Effect of different complexing ligands on europium uptake from aqueous phase by kaolinite: batch sorption and fluorescence studies

Abstract

Clay minerals, a ubiquitous part of the geosphere, can interact with the radioactive contaminants present in aqueous media and can alter their pathways in the geochemical cycle. The final fate of these radiotoxic metal ions in the geosphere is decided not only by nearby clay minerals but also by their organic surroundings. In the present paper, Eu(III) sorption and speciation was studied on a kaolinite–water interface in the presence of complexing ligands, such as oxalic acid, citric acid and humic acid. The % sorption was found to be dependent on the ionic strength of media in the lower pH range but independent in higher pH range. This suggested that the Eu(III) sorption follows an ion-exchange (outer-sphere) mechanism up to pH ∼ 6, beyond which surface complexation (inner-sphere) is predominantly responsible for Eu(III) sorption onto kaolinite. The addition of complexing ligands modifies the sorption profiles. Fluorescence studies showed the sorption of Eu(III) as an Eu(III)–oxalate complex onto the kaolinite surface. The effect of the addition sequence of Eu(III) and humic acid on the sorption and speciation of Eu(III) onto the kaolinite surface was investigated and found to affect the sorption behaviour of Eu(III) onto the kaolinite surface. The effect of kaolinite solubility on Eu(III) sorption and desorption was also investigated.

---

Introduction

Actinide/lanthanide ion sorption and speciation onto clay minerals is an area of research where much of advancements have been made in the past couple of decades.^1–11 These studies have direct relevance to the back end of the nuclear fuel cycle. For the nuclear fuel cycle program, nuclear waste immobilization, followed by burial in deep geological repositories, is fast emerging as the acceptable norm. Furthermore, the vitrified waste blocks are required to be under surveillance, in isolation from the biosphere, for millions of years. For this, different practices and methodologies are being followed to reduce or separate the long-lived actinides and fission products. Although research in reducing or separating long-lived actinides from nuclear waste is an ongoing and evolving task, for most cases, their final disposal in geological repositories is the ultimate goal.^12–14 The search for places having minerals with suitable properties for waste immobilization is an ongoing task in various countries. The earth's crust is mainly rich in silicate minerals of different forms, starting from simple oxides (e.g. silica and alumina) to complex assemblages of minerals (alumino-silicates). These minerals can act as natural reservoirs for radioactive elements if these are released accidentally into the geosphere. A vast amount of literature is available on the sorption and speciation of Ln/An onto clay minerals under different experimental conditions.^15,16 Kautenburger et al. used kaolinite (KGa-1b), an alumino-silicate (1 : 1 layer type), for sorption and desorption studies of Eu(III) and Gd(III). The authors also studied the role of Ca(II) and Mg(II) on the sorption properties of Eu(III)/Gd(III) onto kaolinite.¹⁷ The studies indicated that, with increasing Eu(III)/Gd(III) concentration, the relative desorption of these metals from kaolinite may increase significantly before the cation-exchange capacity (CEC) is reached. This suggests relatively lower metal-ion migration due to higher sorption at more active sites for low lanthanide concentrations, in comparison to that for higher lanthanide concentrations. The presence of Ca(II)/Mg(II) also affected the Eu(III)/Gd(III) sorption.

Humic substances are invariably present in most natural aquatic systems as they mainly originate from the degradation and decomposition of plants and animals, either directly or from their by-products. Humic substances, mainly comprising humic acids (HA), are heterogeneous polyfunctional organic ligands, which can easily bind radionuclides and can alter their migration behaviour.^{9,10,16,18,20} Radionuclides, in the presence of clay minerals, can form a surface complex (inner or outer sphere), which is one of the important factors in deciding the fate of the former. Interactions with HA and clay minerals can influence the migration of the radionuclides. The sorption of Eu(III) onto clay minerals was investigated by several researchers under a variety of experimental conditions.^{1–9,17,20–23} The role of humic acid (HA) on the interaction between Am(III) and kaolinite was investigated by Křepelová et al.²⁴ The authors reported marginal differences in the Am(III) sorption onto kaolinite in the presence and the absence of HA. At lower pH values (pH < 5), the presence of HA slightly enhances the sorption of Am(III), whereas at higher pH values (pH ≥ 5.5), the presence of HA reduces the % sorption of Am(III) due to the formation of aqueous soluble Am(III)–humate complexes.

To understand the mechanism of sorption, molecular level speciation at the mineral–water interface is essential. Spectroscopic techniques, such as TRLFS (Time Resolved Laser Fluorescence Spectroscopy) and EXAFS (Extended X-ray Absorption Fine Structure Spectroscopy), are most commonly used for molecular level speciation at mineral–water interfaces.^22,25–33 Stumpf et al. used TRLFS studies for molecular level speciation of Cm(III) sorbed onto kaolinite and smectite.³² Their studies suggested the formation of an outer sphere complex at low pH. Later, inner-sphere complexation occurs via aluminol edge sites. No incorporation of Cm(III) into the bulk structure was observed for either of the minerals. The same group reported analogous studies with Eu(III) using TRLFS.³³ The effect of carbonate on the sorption mechanism was also investigated for Eu(III) sorption onto smectite and kaolinite. Fluorescence data suggested the formation of a ternary clay/Eu(III)/carbonate complex in the presence and absence of CO₂. TRLFS and parallel factor analysis (PARAFAC) was used by Ishida et al. to study the surface speciation of Eu(III) sorbed onto kaolinite.²⁸ The PARAFAC modelling with TRLFS suggested the formation of three different Eu(III) complexes on the kaolinite surface. Inner-sphere complexation was reported to be dominant at relatively high pH, high salt, and low Eu(III) concentrations. A comparison of similar studies with gibbsite suggested Eu(III) complexation by the edge face of a gibbsite layer of kaolinite.

In the present paper, the authors intended to study the sorption and speciation of Eu(III) (an analogue for trivalent actinides (e.g. Am(III) and Cm(III))) onto kaolinite under different chemical conditions. The role of complexing anions, such as humic acid, oxalic acid and citric acid, on the sorption of Eu(III) onto kaolinite, and the effects of the sequence of addition for humic acid and metal ion were also studied.

Experimental section

Instrumentation

A Labindia pH meter (PHAN) was used with a glass combination electrode (Corning) for the pH measurements. The electrode was calibrated at pH 4.00(±0.01), 7.00(±0.01), and 9.00(±0.01) at room temperature (25(±2) °C) with NIST standard pH buffer solutions (Fisher). The X-ray diffraction (XRD) patterns of kaolinite were recorded on a Philips X-ray diffractometer (model PW 1710) with Ni-filtered Cu Kα radiation, using silicon as the external standard. The surface area and the pore volume analysis were carried out by the standard BET technique using a “Sorptomatic-1990 Analyzer”, procured from M/s C.E. Instruments, Italy. Fourier Transform Infrared (FTIR) spectra of the kaolinite clay were recorded on a JASCO FT/IR-420 spectrometer. A high-speed centrifuge from SIGMA Laborzentrifugen GmbH, Germany was used for separating kaolinite clay from aqueous media after the sorption experiments. Photoluminescence studies were performed using an Edinburgh unit provided with a CD-920 controller and a microsecond flash lamp. The data acquisition and analysis were done with F-900 software provided by Edinburgh Analytical Instruments, UK. The fluorescence decay curves for all the samples were recorded on a 4 ms scale and fitted via an iterative method. ^152–154Eu(III) was estimated by gamma-counting using an NaI(Tl) detector (Para Electronics) interfaced with a multi-channel analyzer (ECIL, India). Details of sample preparation and measurement are given in the ESI.†

Reagents

Kaolinite, procured from Sigma-Aldrich, was used as received. Stock solutions of Eu(III) were prepared by dissolving weighed amounts of Eu₂O₃ (>99.99%, Sigma-Aldrich) in perchloric acid (HClO₄), drying and re-dissolving in HClO₄ (pH ∼ 3). The concentrations of the stock solutions of Eu(III) were determined by complexometric titration using EDTA (ethylenediamine-N,N,N′,N′-tetraacetic acid) as the complexing agent and xylenol orange as the indicator. AR grade citric acid (SD Fine Chemicals, Mumbai), sodium perchlorate (Sigma-Aldrich), oxalic acid (Sigma-Aldrich), and humic acid (Sigma-Aldrich) were used as received. Stock solutions of oxalic acid (10⁻² M), citric acid (10⁻² M), and HA (1 gL⁻¹) were prepared in Milli-Q water (Millipore, USA). Stock solutions of kaolinite suspension (5 gL⁻¹) were also prepared using Milli-Q water. All the reagents were diluted to appropriate concentrations as required for further experiments. An ^152,154Eu radiotracer, obtained from the Board of Radiation and Isotope Technology (BRIT, Mumbai), was used after verifying its radiochemical purity by gamma-ray spectrometry. An ^152,154Eu(III) tracer stock solution (10⁻⁶ M) was prepared by repeatedly drying the ^152,154Eu(III) radiotracer in HClO₄ and finally making it up to a known volume using pH 3 solution (HClO₄). The degree of crystallinity was ascertained by XRD and FTIR studies. The specific surface area was analyzed using the BET isotherm method.

Sorption experiments

Eu(III) sorption studies were carried out in polypropylene tubes (50 mL capacity Tarsons Centrifugation Tubes). The losses due to wall sorption were within the error limits (<5%) under the experimental conditions used in the present study. All the sorption studies were done in aerobic conditions. Sample solutions (10 mL) for the sorption experiments were prepared by taking desired volumes of the stock kaolinite suspension and Eu(III) (spiked with ^152,154Eu(III)) at the desired concentration of the complexing anions (10⁻⁴ M for both oxalic acid and citric acid, and 10 mg L⁻¹ or 100 mg L⁻¹ for humic acid), pH, and ionic strength, as per the experimental requirements. Dilute solutions of HClO₄ and NaOH were used to adjust the pH of the sample solutions before the attainment of equilibrium. The kinetic studies were done at different pH values and ionic strengths for the ^152,154Eu(III) (10⁻⁶ M) sorption onto kaolinite (1 g L⁻¹) (details given in ESI†). For all subsequent sorption studies, duplicate samples (10 mL each) were placed in 50 mL polypropylene tubes and mechanically shaken for 24 h at room temperature to ensure equilibrium, unless stated otherwise. The suspensions were centrifuged for 25 minutes at 20 000 rpm using a high-speed centrifuge (Sigma 3-30K). The % sorption of ^152,154Eu(III) on the kaolinite suspension was determined by radiometric assay of 1 mL of the supernatant sample for 5 minutes before and after the equilibrations with kaolinite, using eqn (1).

% sorption = [(A_i − A_f)/A_i]/100 (1)

where, A_i = initial counts and A_f = final counts.

Results and discussion

Characterization of kaolinite

The XRD data of the kaolinite sample matched very well with JCPDS card no. [14-164], [29-1490], and [78-2109] for kaolinite phases (Fig. 1a). The well-resolved and sharp peaks in the XRD spectra suggested an ordered structure for the kaolinite used in the present studies. FTIR has been extensively used for the characterization of clay minerals.^34–36 L. Vaculíková et al. have used FTIR spectra and thermal analysis to characterize kaolinite from different sources.³⁶ In the present study, FTIR spectral analysis was also used to ascertain the structural disorder of kaolinite (Fig. 1b), and the details are outlined below.

Fig. 1 Characterization of kaolinite (a) XRD and (b) FTIR spectra of kaolinite (inset: magnified region for –OH stretching bands).

(a) The empirical approach (FTIR_E) is based on the resolution and the relative intensities of OH stretching and bending bands.

The kaolinite structure can be considered as ordered or well ordered if the OH stretching and bending bands are clearly resolved, while it can be considered as partially ordered if the individual bands (OH stretching and bending bands) at 3669 cm⁻¹, 3650 cm⁻¹, and 938 cm⁻¹ can be identified with low intensities. Finally, the kaolinite structure is poorly ordered if a single band near 3660 cm⁻¹ or inflexions near 3669, 3650, and 938 cm⁻¹ are observed in the spectra.³⁷

(b) Numerical approach (FTIR_N) based on crystallinity indices CI₁ and CI₂, based on the intensity ratio of selected vibrational modes using the following equations:

CI₁ = I₃₆₉₅/I₉₁₂ (2)

CI₂ = I₃₆₂₀/I₃₆₉₅ (3)

where I₃₆₉₅ and I₃₆₂₀ are the intensities of the OH bond stretching at 3695 cm⁻¹ and 3620 cm⁻¹ respectively and I₉₁₂ is the intensity of the OH-bending band at 912 cm⁻¹. The ranges and the degrees of disorder are as follows: poorly ordered (CI₁ < 0.7, CI₂ > 1.2); partially ordered (0.7 < CI₁ < 0.8, 0.9 < CI₂ < 1.2), and ordered structures (CI₁ > 0.8, CI₂ < 0.9).^38,39

The complete assignment of the FTIR bands for kaolinite can be found elsewhere.³⁶ The doublet at 3695 cm⁻¹ and 3619 cm⁻¹ is characteristic of the kaolin group. The four clearly resolved bands at about 3695 cm⁻¹, 3669 cm⁻¹, 3650 cm⁻¹, and 3619 cm⁻¹ reflect the high structural ordering of the kaolinite sample used in this study. The values for the present system lie in the ordered structure range (within experimental/fitting errors) e.g. CI₁ ∼ 0.98, CI₂ ∼ 0.97; the CI₂ value appears to be on the high side (may be due to peak fitting errors) but the four well-resolved peaks at 3695, 3669, 3650 and 3619 cm⁻¹, and two peaks at 937 cm⁻¹ and 912 cm⁻¹, along with the XRD spectra, confirmed the ordered nature of the sample. The specific surface area of the kaolinite was found to be 23.04 m² g⁻¹ from BET measurements.

Kinetics of Eu(III) sorption onto kaolinite

The kinetics of Eu(III) sorption onto kaolinite was investigated at three different pH values (approximately 3.0, 5.5, and 7.5) and ionic strengths (0.01 M, 0.1 M, and 1 M) (Fig. 2). Equilibrium was reached in about 60 minutes under the present experimental conditions. The % sorption of Eu(III) varies with ionic strength at pH 3.0 and 5.5, whereas the analogous data obtained at different ionic strengths are comparable at pH 7.5. This behaviour for Eu(III) sorption onto kaolinite suggests the prevalence of different mechanisms of Eu(III) sorption at different pH values.

Fig. 2 Kinetics of Eu(III) (10⁻⁶ M) sorption onto kaolinite (1 g L⁻¹) at different pH and ionic strength values.

These preliminary data suggest that the Eu(III) sorption follows an ion-exchange mechanism of sorption at lower pH, while at higher pH values (pH ∼ 7.5), the sorption is dominated by complexation reactions with the surface hydroxyl groups. To obtain the range of pH values where the sorption mechanism changes from ion-exchange to surface complexation, % sorption experiments were done as a function of pH at different ionic strengths (see next section).

Sorption of Eu(III) onto kaolinite

From the kinetics studies of Eu(III) sorption onto kaolinite, two broad observations are made, viz. the sorption kinetics is fast, as only 60 minutes is required to attain equilibrium at different pH and ionic strength values, and there is a change in the sorption mechanism with varying pH values.

To investigate further, the % sorption experiments were carried out with varying pH and ionic strength values (0.01 M and 1 M). From the sorption profiles obtained at different ionic strength values (Fig. 3), it can be seen that at lower pH, the % sorption is quantitative at lower ionic strength (0.01 M), whereas this is not true for sorption studies carried out at higher ionic strength. The % sorption at higher pH is quantitative at both the ionic strengths investigated. The data suggest that the Eu(III) sorption follows an outer-sphere complexation mechanism at the kaolinite surface up to pH ∼ 6, beyond which, surface complexation (inner-sphere) is predominantly responsible for Eu(III) sorption onto kaolinite.⁴⁰ This behaviour at different ionic strengths can be correlated with the aqueous speciation of Eu(III). The speciation diagram suggests the presence of Eu³⁺ up to pH 6, beyond which the carbonato, hydroxy, and mixed carbonato–hydroxo species dominate (ESI, Fig. S1†). Lower % sorption at higher ionic strength and low pH, can be attributed to competition between Na⁺ and Eu³⁺ for the binding sites available on the kaolinite surface, whereas at higher pH, the nature of the interaction changes and the % sorption becomes similar in both the cases.

Fig. 3 pH edge for Eu(III) (10⁻⁶ M) sorption onto kaolinite (1 g L⁻¹) at different ionic strengths (0.01 M and 1 M NaClO₄).

Role of different complexing anions

The presence of different complexing agents in the geosphere can affect the interaction mechanism between the clay and the metal ions of interest. The changes take place either by the direct complexation of the metal ion with the complexing anions, followed by interaction with clay minerals, or vice versa.^{9,10,16,18,20,41,42}

Oxalic acid (10⁻⁴ M), citric acid (10⁻⁴ M), and humic acid (10 mg L⁻¹) were used as representatives of the organic complexants found in the ecosphere, as these molecules owe their complexing ability due to the presence of carboxylate and/or hydroxyl groups, which are the most common functional groups found in naturally occurring organic molecules. The sorption experiments were performed as described in the experimental section, the only modification being the addition of appropriate concentrations of the complexing agents before equilibration.

The sorption of Eu(III) in the presence of oxalic acid initially decreased by up to 30% from pH 3 to 5.5 and subsequently increased to yield quantitative sorption (pH ∼ 8). The initial decrease is due to the aqueous complexation of Eu(III) with oxalic acid up to pH 5.5, beyond which the formation of neutral species (Fig. 4) reduces the complexing ability of Eu(III) with oxalic acid, leading to a rise in Eu(III) sorption. In the case of citric acid, the % sorption does not change much as compared to the bare Eu(III) sorption profile. The difference in the nature of the Eu(III) sorption profile in the two cases is still not understood and will be investigated separately. For studies involving HA, the sorption profile follows a similar trend, as in the case of oxalic acid, and hence similar logic can be used to explain the sorption profile. To further investigate the interaction of Eu(III) with kaolinite at the molecular level, TRLFS studies were attempted for oxalic acid and the HA case.

Fig. 4 Effect of oxalic acid (10⁻⁴ M), citric acid (10⁻⁴ M), and humic acid (10 mg L⁻¹) on Eu(III) (10⁻⁶ M) sorption onto kaolinite (1 g L⁻¹).

Speciation at molecular level: TRLFS studies

The interaction between Eu(III), kaolinite and oxalic acid/HA was studied by luminescence spectroscopy to obtain an insight into the nature of the complexes (inner or outer sphere) of Eu(III) with kaolinite in the presence/absence of the complexing anions. The TRLFS experiments were performed with varying pH (pH 3–8) for oxalic and addition of sequence for HA. The surface speciation of the sorbed metal ion in the presence of the complexing ligands can be of two types. Firstly, the metal/ligand can act as a bridge between the ligand/metal and the clay surface. Secondly, the metal–ligand complex can precipitate on the surface of the clay (ESI Fig. S2†). The luminescence spectra of Eu(III)_aq, Eu(III)–kaolinite, and Eu(III)–oxalic acid/HA were recorded and compared with the ternary system, e.g., Eu/kaolinite–oxalic acid/HA, to correlate the luminescence data with the speciation of Eu(III) in the different systems. The luminescence studies for the ternary systems were carried out using 10⁻⁴ M Eu(III) for better signal to noise ratios. The luminescence spectra of the suspensions were recorded for both the systems.

In the case of oxalic acid, the luminescence spectra of the suspensions were recorded with varying pH at a higher kaolinite concentration of 2.5 g L⁻¹ (Fig. 5). The luminescence spectrum of the Eu–kaolinite–oxalic acid ternary system was similar to that of the Eu(III)–oxalic acid complex (binary system) in aqueous media. This suggests the presence of an Eu(III)–oxalate complex, even in the ternary system. The lifetime data conform to a mono-exponential decay pattern with a lifetime of 232 ± 10 μs, which is the same as the lifetime of the Eu(III)–oxalate complex (within the limits of experimental error). Since the suspension was directly used for luminescence studies, the Eu(III)–oxalate complex may be present as an aqueous soluble species or may be sorbed onto the kaolinite surface.

Fig. 5 Life time (upper) and luminescence spectra (lower) of Eu(III) (10⁻⁴ M) sorbed onto kaolinite (2.5 g L⁻¹) as a function of varying pH.

To confirm this, the suspension was centrifuged at 20 000 rpm and the supernatant and the solid residue were used for the luminescence studies. The luminescence spectra of the supernatant show no Eu–oxalate complex signal (no Eu–oxalate complex or below the detection limit of the instrument). A thin layer of the slurry kept on a glass plate was air-dried at room temperature and used for the luminescence measurements.

The recorded spectra were very similar to those of the aqueous Eu(III)–oxalic acid complex after normalization (Fig. 6). The peak ratio values and lifetime data are in close agreement (within the experimental uncertainties, ESI†) with those of the aqueous Eu(III)–oxalic acid complex. The glass plate, also seen under a UV lamp at 254 nm excitation, emits red luminescence from the surface, confirming the presence of Eu(III) as an Eu(III)–Ox complex on the kaolinite surface (ESI, Fig. S2†).

Fig. 6 Comparison of luminescence spectra of Eu(III) (10⁻⁴ M) sorbed onto kaolinite (2.5 g L⁻¹) in the presence and absence of oxalic acid (10⁻⁴ M) (a) solid deposited on film; (b) suspension.

The addition sequence of metal and ligands onto the clay surface can affect the speciation/sorption of the metal ions at the mineral–water interface.⁴³ It was of interest to see the effect of the addition sequence of Eu(III), HA, and kaolinite to the sorption profile of Eu(III) onto kaolinite. For this, two sets of experiments were done, viz. (a) HA was added after Eu(III) and kaolinite were equilibrated (Set 1) and (b) Eu(III) was added after HA and kaolinite were equilibrated (Set 2).

In both cases, the equilibration was done for 16 h, followed by centrifugation. Fig. 7 shows the Eu(III) sorption profile onto kaolinite with varying orders of addition of HA and Eu(III). For Set 1, the Eu(III) sorption profile looks like a desorption phenomenon, in which Eu(III) sorbed onto kaolinite is desorbed in the presence of the highly complexing HA to the aqueous phase, whereas in Set 2, the sorption of Eu(III) onto HA-equilibrated kaolinite shows a sharp decrease after pH 4. This is explained on the basis of the fact that, although an increase in the sorption of HA onto kaolinite is expected with increasing pH, simultaneous deprotonation of HA leads to increasing complex formation with the Eu(III), resulting in predominantly the Eu(III)–HA complex being present in the aqueous phase.

Fig. 7 Effect of sequence of addition of humic acid (10 mg L⁻¹) and Eu(III) on Eu(III) (10⁻⁶ M) sorption onto kaolinite (1 g L⁻¹) with varying pH; I: 0.01 M NaClO₄. KA + HA/Eu + Eu/HA means KA and HA/Eu were shaken for 16 h then Eu(III)/HA was added and again shaken for 16 h before calculation of % sorption.

To obtain an insight into the nature of the complex (inner/outer sphere or ternary) formed in the sorption process, luminescence studies were carried out using samples containing different Eu(III) (mg L⁻¹) : HA (mg L⁻¹) ratios, viz. Eu(III) : HA, 1.5 : 1 and Eu(III) : HA, 0.15 : 1 at a fixed kaolinite concentration (1 g L⁻¹). The luminescence spectra were recorded with varying orders of addition of Eu(III), kaolinite and HA. Table 1 shows the lifetime and peak ratio data for various samples. The lifetime data suggests species of a similar nature in all the samples, probably coordinated with 9–10 nearest –OH groups, whereas the peak ratio varies slightly in various samples. As compared to Eu(III)_aq, the peak ratio for Eu(III) sorbed onto kaolinite is higher and is attributed to the distortion in the Eu(III)_aq structure due to interaction with the surface of kaolinite. However, the nature of the interaction is not very strong and is considered as outer-sphere in nature.

Table 1 Effect of the sequence of addition of Eu(III) (10⁻⁴ M) and humic acid (10 mg L⁻¹) on fluorescence lifetime and peak ratio (asymmetric ratio)^c

Sample	Life time	Intensity ratio (617/591)
a Suspension.b Supernatant.c The two entities in the bracket were mixed first, then the third (outside bracket) was added.
Eu(III): 10^−4 M, KA: 1 g L^−1, HA: 10 mg L^−1
Euaq	114–115 μs	0.56 or 0.65
Eu + HA	108 μs	0.76
Eu + KA^a	104–113 μs	0.68 or 0.69
Eu + KA^b	114 μs	0.54
(Eu + KA) + HA^a	107 μs	0.92
(Eu + KA) + HA^b	112 μs	0.67
(Eu + HA) + KA^a	103 μs	0.83
(Eu + HA) + KA^b	110 μs	0.65
(KA + HA) + Eu^a	104 μs	0.74
(KA + HA) + Eu^b	110 μs	0.62
Eu(III): 10^−4 M, KA: 1 g L^−1, HA: 100 mg L^−1
Eu + HA	115 μs	1.24
(Eu + HA) + KA^a	87 μs	1.66
(Eu + KA) + HA^b	75 μs	1.41

---

The addition of HA to Eu(III) increases the overall intensity of the emission spectra of Eu(III), although the lifetime remains unchanged. The changes in the peak ratio (I₆₁₇/I₅₉₁) suggest distortion in the Eu(III) structure upon HA complexation.

In the presence of both kaolinite and HA, the intensity of the emission spectra was found to be enhanced, as in the case of pure HA complexation with Eu(III). To confirm this, the suspension was centrifuged at 20 000 rpm for 25 minutes and the supernatant was used for luminescence measurements. The emission spectra of the supernatant were found to be similar to that obtained for a suspension, with the latter showing emission lines of marginally lower intensities.

The lifetime values of the supernatant and the suspension are comparable, suggesting the presence of similar emitting species. The luminescence spectra were also recorded at lower Eu : HA ratios under identical conditions. With increasing concentration of HA, the peak ratio increases, showing more distortion of the complex structure, although the lifetime values suggest a similar nature of the emitting Eu(III) species, as in Eu(III)_aq. The luminescence spectra of the suspensions at higher HA concentrations also show similar lifetime values. However, a higher intensity of the emission lines, as compared to those obtained for an Eu : HA of 1.5, suggests that, in spite of stronger complexation of Eu(III) with HA to make the complexed species present in the aqueous phase, the primary coordination sphere of Eu(III) appears unchanged. This was reflected in similar lifetime values for all the samples. On the other hand, the enhancement in the peak area ratios suggests perturbation of local symmetry on HA complexation, as suggested by the higher intensities of the Eu(III) emission lines.

Effect of kaolinite solubility on Eu(III) uptake

In any metal ion sorption studies, it is ideal that the aqueous phase composition should not change during the course of the experiment. However, in practice, the clay dissolution can give rise to some side reactions and affect the metal-ion sorption and speciation.⁴⁴ Various groups studied the dissolution of kaolinite, and reported that the dissolution was minimal in neutral pH range and increased on both acidic and basic sides.⁴⁵ To confirm this observation, two sets of experiments were planned. In the first set, kaolinite (1 g L⁻¹) was suspended in pH ∼ 3 solution (to obtain somewhat faster dissolution), the aged kaolinite suspension (10 mL) was taken at different time intervals, and the % Eu(III) (10⁻⁶ M) sorption was measured. In the second set, Eu(III) was sorbed onto fresh kaolinite under similar conditions to those used in the first set, and the % desorption was followed with time. The % sorption was found to decrease with time from ∼97% in 2 h to 75% in 185 h (Fig. 8a). It can be inferred from the above results that the dissolution products of kaolinite hold the Eu(III) in the aqueous media, thereby reducing the % sorption. Whereas in the second set, changes in the % sorption values are insignificant (within experimental error limits) and show no or very slow desorption of Eu(III) with time due to a higher ratio of sorption sites to the Eu(III) concentration.

Fig. 8 Effect of aging of (a) kaolinite (1 g L⁻¹) solution on Eu(III) (10⁻⁶ M) sorption; pH ∼ 3, I: 0.01 M NaClO₄; (b) kaolinite solution on Eu(III) (10⁻⁴ M) sorption with varying kaolinite concentration; pH ∼ 5 and I: 0.01 M NaClO₄.

Similar experiments were repeated with kaolinite (1 g L⁻¹) in the presence of oxalic acid (10⁻⁴ M) and citric acid (10⁻⁴ M) (Fig. 9a and b). The results suggested a decrease in the % sorption data with the ageing of the suspension in the case of citric acid, whereas for oxalic acid, the changes are not significant.

Fig. 9 Effect of aging of kaolinite (1 g L⁻¹) solution on Eu(III) (10⁻⁶ M) sorption in the presence of (a) oxalic acid (10⁻⁴ M); (b) citric acid with varying pH; I: 0.01 M NaClO₄.

Variation of M : S ratio and Eu(III) desorption

The sorption of Eu(III) (10⁻⁴ M) onto kaolinite was studied as a function of kaolinite concentration (1 to 10 g L⁻¹) at pH 5, with an ionic strength of 0.01 M (NaClO₄). The % Eu(III) sorption was found to increase from 40% to 70% with increasing kaolinite concentration (Fig. 8b). A systematic decrease in the % Eu(III) sorption was observed with the ageing of the suspension. This decrease in the % Eu(III) sorption can be attributed to the release of the sorbed Eu(III) into the aqueous phase with time due to the slow surface dissolution of kaolinite. The effect of such dissolution is higher at lower kaolinite concentrations, as compared to the cases with higher kaolinite concentration. This could be attributed to a greater number of available sites for Eu(III) sorption at higher kaolinite concentrations, as compared to that with lower kaolinite concentrations.

Conclusions

Eu(III) sorption and speciation onto kaolinite surfaces was studied under varying conditions. The sorption of Eu(III) onto kaolinite was relatively fast as equilibrium was attained in ∼1 h at different pH values and ionic strengths. The Eu(III) sorption onto a kaolinite surface was found to be dependent on ionic strength at lower pH (<6), although this was independent of the ionic strength of the medium at higher pH. This suggests different mechanisms of Eu(III)–kaolinite interaction for different pH ranges. The Eu(III) sorption follows an ion-exchange (outer-sphere) mechanism up to pH ∼ 6, beyond which surface complexation (inner-sphere) is predominantly responsible for Eu(III) sorption onto kaolinite. The effect of the addition of complexing agents modifies the Eu(III) % sorption profile. In the case of oxalic acid, sorption minima are found at pH 5.5 and the % sorption rises on both sides of the minima. Fluorescence studies suggested the sorption of Eu(III) as an Eu(III)–oxalate complex in the presence of oxalic acid. The sorbed Eu(III) may be forming a metal ion bridge between oxalic acid and the clay surface, or Eu(III) may be precipitated as the Eu(III)–oxalate complex. A clear distinction is not made in this work. Further spectroscopic studies, such as EXAFS, can be helpful in differentiating the bridged complex or surface precipitate. The addition sequence of Eu(III) and humic acid onto the kaolinite surface affected the sorption profile. Fluorescence studies showed that the nature of species present in the suspension and supernatant is the same. The observed lifetime data and fluorescence lines match well with Eu(III)_aq. In view of this, it can be concluded that the observed fluorescence is from Eu(III)_aq remaining in the solution and the sorbed species is either non-luminescent or the luminescence was quenched by the presence of humic acid. The % sorption was also found to decrease as the kaolinite suspension aged.

Acknowledgements

Authors thank Dr P. K. Pujari, Head, Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai, India for his keen interest in this work. They also thank Dr M. Mohapatra and Dr A. Sengupta for their help in fluorescence measurements.

Notes and references

1. M. H. Bradbury and B. Baeyens, Geochim. Cosmochim. Acta, 2002, 66, 2325.
2. Y. Takahashi, T. Kimura, Y. Kato and Y. Minai, Environ. Sci. Technol., 1999, 33, 4016.
3. M. H. Bradbury, B. Baeyens, H. Geckeis and T. Rabung, Geochim. Cosmochim. Acta, 2005, 69, 5403.
4. G. D. Sheng, D. D. Shao, Q. H. Fan, D. Xu, Y. X. Chen and X. K. Wang, Radiochim. Acta, 2009, 97, 621.
5. K. Fouchard, R. Drot, E. Simoni, N. Marmier, F. Fromage and J. J. Ehrhardt, New J. Chem., 2004, 28, 864.
6. B. Grambow, M. Fattahi, G. Montavon, C. Moisan and E. Giffaut, Radiochim. Acta, 2006, 94, 627.
7. M. Olin, E. Puhakka and J. Lehikoinen, Working Report 2007-82, Technical Research Centre of Finland (VTT), 2007.
8. Q. H. Fan, X. L. Tan, J. X. Li, X. K. Wang, W. S. Wu and G. Montavon, Environ. Sci. Technol., 2009, 43, 5776.
9. C. Chen, X. Yang, J. Wei, X. Tan and X. Wang, J. Colloid Interface Sci., 2013, 393, 249.
10. M. Smadfam, T. Jintoku, S. Sato, H. Ohashi, T. Mitsugashira, M. Hara and Y. Suzuki, Radiochim. Acta, 2000, 88, 717.
11. C. Degueldre, H. J. Ulrich and H. Silby, Radiochim. Acta, 1994, 65, 173.
12. Engineered Barrier Systems and the Safety of Deep Geological Repositories: State-of-the-art Report, Nuclear Energy Agency Organisation for Economic Co-Operation and Development, OECD, 2003, ISBN 92-64-18498-8, in co-operation with the European Commission EUR 19964 EN Search.
13. R. Pusch, Nucl. Eng. Technol., 2006, 38, 483.
14. C. Steefel, J. Rutqvist, C.-F. Tsang, H.-H. Liu, E. Sonnenthal, J. Houseworth and J. Birkholzer, LBNL report, 2010.
15. H. Geckeis, J. Lutzenkirchen, R. Polly, T. Rabung and M. Schmidt, Chem. Rev., 2013, 113, 1016.
16. K. Maher, J. R. Bargar and G. E. Brown Jr, Inorg. Chem., 2013, 52, 3510.
17. R. Kautenburger and H. P. Beck, J. Environ. Monit., 2010, 12, 1295.
18. A. Křepelová, S. Sachs and G. Bernhard, Radiochim. Acta, 2011, 99, 253.
19. P. N. Pathak and G. R. Choppin, J. Radioanal. Nucl. Chem., 2007, 274, 517.
20. D. I. Kaplan, S. M. Serkiz and J. D. Allison, Appl. Geochem., 2010, 25, 224.
21. M. Baik, W. Cho and P. Hahn, Environ. Eng. Res., 2004, 9, 160.
22. P. N. Pathak and G. R. Choppin, Radiochim. Acta, 2007, 95, 267.
23. M. Schmidt, T. Stumpf, C. Walther, H. Geckeis and T. Fanghänel, Dalton Trans., 2009, 6645.
24. A. Křepelová, S. Sachs and G. Bernhard, Radiochim. Acta, 2011, 99, 253.
25. M. A. Denecke, Coord. Chem. Rev., 2006, 250, 730.
26. X. Tan, M. Fang and X. Wang, Molecules, 2010, 15, 8431.
27. A. Schnurr, R. Marsac, T. Rabung, J. Lützenkirchen and H. Geckeis, Geochim. Cosmochim. Acta, 2015, 151, 192.
28. K. Ishida, T. Saito, N. Aoyagi, T. Kimura, R. Nagaishi, S. Nagasaki and S. Tanaka, J. Colloid Interface Sci., 2012, 374, 258.
29. N. Huittinen, T. Rabung, P. Andrieux, J. Lehto and H. Geckeis, Radiochim. Acta, 2010, 98, 613.
30. M. M. Fernandes, T. Stumpf, B. Baeyens, C. Walther and M. H. Bradbury, Environ. Sci. Technol., 2010, 44, 921.
31. A. Křepelová, V. Brendler, S. Sachs, N. Baumann and G. Bernhard, Environ. Sci. Technol., 2007, 41, 6142.
32. T. Stumpf, C. Hennig, A. Bauer, M. A. Denecke and T. Fanghänel, Radiochim. Acta, 2004, 92, 133.
33. T. Stumpf, A. Bauer, F. Coppin, T. Fanghänel and J. I. Kim, Radiochim. Acta, 2002, 90, 345.
34. J. Madejová, Vib. Spectrosc., 2003, 31, 1.
35. R. G. Wolff, Am. Mineral., 1963, 48, 390.
36. L. Vaculíková, E. Plevová, S. Vallová and I. Koutník, Acta Geodyn. Geomater., 2011, 8, 59.
37. J. Madejová, I. Kraus, D. Tunega and E. Šamajová, Geol. Carpathica--Clays, 1997, 6(1), 3.
38. J. D. Russell and A. R. Fraser, Infrared methods, in Clay Mineralogy: Spectroscopic and Chemical Determinative Methods, ed. M. J. Wilson, Chapman & Hall, London, 1994, pp. 11–67.
39. J. Madejová and P. Komadel, Clays Clay Miner., 2001, 49, 410.
40. W. Stumm, Chemistry of the Solid-Water Interface: Processes at the Mineral-Water and Particle-Water Interface in Natural Systems, John-Wiley & Sons, Inc., 1992, ch. 2, pp. 21–24.
41. T. Sakuragi, S. Sato, T. Kozaki, T. Mitsugashira, M. Haraand and Y. Suzuki, Radiochim. Acta, 2004, 92, 697.
42. G. Montavon, S. Markai, Y. Andrés and B. Grambow, Environ. Sci. Technol., 2002, 36, 3303.
43. X. Wanga, Z. Chena, X. Tanb, T. Hayatd, B. Ahmadd, S. Daia and X. Wang, Chem. Eng. J., 2016, 287, 313.
44. N. Huittinena, T. Rabungb, A. Schnurrb, M. Hakanena, J. Lehtoa and H. Geckeis, Geochim. Cosmochim. Acta, 2012, 99, 100.
45. S. A. Carroll and J. V. Walther, Am. J. Sci., 1990, 290, 797.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17984c

---