Solid-water distribution coefficient

Sorption Isotherms, Solid-Water Distribution Coefficients (A/d), and the Fraction Dissolved (fP)... 

To assess the extent to which a compound is associated with solid phases in a given system at equilibrium (see below), we need to know the ratio of the compound s total equilibrium concentrations in the solids and in the aqueous solution. We denote this solid-water distribution coefficient as Kid (e.g., in L kg 1 solid) ... 

So the assumption about the constancy of Kid is equivalent to presuming either (a) the overall process is described by a linear isotherm (n, - 1= 0), or (b) the relative concentration variation, (concentration range is less than a factor of 10, when multiplied by (n, - 1) with an n, value of 0.7, then the solid-water distribution coefficient would vary by less than a factor of 3. 

Therefore, we may not be too surprised to find that nonionic chemicals show increasing solid-water distribution ratios for soils and sediments with increasing amounts of natural organic matter. This is illustrated for tetrachloromethane (carbon tetrachloride, CT) and 1,2-dichlorobenzene (DCB) when these two sorbates were examined for their solid-water distribution coefficients using a large number of soils and sediments (Fig. 9.7, Kile et al., 1995.)... 

In all of these cases, the structure of the organic sorbate, the composition of the surface, and the conditions of the vapor or solution exchanging with the solid must be considered. However, it is important to note that with some experience in thinking about the organic chemicals and environmental situation involved, we can usually anticipate which one or two sorption mechanisms will predominate. For example, in Chapter 9 we wrote an expression reflecting several simultaneously active sorption mechanisms, each with their own equilibrium descriptor, to estimate an overall solid-water distribution coefficient for cases of interest (Eq. 9-16) ... 

Occurrence of different classes of perfluorinated compoimds in Greek waste-water treatment plants and determination of their solid-water distribution coefficients. 239-240 24-31. 

Fig. 16.2 Octanol-water distribution coefficient versus pH for a weak acid with pfC=4, logPHA=4 and logP =0 (solid line) or 2 (dashed line).

Since the open water column is nearly pure water (op =1, rs p = 0), for compounds with small to moderate solid-water distribution ratios (Kd < 10 m3kg ), the above equilibrium partition coefficient can be simplified to ... 

Figure 9.28. The distribution of nonpolar organic substances between aquatic solids and water (as given by the distribution coefficient Kp) is dependent upon the lipophilicity of the compound and the organic C content of the adsorbing material (foe = weight fraction). The solid phases considered here are coastal sea and lake sediments, river sediments, solids from aquifers and biomass (activated sludge). The octanol/water distribution coefficients are, respectively 500, 2400, 11,200 and 52,000 for chlorobenzene, 1,4-dichlorobenzene, 1,2,4-trichlorobenzene and 1,2,4,5-tetrachlorobenzene. (Modified from Schwarzenbach and Westall, 1980).

In Figure 1 dashed squares refer to water bodies, while solid squares refer to solid matter (also containing some water). Plutonium may appear in any of these squares. The ratio of the concentration of plutonium in two adjacent squares is usually referred to as the concentration factor (CF usually from the water to the solid substance), the transfer coefficient (TC usually between two biological species), or the sorption ratio (or between minerals and water). To avoid ambiguity, we shall use the expression distribution coefficient (abbreviated Kd) with unit dimension (Pu amount per kg product divided by Pu amount per kg source). For the transfer of plutonium from A to B, Pu(A) ->- Pu(B), we define... 

The partitioning of As in the aquifer solid-water interface can best be explained with the distribution coefficient, Kd (a ratio of solute adsorbed in sediment to that of dissolved in groundwater). Due to being simplistic in nature, Kd has long been well appreciated as well as applied by geochemical modelers. 

Distribution coefficients describe in a summarizing way the distribution of an element between the dissolved and solid phases. They are conditional constants valid for a given pH, temperature and other conditions they are independent of the concentrations of solids in water. They are usually defined (Chapter 4.8) as... 

The distribution coefficients are independent of the concentration of suspended solids in water, which can vary over a wide range they thus give a better picture than the fraction of metal ions in solution. Such distribution coefficients can be predicted on the basis of the equilibrium constants defining the complexation of metals by surfaces and their complexation by solutes (Table 11.1). 

Distribution coefficients based on adsorption equilibria are independent of the total concentrations of metal ions and suspended solids, as long as the metal concentrations are small compared with the concentration of surface groups. Examples of the Kd obtained from calculations for model surfaces are presented in Fig. 11.1. A strong pH dependence of these Kd values is observed. The pH range of natural lake and river waters (7 - 8.5) is in a favorable range for the adsorption of metal ions on hydrous oxides. 

Paschke, A. and Popp, P. 2003, Solid-phase microextraction fibre-water distribution constants of more hydrophobic organic compounds and dieir correlations widi octanol-water partition coefficients. J. Chromatogr. A 999 35-42. 

Experimental Technique. The solid material (1-3 g) with known particle size and standard water (30-50 ml) containing the radionuclide of interest were shaken in glass bottles for 8-12 hours at constant temperature (25°C or 65 C). The phases were separated by centrifugation (50 min, 7000 rpm) and the distribution coefficient of the radionuclide was determined from measurements of the remaining activity of the water. Filtration of the samples through 0.2 pm membrane filter did not change the values. 

Effect of the Mater Composition. For Cs, Sr, U and Np somewhat higher distribution coefficients are obtained for Aq ga than for Aqiios (an increase by a factor of three or less). This effect is expected, since an increased salt concentration would give a larger number of charged particles in the water which may compete for the available sites on the solid sorbent. 

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