Soil-Pesticide Interactions

Jardine et al. (1985b) employed a two-site nonequilibrium transport model to study Al sorption kinetics on kaolinite. They used the transport model of Selim et al. (1976b) and Cameron and Klute (1977). Based on the above model, Jardine et al. (1985a) concluded that there were at least two mechanisms for Al adsorption on Ca-kaolinite. It appeared that there were equilibrium (type-1) reactions on kaolinite that involved instantaneous Ca-Al exchange and rate-limited reaction sites (type-2) involving Al polymerization on kaolinite. The experimental breakthrough curves (BTC) conformed well to the two-site model. 

A number of kinetically based models have been used to study soil-pesticide reactions. In many cases, sorption of pesticides has been treated as a rapid-equilibrium, single-valued, reversible process. Some of these models are briefly outlined below. 

Van Genuchten et al. (1974) assumed that the amount of pesticide sorbed with time followed reversible nonlinear kinetics such that 

The sorption kinetics of 2,4-D on illite, kaolinite, and montmorillonite was modeled by Haque et al. (1968) using 

Kinetic Modeling of Inorganic and Organic Reactions in Soils 

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Spencer, W. F., M. M. Cliath, and S. R. Yates. Soil-pesticide interactions and their impact on the volatilization process, in Environmental Impact of Soil Component Interactions—Natural and Anthropogenic Organics, Vol. 1, CRC Press, Boca Raton, FL, 1995, pp. 371-381. 

Most soil-pesticide sorption-desorption studies have used batch techniques, which create several problems. In many batch studies the slow portion of the soil-pesticide interactions may not be seen if observation times are too short (McCall and Agin, 1985). Additionally, desorption is usually begun by centrifuging the equilibrated soil-pesticide system, removing a known volume of pesticide solution, replacing with the same volume of pesticide-free solution, and resuspending the soil-pesticide solution. This procedure is then repeated to develop desorption isotherms initiated from a particular point on the sorption isotherm. Then there is... 

Modeling of Inorganic Reactions 174 Nitrogen Reactions 174 Phosphorus Reactions 177 Potassium Reactions 181 Aluminum Reactions 183 Modeling of Soil-Pesticide Interactions 183 Modeling of Organic Pollutants in Soils 186 Supplementary Reading 189... 

Certain cautions should also be mentioned so that the meaning of these correlations is not extended beyond what the data warrant. A significant correlation between sorption and soil organic carbon contents does not imply that only one mechanism of sorption is involved or that all pesticides interact with all components of soil organic matter by the same mechanism. Furthermore, the lack of... 

The analytical solution to Equation 2 for a range of boundary conditions is a model of pesticide fate that has been used under a variety of laboratory situations to study the basic principles of soil-water-pesticide interaction. It is in fact limited to such laboratory cases, as steady state water flow is an assumption used in deriving the equation. As a modeling approach it is useful in those research studies in which careful control of water and solute fluxes can be used to study degradation and adsorption. For example, Zhong et al. (11) present a study of aldicarb in which the adsorption and degradation of aldicarb, aldicarb sulfone and aldicarb-sulfoxide were simultaneously determined from laboratory soil column effluent data. The solution to a set of equations of the form of Equation 2 was used. A number of similar studies for other chemicals could be cited that have provided useful basic information on pesticide behavior in soil (4,12,13). Yet, these equations are not useful in the field unless re-formulated to describe transient water and solute fluxes rather than steady ones. Early models of pesticide fate based upon Equation 2 (14) were constrained by such assumptions, but were... 

In an effort to determine the criteria that should be used to invoke cases of enhanced degradation, an experimental approach for Its study was developed that focused on laboratory investigations with field-collected soils. It was obvious that Insecticide control failures were common occurrences and certainly not all due to enhanced degradation, as Investigations of faulty application methods and unusual environmental conditions have shown (18). The ideal approach to the study of enhanced degradation would Involve controlled field research in which pesticide persistence and control efficacy were both measured at many locations over a number of years. However, the tremendous cost In time and effort and confounding of results by environmental variables make a controlled laboratory approach desirable. The limitation of laboratory efforts focused exclusively on the soil-lnsecticlde Interaction is that they cannot fully address the additional insect-insecticide and Insect-crop interactions present in the field. This means that caution must be excercised when proof of enhanced degradation is discovered In the laboratory, for this does not necessarily mean that Insect control and crop yield will be adversely affected under field conditions. 

Persistence of pesticides in the environment is controlled by retention, degradation, and transport processes and their interaction. Retention refers to the abihty of the soil to bind a pesticide, preventing its movement either within or outside of the soil matrix. Retention primarily refers to the sorption process, but also includes absorption into the soil matrix and soil organisms, both plants and microorganisms. In contrast to degradation that decreases the absolute amount of the pesticide in the environment, sorption processes do not affect the total amount of pesticide present in the soil but can decrease the amount available for transformation or transport. 

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. 

It appears that pesticides with solubiHties greater than 10 mg/L are mainly transported in the aqueous phase (48) as a result of the interaction of solution/sediment ratio in the mnoff and the pesticide sorption coefficient. For instance, on a silt loam soil with a steep slope (>12%), >80% of atra2ine transport occurs in the aqueous phase (49). In contrast, it has been found that total metolachlor losses in mnoff from plots with medium ground slopes (2—9%) were <1% of appHed chemical (50). Of the metolachlor in the mnoff, sediment carried 20 to 46% of the total transported pesticide over the monitoring period. 

E1 Beit lOD, Wheelock JV, Cotton DE. 1981b. Pesticide-microbial interaction in the soil. IntJ Environ Stud 16 171-179. 

The physicochemical properties of a pesticide and its interaction with soil greatly influences both its mobility and biological a-vailability in a soil environment (1). Reviews on this subject have been published by Goring and Hamaker (2 ) and Greenland and Hayes ( 3). 

One particularly important application of chromatography has been to the analysis of pesticides, their degradation and movement. Small amounts of pesticides can be determined and their interaction with soil can be modeled using chromatographic methods [30], It is unlikely that all types of chromatographic separation have been developed or even conceived. New variants such as ultrahigh-pressure liquid and hydrolitic interaction liquid chromatography are but two examples. 

Previous research has shown that contaminant biodegradation by specific microorganisms can alter desorption rates of contaminants from sorbing surfaces [226, 357-359]. For pesticides, biodegradation has been shown to contribute to significant residue accumulation in soil at rates much greater than surface sorptive interactions [352]. 

Weber, J.B. Interaction of organic pesticides with particulate matter in aquatic and soil systems, in Fate of Organic Pesticides in the Aquatic Environment, Advances in Chemistry Series, Gould, R.F., Ed. (Washington, DC American Chemical Society, 1972), pp. 55-120. 

Bollag JM, LoU Ml (1983) Incorporation of xenobiotics in soil humus. Experentia 39 1221-1225 Bollag JM, Myers CJ, Minard RD (1992) Biological and chemical interactions of pesticides with soil organic matter. Sci Total Environ 123/124 205-217 Bolt GH (1955) Ion adsorption by clays. Soil Sd 79 267-278... 

Bolt GH, De Boodt MF, Hayes ME, McBride MB (eds) (1991) Interactions at the soil coUoid-solution interface. NATO ASI Series—Applied Science—Series E vol 190, Kluwer, Dordrecht Bowman BT (1979) Method of repeated additions for generating pesticide adsorption-desorption isotherm data. Can J Soil Sci 59 435-437... 

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