Sparsely soluble components

To evaluate the leaching performance of the waste streams, we assume that soluble and sparsely soluble compounds will leach out and fail the TCLP and, hence, should be target contaminants for stabilization. These soluble or sparsely soluble components may directly be treated with phosphates and converted to their insoluble, nonleachable forms. The literature is full of studies on stabilization of such divalent hazardous metal contaminants (Pb, Cd, and Zn, in particular), where treatment with various phosphates has been elfective. These studies are summarized in Section 16.3. 

The first component on the left-hand side of Eq. 13.10 is alkaline, and the second one is acidic. Thus, this reaction is an acid-base reaction that yields HAP at a near neutral pH in an aqueous environment, where both reacting components are sparsely soluble. 

As discussed in Chapter 16, chemical stabilization is a result of conversion of contaminants in a radioactive waste into their insoluble phosphate forms. This conversion is solely dependent on the dissolution kinetics of these components. In general, if these components are in a soluble or even in a sparsely soluble form, they will dissolve in the initially acidic CBPC slurry and react with the phosphate anions. The resultant product will be an insoluble phosphate that will not leach into the groundwater. On the other hand, if a certain radioactive component is not soluble in the acid slurry, it will not be soluble in more neutral groundwater, because the solubility of such components is lower in neutral than in acidic solutions. Such a component will be simply microencapsulated in the phosphate matrix of the CBPC. Thus, the solubility of hazardous and radioactive components is key to chemical immobilization. 

The question we now wish to address is whether Henry s law can be extended to mixtures of sparsely soluble gases. We note from Table 6.2 that most of the gases listed have solubilities at 1 atm pressure, of the order of 0.01 mol%. Even a moderately soluble gas such as carbon dioxide falls below the concentration level of 0.1 mol%. These values are so low, and the distance between dissolved molecules so large, that their solubilities will not be significantly altered by the presence of other, sparely soluble components. In other words, we feel jushfied in applying Henry s law for a pure gas to each individual component in a mixture, with the understanding that p is now the partial pressure of the gas in question. We apply this principle in the following illustration. 

It is logical to conclude -with the case where the new component is not appreciably soluble in either phase. At first sight it would appear unlikely that the interfacial tension could then be affected in either direction, yet this is possible. If the interfacial tension is increased by its addition to the system this substance will be adsorbed at the interface if decreased, the added body will be more sparsely distributed at the interface or not appear there at all. It is easy to observe, at any rate, the appearance of a film of non-transparent matter at the interface, and the experiment has been carried out for a number of liquid pairs and solids by Hofmann. 

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