Pesticide degradation rates

Numerous studies have shown that several factors affect pesticide degradation rates, including soil type, water content, pH, temperature, and clay and organic matter content (Rao and Davidson, 1980). Hamaker (1972) has published an excellent review on the quantitative aspects of pesticide degradation rates in soils. He consider two types of rate models ... 

Soil-water content ( ) and temperature (T) are the two major soil environmental factors that control pesticide degradation rates. Walker ( 34 ) and Walker and Barnes ( 35 ) have proposed a model for pesticide persistence in soils. In their model the 0-depgndence of described by a power function [two =... 

Pesticide Degradation Rate (half life in days) ... 

Accelerated pesticide degradation rate, characterization, 273-274 Acridine orange direct counting, 156 Adaptation characterization, 181 definition, 168 mechanisms, 168-174 microbial, See Microbial adaptation occurrence, 168... 

TABLE 9.6 Pesticide Degradation Rates under Tropical Conditions... 

The rate and extent of pesticide metaboHsm can vary dramatically, depending on chemical stmcture, the number of specific pesticide-degrading microorganisms present and their affinity for the pesticide, and environmental parameters. The extent of metaboHsm can vary from relatively minor transformations which do not significantly alter the chemical or toxicological properties of the pesticide, to mineralisation, ie, degradation to CO2, H2O, NH" 4, Cf, etc. The rate of metaboHsm can vary from extremely slow (half-life of years) to rapid (half-life of days). 

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. 

The objectives of the soil persistence experiments were (1) to learn the effect of soil type and concentration on the TCDD degradation rate, (2) to isolate and characterize degradation products from DCDD and TCDD, and (3) to determine whether chlorodioxins could be formed from chlorophenol condensation in the soil environment. This last study was essential since quality control at the manufacturing level could reduce or eliminate the formed dioxin impurity. But the biosynthesis of chlorodioxins by chlorophenol condensation in the soil environment could not be controlled and would have connotations for all chlorophenol-de-rived pesticides if formation did occur. The same question needed to be answered for photochemical condensation reactions leading to chloro-... 

In order to Increase the degradation rate of certain pesticides. In most of the beds approximately 1 ton of hydrated... 

PRZM was applied to a hypothetical situation of a pesticide In a Georgia agricultural environment. An overall, pseudo-first-order degradation rate coefficient of 0.001 day was used, along with a series of values. A cover crop of peanuts was assumed. The simulation was done for a 900 g/ha application to a class A soil (well drained) and a class D soil (poorly drained). Movement through the root zone was simulated using rainfall records. In the hypothetical 1-ha plot, 800 g and 550 g of the pesticide leached past 60 cm In the class A and D soils, respectively, when a Kj value of 0.06 was used 40 g and 5 g leached past 60 cm In the class A and D soils, respectively, when a Kj value of 1.5 was used. These computational results support the conclusion on Kj values stated at the end of this paper. 

Nonsingularity of Pesticide-Soil Interactions 136 Degradation Rates of Pesticides 139 Reaction Rates and Mechanisms of Organic Pollutants 143 Supplementary Reading 144... 

Another aspect of pesticide-soil interactions that is very important in predicting the effect of pesticides on environmental quality is the degradation rate. It is not the purpose of this discussion to give an in-depth discussion of pesticide degradation in soils. Numerous reviews are available on transformations, metabolic pathways, persistence, and tl/2 values of... 

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