Evaluation of Speciation Technology - OECD Nuclear Energy Agency

Introduction
Appraisal of actinide chemical reactions in a dynamic natural aquatic system requires direct
speciation methods that allow the characterisation of their physical and chemical states without
perturbation. The solubility of actinides under natural aquatic conditions is sparingly low in the
absence of strongly complexing ligands. As a result, the geochemical behaviour of aquatic actinide
ions is constrained by nano-mole or subnano-mole scale reactions. The appraisal of such chemical
reactions demands, therefore, non-conventional speciation methods that are operational without
perturbation or with minimal perturbation of the system under investigation.
Mobile and immobile actinide species possibly present in aquatic systems [1] are schematically
illustrated in Figure 1.
Figure 1. Mobile and immobile actinide species in natural aquatic system
Ionic species
Colloid-borne species
M m+
M x (OH) y
(mx-y)+
M
O
O
M x (OH) y L z
(mx-y-pz)+
H O
H O
M O
Colloid
M m+
OML
H
O
Surface sorbed species
M
O
M
O O
L
M
O
H
O
H
O
M m+
M m+
Solid solution
species
Ionic actinide species present in aqueous phase are hydrated metal ions, hydrolysed ions and
binary or ternary complexes. Immobile species are present as pure homogeneous solids, as surface
sorbed species or as species incorporated into mineral phases. Additionally, actinides may form
“real-colloids” by homogeneous nucleation of hydrolysed species or may be sorbed onto natural
colloids in groundwater and thus present as “pseudo-colloids” [2]. The migration behaviour of ionic
and colloid-borne species depends on the extent of their interactions with available solid matrices.
Spectroscopic speciation of actinides in various physical and chemical states, as shown in Figure 1,
can be performed without perturbation of a given system. Absorption spectroscopy on the transitions
of the 5f-electron shell is a useful speciation method for the characterisation of oxidation and
complexation states. The spectroscopic resolution of actinide transition bands is in general very high
but their intensity is relatively weak (molar absorbance < 500 L/mole cm) [3]. Therefore, the
application of conventional absorption spectroscopy to the speciation of actinide ions under natural
conditions is much limited, because of the poor spectroscopic sensitivity. The low solubility of
actinides under given conditions (usually below 10 -6 mole/L) necessitates a speciation method of high
sensitivity.
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