Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 1, pp. 1-5, 2005

ArticlesActinide Science: Fundamental and Environmental Aspects
Gregory R. Choppin*
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306-4390, USA
Received: November 15, 2004; In Final Form: November 15, 2004
Nuclear test explosions and reactor wastes have deposited an estimated 16 × 1015 Bq of plutonium into the world’s
aquatic systems. However, plutonium concentration in open ocean waters is orders of magnitude less, indicatingthat most of the plutonium is quite insoluble in marine waters and has been incorporated into sediments. Actinide
ions in waters often are not in a state of thermodynamic equilibrium and their solubility and migration behavior isrelated to the form in which the nuclides were introduced into the aquatic system. Actinide solubility depends onsuch factors as pH(hydrolysis), EH(oxidation state), reaction with complexants (e.g. carbonate, phosphate, humic
acid, etc.), sorption to surfaces of minerals and/or colloids, etc., in the water. The most significant of these variables is the oxidation state of the metal ion. The simultaneous presence of more than one oxidation state for someactinides (e.g. plutonium) in a solution complicates actinide environmental behavior. Both Np(V)O 2 + andPu(V)O2+, the most significant soluble states in natural oxic waters are relatively noncomplexing and resistant tohydrolysis and subsequent precipitation but can undergo reduction to the Pu(IV) oxidation state with its different
elemental behavior. The solubility of NpO2+ can be as high as 10−4 M while that of PuO2+ is more limited by
reduction to the insoluble tetravalent species, Pu(OH)4, (pKSP~56). The net solubility of hexavalent UO22+ in sea
water is also limited by hydrolysis; however, it has a relatively high concentration due to formation of carbonate
complexes. The insoluble trivalent americium hydroxocarbonate, Am(CO)3(OH), is the limiting species for thesolubility of Am(III) in sea water. Thorium is found exclusively as the tetravalent species and its solubility islimited by the formation of quite insoluble Th(OH)4. The chemistry of actinide ions in the environment isreviewed to show the spectrum of reactions that can occur in natural waters which must be considered in assessing
the environmental behavior of actinides. While much is understood about sorption of actinides on surfaces, themode of migration of actinides in such waters and the potential effects of these radioactive specials on marine
biota, much more information is needed for a satisfactory understanding of the behavior of the actinides in theenvironment.

1. Introduction
Since 1945, over 2 × 10 20 Bq of radioactivity have beenreleased into the atmosphere due to nuclear weapons tests.
Controlled and uncontrolled releases of radioactivity from
nuclear reactor systems amount to a total of 5.3 × 1018 Bq (ca.
2.5% of that from tests). The large majority of the total releasehas been deposited in the oceans of the world. Plutonium
concentration in surface ocean waters is on the order of 10−5
Bq/kg, indicating that most of the 16 × 1015 Bq of plutonium
estimated to be deposited by nuclear weapons testing has
already been incorporated in sea bottom sediments. This isconsistent with the estimate that of the 8 × 1014 Bq of plutonium discharged into the Irish Sea from controlled releases of
low level waste from the nuclear fuel reprocessing plant at
Sellafield, U.K., only about 7% has moved into the Atlantic
ocean. These data have led to studies on the nature of actinide
sorption, the mode of migration, and the potential effects ofthese radionuclides on marine biota.
2. Complexation
The actinides form strongly ionic bonds. Such cations areclassified as “hard” acids, and interact preferentially with“hard” base complexants containing oxygen and fluorine
donors (including the oxygen atoms of water molecules).
Binding in aqueous media to softer donor atoms, such as
nitrogen, sulfur, etc., occurs via chelation with multidentate*Corresponding author. E-mail: choppin@chemmail.chem.fsu.edu.
FAX: +1-850-576-5920.

ligands, which contain bonds to both the soft donor and hard
oxygen donor.
In ionic bonds, the bond strength increases with the effective charge density of the metal, the order of increasing
complexation strength of actinide ions is:Na+ « Ca2+ < AnO2+ < An3+ < AnO22+ < An4+

(1)This trend reflects the experimental evaluation of the effective charge density of NpO2+ as + 2.2 and of UO22+ as + 3.3.1
Electrostatic interaction and steric interference are theprimary factors in the structure of the complexes and in thenumber of water molecules bound in the first coordination
sphere of the actinide cation. Observed total coordination
numbers in the solid state range from 6 to 12 for simple
actinide complexes and from 2 to 8 for dioxo actinide (V) and(VI) compounds. The similarities between lanthanide andactinide aminocarboxylate complexes (formation constants,
bonding enthalpies and number of inner sphere waters)2 andthe data for lanthanide complexes in solution indicate that theanalogous actinide compounds should have coordination
numbers of 8 or 9 for trivalent, An(III),3 7 or 8 for tetravalent,An(IV),4 4 for pentavalent, An(V)O2+, and 6 for haxavalent,An(VI)O22+ cations.
Actinide cations form inner sphere complexes (anion andcation in direct contact) with strongly negative anions such as
humics, carbonate, bicarbonate, sulfate, fluoride, and mostcarboxylate ligands. Conversely, weaker, outer spherecomplexes (anion and cation separated by bridging watermolecule) form for actinides with weakly basic ligands such as
chloride, bromide, iodide, and perchlorate ions.

© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 07/31/2005

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ChoppinJ. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

3. Actinide Oxidation States

4. Environmental Species

The variety of oxidation states, Table 1, of the actinide
cations, and the simultaneous presence in solution of morethan one of these states complicate actinide behavior in solution amd makes it difficult to predict the behavior withchanges in EH, pH, ligand concentrations, etc. For some of theactinides of environmental interest, (e.g. Am(III), Cm(III),
Th(IV)) only one oxidation state is formed. Uranium is in thetetravalent state in reducing waters and in the hexavalent statein oxidizing waters. In oxic waters, neptunium is found in the
pentavalent state as NpO 2 + unless the solution is strongly
oxidizing when NpO22+ forms.5 Plutonium may exist in several
oxidation states, from IV through VI, in solution. Trivalent
plutonium is present only in anoxic or strongly acidic waters.
The dominant soluble specie of plutonium in natural aerobic
waters is PuO2+.6−10 Reduction of PuO2+ to Pu(IV) due to theinsolubility of Pu(OH)4 is a major factor in plutonium behavior
in environmental waters. Under reducing conditions and in thepresence of enhanced levels of organics, Pu(III) is the dominant state and is often complexed to humics.11−15 In surface
ocean waters both Pu(V)O 2 + and Pu 4 + forms have beenreported. 16, 17 However, the concentration of “dissolved”
Pu(IV) measured is very dependent on the size of the filters
used, indicating that most of the reported “dissolved” Pu(IV)in natural waters is in colloidal form.

The environmental behavior of the actinide species isdependent on whether the actinides are injected in the environment as source-dependent or source-independent species.16a
The oxides of uranium and plutonium formed in the very hightemperature weapon explosions are highly refractory oxides
which do not react with the surrounding media but remain in
the forms in which they are injected into the environment,
usually as solid particles, for long periods of time. Over thisperiod of time, these refractory “source specific” species are
modified to become “source independent” species whose
behavior is dependent on the conditions of the ecosystem inwhich they exist. Source independent species not only originate from the release of nuclides which are originally sourcedependent but, also, include actinides and other radionuclides
released into natural waters from nuclear processing plants,
laboratory research, etc. which are never in a refractory form.
The kinetics of these source independent species are sufficiently rapid that either an equilibrium, or, a steady statemodel, can describe their behavior over time. Development ofa useful geochemical database for prediction of the behavior
for such source-independent actinide species has been a major
goal in research on actinide environmental behavior.

TABLE 1: Available Actinide Oxidation States* in Sea Water,
pH = 8.2, I = 0.7m

Actinide solubility depends on such factors as pH, EH, reaction with complexants (e.g. carbonate, phosphate, humic acid,etc.), sorption to surfaces of minerals and/or colloids, etc., in
the water.18−20 Table 2 lists some techniques for oxidation statemeasurements and their sensitivities. Actinide cations havestrong ion–dipole interaction with water molecules and bindingof water of hydration extends beyond the first hydration
sphere. The total hydration is dependent on the effective
charge of the actinide cation and the number of coordinationsites not occupied by ligands.

Actinide

Oxidation State

Th

IV

U

IV, VINp

IV, V, VI

Pu

III, IV, V, VI

Am

III

Cm

III

*The most common oxidation states denoted in bold

The concentrations of the different oxidation states ofactinides are dependent on pH, E H , and the presence ofsuspended particulates. Each actinide oxidation state has
different chemical behavior, complicating prediction of thegeochemical behavior. In general, increasing pH and/or EH
favors higher oxidation states. Also, the plutonium redox
behavior may be affected by photolytic peroxide formation atorganic and inorganic surfaces.4 The reduction potentials of U,
Np and Pu at pH 0, 8, and 14 are presented in Figure 1.

5. Solubility, Hydrolysis and Carbonate Complexation

TABLE 2: Approximate Detection Limits of Actinides in
Oxidation State Speciation
TechniqueApproximate
Detection Limits

Nuclear Magnetic Resonance10−1 – 10−4 M

X-ray Absorption Spectroscopy

10−1 – 10−4 M

Visible Absorption Spectroscopy≥ 10−5 M

Laser-Induced Photoacoustic Spectroscopy

10−5 – 10−9 M

Laser-Induced Thermal Lensing Spectroscopy

10−5 – 10−9 M

Time-Resolved Laser Fluorescence Spectroscopy 10−5 – 10−9 M
Techniques to determine oxidation state of actinides and theirconcentration sensitivities.

Figure 1. Redox potentials of U, Np and Pu at pH = 0, 8, and 14.

In carbonate-free environments, Am(OH)2+ and Am(OH)2+
form the major species of Am(III) at pH 8.2, in carbonate-richwaters, Am(CO3)+, Am(CO3)2−, and Am(OH)2+ are significant
components. Because of the strong sorption characteristics ofthe hydroxyl species, americium is sorbed onto colloids, sediments and humics. The OECD–NEA21 has reviewed americium hydrolysis while general actinide hydrolysis has beenreviewed by Fuger22 and Rizkalla and Choppin.23 The speciation of soluble Am(III) in sea water is given in Figure 2.
Hydrogen peroxide has been shown to be a reducing agent
in sea and natural waters for the higher oxidation states of
plutonium.24 The reduction of plutonium in natural waters canresult from the photolytic formation of peroxide from theorganics present. Tracer level Pu(VI) in organic-free synthetic
sea water at pH 8 has been shown to be reduced to Pu(V) inJ. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

Actinide Science: Fundamental and Environmental Aspects

Figure 2. Speciation of Am(III) in sea water as a function of pH.

3

Figure 4. Speciation of U(VI) O22+ as a function of pH in oxic waters.

account for 90 – 100% of the uranium dissolved in ocean
waters. 33 Uranyl carbonate complexes exist as UO 2 CO 3 ,UO2(CO3)22−, and (UO2)(CO3)34− and are discussed in the literature.32, 34 Kinetics of exchange and activation parameters havebeen studied by 13C NMR.35, 36 Figure 4 presents the speciationof UO22+ as a function of pH.
6. Role of Organics

Figure 3. Pattern of hydrolysis of Pu(IV) as a function of pH in oxic
waters.

only a few minutes. Reduction of Pu(V) has a slower rate dueto the weak tendency of pentavalent plutonium to complex orreact with other components of the solution.25
Thorium(IV) is capable of forming both mono-and polymeric hydrolysis products. In dilute solution (≤ 10−4 M), as the
pH increases above 2, the species, Th(OH)3+, Th(OH)3+ and, at
higher pH, Th(OH)4 were identified.26, 27 An(IV) cations do notform carbonate complexes in significant levels as hydrolysis isa stronger dominant reaction, leading to the very insoluble
An(OH)4 species. Figure 3 shows the pattern of hydrolysis ofPu(IV) as a function of pH.
The pentavalent cation AnO2+ comprises ca. 95% of the total
concentration of soluble neptunium and plutonium in solutions
with carbonate concentrations ≤10 −4 M. The solubility of
NpO 2 + may be as high as 10 −4 M, 28 while the solubility of
PuO2+, 10−8 – 10−6 M, is limited by the formation of the insoluble tetravalent species, Pu(OH)4, (pKSP ~56). Pentavalent
neptunium hydrolyzes in solutions with pH > 9. In waters
where [CO 3 2− ] ≥ 10 −4 M, NpO 2 (CO 3 ) − , NpO 2 (CO 3 ) 2 3− , andNpO2(CO3)35− species may be present. For more complete dataof neptunium carbonate complexation, see References 29− 31.
The stability of NpO 2 + makes it very useful as a chemical
analogue for PuO2+. The redox potential of Pu(V) – Pu(IV)and the strong hydrolysis of Pu (IV) limits the concentration of
PuO2+ in marine and natural waters.
Study of uranyl (VI) hydrolysis is complicated by theformation of polymeric species. In concentrations below 10−6
M, UO2(OH)+ is the dominant hydrolysis species in solution
while above this concentration, polymeric forms such as(UO 2 ) 2 (OH) 2 2 + , (UO 2 ) 3 (OH) 4 2 + and (UO 2 ) 3 (OH) 2 4 + and(UO2)3(OH)5+ form.32 Carbonate complexes are estimated to

An important factor in actinide redox and complexation
environmental behavior is the effects of organics in aquatic
systems. Of particular interest is the role of humic acids sincethese are ubiquitous in natural waters, including sea water.
The size of humics in sea water and other natural waters
normally falls in the range of colloidal particles. As thedissolved organic carbon content increases the concentration of
plutonium in the aqueous phase usually decreases due toreduction of PuO2 + to hydrolyzed, insoluble Pu(IV) by thehumics. Addition of An(V)O2+ and An(VI)O22+ to solutions ofhumic acids has shown that the humics reduce the plutonium
fairly rapidly to the tetravalent state at which point hydrolysis
occurs.37 The hydrolytic product may precipitate or sorb tosolids or to colloidal species. Colloids in sea water generally
are usually carbonate or oxide mineral systems; however, inmany cases, they have a surface coating of humic acid (pseudocolloids). Colloids of this latter type sorb the hydrolyzed
species even more strongly than simpler colloid systems.
To summarize the role of humics, humic interactions withactinides include redox activity, coulombic attraction due tothe net anionic charge of the humic (condensation binding),and direct site-specific binding to the humics. The existenceof these two types of binding, accompanied by the different
dissociative kinetic rates requires that predictive models for
actinides in marine and natural waters incorporate both kinetic
and thermodynamic behavior. The source specific nature ofhumics complicates significantly the characterization ofactinide-humic interactions.
7. Sorption
Since carbonate anions form strong complexes withactinides in solution, carbonate minerals can be expected toprovide excellent surfaces for actinide sorption. The importantrole of sorption in actinide immobilization and transport has
been discussed by many authors.38−40 As actinide sorption is
influenced by the charge of the cations as well as by stericeffects, sorption strength follows the same trend as complexation. In neutral and basic media, actinides interact withhydroxyl moieties of polysilicic acid, hydrated iron oxides, aswell as the surface hydroxyl groups of colloidal materials.
Particles may have net positive or negative surface charges

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J. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

Figure 5. Schematic of behavior of plutonium in natural waters.

depending on the pH. For a more complete discussion of
surface charge models the reviews of Silva and Nitsche21 andStumm and Morgan41 are recommended.
Am(OH)3 sorbs readily to many types of surface. Am(III)sorption42 onto calcite and aragonite surfaces is both rapid andextensive, and less than 1% could be desorbed readily.
Americium(III) is more extensively sorbed onto aragonite
compared to calcite, although both minerals consist of CaCO3.
The preference for aragonite was attributed to competition by
Mg2+ for aragonite binding sites, resulting in suppression of
Am sorption.
NpO2+ and UO22+ sorb very weakly onto colloids. KeeneyKennicutt and Morse 5 report that NpO 2+ sorbs onto marine
carbonate sediment irreversibly; however, NpO 2+ sorbs lessstrongly onto minerals other than carbonates37 and organics.43
Plutonium sorbs more extensively, by several orders ofmagnitude, in the (III) and (IV) oxidation states than in thehigher V and VI states.8 In the Baltic Sea, 99% of the total
plutonium inventory sorbs onto sediment as Pu(IV),19 consistent with observations that Pu sorbed onto coastal soils along
the Irish Sea exists exclusively as Pu(IV).44 Because of thestrong affinity of Pu(OH)4 for sediments and suspended particulates, Pu(IV) sorption is the limiting factor in Pu transport
processes.4 Figure 5 summarizes the behavior of plutonium innatural waters.
8. Summary
The chemical state of actinide elements released to the environment, combined with the complexity and diversity of theirenvironment behavior, creates difficulties in modeling their
behavior over the lifetime of their radioactivity. Some of thatdiversity and the associated problems in assessing their
behavior over long periods have been addressed in this paper.
Much progress has been made in our understanding of theenvironmental behavior of actinides but much remains a challenge to understand in order to predict their future safety inrepositories.
Preparation of this paper was done under a grant from the
USDOE-OBES-Division of Chemical Science to Florida State
University.
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