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TRLFS study of hydrolyzed Eu(III) species
Journal of Luminescence 202 (2018) 469–474
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TRLFS study of hydrolyzed Eu(III) species a,⁎
b
T
a
a
b
Hee-Kyung Kim , Seonggyu Choi , Euo Chang Jung , Hye-Ryun Cho , Jong-Il Yun , Wansik Cha a b
a
Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute, Daejeon 34057, Republic of Korea Department of Nuclear and Quantum Engineering, KAIST, Daejeon 34141, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Europium hydrolysis TRLFS Luminescence LIBD PARAFAC
The hydrolysis of Eu(III) and Am(III) begins near neutral pH conditions, above which precipitation occurs due to low solubility. Careful sample characterization steps are a prerequisite for the speciation study of hydrolyzed Eu (III) complexes without the interference of colloidal particles or precipitation. Herein, the hydrolysis reaction of Eu(III) was studied using time-resolved laser fluorescence spectroscopy (TRLFS). Laser-induced breakdown detection was employed to monitor the presence of colloidal particles. The luminescence properties of Eu(III) and precipitated Eu(OH)3(s) were investigated and comparisons were made to those of aqua Eu3+. Parallel factor analysis was applied for the deconvolution of the TRLFS results. Both primary hydrolyzed Eu(III), EuOH2+ and precipitated Eu(OH)3(s) showed distinct luminescence spectral properties, including an enhanced hypersensitive luminescence peak (J = 2). An increased luminescence lifetime was observed for both EuOH2+ (τ = 130 ± 1 μs) and precipitated Eu(OH)3(s) (τ = 158 ± 7 μs) compared to that of aqua Eu3+ (τ = 112 ± 1 μs). A formation constant log*β1,1 of EuOH2+ at I = 0.1 M NaClO4 and 25 °C was measured to be –8.28 ± 0.22, which was converted to log*β°1,1 = –7.87 ± 0.22 at an infinitely diluted condition of I = 0 based on the specific ion interaction theory.
1. Introduction
americium (Eu(III): 4f6 and Am(III): 5f6). It displays strong luminescent properties [7,11,12], while Am(III) exhibits a very weak luminescence [13]. Therefore, Eu(III) has been extensively studied using luminescence spectroscopy to understand the chemical behaviors of Am(III) under geochemical environments [14–17]. Trivalent actinide and lanthanide ions, including Am(III), Eu(III), and Cm(III), are well- known for linear correlations between their luminescence decay constants (1/t, reciprocal of luminescence lifetimes) and the number of water molecules coordinated in the inner-spheres (n(H2O)) [18–20]. Eq. (1) describes the relation for the case of Eu(III) [19,21]:
Americium is one of the minor actinides in spent nuclear fuels and significantly contributes to the long-term radio-toxicity of the spent nuclear fuels [1,2]. It is important to understand the chemical behaviors of americium under geochemical environments for the long-term safety assessment of high level radioactive waste disposal, including the spent nuclear fuels [1–4]. Hydrolysis is one of the most fundamental reactions that takes place in natural aquatic systems, and thus has been a long lasting topic in the thermodynamic study of chemical reactions of trivalent lanthanides and actinides [1,3]. Typical classical methods used for the thermodynamic study of hydrolysis reactions include solvent extractions, solubility, and potentiometric titrations [5]. Time-resolved laser fluorescence spectroscopy (TRLFS) is a powerful method for the speciation of actinides and lanthanides [6–8] and has been also successfully applied for measuring the formation constants of Cm(III) hydrolysis reactions [9,10]. TRLFS measures the luminescence spectral shifts and lifetime changes of the metal ions upon complex formations, which enables the speciation of the complexes [6–8]. The hydrolyzed Cm(III) species were identified with red-shifted luminescence spectra and increased luminescence lifetimes in comparison to Cm3+ [9,10]. Eu(III) is generally adopted as a non-radioactive analogue of
⁎
Corresponding author. E-mail address: [email protected] (H.-K. Kim).
https://doi.org/10.1016/j.jlumin.2018.06.003 Received 13 December 2017; Received in revised form 30 May 2018; Accepted 2 June 2018 Available online 04 June 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
n(H2 O) ± 0.5 = 1.07·
τ
1 (ms )
− 0.62
(1)
The extent of the increase in the luminescence lifetime provides information on the number of ligand bindings that replace the water molecules bound in the inner-sphere of the metal ions. In addition, Eu (III) generally shows an enhanced hypersensitive luminescence peak (J = 2, in 5D0 → 7FJ) relative to the J = 1 peak, which provides information on the changes in the inner-sphere coordination upon complexation [22]. The appearance of the forbidden 5D0 → 7F0 transition based on Judd-Ofelt theory is an indication of Eu(III) occupying a low symmetry site due to the asymmetric binding of ligands [22]. To utilize Eu(III) luminescence as a general probe for the
Journal of Luminescence 202 (2018) 469–474
H.-K. Kim et al.
2.2. LIBD
interactions with various ligands in natural aquatic systems, it is necessary to characterize the luminescence properties of the Eu(III) hydrolyzed species. The hydrolysis reactions of Eu(III) start taking place near neutral pH conditions, after which hydrolyzed colloidal particles also start forming [23,24]. Due to the narrow pH windows for the hydrolysis reactions to take place before colloidal particle formation, the study of aqueous hydrolyzed species is often interrupted by colloidal particles. This might explain that while there are numerous TRLFS studies on diverse Eu(III) reactions [14–17], only a single TRLFS study has been reported on Eu(III) hydrolysis by Plancque et al[25]. In the study, theyused 6.6–660 μM Eu(III) in the pH range of 8.5–12, under which Eu(III) is predicted to be present mostly as solid-species due to the low solubility of the Eu(OH)3 (log*K°s,0(am) = 17.6 ± 0.8) [24,26]. It implies that the reported spectroscopic properties of hydrolyzed Eu(III) species in the study are likely from mixtures of solid and aqueous species. Thus, spectroscopic characterizations of the hydrolyzed Eu(III) species are still needed. In this study, we investigated the luminescence spectral properties of Eu(III) hydrolysis species using TRLFS. To ensure that the aqueous hydrolysis species are not interrupted by colloidal particles, laser-induced breakdown detection (LIBD) was employed to monitor the formation of colloidal particles. Spectral changes along with pH changes were measured by TRLFS and the results were deconvoluted by parallel factor (PARAFAC) analysis for speciation. The formation the primary hydrolyzed Eu(III) species, EuOH2+ was characterized with distinct spectral changes of an enhanced hypersensitive peak, an appearance of J = 0 peak, and a longer luminescence lifetime. The formation constant, *β1,1 of EuOH2+, was determined. The luminescence characteristics of the precipitated Eu(OH)3(s) were also investigated.
Colloidal particle formation, especially under high pH conditions, was monitored by LIBD, as described elsewhere [27]. Briefly, a nanosecond pulsed 532 nm laser beam from Nd:YAG (Continuum) was tightly focused on the center of a sample cell. Images of plasma produced by laser-induced breakdowns were recorded by a charge-coupled device (CCD) camera and the number of breakdown events was counted using a homemade LabVIEW program. Breakdown probability is defined as the ratio of the number of breakdown events to the number of incident laser pulses. The breakdown probability was measured at the threshold energy of Millipore water (approximately 0.6 mJ of the incident laser pulse energy), at which the breakdown of Millipore water begins. An increase in the breakdown probability at the threshold energy of Millipore water was considered as the presence of Eu(OH)3(s) colloidal particles. 2.3. TRLFS A wavelength-tunable optical parametric oscillator (OPOTEK Inc. Vibrant B) laser (394 nm) was used as the excitation source in the TRLFS setup. Luminescence was collected by an optical fiber bundle connected to a spectrograph (Andor Technol. SR-303i.). Typically, a 300 lines/mm grating was used to record the luminescence spectrum. For Eu(OH)3(s) samples, a grating of 1200 lines/mm was also adopted to record the luminescence spectrum with better spectral resolution. An intensified CCD (ICCD, iStar, Andor Technol.) was employed for the gated detection of luminescence. A delay generator (Stanford Research, DG535) was configured to synchronize the ICCD gate opening and the incident laser pulse. For the luminescence lifetime measurements of hydrolyzed Eu(III), a kinetic mode of the ICCD was set with a series of 20 μs step-wise gate-delays and a constant gate width of 1 ms. Eu(III) samples were excited with a laser wavelength of 394 nm (2.6 mJ). Luminescence spectra from 5D0 → 7FJ transitions were recorded in the wavelength range of 570–710 nm. Each spectrum was obtained by accumulating the luminescence intensity from 1000 laser pulses. The lifetimes of the precipitated Eu(OH)3(s) were measured in short periods (30–90 s), with accumulations of 20–50 laser pulses. A kinetic mode was set with a series of 30 μs step-wise gate-delays. All the measurements were carried out at 25 °C.
2. Experimental 2.1. Sample preparations Europium oxide (Eu2O3, Sigma-Aldrich) was dissolved in dilute HClO4 and filtered with a 0.2 µm membrane filter (Mixed cellulose ester A020A013A, ADVANTEC MFS, Inc). The membrane filter was washed with water first and was primed with Eu(III) stock solution before collecting the filtered Eu(III) solution. The concentration of Eu (III) in the filtered stock solution was measured by using ICP-AES. NaClO4 stock solution was prepared using re-crystallized NaClO4. To remove dissolved CO2 from water, the Millipore water (Milli-Q element, Millipore) used for sample preparations was conditioned with 10−5 M HClO4, degassed by Ar bubbling for 2 h, and stored in an Arconditioned glove box. NaOH (Semiconductor grade, Na2CO3 < 1%, Sigma-Aldrich) solution was prepared with the degassed water in the glove box. All the sample preparations and handlings were also carried out in an Ar-conditioned glove box to prevent dissolution of CO2, especially under high pH conditions. A series of 2 μM Eu(III) samples in 0.1 M NaClO4 was prepared in a pH range of 2–8.5. To minimize the local saturation of OH– ions around Eu(III), a known amount of stock solution was added to the Millipore water in the following order with stirring: 5.77 M NaClO4 first, followed by 10 mM NaOH, and then 10 mM Eu(III) at the last step. The final volume of the samples was 20 mL. The prepared samples sat for two days or longer at room temperature with occasional manual swirling to achieve equilibrium, after which 10 mL of the samples were subjected to centrifugation at 34000 rpm for one hour. The supernatants were transferred to quartz cells for TRLFS measurements. Precipitated europium hydroxide samples were prepared with 100 μM Eu(III) in 0.1 M NaClO4 under alkaline conditions (pH 8–10). The precipitants were equilibrated for 6–10 days with occasional swirling until no further spectral changes were observed. The pH of each sample was measured in quartz cells after the TRLFS experiments by using a glass combination electrode (Orion™, Ross Ultra).
3. Results 3.1. LIBD for colloidal particle detection LIBD is a powerful tool for detecting colloidal particles [27]. In order to select 2 μM Eu(III) samples not contaminated by Eu(OH)3(s) colloidal particles, the prepared samples were examined for the presence of Eu(OH)3(s) colloidal particles by LIBD. The breakdown probabilities of the samples were compared at the threshold energy of Millipore water. For the samples below pH 8.3, breakdown probabilities were as low as Millipore water, confirming that no colloidal particles formed under these pH conditions (Fig. 1). When the pH was above 8.3, the breakdown probability abruptly increased, indicating that colloidal particles formed under the conditions. Thus, the samples between pH 2 and pH 8.15 were selected for the hydrolysis study of Eu(III) without colloidal particles interferences. 3.2. Luminescence spectral changes of hydrolyzed Eu(III) The typical luminescence spectrum of the aqua Eu3+ ion in an acidic condition is shown in black in Fig. 2(a). Four peaks at 591, 616, 650, and 697 nm correspond to transitions of 5D0 → 7FJ with J = 1–4. A ratio of the J = 1 peak height to the J = 2 peak height, IJ = 1/IJ = 2, was approximately 3 (Fig. 2(b)). The luminescence lifetime of the aqua Eu3+ at pH 2 was measured to be 112 ± 1 μs from a luminescence 470
Journal of Luminescence 202 (2018) 469–474
H.-K. Kim et al.
Table 1 Summary of the measured Eu(III) luminescence properties. Species 3+
Eu (pH 2) Eu3+ (Species 1)a EuOH2+ (Species 2)a Eu(OH)3(s) (pH 9.6, pH 10)
IJ=1/IJ=2
Lifetime
n(H2O)
3.0 ± 0.1 3.0 0.76 0.9
112 112 130 158
9.0 8.9 7.6 6.2
± ± ± ±
1 µs 0.1 µsb 1 µsb 7 µs
± ± ± ±
0.5 0.5 0.5 0.5
a
Deconvoluted species by PARAFAC analysis as shown in Fig. 3. Errors in the deconvolution results were estimated by using a jack-knife method [31]. b
decay plot (Fig. 2(c)); the error corresponds to a standard deviation of four different samples. These properties are in good agreement to previously reported values [21,28]. The luminescence properties of Eu(III) are summarized in Table 1. As the pH of the Eu(III) samples increases, the luminescence of the J = 2 peak, called hypersensitive, gradually grew relative to the J = 1 peak. An apparent decrease of IJ = 1/IJ = 2 was reproducibly observed as a function of the pH (Fig. 2(b)). The strength and shape of the hypersensitive transition are highly sensitive to the environmental changes of the Eu(III) ions [22]. These spectral changes are well-known features for complexed Eu(III) species with organic or inorganic ligands [7,11,12,25]. In addition, the J = 0 peak at 577 nm appeared from the
Fig. 1. Laser-induced breakdown detection (LIBD) of 2 μM Eu(III) samples at different pH conditions. A laser wavelength of 532 nm was set with threshold energy of 0.6 mJ, at which the Millipore water starts to show breakdown events with a low probability (0.004, dotted line). Samples, with breakdown probability as low as water, were considered not to contain colloidal particles.
Fig. 2. pH-dependent luminescence spectral changes of 2 μM Eu(III) in 0.1 M NaClO4. (a) Normalized luminescence spectra of 5D0 → 7FJ transitions. (b) pHdependent change of the luminescence intensity ratio of the J = 1 peak to J = 2 peak. (c) Luminescence decay plots and linear regression fittings (solid lines). (d) pHdependent lifetime changes measured from the slope (1/τ) of the linear fitting of the decay plots in (c). 471
Journal of Luminescence 202 (2018) 469–474
H.-K. Kim et al. 5 D0 → 7F0 transition (Fig. 2(a)). This transition was not observed for the high symmetry aqua Eu3+ ion (D3h), as it is forbidden according to Judd-Ofelt theory [22]. The appearance of the J = 0 peak indicates that the binding of the –OH group generates a low symmetry site of Eu(III) occupancy, thereby allowing for the forbidden transition. The lifetime at each pH condition was obtained from a series of luminescence spectra taken with a step-wise delayed gating of ICCD. In all the pH conditions, luminescence intensity in a natural logarithmic scale linearly decayed as a function of delay time (Fig. 2(c)), thereby resulting in single lifetime values (k = 1/τ). With an increase in the pH conditions, the luminescence lifetime also tends to increase, even though the errors are large (Fig. 2(d)). These observations indicate that the hydrolyzed Eu(III) species exhibits a longer luminescence lifetime similar to Eu(III) complexes with organic or inorganic ligands [16,17,22] and the relative amount of the hydrolyzed species in a sample increases as the pH increases.
Fig. 4. Relative amount of the second species compared to free aqua Eu3+, as function of proton concentration, pHc = –log[H+]. Linear fitting of the plot results in a slope of 0.9, which is close to a value of 1, suggesting formation of a primary hydrolysis complex, EuOH2+, under the experimental conditions.
3.3. Deconvoluted spectra PARAFAC analysis has been demonstrated as a useful tool for the deconvolution of TRLFS data [28–30]. PARAFAC analysis was carried out by implementing N-way331 in Matlab [29]. Twenty five kinetic data sets of 2 μM Eu(III) in a pH range of 2–8.15 were subjected to the PARAFAC analysis. Deconvolution with two components converged well with a high core-consistency value (87%), while an analysis with three components resulted in a low core-consistency value (15%). The summarized results of the PARAFAC analysis with two components are shown in Fig. 3. Spectral properties of the species 1 are well matched to those of the free aqua Eu3+ ion in acidic conditions, as summarized in Table 1. The luminescence spectrum of the species 2 is characterized with an appearance of a J = 0 peak at 577 nm and an increased intensity of the hypersensitive peak (J = 2) relative to the J = 1 peak (Fig. 3(a)). In terms of the positions of the peaks, there is no considerable spectral shift between the two species. The luminescence lifetime of the species 2 was calculated to be 130 ± 1 μs (Fig. 3(b)). The errors were estimated from a jack-knife technique in the PARAFAC model [31]. This lifetime corresponds to 7.6 ± 0.5 water molecules bound in the inner-sphere of the europium ion, according to Eq. (1). The relative species distribution obtained from the PARAFAC analysis shows that the species 2 gradually increases up to 50% as the pH reaches 8.15 (Fig. 3(c)).
3.4. Formation constant of Eu(III) hydrolysis The hydrolysis of Eu(III) is described in the following reactions:
Eu3 +
+ nH2 O ⇋ Eu (OH )3n− n + nH+
(2)
The formation constants of the reactions, βn, are defined as *
*β = 1, n
[Eu (OH )3n− n][H+]n [Eu3 +]
(3)
which can be rearranged as
log ⎛⎜ ⎝
[Eu (OH )3n− n] ⎞ −n⋅ log [H+] ⎟ = log *β 1, n [Eu3 +] ⎠
(4)
In Fig. 4, the relative amount of the two Eu(III) species obtained from the deconvolution using the PARAFAC analysis (Fig. 3(c)) is replotted in log ([Eu species 2]/[Eu3+]) as a function of pHc = –log[H+], as described by Eq. (4). pHc was obtained by adding an activity coefficient term (γH+) to the measured pH value as following:
Fig. 3. Deconvolution results by PARAFAC analysis with two components, species 1 (S1, Eu3+, black) and species 2 (S2, red). (a) Normalized deconvoluted luminescence spectra of the two species. (b) Luminescence decays plots of the two species in a natural log scale, as a function of delay time. Lifetimes were obtained from the slope (1/τ) of the linear fittings (solid lines). (c) Relative species distribution of the two components, depending on pH. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
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Journal of Luminescence 202 (2018) 469–474
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3.5. Luminescence of precipitated Eu(OH)3(s)
Table 2 Formation constants calculated from experimental data at each pHc condition. pHca
log*β1,1 (I = 0.1 M NaClO4)
6.585 6.805 7.205 7.315 7.355 7.375 7.685 7.725 7.755 7.765 7.795 7.845 7.905 8.085 8.165 8.215 8.215 8.225 8.235
–8.25 –8.46 –8.39 –7.92 –7.92 –8.01 –8.03 –8.15 –8.57 –8.73 –8.13 –8.40 –8.36 –8.22 –8.53 –8.44 –8.34 –8.21 –8.28 –8.28 ± 0.22b
To examine the luminescence properties of precipitated Eu(OH)3(s), 100 µM of Eu(III) was prepared under alkaline conditions (pH = 8–10). The precipitated particles were stirred during TRLFS measurements. Depending on the pH conditions and aging times of the precipitants, different luminescence spectral shapes were observed (data not shown). A representative luminescence spectrum of the precipitants formed at pH 10 was compared to that of aqua Eu3+ at the same concentration in Fig. 5(a). The luminescence spectra of the Eu(OH)3(s) at pH 10 progressively changed in the first week of preparation. The spectrum in Fig. 5 was taken after equilibrating the sample for 8 days, after which no further spectral changes were observed. The precipitated Eu(OH)3(s) samples exhibited a comparable luminescence intensity as an aqua Eu3+ ion, but the spectral shape was apparently different for both EuOH2+ and aqua Eu3+ (Fig. 5). The characteristic differences in the luminescence spectrum of Eu(OH)3(s) compared to those of aqua Eu3+ and EuOH2+ include the splitting of the J = 4 peak and shoulders in J = 1 and J = 2 peaks. The considerable growth of the J = 4 peak, as well as the J = 2 peak relative to the J = 1 peak, is another distinct change in comparison to the luminescence of EuOH2+. In spite of continuous stirring, precipitated particles were often inhomogeneously dispersed in the solution after a while, which caused a considerable bias in the lifetime measurements towards shorter lifetime values. To overcome this problem, lifetime measurements of the precipitated samples were completed in 60–90 s, during which no such effect was pronounced. The luminescence intensity of the precipitated species in a natural log scale linearly decayed as a function of the gate delay time of the ICCD (Fig. 5(b)). The lifetimes of the luminescence were measured from the decay constant (k = 1/τ). The lifetimes of Eu (OH)3(s) at a pH 9.6 and pH 10 exhibit similar values within an error. Therefore, the data from the two pH conditions were pooled together. The measured lifetime was 158 ± 7 μs; the error was estimated from the standard deviation of three aliquots each of the pH 9.6 and pH 10 samples.
pHc = –log[H+]. An average value and a standard deviation of the 19 data sets. a
b
pHc = pH + log γH+
(5)
where log γH+ is 0.095 in 0.1 M NaClO4, in consideration of the ion interaction coefficient of 0.14 ± 0.02 between H+ and ClO4– [23]. Fitting the plot to a linear regression shows a slope of 0.9, which is close to + 1. The result suggests that the second species corresponds to the EuOH2+ species and the observed spectral changes are mostly from the formation of the EuOH2+ species. The formation constants of log*β1,1, were calculated from the relative amount of Eu3+ and EuOH2+ obtained from the deconvolution results (Fig. 3(c)) at each pHc condition, according to Eq. (3) with n = 1 (Table 2). On average, log*β1,1 is measured to be –8.28 ± 0.22 at 25 °C and I = 0.1 M NaClO4. The error corresponds to a standard deviation of the 19 experimental data sets listed in Table 2. To compare this value to the literature values, a formation constant at the I = 0 M condition, * β°1,1, was calculated by extrapolating the obtained *β1,1 at I = 0.1 M NaClO4 using Δε = –0.28 ± 0.07 for EuOH2+ [5] and aH2O = 0.9966 according to the specific ion interaction theory (SIT) [23]. The converted formation constants, log*β°1,1, is estimated to be –7.87 ± 0.22 on average.
4. Discussion Eu(III) samples were prepared under de-carbonated conditions and confirmed in not containing colloidal particles by LIBD. PARAFAC analysis on the TRLFS results suggests that two Eu(III) species were dominantly present under our experimental conditions. The first species was identified as aqua Eu3+ ions from the spectral consistency. A slope analysis of the relative amount of the two species as a function of pH values indicated that the second species corresponded to the EuOH2+ species. EuOH2+ exhibited characteristic luminescence properties distinct from those of aqua Eu3+. The hypersensitive peak (J = 2) was Fig. 5. (a) A representative luminescence spectra of precipitated Eu(OH)3(s) at pH 10 (orange) with a grating of 300 lines/mm (bottom) and 1200 lines/mm (upper). For the comparisons, the luminescence spectrum of aqua Eu3+ at a pH 2.3 is displayed in black. Both samples were prepared with 100 μM Eu(III) in 0.1 M NaClO4. Precipitants (pH 10) were equilibrated for 8 days, after which no spectral changes were observed. Samples were stirred during TRLFS measurements. (b) Lifetimes were measured from the linear decays of the luminescence in a natural log scale, as a function of delay time. An error range was the standard deviation of multiple measurements of two different samples at a pH 9.6 and pH 10. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
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References
considerably enhanced relative to the J = 1 peak, and the luminescence lifetime was lengthened to 130 ± 1 μs. These results are consistent with earlier TRLFS studies on the hydrolysis of Cm(III), which showed a redshifted luminescence peak and longer luminescence lifetimes [9,10]. The secondary hydrolyzed Eu(OH)2+ species could not be resolved under our experimental conditions. Ultrafiltration often suffered from pH changes of the samples during the filtration process, especially for the near neutral pH conditions. In addition, the low solubility at higher pH conditions made it difficult to further pursue TRLFS detection of Eu (OH)2+. Our results contradict an earlier TRLFS study, in which Eu(III) hydrolysis was reported to exhibit a shortened luminescence lifetime of 50 ± 5 μs [25]. In addition, their spectra of hydrolyzed Eu(III) species showed a similar peak height of J = 1 and J = 2. However, their experimental conditions were high concentrations of Eu(III) (6.6 – 660 μM) prepared at alkaline pH conditions of 8.5 and above, under which most of the Eu(III) must have precipitated based on the solubility product (log*K°s,0(am) = 17.6 ± 0.8) [24]. Indeed, our LIBD results showed that 2 μM Eu(III), which is a much lower concentration, started to form colloidal particles around pH 8.3. In our study, precipitated Eu (OH)3(s) exhibited a longer luminescence lifetime of 158 ± 7 μs, which does not explain the shortened lifetimes of 50 ± 5 μs reported by Plancque et al. Another earlier study also reported a shortened lifetime of 21.6 ± 3.3 μs for crystalline Eu(OH)3(s) [14]. Shortened lifetimes are usually explained by the efficient energy transfer between nearby positioned Eu(III) ions in solid states [32]. Different structures and crystallinities of the solid species likely resulted in different luminescence properties. Further studies are needed to specify the observed differences in the luminescence lifetimes of the precipitates. This study is the first spectroscopic measurement of the Eu(III) hydrolysis formation constant, which is determined to be log*β1,1 = –8.28 ± 0.22 in 0.1 M NaClO4 at 25 °C. A formation constant at an I = 0 condition was calculated to be log*β°1,1 = –7.87 ± 0.22. Our result fairly agrees with the reported formation constant of EuOH2+, log*β°1,1 = –7.66 ± 0.05, which was evaluated by critical reviews on many studies by classical methods, including solubility studies, solvent extractions, and potentiometric titrations [5,26]. Our result is slightly lower than the formation constant of AmOH2+ and CmOH2+, log*β°1,1 = –7.2 ± 0.5 reported in NEA-TDB thermodynamic database review [23], but it is still in the error range.
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Acker, Americium(III) and europium (III) complex formation with lactate at elevated temperatures studied by spectroscopy and quantum chemical calculations, Dalton Trans. 43 (2014) 11221–11232. [18] W.D. Horrocks, D.R. Sudnick, Lanthanide ion probes of structure in biology. Laserinduced luminescence decay constants provide a direct measure of the number of metal-coordinated water molecules, J. Am. Chem. Soc. 101 (1979) 334–340. [19] T. Kimura, G.R. Choppin, Luminescence study on determination of the hydration number of Cm(III), J. Alloy. Compd. 213 (1994) 313–317. [20] T. Kimura, Y. Kato, Luminescence study on determination of the inner-sphere hydration number of Am(III) and Nd(III), J. Alloy. Compd. 271 (1998) 867–871. [21] T. Kimura, R. Nagaishi, Y. Kato, Z. Yoshida, Luminescence study on solvation of americium(III), curium(III) and several lanthanide(III) ions in nonaqueous and binary mixed solvents, Radiochim. Acta 89 (2001) 125–130. [22] K. Binnemans, Interpretation of europium(III) spectra, Coord. Chem. Rev. 295 (2015) 1–45. [23] R. Guillaumont, T. Fanghaenel, V. Neck, J. Fuger, D. Palmer, I. Grenthe, M.H. Rand, Chemical Thermodynamics Vol. 5: Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium, and Technetium, Elsevier, Amsterdam, The Netherlands, 2003. [24] W. Hummel, U. Berner, E. Curti, F.J. Pearson, T. Thoenen, Nagra/psi chemical thermodynamic data base 01/01, Radiochim. Acta 90 (2002) 805–813. [25] G. Plancque, V. Moulin, P. Toulhoat, C. Moulin, Europium speciation by time-resolved laser-induced fluorescence, Anal. Chim. Acta 478 (2003) 11–22. [26] W. Hummel, U. Berner, E. Curti, F.J. Pearson, T. Thoene, Technical report 02-16 Nagra / PSI Chemical Thermodynamic Data Base 01/01, Nagra. Wettingen, Switzerland, 2002. [27] E.C. Jung, H.R. Cho, Characteristics of the laser-induced breakdown detection of colloidal nanoparticles for determining particle size, in: A.A. Hashim (Ed.), The Delivery of Nanoparticles, InTech, Rijeka, Croatia, 2012. [28] T. Saito, H. Sao, K. Ishida, N. Aoyagi, T. Kimura, S. Nagasaki, S. Tanaka, Application of parallel factor analysis for time-resolved laser fluorescence spectroscopy: implication for metal speciation study, Environ. Sci. Technol. 44 (2010) 5055–5060. [29] R. Bro, PARAFAC. Tutorial and applications, Chemom. Intell. Lab. Syst. 38 (1997) 149–171. [30] B. Drobot, A. Bauer, R. Steudtner, S. Tsushima, F. Bok, M. Patzschke, J. Raff, V. Brendler, Speciation studies of metals in trace concentrations: the mononuclear uranyl(VI) hydroxo complexes, Anal. Chem. 88 (2016) 3548–3555. [31] J. Riu, R. Bro, Jack-knife technique for outlier detection and estimation of standard errors in PARAFAC models, Chemom. Intell. Lab. Syst. 65 (2003) 35–49. [32] Y. Takahashi, T. Kimura, Y. Kato, Y. Minai, T. Tominaga, Characterization of E(III) species sorbed on silica and montmorillonite by laser-induced fluorescence spectroscopy, Radiochim. Acta 82 (1998) 277-232.
5. Conclusion We investigated the luminescence properties of Eu(III) hydrolysis using TRLFS and PARAFAC analysis. LIBD measurements ensured that the examined samples do not contain colloidal particles or precipitation. The formation of EuOH2+ exhibited an enhanced hypersensitive peak (J = 2) and appearance of a J = 0 peak in the luminescence spectrum. An increased luminescence lifetime of 130 ± 1 μs was measured for EuOH2+. A thermodynamic formation constant of the primary hydrolysis reaction was obtained to be log*β1,1 = –8.28 ± 0.22 in 0.1 M NaClO4 at 25 °C. The conversion of the constant to an I = 0 condition yielded log*β°1,1 = –7.87 ± 0.22, which is in general agreement with the reported value of log*β°1,1 = –7.66 ± 0.05 [5,26]. The precipitated Eu(III) species at approximately pH 10 exhibited a distinct luminescence spectrum different from aqua Eu3+ and EuOH2+ and a longer lifetime of 158 ± 7 μs.
Acknowledgements This study is supported by the Nuclear Research and Development program of the National Research Foundation of Korea (Grant code: 2017M2A8A5014719).
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
TRLFS study of hydrolyzed Eu(III) species a,⁎
b
T
a
a
b
Hee-Kyung Kim , Seonggyu Choi , Euo Chang Jung , Hye-Ryun Cho , Jong-Il Yun , Wansik Cha a b
a
Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute, Daejeon 34057, Republic of Korea Department of Nuclear and Quantum Engineering, KAIST, Daejeon 34141, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Europium hydrolysis TRLFS Luminescence LIBD PARAFAC
The hydrolysis of Eu(III) and Am(III) begins near neutral pH conditions, above which precipitation occurs due to low solubility. Careful sample characterization steps are a prerequisite for the speciation study of hydrolyzed Eu (III) complexes without the interference of colloidal particles or precipitation. Herein, the hydrolysis reaction of Eu(III) was studied using time-resolved laser fluorescence spectroscopy (TRLFS). Laser-induced breakdown detection was employed to monitor the presence of colloidal particles. The luminescence properties of Eu(III) and precipitated Eu(OH)3(s) were investigated and comparisons were made to those of aqua Eu3+. Parallel factor analysis was applied for the deconvolution of the TRLFS results. Both primary hydrolyzed Eu(III), EuOH2+ and precipitated Eu(OH)3(s) showed distinct luminescence spectral properties, including an enhanced hypersensitive luminescence peak (J = 2). An increased luminescence lifetime was observed for both EuOH2+ (τ = 130 ± 1 μs) and precipitated Eu(OH)3(s) (τ = 158 ± 7 μs) compared to that of aqua Eu3+ (τ = 112 ± 1 μs). A formation constant log*β1,1 of EuOH2+ at I = 0.1 M NaClO4 and 25 °C was measured to be –8.28 ± 0.22, which was converted to log*β°1,1 = –7.87 ± 0.22 at an infinitely diluted condition of I = 0 based on the specific ion interaction theory.
1. Introduction
americium (Eu(III): 4f6 and Am(III): 5f6). It displays strong luminescent properties [7,11,12], while Am(III) exhibits a very weak luminescence [13]. Therefore, Eu(III) has been extensively studied using luminescence spectroscopy to understand the chemical behaviors of Am(III) under geochemical environments [14–17]. Trivalent actinide and lanthanide ions, including Am(III), Eu(III), and Cm(III), are well- known for linear correlations between their luminescence decay constants (1/t, reciprocal of luminescence lifetimes) and the number of water molecules coordinated in the inner-spheres (n(H2O)) [18–20]. Eq. (1) describes the relation for the case of Eu(III) [19,21]:
Americium is one of the minor actinides in spent nuclear fuels and significantly contributes to the long-term radio-toxicity of the spent nuclear fuels [1,2]. It is important to understand the chemical behaviors of americium under geochemical environments for the long-term safety assessment of high level radioactive waste disposal, including the spent nuclear fuels [1–4]. Hydrolysis is one of the most fundamental reactions that takes place in natural aquatic systems, and thus has been a long lasting topic in the thermodynamic study of chemical reactions of trivalent lanthanides and actinides [1,3]. Typical classical methods used for the thermodynamic study of hydrolysis reactions include solvent extractions, solubility, and potentiometric titrations [5]. Time-resolved laser fluorescence spectroscopy (TRLFS) is a powerful method for the speciation of actinides and lanthanides [6–8] and has been also successfully applied for measuring the formation constants of Cm(III) hydrolysis reactions [9,10]. TRLFS measures the luminescence spectral shifts and lifetime changes of the metal ions upon complex formations, which enables the speciation of the complexes [6–8]. The hydrolyzed Cm(III) species were identified with red-shifted luminescence spectra and increased luminescence lifetimes in comparison to Cm3+ [9,10]. Eu(III) is generally adopted as a non-radioactive analogue of
⁎
Corresponding author. E-mail address: [email protected] (H.-K. Kim).
https://doi.org/10.1016/j.jlumin.2018.06.003 Received 13 December 2017; Received in revised form 30 May 2018; Accepted 2 June 2018 Available online 04 June 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
n(H2 O) ± 0.5 = 1.07·
τ
1 (ms )
− 0.62
(1)
The extent of the increase in the luminescence lifetime provides information on the number of ligand bindings that replace the water molecules bound in the inner-sphere of the metal ions. In addition, Eu (III) generally shows an enhanced hypersensitive luminescence peak (J = 2, in 5D0 → 7FJ) relative to the J = 1 peak, which provides information on the changes in the inner-sphere coordination upon complexation [22]. The appearance of the forbidden 5D0 → 7F0 transition based on Judd-Ofelt theory is an indication of Eu(III) occupying a low symmetry site due to the asymmetric binding of ligands [22]. To utilize Eu(III) luminescence as a general probe for the
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2.2. LIBD
interactions with various ligands in natural aquatic systems, it is necessary to characterize the luminescence properties of the Eu(III) hydrolyzed species. The hydrolysis reactions of Eu(III) start taking place near neutral pH conditions, after which hydrolyzed colloidal particles also start forming [23,24]. Due to the narrow pH windows for the hydrolysis reactions to take place before colloidal particle formation, the study of aqueous hydrolyzed species is often interrupted by colloidal particles. This might explain that while there are numerous TRLFS studies on diverse Eu(III) reactions [14–17], only a single TRLFS study has been reported on Eu(III) hydrolysis by Plancque et al[25]. In the study, theyused 6.6–660 μM Eu(III) in the pH range of 8.5–12, under which Eu(III) is predicted to be present mostly as solid-species due to the low solubility of the Eu(OH)3 (log*K°s,0(am) = 17.6 ± 0.8) [24,26]. It implies that the reported spectroscopic properties of hydrolyzed Eu(III) species in the study are likely from mixtures of solid and aqueous species. Thus, spectroscopic characterizations of the hydrolyzed Eu(III) species are still needed. In this study, we investigated the luminescence spectral properties of Eu(III) hydrolysis species using TRLFS. To ensure that the aqueous hydrolysis species are not interrupted by colloidal particles, laser-induced breakdown detection (LIBD) was employed to monitor the formation of colloidal particles. Spectral changes along with pH changes were measured by TRLFS and the results were deconvoluted by parallel factor (PARAFAC) analysis for speciation. The formation the primary hydrolyzed Eu(III) species, EuOH2+ was characterized with distinct spectral changes of an enhanced hypersensitive peak, an appearance of J = 0 peak, and a longer luminescence lifetime. The formation constant, *β1,1 of EuOH2+, was determined. The luminescence characteristics of the precipitated Eu(OH)3(s) were also investigated.
Colloidal particle formation, especially under high pH conditions, was monitored by LIBD, as described elsewhere [27]. Briefly, a nanosecond pulsed 532 nm laser beam from Nd:YAG (Continuum) was tightly focused on the center of a sample cell. Images of plasma produced by laser-induced breakdowns were recorded by a charge-coupled device (CCD) camera and the number of breakdown events was counted using a homemade LabVIEW program. Breakdown probability is defined as the ratio of the number of breakdown events to the number of incident laser pulses. The breakdown probability was measured at the threshold energy of Millipore water (approximately 0.6 mJ of the incident laser pulse energy), at which the breakdown of Millipore water begins. An increase in the breakdown probability at the threshold energy of Millipore water was considered as the presence of Eu(OH)3(s) colloidal particles. 2.3. TRLFS A wavelength-tunable optical parametric oscillator (OPOTEK Inc. Vibrant B) laser (394 nm) was used as the excitation source in the TRLFS setup. Luminescence was collected by an optical fiber bundle connected to a spectrograph (Andor Technol. SR-303i.). Typically, a 300 lines/mm grating was used to record the luminescence spectrum. For Eu(OH)3(s) samples, a grating of 1200 lines/mm was also adopted to record the luminescence spectrum with better spectral resolution. An intensified CCD (ICCD, iStar, Andor Technol.) was employed for the gated detection of luminescence. A delay generator (Stanford Research, DG535) was configured to synchronize the ICCD gate opening and the incident laser pulse. For the luminescence lifetime measurements of hydrolyzed Eu(III), a kinetic mode of the ICCD was set with a series of 20 μs step-wise gate-delays and a constant gate width of 1 ms. Eu(III) samples were excited with a laser wavelength of 394 nm (2.6 mJ). Luminescence spectra from 5D0 → 7FJ transitions were recorded in the wavelength range of 570–710 nm. Each spectrum was obtained by accumulating the luminescence intensity from 1000 laser pulses. The lifetimes of the precipitated Eu(OH)3(s) were measured in short periods (30–90 s), with accumulations of 20–50 laser pulses. A kinetic mode was set with a series of 30 μs step-wise gate-delays. All the measurements were carried out at 25 °C.
2. Experimental 2.1. Sample preparations Europium oxide (Eu2O3, Sigma-Aldrich) was dissolved in dilute HClO4 and filtered with a 0.2 µm membrane filter (Mixed cellulose ester A020A013A, ADVANTEC MFS, Inc). The membrane filter was washed with water first and was primed with Eu(III) stock solution before collecting the filtered Eu(III) solution. The concentration of Eu (III) in the filtered stock solution was measured by using ICP-AES. NaClO4 stock solution was prepared using re-crystallized NaClO4. To remove dissolved CO2 from water, the Millipore water (Milli-Q element, Millipore) used for sample preparations was conditioned with 10−5 M HClO4, degassed by Ar bubbling for 2 h, and stored in an Arconditioned glove box. NaOH (Semiconductor grade, Na2CO3 < 1%, Sigma-Aldrich) solution was prepared with the degassed water in the glove box. All the sample preparations and handlings were also carried out in an Ar-conditioned glove box to prevent dissolution of CO2, especially under high pH conditions. A series of 2 μM Eu(III) samples in 0.1 M NaClO4 was prepared in a pH range of 2–8.5. To minimize the local saturation of OH– ions around Eu(III), a known amount of stock solution was added to the Millipore water in the following order with stirring: 5.77 M NaClO4 first, followed by 10 mM NaOH, and then 10 mM Eu(III) at the last step. The final volume of the samples was 20 mL. The prepared samples sat for two days or longer at room temperature with occasional manual swirling to achieve equilibrium, after which 10 mL of the samples were subjected to centrifugation at 34000 rpm for one hour. The supernatants were transferred to quartz cells for TRLFS measurements. Precipitated europium hydroxide samples were prepared with 100 μM Eu(III) in 0.1 M NaClO4 under alkaline conditions (pH 8–10). The precipitants were equilibrated for 6–10 days with occasional swirling until no further spectral changes were observed. The pH of each sample was measured in quartz cells after the TRLFS experiments by using a glass combination electrode (Orion™, Ross Ultra).
3. Results 3.1. LIBD for colloidal particle detection LIBD is a powerful tool for detecting colloidal particles [27]. In order to select 2 μM Eu(III) samples not contaminated by Eu(OH)3(s) colloidal particles, the prepared samples were examined for the presence of Eu(OH)3(s) colloidal particles by LIBD. The breakdown probabilities of the samples were compared at the threshold energy of Millipore water. For the samples below pH 8.3, breakdown probabilities were as low as Millipore water, confirming that no colloidal particles formed under these pH conditions (Fig. 1). When the pH was above 8.3, the breakdown probability abruptly increased, indicating that colloidal particles formed under the conditions. Thus, the samples between pH 2 and pH 8.15 were selected for the hydrolysis study of Eu(III) without colloidal particles interferences. 3.2. Luminescence spectral changes of hydrolyzed Eu(III) The typical luminescence spectrum of the aqua Eu3+ ion in an acidic condition is shown in black in Fig. 2(a). Four peaks at 591, 616, 650, and 697 nm correspond to transitions of 5D0 → 7FJ with J = 1–4. A ratio of the J = 1 peak height to the J = 2 peak height, IJ = 1/IJ = 2, was approximately 3 (Fig. 2(b)). The luminescence lifetime of the aqua Eu3+ at pH 2 was measured to be 112 ± 1 μs from a luminescence 470
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Table 1 Summary of the measured Eu(III) luminescence properties. Species 3+
Eu (pH 2) Eu3+ (Species 1)a EuOH2+ (Species 2)a Eu(OH)3(s) (pH 9.6, pH 10)
IJ=1/IJ=2
Lifetime
n(H2O)
3.0 ± 0.1 3.0 0.76 0.9
112 112 130 158
9.0 8.9 7.6 6.2
± ± ± ±
1 µs 0.1 µsb 1 µsb 7 µs
± ± ± ±
0.5 0.5 0.5 0.5
a
Deconvoluted species by PARAFAC analysis as shown in Fig. 3. Errors in the deconvolution results were estimated by using a jack-knife method [31]. b
decay plot (Fig. 2(c)); the error corresponds to a standard deviation of four different samples. These properties are in good agreement to previously reported values [21,28]. The luminescence properties of Eu(III) are summarized in Table 1. As the pH of the Eu(III) samples increases, the luminescence of the J = 2 peak, called hypersensitive, gradually grew relative to the J = 1 peak. An apparent decrease of IJ = 1/IJ = 2 was reproducibly observed as a function of the pH (Fig. 2(b)). The strength and shape of the hypersensitive transition are highly sensitive to the environmental changes of the Eu(III) ions [22]. These spectral changes are well-known features for complexed Eu(III) species with organic or inorganic ligands [7,11,12,25]. In addition, the J = 0 peak at 577 nm appeared from the
Fig. 1. Laser-induced breakdown detection (LIBD) of 2 μM Eu(III) samples at different pH conditions. A laser wavelength of 532 nm was set with threshold energy of 0.6 mJ, at which the Millipore water starts to show breakdown events with a low probability (0.004, dotted line). Samples, with breakdown probability as low as water, were considered not to contain colloidal particles.
Fig. 2. pH-dependent luminescence spectral changes of 2 μM Eu(III) in 0.1 M NaClO4. (a) Normalized luminescence spectra of 5D0 → 7FJ transitions. (b) pHdependent change of the luminescence intensity ratio of the J = 1 peak to J = 2 peak. (c) Luminescence decay plots and linear regression fittings (solid lines). (d) pHdependent lifetime changes measured from the slope (1/τ) of the linear fitting of the decay plots in (c). 471
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H.-K. Kim et al. 5 D0 → 7F0 transition (Fig. 2(a)). This transition was not observed for the high symmetry aqua Eu3+ ion (D3h), as it is forbidden according to Judd-Ofelt theory [22]. The appearance of the J = 0 peak indicates that the binding of the –OH group generates a low symmetry site of Eu(III) occupancy, thereby allowing for the forbidden transition. The lifetime at each pH condition was obtained from a series of luminescence spectra taken with a step-wise delayed gating of ICCD. In all the pH conditions, luminescence intensity in a natural logarithmic scale linearly decayed as a function of delay time (Fig. 2(c)), thereby resulting in single lifetime values (k = 1/τ). With an increase in the pH conditions, the luminescence lifetime also tends to increase, even though the errors are large (Fig. 2(d)). These observations indicate that the hydrolyzed Eu(III) species exhibits a longer luminescence lifetime similar to Eu(III) complexes with organic or inorganic ligands [16,17,22] and the relative amount of the hydrolyzed species in a sample increases as the pH increases.
Fig. 4. Relative amount of the second species compared to free aqua Eu3+, as function of proton concentration, pHc = –log[H+]. Linear fitting of the plot results in a slope of 0.9, which is close to a value of 1, suggesting formation of a primary hydrolysis complex, EuOH2+, under the experimental conditions.
3.3. Deconvoluted spectra PARAFAC analysis has been demonstrated as a useful tool for the deconvolution of TRLFS data [28–30]. PARAFAC analysis was carried out by implementing N-way331 in Matlab [29]. Twenty five kinetic data sets of 2 μM Eu(III) in a pH range of 2–8.15 were subjected to the PARAFAC analysis. Deconvolution with two components converged well with a high core-consistency value (87%), while an analysis with three components resulted in a low core-consistency value (15%). The summarized results of the PARAFAC analysis with two components are shown in Fig. 3. Spectral properties of the species 1 are well matched to those of the free aqua Eu3+ ion in acidic conditions, as summarized in Table 1. The luminescence spectrum of the species 2 is characterized with an appearance of a J = 0 peak at 577 nm and an increased intensity of the hypersensitive peak (J = 2) relative to the J = 1 peak (Fig. 3(a)). In terms of the positions of the peaks, there is no considerable spectral shift between the two species. The luminescence lifetime of the species 2 was calculated to be 130 ± 1 μs (Fig. 3(b)). The errors were estimated from a jack-knife technique in the PARAFAC model [31]. This lifetime corresponds to 7.6 ± 0.5 water molecules bound in the inner-sphere of the europium ion, according to Eq. (1). The relative species distribution obtained from the PARAFAC analysis shows that the species 2 gradually increases up to 50% as the pH reaches 8.15 (Fig. 3(c)).
3.4. Formation constant of Eu(III) hydrolysis The hydrolysis of Eu(III) is described in the following reactions:
Eu3 +
+ nH2 O ⇋ Eu (OH )3n− n + nH+
(2)
The formation constants of the reactions, βn, are defined as *
*β = 1, n
[Eu (OH )3n− n][H+]n [Eu3 +]
(3)
which can be rearranged as
log ⎛⎜ ⎝
[Eu (OH )3n− n] ⎞ −n⋅ log [H+] ⎟ = log *β 1, n [Eu3 +] ⎠
(4)
In Fig. 4, the relative amount of the two Eu(III) species obtained from the deconvolution using the PARAFAC analysis (Fig. 3(c)) is replotted in log ([Eu species 2]/[Eu3+]) as a function of pHc = –log[H+], as described by Eq. (4). pHc was obtained by adding an activity coefficient term (γH+) to the measured pH value as following:
Fig. 3. Deconvolution results by PARAFAC analysis with two components, species 1 (S1, Eu3+, black) and species 2 (S2, red). (a) Normalized deconvoluted luminescence spectra of the two species. (b) Luminescence decays plots of the two species in a natural log scale, as a function of delay time. Lifetimes were obtained from the slope (1/τ) of the linear fittings (solid lines). (c) Relative species distribution of the two components, depending on pH. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
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3.5. Luminescence of precipitated Eu(OH)3(s)
Table 2 Formation constants calculated from experimental data at each pHc condition. pHca
log*β1,1 (I = 0.1 M NaClO4)
6.585 6.805 7.205 7.315 7.355 7.375 7.685 7.725 7.755 7.765 7.795 7.845 7.905 8.085 8.165 8.215 8.215 8.225 8.235
–8.25 –8.46 –8.39 –7.92 –7.92 –8.01 –8.03 –8.15 –8.57 –8.73 –8.13 –8.40 –8.36 –8.22 –8.53 –8.44 –8.34 –8.21 –8.28 –8.28 ± 0.22b
To examine the luminescence properties of precipitated Eu(OH)3(s), 100 µM of Eu(III) was prepared under alkaline conditions (pH = 8–10). The precipitated particles were stirred during TRLFS measurements. Depending on the pH conditions and aging times of the precipitants, different luminescence spectral shapes were observed (data not shown). A representative luminescence spectrum of the precipitants formed at pH 10 was compared to that of aqua Eu3+ at the same concentration in Fig. 5(a). The luminescence spectra of the Eu(OH)3(s) at pH 10 progressively changed in the first week of preparation. The spectrum in Fig. 5 was taken after equilibrating the sample for 8 days, after which no further spectral changes were observed. The precipitated Eu(OH)3(s) samples exhibited a comparable luminescence intensity as an aqua Eu3+ ion, but the spectral shape was apparently different for both EuOH2+ and aqua Eu3+ (Fig. 5). The characteristic differences in the luminescence spectrum of Eu(OH)3(s) compared to those of aqua Eu3+ and EuOH2+ include the splitting of the J = 4 peak and shoulders in J = 1 and J = 2 peaks. The considerable growth of the J = 4 peak, as well as the J = 2 peak relative to the J = 1 peak, is another distinct change in comparison to the luminescence of EuOH2+. In spite of continuous stirring, precipitated particles were often inhomogeneously dispersed in the solution after a while, which caused a considerable bias in the lifetime measurements towards shorter lifetime values. To overcome this problem, lifetime measurements of the precipitated samples were completed in 60–90 s, during which no such effect was pronounced. The luminescence intensity of the precipitated species in a natural log scale linearly decayed as a function of the gate delay time of the ICCD (Fig. 5(b)). The lifetimes of the luminescence were measured from the decay constant (k = 1/τ). The lifetimes of Eu (OH)3(s) at a pH 9.6 and pH 10 exhibit similar values within an error. Therefore, the data from the two pH conditions were pooled together. The measured lifetime was 158 ± 7 μs; the error was estimated from the standard deviation of three aliquots each of the pH 9.6 and pH 10 samples.
pHc = –log[H+]. An average value and a standard deviation of the 19 data sets. a
b
pHc = pH + log γH+
(5)
where log γH+ is 0.095 in 0.1 M NaClO4, in consideration of the ion interaction coefficient of 0.14 ± 0.02 between H+ and ClO4– [23]. Fitting the plot to a linear regression shows a slope of 0.9, which is close to + 1. The result suggests that the second species corresponds to the EuOH2+ species and the observed spectral changes are mostly from the formation of the EuOH2+ species. The formation constants of log*β1,1, were calculated from the relative amount of Eu3+ and EuOH2+ obtained from the deconvolution results (Fig. 3(c)) at each pHc condition, according to Eq. (3) with n = 1 (Table 2). On average, log*β1,1 is measured to be –8.28 ± 0.22 at 25 °C and I = 0.1 M NaClO4. The error corresponds to a standard deviation of the 19 experimental data sets listed in Table 2. To compare this value to the literature values, a formation constant at the I = 0 M condition, * β°1,1, was calculated by extrapolating the obtained *β1,1 at I = 0.1 M NaClO4 using Δε = –0.28 ± 0.07 for EuOH2+ [5] and aH2O = 0.9966 according to the specific ion interaction theory (SIT) [23]. The converted formation constants, log*β°1,1, is estimated to be –7.87 ± 0.22 on average.
4. Discussion Eu(III) samples were prepared under de-carbonated conditions and confirmed in not containing colloidal particles by LIBD. PARAFAC analysis on the TRLFS results suggests that two Eu(III) species were dominantly present under our experimental conditions. The first species was identified as aqua Eu3+ ions from the spectral consistency. A slope analysis of the relative amount of the two species as a function of pH values indicated that the second species corresponded to the EuOH2+ species. EuOH2+ exhibited characteristic luminescence properties distinct from those of aqua Eu3+. The hypersensitive peak (J = 2) was Fig. 5. (a) A representative luminescence spectra of precipitated Eu(OH)3(s) at pH 10 (orange) with a grating of 300 lines/mm (bottom) and 1200 lines/mm (upper). For the comparisons, the luminescence spectrum of aqua Eu3+ at a pH 2.3 is displayed in black. Both samples were prepared with 100 μM Eu(III) in 0.1 M NaClO4. Precipitants (pH 10) were equilibrated for 8 days, after which no spectral changes were observed. Samples were stirred during TRLFS measurements. (b) Lifetimes were measured from the linear decays of the luminescence in a natural log scale, as a function of delay time. An error range was the standard deviation of multiple measurements of two different samples at a pH 9.6 and pH 10. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
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References
considerably enhanced relative to the J = 1 peak, and the luminescence lifetime was lengthened to 130 ± 1 μs. These results are consistent with earlier TRLFS studies on the hydrolysis of Cm(III), which showed a redshifted luminescence peak and longer luminescence lifetimes [9,10]. The secondary hydrolyzed Eu(OH)2+ species could not be resolved under our experimental conditions. Ultrafiltration often suffered from pH changes of the samples during the filtration process, especially for the near neutral pH conditions. In addition, the low solubility at higher pH conditions made it difficult to further pursue TRLFS detection of Eu (OH)2+. Our results contradict an earlier TRLFS study, in which Eu(III) hydrolysis was reported to exhibit a shortened luminescence lifetime of 50 ± 5 μs [25]. In addition, their spectra of hydrolyzed Eu(III) species showed a similar peak height of J = 1 and J = 2. However, their experimental conditions were high concentrations of Eu(III) (6.6 – 660 μM) prepared at alkaline pH conditions of 8.5 and above, under which most of the Eu(III) must have precipitated based on the solubility product (log*K°s,0(am) = 17.6 ± 0.8) [24]. Indeed, our LIBD results showed that 2 μM Eu(III), which is a much lower concentration, started to form colloidal particles around pH 8.3. In our study, precipitated Eu (OH)3(s) exhibited a longer luminescence lifetime of 158 ± 7 μs, which does not explain the shortened lifetimes of 50 ± 5 μs reported by Plancque et al. Another earlier study also reported a shortened lifetime of 21.6 ± 3.3 μs for crystalline Eu(OH)3(s) [14]. Shortened lifetimes are usually explained by the efficient energy transfer between nearby positioned Eu(III) ions in solid states [32]. Different structures and crystallinities of the solid species likely resulted in different luminescence properties. Further studies are needed to specify the observed differences in the luminescence lifetimes of the precipitates. This study is the first spectroscopic measurement of the Eu(III) hydrolysis formation constant, which is determined to be log*β1,1 = –8.28 ± 0.22 in 0.1 M NaClO4 at 25 °C. A formation constant at an I = 0 condition was calculated to be log*β°1,1 = –7.87 ± 0.22. Our result fairly agrees with the reported formation constant of EuOH2+, log*β°1,1 = –7.66 ± 0.05, which was evaluated by critical reviews on many studies by classical methods, including solubility studies, solvent extractions, and potentiometric titrations [5,26]. Our result is slightly lower than the formation constant of AmOH2+ and CmOH2+, log*β°1,1 = –7.2 ± 0.5 reported in NEA-TDB thermodynamic database review [23], but it is still in the error range.
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5. Conclusion We investigated the luminescence properties of Eu(III) hydrolysis using TRLFS and PARAFAC analysis. LIBD measurements ensured that the examined samples do not contain colloidal particles or precipitation. The formation of EuOH2+ exhibited an enhanced hypersensitive peak (J = 2) and appearance of a J = 0 peak in the luminescence spectrum. An increased luminescence lifetime of 130 ± 1 μs was measured for EuOH2+. A thermodynamic formation constant of the primary hydrolysis reaction was obtained to be log*β1,1 = –8.28 ± 0.22 in 0.1 M NaClO4 at 25 °C. The conversion of the constant to an I = 0 condition yielded log*β°1,1 = –7.87 ± 0.22, which is in general agreement with the reported value of log*β°1,1 = –7.66 ± 0.05 [5,26]. The precipitated Eu(III) species at approximately pH 10 exhibited a distinct luminescence spectrum different from aqua Eu3+ and EuOH2+ and a longer lifetime of 158 ± 7 μs.
Acknowledgements This study is supported by the Nuclear Research and Development program of the National Research Foundation of Korea (Grant code: 2017M2A8A5014719).
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