Sorption and leaching

Reddy. K.N., Singh, M., and Alva. A.K. Sorption and leaching of bronracil and sinrazine in Florida flatwoods soils. Bull Environ. Contam. Toxicol, 48(5) 662-670, 1992. 

Sorption and Leaching of 4-Amino-3,5,6-trichloropicolinic Acid in Soils... 

Sorption and Leaching Behavior of Perfluorinated Compounds in Soil... 

To be able to predict the fate and behavior in the environment the knowledge on sorption and leaching behavior of PFC in soil is an important tool, which is addressed in a separate chapter. 

Considerable research has been conducted to investigate the soil sorption and mobiUty of dinitroaniline herbicides. In general, these herbicides are strongly sorbed by soil (354), and sorption has been correlated to both soil organic matter and clay content (355). Dinitroaniline herbicides are not readily leached in most soils (356), although leaching of triduralin is enhanced by addition of surfactants (357). 

Desorption is the reverse of the sorption process. If the pesticide is removed from solution that is in equdibrium with the sorbed pesticide, pesticide desorbs from the sod surface to reestabUsh the initial equdibrium. Desorption replenishes pesticide in the sod solution as it dissipates by degradation or transport processes. Sorption/desorption therefore is the process that controls the overall fate of a pesticide in the environment. It accomplishes this by controlling the amount of pesticide in solution at any one time that is avadable for plant uptake, degradation or decomposition, volatilization, and leaching. A number of reviews are avadable that describe in detad the sorption process (31—33) desorption, however, has been much less studied. 

As an example, we can compare the solubility of lindane and 2,4-D pesticides with respect to their major loss pathways. The Ku values of these pesticides are 1.33 x 1CT4 and 5.5 x 10 9 mol L 1, respectively. In addition, the retardation coefficient or degree of sorption of lindane is much higher than that of 2,4-D. Therefore, 2,4-D is more likely to be leached, whereas lindane is more likely to remain near the soil surface from whichit can vaporize. Volatilization is thus the major pathway of lindane, and degradation and leaching are the major loss pathways of 2,4-D in the Asian environments. 

The fate of a given compound in the soil depends upon its partition between the soil particles themselves and the water, air and organisms it contains. Certain physicochemical properties of pesticides can be used to predict which are most likely to be leached. The most diagnostic properties are the sorption and degradation characteristics. 

The sorption and degradation characteristics listed for most pesticide compounds in terms of partition coefficients and half-lives relate only to a (standard) fertile, organic clayey soil and must not to be taken as representative of the permeable sandy soils widely developed on aquifer outcrops. Thus leaching of 1% of original application rates, and perhaps significantly higher, could easily occur for certain compounds on permeable soils. 

The conclusions of retardation are applicable to a wide range of sorption and transport problems including artificial groundwater recharge, and leaching of pollutants from landfills. 

Study soil sorption and desorption of pollutants in complex mixtures and/or leached from SWMs... 

Whereas batch equilibrium tests are designed to study equilibrium sorption of solid phase particles with various pollutants, singly or in combination with other pollutants, solid phase column-leaching tests study both sorption and diffusion of organic pollutants through the subsurface environment [10,11,127, 141,142]. 

These differences are exemplified in Figure 7.5 which shows the results of an experiment in which Cd +, Zn +, Ni + and Cn + salts were applied on the surface of an acid soil, with and without lime, and the soil leached with 0.01 MCaCF for several hours (McBride, 1994). In the unlimed soil, Cd +, Zn + and Ni moved readily to depth, but Cu + remained near the snrface becanse it was strongly sorbed on soil solids. In the limed soil, with pH 6.5, increased sorption and... 

Because chlorite is an anion, sorption of chlorite ions onto suspend particles, sediment, or clay surfaces is expected to be limited under enviromnental conditions. Thus, chlorite ions may be mobile in soils and leach into groundwater. However, chlorite (ions or salts) will undergo oxidation-reduction reactions with components in soils, suspend particles, and sediments (e.g., Fe, Mn ions see Section 6.3.2.2). Thus, oxidation-reduction reactions may reduce the concentration of chlorite ions capable of leaching into groundwater. 

However, as pH drops, the ability of natural attenuation mechanisms in the ash (e.g., carbona-tion, sorption to hydrous ferric oxides) becomes more dominant and leaching is reduced. 

Wolfsberg, K., "Sorption-Desorption Studies of Nevada Test Site Alluvium and Leaching Studies of Nuclear Explosion Debris," Los Alamos Scientific Laboratory, Kept. LA-7216-MS, (April, 1978). 

Dye-containing solid wastes, including cleanup wastes from manufacturing and textile dyeing operations, are likely to enter landfills, where aerobic and anaerobic biodegradation, sorption to soils, and leaching may occur. 

The chapters of this volume are organized into sections that cover the chemical aspects that are important to understanding the behavior of disposed radioactive wastes. These aspects include radionuclide sorption and desorption, solubility of radionuclide compounds, chemical species of radionuclides in natural waters, hydrothermal geochemical reactions, measurements of radionuclide migration, solid state chemistry of wastes, and waste-form leaching behavior. The information in each of these sections is necessary to predict the transport of radionuclides from wastes via natural waters and thus to predict the safety of the disposed waste. 

---