Actinide sequestration using self-assembled monolayers on mesoporous supports

Article abstract of DOI:10.1021/es049201j

Surfactant templated synthesis of mesoporous ceramics provides a versatile foundation upon which to create high efficiency environmental sorbents. These nanoporous ceramic oxides condense a huge amount of surface area into a very small volume. The ceramic

Full text of DOI:10.1021/es049201j

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Environ. Sci. Technol. 2005, 39, 1324-1331
remediation process. In addition, the short-term risk as-
sessment for tank closure requires a complete and accurate
accounting of actinide speciation. This can be done either
via direct separation or by concentration of low-level actinides
and subsequent separation from the matrix. Isolation of
individual actinides such as americium is a key parameter
in the risk assessment necessary for tank closure. There are
currently no methods available to distinguish or separate
americium from plutonium at extremely low concentrations.
This is essential information for the short-term risk assess-
ment for HLW tank closure.
These needs dictate the development of selective and
efficient separation of actinides from complex waste streams
so as to minimize HLW volume, reduce waste management
costs, and enhance long-term stability of the HLW form.
Recently the U.S. DOE has placed emphasis on the need to
significantly reduce the volume of material put through the
vitrification process. Thus, selective separation of the ac-
tinides and radiocesium from tank waste forms a critical
need for this waste cleanup strategy.
Actinide Sequestration Using
Self-Assembled Monolayers on
Mesoporous Supports
G L E N E . F R Y X E L L , * Y U E H E L I N ,
S A N D Y F I S K U M ,
J E R O M E C . B I R N B A U M , A N D H O N G W U
Materials Synthesis & Modification Group, Pacific Northwest
National Laboratory, P.O. Box 999, Mailstop K2-44,
Richland, Washington 99352
K E N K E M N E R A N D S H E L L E Y K E L L Y
Argonne National Laboratory, 9700 South Cass Avenue,
Argonne, Illinois 60439
Surfactant templated synthesis of mesoporous ceramics
provides a versatile foundation upon which to create high
efficiency environmental sorbents. These nanoporous
ceramic oxides condense a huge amount of surface area
into a very small volume. The ceramic oxide interface is
receptive to surface functionalization through molecular self-
assembly. The marriage of mesoporous ceramics with self-
assembled monolayer chemistry creates a powerful
new class of environmental sorbent materials called self-
assembled monolayers on mesoporous supports (SAMMS).
These SAMMS materials are highly efficient sorbents whose
interfacial chemistry can be fine-tuned to selectively
sequester a specific target species, such as heavy metals,
tetrahedral oxometalate anions, and radionuclides.
Details addressing the design, synthesis, and characterization
of SAMMS materials specifically designed to sequester
actinides, of central importance to the environmental cleanup
necessary after 40 years of weapons-grade plutonium
production, as well as evaluation of their binding affinities
and kinetics are presented.
The area of functionalized nanoporous ceramics has
received a great deal of attention in recent years (1-28). The
field has been reviewed (29-31). We have developed self-
assembled monolayers on mesoporous supports (SAMMS)
as a superior method of mercury and heavy metal seques-
tration, proving to be orders of magnitude faster and more
effective than existing mercury-scavenging methods (32-
38) (see also refs 10-12). This research was extended to
include anions (39, 40) (see also refs 41 and 42), cesium (43),
and radioiodine (44). This background provided the founda-
tion for the current work, which extends the interfacial
chemistry of monolayer-coated mesoporous ceramics to the
sequestration of actinides.
The highly ordered nanostructure of SAMMS is the
culmination of three successive generations of molecular
self-assembly. The first generation is the aggregation of the
surfactant molecules to create the micelle template; the
second is the aggregation of the silicate coated micelles into
the mesostructured greenbody, and the third is the self-
assembly of the silane molecules into an ordered monolayer
structure across the pore interface (see Figure 1). This
functionalized hexagonal honeycomb structure is a powerful
foundation upon which to build an environmental sorbent
material.
Introduction
The SAMMS concept allows for significant freedom in
the design and synthesis of tailored materials for actinide
separation. The mesoporous ceramic synthesis is quite
general and can be used to prepare a variety of high surface
area ceramic oxide supports that are stable in different
environments (acidic, corrosive, oxidizing, etc.). The high
surface area of the mesoporous support (ca. 1000 m2/g)
coupled with the high population density of binding groups
creates a high loading capacity in the final SAMMS material.
The rigid, open pore structure of the mesoporous support
makes all of the interfacial binding sites available to solution-
borne species and allows for facile diffusion into the porous
matrix, resulting in rapid sorption kinetics. The chemical
specificity of a given type of SAMMS is governed by the
monolayer interface. The ability to install chemically different
monolayers, along with the ability to synthetically elaborate
those monolayers post-installation, allows for a wide variety
of binding chemistries to be installed, making the SAMMS
concept easily tailored for a variety of environmental targets
(Pu, Cs, TcO4, etc.). SAMMS, being a silica-based technology,
would be readily incorporated into a vitrification process
stream. This would reduce the volume of waste needing
Over the past decade, a great deal has been learned about
making nanostructured materials. In recent years there has
been a shift from asking “What shapes can be made?” to
asking “What can these shapes be made to do?” This has
required that the nanostructured materials be inherently
functional. One of the areas that shows great promise for
these functional nanomaterials is their use in the field of
environmental remediation. Recently, significant advances
have been made in the synthesis of functional nanomaterials
(1-45) creating exciting new possibilities for the sequestration
of toxic materials from the environment.
The U.S. Department of Energy is faced with a daunting
environmental cleanup resulting from 40 years of weapons-
grade plutonium production. A central focus of this cleanup
effort is the ability to selectively and completely remove the
radionuclides from complex mixtures so that high-level waste
(HLW) volume can be minimized and the nonradioactive
components can be segregated and disposed of as low-level
waste (LLW), thus substantially reducing the cost of the
* Corresponding author phone: (509)375-3856; fax: (509)375-2186;
e-mail: glen.fryxell@pnl.gov.
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1324 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 5, 2005
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2005 American Chemical Society
Published on Web 01/12/2005

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FIGURE 1. Genesis of SAMMS.
vitrification significantly since only the volume of the
radionuclide-laden SAMMS would have to be vitrified and
not the bulk of the waste. This paper summarizes some of
our work in the design, synthesis, and performance evaluation
of actinide-specific SAMMS.
coverage. The pore size is estimated from the desorption
branch, using a standard Barrett-Joyner-Halenda (BJH)
method.
(c) ICP-MS. For non-radioactive samples, solution metal
concentrations were determined using ICP-MS, on a Hewlett-
Packard HP4500. Solutions were acidified using either HCl
or HNO3.
Experimental Section
Materials. MCM-41 was prepared as described previously
(32, 34). 3-Aminopropyl trimethoxysilane and (2-aminoethyl)-
3-aminopropyl trimethoxysilane were obtained from United
Chemical Technologies. The acetamidephosphonate (“Ac-
Phos”) silane and propionamidephosphonate (“Prop-Phos”)
silane were prepared as described previously (45).
The self-assembled monolayers were deposited in MCM-
41 (60-65 Å pores, 880-900 m2/g) by carefully prehydrating
the ceramic with approximately 2-2.5 monolayers worth of
water (approximately 0.32 mL of water for each 1.0 g of MCM-
41), followed by treatment with a slight (ca. 10-15%) excess
of silane (relative to available surface area), and the mixture
was held at toluene reflux for 4-6 h. The mixture was then
cooled to ambient temperature and filtered, and the collected
solid was washed copiously with 2-propanol. The SAMMS
were then air-dried to constant weight in a fume hood and
characterized using solid-state 29Si and 13C NMR, BET, and
in some cases EXAFS and XPS.
Batch Contacts with Actinide Solutions. Solutions of 1
M NaNO3 were prepared from reagent-grade chemicals.
Concentrated HNO3 was added on a volume basis to make
1 M NaNO3-0.1 M HNO3 solution. Additionally concentrated
HNO3 was titrated into aliquots of 1 M NaNO3 to pH end
points 4, 2, and 1 as indicated potentiometrically. Aliquots
of these solutions were spiked with 230Th, 237Np, 241Am(III),
239Pu(IV), or 233U(VI) resulting in nominal 2000 dpm/mL
concentrations. The radiotracers 233U (0.15 M HNO3), 239Pu
(7 M HNO3), and 241Am (0.01 M HNO3) were obtained from
in-house stocks. Interference test solutions were prepared
by adding aliquots of 10 mg/mL standard solutions to aliquots
of 1 M NaNO3-0.1 M HNO3. The solution/solid ratio was
varied from 50 to 5000 to explore the effect on binding
efficiency. No significant variation was observed over this
range, so a solution-to-solids ratio of 100 was adopted as
standard. Samples were shaken in an orbital shaker for 1-4
h (control experiments revealed that equilibrium was reached
in less than 30 min, so these results represent true equilibrium
conditions). After the mixtures were shaken for the specified
time, they were filtered through a 0.2 µm syringe filter. Test
solution aliquots were suspended in Ultima Gold (Packard
Instrument, Meriden, CT) scintillation cocktail, and R activi-
ties were measured using a 2550 TR/AB liquid scintillation
counter (Packard Instruments). In the cases where 241Am-
(III) and 239Pu(IV) were tested in the same contact solution,
the isotope-specific activity was determined using R energy
analysis on a precipitation-plated mount. Experiments were
also performed in which competing ions and complexants
were included in the me´lange.
The phosphonic ester was cleaved post-deposition using
trimethylsilyl iodide (TMSI), either in anhydrous acetonitrile
for 18 h at ambient temperature, or pure in the vapor phase
at 80 °C, followed by aqueous hydrolysis for 1 h.
Measurements. (a) NMR. Solid-state 29Si and 13C NMR
spectra were determined at 19.944 and 25.2458 MHz,
respectively, using a Chemagnetics CMX-100 NMR quadruple
channel spectrometer system. The probe was a 7-mm pencil-
type probe with magic angle spinning at 4 kHz. The 4-µs
90-deg pulses were utilized for both 29Si and 13C, with proton
decoupling power at 62.5 kHz.
29Si spectra were acquired using the Freeman-Hill T1 (t1fh)
pulse sequence with a short recovery time, 50 µs, to suppress
acoustic ringing. Pulse delays of 60 s were employed. Samples
were prepared by loading a spinner with a Teflon spacer
followed by ca. 65 mg of sample and 15 mg of tetrakis-
(trimethylsilyl)silane (TTMS), followed by an additional 65
mg of sample, and a sample end spacer. Chemical shifts
were referenced to internal TTMS.
Kinetics. The kinetics experiments were performed
similarly to the batch contacts. The test solution was 1 M
NaNO3-0.1 M HNO3. Sample aliquots were removed at the
specific time intervals ranging from 1 min to 6 h. At specific
time intervals, aliquots (1 vol %) were removed, filtered
through 0.2 µm filters, and counted using liquid scintillation.
(b) BET. The N2 adsorption/desorption isotherms were
obtained from a Quantachrome BET instrument (Autosorb-
6). The Brunauer-Emmett-Teller (BET) surface area of the
pure silica was found to be 886-928 m2/g (depending on
batch), and the BET surface of the functionalized silica is
approximately half this value, depending upon the degree of
Results and Discussion
Synthesis. We employed three related synthetic pathways to
prepare our functionalized SAMMS materials, exemplifying
the versatility of our convergent SAMMS synthesis. Glycinyl-
urea SAMMS were prepared by treating an isocyanate
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VOL. 39, NO. 5, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1325

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The third synthetic route was used to prepare the
phosphonate SAMMS and involved displacement of trifluo-
roethanol from the corresponding ester, as shown in Figure
4 (45). This silane achieved a surface coverage of 2.0-2.2
silanes/nm2.
Distribution Coefficient Measurements. The actinide
target species commonly form insoluble hydroxides or
polymeric oxides under alkaline conditions, so our binding
studies were performed over a pH range of 1.0-6.5. Cation
binding is an equilibrium process and can be influenced by
the stoichiometric ratio of binding sites to solution cation
content. The solution/solids ratio employed in these experi-
ments was typically 200 but was also studied in a range from
50 to 1000. To ensure that all distribution coefficients reflected
genuine equilibrium conditions, contact times were several
hours (typically 1-4; as will be shown later, equilibrium under
these conditions is reached within only a few minutes):
FIGURE 2. Preparation of glycinyl-urea (Gly-UR) SAMMS.
(Co - Cf)
V
M
Kd )
×
Cf
The distribution coefficients (Kd) are simply a mass-
weighted partition coefficient between the aqueous super-
natant phase and SAMMS solid phase. The higher the Kd
value, the more effective the sorbent material is at seques-
tering the target species. For an arbitrarily chosen solution-
to-solids ratio of 100 (i.e., 1% sorbent loading), a Kd of 1000
indicates that 10 times as much of the target analyte is in the
sorbent phase as is left in the supernatant solution (in other
words, 90% sequestration). Kd values above 500 are consid-
ered acceptable, those above 5000 are considered very good,
and Kd values in excess of 50 000 are considered outstanding.
The units of Kd in this case are mL/g. The uncertainty of the
Kd values obtained in these studies is estimated to be (5%.
Binding Studies. Much has been learned in terms of
metal-ligand interactions from actinide solvent extraction
studies, as well as those studies aimed at designing ligands
to remove actinides from biological systems. The key lessons
arising from these studies are that the actinide cations are
hard Lewis acids that are considerably larger than the typical
transition metal cation and that ligand synergy is an
important attribute to design into a ligand field. Noteworthy
examples of how these concepts have been joined into
powerful rare earth ligand systems include the hydroxyp-
FIGURE 3. Preparation of the salicylamide ligand.
terminated silane with a triethylamine-buffered solution of
glycine. This approach tethers the acid to the silane via a
urea linkage (see Figure 2). In this fashion, we obtained a
glycine terminated SAMMS material with a surface coverage
of approximately 4.0 silanes/nm2 (as determined by solid-
state 29Si NMR).
The second of our strategies is exemplified by the
incorporation of the salicylate ligand via an amide linkage
to commercially available aminopropylsiloxane (APS), ac-
complished by activating the carboxylate with carbonyl
diimidazole (see Figure 3). All attempts to effect this amidation
via the corresponding methyl and ethyl esters failed. The
CDI amidation reaction was followed immediately by depo-
sition in freshly hydrated MCM-41. This protocol resulted in
the deposition of 1.1 silanes/nm2, which is the expected level
of coverage for an aromatic terminated silane of this bulk.
This strategy was also used to make the p-NO2 analogue and
other related aromatic interfaces, resulting in similar coverage
densities.
FIGURE 4. Preparation of acetamide-phosphonate (“Ac-Phos”) acid SAMMS.
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ridinone (HOPO) and desferrioxamine (DFO) chelators (46-
48) and the carbamoylphosphine oxide (CMPO) extractants
(49, 50). Our previous lanthanide model studies suggested
that SAMMS are a viable method for sequestering actinides
and that the synergistic ligand field concept outlined above
is indeed valid at the monolayer interface inside a meso-
porous matrix.
Salicylamide. One of the simplest and most direct entries
into a synergistic ligand field is provided by the salicylamide
ligand due to the combination of the resonance stabilized
phenolic OH group and the adjacent amide carbonyl group.
Salicylamide-based ligands have been used to chelate a wide
variety of metal cations (51-53), and recently salicylamide-
based ligand systems have been very elegantly employed to
create elaborate supermolecular complexes (54). The pH
limitation found with this ligand was initially thought to arise
from the modest acidity of the phenolic hydroxyl group. As
a means of testing this hypothesis, we also prepared and
studied the 5-nitro analogue as well as phthalamide SAMMS
(the corresponding ortho carboxylic acid). Neither one of
these more acidic ligands demonstrated any notable im-
provement over the original Sal-SAMMS.
TABLE 1. Actinide Distribution Coefficient Values for the
Carboxylic Acid SAMMSa
Kd (mL/g)
Phthal-SAMMS
Kd (mL/g)
Gly-UR SAMMS
actinide
pH
Am(III)
0.78
1.9
0
0
0
1
4.26
5.24
0.66
1.05
2.08
0.72
1.55
3.68
4.73
0.7
51
92 000
240 000
160
5 100
45 000
0
1 400
0
Pu(IV)
Np(V)
46
9 000
0
0
0
0
86
200
0
U(VI)
57
160
2.36
4.55
0.76
2.43
130
2 800
34
12 000
160 000
900
Th(IV)
2 000
91 000
a [Ac] ) 2000 dpm/mL. 0.1 M NaNO3. 0.10 g SAMMS in 10 mL.
Eu(III) was chosen as a model system to mimic the Am-
(III). Detailed EXAFS analysis of the Eu(III) adduct of Sal-
SAMMS revealed an average Eu-O coordination number of
8 ( 1.2, a Eu-O radial distance of 2.40 ( 0.015 Å, and an
XAFS Debye-Waller term of 0.0145 ( 0.0025 Å-2. These
results are consistent with the Eu(III) cation being 8-coor-
dinate (not unusual for the lanthanides), in either a cubic or
distorted square antiprism geometry. Taking into account
the geometric considerations of the monolayer interface, a
distorted square antiprism is the most likely bonding
geometry for this complex. These EXAFS results clearly
support the conclusion that the close proximity of the ligands
in the monolayer interface allows for multiple ligands to
interact with a single metal cation. In this case we observe
a 4:1 ligand:metal interaction, which clearly contributes to
enhancing the binding affinity between a given ligand and
metal cation.
TABLE 2. Acid Regeneration of Glycinyl-Urea SAMMS and
Eu(III) Binding Affinitya
cycle
Cini (ppb)
Cfin (ppb)
Kd (mL/g)
1
2
1708
1800
1922
1826
1730
1892
1870
1742
1814
1438
76
73
88
74
57
50
50
49
67
43
4300
4700
4200
4700
5900
7400
7300
6900
5200
6500
3
4
5
6
7
8
9
10
a At pH 4.
Glycinyl-Urea. The rigid nature of the benzene ring
precludes coplanar chelation of the metal cation if the ionic
radius of the cation is large (as with the early lanthanides
and low oxidation state actinides). Thus, we chose to explore
the conformationally more flexible sp3 hybridized ligand
systems, in conjunction with seven-membered chelates.
While the actinide binding affinity of the salicylamide
SAMMS was found to be modest at best, the glycinyl-urea
SAMMS were found to be far more effective. However, the
binding is heavily dependent on pH, with the better binding
efficiencies occurring at higher pH values, with binding
affinity for the actinides dropping off below a pH of 2. This
is not unexpected because of the basicity of the carboxylate
ligand. However, as a result of the ubiquitous nature and low
cost associated with carboxylate ligands (e.g., glycine deriva-
tives) and the fact that they function well in the pH range of
2-4, these ligands may offer a cost-effective compromise for
sequestering actinides from complex mixtures. In summary,
Gly-UR SAMMS were found to bind all of the actinides
studied, except for Np(V), which is difficult to sequester (see
Table 1) and Gly-UR SAMMS were found to have a broader
pH window, effectively binding the target actinides to below
a pH of 4. The conformationally flexible Gly-UR SAMMS were
also found to bind both large and small cations with relative
ease.
as a model system. This was done by stripping the sorbed
europium with an acid wash (0.5 M HCl) and re-exposing
the SAMMS to fresh europium solution, and then we re-
measured the sorption affinity. Repeating this cycle 10 times
revealed no loss in binding affinity (see Table 2). Clearly,
regeneration of Gly-UR SAMMS is both easy and effective,
making recycling of these nanostructured sorbent materials
a viable option in their deployment.
Phosphonate Ligands. Ac-Phos and Prop-Phos ester
SAMMS have both the carbonyl amide and the PdO ester
ligands incorporated as a part of the monolayer interface.
Both of these groups activate the central enolizable meth-
ylene, creating the protic portion of the ligand (similar to the
classic acetonylacetonate, or acac, ligand). The ambiphilic
wettability of the phosphonate ester interface makes these
SAMMS materials particularly well-suited for sequestering
species from nonaqueous waste streams (e.g., oils, cutting
fluids, solvents, etc.).
The homologation of Ac-Phos ester SAMMS to form Prop-
Phos ester SAMMS does two things. First, it significantly
attenuates the acidity of the enolizable methylene, and second
it increases the chelate ring size to 7. In a series of neutral
diamide ligands designed as solution extractants, seven-
membered chelates are known to effectively ligate the
lanthanide cations (55).
The ability to regenerate any sorbent material is a key
factor in determining life cycle costs and field application
strategy. The fact that these carboxylate SAMMS show lesser
Kd values at lower pH values suggests that it might be possible
to strip the target species from the SAMMS phase with an
acid wash. To test this, we studied regeneration using Eu(III)
The Ac-Phos ester was found to be effective for all of the
actinides studied, except for the difficulty to bind Np(V) (see
Table 3). Distribution coefficients were commonly found to
be in excess of 20 000 and in some cases over 200 000. In
contrast, the Prop-Phos ester was found to be selective for
Pu(IV), and especially so at low pH values. In fact, it is possible
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