Supplemental Online Material
Sorption of Eu3+ on Na-montmorillonite studied by time-resolved laser fluorescence
spectroscopy and surface complexation modeling
Takayuki Sasaki a,*, Kenyou Ueda a, Takumi Saito b, Noboru Aoyagi c, Taishi Kobayashi a, Ikuji Takagi a,
Takaumi Kimura c, Yukio Tachi d
a
Department of Nuclear Engineering, Kyoto University, Kyoto daigaku-Katsura C3, Nishikyo-ku, Kyoto
615-8540, Japan
b
Advanced Science Research Center, Japan Atomic Energy Agency (JAEA), 2-4 Shirakata-Shirane,
Tokai, Ibaraki 319-1195, Japan
c
Nuclear Science and Engineering Center, Japan Atomic Energy Agency (JAEA), 2-4 Shirakata-
Shirane, Tokai, Ibaraki 319-1195, Japan
d
Sector of Decommissioning and Radioactive Waste Management, Japan Atomic Energy Agency
(JAEA), 4-33 Muramatsu, Tokai, Ibaraki 319-1194, Japan
*corresponding author. E-mail address: sasaki@nucleng.kyoto-u.ac.jp (T. Sasaki).
This Supplemental Online Material consists of four pages, including three figures and one table.
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1. PARAFAC modeling of the aqueous Eu3+ samples
The solution samples prepared at different pH and NaNO3 concentrations with 1 mM Eu3+ in 1-cm quartz
cuvettes were measured by TRLFS under continuous stirring for comparison. After 5-pint moving average
smoothing, background subtraction, and normalization by the intensity of the ΔJ = 1 peak of the first spectra
in the temporal scan, the entire data were stored in single 3D array and processed by PARAFAC. The sum of
squared residuals (SSR) and core consistency (CC) of the PARAFAC decomposition with the number of
factors, M, are given in Fig. S1 (a). We chose the case with M = 4, although the CC value was small (CC =
9.6). This was meant to include a factor corresponding to a nitrato complex of Eu3+, EuNO32+, which
appreciably contribute to the speciation at higher NaNO3 concentrations (Fig. S2). Inner-sphere complexation
of Eu3+ with NO3- has been reported by others [1-3] and clearly shown in the enhanced intensity of the ΔJ = 2
peak of the sample at 1 M NaNO3 and pH 3.9, compared with that at 0.01 M NaNO3 and pH 4.3 (Fig. S3).
Fig. S1. Results of PARAFAC modeling of the Eu3+ solution samples prepared as a function of pH
and NaNO3 concentration with 1 mM Eu3+. (a) sum of squared residuals (SSR) and core consistency
(CC), (b) normalized fluorescence spectra, (c) normalized decays, and (d) relative intensity profile.
Four factors (factors D, E, F, and G) were chosen to decompose the entire TLRFS data. The curves
in (c) are the results of exponential fitting with the lifetimes given in Table S1.
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The small CC for M = 4 is caused by the close similarity of the factors; they largely exist similar pH ranges
(Fig. S1 (d)); their lifetimes differ only by 21 μsec (Table S1). Such collinear factors are difficult to be
separated and result in a decrease of CC. Note that with M = 3 the factors corresponding to D, F, and G were
obtained, which the known chemistry of the system under consideration, as mentioned above.
The spectra, decay curves, and relative intensity profiles of the four factors are presented in Fig. S1 (b),
(c), and (d), respectively, and the corresponding lifetimes and the peak ratios are tabulated in Table S1.
Comparing the relative intensity profiles with the speciation diagrams in Fig. S3, it is apparent that factor D
corresponds to Eu3+ and factor E to EuNO32+. The spectral shapes (Fig. S2 and the peak ratios in Table S1) of
factors D and E well agree with these assignments. The hydration numbers calculated according to Kimura
and Kato [3] are given for these factors in Table S2, which indicates displacement of one water molecule
from the first coordination sphere of Eu3+ upon complexation of Eu3+ with NO3-. Both factors F and G appear
at pH > 7 and the former dominates the speciation at 0.01 M NaNO3 and the latter at 1 M NaNO3.
Considering the relatively high concentration of Eu3+ in this experiment (1 mM Eu3+) and the speciation
diagrams, they likely correspond to Eu(OH)3 (s). The appearance of two factors for Eu(OH)3 (s) can be
explained by two different Eu(OH)3 colloids, reflecting the differences of salt effects in the nucleation or
growth stages, probably electrostatic in nature. Further research on the size distribution and composition of
Eu(OH)3 colloids prepared at different NaNO3 concentrations is needed to elucidate this. The spectra of
factors F and G can be compared with that of Eu(OH)3 (aq) reported by Planeque, et al. [4]. The spectrum of
Eu(OH)3 (aq) are characterized by broadening of the ΔJ = 1 peak and a shoulder around 620 nm, which are in
line with the spectral shapes of actors F and G. On the other hand, the lifetime of Eu(OH)3 (aq) (40 ± 5 μsec)
is much shorter than those of factors F and G. This may correspond to the exclusion of hydration water
molecules upon Eu(OH)3 (s) colloid formation.
Fig. S2. Speciation diagrams of 1 mM Eu3+ as a function of pH at three NaNO3 concentrations. The
stability constants and solubility product given in the main text are used. The Davis equation was
used to calculate the activity coefficients of the Eu3+ species.
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Fig. S3. Normalized fluorescence spectra of 1 mM Eu3+ at acidic pH and 1 and 0.01 M NaNO3.
Table S1. The fluorescence peak ratios and lifetimes of the species D and E obtained by the
PARAFAC modeling of the TRLFS data for the Eu3+ solution samples.
Peak ratio
Lifetime (µsec)
NH2O a
Species
Factor D
0.30
111.2 ± 0.4
9.0
Eu3+
Factor E
1.01
123.0 ± 0.6
8.1
EuNO32+
Factor F
1.74
102.2 ± 0.7
-
Eu(OH)3 (s)
Factor G
1.72
174 ± 3
Eu(OH)3 (s)
a. The hydration number was calculated according to NH2O = 1.05 x 10–3 kobs
– 0.44 given in ref. 3.
Literature cited in the supporting information
[1] Bunzli, JCG., Yersin, JR. Fluorescence-spectra and lifetime measurements of aqueous-solutions of
europium nitrate and perchlorate. Inorg. Chem. 1979;18:605-607.
[2] Breen, PJ., Horrocks Jr., WD. Europium(III) luminescence excitation spectroscopy. Inner-sphere
complexation of europium(III) by chloride, thiocyanate, and nitrate ions. Inorg. Chem. 1983;22:536-540.
[3] Kimura, T., Kato, Y. Luminescence study on the inner-sphere hydration number of lanthanide(III) ions in
concentrated aqueous salt solutions in fluid and frozen states. J. Alloys Compd. 1998;278:92-97.
[4] Plancque, G., Moulin, V., Toulhoat, P. Europium speciation by time-resolved laser-induced fluorescence.
Anal. Chim. Acta, 2003;478:11-22.
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