Radionuclide speciation waters

It is clear from previous comments that radionuclide speciation studies must consider not only oxidation states and specific inorganic forms but also complex species arising through association with natural organic matter and the possibility of different physical states. The relative importance of these various physicochemical forms will, in practice, be dictated by a combination of the basic elemental characteristics of the radionuclide and the type of environment into which it is placed. Thus in seawater, colloidal organic complex species are likely to be far less dominant than for the same radionuclide in a very low ionic strength freshwater. In the case of soil and sediment interstitial waters or groundwaters,... 

Keywords Trace elements Radionuclides Environment Water Soil Aerosol Plant Neutron activation analysis Atomic absorption spectrometry Inductively coupled plasma-atomic emission spectroscopy Inductively coupled plasma-mass spectrometry X-ray fluorescence Electrochemical methods Speciation... 

For the purposes of this study, five actinide elements are considered Am(lll), Np(V), Pu(V), Th(IV), and U(VI). All five of these radionuclides are an important part of the radionuclide inventory intended for Yucca Mountain (Kerrisk, 1985 Oversby, 1987). With the exception of Pu, all are considered to be strongly dominated by a single oxidation state in the oxidizing groundwaters assumed for Yucca Mountain. Although the redox chemistry of Pu is very complex, U.S. Department of Energy research on radionuclide speciation and solubility indicates that Pu(V) is the dominant oxidation state for Pu in water from wells J-13 and UE-25 p l (Nitsche et al., 1993 Triay et al., 1997). Plutonium also may be present in the +4 and +6 oxidation states, but only Pu(V) is considered here. 

The presence of relatively high levels of strong organic chelators like EDTA in the water samples prompted a detailed chemical speciation study aimed at determining whether the organic compounds identified in the survey study are chelated or complexed to radionuclides. Water samples from waste trenches 19S and 27 and inert atmosphere wells WIN and W2NA were fractionated by steric exclusion chromatography and subsequently analyzed for their... 

Part II considers speciation in specific compartments of the environment viz. the atmosphere, biological systems, soils, sediments and natural waters, and with particular aspects of the speciation of environmentally important radionuclides. Two new chapters have been added to make the coverage even more comprehensive. These new chapters are Chapter 10, Chemical Speciation in Soib and Related Materials by Selective Chemical Extraction by the editors, and Chapter 12, Speciation in Seawater by R.H. Byrne of the University of South Florida. 

Computer simulation is now used extensively as a tool to help to understand and predict the transport of radionuclides through environmental systems. Most models relate to waste disposal and are based on measured parameters such as water movements, salinity, suspended load and the radionuclide concentration in the solute, suspended particulate matter and bottom deposits. Comparatively few attempts appear to have been made to include chemical speciation into this type of model, presumably because of the added complexity involved. Some modellers have attempted to take into account the characteristics of the major chemical phases such as those present in different particles or coatings (e.g. Martinez-Aquirre et al., 1994). Others have noted the importance of including details of particular chemical species present in industrial waste releases when constructing models to predict dispersion (Abril and Fraga, 1996). 

Fig. 7-2. Summary of environmental pathways by which terrestrial plants may become contaminated with radionuclides. In the case of an input from atmosphere, or as a result of the process of resuspension , any external radionuclide burden may be reduced by field loss mechanisms conversely, an initially external radionuclide deposit (Rat) may become internalised (i int) following foliar absorption and translocation. Radioactive contaminants of soils may be derived either from atmospheric inputs or from seepage in ground waters. Partitioning of radionuclides in soil—soil water systems controls their availability for root absorption, which normally occurs exclusively from the liquid phase. The chemical speciation of the nuclide in this phase, however, provides a further control on bioavailability which is highly radionuclide specific.

Solubility and speciation. Minimum requirements for reliable thermodynamic solubility studies include (i) solution equilibrium conditions (ii) effective and complete phase separation (iii) well-defined solid phases and (iv) knowledge of the speciation/oxidation state of the soluble species at equilibrium. Ideally, radionuclide solubilities should be measured in both oversaturation experiments, in which radionuclides are added to a solution untU a solid precipitates, and undersaturation experiments, in which a radionuchde solid is dissolved in aqueous media. Due to the difference in solubilities of crystalline versus amorphous solids and different kinetics of dissolution, precipitation, and recrystalhzation, the results of these two types of experiments rarely agree. In some experiments, the maximum concentrahon of the radionuchde source term in specific water is of interest, so the sohd that is used may be SF or nuclear waste glass rather than a pure radionuclide solid phase. 

A variety of methods have been used to characterize the solubility-limiting radionuclide solids and the nature of sorbed species at the solid/water interface in experimental studies. Electron microscopy and standard X-ray diffraction techniques can be used to identify some of the solids from precipitation experiments. X-ray absorption spectroscopy (XAS) can be used to obtain structural information on solids and is particularly useful for investigating noncrystalline and polymeric actinide compounds that cannot be characterized by X-ray diffraction analysis (Silva and Nitsche, 1995). X-ray absorption near edge spectroscopy (XANES) can provide information about the oxidation state and local structure of actinides in solution, solids, or at the solution/ solid interface. For example, Bertsch et al. (1994) used this technique to investigate uranium speciation in soils and sediments at uranium processing facilities. Many of the surface spectroscopic techniques have been reviewed recently by Bertsch and Hunter (2001) and Brown et al. (1999). Specihc recent applications of the spectroscopic techniques to radionuclides are described by Runde et al. (2002b). Rai and co-workers have carried out a number of experimental studies of the solubility and speciation of plutonium, neptunium, americium, and uranium that illustrate combinations of various solution and spectroscopic techniques (Rai et al, 1980, 1997, 1998 Felmy et al, 1989, 1990 Xia et al., 2001). 

Surface Complex Formation, Ion Exchange, and Transport in Ground-water and Soil Systems The retardation equation can also be applied to inorganic soluble substances (ions, radionuclides, metals). But here we have to consider, in addition to the sorption or ion exchange process, that the speciation of metal ions or ligands in a multicomponent system influences the specific sorption process and varies during the pollutant transport in the groundwater chemistry then becomes an important part of the transport. 

In addition to the solution speciation of actinides, another critical aspect of the risk assessment of nuclear waste repositories is the determination of upper limits of radionuclide concentrations for very site-specific conditions (Nitsche et al., 1992). Solubility studies provide data that are used to calculate these upper-limit concentrations and enable a conservative predictive approach. We studied the solid phases containing U, Np, and Pu that precipitate out of aqueous solutions of compositions mimicking waters surrounding the YM and WIPP sites. 

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