Clay distribution coefficients

The first of these environmentally-important parameters can be expressed as a partition coefficient. In aqueous solution many, but not all pesticide compounds exhibit strong affinity for soil organic matter or concentrate in the lipid phase of soil organisms. Some, notably the cationic group, also exhibit marked affinity for clay or other mineral surfaces. An overall partition (or distribution) coefficient (kD) can be defined ... 

Now, because the water-borne radioactive element is predominantly associated with the colloids, we no longer have a need for the distribution coefficient. There will still be a partitioning because the major portion of the radioactive elements will still be adsorbed to the sediment. This is a separate equilibrium partitioning coefficient, requiring a new experiment on the clay sediments and the colloids present. The partitioning colloid-clay ratio would most likely be dependent on the surface areas of each present in the sediments. A separate size distribution analysis has resulted in a sediment-colloid surface area ratio of 99 1 for the sediment. This results in a colloid retardation coefficient oiRc = 100 rather than Ri = 4.2 x 10 or i 2 = 6 x 10. ... 

The Distribution Coefficient. As a measure of the sorption and retarding capacity of the rock and clay the mass related distribution coefficient Kj [ms/kg] was used, defined as... 

Distribution coefficients were measured employing batch methods. The solution (ml) to clay (mg) ratios were aporoximate-ly as follows 1 5 attapulgite 1 60 montmorilIonite and 1 25 kaolinite. The solutions were brines (NaCl) buffered with pH 5 acetate solution. The original stock solution contained 3 M NaCl and 1 M Na acetate buffer. The lower [Na] solutions were made by... 

In most mathematical analyses used to establish bounds for radionuclide migration rates through the abyssal red clays, the sorption properties of the sediment are generally represented mathematically by the sorption equilibrium distribution coefficients for each of the species involved. These coefficients are usually denoted by Kp. and are defined by... 

The principal rubidium salts which would probably have been present in the sediment (chloride, sulfate, bicarbonate, etc.) are all soluble in water. As discussed later, the red clay was thoroughly dialyzed prior to use (including prior to analysis by emission spectroscopy). Any rubidium salts initially present in the clay samples would, therefore, have been removed by the dialyzing solution. Hence, it was assumed that the rubidium concentration given in Table I represented sorbed rubidium which had been in equilibrium with the rubidium in the original interstitial seawater. Then when calculating distribution coefficients from experimental data, the concentration given in Table I was used as the initial clay-phase rubidium concentration, rather than zero as used with most of the other species studied. 

Therefore, based on available literature, the following sorption results were expected (l) as a result of the smectite minerals, the sorption capacity of the red clay would be primarily due to ion exchange associated with the smectites and would be on the order of 0.8 to I.5 mi Hi equivalents per gram (2) also as a result of the smectite minerals, the distribution coefficients for nuclides such as cesium, strontium, barium, and cerium would be between 10 and 100 ml/gm for solution-phase concentrations on the order of 10"3 mg-atom/ml (3) as a result of the hydrous oxides, the distribution coefficients for nuclides such as strontium, barium, and some transition metals would be on the order of 10 ml/gm or greater for solution-phase concentrations on the order of 10 7 mg-atom/ml and less (U) also as a result of the hydrous oxides, the solution-phase pH would strongly influence the distribution coefficients for most nuclides except the alkali metals (5) as a result of both smectites and hydrous oxides being present, the sorption equilibrium data would probably reflect the influence of multiple sorption mechanisms. As discussed below, the experimental results were indeed similar to those which were expected. 

Distribution Coefficients. The distribution coefficients determined for rubidium (at ll C) and for cesium (at ll C for -log Ci less than 5 and at for -log greater than 5) are summarized in Figure 1. Over the range of solution-phase concentrations in which both rubidium and cesium were studied, the rubidium coefficients appear to behave very similarly to those for cesium. For solution-phase concentrations on the order of 10 3 mg-atom/ml, the coefficients are on the order of 100 ml/gm, as was expected. Furthermore, the distribution coefficients obtained for cesium generally appear consistent with the corresponding coefficients obtained for similar oceanic sediments and related clay minerals found within the continental United States (6,7,8,9510,12,13). In the pH range of 6.3 to 8.0, the cesium coefficients appear to... 

For the nuclides studied (rubidium, cesium, strontium, bariun silver, cadmium, cerium, promethium, europium, and gadolinium) the distribution coefficients generally vary from about 10 ml/gm at solution-phase concentrations on the order of 10 mg-atom/ml to 10 and greater at concentrations on the order of 10 and less. These results are encouraging with regard to the sediment being able to provide a barrier to migration of nuclides away from a waste form and also appear to be reasonably consistent with related data for similar oceanic sediments and related clay minerals found within the continental United States. 

For the distribution coefficients, an acetate buffer adjusted to pH 5 was used and the sodium chloride concentration of it varied from 0.25 to 4 M by dissolving sodium chloride in the buffer. Aliquots of the Es solution were mixed with the buffer and a weighed quantity of the solid clay was mixed with this solution. 

Distribution Coefficient. Distribution coefficients were determined using conventional batch equilibrations, in which a known amount of clay is contacted with a known volume of solution and shaken until equilibrium is essentially reached, at least overnight at room temperature. In tests with several ions and montmorillonite, equilibrium was reached in less than two hours. 

Distribution coefficients for potassium on one clay are summarized in Figure 9. The ideal slope of -1 for log vs log sodium concentration was closely approximated. Values of D were about 1/3 those for cesium on the same clay. A value of K/T [(K )clay(Na+)/(K+)(Na+)ciay] of about 3.75 can be estimated from Figure 9. This agrees satisfactorily with values of about 2.5 reported both by Cast (10) for 0.001 M Na+ and by Shainberg and Kemper (20) for 0.05 Af Na+. 

In deriving the shapes of these isotherms, we first must define isotherms for ideal clays and ideal oxides. We then combine these ideal functions into overall isotherms and compare the derived functions to experimental isotherms determined for various adsorbents. We will show that observations which have been said to preclude ion exchange are, in fact, quite consistent with ion-exchange behavior. We will not attempt to derive actual values of equilibrium distribution coefficients, but rather we seek only to define the shapes of the isotherms. 

Figure 1. Theoretical plots of distribution coefficient vs concentration of NaCl for sorption of Cs(I), Sr(II), and Hu (III) on a hypothetical clay with a capacity of one equivalent per kilogram.

In Figure 6, we show a theoretical calculation using Equations 3-6 of the effect of pH on the distribution coefficient for a case where the principal adsorbent is a hypothetical clay which contains a total of 0.8% oxides. In this case, we chose a mixture of three oxides as follows oxide 1, 0.1%, Kox =. 01, C =. 5 oxide 2, 0.2%, -. 005,... 

Figure 6. Theoretical distribution coefficients for trivalent ion sorption on sorbent containing 99.2% clay and 0.8% oxide. Arbitrary constants (see text).

H. E. Jensen, Selectivity coefficients of mixtures of ideal cation-exchangers, Agrochimica XIX.-247 (1975). See also K. Bunzl and W. Schultz, Distribution coefficients of 137Cs and 85Sr by mixtures of clay and humic material, J. Radioanalyti. Nucl. Chem. 90(1) 23 (1985). 

The other clay mineral, illite, also has some strontium sorption, but the distribution coefficient is about one order of magnitude lower than that of montmorillonite and rectorite. Since, in illite, the layer charge is compensated by non-exchangeable cations (Chapter 1, Table 1.2), cation sorption can takes place only on the deprotonated edge sites. This is the case for tectosilicates (quartz, cristobalite). 

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