Full text of DOI:10.1557/mrs2001.181

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mediation and cleanup of contaminated
sites, (2) predict the transport behavior,
(
3) carry out performance-assessment
studies to determine if the repositories can
safely contain HLW, and (4) engineer
methods to retard their release and migra-
tion rates. To make this task even more
challenging, predictive modeling has to
cover time frames of up to 10,000 years,
according to regulatory requirements in
certain countries.
E
nvironmental
Actinide Science
Robert J. Silva and Heino Nitsche
Plutonium can migrate in the environ-
ment via aqueous media, such as ground-
water and surface, river, lake, and seawater,
and then become fixed as a precipitate, col-
loid, or mineral-adsorbed species. Models pre-
dicting the hydrological transport through
the environment require as input a pluto-
nium concentration, the true amount that is
actually available for transport. It is defined
as the plutonium source term and not as
true solubility, because it may be a combi-
nation of dissolved and colloidal material.
Plutonium can undergo a variety of com-
plex chemical reactions in the environ-
ment. In addition to the formation of solid
precipitates, colloids, and dissolved solu-
tion species common to aqueous systems,
Pu ions can interact with the surrounding
geomedia to change oxidation states or
sorb on surfaces and colloids. Four im-
portant processes that can control the
amounts and forms of the plutonium in
solution are (1) precipitation, (2) complexa-
tion, (3) sorption, and (4) colloid formation.
These processes control the overall chemi-
cal behavior of Pu in the environment and
determine how much and how rapidly
plutonium will move around in environ-
mental systems. All four processes must
be considered if one wants to understand
and predict the chemical behavior and
transport properties of plutonium in the
environment. Detailed knowledge of the
plutonium species and charges is essential
because the nature and extent of precipita-
tion, complexation, sorption, and colloid
formation are strongly dependent on them.
In addition, because these processes are
not independent of one another, they must
be considered simultaneously.
This article is a short qualitative summary
duced and are stored in 149 single-shell
4,5
of a chapter in the forthcoming book Advances
in Plutonium Chemistry: 1967–2000, edited
by Darleane Hoffman for the Amarillo National
Research Center. Readers are referred to the
chapter for more technical details.
and 28 double-shell steel tanks. As much
3
as 3800 m of radioactive contaminated
liquid were released into the environment.
During the earlier times of operation, some
high-level wastes (HLW) and other contami-
nated liquids were discharged directly into
the ground through trenches and shallow
subsurface drainage systems, so-called cribs.
The released volume of more than one bil-
Introduction
Considerable progress has been made
in the study of environmental plutonium
science in the last 30-plus years, driven to
a large extent by concerns about the re-
lease and migration of large amounts of
plutonium into the accessible geosphere.
Plutonium has been introduced into the
environment through several pathways.
Environmental contamination has been
caused by nuclear-weapons production
and testing, nuclear-reactor accidents, and
accidents during the transport of nuclear
weapons. Above-ground testing of more
than 420 nuclear weapons has produced
large amounts of radionuclides through
fission and neutron activation products.
More than three metric tons of plutonium
have been distributed on the earth’s sur-
3
2
lion m contaminated around 500 km with
radionuclides and toxic chemicals. At
Oak Ridge National Laboratory (ORNL),
3
160,000 m of liquid waste containing fis-
sion products and transuranium elements
were directly discharged into the ground,
thus creating a radioactive burden of about
44.4 TBq (1.2 kCi). In the whole U.S.
weapons complex, 5700 contaminated
groundwater plumes exist, and estimates
for the volume of contaminated soil range
3
from 73 million to 200 million m .
At Sellafield, the British nuclear-fuel re-
processing plant, between 1950 and 1992,
discharges were made into the Irish Sea of
238
120 TBq (3.2 kCi) of Pu and 600 TBq
1
239,240
face by global fallout. For example, the
(16.2 kCi) of
Pu. As of 1977, 280 TBq
Pu were strongly sorbed
239,240
MAYAK plutonium production complex
in the former Soviet Union is located in
the southern Urals, about 70 km north of
Chelyabinsk and 15 km east of Kyshtym.
Between 1949 and 1951, about 76 million
(7.6 kCi) of
6,7
to the seabed sediments.
Proposed disposal of radioactive HLW
and direct disposal of spent fuel in deep
underground geologic repositories, as well
as the storage and disposal of plutonium
from nuclear-weapons dismantlement
(it has been estimated that in excess of
250 metric tons of plutonium, primarily
3
m of liquid radioactive waste with a total
activity of 100 PBq (2.7 MCi) were dis-
charged into the Techa River.2
,3
The reactor explosions in 1986 in Cher-
2
239
8
nobyl released 23 kg of plutonium iso-
Pu, is stored as nuclear weapons ), are
topes totaling 6.5 PBq (0.18 MCi). Within a
further potential sources for plutonium
releases into the environment. Although
much of the radioactivity of the waste is
caused by the relatively short-lived iso-
Oxidation States
239,240
3
0-mi radius, the concentration of
Pu
The single most important property of
plutonium is its oxidation state because
the processes of precipitation, complexa-
tion, sorption, and colloid formation differ
considerably from one oxidation state to
another. Plutonium is known to exhibit9
several oxidation states, ꢁ3 through ꢁ7,
depending on solution conditions, and
because of similar redox potentials con-
necting the various oxidation states, it is
possible for two or more to coexist in so-
2
2
is between 3.7 GBq/km and 185 GBq/km
2
(
0.1–5 Ci/km ). Large areas in Belorussia
90
137
and Ukraine were contaminated also.
Severe contaminations occurred in the
United States at sites within the nuclear-
weapons complex. The Hanford Site in
southeastern Washington State was the first
industrial-scale plutonium production site.
topes Sr (29 yr), Cs (30.2 yr), and other
fission products, a potential long-term
hazard comes from the following long-
238
lived plutonium isotopes: Pu (87.7 yr),
2
39
41
4
240
3
Pu (2.4 ꢀ 10 yr), Pu (6.4 ꢀ 10 yr), and
Pu (14.4 yr).
2
Between 1944 and 1981, approximately
It is essential to understand the behavior
3
2
20,000 m of liquid reprocessing waste,
of plutonium in the environment in order
to (1) develop efficient methods for the re-
10
totaling 11.8 EBq (320 MCi), were pro-
lution at one time. Water has an impor-
MRS BULLETIN/SEPTEMBER 2001
707

Environmental Actinide Science
tant oxidation–reduction chemistry that
puts limits on the oxidation states that plu-
tonium can exhibit in aqueous solutions.
mation about the nature of the solution
species. Absorption spectroscopy meth-
ods are considered to be one of the most
reliable techniques for the detection and
between 0.01 mM and 1 mM for pluto-
24
nium, depending on their molar absorp-
tivities, while the solubilities of plutonium
solids in aqueous solutions of near-neutral
pH are generally 2 or more orders of
magnitude less. With the advent of high-
powered pulsed lasers in recent years,
new laser-based techniques have been de-
veloped for measuring very weak sample
19,20
Identification of Oxidation States
The method to be used for the quantita-
tive determination of the oxidation states
of plutonium in aqueous solutions de-
pends on the concentration range of the
plutonium. For concentrations of Pu less
than micromolar, chemical separation
techniques offer the only avenues. Above
this concentration, spectroscopic tech-
niques may be used.
characterization of solution species
and have been widely used for the charac-
2
1,22
terization of actinides in solutions.
For
example, spectra are shown in Figure 1 for
3ꢁ
4ꢁ
ꢁ
2
2ꢁ 21
Pu , Pu , PuO , and PuO
.
The
2
25
major absorption peak wavelengths for
Pu in the oxidation states of environmen-
tal importance are given in Table II. In ad-
dition to the identification, the quantity of
the solution species can be obtained from
the measured absorption peaks through the
absorbance. These techniques—photo-
26
acoustic spectroscopy (PAS),2 thermal
deflection spectroscopy (TDS), 7 and ther-
28
mal lensing spectroscopy (TLS) —are re-
ferred to as photothermal spectroscopies
and are potentially 3 orders of magnitude
more sensitive than absorption spectros-
copy using conventional spectrometers,
while supplying the same information.29
Coprecipitation, selective sorption, liquid
chromatography, and solvent-extraction
techniques have been used with varying
degrees of success to isolate and deter-
mine the oxidation states of Pu in aqueous
systems and have been discussed in the
23
use of Beer’s law.
Unfortunately, conventional absorption
spectrometers have detection limits of
11,12
literature.
Of these, solvent-extraction
Table I: Chemical Separation Methods for DeterminingTrace Concentrations of
Pu(III,IV,V,VI) in Environmental Samples.
schemes have been used most often in re-
cent times because they can give a rapid
and nearly complete separation of oxida-
tion states at trace levels. Choppin and
Bertrand have proposed a series of solvent
Oxidation-State Distribution
Method
Organic Phase
Aqueous Phase
PMBP extraction at pH ꢀ 0
PMBP extraction at pH ꢀ 0 w/Cr
HDEHP extraction at pH ꢀ 0
(ꢁ4)
(ꢁ3,ꢁ4)
(ꢁ4,ꢁ6)
(ꢁ3,ꢁ4,ꢁ5,ꢁ6)
(ꢁ3,ꢁ5,ꢁ6, p)a
(ꢁ5,ꢁ6,p)
(ꢁ3,ꢁ5,p)
(p)
extractions at differing pH values using
TTA (thenoyltrifluoroacetone).1
3,14
How-
2ꢂ
2
O
7
ever, Schramke et al. later showed that
2ꢁ
significant amounts of PuO2 can be re-
2ꢂ
HDEHP extraction at pH ꢀ 0 w/Cr
2
O
7
ꢁ
duced to PuO2 by the organic extractant
a
during the separation procedure if an oxi-
p ꢀ Pu(IV) polymer.
15
12
dant is not present. Choppin et al. have
proposed an actinide oxidation-state sepa-
ration scheme for environmental samples,
adapted from earlier work by Saito and
PMBP is 1-phenyl-3-methyl-4-benzoyl-pyrazol-5-one.
HDEHP is di-2-ethylhexylphosphoric acid.
16
Choppin based on the organic extractant
DBM (dibenzoylmethane) in chloroform.
The use of this extractant has the advan-
tage that the near-neutral pH of most
environmental samples does not require
adjustment to more acid conditions for
the extraction step, as is the case with
the other extractants mentioned here,
thus minimizing the possible effects of
pH adjustment on the oxidation-state dis-
tribution. Nitsche and co-workers used
a combination of TTA, HDEHP (di-2-
ethylhexylphosphoric acid), and hexone
extractions at varying pH for oxidation-
state determinations of Pu.17 This same
group later changed the scheme to substi-
tute PMBP (1-phenyl-3-methyl-4-benzoyl-
pyrazol-5-one) for TTA and added a step
that included oxidation by dichromate for
oxidation-state determinations at pH ꢀ 0
in synthetic brine solutions.18 PMBP was
found to be more efficient as an extractant
than TTA and was more resistant to de-
composition by oxidants such as dichro-
mate. This is shown in Table I.
While chemical separation methods
provide a sensitive method for determin-
ing the amounts of Pu in solution in a
given oxidation state, they give no infor-
Figure 1. Optical-absorption spectrum of (a) Pu3ꢁ, (b) Pu4ꢁ, (c) PuO2 , and (d) PuO22ꢁ.
(From Reference 21.)
ꢁ
708
MRS BULLETIN/SEPTEMBER 2001

Environmental Actinide Science
The detection limits obtained by PAS for
Pu in the oxidation states of environmen-
tal importance are given in Table II.
Table II: Optical-Absorption Spectroscopy Parameters for Major Absorption Peaks
of Plutonium Ions of Environmental Importance.
Molar
Solid Phases
Important Solid Phases
Precipitation can occur if there is a suf-
Wavelength
(nm)
Absorption
Sensitivity
(M)
Sensitivity
(ppm)
ꢀ
1
ꢀ1
Ion
(M cm )
ꢂ7
ficient concentration of the plutonium in
solution to exceed the solubility-product
constant for the formation of a solid phase.
This effect will limit the amount of the
plutonium in solution in the vicinity of the
solid phase at any given time and thus
will tend to have a retarding effect on the
rate of movement of Pu in the environment.
In Table III, the compositions of ground-
waters taken from a number of sources
Pu(III)
Pu(IV)
600
470
568
830
38
55
19
1 ꢀ 10
0.03
0.03
0.10
0.03
ꢂ7
ꢂ7
ꢂ7
1 ꢀ 10
5 ꢀ 10
1 ꢀ 10
ꢁ
PuO
PuO
2
2ꢁ
2
550
a number of uranium and neptunium
compounds (e.g., see References 40–43).
X-ray absorption near edge structure
(XANES) spectroscopy, usually within
50 eV of the adsorption edge, can provide
information about the local structure, co-
ordination number, and oxidation state of
an actinide in solution, solid form, or at
a solution–solid interface. Peak shifts in
the spectra with oxidation state are easily
detected. Conradson et al. have shown
that XANES spectroscopy is an efficient
method for determining the oxidation
states of Pu in a variety of matrices.44 An
example of plutonium XANES spectra
from Reference 44 is shown in Figure 2.
The edge energies were observed to shift
progressively to higher energy with in-
creasing valence, with an average 1.68-eV
increase per formal oxidation-state increase.
In addition, the general spectral shape of
the (III) and (IV) species was clearly dif-
ferent from the dioxo-containing (V) and
(VI) species.
30
are shown. The common anions are hy-
droxide, carbonate, sulfate, phosphate,
fluoride, chloride, nitrate, and silicate.
Hydroxide, carbonate, sulfate, phosphate,
and fluoride form insoluble compounds
with all four oxidation states of plutonium,
while chloride and nitrate compounds are
3
5
31
quite soluble. The silicate compounds of
plutonium have not been investigated,
4ꢁ
6ꢁ
however, U and U form insoluble com-
pounds with silica—for example, coffinite,
uranophane, sodium boltwoodite, soddyite,
Table III: Composition of Various Natural Groundwaters (from Reference 30).
32
and sodium weeksite —and plutonium
would be expected to exhibit similar be-
havior. The solubilities and solid phases
exhibited by Pu in all of the oxidation
states possible under environmental con-
ditions have not been investigated. Be-
cause of the similarity in the electronic
YMPc
J-13
Water
YMPc
UE-25p#1
Water
Mono
Lake
Ocean Brine
Surface
Watera
Bedrock
Waterb
Analysis
Rainwater
Depth (m)
Age (yr)
pH
0
0
0.50
ꢃ10
ꢃ500
ꢄ100
7.3–8.4 (7.9) 7–10
...
...
6.9
0.7
5.7
45
5.3
11.5
1.76
0.04
2.1
6.4
...
...
...
0–20
...
8.1
0.8
ꢃ9
...
...
...
...
...
33
34
structure and charge to ion size, ac-
tinides in a given oxidation state tend to
exhibit similar chemical properties, and
this effect can be used to estimate pluto-
nium behavior from the known behavior
of other actinides.
4–6
0.9
6.7
0.36
...
171
13.4
87.8
31.9
ꢅ0.1
3.5
37
E
h
(V)
0.0–0.3
1–10
ꢂ0.05
ꢅ0.1
10–100e
d
O (mg/l)
2
10
ꢁ
Na
K
0.3–20
0.1–4
0.5–5
0.1–0.5
...
1–20 (12)
0.3–8 (4)
2–100 (25)
3–30 (10)
0.1–1
ꢃ0.1
0.5–90 (10)
...
10,766 22,000
ꢁ
1–5
399
413
1292
0.02
1.4
...
...
...
1
2ꢁ
Ca
Mg
20–60
15–30
5–30
0.5–2
5–50e
...
5–400
ꢅ1
ꢃ0.1
1–15
5–30
ꢅ1
2ꢁ
Identification of Solid Phases
Fe (total)
The identification and characterization
of solid phases have long relied on the
results from the measurement of x-ray
powder diffraction patterns of the solids.
However, solids obtained under environ-
mental conditions are frequently amor-
phous and not amenable to this method of
analysis. With the availability of third-
generation synchrotrons35 and the devel-
opment of new fluorescence detectors36
that can process high count rates, x-ray ab-
sorption spectroscopy (XAS) is becoming
a powerful technique for studying the
speciation of actinides in the environ-
ꢂ
F
...
48
ꢂ
Cl
0.1–20
...
(total)e ꢃ1
19.353 ...
ꢂ
Br
...
67
...
28,300
...
20
8300
2
ꢂ
CO
3
60–200
ꢃ10
ꢃ0.1
118–143
10.1
...
960
ꢅ0.1
...
129
66
...
...
...
ꢀ140
ꢅ0.7
ꢃ0.1
2712
ꢂ
NO3
0.1–4
3ꢂ
PO
SO
4
(total)
0
2ꢂ
(total)f ꢃ20 (1–5) 3–300 (20)
4
18.1
66
...
...
0.15
SiO
SH
2
ꢂ g
(total)
0
0
3–15
0
ꢅ0.1
1–50
0.01–7 ...
ꢅ0.05 ...
ꢅ0.05 ...
g
NH
3
ꢅ0.5
...
ꢅ0.5
ꢅ1
Organic
carbon
ꢀ1
...
37–39
ment.
XAS is particularly useful for in-
a
Lakes, rivers, and shallow wells; typical values within parentheses.
Swedish granite.
Yucca Mountain Project (Nevada) wells.
All concentrations in mg/l.
vestigating noncrystalline and polymeric
actinide compounds that cannot be identi-
fied by x-ray diffraction analysis. Reports
on the use of this technique for the charac-
terization of plutonium solids are rather
sparse; however, it has been used to study
b
c
d
e
f
ꢂ
Mainly as HCO
3
.
For relict water, the value may be 500–3000 mg/l.
g
ꢂ
SH and NH
3
only occur in reducing waters(E
h
ꢅ 0), except for surface waters in industrial areas.
MRS BULLETIN/SEPTEMBER 2001
709

Environmental Actinide Science
ate, sulfate, phosphate, chloride, fluoride,
nitrate, and silicate. Complexation can occur
with more than one ligand or plutonium
ion, so several species may be associated
with a given ligand.
plutonium source term. They exhibit the
highest biodiversity of any living organ-
ism and sometimes can adapt quickly to
changing living conditions.5
6–58
Certain
strains can even survive under harsh envi-
ronmental conditions such as low or high
temperature, high pressure, highly acidic
and basic media, and high-radiation
In addition to the inorganic ligands in
groundwater, there are naturally occur-
ring organic ligands that can complex plu-
tonium rather strongly and could affect
plutonium transport. The interaction of
plutonium ions with environmental organic
material such as anthropogenic and natu-
ral dissolved organics and microbes is
complex. These substances can also form
colloids and therefore contribute to the
formation of actinide pseudocolloids. The
transport of organic actinide complexes,
especially with EDTA (ethylenediaminete-
tracetic acid), was observed at ORNL and
59–61
fields.
Compared with processes in-
volving inorganic constituents, relatively
little is known about bacterial interaction
with plutonium. Several comprehensive
reviews of the current understanding are
62–67
available in the literature.
Actinides may be stabilized or precipi-
tated either by enzymatic and direct or in-
direct microbial action. The chemical form
of the actinide, the presence of electron ac-
ceptors and donors, and the environmen-
tal condition play important roles in the
type, rate, and magnitude of microbial
activity. Redox conditions influence the
solubility of actinides. Enzymatic reduc-
tion from a higher to a lower, less soluble
oxidation state can occur. Microbes can
produce extracellular chelating agents in
response to a low availability of soluble
iron. Siderophores, low-molecular-weight
iron chelators, were identified to have a
very high complexation affinity for Pu(IV),
U(IV), U(VI), and Th(IV), and were shown
4
6,47
at Maxey Flats Disposal Site in Kentucky.
Humic and fulvic acids were identified
as the most important natural organic lig-
ands.4
8–52
They are ubiquitous to natural
waters and range in concentration from
1
ppm at the ocean’s surface, to 5 ppm in
Figure 2. X-ray absorption near edge
structure (XANES) spectra for aqueous
surface waters, to 50 ppm in dark-brown
53
3ꢁ
4ꢁ
ꢁ
2
2ꢁ
swamps. Their names are derived from
their method of isolation. Humic and fulvic
acids are the alkaline-soluble fraction of
soils, leaving behind insoluble humin.
Acidification of this extract to pH 2 pre-
cipitates humic acid, and fulvic acid re-
mains in solution. Humic and fulvic acids
are polyelectrolytes with molecular weights
ranging from 1000 amu to more than
10,000 amu, and from 500 amu to 2000 amu,
Pu , Pu , PuO
, and PuO
2
. Reprinted
from Reference 44 with permission of
Elsevier Science.
Extended x-ray absorption fine structure
EXAFS) spectroscopy gives additional in-
68–71
(
to solubilize PuO
.
2
Tetravalent actinides
formation on coordination numbers and
bond lengths to first, second, and even
more distant neighbor atoms.3 This in-
formation is very useful for understand-
ing the chemical state, bonding, and bond
lengths of actinide related to (1) speciation
and complexation studies in aqueous and
nonaqueous solutions; (2) adsorption proc-
esses at solid–solution interfaces involving
minerals, mineral assemblies, rocks, and
soils; and (3) adsorption and incorporation
of radionuclides in microorganisms and
other biological materials. Conradson has
recently reported on the study of the Pu–O
bonding in Pu(IV) colloid and dioxide.45
are bound preferentially because their
charge-to-radius ratios are similar to Fe(III).
Panak and co-workers have shown that
5,45
2ꢁ
the uptake of UO
by strains of the bac-
2
54
respectively. Their constitution, and thus
their functionality toward metal actinide
binding, depends very much on their
origin. Although no two humic acids are
completely alike, they all have in common
many structural elements that can co-
ordinate with plutonium. A proposed
structure, representative for humic acids,
teria Thiobacillus ferrooxidans is primarily
through the formation of stable inner-
sphere complexes with the biomass.72
These authors have also shown that the
2
ꢁ
3ꢁ
bacteria can oxidize Fe to Fe , and the
3ꢁ
4ꢁ
2ꢁ
Fe can in turn oxidize U to UO2 , thus
7
2,73
2ꢁ
solubilizing the UO2.
UO2 can also be
reduced by sulfur-reducing bacteria with
subsequent precipitation of less soluble
55
is shown in Figure 3.
4ꢁ
73
U
compounds.
Microbial Interaction
Microbes in soil, sediment, and water
can have a significant influence on the
Identification of Solution Species
Complexation or colloid formation pro-
duces changes in the optical-absorption
spectra that can frequently be used to
identify the complex species or detect
Aqueous Species
A number of inorganic and organic lig-
ands that can form complexes with pluto-
nium ions in solution may be present in
groundwater. The charge of the solution
complex can vary widely. Complexation
tends to increase the amount of plutonium
in solution and thus increase migration
rates. This represents an important path
for the migration of actinides in the
environment.
4ꢁ
colloid formation. In the spectra of Pu ,
the absorption wavelength of the un-
complexed hydrated ion is shifted from
4
70 nm to 486 nm with the formation of
carbonate complexes. Because the concen-
trations of both complexed and uncom-
plexed species can be obtained from an
analysis of the absorption spectra, this
method is frequently used to measure
complex-formation constants of actinides,
for example, the carbonate complexation
Inorganic and Organic Complexes
The inorganic ligands in groundwater
responsible for the complexation of pluto-
nium are the same ones discussed under
precipitation, namely, hydroxide, carbon-
Figure 3. Hypothetical structural
74
75
relationships in aqueous humic material
based on degradation products of
permanganate. (From Reference 55.)
of Pu(IV) and Pu(V).
XAS is a new and powerful method for
the study of the nature of solution species.
710
MRS BULLETIN/SEPTEMBER 2001

Environmental Actinide Science
locity.95,96 The sign and magnitude of the
electrical charge of a colloid, the ꢆ poten-
tial, generally is a function of the pH of the
solution, and there is a specific pH where
they are uncharged. When uncharged par-
ticles pass through a porous media or thin
fracture, they are usually transported with-
out retention by convection and diffusion
due to Brownian motion. The water veloc-
ity distribution is generally parabolic, the
maximum velocity at the center being
about two times the average velocity of
Not only can species be identified and
thermodynamic constants evaluated, but
bond strengths and distances to near neigh-
bors can also be calculated. The structures
of the aqueous nitrate complexes have
been studied by EXAFS as a function of
calculated adsorption curves using the
86,87
model to experimental data.
Because
this process typically involves a large
number of potentially adjustable parame-
ters, it is likely to lead to nonunique
parameter-fitting and does not always
lead to consistent sets of parameters for
the same systems. However, recently XAS
has been applied to the characterization of
76
nitrate concentration. The Pu was shown
to be highly coordinated (coordination
numbers of 11–12) for the first shell of
oxygen nearest neighbors. The average
plutonium–oxygen (nitrate) bond length
was found to be 2.49 Å, and the average
plutonium–oxygen (water) bond length
was estimated to be 2.38 Å. EXAFS has
also recently been utilized for the investi-
gation of the chloride complexation of
the adsorbed species and surface reac-
88–90
tions,
and a direct determination of the
sorbed species and surface reactions ap-
pears possible. The information that can
be obtained by this method includes the
mode of sorption (inner- or outer-sphere
complexes or precipitation: mono-, bi-, or
tridentate binding), the structure of the
surface complex or precipitate (monomers,
dimers, and/or more complex hydroxo-
bridged metal-ion oligomers), and the
composition of the surface complex or
95,96
the water. Particles will randomly sample
this velocity variation, but will not reach
the walls due to their size. Therefore, their
average velocity will be greater than the
average groundwater velocity; the larger
colloids will travel more rapidly than the
smaller ones. If the colloids have the same
charge as the surrounding media, repul-
sion will tend to increase the colloids’ rela-
tive velocity even more as they are kept
even further away from the walls and will
tend to inhibit wall adsorption. In addi-
tion, colloid particles that are larger than
some pore diameters will be excluded from
passage through them and experience a
shorter overall path length (size exclusion)
and, again, an apparent higher velocity
than the average water velocity. If the par-
ticles have a charge that is opposite that of
the surrounding surfaces, retention mecha-
nisms (wall absorption) will decrease the
particle velocity relative to that of the
groundwater. Failure to account for col-
loid formation and transport could lead
to serious underestimations of the rate of
transport of plutonium contaminants in
the environment.
3ꢁ 77
several actinides, including Pu . At low
chloride concentrations, a hydration num-
ber of 10.2 and chloride bond length of
3ꢁ
2
.51 Å were found for Pu . This ion
showed a decrease in the hydration num-
ber (40%) and no inner-sphere complexa-
tion for chloride concentrations ꢅ14 M.
A number of U(VI) inorganic and organic
complexes have been investigated via
91
precipitate.
Colloids
Plutonium ions can form or become as-
sociated with colloidal-sized particles. This
represents another means for transmitting
actinides through the geosphere. Pluto-
nium in this form will not behave like a
dissolved species and may exhibit consid-
erably different migration behavior than
the dissolved species. Depending on the
nature of the colloid and the solution con-
ditions, this process may enhance or retard
migration of the plutonium. The role of
colloids in facilitating actinide transport is
far from clear; however, greatly enhanced
transport of plutonium associated with
colloids over that predicted for ionic
species has been observed. At a site at Los
Alamos, plutonium and americium were
detected in monitoring wells over a mile
away from a liquid-waste source and
found by ultracentrifugation to be present
78–81
EXAFS.
Adsorption Processes
The plutonium ions may attach them-
selves to mineral or rock surfaces in con-
tact with the aqueous phase. This process
is similar to the precipitation process since
it removes plutonium ions from solution.
Thus, this process tends to reduce the con-
centration of plutonium ions in solution
and produce a retarding effect on the mi-
gration rate. Plutonium ions in ground-
water can attach themselves reversibly or
irreversibly onto rock or mineral surfaces,
solids, and sediments. According to Stumm
82
and Morgan, this adsorption process may
result from short-range chemical forces (e.g.,
covalent bonding, hydrophobic bonding,
and hydrogen bridges). These processes
are frequently irreversible, for example,
precipitation of a solid phase. Sorption may
also result from long-range forces, (i.e.,
electrostatic and van der Waals attractions).
Electrostatic adsorption is frequently rapid
and reversible (e.g., ion-exchange reactions).
Summary and Outlook
In this article, a number of major sources
and interaction processes for environmen-
tal contamination with plutonium have
been outlined. While emphasis has been
on laboratory studies, we should grasp the
opportunity to study the contaminated
sites, not merely assess their level of con-
tamination, in order to gain a better un-
derstanding of actinide behavior in these
more complex systems, which even the
most sophisticated laboratory experiments
cannot accurately simulate. The contami-
nated sites can be used as anthropogenic
analogues that should aid us to better
identify and prioritize the immobilization
and remobilization processes. Field meas-
urements should guide the design of
laboratory experiments, and in turn, the
understanding that is gathered in the lab-
oratory should be used in directing the
field measurements. This is an iterative
process, and it should aid in designing
more complex laboratory experiments
that still can be described by first princi-
92
as colloids. The results of an actinide
analysis of samples taken from an artifi-
cial, oligotrophic lake in northwest Wales,
which was used as a source of cooling
water for a nuclear-power plant, showed
2
39,240
Pu concentrations in the range of
.4–12.5 fCi/l and associated primarily
6
93
Identification of Surface Species
with colloidal-sized particles. Recently,
Kersting and co-workers have reported
The so-called surface-complexation, or
site-binding, model has been successfully
used to describe the adsorption reactions
of solution species on solid structures.8
The disadvantage of this model is the dif-
ficulty in experimentally determining the
model parameters and surface-reaction
constants for a given solute. Usually, a set
of surface reactions and species are pro-
posed based on the knowledge of the so-
lution speciation of the solute. The
reaction constants for the proposed reac-
tions are then derived by fitting computer-
239,240
detecting
Pu in groundwater samples
taken from aquifers at the Nevada Test
3–85
94
240
239
Site. The Pu/ Pu isotope ratio of the
samples established that an underground
nuclear test 1.3 km north of the sample site
was the origin of the plutonium. The plu-
tonium was found to be associated with
colloidal-sized particles.
Depending on their size and charge
relative to the surrounding porous media,
colloids can move more rapidly or more
slowly than the average groundwater ve-
MRS BULLETIN/SEPTEMBER 2001
711

Environmental Actinide Science
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at Synchrotron Light Sources, edited by N. Edel-
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1999) p. 15.
39. P.G. Allen, J.J. Bucher, M.A. Denecke, N.M.
Edelstein, N. Kaltsoyannis, H. Nitsche, T. Reich,
and D.K. Shuh, in Synchrotron Radiation Tech-
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complex laboratory system is understood,
a new component can be added to it, and
the influence of this added component on
the overall system can be determined.
Future directions should focus on the
development of a better understanding of
the plutonium sorption processes at the
molecular level through improvements in
the detection limits of existing methods,
for example, x-ray absorption spectroscopy
and laser-based spectroscopic techniques,
and the development of new techniques.
This will include the development of lower
detection limits for the measurement of
plutonium speciation in real systems.
In order to provide a better understand-
ing of environmental plutonium behavior,
the research disciplines involved need to
work more closely together. Actinide
chemists, materials scientists, geochemists,
soil scientists, mineralogists, microbiolo-
gists, physicists, and other pertinent scien-
tists need to form multidisciplinary teams
that are able to go beyond the borders of
their individual disciplines. Only then will
we be able to identify and better under-
stand the major transport-controlling phe-
nomena for plutonium migration in the
environment. Accepting this challenge
will contribute to better, more efficient,
and possibly cheaper remediation and
cleanup methods, as well as secure a
bright and very interesting future for ac-
tinide materials chemistry.
Chemistry of the Actinide Elements, 2nd ed., Vol. 2
(
Chapman & Hall, New York, 1986) p. 1139.
0. G.R. Choppin, Radiochim. Acta 43 (1988) p. 82.
1. A. Kobashi, G.R. Choppin, and J.W. Morse,
Radiochim. Acta 43 (1988) p. 211.
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1
1
1
Hromadka, J. Radioanal. Nucl. Chem. 219
(2) (1997) p. 203.
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1
4. P.A. Bertrand and G.R. Choppin, Radiochim.
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1
(
1
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1
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ꢀ
WORKSHOPS
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the MRS Fall and Spring Meeting symposia. However, Workshops differ
significantly in that they:
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000-2001 Workshop Program Subcommittee:
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Howard Katz
Lucent Technologies
Rm 1D-249
Amy Moll
Boise State Univ.
Mechanical Engineering Dept of Physics, Cox Hall
1910 University Dr.
Robert Nemanich
North Carolina State Univ.
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Tel: 919-515-3225
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Tel: 908-582-6968
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www.mrs.org/meetings/workshops
MRS BULLETIN/SEPTEMBER 2001
www.mrs.org/publications/bulletin
713