Speciation of Radionuclides in the Environment

Abstract

To obtain information on physicochemical forms of radionuclides in the environment, speciation techniques should be applied in situ, at‐site or in laboratories shortly after sample collection. Among speciation techniques for radionuclides in waters, combined in situ ultrafiltration and exchange chromatography are most promising as radionuclides species are fractionated with respect to nominal molecular mass and change properties simultaneously. However, the fractionation and characterization of chemically well‐defined low molecular mass (LMM) species in waters is still a challenge. Surface analytical techniques (e.g. electron microscopy) are useful for characterizing colloids and particles with respect to structure and radionuclide distribution. Recent development within synchrotron‐based X‐ray microbeam techniques allows detailed information of crystallographic structure (micro‐X‐ray diffraction (μ‐XRD)), oxidation states (micro‐X‐ray absorption near‐edge spectroscopy (μ‐XANES)) and volume distribution (μ‐XANES tomography) of particle‐associated radionuclides on the micrometer scale. Information on radionuclides reversibly or irreversibly associated with solid surfaces (binding mechanisms) is attained by different extraction techniques, while dynamic tracer experiments can provide information on the kinetics involved.

Speciation of radionuclides – analytical challenges within environmental impact and risk assessments

“…To separate particles and colloids in waters, filtration (0.45 mm) and ultrafiltration systems (tangential flow systems or hollow fibres) having cut-off levels in the nanometer to micrometer range are beneficial. For LMM species in water, in situ ultrafiltration systems interfaced with ion chromatography are recommended, allowing species to be fractionated according to nominal molecular mass and charge properties simultaneously (Salbu, 2000).…”

“…Among solid-state speciation techniques, scanning or transmission electron microscopy (SEM and TEM) as well as X-ray spectroscopy (m-XAS, m-XRF, m-XRDs, m-XANES) utilising synchrotron X-ray radiation have proved most useful in providing such information (Salbu et al, 1998(Salbu et al, , 2003. Table 1 Applicable fractionation techniques for radionuclide species (Salbu, 2000) Size Information on particle size distributions and morphology (structures) can easily be obtained using SEM and TEM, where TEM can be applied for species in the submicron size range. Using the backscattered electron mode (BEI), the distribution of high atomic elements on particle surfaces is reflected as bright, localized areas.…”

“…Furthermore, impact and risk assessments utilising K d , T c , T agg and CR data, often derived from the literature, are generally not taking the speciation of radionuclides into account in their (Salbu, 2000).…”

Fractionation of radionuclide species in the environment

“…However, IUPAC 2000 differentiates between fractionation techniques and speciation techniques, where fractionation is defined as ''the process of classification of an analyte or a group of analytes from a certain sample according to physical (e.g., size, solubility) or chemical (e.g., bonding, reactivity) properties''. As the activity concentrations for most radionuclide species in the environment are too low for any species-specific methods to be applied, fractionation techniques are needed and fractions must be concentrated prior to measurements (Salbu, 2000). Thus, speciation analysis should be defined as the analytical activity of fractionating, isolating, identifying and quantifying one or more individual radionuclide species in a sample, and should include in situ, at site, online, in laboratory fractionation techniques applied prior to analysis.…”

A novel approach to the sequential extraction of plutonium from oxic and anoxic sediment using sodium citrate to inhibit post-extraction resorption

“…Although significant advances have been made in the direct determination of the solid phase speciation of radionuclides (e.g., secondary ion mass spectrometry, X-ray diffraction, scanning electron microscopy with energy dispersive microanalysis, EXAFS, XANES), the task is often hampered by the low concentrations found in most contaminated sediments (Salbu, 2001). An alternative, (indirect) approach is sequential extraction analysis, which can provide important information on the chemical and physical conditions that govern the remobilisation of radionuclides in natural water systems.…”