Sorption of organic pesticides

The composition and pH of these soils were determined by standard methods (3), and the results are presented in Table II. Comparing Tables I and II shows that there is no evident relationship between sorption and the proportions of sand, silt, and clay. However, soils showing high sorption for the three herbicidal materials are also acidic—e.g., soils 3 and Bi through Q . This is caused partly by a tendency of highly organic soils to be acidic. A positive correlation between sorption and low pH would be expected from the number of instances reported in the review on sorption of organic pesticides by Bailey and White (2). 

Sorption of Organic Pesticides from Aqueous Solution... 

The effects of temperature on the equilibrium capacity of active carbon for adsorption of 2,4-D and DNOSBP have been studied. The data are plotted in Figure 11, and the experimentally determined heats of adsorption are listed in Table IV. The values for AH for the 2,4-D and DNOSBP are remarkably similar, and as one would anticipate from considering the thermodynamics of adsorption, both values are negative. In view of the general range of the values for AH it may be anticipated that normal temperature variations in practical applications will not significantly affect ultimate capacity for sorption of organic pesticides on carbon. 

Because many studies have shown a direct relationship between pesticide sorption and organic carbon content of sod, attempts have been made to develop a universal sorption coefficient based on sorption of the pesticide to sod organic carbon (44). Sorption based on sod organic carbon is expressed as C, where is pesticide sorbed per unit mass sod organic carbon, and C is pesticide solution concentration after equdibration. If. is the fraction of organic carbon, can be obtained from i in the equation. Assumptions in the use of this approach include... 

Torrents, A., Jayasundera, S., and Schmidt, W.J. Influence of the polarity of organic matter on the sorption of acetamide pesticides, J. Agric. Food Chem., 45(8) 3320-3325, 1997. 

Delle Site A (2001) Factors affecting sorption of organic compounds in natural sorbent-water systems and sorption coefficients for selected pesticides a review. J Phys Chem Ref Data 30 187-439... 

Acidic pesticides such as 2,4-D, 2,4,5-T, picloram, and dinoseb can ionize in aqueous solutions forming anionic species (Saltzman and Yaron, 1986). Sorption of these pesticides on soils has also been correlated with soil organic matter content (Hamaker et al., 1966), and in their anionic form they can be sorbed on soils, clays, and amorphous materials at low pH. The mechanisms of sorption for these compounds are proton association and, for the molecular form, van der Waals sorption (Saltzman and Yaron, 1986). Hydrogen bonding and electrostatic interactions are other possible mechanisms for sorption. 

Chiou C. T. (1998) Soil sorption of organic pollutants and pesticides. In Encyclopedia of Environmental Analysis and Remediation (ed. R. A. Meyers). Wiley, New York, NY, pp. 4517-4554. 

Humic substances constitute the bulk of the organic matter in most terrestrial soils. The functions they perform are multiple and varied and include the weathering of rocks and minerals, mobilization and transport of metal ions, and formation of stable aggregates by combination with clay minerals. Humic substances make a significant contribution to the cation-exchange capacity of the soil, and they are involved in the sorption of organic molecules applied to soils as pesticides. 

The last part of the book, Chapters 10—17, reports on different applications of polymers in the field of analytical chemistry sorption of organic compounds, Hke phenols, pesticides or caffeine, the use of polymeric materials for size-exclusion chromatography, for HPLC packing materials or as solid phase extraction sorbents. The examples cover a broad variety of chemical compounds, fike pesticides, pharmaceuticals, organic acids in different matrices, such as food, biological fluids, non-aqueous media, air and the use of hemosorbents in blood purification. 

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 analysis techniques used were FTIR to study this effect and the optional use of theoretical calculations to justify the obtained results by means of computational chemistry tools. Using QSAR properties, we can obtain an estimate of the activity of a chemical from its molecular structure only. The QSARs have been successfully applied to predict soil sorption coefficients of non-polar and nonionizable organic compounds, including many pesticides. Sorption of organic chemicals in soils or sediments is usually described by sorption coefficients. The molecular electrostatic potential (MESP) was calculated using the AMBER/AM 1 method. These methods give information about the proper region by which compounds have intermolecular interactions between their units. 

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. 

Sorption and Desorption Processes. Sorption is a generalized term that refers to surface-induced removal of the pesticide from solution it is the attraction and accumulation of pesticide at the sod—water or sod—air interface, resulting in molecular layers on the surface of sod particles. Experimentally, sorption is characterized by the loss of pesticide from the sod solution, making it almost impossible to distinguish between sorption in which molecular layers form on sod particle surfaces, precipitation in which either a separate soHd phase forms on soHd surfaces, covalent bonding with the sod particle surface, or absorption into sod particles or organisms. Sorption is generally considered a reversible equdibrium process. 

Although most nonionic organic chemicals are subject to low energy bonding mechanisms, sorption of phenyl- and other substituted-urea pesticides such as diuron to sod or sod components has been attributed to a variety of mechanisms, depending on the sorbent. The mechanisms include hydrophobic interactions, cation bridging, van der Waals forces, and charge-transfer complexes. 

Preferential flow through root-mediated soil pores has been demonstrated for chloride, nitrate, and other ions that are not sorbed onto soil organic matter and clays. However, pesticide sorption onto soil affects both mobiUty of the pesticide as well as its residual life in the soil. Pesticide sorption onto root organic matter or organic linings of worm burrows may also slow transport of pesticides relative to water (72), thus countering the effects of increased permeabihty caused by roots. 

Ahmad R, Kookana R, Alston A, Skjestad J (2001) The nature of soil organic matter affects sorption of pesticide. 1. Relationships with carbon chemistry as determined by C-13 CPMAS NMR spectroscopy. Environ Sci Technol... 

Historically, organic environmental pollutants were hydrophobic, often persistent, neutral compounds. As a consequence, these substances were readily sorbed by particles and soluble in lipids. In modern times, efforts have been made to make xenobiotics more hydrophilic - often by including ionisable substituents. Presumably, these functional groups would render the compound less bioaccumulative. In particular, many pesticides and pharmaceuticals contain acidic or basic functions. However, studies on the fate and effect of organic environmental pollutants focus mainly on the neutral species [1], In the past, uptake into cells and sorption to biological membranes were often assumed to be only dependent on the neutral species. More recent studies that are reviewed in this chapter show that the ionic organic species play a role both for toxic effects and sorption of compounds to membranes. 

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 second example includes the influence of sorption and sediment-water interaction, processes which were not relevant for the case of chloroform. We choose the real case of a chemical pollution of the River Rhine. On November 1, 1986, a fire destroyed a storehouse at Schweizerhalle near Basel (Switzerland). During the fighting of the fire, several tons of various pesticides and other chemicals were flushed into the River Rhine (Wanner et al., 1989). One of the major constituents discharged into the river was disulfoton, an insecticide. An estimated quantity of 3.3 metric tons reached the river within a time period of about 12 hours leading to a massive killing of fish and other aquatic organisms. 

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. 

Gerstl, Z., and L. Kliger. 1990. Fractionation of the organic matter in soils and sediments and their contribution to the sorption of pesticides. J. Environ. Sci. Health B-Pesticides 25(6) 729-741. 

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. 

Soil sorption of most hydrophobic organic compounds (e.g., nonpolar pesticides) is directly related to SOM content. HS are the major SOM components (Ferreira et al., 2002). FTIR spectroscopy makes possible the observation of how some chemical functions, present in humic structures, are involved in the sorption process. 

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. 

As with other pesticides, organic matter appears to be the soil constituent most important for basic pesticide sorption. Weber et al. (1969) showed that maximum sorption of several s-triazines on organic matter occurred at pH levels close to the pKa values of the compounds. The molecular structure of the pesticide and the pH of the sorbent strongly affected the degree of sorption. The pH-dependent sorption and the relationship between pH and dissocation constant with pH suggests an ion exchange mechanism (Saltzman and Yaron, 1986). 

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