Leach rates materials

Sulfur Polymer Cement. SPC has been proven effective in reducing leach rates of reactive heavy metals to the extent that some wastes can be managed solely as low level waste (LLW). When SPC is combined with mercury and lead oxides (both toxic metals), it interacts chemically to form mercury sulfide, HgS, and lead sulfide, PbS, both of which are insoluble in water. A dried sulfur residue from petroleum refining that contained 600-ppm vanadium (a carcinogen) was chemically modified using dicyclopentadiene and oligomer of cyclopentadiene and used to make SC (58). This material was examined by the California Department of Health Services (Cal EPA) and the leachable level of vanadium had been reduced to 8.3 ppm, well below the soluble threshold limit concentration of 24 ppm (59). 

It was found that the dissolution rate of the material depends both on its surface area and on its crystalline size, but the importance of the crystalline size seems to be greater. The empiric equation describing the above dependence of the leaching rate on these two parameters is as follows (130) ... 

Owing to the use of these different methods, the results are often difficult to compare. Different procedures, however, are required to leant how waste-form materials behave under different conditions. Unless specified otherwise, all leach rates given in this chapter represent values determined with deionized water as the leachant. 

Fig. 7. Plot of the Pu leach rate as a function of time for pyrochlore- and zirconolite-rich LLNL-type waste forms in experiments performed at 90 CC in pure water (after Hart et al. 2000). Power law curve fits illustrate the normal trends for these materials initial rapid decrease in leach rates followed by slower release rales, which decrease to 10"5 g/m2/d or less after time periods of several months to one year.

Leaching with Soxhlet Apparatus. A Soxhlet leaching apparatus has been used extensively( ) in radioactive waste measurements. The sample is contacted with continuously replenished distilled water. Leaching rates have been shown to be inhibited by ions previously leached from the solid material( ). This technique eliminates interference from ions in solution. 

Form of Reported Data. The mass of material leached can be determined either by any standard gravimetric method or, in the case of radioactive samples, by measurement of the radioactivity dissolved in the leaching medium. Since different ions are often leached at different rates, it is important to specify the ion when quoting leach rate results. To avoid confusion on this matter, it has been suggested( ) that long-term leaching results be reported as (fraction of A leached) (cm /g) day) , where A is the specific ion analyzed for. 

MCC-1 American Society for Testing and Materials [16] Evaluates long-term leaching rate of radioactive contaminants from high- and low-level radioactive waste forms... 

Samples. Three standard stone samples were obtained from the collection of the Ball State University Geology Department. The primary sample was a specimen of Salem Limestone (calcite, CaCOo), commonly known as Indiana Building Limestone, which was obtained in the Bloomington-Bedford area. Because of its popularity as a construction material, the Salem Limestone was the focus of this study. It was subdivided into three equal sections for the testing of leaching rates at pH=3.0, 4.0, and 5.6. Henceforth, these three samples will be labeled Salem 3.0, Salem 4.0, and Salem 5.6. The somewhat porous crystalline face of each surface was cut smooth and square, but not polished. 

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... 

The flat plate test determines the leaching rate from a defined surface where mass transfer across a solid/Hquid boundary controls the leaching or flux rate (expressed in mg/cm h). The fiat plate test focuses on release by diffusion from surface pavement construction materials in a simulated on-site experiment. The test material is formed into a 10 cm diameter disk using the Marshall... 

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