PHOSPHATE WASTE FORMS MINERALS

Ionizing radiation can also cause radiation-enhanced diffusion that leads to defect annealing. Ouchani et al. (1997) demonstrated in a dual-beam irradiation (220 keV Pb and 0.3 to 3.2 MeV He) that the critical amorphization dose increases due to defect annealing caused by electron energy loss processes. Soulet et al. (2001) recently studied 

Summary. Phosphate and silicate apatite offer a number of advantages as nuclear waste forms (1) a high capacity for the incorporation of actinide elements, as well as selected fission products such as °Sr (2) a reasonable chemical durability depending on the geochemical environment and (3) a propensity for rapid annealing of radiation damage for the phosphate compositions. Considerable work remains to be done, mainly systematic studies, under relevant repository conditions, of the effects of composition on chemical durability. 

Chemical durability. Leaching studies on synthetic monazite containing 20 wt % simulated Savannah River waste (MCC-1 leach test, 28 days at 90°C) showed release rates of uranium to be on the order of 0.001 g/m d (Sales et al. 1983). The leach rate of the host matrix of a synthetic monazite, LaP04, containing simulated waste remained low even after the material had been transformed to an amorphous state by irradiation with 250 keV Bi ions (Sales et al. 1983). Perhaps, the most interesting aspect of the work by Sales et al. (1983) was the variety of techniques used to measure the leach rate. In addition to the standard MCC-1 leach test, they also used changes in the ionic 

Radiation effects. As with apatite, the increased release rates of radionuclides as a function of radiation damage has lead to rather detailed studies of the behavior of monazite under a variety of irradiation conditions. Karioris et al. (1981) and Cartz et al. (1981) established that natural monazite can be readily transformed to an amorphous state by irradiation with 3 MeV Ar ions at moderate doses. Robinson (1983) simulated the cascades that formed in monazite and found them to be similar in size and shape to simulated cascades in metals. The lack of observed radiation damage effects in monazite is related to the dominance of annealing processes. These early studies lead to detailed studies of damage accumulation as a function of temperature. 

The lines connecting the data points were calculated using an equation described by Meldrum et al. 

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Ceramicrete cures to create final waste forms that are analogs of naturally occurring phosphate minerals. These minerals have been shown to be relatively insoluble over geologic time scales. The final waste form is stronger than typical room temperature, hydraulic cements and performs in the manner of high-temperature fused ceramics. The technology has been evaluated in bench-and operational-scale tests on contaminated wastewater, sedimenL ash, and mixed wastes. 

In most applications, a small amount of binder powders is mixed with a large volume of inexpensive hllers and then the entire mixture is stirred in water to form the reaction slurry. For example, if the phosphate binders are used for manufacturing construction products, invariably the hllers are sand, gravel, ash, soil, or some mineral waste. The phosphate binders provide adhesion between the particles of these hllers and bind them into a solid object. Thus, these mixtures mimic conventional concrete mixmres in which Portland cement binder is mixed with large volume of sand and gravel to produce cement concrete. When phosphate binders are used, the products may be termed as phosphate concrete . In waste stabilization, the waste itself becomes the hller and the hnal product is termed as a waste form . 

The crystal chemistry of monazite, apatite, and related phosphate minerals, has been discussed in detail (see this volume. Chapters 1, 2, and 4 by Hughes and Rakovan, Pan and Fleet, and Boatner, respectively) and will not be repeated here. Rather, we will summarize the work relevant to the consideration of these phosphate phases as nuclear waste forms. 

Dacheux N, Clavier N, Le Coustumer P, Podor R (in press) Immobilization of tetravalent actinides in the TPD structrrre. Proc 10th Inti Ceramics Congress. Vincenzini P (ed) Techna Publishers, Florence, Italy Davis DD, Vance ER, McCarthy GJ (1981) Crystal chemistry and phase relations in the synthetic miner s of ceramic waste forms. II. Studies of uranirrm-containing monazites. In Scientific Basis for Nuclear Waste Management, vol. 3. Moore JG (ed) Plentrm Press, New York, p 197-200 Day DE, Wu Z, Ray CS, Hrma P (1998) Chemically durable iron phosphate glass waste forms. J Non-Crystalline Solids 241 1-12... 

Apatite, a natural calcium fluoride phosphate, can adsorb low to moderate levels of dissolved metals from soils, groundwater, and waste streams. Metals naturally chemically bind to the apatite, forming extremely stable phosphate phases of metal-substituted apatite minerals. This natural process is used by UFA Ventures, Inc., and is called phosphate-induced metals stabilization (PIMS). The PIMS material can by used in a packed bed, mixed with the contaminated media, or used as a permeable barrier. The material may be left in place, disposed of, or reused. It requires no further treatment or stabilization. Research is currently being conducted on using apatite to remediate soil and groundwater contaminated with heavy metals, and the technology may also be applicable to radionuclides. The technology is not yet commercially available. 

The studies by Eighmy and Eusden [10] are limited to stabilization of Pb, Cd, and Zn, which are sole contaminants of the MSW ash. By extension, it is believed that similar minerals are formed during phosphate treatment of other waste streams. For example, Singh et al. [50] have studied the interaction of Pb, Cd, and Zn with phosphatic clay and reported their investigations on the sorption and desorption of these metals. They report precipitation of Pb as fluoropyromorphite. In the case of Cd and Zn, their studies are not as conclusive, but Singh et al. suspect sorption and coprecipitation of phosphates are still the main mechanisms of stabilization. 

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... 

The exposure of sulfide minerals contained in mine wastes to atmospheric oxygen results in the oxidation of these minerals. The oxidation reactions are accelerated by the catalytic effects of iron hydrolysis and sulfide-oxidizing bacteria. The oxidation of sulfide minerals results in the depletion of minerals in the mine waste, and the release of H, SO4, Fe(II), and other metals to the water flowing through the wastes. The most abundant solid-phase products of the reactions are typically ferric oxyhydroxide or hydroxysulfate minerals. Other secondary metal sulfate, hydroxide, hydroxy sulfate, carbonate, arsenate, and phosphate precipitates also form. These secondary phases limit the concentrations of dissolved metals released from mine wastes. 

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