CBPC waste form

In the phosphate washing discussed above, only a small amount of acid phosphate is used to convert contaminants into their insoluble phosphate forms. To fabricate CBPC waste forms, however, a larger amount of binder is needed, so compared to the phosphate washing, the cost of the binder is high. This condition does not mean that the volume of the stabilized waste will increase. Typically, the washed waste is loosely packed, but the fully stabilized ceramic matrix is dense. As a result, the volume does not increase and, hence, the disposal cost will remain the same. Depending on the nature of the waste and amount of the phosphate binder used, the binder cost may be the only higher cost in the CBPC treatment compared to simple acid washing. 

The samples were stored for 3 weeks for curing. Each sample was then crushed and was subjected to the TCLP test. The TCLP test results on both the waste stream and the treated CBPC waste form are given in Table 16.6. The results on the untreated waste streams show that the leaching levels far exceed the regulatory limits. The results for the waste forms, on the other hand, are an order of magnitude below the EPA limit. These results indicate superior stabilization of Hg in the phosphate ceramic waste forms coupled with sulfide immobilization. 

Based on the half-life and specific activity of commonly occurring isotopes in radioactive waste streams given in Table 17.2, each disposal site or repository of the treated waste has developed waste acceptance criteria (WAC). The WACs require that the treated waste pass certain test criteria, which vary from site to site depending on the nature of the waste stored. In a number of case studies discussed in this chapter, we will use these WACs to demonstrate compliance of the CBPC waste form. 

Na, and B from a glass waste form. If, on the other hand, this test is to be adopted for a CBPC waste form, such as Ceramicrete, one may look for Mg, K, and P as the matrix components. Thus, the PCX evaluates the durability of the matrix material, which is a result of the integrity of the individual elements within the matrix. 

This incremental cost increase may be justified when stabilizing mixed waste streams or waste streams containing As, Cr, and Hg, because simple acid washing will not stabilize these waste streams. To stabilize Hg, in addition to the CBPC formation, a sulfide pretreatment is used [55]. The pretreatment converts the contaminants into their most insoluble sulfide forms, then the CBPC formation produces a waste form that is far superior to any other treatment. This dual treatment has the advantage of being performed at room temperature in a one-step mixing process. 

Again, as in the case of hazardous contaminants discussed in Chapter 16, the solubility of a radioactive contaminant plays a major role in its stabilization in a phosphate matrix. Therefore, one needs to understand the aqueous behavior of a radioactive contaminant prior to selecting the acid-base reaction that will form the CBPC used for fabricating the waste form matrix. In this respect, actinides, fission products, and salts have unique solubility behavior. This behavior is discussed below. 

The densities of the final waste forms were 1.70 and 1.9 g/cm, and the compression strengths ranged from 1400 to 1900 psi (9.8-11.9 MPa) for both the chloride and nitrate waste forms. These values are significantly higher than the NRC land disposal requirement of 500 psi (3.5 MPa) for cement-based waste forms [17]. Thus, salt waste forms of the CBPC at high loadings are relatively dense hard materials that are suitable for salt waste... 

Several additional favorable properties of CBPCs make them an even better candidate for stabilization. The waste form is a dense matrix, generally with very good mechanical properties. Also it is nonleachable, does not degrade over time, is neutral in pH, converts even flammable waste into nonflammable waste forms, performs well within acceptable levels in radiolysis tests, and can incorporate a range of inorganic waste streams (solids, sludge, liquids, and salts). 

Low-temperature treatment of low-level mixed wastes has also been accomplished by solidification/stabilization with chemically bonded phosphate ceramics (CBPC). These are made by hydrothermal chemical reaction rather than by sintering. Chemical bonding develops when acid phosphates react with oxides to form crystalline orthophosphate (Singh et al. 1997). The ceramic matrix stabilizes the wastes by microencapsulation. The low temperature of the reaction allows volatile radionuclides to be treated (Singh et al. 1997). 

In the acid-base reactions that form ceramics, this alkaline region is of little interest to us, because the reaction products that constitute the ceramic are neutral, and hence, the reaction is not driven to the alkaline side. We will not elaborate on the reactions in alkaline regions, except in waste management applications in Chapters 16 and 17, where we discuss the stability of CBPC products in highly alkaline waters. 

Inorganic contaminants are immobilized by washing the waste with soluble phosphates. This treatment uses a very small amount of phosphate, does not change other characteristics of the waste such as its granular nature or volume, and is relatively inexpensive. If the waste contains radioactive contaminants, phosphate washing is not sufficient because the dispersibility of the radioactive contaminant powders needs to be reduced, and hence, the waste needs to be solidified. Solidification requires generating phosphate ceramics of the waste in the form of a CBPC. In the case of radioactive waste, both stabilization and solidification are needed because they not only immobilize the contaminants, but also solidify the entire waste. As we will see in this and the next chapter, whether phosphate treatment is used only for stabilization or for both stabilization and solidification, it is very effective for a wide range of waste streams. 

As in the case of hazardous contaminants discussed in Chapter 16, CBPC treatment converts radioactive constituents of waste streams into their nonleachable phosphate mineral forms. It follows the philosophy [7] that, if nature can store radioactive minerals as phosphates (apatite, monozites, etc.) without leaching them into the environment, researchers should be capable of doing the same by converting radioactive and hazardous... 

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. 

Salts of actinides are very common in waste streams. In particular, nitrates, chlorides, and sulfates are found in tank waste streams that were formed by neutralization of highly acidic solutions at several DOE sites, such as Hanford and Savannah River. The aqueous solubility of these salts is very high, and hence, it is a challenge to stabilize them. As we shall see in case studies, the CBPC matrix has good promise in handling these waste streams. 

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