Surface sorption coefficients

The surface sorption capacity is usually expressed as a surface sorption coefficient. In general, the surface distribution coefficient of U(VI) between the solution and the granite surface, K, (cm), is defined as ... 

Ka is the surface sorption coefficient and a. the FWS per volume of water in the fracture. The latter is related to the mean fracture aperture < by... 

In the same way the ratios of the surface sorption coefficient from field and laboratory data are compared. Also here, a difference of one to two orders of magnitude were found. 

The values for surface sorption coefficient K are obtained from the modelling input data set (MIDS) (Byegard et al., 1998) (for Sr-85). The data for other tracers (Co-58, Tc-99m and Am-241) provided by Tasks 6A and 6B modelling task specification (Selroos and Elert, 2001) are used. 

The values of sorption coefficient Kd for different tracers are based on the batch data (fraction 1 - 2 mm) (Byegard et al., 1998), which are about 10 times of the MIDS value. These values are also consistent with the TRUE-1 evaluation. In the modelling, Kd values are either chosen to be 10 times of the MIDS values or 10 times to the values specified in Task Specification. The values for K (the surface sorption coefficient) are chosen to be the same as the MIDS values since K has relatively less effect on retention. 

The surface sorption capacity is usually expressed as a surface sorption coefficient. In... 

Ksa can be estimated from the snow surface sorption coefficient Tia by multiplication with the specific snow surface area SSA (m g ) and the density p of melt... 

Indirect methods of estimating sorption have been used when actual measurement of sorption isotherm is impossible (44). For instance, sorption coefficients have been estimated from soil organic carbon and a specific surface of soil, and from semiempidcal equations using pesticide properties. 

Table I summarizes some typical distribution coefficients. Sediments become enriched in plutonium with respect to water, usually with a factor of vlO5. Also living organisms enrich plutonium from natural waters, but usually less than sediments a factor of 103 - 101 is common. This indicates that the Kd-value for sediment (and soil) is probably governed by surface sorption phenomena. From the simplest organisms (plankton and plants) to man there is clear evidence of metabolic discrimination against transfer of plutonium. In general, the higher the species is on the trophic level, the smaller is the Kd-value. One may deduce from the Table that the concentration of plutonium accumulated in man in equilibrium with the environment, will not exceed the concentration of plutonium in the ground water, independent of the mode of ingestion.

Equilibrium Partitioning Experiments. The equilibrium partitioning of americium-III between gray hornblende schist and rock equilibrated water was determined in batch partitioning experiments with rectangular blocks of gray hornblende schist ( 5). The surface area sorption coefficient, K, was determined to be 4.5 . 5 mL/cm where... 

While the exact sorption mechanism may not be clear, the available literature suggests the following generalizations. First, coarser particles (e.g., silica sand) exhibit less binding per mass of solid than corresponding finer particles made of the same material (e.g., porous silica). This is presumably due to the influence of increased solid surface area per mass of sorbent. Thus, values of sorption coefficients for minerals (Kimm = Cimin/Ciw.Mr) are more useful if they are normalized to the solid s surface area rather than its mass. The second tendency we see is that for any... 

Sorption coefficients quantitatively describe the extent to which an organic chemical is distributed at equilibrium between an environmental solid (i.e., soil, sediment, suspended sediment, wastewater solids) and the aqueous phase it is in contact with. Sorption coefficients depend on (1) the variety of interactions occurring between the solute and the solid and aqueous phases and (2) the effects of environmental and/or experimental variables such as organic matter quantity and type, clay mineral content and type, clay to organic matter ratio, particle size distribution and surface area of the sorbent, pH, ionic strength, suspended particulates or colloidal material, temperature, dissolved organic matter (DOM) concentration, solute and solid concentrations, and phase separation technique. 

Models use sorption coefficients to predict concentrations of organic solutes in water leaching through the soil profile or in runoff from land surfaces into lakes or streams. Along with other properties such as Henry s law constants and bioconcentration factors, sorption... 

For neutral chemicals, sorption coefficients usually are unaffected by pH. However, for ion-izable organic chemicals, sorption coefficients can be affected greatly, since pH affects not only the speciation but also the surface characteristics of natural sorbents. 

Salts can affect sorption of organic compounds by displacing cations from the soil ion exchange matrix, by changing the activity of the sorbate in solution, and by changing the charge density associated with the soil sorption surface (Hamaker and Thompson, 1972). Salt effects are most important for basic sorbates in the cation state, where an increase in salinity can significantly lower the sorption coefficient. Salt effects are least important for neutral compounds, which may show either increases or decreases in sorption as salinity increases. 

A variety of studies have shown that as temperature increases, sorption may increase, decrease, or remain the same, with isosteric heats of sorption being very low (Table 21.7). These studies have used the batch slurry technique, so the impact of temperature on water-triazine interactions may mask surface-triazine interactions. In contrast, at 10% soil moisture isosteric heats of atrazine sorption ranged from -10 to -12kcal/mol determined with the SF technique (Koskinen and Rochette, 1996). Sorption coefficients in field-moist soils were much greater than are typically obtained with the batch slurry system, while heats of sorption were much more negative, indicating greater sorption at low moisture contents. 

Sorption overview. Both empirical and mechanistic approaches have emerged since the 1970s to describe interactions between radionuclides and geomedia. These are based on conditional constants, which are valid for specific experimental conditions, or more robust intrinsic constants, which are valid over a wider range of conditions. The empirical approach involves measurements of conditional radionuclide distribution or sorption coefficients or Rsite-specific water-rock systems using synthetic or natural ground waters and crushed rock samples. Mechanistic-based approaches produce intrinsic, thermodynamic surface-complexation constants for simple electrolyte solutions with pure mineral phases. 

Seme and Muller (1987) describe attempts to hnd statistical empirical relations between experimental variables and the measured sorption ratios (R(js). Mucciardi and Orr (1977) and Mucciardi (1978) used linear (polynomial regression of first-order independent variables) and nonlinear (multinomial quadratic functions of paired independent variables, termed the Adaptive Learning Network) techniques to examine effects of several variables on sorption coefficients. The dependent variables considered included cation-exchange capacity (CEC) and surface area (S A) of the solid substrate, solution variables (Na, Ca, Cl, HCO3), time, pH, and Eh. Techniques such as these allow modelers to constmct a narrow probability density function for K s. 

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