substantiate this assumption, although some field experiments on water and solute movement (discussed below) indicate it to be reasonable at this point in model development. 

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