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r2011 American Chemical Society 4686 dx.doi.org/10.1021/ie102494r |Ind. Eng. Chem. Res. 2011, 50, 4686–4696
ARTICLE
pubs.acs.org/IECR
Surface Modified, Collapsible Controlled Pore Glass Materials for
Sequestration and Immobilization of Trivalent Metal Ions
Ilya A. Shkrob,* Angela R. Tisch, Timothy W. Marin,
†
John V. Muntean, Michael D. Kaminski, and
A. Jeremy Kropf
Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439, United States
b
SSupporting Information
ABSTRACT: We report a one-pot method for sequestration, containment, and immobilization of lanthanide (Ln) ions from dilute
aqueous waste streams. The approach is based on the use of collapsible, surface modified controlled pore glass (CPG)
nanomaterials. We present several approaches for a single-step chemical modification of 3-propylaminated CPGs that yield highly
efficient Ln-extracting materials with distribution coefficients exceeding 10000 mL/g. The resulting Ln complexes were studied
using X-ray absorption, magnetic resonance, and time-resolved luminescence spectroscopies. One of these CPG materials involving
an imidodi(methanediphosphate) moiety demonstrated high extraction efficacy, significant ionic radius sensitivity, and exceptional
tolerance to masking agents, which is conducive to its use for removal of traces of radionuclide ions from aqueous TALSPEAK
raffinate (trivalent actinidelanthanide separation by phosphorus reagent extraction from aqueous complexes process used in
processing of spent nuclear fuel). The glass loaded with the extracted metal ions can be calcined and sintered at 1100 °C, yielding
fused material that buries Ln ions in the vitreous matrix. This processing temperature is significantly lower than 1700 °C that is
required for direct vitrification of lanthanide oxides in high-silica glass. X-ray absorption spectroscopy and acid leaching tests indicate
that the immobilized ions are isolated and dispersed in the fused glass matrix. Thus, the method integrates Ln ions into the glass
network. The resulting glass can be used for temporary storage or as the source of silica for production of borosilicate waste forms
that are used for long-term disposal of high level radioactive waste.
1. INTRODUCTION
Due to its superb thermal and chemical durability, silica is an ideal
vitreous form for containment of nuclear waste, including fission
products, but using this matrix is impractical due to its prohibitively
high processing temperature. In this study, we suggest a method for
circumventing this inherent limitation and delivering radionuclide
ions of interest into an almost pure silica matrix. Specifically, we
suggest using a surface modified nanoporous glass matrix as (i) a
solid support for separations of radionuclides by ion-specificligands
covalently bound onto the surface of nanochannels and (ii) the
subsequent immobilization of these ions in compact glass as these
channels collapse during heat treatment of the porous glass. In such
applications, a packed column filled with microparticles of modified
controlled pore glass (mCPG) having channel surfaces modified
with ligands that bind to f-element ions (such as Ln
3þ
) removes
these ions from the aqueous solution passing through the column.
Alternatively, a slurry of such microparticles is added to the waste
stream and the solid material with the captured lanthanides is
filtered out. The glass loaded with the immobilized ions is dried,
calcined in air, and sintered at 10001200 °C, burying the
sequestered radionuclides in a silica matrix.
We note that the same surface modification chemistry can be
used to prepare ion-exchange (magnetic) silica microspheres for
analytical chemistry and nuclear forensics applications.
1,2
Such
microspheres are used to preconcentrate, sequester, separate,
and transport metal ions in a microfluidic system. Another
application of this surface chemistry is modification of porous
silica materials for magnetic resonance imagining (see below).
For brevity, the synthetic methods for CPG modification,
assay protocols, and details of spectroscopic characterization
were placed in the Supporting Information. Sections, tables, and
figures given in this supplement have designator “S”(e.g., Table
1S). Controlled porous glass is produced either through a
solgel process or Vycor process. The latter involves acid
dissolution of the borate phase produced via near-critical spino-
dal decomposition of sodium borosilicate melt. Below the
liquidus, this sodium borate phase forms a network of inter-
connected nanochannels. As the hydrochloric acid leaches this
borate phase out, the residual matrix is 97% SiO
2
. The resulting
void space is ∼30% and the internal surface is 100250 m
2
/g.
Such glasses readily sinter and fuse at 1100 °C yielding compact
silica glass. This process was used originally to manufacture heat-
and chemical- resistant kitchen and laboratory glassware.
The embedding of Ln
3þ
ions in sintered Vycor glass from
Dow Corning (Corning, NY)
3
and Diphosil (which is a Diphonix
polymer impregnated with Davisil gels from Eichrom (Lisle,
IL))
4,5
has been demonstrated in several previous studies. In the
latter case, isolated, dispersed LnPO
4
nanoclusters of 2.6 nm
have been observed in material collapsed at 1150 °C. Synthetic
approaches to modification of silica surfaces already exist for a
mesoporous silicate MCM-41
617
and similar chemistry can be
Received: December 13, 2010
Accepted: March 2, 2011
Revised: March 1, 2011
4687 dx.doi.org/10.1021/ie102494r |Ind. Eng. Chem. Res. 2011, 50, 4686–4696
Industrial & Engineering Chemistry Research ARTICLE
used for modification of CPG materials. Scheme 1S in the
Supporting Information demonstrates some of these previously
implemented graft designs (15). MCM-41 has an average pore
diameter of ca. 2.53 nm and an inner surface area of 1000 m
2
/g,
which is considerably greater than CPG (Table 1S). In a recent
study Steanbach et al.
18
and Chen et al.
19
modified amorphous
mesoporous silica with a surface area of 7001000 m
2
/g. This
material was modified with a dianhydride of pentetic acid
(DTPA). These workers achieved 46 wt % loading of Gd
3þ
onto this silica matrix
18
(the resulting Gd-loaded porous micro-
spheres were used for in situ magnetic resonance imaging and
doubled as a means for drug delivery).
18,19
Unlike CPG, neither
MCM-41 nor this amorphous material can be fused at relatively
low temperature.
An example of a synthetic approach used in these previous
studies involved activation of surface siloxyl groups of MCM-41
by bromopropyltrichlorosilane and subsequent attachment of
organic moieties (NaH activated malonamides) to the bromide
terminus of the graft. In the studies of Trens et al.,
15,16
trivalent
241
Am and
152
Eu ions were extracted into thus-modified MCM-
41. Fryxell and co-workers
8,11,12
attached glycinyl-urea ligand 2
to the isocyanate terminus of the silane graft; alternatively, the
salicylate groups were grafted by reacting the carboxyl group of
the ligand with 3-aminopropylsiloxane and then reacting this
conjugate with the surface SiOH groups. Other designs from
the same group involved grafting salicylamide 3, 2,3-hydroxypyr-
idinone 4, and phosphonic acid 5ligands. For Ln
3þ
ions (2
ppm), such as La, Nd, Eu, and Lu, the salicylamide and glycinyl-
urea grafts exhibited distribution coefficients (D
Ln
) greater than
10000 mL/g in 0.1 M NaNO
3
solutions at a pH of 4.56.5. X-ray
absorption spectroscopy (XAS) on the 2:1 complex of 3with
Eu
3þ
suggested that the ion is 8-coordinated with a mean EuO
distance of 240 pm.
14
While these elegant but rather complicated syntheses can be
adapted to modification of CPG, we chose a different synthetic
strategy based on the modification of 3-aminopropylaminated
silica, as (i) such materials are available commercially and (ii)
only the simplest designs can be produced on the large scale that
is required for the processing of nuclear waste, which is the
designated application (section 4). As shown in what follows,
modified silica materials exhibiting high distribution coefficients
for Ln
3þ
ions can be obtained using very simple synthetic
methods and inexpensive reagents.
2. EXPERIMENTAL APPROACH
2.1. Materials. The studies described below involved CPG
“Trisoperl”obtained from VitraBio GMbH (Steinach, Thuringen,
Germany). This material has a mean pore diameter of 47 nm and
an inner surface of 107 m
2
/g (see section 1S.1 and Table 1S
in the Supporting Information for comparison with other
materials). Approximately 83% of the glass beads have a diameter
between 100 and 200 μm. This CPG softens above 700 °C and
fuses above 1100 °C. Another starting material was 3-aminopro-
pylated Trisoperl containing 0.22 eq/kg of amino groups, which
is equivalent to 1.4 per nm
2
. For comparison, the density of the
siloxyl groups is 57 per nm
2
. If each 3-aminopropyl group were
conjugated to a ligand forming a 1:1 complex and the occupancy
of these ligands were 100%, this would be equivalent to 3.3 wt %
Ln
3þ
for the maximum loading. Both the starting material and
fused glass were found to be microscopically disordered by
powder X-ray diffractometry.
2.2. Glass Modification. Currently, one of the most popular
methods for surface modification to introduce a ligand for
transition metal ion binding makes use of a conjugate of 3-(gly-
cidoxypropyl)trimethoxysilane (GLYMO) and iminodiacetic
acid (IDA).
13,20,21
This GLYMO-IDA conjugate can be grafted
onto mesoporous silica. As an alternative to this common
synthetic route, Fryxell and co-workers recently developed
synthesis for modification of MCM-41 based on the direct
modification of 3-aminopropyltrimethoxysilane with ethyl
bromoacetate.
13
However, neither this method nor a more
commonly used approach
20,21
produced an mCPG material with
the desired properties (section 1S.2.1). While the material
exhibited a high distribution coefficient D
Ln
(that is, the ratio
of the concentrations of Ln in the solid and liquid phases at the
equilibrium) for traces of Eu
3þ
(D
Eu
≈8700 mL/g for 0.5
ppm Eu), the efficiency of the extraction decreased 10-fold at
510 ppm, suggesting extremely low loading capacity. Examina-
tion of commercial GLYMO-IDA modified silica microspheres
(BioClone, San Diego, CA) with a binding capacity of 55 meq/kg
for Ni
2þ
also showed prohibitively low capacity for lanthanides.
For 20 ppb of Eu
3þ
,D
Eu
was 11 000 mL/g, which is comparable
to the best values for this parameter reported in the litera-
ture,
616
and the estimated binding constant K(assuming 1:1
binding to the GLYMO-IDA ligand) was 10
7.6
M
1
. However,
the maximum loading capacity was only 0.65 meq/kg (0.14
per nm
2
), suggesting that only ∼1% of the GLYMO-IDA ligands
that bind d-ions can bind f-ions. Given that the coordination
number for lanthanide ions is 89 vs 6 for the transition metal
ions, this difference suggests that more than one GLYMO-IDA
ligand is involved in the complexation of the lanthanide ions,
implicating the occurrence of cooperative binding. This is
consistent with the X-ray absorption spectroscopy results of
Fryxell and co-workers
14
for the ligands they studied on MCM-
41 that also implicate the occurrence of cooperative binding. As
the mean distance between the GLYMO-IDA ligands on the
silica surface was ∼0.8 nm, such cooperative binding involves
fortuitously clustered ligands. The fraction of such clusters is
statistically small, reducing the loading capacity.
Given that the GLYMO-IDA-based approach failed for our
CPG materials, two other approaches were pursued. In the first
method, the 3-aminopropyl group of the CPG was reacted with
(di) anhydrides of (amino) carboxylic acids (structures 6and 7in
Scheme 1 and section 1S.2) To address the issue of the
cooperative vs noncooperative binding, we also synthesized a
macrocyclic ligand 8based on 1,4,7,10-tetraazacyclododecane-
1,4,7,10-tetraacetic acid, DOTA
20
(Scheme 1 and section
1S.2.3). In the second approach, a disubstituted imide was
formed by reacting the surface amino group with chloropho-
sphates (structures 9and 10 in Scheme 1 and section 1S.3).
The extent of modification for the 3-propylamiine groups was
characterized using cross-polarization magic angle spinning (CP-
MAS) solid state
13
C NMR (see sections 1S.23 and 2S.1 for
more detail). CP can give up to 4 times signal enhancement by
the dipoledipole transfer of magnetization from the abundant
spin reservoir (
1
H) to the rare nucleus (
13
C). Anything that
attenuates this interaction, distance or motion, distorts the
relative intensities of the carbon-13 resonances. So carbons that
are remote from the proton spins or that are rapidly rotating
(methyl groups) are underrepresented in the spectrum. Unfor-
tunately, without
13
C enrichment, low concentration of organic
material in the CPG matrix obviated the possibility of using other
techniques, such as the Bloch decay NMR. With the latter
4688 dx.doi.org/10.1021/ie102494r |Ind. Eng. Chem. Res. 2011, 50, 4686–4696
Industrial & Engineering Chemistry Research ARTICLE
spectroscopy, one must use a recycle delay with respect to the
13
C nuclei as opposed to CP where the experiment can be
repeated on the relaxation time of the
1
H nuclei. With the 4-fold
loss of intensity and the factor of 20 in repetition rate for signal
averaging, our 12 h
13
C CP-MAS NMR experiment would be a
month long Bloch decay experiment.
For CPG-DTPA conjugate 8(Figure 1S), the resonance line
from the carbonyl at 170180 ppm appeared following glass
modification and a side line from methylene protons was
observed at 60 ppm (vs tetramethylsilane, TMS). The ratio of
the integrals suggested 50% derivatization. The estimated yield
for CPGdGLY modification 7was 40%. Because of the
occurrence of nuclear spin polarization transfer, these estimates
are tentative. It is even more difficult to obtain a reliable estimate
for the extent of the imididiphosphate derivatization, as the
proton polarization can be transferred to the
31
P nuclei. To
address this problem we synthesized CPGNP
2
conjugate 9b by
derivatizing the 3-aminopropyltriethoxysilane and then graft-
ing the resulting product on unmodified CPG. The ratio of
integrals for aromatic and aliphatic protons observed in this
material (which was 1.5:1 instead of the stoichiometric ratio of
8:1) was used to estimate the extent of derivatization via the
reaction of the dichlorophosphate with 3-aminopropylated
CPG (section 1S.3.1), suggesting quantitative conversion of
the amino groups. For CPGNDP
2
conjugate 10 on smooth
silica surfaces, the conversion of the amino groups was also
close to 100%, as suggested by extraction assays (sections 2.3
and 3.2).
2.3. Assaying the Extraction Properties of mCPG Materi-
als. The uptake of lanthanide ions was characterized using time
resolved fluorescence spectroscopy (TRLF), for europium(III)
solutions at pH 47, or inductively coupled plasma mass
spectrometry (ICPMS), for other lanthanide ions and for
Eu
3þ
in acidic solutions. The TRLF assays followed the method
described by Shkrob et al.
1,2
In a typical trial, a suspension of
surface-modified CPG microbeads or smooth silica microspheres
was mixed with 15mLofEu
3þ
solution in 10
4
HNO
3
for 5
min on an orbital shaker, and then the solid phase was separated
by centrifugation; the lanthanide concentration in the aqueous
phase before and after the exposure to the extracting material was
determined (section 2S.2) and distribution coefficients were
assigned accordingly. The site occupancy given below was
calculated assuming that 3-aminopropyl groups were modified
to 1:1 ion-binding sites with a molality of 133 meq/kg (corrected
for the conversion yield according to our solid state NMR
estimates). The mCPG samples loaded with lanthanide ions
were dried at 80 °C, and then sintered and fused for 38 h in an
oxidizing atmosphere at 11001200 °C. The sintering protocols
and analytical methods for analysis of the solid samples are given
in section 2S.3, and the details of the acid leaching test on these
samples are given in section 2S.4.
2.4. Structural Characterization. The structure of Eu
3þ
and
Gd
3þ
complexes was inferred using L-edge extended X-ray
absorption fine structure (EXAFS, section 2S.5) and X-band
electron paramagnetic spectroscopy (for Gd
3þ
, section 2S.6).
3. RESULTS AND DISCUSSION
3.1. Extraction Properties of the mCPG Materials. We first
examine the carboxylated ligands discussed in section 2.2. At 5
ppb of Eu
3þ
, the distribution coefficient D
Eu
for the CPG-DTPA
conjugate 7was 106 000 mL/g, which is comparable to the best
results reported in the literature for MCM-41 modifications.
617
Figure 1a exhibits the loading curve for this material, that is the
fraction of unbound Eu
3þ
ions remaining in aqueous solution vs
the ratio of total Eu
3þ
per site. According to these measurements,
∼60% of Eu
3þ
is bound when 10% of the sites are occupied. At
higher concentrations of Eu
3þ
, the extraction efficiency is
reduced, suggesting that only 10% of the potential Eu
3þ
-binding
sites can be occupied. As noted in section 2.2, inefficient loading
of Eu
3þ
was also observed for smooth silica microspheres
decorated by GLYMO-IDA ligands. Similar results were ob-
tained for the conjugate of ethylenediamine tetraacetate (EDTA)
and CPG-DOTA derivative 8. The common feature of such CPG
modifications was a high D
Eu
value at trace levels of Eu
3þ
ions
and a sloping loading curve (Figure 1). The loading capacity of
the CPG-DTPA and CPG-DOTA conjugates (estimated from
the concentration corresponding to 50% loading) were similar to
other designs examined in this study.
Surprisingly, these complex, polydentate ligands did not per-
form significantly better than a much simpler CPGdGLY
modification. For conjugate 6, >97% of Eu
3þ
was removed from
the aqueous phase when the occupancy was <0.03 per site, but at
0.3 per site, only 12% of the ions were bound in the glass
(Figure 1a). At 40 ppb Eu
3þ
, only 0.4% of Eu
3þ
ions remained in
Scheme 1. Ln
3þ
Binding Ligands Obtained via Chemical Modification of 3-Aminopropylated Silica
4689 dx.doi.org/10.1021/ie102494r |Ind. Eng. Chem. Res. 2011, 50, 4686–4696
Industrial & Engineering Chemistry Research ARTICLE
the solution (140:1 g/g mass ratio), so D
Eu
was 48 000 mL/g.
While the loading curve for this conjugate is more steplike, the
extraction efficiency for Eu
3þ
dramatically decreases at 10%
occupancy of the potential binding sites. We have assayed the
Ln
3þ
uptake in phosphate buffer saline (pH 67), and 5 wt %
and 20 wt % sodium chloride; the increase in ionic strength and
buffering had little effect on the extraction efficiency in the high
loading regime.
Turning to the CPGNP
2
modification 9, which is the only
neutral ligand in Scheme 1, it is possible to directly demonstrate
the formation of the mixed ligand surface complexes using TRLF
spectroscopy (section 2S.2). For conjugates 9, the luminescent
mixed ligand complex of CPGNP
2
-Eu
3þ
and the base form of
2-thenoyltrifluoroacetone (TTA
) is formed at pH 4. The
lifetime τof this complex emitting light at 620 nm
(corresponding to the
7
F
2
r
5
D
0
transition) was ∼0.45 ms. Once
this complex was formed, the luminescence persisted in the
presence of 10 mM EDTA, suggesting strong Ln
3þ
binding by
the complex (see below). The number n
OH
of luminescence
quenching OH groups in the first coordination sphere is related
to τthrough the empirical equation
23
n
OH
= 2.1/τ(ms) 1.4
suggesting that the CPGNP
2
Eu(TTA)
3
complex has, on
average, one or two water molecules coordinated to the lantha-
nide ion (Scheme 2). As the TTA
is a bidentate ligand and Eu
3þ
typically is 810 coordinated, this suggests that the iminodipho-
sphate conjugate 9is a bidentate ligand that coordinates the
lanthanide ion through its PfO groups as shown in Scheme 2
(Hyperchem structural models are shown in Figure 6S).
For the NP
2
modified aminated polystyrene microspheres, a
loading capacity of 50 meq/kg was estimated in 10
4
HNO
3
and
log Kwas determined to be 7.1.
1,2
For NDP
2
modified silica
microspheres, the uptake of Eu
3þ
from 0.1 mM nitric acid
solutions was estimated at 30 meq/kg (Figure 2a), which is close
to the concentration of the amino groups (20 meq/kg). There-
fore, it seems likely that Eu
3þ
forms the 1:1 complex with
conjugate 10 on silica. Figure 2b exhibits the acidity dependence
for Eu
3þ
extraction by such silica microspheres. The distribution
coefficient D
Eu
at infinite dilution decreases from 30 000 mL/g at
0.1 mM HNO
3
to 300 mL/g at 0.1 M HNO
3
. This acidity
dependence allows for extraction of Ln
3þ
ions at low acidity and
stripping of these ions at high acidity. This acidity dependence
implies that some of the hydroxyls in the methanediphosphate
units of the NDP
2
are dissociated. By analogy with similar
complexes studied by Herlinger et al.
24
we hypothesize that
Ln
3þ
is coordinated in a manner shown in Scheme 2 (see also the
Hyperchem structural model shown in Figure 7S). We stress that
this structure remains conjectural, although it is consistent with
X-ray spectroscopy data given below.
In CPG nanochannels, the loading curves for 9a and 10 are
similar (Figure 5), suggesting a loading capacity of 34 mmol/
kg. The D
Eu
values at infinite dilution are also similar, about
17 000 and 30 000 mL/g, respectively. Both of these modifica-
tions showed a sigmoidal loading curve similar to the one
observed for 6.
Figures 3, 4, and 8S exhibit the ionic radius dependence of D
Ln
at fixed nominal loading. Figure 3 exhibits the dependency of the
distribution coefficient obtained for the lanthanides by modifica-
tion 6on the lanthanide ionic radius (33:1 extraction from 10
4
M HNO
3
containing five ions at a total concentration of 24 μM;
the occupancy of sites is 0.6%, 120 ppm loading of glass). The
distribution coefficients for EuLu correspond to about
6070% that of La. Very similar size dependency was obtained
for modification 10 (Figure 4), whereas for 9a (Figure 8S) there
was no size dependence (excepting La
3þ
). For 6and 10, the
gradient of D
Ln
with ionic radius (“Gd number”) compares with
this parameter for ion-exchange resins,
25
suggesting that these
mCPG materials can be used for chromatographic separation of
lanthanides.
A question of considerable practical interest is whether these
mCPG materials can extract Ln
3þ
ions in the presence of
2050 mM DTPA and 12 M HL/L
buffer (where L is
lactate) which is present in the spent raffinate of the TALSPEAK
Scheme 2. Possible Structures for the Complexes of Conju-
gates 9 and 10 with Lanthanide Ions
Figure 1. (a) The fraction of free Eu
3þ
ions remaining in aqueous
solution (10
4
M nitric acid) following extraction using CPG-DTPA
(open circles) and CPGdGLY (filled diamonds). The sample-average
concentration of Eu
3þ
is given in the units of sites, assuming 1:1 binding
to the ligands and taking the surface density of the binding sites to be 133
meq/kg. Unless stated otherwise, the extraction was carried out from
0.1 mM nitric acid. (b) The same, for CPG-DOTA, CPGNP
2
, and
CPGNDP
2
(conjugates 8,9a, and 10 in Scheme 1). See the inset for
disambiguation; 50% loading corresponds to a nominal capacity of 3.5
meq/kg, and the total concentration of the Eu
3þ
ions is given in mol/kg.
4690 dx.doi.org/10.1021/ie102494r |Ind. Eng. Chem. Res. 2011, 50, 4686–4696
Industrial & Engineering Chemistry Research ARTICLE
process used for separation of trivalent lanthanides from minor
actinides in spent nuclear fuel reprocessing.
2628
Our trials
indicated that modifications 6and 9a (Figure 5a) cannot
compete with DTPA at pH 4 when the concentration of DTPA
exceeds 0.1 mM. In this experiment the phase ratio was 200:1 g/g
and the nominal loading for 9 μMEu
3þ
was 2 mmol/kg, so that
the liquid sample average concentration of the binding sites on
mCPG was 30 μM. It is seen from the plot that the DTPA begins
to compete for Eu
3þ
with the bound NP
2
ligands when the
number of the DTPA molecules is comparable to that of the NP
2
biding sites as estimated from the loading curves). In contrast,
conjugate 10 (under the same trial conditions) was competing
with the DTPA up to [DTPA] = 10 mM, suggesting much
stronger binding of Ln
3þ
at the NDP
2
modified surface. Even at
2 mM, the D
Eu
was 800 mL/g, which is only three times lower
than D
Eu
at [DTPA] = 10 μm. In this experiment, the pH was
buffered at 4.0, so the degree of ionization of DTPA was fixed. In
Figure 5b, we plotted D
Eu
as a function of lactic acid (pK
a
=
3.67
23
) concentration, where the latter serves as a buffer in the
TALSPEAK process. The pH of the solution was maintained at
4.0 throughout the series by the addition of ammonia. As seen
from this plot, in the decimolar to molar range, the distribution
coefficient D
Eu
rapidly decreases with increasing concentration of
lactate (HL and L
) in the 0.11 M range, suggesting the
formation of [EuL
n
]
3-n
complexes in solution
23
which can
compete with the CPGNDP
2
groups. Despite this competi-
tion, even in 0.1 M lactate solution D
Eu
≈1000 mL/g. Combin-
ing the data from Figure 5, we conclude that direct extraction of
trivalent elements from TALSPEAK solutions is feasible using
the CPGNDP
2
.
While such performance is remarkable, our data point to a
systemic problem with the CPG materials, that being low loading
capacity (partly, due to their relatively low specific surface). This
low capacity cannot be explained by inefficient modification or
the presence of unmodified 3-aminopropyl groups. Indeed, this
Figure 3. Ionic radius dependence of D
Ln
for the CPGdGLY
conjugate 6in extraction of 0.61 ppm lanthanide ions (33:1 g/g in
10
4
M HNO
3
). The concentrations were determined using ICPMS;
the data were collected using two lanthanide ion mixtures of five
elements each at a total concentration of 3.6 ppm of Ln
3þ
. After the
extraction, the concentration of Ln
3þ
in the glass was 0.8 meq/kg (120
ppm), which is equivalent to a Ln
3þ
/site ratio of 1.8 10
2
(assuming
133 meq/kg of the active dGLY sites and 3:1 binding).
Figure 4. As Figure 3, for CPGNDP
2
. Several nominal loadings are
shown in the plot. The lines are guides for the eye.
Figure 2. (a) Loading-dependence for NDP
2
modified smooth 1 μm
silica spheres. The fraction of Eu
3þ
remaining in solution is plotted vs
nominal loading. Two different syntheses are plotted together. (b) The
dependence of D
Eu
(at infinite dilution) on the concentration of nitric
acid in the aqueous solution.
4691 dx.doi.org/10.1021/ie102494r |Ind. Eng. Chem. Res. 2011, 50, 4686–4696
Industrial & Engineering Chemistry Research ARTICLE
loading capacity remains low even when the ligands are grafted
on CPG. This capacity does not improve with increasing ionic
strength and buffer concentration, which is also inconsistent with
the interference from unmodified 3-aminopropyl groups.
One way to rationalize these observations is to postulate that a
small fraction of the ion-binding sites has high binding constants
whereas the majority of such sites have much lower binding
constants.
For some ligands, such speciation can be accounted for by the
occurrence of cooperative binding involving neighboring ligands
(see Scheme 3, panel a). The graft density of 1.5 per nm
2
corresponds to a mean spacing of 0.8 nm between the grafts,
which is conducive to cooperative binding by the sufficiently large
and flexible ligands. Because grafting imposes conformational
constraints, the individual polyaminopolycarboxylate ligand may
not fold around the ion as freely as in solution, so 1:1 binding
might be inefficient, especially when neighboring grafts already
form a complex that makes a large footprint (Scheme 3, panel b).
At low ion concentrations, several grafts can bind the ion
cooperatively. At higher concentration of the lanthanide ions,
there is not a sufficient number of such sites to provide efficient
binding, whereas if the ion is already associated with some groups
on a given graft, that graft cannot bind more Ln
3þ
ions. This leads
to the situation shown in panel a in Scheme 3, where a single ion
is bound by three dGLY ligands. The binding by such clustered
groups can be very efficient, and yet the loading capacity cannot
be as high as assumed by extrapolation from the low-loading
regime. In Figure 9S, it is shown that the flexibility in grafts 6is
sufficient for the occurrence of cooperative binding of the
lanthanide ions provided that the tripod (dGLY)
3
grafts are
located either on next- or second-neighboring SiO
4
tetrahedra.
Assuming that only grafts with stems separated by 0.5 nm can
bind the f-ion in a cooperative fashion and 50% modification
suggested for this mCPG by NMR, for a surface density of 1.4
groups per nm
2
, there are only 14% of the active (dGLY)
2
sites
and 2% of the (dGLY)
3
sites.
3.2. Fusion of CPG Materials. In these experiments, dry
mCPG samples containing Ln
3þ
ions were heated to 1200 °C.
Pyrolysis of the organic component begins at 500680 °C. In
vacuum-sealed tubes, the sample remains in this carbonized form
at 1200 °C (Figure 6a). When the sample is exposed to air, O
2
diffuses in and the charred organic component is fully oxidized
over 14 h as the glass sinters, resulting in a completely carbon-
free, fused glass sample (Figure 6b). When the extraction of Ln
3þ
occurs in the presence of 0.1 M NaNO
3
,∼180 meq/kg of nitrate
Scheme 3. Cooperative (a) vs Non-cooperative (b) Binding
of f-Element Ions by Ligands (6)
Figure 6. A CPGdGLY-Gd
3þ
sample heated at 1200 °C for 3 h (a) in
vacuum and (b) in air.
Figure 5. (a) The effect of DTPA (all forms) on the extraction of Eu
3þ
by CPGNDP
2
(10, circles) and CPGNP
2
(9a, squares) (200:1 g/g,
9μMEu
3þ
). A pH of 4.0 was maintained throughout the series, and
[DTPA] stands for all forms of the pentetic acid. For CPGNP
2
two
series of measurements analyzed using TRLF (filled squares) and
ICPMS (open squares) are shown separately. (b) The effect of
lactate/lactic acid on the extraction of Eu
3þ
by CPGNDP
2
; pH 4.0
was maintained throughout this series, and [lactate] stands for all forms
of lactic acid. The line is a guide to the eye.
4692 dx.doi.org/10.1021/ie102494r |Ind. Eng. Chem. Res. 2011, 50, 4686–4696
Industrial & Engineering Chemistry Research ARTICLE
anions are trapped in the glass, and the decomposition of the
nitrate accelerates the oxidation. While this method introduces
0.4 wt % sodium into the fused CPG, this is comparable to trace
amounts of sodium that is already present in the glass matrix. The
sintering and fusion of the glass was observed both for organic
and organophosphate ligands.
A particular concern for the use of mCPG as a matrix for
radionuclides and toxic elements is the loss of metal ions during
the fusion. As seen from the digestion analyses of the loaded,
fused samples (section 2S.3), the loss of lanthanide ions during
the fusion was negligible. To test whether the channels com-
pletely collapsed, sealing Ln
3þ
ions in the glass during heat
treatment (section 2S.4), a sample of fused glass doped with
Eu
3þ
was finely ground, dispersed in 1 M nitric acid, and stirred
vigorously for 40 h at 25 °C. The solution was periodically
centrifuged and aliquots were taken, filtered, and then analyzed
using ICPMS. Upon the contact with the acid, 7.5% of the
lanthanide was promptly dissolved from the surface of the
ground glass. Over the next 40 h, only 0.85% more of the original
Eu
3þ
leached from the sample (Figure 7), suggesting that the
bulk of the lanthanide resides in the collapsed pores that are
inaccessible to the nitric acid in the solution. Once the surface
material is dissolved by the acid, there is very little further
dissolution. This is an extreme test as compared to the standard
product consistency test for glass waste forms that involves
exposure of ground glass to demineralized neutral water.
29
This
test suggests that the nanochannels collapse in the sintered
sample with the lanthanide ions buried in the bulk of the glass
form. The same conclusion was reached in the structural studies
examined below.
3.3. Structural Characterization of Ln
3þ
Complexes. For a
given CPG modification, the EXAFS patterns were the same for
all Eu-doped samples reagrdless of the doping level (section 2S.5
and Table 3S) and were indistinguishable between Eu- and Gd-
doped samples (Figure 10S). The loading of 6from 0.1 M
NaNO
3
solutions yielded the same EXAFS spectra as for the
fused and unfused samples loaded from aqueous solutions with-
out the nitrate.
As shown in Supporting Information Figure 10S and
Figure 8a, the EXAFS spectra for CPGdGLY divided into
two types observed before and after 11001200 °C glass fusion.
The phase corrected modulo Fourier transform of k
2
χ(k)
oscillations (Figure 8b) maps on the distances between the
source Ln atom and the scattering atoms, such as oxygen. The
height of the first large peak in the plot (at ∼250 pm) increases
with the coordination number of the lanthanide ion. Semiqua-
litative analysis of the data suggests that Ln
3þ
ions in 6before
fusion have a greater coordination number (89) than the ions
in fused glass (67) and the LnO distances are considerably
shortened in the fused sample (by 1020 pm).
Using crystallographic data from refs 30 and 31 in Figure 11S,
we simulated the EXAFS spectrum in cubic Gd
2
O
3
and mono-
clinic Eu
2
O
3
(as both forms of the oxides are commonly
occurring) and juxtaposed it against the experimental data for
fused samples. It is seen that the first coordination shell observed
in the oxides (with LnO distances of 220225 pm and
coordination number of 67) provides a good fit for the data.
On the other hand, the experimental spectra would be incon-
sistent with the oxide, as the LnLn peaks are absent, suggesting
that Ln
3þ
ions are dispersed rather than clustered. For Ln
3þ
-
loaded samples before fusion, crystallographic data for sodium
tris(oxidiaceto) europium(III) hexahydrate
32
were used to si-
mulate the EXAFS (Figure 12S). In this structure, Eu
3þ
is
coordinated by three (free) dGLY moieties with nine nearest-
neighbor oxygens at 233254 pm from the central ion. Com-
parison with the known structures in which the carboxylic groups
were amidated suggests that the coordination always involves the
carbonyl oxygen as opposed to the nitrogen of the peptide bond,
so the mode of Ln
3þ
binding to the tripods shown in Scheme 3a
and Supporting Information Figure 9S should be similar to this
complex (shown at the top of Figure 12S). The simulated
Figure 7. Loss of Eu
3þ
from fused CPGdGLY glass doped with 1000
ppm Eu
3þ
vs exposure to 1 M nitric acid. The fused, sintered glass was
finely ground before dispersal in solution. The 10 mL sample was
continuously stirred during this trial.
Figure 8. (a) Comparison between EXAFS spectra obtained for Gd-
and Eu-doped CPGdGLY samples (∼5 meq/kg). (b) Phase corrected
modulo Fourier transform of traces shown in panel a. Correspondence
to Table 3S: for Eu, (i) 1, (iv) C; for Gd, (ii) 4, (iv) B. The arrows
indicate the centroids of the EuO ligand shells. When the glass is fused,
the coordination number and the EuO distances decrease.
4693 dx.doi.org/10.1021/ie102494r |Ind. Eng. Chem. Res. 2011, 50, 4686–4696
Industrial & Engineering Chemistry Research ARTICLE
R-domain EXAFS closely resembles the one obtained for
CPGdGLY in Figure 8b suggesting that the binding is co-
operative and involves three dGLY grafts. As the sample is
calcined and fused, the Ln
3þ
ions become hexacoordinated.
The absence of prominent LnLn peaks indicates that no new
phase is formed.
In Figure 13S we compare EXAFS for the five CPG modifica-
tions shown in Scheme 1. It is seen from this plot that with
exception of dGLY, the rest of the modifications yield surpris-
ingly similar EXAFS patterns. In Figure 14S, we compare EXAFS
for fused mCPG samples for dGLY, NP
2
, and NDP
2
modifica-
tions. In Figure 9 we compare the corresponding R-domain
Fourier transforms of the k
2
χ(k) plots (no phase shift
corrections). As seen from these plots, the position of the first
prominent peak at ∼180 pm is the same for all unfused samples;
the main difference is in the atoms at distances beyond 200 pm.
Fusion decreases the coordination number in CPGdGLY (as
seen from the reduced height of the first peak), but the
coordination changes with fusion much less for NP
2
and NDP
2
ligands, suggesting that lanthanide ions are coordinated through
the phosphate groups residing in the silica matrix. This argues
against the migration of the Ln
3þ
ions into the bulk: the ions
remain sealed in the collapsed channels bound to the phosphate
groups. The position of the second peak around 270 nm
corresponds to such positions for LnPO
4
,
33
but the third peak
at 320 pm is greatly reduced as compared to crystalline Ln
2
O
3
and LnPO
4
, suggesting that no Ln oxide or phosphate phases
were formed. Thus, even at the maximum attainable loading in
our mCPG samples, the Ln
3þ
ions remain dispersed and isolated
in the collapsed glass channels.
Counterintuitively, for unfused mCPG samples (Figure 9) the
dGLY modification results in the most ordered Ln complex,
while DTPA yields a less ordered complex and DOTA results in
still less ordered complex. In the latter two EXAFS patterns, there
are prominent peaks at 250 and 300320 nm. The same peaks
with similar amplitudes were observed in aqueous solutions and
crystals of Gd(aq)(DOTA)
and Gd(aq)(DTPA)
2
.
34
The
main peaks were attributed to scattering on the carboxylate
and water oxygens (at 238 and 249 pm, respectively), skeletal N
atoms (at 265 pm), and C atoms (320360 pm). The R-domain
EXAFS pattern for DOTA is remarkably similar to the pattern for
Gd (DOTA)
in aqueous solution, thereby suggesting the
formation of a 1:1 complex, while for DTPA the first peak is
significantly reduced in the amplitude as compared to Gd(aq)-
(DTPA)
2
, suggesting less coordination. The latter could be
caused both by the elimination of one of the carboxyl groups
(through peptide bond formation in 7) or conformational strain
due to crowding at the surface. However, it transpires that for
GPG-DTPA and CPG-DOTA the complex is formed without
the occurrence of cooperative binding, whereas the structure of
the complex for CPGdGLY is indicative of this cooperative
binding.
To further ascertain the nature of CPG-DTPA complexation
of Ln
3þ
, we used EPR spectroscopy. Gd
3þ
(f
7
) is a spin-
7
/
2
paramagnetic ion (
8
S
7/2
) which makes it a convenient reporting
element, as it is an orbital singlet, which greatly simplifies the
EPR spectra that look like a broad singlet from its
7
/
2
f
5
/
2
spin transition. The width of the spectrum is characterized by
ΔB
pp
which is the difference between the field where the first
derivatives of the EPR signal reaches their highest (positive) and
lowest (negative) values. This width is mainly controlled by spin
relaxation in this ion which is determined by the rate of
modulation and the magnitude of the static zero-field splitting
(ZFS) tensor D,defined from the corresponding contribution to
the spin-Hamiltonian, S3D3S=D(S
z2
S(Sþ1)/3) þE(S
x2
S
y2
), where the typical ZFS parameter |D| and the anisotropy ξ=
|E/D| are 0.030.06 cm
1
and 0.140.2, respectively.
3537
The
accepted model for spin relaxation of Gd
3þ
complexes in
solution involves modulation of the ZFS tensor by rotation of
the complex and collisions with solvent molecules. In dilute
aqueous solutions, the observed species is the [Gd(aq)
8
]
3þ
complex with D
4d
symmetry and a rotation correlation time of
20 ps.
36,38
For chelated Gd
3þ
, this time is generally longer, about
80 ps, which results in a different EPR spectrum.
Figure 15S exhibits the first derivative 9.445 GHz spectra
obtained at 50 K. In crystalline [Gd(aq)
6
]
3þ
nitrate (trace iii),
ΔB
pp
is >4 kG (1 G =10
4
Tesla). For 3 mM Gd
3þ
in 0.1 mM
nitric acid (trace i) this parameter is 1183 G, whereas in the
solution containing 0.1 M EDTA (trace ii), Gd
3þ
is bound to
EDTA, and for the complex ΔB
pp
is much smaller. For
[Gd(aq)(DTPA)]
2
,ΔB
pp
= 570 G has been reported in ref
38. For the aqueous suspension of 50 g/L slurry of CPG-DTPA-
Gd
3þ
beads (>98% extraction into the glass), the width of the
EPR spectrum reduces from 1183 to 733 G (trace iv), suggesting
that Gd
3þ
is in a bound form. When the beads are allowed to
precipitate and form a slurry at the bottom of a sample tube, the
line width for this slurry is 680 G (trace v). This line narrowing
indicates polydentate Gd
3þ
binding in a form that resembles
binding by isolated DTPA molecules. While the resonance line in
this surface complex is broader than in the free [Gd(aq)
(DTPA)]
2-
complex, the difference can be readily accounted
for by conformational strain and the missing carboxylate group in
the CPGDTPAGd
3þ
complex.
The removal of water by heating the sample at 200 °C changes
the EPR spectrum to the one observed for solid Gd(aq)
6
(NO
3
)
3
. This extreme line broadening indicates the occurrence
of Gd
3þ
ion clustering:
39,40
the paramagnetic ions are sufficiently
close to each other that their magnetic dipoles interact, broad-
ening the EPR line. This broadening becomes prominent even at
distances of 12 nm, so this “clustering”does not imply the
Figure 9. R-domain EXAFS spectra for mCPG-Eu
3þ
glasses before
(solid lines) and after (dashed lines) high temperature fusion (no phase
correction). The first peak indicated by an arrow corresponds to the
oxygen atoms in the first coordination sphere. The dashed lines
correspond to fused glass samples: (i) dGLY (6), (ii) DTPA (7), (iii)
DOTA (8), (iv) NDP
2
(10), and (v) NP
2
(9a). See the inset for
disambiguation. The loaded samples were dried at 80°for 24 h. The
concentration of Eu
3þ
was 5 mmol/kg.
4694 dx.doi.org/10.1021/ie102494r |Ind. Eng. Chem. Res. 2011, 50, 4686–4696
Industrial & Engineering Chemistry Research ARTICLE
formation of binuclear complexes or new phases. We conclude
that both the EXAFS and EPR data are consistent with the
formation of the CPG-DTPA-Gd
3þ
complex, which exhibits less
order than the [Gd(aq)(DTPA)]
2-
complex but has the same
structural motif. There is no supportive evidence for cooperative
binding in this system.
3.4. Possible Causes for Reduced Ln
3þ
Capacity. Given the
results examined in the previous two sections, the occurrence of
cooperative binding is unlikely for all but one of our ligand
designs. Solid state NMR spectra exclude low yield of modifica-
tion, and Ln
3þ
ion uptake assays exclude the effect of unmodified
surface groups. To probe whether the relatively low loading
capacity is an exclusive property of Trisoperl CPG, we modified
several chromatographic silicas with 1 mol/kg of the NH
2
groups
and pore diameters of 5-to-10 nm. The loading capacity for such
materials was still in the range of 13 mmol/kg (<1 wt %),
suggesting a systemic problem. We note that the Gd
3þ
loading
capacity of DTPA-modified amorphous mesoporous silica (after
normalization by the surface area)
18
was not significantly differ-
ent from these mCPG materials. This suggests a general pattern
that needs be explained.
One possibility (that we were suggested by an anonymous
reviewer) is that the nitrogen in some of the modified 3-amino-
propyl groups is protonated and the resulting positively charged
group forms a bond with the SiOH group at the surface.
41
The
formation of this complex interferes with the chelating of the
metal ions. Several observations argue against the occurrence of
such interference: (i) the
13
C resonance of C
β
in the amino
propyl group is more consistent with unprotonated site, (ii) there
is very little effect of the ionic strength and buffers on the metal
ion uptakes, whereas the strength of the interaction should be
affected by the conditions in the solution, and (iii) while the
primary amines can be readily protonated, this protonation
cannot be facile for amides and imides.
Another possible explanation for these observations hinges on
the morphology of the amorphous nanoporous materials: since
the pore size distribution is extremely broad, some channels are
connected through fairly narrow “bottlenecks.”Once the surface
of the nanochannels is modified, the channel diameter narrows
further and the ions may have limited access to some parts of the
glass interior, especially when the grafted ligands form complexes
with metal ions. If this rationale is correct, the relatively low
loading capacity may be inherent to this class of nanomaterials.
4. CONCLUDING REMARKS
We demonstrated several methods for chemical modification
of CPG by ligands that sequester trivalent ions from pH 36
aqueous solutions. These carboxylated or organophosphate
ligands are destroyed during high-temperature (1100 °C) oxida-
tion of these nanoporous materials that fuse and bury the
embedded metal ions. This temperature range favorably com-
pares to >1700 °C that is required for vitrification of lanthanide
oxides in borosilicate glass.
29
The fused, ground CPG glass
retained the embedded lanthanide ions, and spectroscopic
studies indicate that the Ln
3þ
ions were dispersed in the glass
without clustering. These Ln
3þ
ions were fully integrated into the
silica network.
Counterintuitively, structurally complex polydentate ligands
(that are currently pursued by several other groups)
620
did not
exhibit superior performance as compared to simple modifica-
tions 6,9, and 10 in Scheme 1. The mCPG showed considerable
activity toward trace amounts of Eu
3þ
with distribution coeffi-
cients D
Ln
in excess of 10 000 mL/g. The most efficient
modification using nonphosphate ligands proved to be 6, for
which ca.1020% of the ligands can be bound to the Ln
3þ
ion.
We showed that this binding is cooperative: it takes three
neighboring ligands to bind one trivalent ion. Two of our
organophosphate designs (9a and 10) were also very efficient.
The extracted lanthanide ions can be immobilized by glass fusion
or back extracted using concentrated nitric acid.
The tolerance of imidodi(methanediphosphate) 10 to the
presence of strong complexants in aqueous solution (section 3.1)
presents considerable practical interest. In the TALSPEAK process
(which is an important part of reprocessing for advanced nuclear
cycle concepts),
26
the aqueous raffinate consists of 0.52 M lactic
acidlactate (serving primarily as a buffer) and 2050 mM DTPA,
while the organic solvent contains bis-(2-ethylhexyl) phosphoric
acid (HEDHP). In section 3.1, we foundthat NDP
2
ligand 10 can
readily compete with DTPA: the distribution coefficients D
Eu
remain high (500800 mL/g) even when 2050 mM DTPA is
present in the solution. Conversely, the distribution coefficients are
1000100 mL/g when 0.10.5 M lactic acid is present in the
solution (200:1 g/g phase ratio). Thus, using this CPGNDP
2
material, lanthanide/actinide ions can be extracted directly from
TALSPEAK solutions without pH adjustments (which are com-
plicated by the presence of the lactate buffer).
The end point ofthe TALSPEAK process has yet to be decided
upon.
26,27
Current methods demonstrate that removal of trivalent
ions from the spent TALSPEAK aqueous phase can be achieved
using 1 M HDEHP extractions with large aqueous-to-organic
ratios,
28
and the residual solution (still having expensive
chemicals) can be evaporated and calcined to produce small
quantities of lanthanide oxides that are subsequently vitrified. This
is a multistepprocess yielding small quantities of Ln in a dispersed
form. Using CPGNDP
2
the Ln
3þ
ions can be sequestered from
the TALSPEAK raffinate (which can be subsequently reused
rather than sacrificed). No adjustments of pH are necessary before
and after the extraction. The sequestered ions can be imbedded
into a matrix for further vitrification (see below).
The problematic part of using the mCPG nanomaterials we
developed is their relatively low loading capacity which makes
such materials unsuitable as a high-level waste form for storage of
the radionuclides. The benefit of lowering of the processing
temperature and the ease of sequestration is opposed by this
insufficient loading capacity. Nevertheless, the mCPG materials
can be used to concentrate, sequester, and immobilize fission
radionuclides from dilute aqueous waste streams, thereby closing
various processing cycles. The resulting (fused or unfused) glass
can be used to introduce SiO
2
into the borosilicate glass and used
for further disposal of concentrated radionuclides.
’ASSOCIATED CONTENT
b
SSupporting Information. The synthetic methods and
assay protocols; spectroscopy methods; details of the fusion
and leaching experiments, and additional tables and figures with
captions. This material is available free of charge via the Internet
at http://pubs.acs.org.
’AUTHOR INFORMATION
Corresponding Author
*Tel.: 630-252-9516. E-mail: shkrob@anl.gov.
4695 dx.doi.org/10.1021/ie102494r |Ind. Eng. Chem. Res. 2011, 50, 4686–4696
Industrial & Engineering Chemistry Research ARTICLE
Notes
†
Permanent affiliation: Chemistry Department, Benedictine
University, 5700 College Road, Lisle, IL 60532.
’ACKNOWLEDGMENT
The authors gratefully acknowledge the work of collaborators
and those providing support, technical guidance, and review:
W. Ebert, A. Guelis, Y. Tsai, S. Naik, D. Gracszyk, L. Soderholm,
J. Schlueter, S. Skanthakumar, and other colleagues at Argonne.
Programmatic guidance provided by T. Todd and J. Vienna is
gratefully acknowledged. The submitted manuscript has been
created by UChicago Argonne, LLC, Operator of Argonne
National Laboratory. Argonne, a U.S. Department of Energy,
Office of Science laboratory, is operated under Contract No. DE-
AC02-06CH11357. Financial support from the Department of
Energy, Office of Nuclear Energy’s Fuel Cycle Research and
Development Separations and Waste Campaign, Contracts No.
AN1015030401 and FTAN11SW090, is gratefully acknowledged.
’NOMENCLATURE
CPG = controlled pore glass
CP-MAS = cross-polarization magic angle spinning
DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid
D
Ln
= distribution coefficient for lanthanide ions
DTPA = diethylamine pentaacetic acid
dGLY = oxidiacetate moiety
EDTA = ethylenediamine tetraacetic acid
EPR = electron paramagnetic resonance
EXAFS = extended X-ray absorption fine structure
FT = Fourier transform
fwhm = full width at half-maximum
GLYMO = 3-glycidoxypropyltrimethoxysilane
HDEHP = bis-(2-ethylhexyl) phosphoric acid.
ICPMS = inductively coupled plasmamass spectrometry
IDA = iminodiacetic acid
Ln = lanthanide
mCPG = surface modified controlled pore glass
MCM-41 = Four nm diameter mobile crystalline material
NP
2
= imidodi(diphopshate)
NDP
2
= imidodi(methanediphoshate)
TALSPEAK = trivalent actinidelanthanide separation by phos-
phorus reagent extractionfromaqueouskomplexes
TMS = tetramethylsilane
TTA = 2-thenoyltrifluoroacetone
TRLF = time-resolved laser fluorescence spectroscopy
NMR = nuclear magnetic resonance
ppm/ppb = part per million/billion, g/g
XAS = X-ray absorption spectroscopy
ZFS = zero-field splitting
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