Investigation of gamma radiation effect on Amberlite IRA-400 hydroxide
anion exchange resin by FTIR and 13C NMR spectroscopy
A. Traboulsia,b, N. Dupuyb, C. Rebufab*, M. Sergentc and V. Labeda
a CEA
Marcoule, DEN/DTCD/SPDE/SPDE, BP 17171, 30207 Bagnols-sur-Cèze, France
UMR-CNRS 6263, Université Paul Cézanne, case 451, 13397 Marseille cedex 20, France
c ISM2, UMR-CNRS 6263, Université Paul Cézanne, 13397 Marseille cedex 20, France
b ISM2,
Abstract
Radiation-induced decomposition of the anion exchange resin Amberlite IRA-400 in
hydroxide form by gamma radiolysis has been studied under different irradiation doses and
irradiation atmospheres. In this work, we focused on the degradation of the solid part resin by
Fourier Transformed Infra-Red (FTIR) and
13C
Nuclear Magnetic Resonance (NMR)
spectroscopy associated with chemometric treatments. FTIR and
13C
NMR spectroscopy
showed that only −CH2N+(CH3)3 groups were detached from the resin whereas the polystyrene
divinylbenzene backbone remains intact. The presence of water was found to be without effect
on resin degradation whereas oxidation occurs in presence of oxygen leading to the formation
of carbonyl groups. Principle Components Analysis (PCA) was used to classify the degraded
resins according to their irradiation conditions. The PCA loadings have shown spectral regions
which discriminate the irradiated resins whereas SIMPLe-to-use Interactive Self-modeling
Mixture Analysis (SIMPLISMA) allows to identify component families characterizing the
chemical structure of resins in terms of their irradiation conditions.
Keywords: ion exchange resin, FTIR, 13C NMR, gamma irradiation, chemometric.
1. Introduction
Ion exchange resins are widely used in decontamination processes in the nuclear industry.
After use, these resins are considered and managed as low- and intermediate-level nuclear
wastes, although their management is still studied in some cases. During interim storage and
after disposal, ion exchange resins are subjected to ionizing irradiation due to the presence of
nearby radionuclides in the containers, which causes their decomposition and modifications in
their chemical structure.
As reported in the literature, most gamma-induced decomposition of ion exchange resins
is related to matrix aging, which leads to the production of gases and other degradation
products [1−4]. This has a potential impact on the integrity and the security of waste packages.
Radiolysis gases may present overpressure and even explosion and flammability risks if not
removed, and depending on their nature they may also be corrosive [4]. Therefore
2
investigating the effect of ionizing irradiation on ion exchange resins is very important to
ensure secure and safe management of this kind of nuclear waste. Resins mostly used by the
nuclear industry are commercial materials of nuclear grade, constituted by sulfonic (cation
resin) and trimethylammonium (anion resin) functional groups fixed on a polystyrenedivinylbenzene backbone, or a mixture of them (mixed bed ion exchange resin). Most studies
of ion exchange resins investigate radiation-induced modifications in their physical and
chemical properties such as their total exchange capacity, pH and moisture content [2, 5, 6, 7].
Other studies have also investigated gas or other species generated during the resin radiolysis
[1, 2, 8, 9]. In these works, various ion exchange resins in different ionic forms were irradiated
under different irradiation conditions (moisture content, irradiation environments, irradiation
doses, etc.). Modifications in the resin chemical structure were explained by probable
mechanisms involving radiolytic splitting of the functional groups and deterioration of the
resin matrix (cross-linking, oxidation, etc.).
In our work we were interested in studying the gamma radiation effects on the solid state
of the anion exchange resin Amberlite IRA-400 in its hydroxide form under different
irradiation doses and three atmospheric conditions (aerobic, anaerobic environments and
anaerobic atmosphere with water). This resin is constituted of a polystyrene-divinylbenzene
backbone with trimethylammonium functionality (−CH2N+(CH3)3 …OH-). Gas evolution and
leaching studies on irradiated resins were also done in parallel but will not be discussed in this
paper. Earlier papers relate that anion exchange resins exhibit low gamma radiation stability
even at low doses. Their degradation starts at 0.1 MGy by splitting off the functional groups
from the organic backbone due to the low binding energy between them [6, 10]. In presence of
oxygen and water, oxygenated compounds could be formed by oxidation occurring during
irradiation which could accelerate the resin decomposition [5, 11]. Many attempts to explain
such radiation damage by proposing degradation mechanisms have been reported [2, 5, 12,
13]. These mechanisms are not proven because of possible interactions between the resin
radiolysis products and other compounds present in the medium, and between the resins
themselves. The polystyrene backbone is known to be very stable under radiation because of
aromatic ring resonance. Very high irradiation doses are required to produce any modification
in its chemical structure [14].
In order to follow the solid state degradation of the Amberlite IRA-400 hydroxide form,
FTIR (Fourier Transformed Infra-Red spectroscopy) and
13C
NMR (Nuclear Magnetic
Resonance) were used because these techniques are known to be very efficient in the
characterization of solid state materials. A first interpretation of the infrared (IR) spectra of
ion-exchangers on polystyrene base (Amberlite IRA-400 in Cl- form and Dowex AG 50) was
3
published by Strasheim et al. [15]. Narebska et al. also used infrared spectroscopy to follow
the radiation decomposition of the matrix of the Amberlite IRA-400 in chloride form
irradiated from 2 to 20 MGy [5]. More recent papers have been published by [16] where band
assignments were carried out for strong anion exchange systems used in the food industry and
particularly on resins with quaternary amines in hydroxide form. Very little information is
known about using 13C NMR spectroscopy to investigate the effect of gamma radiation on ion
exchange resins. An allied area of research is represented by 13C NMR studies on substituent
effects on nitrogen compounds [17]. This method was also applied to study a type of
commercial polystyrene [18].
It is known that radiation-induced decomposition of ion exchange resins depends on
several factors, three of which will be discussed in detail here: the irradiation dose, the
irradiation atmosphere, and the presence of liquid water. In order to study the effect of these
factors on the deterioration of Amberlite IRA-400 OH- anion exchange resin, an experimental
design was established to reduce the number of experiments required and to determine the
most influential factors.
Chemometric methods were used to rapidly and qualitatively estimate the effect of gamma
radiation on resin degradation. The 13C NMR data treatment was performed with a classical
method using area ratios of 13C NMR peaks calculated from DMFit software. To compare the
evolution of the values of these area ratios under the different irradiation conditions, a PCA
(Principal Component Analysis) was done to differentiate the influential factors (irradiation
dose and environment). The associated correlation circle allowed us to establish a correlation
or an anti-correlation for these factors, and relates the presence of some 13C NMR peaks to
irradiation conditions. A PCA on 13C NMR and FTIR spectra also permitted us, from the
component loadings, to study the spectroscopic signals modified by radiolysis and to identify
chemical structural and functional groups present on the resin matrix. Matrices of IR and
NMR data have also been resolved into pure compound spectra associated with concentration
profiles using a self-modeling curve resolution technique, SIMPLISMA (SIMPLe-to-use
Interactive Self-modeling Mixture Analysis). A few mathematical spectra characterizing water
trapped in the resin matrix or compound families present on the resin structure have been
extracted. These compound families have specific functional groups whose presence depends
on the irradiation conditions.
4
2. Materials and methods
2.1
Resin
The resin used in this work is a strong base anion exchange resin Amberlite IRA-400 in
hydroxide form (AmbOH) manufactured by Rohm and Haas Company (Philadelphia, PA,
US). Its macroporous structure is formed by quaternary ammonium functional groups
(−CH2N+(CH3)3) fixed in meta and para position on a polystyrene/divinylbenzene copolymer
backbone. This anion resin contains around 57% hydrated water. Its physical properties are
given in Table 1. AmbOH is used industrially in powder form. Supplied in bead form, it was
then cryoground to powder with a mean size of 0.8 µm. Cryogrinding was used to avoid prior
heat degradation.
2.2
Backbone
The backbone is a copolymer of polystyrene cross-linked with 6.9% divinylbenzene.
2.3
Gamma irradiation of the AmbOH
2.3.1 Sample packaging
3 g of resin were introduced in a glass tube which was then flame-sealed under controlled
atmosphere predetermined by an experiment plan.
Anaerobic conditions with or without liquid water were obtained by evacuating the
atmospheric air from the glass tube and then backfilling it with argon (vacuum pressure was
limited to 15−17 mbar to maintain the sample moisture content). To eliminate the residual air,
this operation was repeated three times consecutively before sealing the tube at a pressure of
800 mbar. This value, slightly lower than atmospheric pressure, was chosen to facilitate the
sealing operation.
Irradiation in aerobic atmosphere was carried out in open tubes to ensure the presence of
oxygen throughout the irradiation time.
2.3.2 Irradiator
Irradiation was performed at a temperature of around 37°C using a Cobalt-60 irradiation
source with a dose rate of around 4 kGy/h.
2.3.3 Samples treatment before spectroscopic analysis
Irradiated samples were dried at 25° for about 15 days until their mass remained constant.
This does not mean that the resins are totally dry. Due to strongly polar quaternary ammonium
5
groups, the polymer structure of the resin can always contain some residual water. Samples
were then stored at room temperature in closed vials shielded from UV rays.
2.4
Experimental design
2.4.1 Selection of experimental factors
The irradiation parameters, irradiation atmosphere (X1) and irradiation dose (X2) which
have effect on the resin degradation were studied. Xi designates the unidimensional variables
corresponding to real variables Ui.
2.4.2 Experimental design methodology
In a first step, a screening study was purchased in order to compare the behavior of the
different levels of the studied factors.
In a screening design, the interaction effects are assumed negligible; consequently, the
postulated model is additive. The reduced reference state model used in a screening design for
1 variable with 3 levels and 1 variable with 4 levels is the following:
η= α0 + α1X1A + α2 X1B + β1X2A + β2X2B + β3X2C
where Xi = 0 or 1 (presence-absence variables) depending on the level present for the two
factors. The coefficients α1, … β1, β2
…
define the differential effect on the properties of
replacing a level (the reference state) by another one.
To estimate the coefficients of the model, an optimal experimental design [19–21] has
been chosen: this asymmetrical design, derived from the experimental values in Table 2,
comprises 11 distinct experiments described in Table 3. Some of the experiments were
replicated.
2.5
FTIR characterization
Solid samples were directly deposited on the Bruker “Golden Gate” attenuated total
reflectance (ATR) accessory consisting of a diamond crystal prism (brazed in only one
tungsten carbide part), four mirrors and two ZnSe focusing lenses in to reflect the optical path.
FTIR-ATR spectra were acquired using a Thermo Nicolet IS10 spectrometer equipped with a
MCT detector, an Ever-Glo source and a KBr/Ge beam-splitter, at room temperature. Data
acquisition with an absorbance scale, was done from 4000 to 650 cm−1 with a 4 cm−1 nominal
resolution. For each spectrum, 100 scans were co-added. A background scan in air (in the
same resolution and scanning conditions used for the samples) was carried out before the
6
acquisition. The ATR crystal was carefully cleaned with ethanol to remove any residual traces
of the previous sample. Three spectra designated a, b and c were recorded for each sample.
2.6
13C
NMR characterization
The solid-state 13C NMR spectra were obtained on a Bruker Avance-400 MHz NMR
spectrometer operating at a 13C resonance frequency of 106 MHz and using a commercial
Bruker double-bearing probe. About 100 mg of sample were placed in zirconium dioxide
rotors of 4 mm outside diameter and spun at a Magic Angle Spinning rate of 10 kHz. The CP
(Cross Polarization) technique [22] was applied with a ramped 1H pulse starting at 100%
power and decreasing to 50% during the contact time (2 ms) in order to circumvent HartmannHahn mismatches [23, 24]. To improve the resolution, a dipolar decoupling GT8 pulse
sequence was applied during the acquisition time [25]. To obtain a good signal-to-noise ratio
in the 13C CPMAS (Cross Polarization Magic Angle Spinning) experiment, 10 000 scans were
accumulated with a 3 s delay. The 13C chemical shifts were referenced to tetramethylsilane and
calibrated with a glycine carbonyl signal, set at 176.5 ppm. In order to have sufficient
quantities for 13C NMR analysis, samples irradiated under the same experimental conditions
were mixed.
2.7
Principal Component Analysis
Principal Component Analysis is a method for extraction of the systematic variations in a
data set [26]. PCA is a tool for unsupervised learning, e.g. extracting information directly from
the input data without referring to classes known in advance. This method can be used for
classification as well as for description and interpretation. PCA is oriented toward modeling
variance/covariance structure of the data matrix into a model which represents the significant
variations and which considers noise as an error. The components are found one by one during
calibration and each principal component represents the main systematic variation in the data
set, which can be modeled after extraction of the previous ones. The characteristics common to
all the spectra are modeled in one or more principal components for which the scores are not
significantly different according to the species. On the contrary, the information which
differentiates the species contributes to principal components whose scores are significant
[27]. The classification may be done on the basis of the scores, and the characteristics of each
species are established by the interpretation of these specific principal components.
7
2.8
SIMPLISMA approach
This interactive method [28, 29] is used for self-modeling mixture analysis by resolving
mixture data in pure component spectra and concentration profiles without prior information
about the mixture. When overlapping spectral features are present in spectroscopic data, this
tool is unable to resolve broad spectral components and separates spectral absorption bands
characterizing one component. Its concept is based on the determination of pure variables (e.g.
a wavelength, a wavenumber in spectroscopic terms) that have contributions from only one
component. In mathematical terms, a pure variable is a variable with the maximum ratio of the
standard deviation to the mean. This ratio, called the purity, is given by the following
expression:
Pij = Wij *
σi
μi + α
(1)
where Pij is the purity value of the variable (i is the variable index) from which the jth pure
variable will be selected. µi and σi represent the mean and the standard deviation of variable i.
The constant α is added to give pure variables with a low mean value (i.e. in the noise range) a
lower purity value Pij. Typical values for α range from 1 to 5% of the maximum of µ i. The
weight factor Wij is a determinant-based function that corrects for previously chosen pure
variables. The value of Wij also depends on the value of α. For more details of this function see
Ref. [29]. The purity values are represented in the form of spectra. Along with the purity
spectrum, the standard deviation spectrum is available, described by:
S ij = Wij × σ j
(2)
This spectrum has more similarities with the original spectra and is used to facilitate the
validation of the pure variables. The interactive process makes it possible to guide the pure
variable selection by changing the value of α in combination with the option to exclude certain
spectral ranges for the selection of pure variables. This capability is especially useful since
pure variables may describe unwanted features in the dataset. As a result, the intensity of the
pure variable can be considered as proportional to a contribution or a relative “concentration”
of that component in the mixture. When the pure variables have been determined by iteration
with the maximum ratio (called purity) of standard deviation to mean intensity of each
spectrum, the original dataset can be resolved into pure components and their contributions in
the original mixture spectra. The task of the mixture analysis is to express the dataset as a
product of a matrix containing the relative concentrations and a matrix with the spectra of the
pure components:
8
D = C* PT + E
(3)
D is the matrix with the original data with the spectra in rows; its size is cv, where c is the
number of cases (spectra) and v the number of variables (wavenumbers). The C matrix (size
cn, where n is the number of pure components) contains the concentrations of the pure
components in the mixture. The P matrix (size nv) contains the resolved spectra. PT represents
the transpose of P, and E the residual error. Typically, D exists, but C and P are not known.
When the D and C matrices are known through the SIMPLISMA algorithm, the estimate of
the pure spectra P̂ can be calculated by standard matrix algebra.
P̂ = D T C( C T C )-1
(4)
In the next step, the contributions are now calculated from P̂ , which is basically a
projection of the original pure variable intensities in the original dataset. This step reduces the
noise in the contributions. The equation is:
C * = D( P̂ T P̂ )-1
(5)
where C* stands for the projected C.
Because matrix C does not contain relative concentrations but intensities proportional to
concentrations, scaling procedures such as normalization of the resulting spectra in P̂ and the
associated inverse normalization of C are often used for the quantitative information we refer
to as the contributions of the components. The reconstructed dataset can be obtained from:
D recontstructed = C* P̂T
(6)
When the proper number of components has been determined, the difference between D
and Dreconstructed is estimated from the relative root of sum of square differences (RRSSQ) as
follows:
RRSSQ =
∑ ∑ (d d
∑ ∑ d
nspect
n var
t =1
j =1
nspect
i =1
reconstructed
ij
ij
n var
j =1
2
ij
)2
(7)
where dij is the ith row and jth column element of D, d ijreconstructed is the ith row and jth column
element of Dreconstructed, nspect is the number of mixture spectra and nvar is the number of
recorded intensities. The value expresses the difference with respect to the original data
intensities. For a perfect match, the value is 0.
9
2.9
Calculation of 13C NMR peak area ratios
13C
NMR spectra were imported to DMFit software version 2010 in order to deconvolve
them in the chemical shift zone between 0 and 200 ppm in which we are interested (Fig. 1).
Twelve
13C
NMR peaks were then obtained. For each peak i, its area Ai is given by the
software. To overcome the peak intensity variation, relative areas (named Si) were calculated
as follows:
The surface percentage Si for each
13C
NMR peak area was calculated according to
equation (8):
Si =
Ai
× 100
At
(8)
where At is the total area of the 13C NMR spectrum calculated according to equation (9):
i =11
At =
∑A
i
+ A( SSB )
(9)
i =1
where A(SSB) is the 13C NMR peak area of the Spinning Side Bands (SSB).
2.10 Software
OMNIC 8.1 (Thermo Nicolet) was used to record FTIR spectra.
The Unscrambler version 9.6 from CAMO (Computer Aided Modeling, Trondheim,
Norway) was used to perform Principal Components Analysis (PCA) on FTIR and 13CNMR
spectra.
All calculations relating to SIMPLISMA were performed with laboratory routines in the
Matlab 6.5 computer environment.
DMFit (version 2010) [30] was used to determine the 13C NMR peak area ratios.
2.11 Data processing
Before chemometric treatment of FTIR data, the spectral region between 1900 and
2400 cm-1 was subtracted. This zone corresponds to the CO2 response because our apparatus is
not purged.
Two preprocessing steps were performed on FTIR and 13C NMR spectra: a Multiplicative
Scattering Correction (MSC) and a baseline correction to overcome the contribution of particle
size differences to the spectrum signal. PCA was then performed with a full cross validation
on centered data. PCA on 13C NMR peak area ratios was performed by dividing variables by
the standard deviation in order to give them the same variance.
10
A SIMPLISMA approach on FTIR and 13C NMR spectra was performed by using second
derivative data with an offset of 30.
3. Results and discussion
3.1
FTIR-ATR characterization
3.1.1 Analysis of backbone and AmbOH spectra
To estimate the spectral modifications due to the presence of trimethylammonium groups
on the backbone, a comparison was performed between the polystyrene spectrum (Fig. 2) and
the Non Irradiated AmbOH (N.I. AmbOH) spectrum (Fig. 3). Figure 2 shows absorption bands
characterizing vibrations of C−H and C=C bonds in monosubstituted benzene rings and
vibrations of aliphatic chains linking the aromatic rings. The infrared assignments of the two
structures are detailed in Table 3. The N.I. AmbOH spectrum (Fig. 3) reveals important
deformations in the spectral regions between 3600–2500 cm-1, around 1500 cm-1 and between
600–700 cm-1 hiding some of the bands initially present in the backbone spectrum and the
absorption bands characterizing the ring substitution. This is due to hygroscopic water present
in the resin structure and intramolecular hydrogen bonds induced by the hydroxyl group of
(−CH2N+(CH3)3 …OH-). The presence of the quaternary amines in the hydroxide form of the
resin increases the intensity and width of the absorption bands which appear as broad massifs.
New peaks attributed to functional group vibrations are observed around 1378, 1337, 1275,
1220, 991 and 973 cm-1 and are detailed in Table 4.
3.1.2 PCA on irradiated FTIR spectra
In order to estimate the effect of gamma irradiation on the AmbOH chemical structure, a
PCA was carried on FTIR spectra of the irradiated samples. From the PCA scores plot
(Fig. 4A) in the PC1 versus PC2 plane, we can see that the first principal component
discriminates data according to the irradiation dose. The samples irradiated at 4 MGy under
anaerobic conditions with and without water are separated from the other irradiated resins.
This means that these samples are more degraded than the others. The second principal
component differentiates the irradiation atmospheres. It reveals that when the irradiation dose
increases, samples under aerobic atmosphere at 4 MGy are distributed along the positive part
of the PC2 axis whereas the other samples are grouped together. This seems to imply that
water has no effect on the AmbOH gamma radiation in anaerobic conditions.
The positive part of the PC1 loading (Fig. 4B) shows that samples irradiated at 4 MGy in
anaerobic atmosphere with and without water are mainly characterized by the appearance of
11
new absorption bands showing the presence of primary amines −CH2−NH2 (1555 cm-1: N−H
scissor vibrations, 1114 and 1020 cm-1: C−N stretching vibrations, 849 cm-1: N−H out-ofplane deformation and 813 cm-1: NH2 wagging vibration). Out-of-plane deformation vibrations
of C−H bonds in monosubstituted benzene rings are also observed at 911 and 758 cm-1. This
means that under the irradiation conditions cited above, trimethylammonium functional groups
were three times demethylated and/or detached from the resin matrix leading to the presence
of primary amines and monosubstituted benzene rings. In the negative part of PC1 loading, the
other samples are mainly characterized by the IR peaks pointed at 1615, 1386 and 1337 cm-1
attributed to the quaternary trimethylamines initially present in the N.I. AmbOH structure. In
the positive part of PC2 loading (Fig 4C), samples irradiated in aerobic atmosphere (with an
irradiation dose between 0.5 and 4 MGy) are mainly characterized by a broad band centered at
3300 cm-1 in which hydrogen bonding may occur (N−H and/or O−H stretching vibrations), a
band at 1690 cm-1 assigned to C=O stretching vibrations in amide groups, two bands at 1548
and 1368 cm-1 which respectively could be assigned to asymmetric and symmetric CO2vibrations in carboxylic acid salts. If the presence of alcohol functions is considered, then
bands observed at 1368, 1217 and 1173 cm-1 could be then assigned to C−O stretching and
deformation vibrations. The negative part of PC2 loading shows that the remaining samples
are described by the bands initially present in the AmbOH spectrum and those found in
structures degraded at 4 MGy which are substituted or no with amine groups −CH2-NH2. Then
absorption peaks affected to backbone and −CH2N+(CH3)3 vibrations are always observed.
3.1.3 SIMPLISMA on irradiated sample FTIR-ATR spectra
SIMPLISMA multivariate analysis was performed on FTIR data for the irradiated
samples. Four pure spectra were mathematically extracted (Fig. 5). The first extracted
spectrum displays absorption bands of water and carbon dioxide present in all the samples as
shown by its associated concentration profile. The second extracted spectrum is affected to the
N.I. AmbOH structure. It is recognized by absorption peaks of the backbone and
−CH2N+(CH3)3 functional groups given in Table 3. Its associated concentration profile shows
that this structure characterizes all the irradiated samples except those irradiated at 4 MGy.
Based on its associated concentration profile, the third extracted spectrum corresponds to
the samples irradiated at 1 and 4 MGy in anaerobic atmosphere with and without water. The
contribution at 1 MGy is low, indicating the beginning of the degradation. Their structure is a
polystyrene/DVB backbone with or without substitution with−CH2−NH2 groups or amide
groups. Absorption bands of C−H aromatic vibrations are found at 3089, 3048 and 3012 cm-1.
A broad band centered at 3400 cm-1 describes N−H stretching vibrations. In place of broad
12
peak at 1610 cm-1 found in the initial structure of AmbOH, two peaks appear at 1660 and
1563 cm-1 respectively attributed to N−H scissor vibrations of amine groups or amide C=O
stretching vibrations and N−H wagging vibrations masked by an aromatic band. In the spectral
region between 1300 and 1000 cm-1, few bands are observed due to the C−N stretching
vibrations and CH2 vibrations of −CH2−NH2 groups. The NH2 wagging vibrations are also
found at 814 cm-1. The fourth extracted spectrum exhibits stretching vibration peaks of C=O
bonds in amides or carboxylate groups found at 1642 cm-1 and by stretching vibrations of
COO- at 1548 (asymmetric) and 1372 cm-1 (symmetric). Stretching vibration peaks, which
could also be assigned to alcohol compounds, are also observed at 1372, 1216 and 1172 cm-1.
The associated concentration profile shows that this spectrum characterizes a resin structure
irradiated in aerobic atmosphere and further at 1 and 4 MGy. The concentration profile at
1 MGY is low, which reveals that significant degradation starts at this dose.
3.2
13C
NMR characterization
3.2.1 Analysis of 13C NMR spectra
To determine the influence of the trimethylammonium (−CH2−N+(CH3)3) functional
group on the polystyrene-divinylbenzene backbone, chemical structures and 13C NMR spectra
of the backbone and the non-irradiated AmbOH (N.I. AmbOH) are presented in Fig. 6A and
Fig. 6B respectively. For both spectra, Spinning Side Bands (SSB), due to sample rotation
during spectrum acquisition were observed around 28.0, 228.0 and 246.0 ppm. The 13C NMR
spectrum of the backbone shows four peaks assigned to the carbons C1, C2, C3 and C4, C5, and
C6 according to the backbone chemical structure. Chemical shifts of the aliphatic carbons C1
and C2 are around 45.7 and 40.8 ppm. Aromatic carbons are characterized by only two 13C
NMR peaks: one at 146.0 assigned to carbon C3 and another at 128.6 ppm assigned to carbons
C4, C5 and C6 which have the same chemical shift due to the aromatic ring symmetry. These
carbons are designated Caro for simplification [17, 18, 31].
In comparison with the backbone, the N.I. AmbOH 13C NMR spectrum (Fig. 6B) reveals
three new 13C NMR peaks around 52.9, 68.8 and 133.7 ppm. The first two peaks are assigned
to carbons C7 and C8 related to functional groups. The third is assigned to the aromatic carbon
on which the functional groups are grafted which causes its chemical shift to change. This
carbon will be designated C*aro. The other aromatic carbons will still be designated Caro for
simplification.
Based on the experimental design, eleven
13C
NMR spectra were obtained for the
irradiated samples. Whatever the irradiation atmosphere, slight spectral modifications are
observed at low irradiation doses. Significant changes are observed when the AmbOH is
13
irradiated at 4 MGy. 13C NMR spectra of the samples irradiated at 4 MGy will be discussed
and compared with N.I. AmbOH, which is considered as a reference structure. Fig. 6C−E
shows
13C
NMR spectra of the samples irradiated at 4 MGy under anaerobic (ANA4),
anaerobic with water (W4) and aerobic (A4) atmosphere. Chemical shifts of different
13C
NMR peaks of the backbone, the N.I. AmbOH and the samples irradiated under the conditions
mentioned above are indicated in Table 5.
After irradiation, 13C NMR peaks initially assigned to the carbons C1 to C8, according to
the chemical structure of the AmbOH (Fig. 6B), undergo qualitative intensity modifications.
The intensity of the peaks assigned to carbons C7 and C8 decreases while it increases for that
assigned to carbon Caro. For these carbons, a slight chemical shift variation, obviously due to
chemical structural modifications of the N.I. AmbOH after irradiation, is also recorded.
In anaerobic atmosphere with and without water (spectra C and D), NMR peaks assigned
to carbon C*aro tend to disappear and become a shoulder of the Caro peak, suggesting that this
carbon is evolving toward a chemical environment similar to that of Caro. This is probably due
to the loss of −CH2N+(CH3)3 groups after irradiation. A new peak noted C9, absent in the N.I.
AmbOH spectrum, appears around 21.2 ppm. 13C NMR correlation tables assign this chemical
shift to methyl groups. Methyl groups could appear in place of −CH2N+(CH3)3 groups as
shown in Fig. 7. In aerobic atmosphere (spectrum C), no aliphatic carbon (C9) formation was
observed but new peaks annotated C10 and C11, assigned to carbonyl groups, appear around
172.4 and 193.6 ppm. Carbonyl groups could be formed due to oxidation by the oxygen
present in the atmosphere.
3.2.2 PCA on 13C NMR area ratios
In order to analyze quasi-quantitatively 13C NMR spectral modifications, different 13C
NMR peak area ratios were calculated using DMFit. Area ratios (S1 to S11 and S*aro) assigned
to the carbons C1 to C11 and C*aro are given in Table 6. SSB area ratios (not shown in Table 6)
were also calculated to verify that the sum of all area ratios is equal to 100%.
The results show that for some 13C NMR peaks, area ratios undergo important variations.
A PCA was then performed on the area ratio data to correlate their variations with the
irradiation conditions (irradiation atmosphere and irradiation dose). Fig. 8 shows the PCA plot
of the first two principal components (PC1, PC2) and their associated correlation loadings.
The scores plot of PC1 versus PC2 (Fig. 8A) reveals that the first principal component
discriminates the samples according to the irradiation dose with a variance of 79%. The
second principal component, representing a variance of 19%, differentiates only samples A05,
A1 and A4 according to their irradiation atmosphere (aerobic). The negative part of PC1
14
includes N.I. AmbOH and all the samples irradiated between 0.1 and 1 MGy, regardless of the
irradiation atmosphere, and the sample irradiated at 4 MGy in aerobic atmosphere. These
appear to be slightly degraded. In the positive part of PC1, samples ANA4 and W4 irradiated
under anaerobic conditions with and without water are placed near the backbone. The
chemical structure of these highly deteriorated samples appears to be similar to that of the
backbone. The associated correlation loadings (Fig. 8B) show that the S*, S7 and S8 area ratios
are anti-correlated with S1, S2, S3, Saro and S9. This means that when the irradiation dose
increases, S*aro, S7 and S8 increase whereas the others decrease, as shown in Table 5. S10 and
S11 are not correlated with the other area ratios. However, Table 5 shows that they have very
low values which could be the reason of their discrimination. Further irradiation of the resin
(higher than 4 MGy) may change this distribution.
Examination of both scores plot and correlation loadings reveals that S*aro, S7 and S8 area
ratios characterize the samples irradiated at doses from 0.1 to 1 MGy under anaerobic
conditions with and without water as well as all the samples irradiated in aerobic atmosphere.
This means that when the irradiation dose increases, chemical structure modifications are
occurring on the carbons C*aro, C7 and C8 (related to −CH2N+(CH3)3 groups) and these
modifications are less important in aerobic atmosphere. S1, S2, S3, Saro and S9 characterize the
backbone and the samples irradiated at 4 MGy under anaerobic conditions with and without
water. Under these irradiation conditions, when the irradiation dose increases to 4 MGy, the
chemical structure of the resin becomes similar to that of the backbone. This suggests that the
resin backbone is not affected by gamma irradiation at doses between 0.1 and 4 MGy. Only
−CH2N+(CH3)3 groups are being attacked, modified or eliminated. The unalterable resin
backbone and the presence of S9 on the same side of highly deteriorated samples suggest that
some of the functional groups are being modified by breaking the bond between the −CH2 and
the −N+(CH3)3 to form methyl groups (Fig. 7).
Samples irradiated under aerobic conditions between 0.5 and 4 MGy are different from
the others. They are distributed along the PC1 axis and characterized by S 11 and S12
corresponding to carbons C11 and C12 assigned to carbonyl groups. This confirms that in
presence of oxygen, oxidation occurs and leads to the formation of oxidized compounds.
3.2.3 PCA on 13C NMR spectra
As expected, PCA on 13C NMR spectra also discriminates the resins according to the
irradiation dose along PC1 axis (with 95% variance) and according to the irradiation
atmosphere along PC2 axis (Fig. 9A). The sample distribution is identical to that of the PCA
of the area ratio data: along PC1, a separation of the samples irradiated under anaerobic
15
conditions with and without water at 4 MGy (ANA4 and W4) from the other doses, and along
PC2, a discrimination of the samples irradiated under aerobic atmosphere. This confirms our
previous results which show that the resin is less degraded when it is irradiated in aerobic
atmosphere. The positive part of PC1 loading (Fig. 9B) shows that slightly degraded samples
are mainly characterized by 13C NMR peaks at 52.7, 68.8, 132.2 and 148.5 ppm assigned to
the C8, C7, C*aro and C3, respectively. The other samples, in the negative part of the PC1
loading, are characterized by the
13C
NMR peaks at 21.2, 39.0, 43.6 and 128.8 ppm and
assigned to carbons C9, C1, C2 and Caro, respectively. The presence of carbon C9 confirms the
replacement of some −CH2N+(CH3)3 groups by methyl groups when the resin is irradiated at 4
MGy. However, peaks corresponding to carbons C3 and C*aro are also observed but with
different chemical shifts near 135.7 and 142.3 ppm. This is certainly due to changes in their
chemical environment.
The positive part of PC2 loading (Fig. 9C) characterizes the samples irradiated in aerobic
atmosphere by the 13C NMR peaks at 41.1, 53.6, 69.5, 127.0, 134.5 and 150.0 ppm assigned to
carbons C1, C8, C7, Caro, C*aro and C3. In addition to these peaks initially present in the N.I
AmbOH spectrum, two new peaks assigned to carbons C10 and C11 and corresponding to
carbonyl groups appear around 172.4 and 193.6 ppm. Carbonyl groups are formed by
oxidation occurring in presence of oxygen. 13C NMR peaks of aromatic carbons appear to be
undergoing chemical shift variations. This is probably due to the replacement of functional
groups by carbonyl groups, which influence the chemical environment of all aromatic carbons
due to aromatic ring resonance. Samples irradiated under anaerobic conditions with and
without water are characterized in the negative part of PC2 loading by the 13C NMR peaks at
21.2, 39.0, 45.2, 51.7 and 142.3 ppm assigned to the carbons C9, C2, C1, C8 and C3.
3.2.4 SIMPLISMA on 13C NMR spectroscopic data
Three pure spectra (Fig. 10) were extracted from SIMPLISMA method. The first extracted
spectrum represents the N.I AmbOH structure recognized by its carbon chemical shifts around
40.2 (C2), 44.6 (C1), 52.7 (C7), 68.8 (C8), 126.1 (Caro), 132.6 (C*aro) and 146.9 ppm (C3). The
associated concentration profile shows that this structure characterizes all the samples
irradiated between 0.1 and 1 MGy, regardless of the irradiation atmosphere. The second
mathematical spectrum again shows the 13C NMR peaks present in the N.I. AmbOH spectrum
but with slightly different chemical shifts: C2 at 40.2, C1 at 44.6, C8 at 53.0, C7 at 69.5, Caro at
128.6, C*aro at 134.5 and C3 at 142.9 ppm. Beside these peaks, a new peak assigned to methyl
groups is observed near 21.1 ppm. This characterizes a resin structure having undergone some
modifications by replacement of some −CH2N+(CH3)3 groups with methyl groups. Based on
16
the associated concentration profile, these modifications are encountered under anaerobic
conditions with and without water. This spectrum further describes the samples irradiated at
4 MGy. The third pure spectrum reveals also the same 13C NMR peaks of the N.I. AmbOH
appearing at 40.2, 44.6, 58.0, 69.1, 128.0 and 145.0 ppm assigned to the carbons C2, C1, C8,
C7, Caro and C3 respectively. Two more peaks assigned to carbonyl groups appear near 172.4
and 194.2 ppm. From the associated concentration profile, we can see that these groups are
only formed in aerobic atmosphere, and further at 4 MGy.
4. Comparison between FTIR and 13C NMR results
PCA differentiates the irradiated samples according to their irradiation conditions and
correlates this differentiation, by the associated loadings, with chemical modifications in the
resin structure. FTIR-ATR PCA discriminates the irradiated samples according to their
irradiation dose and their irradiation atmosphere. It shows that the degradation level increases
when the irradiation dose increases. Irradiation between 0.1 and 1 MGy does not have
significant impact and does not lead to structural modifications detectable by spectroscopic
techniques. Structural alterations of the functional groups (−CH2N+(CH3)3) have only been
observed for high irradiation doses (4 MGy). Concerning the influence of the environment, the
aerobic atmosphere is differentiated from the other two. The presence of liquid water in
anaerobic atmosphere does not have a particular effect on resin degradation. After radiolysis,
mathematically extracted spectra suggested the presence of amine or amide absorption bands
under anaerobic conditions with and without water, also revealing that −CH2N+(CH3)3 groups
have been demethylated, oxidized and, in certain cases, detached from the resin (presence of
characteristic bands of monosubstituted benzene). Oxygenated compounds (with amide,
carbonyl or alcohol absorption bands) are also formed in aerobic atmosphere due to oxidation.
The
13C
NMR study shows the same distribution of the samples, which confirms the
FTIR-ATR results. The FTIR data suggested the presence of amine or amide bands under
anaerobic conditions with or without water. Superimposing their absorption bands did not
confirm the existence of either.
13C
NMR results confirm the formation of amide or
carboxylate groups by the presence of carbonyl chemical shifts only in aerobic atmosphere.
Under anaerobic conditions with or without water, the appearance of a 13C NMR chemical
shift characterizing –CH3 group further demonstrates the replacement of −CH2N+(CH3)3
groups by methyl groups. PCA performed on the 13C NMR peak area ratios of the backbone
and the irradiated samples show that between 0.1 and 4 MGy, gamma radiation of the AmbOH
causes the loss of its −CH2N+(CH3)3 groups but the resin backbone remains virtually
unaltered.
17
5. Conclusion
In this work, FTIR and 13C NMR spectroscopy were used to study the effect of gamma
radiation on the degradation of the solid part of the anion exchange resin AmbOH IRA-400 in
hydroxide form. The results show that when the irradiation dose increases, the −CH2N+(CH3)3
groups are detached from the resin, whereas the backbone remains intact. They also show that
in aerobic atmosphere, carbonyl carbons are formed by oxidation occurring due to the
presence of oxygen.
Acknowledgments
We are thankful to AREVA for giving us the opportunity to investigate the effect of
gamma radiation on the degradation of ion exchange resins and for its financial support. We
also thank the Spectropole of Aix-Marseille University at Saint Jérôme and especially M.
Fabio Ziarelli for helping us in the acquisition of the 13C NMR spectra.
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