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Radionuclide Speciation ProjectThe natural-matrix SRM program has been successful in providing materials that are used to evaluate radiochemical-measurement techniques. Traditionally, measurements of environmental contamination for site assessments have focused on the determination of total radionuclide concentrations. It is clear, however that total concentration does not adequately describe the environmental behavior or bioavailability of radiological contaminants. Rather, the time dependent spread of contaminating radionuclides is a function of their geochemical "partitioning" or "speciation" within the soil or sediment matrix.In the United States and many other parts of the world, there remains an enormous task ahead for the remediation of radiologically contaminated areas and monitoring the impact of residual anthropogenic radioactivity on the environment. The U.S. currently faces contamination plumes of greater than 2 million cubic meters of groundwater and greater than 150 million cubic meters of contaminated soil. The development of strategies for remediation, restoration, and mitigation of radiologically contaminated areas is necessarily constrained by budgetary concerns. Therefore, for long-term risk assessment analyses, regulatory bodies need information which includes an evaluation of the mobility and bioavailability of radionuclides. The concept of macro-and micro-nutrient metal fractionation or partitioning has a long history. Soil science as a discipline actually began as early as the seventeenth and eighteenth centuries. These early studies were primarily concerned with soil productivity for agricultural purposes. One of the important findings of the time, by German physical chemist Julius von Liebig was that crop yields were directly related to the mineral content of the soil. Although this finding essentially revolutionized soil science and agriculture, it was not the complete story. The flaw in Liebig's theory, as discovered by J.H. Gilbert, J.B. Lawes, and others, was that it was not the total mineral content which governed the productivity of soils, but rather the chemical form of the nutrient that determined the "availability" of the nutrient to plants. This realization led to the modern concept of the "fractionation" or "partitioning" of metal associations and behavior in soils. Just as this concept has been applied to the study of nutrients in agricultural science, the concept has been applied to the study of radionuclide partitioning in soils and sediments.
The practical (time, effort, simplicity) technique of choice for evaluating
low-level radionuclide partioning in soils and sediments is the sequential
extraction approach. This methodology applies operationally-defined chemical
treatments to selectively dissolve specific classes of macro-scale soil or
sediment components. There are many variations on the technique. However,
certain fundamental practices are applied generally. The soil or sediment
sample is subjected to a series of chemical treatments, each designed to
selectively dissolve a specific geochemical fraction of the sample,
see Fig. 1 (e.g., exchangeable ions, carbonate
minerals, reducible oxides of iron and manganese, organic matter, and finally
total dissolution of the residual fraction). Each chemical treatment consists
of a reaction period, during which the sample is reacted with the reagent
designed to attack the fraction of interest. The reactions are conducted in a
sequence that optimizes the dissolution of the target phase, while minimizing
the attack on non-targeted phases. Following each reaction, the solid and
aqueous phases are separated by centrifugation and/or filtration. Radiometric
analyses are combined with the results of stable element measurements to
establish the primary geochemical radionuclide associations in the sample. The
results provide more complete information for decision making in remediation,
mitigation and monitoring of radiologically-impacted areas. 
Figure 1. Solid/Aqueous Phase Environment is Complex Although sequential extraction techniques are used extensively to evaluate radionuclide partitioning, there is currently no standard method or protocol that can be used as a reference point for this research field. The radioactivity group has undertaken the development of a standard methodology to fill this need. Optimizing the Standard Sequential Extraction ProtocolThe Full-Factorial Experimental Design
The experimental plan is based upon the principles for maximizing information,
while minimizing the number of individual experiments, presented in
Statistics for Experimenters [Box, Hunter, and Hunter, John Wiley &
Sons, Inc., New York, 1978]. The purpose of these experiments is to select
(from predetermined alternatives) the time, temperature, and extractant
concentration which produce the maximum fractional release of radionuclides,
coupled with a minimum of dissolution of non-targeted geochemical phases. The
specificity of the extractions is monitored by measuring the co-release of
stable elements Al, Ba, Ca, Fe, K, Mn, Pb, Sr, and Ti. The extraction
efficiency is determined at two extreme values for each variable and a midpoint
experiment is included to identify any curvature in the relationship. The
resulting full-factorial design provides for an estimation of the effect
(change in the % extracted radionuclide or stable metal), as the independent
variables (time, temperature, and reagent concentration) are varied from a low
to a high value. The resultant data is depicted as a three dimensional figure,
with time temperature and concentration in the x, y, and z axes, and the
mid-point experiment at the center. Replication is performed at tetrahedral
points and the midpoint on the cube to estimate the reproducibility of the
extractions.
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