Pesticides sorption behavior

Hicken, S.T. 1993. Sorption behavior of three pesticides on sorbents with varying clay and organic carbon fractions. Master s thesis, Utah State University, Logan, UT. 

Karickhoff (1980) and Karickhoff et al. (1979) have studied sorption and desorption kinetics of hydrophobic pollutants on sediments. Sorption kinetics of pyrene, phenanthrene, and naphthalene on sediments showed an initial rapid increase in sorption with time (5-15 min) followed by a slow approach to equilibrium (Fig. 6.7). This same type of behavior was observed for pesticide sorption on soils and soil constituents and suggests rapid sorption on readily available sites followed by tortuous diffusion-controlled reactions. Karickhoff et al. (1979) modeled sorption of the hydrophobic aromatic hydrocarbons on the sediments using a two-stage kinetic process. The chemicals were fractionated into a labile state (equilibrium occurring in 1 h) and a nonlabile state. 

Many factors affect the mechanisms and kinetics of sorption and transport processes. For instance, differences in the chemical stmcture and properties, ie, ionizahility, solubiUty in water, vapor pressure, and polarity, between pesticides affect their behavior in the environment through effects on sorption and transport processes. Differences in soil properties, ie, pH and percentage of organic carbon and clay contents, and soil conditions, ie, moisture content and landscape position climatic conditions, ie, temperature, precipitation, and radiation and cultural practices, ie, crop and tillage, can all modify the behavior of the pesticide in soils. Persistence of a pesticide in soil is a consequence of a complex interaction of processes. Because the persistence of a pesticide can govern its availabiUty and efficacy for pest control, as weU as its potential for adverse environmental impacts, knowledge of the basic processes is necessary if the benefits of the pesticide ate to be maximized. 

Several studies have shown that sorption of various organic compounds on solid phases could be depicted as an accumulation at hydrophobic sites at the OM/water interface in a way similar to surface active agents. In addition Hansch s constants [19,199-201], derived from the partition distribution between 1-octanol and water, expressed this behavior better than other parameters. Excellent linear correlations between Koc and Kow were found for a variety of nonpolar organic compounds, including various pesticides, phenols, PCBs, PAHs, and halogenated alkenes and benzenes, and various soils and sediments that were investigated for sorption [19,76,80,199-201]. 

The fate of pesticides and organic pollutants in natural waters and in soils is strongly dependent on their sorptive behavior (Karickhoff, 1980). Sorption affects not only physical transport of these materials but also their degradation. It is also important to note that the chemical reactivity of pollutants in a sorbed state may be different from their behavior in aqueous solution. Karickhoff (1980) notes that sorbents such as inorganic and organic soil constituents may affect solution-phase processes by changing the solution-phase pollutant concentration or by affecting the release of pollutants into the solution phase. 

Pesticides Pesticide Properties in the Environment The efficacy and environmental impact of these compounds depends on their distribution and persistence. This monograph contains data on > 340 active ingredients including, aqueous solubility, vapor pressure, and pK. Soil behavior is defined by sorption coefficients and degradation half-lives. In most cases a range of referenced values is sited along with selected values. 

Data on sorption of organic chemicals on dry and partially hydrated soils and of ionic organic compounds from water are limited. Consequently, the present discussion will emphasize the soil sorption of nonionic organic compounds from water. Moreover, the characteristic behavior of nonionic organic chemicals between soil and water is directly applicable in interpreting the fate of the majority of pesticides and industrial pollutants in aquatic systems. 

The effect of formulation and spray adjuvants on insecticide efficacy has received considerable attention from the pesticide industry. However, few detailed mechanistic studies on the role these additives play in environmental fate processes have appeared in the open literature. Application of laboratory-derived process information to field scenarios is hindered by the fact that most laboratory investigations have used technically pure (unformulated) organophosphorus insecticides. Including the effects of formulation ingredients on such processes as volatilization and sorption to soil solids would allow laboratory studies to better predict the environmental behavior of these compounds. 

Sorption to soil solids and plant cuticular material represents an important process influencing the chemodynamic behavior of insecticides, including their transport in surface runoff Sorption phenomena affect the volatilization, hydrolysis, photolysis and microbial transformation of organophosphorus insecticides. Furthermore, species sorbed to soil particles are transported by erosion processes rather than as solutes in the water phase. Sorption to foliar surfaces reduces the amount of pesticide mobilized by washoff. 

There now exists an extensive literature on the uptake and translocation of herbicides in plants. In this brief review, it is intended to elucidate the general principles that govern the transport of pesticides in plants. The transport behavior of the different classes of herbicides will be considered, together with ways of using physicochemical properties to predict the likely behavior of new chemicals. It should be noted that the processes of sorption in soil, of cuticular penetration in the case of foliar-applied compounds, and of metabolism in the plant all influence the availability of herbicides for uptake and redistribution in plants, and these factors are the subjects of separate chapters in this book. Interpretation of the literature on systemic transport usually needs substantial consideration of these additional factors. 

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