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A 1H and 27Al NMR investigation of yttrium(III) and europium(III) interaction with kaolinite
Applied Clay Science 80–81 (2013) 182–188
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Research paper
A 1H and 27Al NMR investigation of yttrium(III) and europium(III) interaction with kaolinite Nina Huittinen a,⁎, Priit Sarv b, Jukka Lehto a a b
Laboratory of Radiochemistry, Department of Chemistry, P.O. Box 55, FIN-00014 University of Helsinki, Finland Institute of Chemical Physics and Biophysics, Akadeemia Tee 23, EST-12618 Tallinn, Estonia
a r t i c l e
i n f o
Article history: Received 13 January 2013 Received in revised form 22 March 2013 Accepted 11 April 2013 Available online 9 May 2013 Keywords: Solid-state NMR Sorption Trivalent metal Kaolinite Surface hydroxyl Chemical shift
a b s t r a c t Sorption of the trivalent metals Eu 3+ and Y3+ on a natural kaolinite mineral was investigated with solid-state 1H and 27Al NMR. The metal ion concentration in the samples was varied between 4 × 10 −6 M and 8 × 10−5 M, while the kaolinite concentration and ionic strength with respect to Na + were kept constant at 5 g/l and 1 mM, respectively. The aqueous samples were equilibrated for at least six days at pH 8.0 ± 0.1 before the mineral was separated from the supernatant, recovered, and dried. Metal ion addition to the kaolinite samples had no influence on recorded 27Al spectra, which show a center peak at 6.15 ppm corresponding to octahedrally coordinated aluminum in the gibbsite-like sheets of kaolinite. The 27 Al measurements were, however, imperative to perform as potential mineral dehydroxylation was monitored from the 27Al spectra before and after drying treatment and normalization of 1H spectra was based on the acquired 27Al data. The attachment of Y3+/Eu 3+ on the kaolinite surface was shown to reduce 1H resonances with chemical shifts at 2.3–3.5 ppm, 1.6, 1.3, 0.9 and 0.7–0.3 ppm. The loss of proton signal could be attributed to metal ion attachment to isolated Al–OH groups on the kaolinite edge surfaces and possibly to bridging Al–OH–Al hydroxyls on the gibbsite-like basal plane of the mineral. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The adsorption of radionuclides on mineral surfaces is regarded as one of the dominant retardation processes in the near and far-field of nuclear waste repositories (Geckeis and Rabung, 2008). As potential host-rock or buffer and backfill materials in these repositories, clay minerals play an important role as sorbent phases for mobilized radionuclides in the geosphere. Kaolinite, Al2Si2O5(OH)4, is one of the most abundant aluminosilicate minerals (Sutheimer et al., 1999) with a 1:1 layered structure of alternating aluminum hydroxide octahedral and silica tetrahedral sheets that share a common plane of oxygen atoms (Bergaya et al., 2006). Even though kaolinite is considered as the least sorbing clay (Cheremisinoff, 1997) it is a very good model mineral in sorption investigations due to its simple structure and practically no sorption of cations in the interlayer space. With no interlayer space for radionuclide sorption, the sorption activity can be assumed to occur solely along the edges and the surfaces of the mineral (Miranda-Trevino and Coles, 2003), which in turn simplifies the interpretation of sorption data. In our previous works (Huittinen et al., 2010, 2012) we studied the sorption and speciation of trivalent curium, a minor actinide in spent nuclear fuel, on natural kaolinite from St. Austell (UK) using ⁎ Corresponding author at: A. I. Virtasen aukio 1 (P.O. Box 55), 00014 University of Helsinki, Finland. Tel.: +358 9191 50136. E-mail address: nina.huittinen@helsinki.fi (N. Huittinen). 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.04.005
time-resolved laser fluorescence spectroscopy (TRLFS). Based on the spectroscopic data the speciation of this trivalent metal in the kaolinite dispersion could be explained over a broad pH range (pH 3–13). Briefly, three inner-sphere sorbed surface complexes differing only in the degree of hydrolysis were found to dominate the speciation between pH 5 and 11. When the pH was further increased above pH 11 a ternary curium-silicate surface complex was found to be the predominant species on the kaolinite surface. In this paper we focus on understanding the role of different hydroxyl groups at the kaolinite surface in the trivalent metal ion sorption process using solid-state 1H and 27Al NMR. The paramagnetic Eu3+ ion is selected as an analogue to the trivalent actinides Pu 3+, Am 3+ and Cm 3+ while Y 3+ is taken under study due to its diamagnetic properties. The study is performed analogous to a previous NMR study of our group where we investigated the specific sorption of Eu 3+ and Y3+ ions on γ-alumina (Huittinen et al., 2011). At constant pH and varying metal ion concentrations we could identify a manifold of chemically very similar hydroxyl groups at the γ-alumina surface that participated in the sorption reaction of the trivalent metals. Here, we aim to identify the proton signals from the various hydroxyl groups on the kaolinite surface and to locate those hydroxyl groups that are involved in the surface complexation reaction of the trivalent metal by monitoring the kaolinite proton spectra before and after metal ion addition. In contrast to γ-alumina where the recorded proton signal stems from the surface only, kaolinite has hydrogen atoms in the bulk structure (Al2Si2O5(OH)4) that will give rise to the majority of the collected proton signal, thus, making it very difficult to extract any surface
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information from the NMR spectra. To increase the chance of locating surface protons we included Eu 3+ in the study, even though we could show that the paramagnetic Eu3+ ion caused a greater loss of proton signal at the γ-alumina surface in comparison to the diamagnetic Y 3+ ion and thus, prevented the straight forward interpretation of recorded data (Huittinen et al., 2011). In this case the greater loss of proton signal caused by the paramagnetic europium ions is employed to facilitate the detection of protons in the close proximity of the trivalent metal, i.e. protons at the kaolinite surface. 2. Material and methods
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55 °C over night. Before NMR experiments the samples were dried for 5 h at 180 °C (0.2 mTorr) in a vacuum oven (Vacuum Systems, Finland) using a ramp temperature of 1°/min to eliminate physisorbed water from the mineral samples. The dried samples were sealed under vacuum (40 mTorr) and transferred into a N2-glove box (MBraun) where the oxygen and moisture contents were monitored to be less than 1 ppm throughout the sample handling, thus, assuring that samples were not rehydrated before NMR measurements. In the N2-glove box 3.2 mm o.d. zirconia rotors were filled with the dried kaolinite samples. Kel-F® (polychlorotrifluoroethylene) caps were used to seal the rotors and some Kel-F® oil was spread between the cap and the rotor wall to ensure air tightness of the rotors after removal from the glove box.
2.1. Kaolinite The kaolinite mineral from St. Austell (UK) used throughout this work has been characterized by Bauer and Berger (1998) and further in our previous work (Huittinen et al., 2010). Results from these characterizations are compiled in Table 1. No crystalline phase impurities such as illite, anatase, quartz or iron oxides found in e.g. the natural source-clay kaolinite minerals KGa-1 and KGa-2 (Chipera and Bish, 2001; Pruett and Webb, 1993) could be detected in the XRD survey (Huittinen et al., 2010). The tabulated Hinckley index (HI) (Hinckley, 1963) reflects the degree of crystallinity in kaolinite minerals. Kaolinite is considered to be well-ordered when the HI exceeds 0.9 (Aparicio and Galan, 1999) implying that our kaolinite material is highly ordered. Before sample preparation for the NMR experiments the kaolinite mineral was washed with ultrapure water (MilliQ) over night under constant stirring. After mineral sedimentation the water was discarded and the mineral was allowed to dry. 2.2. 1H and
27
Al NMR investigations
2.2.1. Sample preparation NMR samples were prepared by weighing 300 mg of kaolinite in HDPE bottles. 50–55 ml of MilliQ water and varying concentrations of the metal ions (either Y 3+ or Eu 3+) were added to the dispersions. Due to pH-dependent protonation and dissociation reactions occurring at the mineral surface, the pH of all samples was adjusted to 8.0 ± 0.1, a typical pH prevailing in deep bedrock, by addition of NaOH. Furthermore, the Na + concentration was kept at 1 mM by adjusting the amount of added Na + after NaOH additions by addition of an appropriate amount of 0.1 M NaClO4 to the samples. The final sample volume of 60 ml, resulting in a solid content of 5 g/l, and a metal ion concentration ranging from 4 × 10 −6 to 8 × 10 −5 M was adjusted with addition of MilliQ water. All samples were allowed to equilibrate for at least 6 days before they were separated by centrifugation at 20,000 rpm. An aliquot of the solution phase after centrifugation at 80,000 rpm was subjected to ICP-MS analysis of the remaining, unsorbed europium and yttrium concentrations, to confirm that the majority (~ 100%) of the metal ions are attached on the mineral surface. The separated minerals were dried in an oven at
Table 1 Characteristics of kaolinite. Kaolinite property (method)
Determined value
Specific surface area (N2-BET)a Grain size (SEM)a
11.72 m2/g Thickness 0.2 μm Diameter 0.4–1.0 μm O — 68.27 ± 3.40 (atom) % Al — 15.26 ± 1.50% Si — 16.47 ± 1.90% Traces (b1%) of K, Na, Sb, Fe 1.34
Elemental composition (SEM-EDX)b
Hinckley index (XRD)b a b
Bauer and Berger (1998). Huittinen et al. (2010).
2.2.2. 1H and 27Al NMR experimental set-up The 1H and 27Al single-pulse MAS NMR experiments were performed on a Bruker BioSpin Avance III NMR spectrometer using a Bruker triple-resonance MAS probehead. Parameters used in the experiments for the two nuclei, 1H and 27Al, are compiled in Table 2. Solid adamantane and alum were used as secondary references because of easier handling in comparison to the liquid standards TMS and Al(NO3)3. All the samples were spun with dry air at 18,000 ± 1 Hz. The spinning-speed was stabilized by a Bruker MAS controller. For our quantitative 27Al experiments a small nonselective pulse (10°) was needed to excite the Al sites with different quadrupolar interactions to the same extent (Lippmaa et al., 1986). Considering our experimental conditions (high magnetic field, fast MAS spinning) and modest (less than 6 MHz) 27Al quadrupolar coupling constant of kaolinite (Ashbrook et al., 2000), we may assume, that all the intensity from the central transition of 27Al is in the central band and spinning-sidebands (SSB) belong to the satellite transitions (Massiot et al., 1990). Subtracting the intensity of one SSB from the central band, we obtain the pure central transition intensity, which was used to calculate the aluminum concentration. The 1H MAS NMR spectrum of the empty rotor (the background) was subtracted from the sample spectra to get the 1H MAS NMR spectra of the pure samples. For such and other manipulations (discussed in the following section) it is important, that the SSBs are positioned at the same frequency throughout the spectra which is why we needed the spinning-speed stabilization. In quantitative calculations the intensities from the SSBs, if present, were taken into account. To ensure compatibility between acquired 27Al and 1H data for a given sample measurements both nuclei were probed in succession, i.e. without stopping the sample rotation between the measurements. 2.2.3. 1H NMR spectral treatment In order to enable direct comparison of the background corrected proton spectra all recorded spectra had to be phase and baseline corrected. In addition, as the proton signal is proportional to the amount (mass) of sample, the slightly different sample amounts (arising from uneven rotor filling) require that all spectra are normalized before they are comparable. Zero-order and first order phase corrections were done to each acquired spectrum. The first order, frequency dependent phase, was acquired from the measured reference material, i.e. adamantane, and the same phase parameter was used for all measured mineral samples. Zero order phase correction is frequency independent and this correction was done manually by adjusting the phase a few degrees for each sample spectrum. Thus, we aimed at obtaining proton peaks with precisely same shapes and phases with one another to allow for direct comparison of the spectra. The baseline correction of the proton spectra was done using a cubic spline function over the 80–(−80) ppm region. Approximately 10–14 points were selected as true baseline points for the spline correction. After the above mentioned spectral treatments a perfect match between all spectra in the yttrium and europium series could unfortunately not be acquired, and these spectra had to be left out from the data interpretations. Omitted spectra are those without any metal
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Table 2 NMR parameters used in the experiments. Nucleus
Resonance frequency (MHz)
Magnetic field (T)
Exciting pulse
Relaxation delay (s)
Number of scans
Chemical shift reference
1
800.13 208.49
18.8 18.8
90°(3.7 μs) 10°(0.5 μs)
40 2
160 240
Adamantanea C10H16 Alumb KAl(SO4)2 × 12H2O
H 27 Al a b
Chemical shifts given relative to TMS. Chemical shifts given relative to alum, which itself is −0.35 ppm relative to 3 M Al(NO3)3 at the magnetic field 18.8 T (this work).
ion addition as well as the spectrum of kaolinite containing 8 × 10−6 M yttrium. Finally, for a direct comparison of the spectra acquired from slightly different sample amounts, the spectra had to be normalized. Gravimetric normalization was not satisfactory due to the very small differences in the proton spectra induced by the adsorption of the trivalent metal to the kaolinite surface. Therefore, proton spectra acquired from the kaolinite samples were normalized with respect to the amount of aluminum in the samples, assuming that the aluminum concentration per unit mass is constant in all samples. By integrating the phase and baseline corrected aluminum spectra and setting one sample integral to 100% and comparing the other integrals to this sample, normalization factors were obtained that were further used to normalize the proton spectra. Normalized spectra could thereafter be compared to one another to acquire information on the different surface protons and the exchange of these due to trivalent metal ion attachment to the surface. The validity of our normalization procedure was evaluated by calculating the number of aluminum and hydrogen atoms in the samples after normalization. According to the chemical formula of kaolinite, Al2Si2O5(OH)4, the hydrogen to aluminum ratio should be 2:1. Values obtained from our normalized spectra yielded a ratio of 1.97 ± 0.08 hydrogens per one aluminum atom, thus, confirming the validity of our normalization procedure. 3. Results and discussion 3.1.
27
Al experiments
27 Al spectra were recorded for all NMR samples. However, no differences between spectra of pure kaolinite and samples including Eu 3+/Y 3+ could be detected. Thus, no information on metal sorption onto kaolinite could be gained from the 27Al NMR spectra. The aluminum data, however, serves two very important purposes. The possible alteration of the mineral due to the drying treatment was monitored from hydrated and dehydrated samples by measuring some kaolinite samples before and after drying treatment. Secondly, the phase, and baseline corrected aluminum data was used to normalize the recorded
proton spectra as explained previously. In Fig. 1 (left) two of the collected spectra with and without Eu(III) are presented. The spectra show one peak with a peak position at 6.15 ppm, corresponding to octahedrally coordinated aluminum in the gibbsite-like aluminum hydroxide layers of kaolinite. This peak position falls within the range of reported values found in literature e.g. ranging from 2.4 ppm (Temuujin et al., 1998) to 8.64 ppm (Crosson et al., 2006). The magnetic field strength will affect the position of the aluminum signal, implying that spectra collected at varying field strengths result in slightly different chemical shift values. Zhou et al. (2009) showed how the peak position of the 27 Al signal changes as a function of magnetic field and performed ab initio calculations to support their findings. Based on their theoretical ab initio quantum mechanical calculations, the δiso for the 27Al center transition in kaolinite is located at 6.25 ppm which is very close to their experimentally obtained value of 6.2 ppm at a magnetic field of 21.06 T. At this magnetic field quadrupolar interactions become insignificant and the δiso is only moved very little by quadrupole induced shifts. Our experiments were performed at 18.8 T leading to a slight quadrupolar shift that moves the peak position of the kaolinite 27Al signal upfield in comparison to the results obtained by Zhou et al. (2009). In Fig. 1 (right) aluminum spectra before and after two different drying treatments are presented. The spinning speed of the hydrated aluminum sample was slightly faster at 20 kHz in comparison to the dehydrated samples that were spun at 18 kHz, which is why the spinning sidebands (SSBs) are located at different chemical shifts. Spinning sidebands are separated by a chemical shift equal to the rate of spinning. The sample dehydrated at 180 °C (0.2 mTorr) i.e. the drying temperature we subjected all samples to, show no evidence of dehydroxylation. To illustrate the effect of a too harsh drying treatment we subjected one kaolinite sample to a much higher drying temperature of 450 °C. At this temperature substantial mineral dehydroxylation occurs that shows up as two peaks around 30, and 57 ppm. In literature aluminum bearing minerals with 27Al chemical shift values in the 30 ppm region, e.g. 28 ppm (Fernandez et al., 2011; Rocha and Klinowski, 1990), 35 ppm (Alemany and Kirker, 1986), 36–37 ppm (Fitzgerald et al., 1997), have been assigned to five-coordinated aluminum. Therefore, in accordance
Fig. 1. Left — 27Al spectra of kaolinite (black) and kaolinite with adsorbed Eu(III) (blue). The asterisks denote spinning sidebands. Right — 27Al spectra of a hydrated kaolinite sample and two dehydrated samples subjected to drying temperatures of 180 °C and 450 °C, respectively.
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Fig. 2. Left — normalized 1H NMR spectra of kaolinite samples containing varying concentrations of yttrium (red colors) and europium (blue colors). Proton spectra of yttrium containing samples were divided by a factor of 1.25 to allow for better visualization in the graph. Asterisks denote spinning sidebands. Right — magnification of the 10–(− 4) ppm region.
to these studies we assign the peak at 30 ppm to AlO5 forming as a result of kaolinite dehydroxylation. Based on chemical shift data for fourcoordinated aluminum (AlO4), e.g. 53 ppm (Temuujin et al., 1998), 55.3 ppm (Lambert et al., 1989), 56 ppm (Fernandez et al., 2011), 57 ppm (Rocha and Klinowski, 1990) the broad peak appearing between 50 and 57 ppm is assigned to AlO4. 3.2. 1H experiments The acquired and normalized proton spectra of kaolinite with yttrium (red colors) and europium (blue colors) are presented in Fig. 2 (left). To separate the two different series of metal ion containing sample spectra in the figure and, thus, allow for better visualization all proton spectra containing yttrium were divided by a factor of 1.25. Due to the very small differences induced by the metal ion addition the spectra that could not be matched and phased perfectly (as explained in Section 2.2.3) in comparison to the others were not considered for data interpretations. Such spectra include those of the pure kaolinite mineral and kaolinite with 8 × 10−6 M Y(III). Thus, the results and discussions are based on data acquired from kaolinite samples containing 4 × 10−6, 4 × 10−5, and 8 × 10−5 M yttrium and 4 × 10−6, 8 × 10−6, 4 × 10−5, and 8 × 10−5 M europium. In the magnification of the proton spectra Fig. 2 (right) a peak at 1.9 ppm and a shoulder at approximately 2.7 ppm can be distinguished. The yttrium containing spectra are close to identical with no apparent loss of proton signal with increasing yttrium concentration in the samples, while a small loss of proton signal is visible in the sample containing 8 × 10−5 M Eu(III). Therefore, to be able to see the possible effect induced by the metal ion addition, difference spectra were plotted in which the spectra containing higher metal ion concentrations (8 × 10−6 M–8 × 10−5 M) were subtracted from the spectrum with the least added amount of metal ion (4 × 10 −6 M). These difference spectra are shown in Fig. 3. The relative differences of the Y(III) and Eu(III) containing difference spectra are illustrated in Fig. 4. The Eu(III)-containing spectra indicate removal of a greater amount of protons than Y(III) from the kaolinite surface (Fig. 4), similarly to what we observed in our γ-alumina study (Huittinen et al., 2011), but the overall effect on the total proton signal is very small. The difference between the spectra with lowest and highest concentrations of added metal ion is only 0.72% in case of Y(III) and three times higher, i.e. 2.16% in case of Eu(III). Using a total surface site density of 12.0 sites/nm 2 (Peacock and Sherman, 2005) the surface protons would amount to 1.5% of the total number of protons in the Al2Si2O5(OH)4 structure. It is clear that Eu(III) removes more protons
than theoretically available at the surface. We believe that this is a combined effect of the paramagnetism of the europium ion and the normalization procedure that we used. Europium is paramagnetic which means that the ion produces a new magnetic field that the probed hydrogen or aluminum nucleus feels in addition to the magnetic field produced by the spectrometer. This induced magnetization causes an isotropic shift of atoms in the vicinity of the paramagnetic substance. Thus, not only protons exchanged in the surface complexation reaction at the kaolinite surface will be absent in the spectra containing europium, but also protons in the close vicinity of the adsorbed ion. In addition, it is also possible that aluminum atoms close to the europium ion are influenced by the paramagnetism. In that case our normalization procedure, that we based on the assumption that the aluminum concentration per unit mass in the samples is constant, will introduce a small additional error to the actual number of protons that are being removed in the surface complexation reaction. In Fig. 4 the relative intensities of the yttrium containing and europium containing difference spectra are shown. Europium can be seen to remove approximately three times more of the proton signal than the corresponding yttrium concentration at the kaolinite surface does. In both sets of difference spectra, Fig. 3, a broad peak in the chemical shift region 2.3–3.5 ppm is observable. The yttrium spectra also show peaks at 1.6, 1.3, 0.9 and 0.7–0.3 ppm. The corresponding chemical shifts of the europium spectra are located at similar chemical shift values but the spectra are less featured than the yttrium ones. In addition, the greater removal of proton signal in the europium containing spectra results in a much larger contribution of the 2.3–3.5 ppm broad peak in the 4 × 10−6–4 × 10−5 M difference spectrum than what is observed for the corresponding Y(III) difference spectrum. The difference spectra of 4 × 10−6–8 × 10−6 M Eu(III) and 4 × 10−6–4 × 10−5 M Y(III) on the other hand are very much alike, both with respect to their relative intensities as well as the chemical shifts of the observed proton signals. To enable the assignment of these proton signals to various hydroxyl groups on the kaolinite surface we will discuss the different hydroxyl groups available in the kaolinite structure. The majority of OH-groups in the kaolinite mineral are confined to the gibbsite-like aluminum hydroxide layers. The exposed aluminum hydroxide basal surface contains doubly coordinated hydroxyls (Al–OH–Al), referred to as inner surface hydroxyls (Frost, 1998). The kaolinite edge surfaces feature three different hydroxyl groups, singly coordinated Al–OH and Si–OH groups and bridged Al–OH–Si groups. These edge hydroxyls are considered to be the major sorbing hydroxyl sites (Morris et al., 1990). Whether or not these surface sites are protonated or not is highly dependent on the
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Fig. 3. 1H difference spectra produced by subtracting the spectra containing higher metal ion concentrations from the spectrum with the lowest added metal ion concentration.
solution pH. Silanol groups of silicate minerals are known to deprotonate already in the acidic pH range (pH 2–4) yielding negatively charged Si–O − groups (Franks, 2002; Schwarz et al., 2000), while the majority of the aluminol groups exist as either protonated positively charged Al–OH2 groups or Al–OH groups up to pH values of 8–11 (Huittinen et al., 2009; Rabung et al., 2000). Newman (1987) showed that the kaolinite edges carry a positive double layer in acidic solution and a negative double layer in alkaline solution, with a point of zero charge close to pH 7. A similar edge isoelectric point (IEP) of 7.3 ± 0.2 has been reported in Rand and Melton (1977). Braggs et al. (1994) modeled the edge IEPs for two different kaolinites obtaining values at pH 5.25 and 6.75. They showed how the deprotonation of the edge hydroxyls depends on the edge iso-electric point (IEP) of the kaolinite mineral, Fig. 5. We prepared our NMR samples at pH 8.0, clearly above any reported edge IEP for kaolinite. Thus, the majority of edge proton signal in the recorded 1H NMR spectra stems from Al–OH groups. All of the silanol groups can be assumed to occur as Si–O−, thus, not contributing to any proton signal in the recorded NMR spectra. Similarly the majority of Al–OH–Si groups can be assumed to exist at deprotonated Al–O−½–Si groups. Thus, our experimental set-up in terms of sample pH does not allow for the detection of silanol or Al–OH–Si groups and very limited or no information on Y(III) or Eu(III) attachment onto these surface sites can be obtained. However, according to the Pauling bond valence principle (Pauling, 1929), aluminol groups are stronger Lewis bases
than silanol groups and, thus, have a stronger affinity towards Lewis acids such as the trivalent metal ions investigated in the present study. Therefore, aluminol groups can be assumed to be the preferential sorption sites on the kaolinite surface. This assumption is supported by spectroscopic findings on Eu(III) or Cm(III) sorption onto kaolinite by Tertre et al. (2006) and Stumpf et al. (2001), respectively. In these investigations the authors compared TRLFS emission spectra and lifetimes of sorbed europium or curium in aluminum (hydr)oxide or silica suspensions with data obtained for Eu(III)/Cm(III) in kaolinite suspensions and they could conclude that the metal ion complex on the clay surface is mainly bound to aluminol sites. Data on kaolinite proton signals in literature occur at a rather broad range of chemical shifts. Wang et al. (2002) assigned signals at 2.4–3.0 ppm to the inner surface hydroxyls on kaolinite. Fitzgerald et al. (1996) made a similar assignment of their pyrophyllite mineral where the 2.4 ppm peak was assigned to OH groups in the gibbsitelike AlOH layer. This would indicate that the broad peak in our kaolinite spectra at 2.3–3.5 ppm could stem from the inner surface hydroxyl proton Al–OH–Al on the exposed aluminum hydroxide basal surface of kaolinite. The Al–OH–Si groups have been reported at higher chemical shifts e.g. 4.0 ± 0.2 ppm and 5.0 ± 0.2 ppm (Hunger et al., 1991) and 5–6 ppm (Fitzgerald et al., 1996). We do not see any proton signals at these chemical shifts but the dissociation of the hydroxyl proton at the sample dispersion pH value is likely and we will, thus, not observe these groups in the acquired proton spectra. Water remaining on the mineral surface after drying was detected in our previous study on Eu3+/Y3+ attachment on γ-alumina at 1.3 and 0.9 ppm (Huittinen et al., 2011). In addition physisorbed water on the exposed siloxane plane of silica or silica-aluminas has been found at >3 ppm (Bronnimann et al., 1987, 1988) and at ≥4 ppm (Bronnimann et al., 1987) on the aluminum oxide plane of silica-aluminas and alumina. Our proton spectra of
Fig. 4. Illustration of the relative intensities between all obtained difference spectra. Solid lines (yttrium series), dashed lines (europium series).
Fig. 5. Edge hydroxyls in kaolinite and their deprotonation as a function of pH. Figure adapted from Braggs et al. (1994).
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kaolinite indeed show a small shoulder at 0.9 ppm that we, in analogy to our previous study, assign to physisorbed water. The proton peak at 1.3 ppm stems most likely from surface hydroxyls but the presence of physisorbed water in this chemical shift region cannot be excluded. Proton shifts at >3 or >4 ppm, corresponding to physisorbed water on the basal planes are not present in our spectra. To facilitate the assignment of peaks at 1.6, 1.3 ppm and b 0.7 ppm we plotted a difference spectrum obtained for Y(III) sorption on kaolinite together with yttrium difference spectra on γ-alumina (Huittinen et al., 2011), Fig. 6. The kaolinite difference spectrum shares identical peak positions at 1.3, 0.9, 0.7 ppm and around 0.5 ppm with γ-alumina. In our γ-alumina study we assigned the signals with chemical shifts below 0.0 ppm to singly coordinated hydroxyls of either four-coordinated Al (AlIV-OH) or six-coordinated Al (AlVI-OH). Peaks between 2 and 0 ppm were assigned to (AlVI)2–OH or AlVI–OH–AlIV groups. We cannot distinguish any signals below 0.0 ppm from the background in any of our kaolinite difference spectra. However, it has to be noted that in contrast to the γ-alumina spectra we could not compare the proton spectra with metal ion attachment to the spectrum of a pure kaolinite surface due to phasing problems. The difference spectra show very similar features, i.e. very similar sorption sites at 0.7 and 0.5 ppm, which cannot be ascribed to background scattering only. Due to the low concentration of these sites and the small shift in reference to TMS, typical for basic protons, these signals are likely to stem from singly coordinated AlVI–OH groups on the kaolinite edges. Proton signals in the higher chemical shift region, i.e. 1 ppm and above are assigned, in accordance to our previous study, to doubly coordinated hydroxyls, (AlVI)2–OH. Whether these groups are located on the gibbsite-like basal plane or at the edges faces of the kaolinite mineral due to e.g. condensation of singly coordinated surface groups as a consequence of surface relaxation is unclear. Both kaolinite and γ-alumina show substantial 1H resonance peak intensities roughly between 1 and 2 ppm. As γ-alumina does not have a surface plane comparable to the gibbsite-like sheet in kaolinite, the 1.3 and 1.6 ppm peaks in the kaolinite difference spectrum are likely to stem from hydroxyl protons located on the kaolinite edge planes.
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Structural proton signals from the mineral gibbsite have been identified over a large range of chemical shifts between 1.8 and 5.8 ppm (Piedra et al., 1996; Xue and Kanzaki, 2007). Together with the assignment by Wang et al. (2002) discussed above, where signals appearing at 2.4– 3.0 ppm were assigned to the inner surface hydroxyls on kaolinite, it is possible to ascribe the rather broad peak at 2.3–3.5 ppm in the kaolinite difference spectrum to doubly coordinated hydroxyls with varying acid strengths on the gibbsite-like basal plane of kaolinite. A definite assignment of these hydroxyls, however, would require simulation of NMR spectra. 4. Conclusions Our study has demonstrated that the loss of surface proton signal upon trivalent yttrium or europium attachment on the kaolinite surface can be obtained in 1H NMR experiments, even thought the vast majority of the recorded proton signal stems from the hydrogen atoms in the kaolinite bulk structure. The extremely small differences between spectra containing varying amounts of metal ion, however, require very precise manipulation and normalization of the recorded spectra. Normalization by sample mass was found to be inadequate due to the insufficient precision of the analytical scale used to weigh the samples. Therefore, available aluminum data had to be applied for normalization purposes. This is the first time, to our knowledge, that aluminum data has been used in this manner to accurately normalize acquired proton spectra. By employing this normalization method differences in the scale of 0.72% and below of the total recorded proton signal could be discerned. The loss of proton signal occurs at chemical shift values of 2.3–3.5 ppm, 1.6 ppm, 1.3 ppm and 0.7–0.3 ppm. By comparing these signals to what we previously obtained for Y3+ sorption on γ-alumina (Huittinen et al., 2011) and building on our assignment of hydroxyl groups on the aluminum oxide surface we can state that singly coordinated Al–OH groups on the kaolinite edge surfaces participate in the sorption reaction of the trivalent metals. In addition to edge surface adsorption our results indicate that the gibbsite-like basal plane is available for trivalent metal ion attachment. This surface is in many studies considered inert when it comes to proton or metal ion sorption reactions (Hiemstra et al., 1999; Weerasooriya et al., 2002). According to the difference spectra the lower metal ion concentrations favor attachment to sites below 2 ppm, while protons with shifts at 2.3–3.5 ppm are removed only for the higher metal ion concentrations. The silanol and mixed Al–OH–Si sites are not protonated under the experimental conditions and, thus, do not show in recorded 1H spectra. Therefore, we cannot rule out the adsorption of Y3+ or Eu3+ to Si–O− or Al–O−½–Si sites, even though we believe that the aluminol groups are the preferential sorption sites for the trivalent metal ion complexes. The potential involvement of these surface sites is planned to be investigated in a series of experiments conducted at lower pH values where the majority of all available surface groups are protonated. Finally, the comparison of difference spectra shows the presence of very similar sorption sites on the different mineral phases of kaolinite and γ-alumina, thus, confirming that aluminum oxides can to some extent be used as model phases for the more complex aluminosilicate minerals. Acknowledgments We want to thank Dr. Juhani Virkanen for his assistance in ICP-MS analyses. This work was funded by the Finnish Research Programme on Nuclear Waste Management (KYT 2014).
Fig. 6. Difference spectra obtained for yttrium sorption on kaolinite (this work) and γ-alumina (Huittinen et al., 2011). The pale yellow (right) and pale red regions (middle) indicate the chemical shift range of singly coordinated, and doubly coordinated hydroxyl protons on the mineral surfaces, respectively. Hydroxyl protons with chemical shifts in the pale blue region (left) are assigned to hydrogen atoms in the gibbsite like basal plane of kaolinite. The spectra are normalized to the highest intensity and do not represent relative loss of proton signal.
References Alemany, L.B., Kirker, G.W., 1986. First observation of 5-coordinate aluminum by MAS 27Al NMR in well-characterized solids. Journal of the American Chemical Society 108, 6158–6162. Aparicio, P., Galan, E., 1999. Mineralogical interference on kaolinite crystallinity index measurements. Clays and Clay Minerals 47, 12–27.
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Ashbrook, S.E., McManus, J., MacKenzie, K.J.D., Wimperis, S., 2000. Multiple-quantum and cross-polarized 27Al MAS NMR of mechanically treated mixtures of kaolinite and gibbsite. The Journal of Physical Chemistry. B 104, 6408–6416. Bauer, A., Berger, G., 1998. Kaolinite and smectite dissolution rate in high molar KOH solutions at 35 and 80 °C. Applied Geochemistry 13, 905–916. Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.), 2006. Handbook of Clay Science, first ed. Elsevier Science, The Netherlands. Braggs, B., Fornasiero, D., Ralston, J., St. Smart, R., 1994. The effect of surface modification by an organosiliane on the electrochemical properties of kaolinite. Clays and Clay Minerals 42, 123–136. Bronnimann, C.E., Chuang, I.-S., Hawkins, B.L., Maciel, G.M., 1987. Dehydration of silicaaluminas monitored by high-resolution solid-state proton NMR. Journal of the American Chemical Society 109, 1562–1564. Bronnimann, C.E., Zeigler, R.C., Maciel, G.M., 1988. Proton NMR study of dehydration of the silica gel surface. Journal of the American Chemical Society 110, 2023–2026. Cheremisinoff, N.P., 1997. Groundwater Remediation and Treatment Technologies. William Andrew, New Jersey. Chipera, S.J., Bish, D.J., 2001. Baseline studies of the clay minerals society source clays: powder X-ray diffraction analyses. Clays and Clay Minerals 49, 398–409. Crosson, G.S., Choi, S., Chorover, J., Amistadi, M.K., O’Day, P.A., Mueller, K.T., 2006. Solidstate NMR identification and quantification of newly formed aluminosilicate phases in weathered kaolinite systems. The Journal of Physical Chemistry. B 110, 723–732. Fernandez, R., Martirena, F., Scrivener, K., 2011. The origin of the pozzolanic activity of calcined clay minerals: a comparison between kaolinite, illite and montmorillonite. Cement and Concrete Research 41, 113–122. Fitzgerald, J.J., Hamza, A.I., Dec, S.F., Bronnimann, C.E., 1996. Solid-state 27Al and 29Si NMR and 1H CRAMPS studies of the thermal transformation of the 2:1 phyllosilicate pyrophyllite. Journal of Physical Chemistry 100, 17351–17360. Fitzgerald, J.J., Piedra, G., Dec, S.F., Seger, M., Maciel, G.E., 1997. Dehydration studies of a high-surface-area alumina (pseudo-boehmite) using solid-state 1H and 27Al NMR. Journal of the American Chemical Society 119, 7832–7842. Franks, G.V., 2002. Zeta potentials and yield stresses of silica suspensions in concentrated monovalent electrolytes: isoelectric point shift and additional attraction. Journal of Colloid and Interface Science 249, 44–51. Frost, R.L., 1998. Hydroxyl deformation in kaolins. Clays and Clay Minerals 46, 280–289. Geckeis, H., Rabung, Th., 2008. Actinide geochemistry: from the molecular level to the real system. Journal of Contaminant Hydrology 102, 187–195. Hiemstra, T., Yong, Han, Van Riemsdijk, W.H., 1999. Interfacial charging phenomena of aluminum (hydr)oxides. Langmuir 15, 5942–5955. Hinckley, D.N., 1963. Variability in “crystallinity” values among the kaolin deposits of the coastal plain of Georgia and South Carolina. Clays and Clay Minerals 11, 229–235. Huittinen, N., Rabung, Th., Lützenkirchen, J., Mitchell, S.C., Bickmore, B.R., Lehto, J., Geckeis, H., 2009. Sorption of Cm(III) and Gd(III) onto gibbsite, α-Al(OH)3: a batch and TRLFS study. Journal of Colloid and Interface Science 332, 158–164. Huittinen, N., Rabung, Th., Andrieux, P., Lehto, J., Geckeis, H., 2010. A comparative batch sorption and time-resolved laser fluorescence spectroscopy study on the sorption of Eu(III) and Cm(III) on synthetic and natural kaolinite. Radiochimica Acta 98, 613–620. Huittinen, N., Sarv, P., Lehto, J., 2011. A proton NMR study on the specific sorption of yttrium(III) and europium(III) on gamma-alumina [γ-Al2O3]. Journal of Colloid and Interface Science 361, 252–258. Huittinen, N., Rabung, Th., Schnurr, A., Hakanen, M., Lehto, J., Geckeis, H., 2012. New insight into Cm(III) interaction with kaolinite — influence of mineral dissolution. Geochimica et Cosmochimica Acta 99, 100–109. Hunger, M., Freude, D., Pfeifer, H., 1991. Magic-angle nuclear magnetic resonance studies of water molecules adsorbed on Brønstedt- and Lewis-acid sites in zeolites and amorphous silica-aluminas. Journal of the Chemical Society, Faraday Transactions 87, 657–662.
Lambert, J.F., Millman, W.S., Fripiat, J.J., 1989. Revisiting kaolinite dehydroxylation: a 29 Si and 27Al MAS NMR study. Journal of the American Chemical Society 111, 3517–3522. Lippmaa, E., Samoson, A., Mägi, M., 1986. High-resolution 27Al NMR of aluminosilicates. Journal of the American Chemical Society 108, 1730–1735. Massiot, D., Bessada, C., Coutures, J.P., Taulelle, T., 1990. A quantitative study of 27Al MAS NMR in crystalline YAG. Journal of Magnetic Resonance 90, 231–242. Miranda-Trevino, J.C., Coles, C.A., 2003. Kaolinite properties, structure and influence of metal retention on pH. Applied Clay Science 23, 133–139. Morris, H.D., Shelton, B., Ellis, P.D., 1990. 27Al NMR spectroscopy of iron-bearing montmorillonite clays. Journal of Physical Chemistry 94, 3121–3129. Newman, A.C.D., 1987. Chemistry of Clays and Clay Minerals. Longman Group UK Limited, London. Pauling, L., 1929. The principles determining the structure of complex ionic crystals. Journal of the American Chemical Society 51, 1010–1026. Peacock, C.L., Sherman, D.M., 2005. Surface complexation model for multisite adsorption of copper(II) onto kaolinite. Geochimica et Cosmochimica Acta 69, 3733–3745. Piedra, G., Fitzgerald, J.J., Dando, N., Dec, S.F., Maciel, G.E., 1996. Solid-state 1H NMR studies of aluminum oxide hydroxides and hydroxides. Inorganic Chemistry 35, 3474–3478. Pruett, R.J., Webb, H.L., 1993. Sampling and analysis of KGa-1b well-crystallized kaolin source clay. Clays and Clay Minerals 41, 514–519. Rabung, Th., Stumpf, Th., Geckeis, H., Klenze, R., Kim, J.I., 2000. Sorption of Am(III) and Eu(III) onto γ-alumina: experiment and modelling. Radiochimica Acta 88, 711–716. Rand, B., Melton, I.E., 1977. Particle interactions in aqueous kaolinite suspensions: I. Effect of pH and electrolyte upon the mode of particle interaction in homoionic sodium kaolinite suspensions. Journal of Colloid and Interface Science 60, 308–320. Rocha, J., Klinowski, J., 1990. 29Si and 27Al magic-angle spinning NMR studies of the thermal transformation of kaolinite. Physics and Chemistry of Minerals 17, 179–186. Schwarz, S., Lunkwitz, K., Kessler, B., Spiegler, U., Killmann, E., Jaeger, W., 2000. Adsorption and stability of colloidal silica. Colloids and Surfaces A: Physicochemical and Engineering Aspects 163, 17–27. Stumpf, Th., Bauer, A., Coppin, F., Kim, J.I., 2001. Time-resolved laser fluorescence spectroscopy study of the sorption of Cm(III) onto smectite and kaolinite. Environmental Science and Technology 35, 3691–3694. Sutheimer, S.H., Maurice, P.A., Zhou, Q., 1999. Dissolution of well and poorly crystallized kaolinites: Al speciation and effects of surface characteristics. American Mineralogist 84, 620–628. Temuujin, J., Okada, K., MacKenzie, K.J.D., Jadambaa, T., 1998. The effect of water vapour atmospheres on the thermal transformation of kaolinite investigated by XRD, FTIR and solid state MAS NMR. Journal of the European Ceramic Society 19, 105–112. Tertre, E., Berger, G., Simoni, E., Castet, S., Giffaut, E., Loubet, M., Catalette, H., 2006. Europium retention onto clay minerals from 25 to 150 °C: experimental measurements, spectroscopic features and sorption modelling. Geochimica et Cosmochimica Acta 70, 4563–4578. Wang, L., Wu, D., Yuan, P., Chen, Z., Chen, Z., 2002. 1H MAS NMR spectra of kaolinite/ formamide intercalation compound. Chinese Science Bulletin 47, 504–508. Weerasooriya, R., Wijesekara, H.K.D.K., Bandara, A., 2002. Surface complexation modeling of cadmium adsorption on gibbsite. Colloids and Surfaces A: Physicochemical and Engineering Aspects 207, 13–24. Xue, X., Kanzaki, M., 2007. High-pressure δ-Al(OH)3 and δ-AlOOH phases and isostructural hydroxides/oxyhydroxides: new structural insights from high-resolution 1H and 27Al NMR. The Journal of Physical Chemistry. B 111, 13156–13166. Zhou, B., Sherriff, B.L., Wang, T., 2009. 27Al NMR spectroscopy at multiple magnetic fields and ab initio quantum modeling for kaolinite. American Mineralogist 94, 865–871.
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Research paper
A 1H and 27Al NMR investigation of yttrium(III) and europium(III) interaction with kaolinite Nina Huittinen a,⁎, Priit Sarv b, Jukka Lehto a a b
Laboratory of Radiochemistry, Department of Chemistry, P.O. Box 55, FIN-00014 University of Helsinki, Finland Institute of Chemical Physics and Biophysics, Akadeemia Tee 23, EST-12618 Tallinn, Estonia
a r t i c l e
i n f o
Article history: Received 13 January 2013 Received in revised form 22 March 2013 Accepted 11 April 2013 Available online 9 May 2013 Keywords: Solid-state NMR Sorption Trivalent metal Kaolinite Surface hydroxyl Chemical shift
a b s t r a c t Sorption of the trivalent metals Eu 3+ and Y3+ on a natural kaolinite mineral was investigated with solid-state 1H and 27Al NMR. The metal ion concentration in the samples was varied between 4 × 10 −6 M and 8 × 10−5 M, while the kaolinite concentration and ionic strength with respect to Na + were kept constant at 5 g/l and 1 mM, respectively. The aqueous samples were equilibrated for at least six days at pH 8.0 ± 0.1 before the mineral was separated from the supernatant, recovered, and dried. Metal ion addition to the kaolinite samples had no influence on recorded 27Al spectra, which show a center peak at 6.15 ppm corresponding to octahedrally coordinated aluminum in the gibbsite-like sheets of kaolinite. The 27 Al measurements were, however, imperative to perform as potential mineral dehydroxylation was monitored from the 27Al spectra before and after drying treatment and normalization of 1H spectra was based on the acquired 27Al data. The attachment of Y3+/Eu 3+ on the kaolinite surface was shown to reduce 1H resonances with chemical shifts at 2.3–3.5 ppm, 1.6, 1.3, 0.9 and 0.7–0.3 ppm. The loss of proton signal could be attributed to metal ion attachment to isolated Al–OH groups on the kaolinite edge surfaces and possibly to bridging Al–OH–Al hydroxyls on the gibbsite-like basal plane of the mineral. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The adsorption of radionuclides on mineral surfaces is regarded as one of the dominant retardation processes in the near and far-field of nuclear waste repositories (Geckeis and Rabung, 2008). As potential host-rock or buffer and backfill materials in these repositories, clay minerals play an important role as sorbent phases for mobilized radionuclides in the geosphere. Kaolinite, Al2Si2O5(OH)4, is one of the most abundant aluminosilicate minerals (Sutheimer et al., 1999) with a 1:1 layered structure of alternating aluminum hydroxide octahedral and silica tetrahedral sheets that share a common plane of oxygen atoms (Bergaya et al., 2006). Even though kaolinite is considered as the least sorbing clay (Cheremisinoff, 1997) it is a very good model mineral in sorption investigations due to its simple structure and practically no sorption of cations in the interlayer space. With no interlayer space for radionuclide sorption, the sorption activity can be assumed to occur solely along the edges and the surfaces of the mineral (Miranda-Trevino and Coles, 2003), which in turn simplifies the interpretation of sorption data. In our previous works (Huittinen et al., 2010, 2012) we studied the sorption and speciation of trivalent curium, a minor actinide in spent nuclear fuel, on natural kaolinite from St. Austell (UK) using ⁎ Corresponding author at: A. I. Virtasen aukio 1 (P.O. Box 55), 00014 University of Helsinki, Finland. Tel.: +358 9191 50136. E-mail address: nina.huittinen@helsinki.fi (N. Huittinen). 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.04.005
time-resolved laser fluorescence spectroscopy (TRLFS). Based on the spectroscopic data the speciation of this trivalent metal in the kaolinite dispersion could be explained over a broad pH range (pH 3–13). Briefly, three inner-sphere sorbed surface complexes differing only in the degree of hydrolysis were found to dominate the speciation between pH 5 and 11. When the pH was further increased above pH 11 a ternary curium-silicate surface complex was found to be the predominant species on the kaolinite surface. In this paper we focus on understanding the role of different hydroxyl groups at the kaolinite surface in the trivalent metal ion sorption process using solid-state 1H and 27Al NMR. The paramagnetic Eu3+ ion is selected as an analogue to the trivalent actinides Pu 3+, Am 3+ and Cm 3+ while Y 3+ is taken under study due to its diamagnetic properties. The study is performed analogous to a previous NMR study of our group where we investigated the specific sorption of Eu 3+ and Y3+ ions on γ-alumina (Huittinen et al., 2011). At constant pH and varying metal ion concentrations we could identify a manifold of chemically very similar hydroxyl groups at the γ-alumina surface that participated in the sorption reaction of the trivalent metals. Here, we aim to identify the proton signals from the various hydroxyl groups on the kaolinite surface and to locate those hydroxyl groups that are involved in the surface complexation reaction of the trivalent metal by monitoring the kaolinite proton spectra before and after metal ion addition. In contrast to γ-alumina where the recorded proton signal stems from the surface only, kaolinite has hydrogen atoms in the bulk structure (Al2Si2O5(OH)4) that will give rise to the majority of the collected proton signal, thus, making it very difficult to extract any surface
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information from the NMR spectra. To increase the chance of locating surface protons we included Eu 3+ in the study, even though we could show that the paramagnetic Eu3+ ion caused a greater loss of proton signal at the γ-alumina surface in comparison to the diamagnetic Y 3+ ion and thus, prevented the straight forward interpretation of recorded data (Huittinen et al., 2011). In this case the greater loss of proton signal caused by the paramagnetic europium ions is employed to facilitate the detection of protons in the close proximity of the trivalent metal, i.e. protons at the kaolinite surface. 2. Material and methods
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55 °C over night. Before NMR experiments the samples were dried for 5 h at 180 °C (0.2 mTorr) in a vacuum oven (Vacuum Systems, Finland) using a ramp temperature of 1°/min to eliminate physisorbed water from the mineral samples. The dried samples were sealed under vacuum (40 mTorr) and transferred into a N2-glove box (MBraun) where the oxygen and moisture contents were monitored to be less than 1 ppm throughout the sample handling, thus, assuring that samples were not rehydrated before NMR measurements. In the N2-glove box 3.2 mm o.d. zirconia rotors were filled with the dried kaolinite samples. Kel-F® (polychlorotrifluoroethylene) caps were used to seal the rotors and some Kel-F® oil was spread between the cap and the rotor wall to ensure air tightness of the rotors after removal from the glove box.
2.1. Kaolinite The kaolinite mineral from St. Austell (UK) used throughout this work has been characterized by Bauer and Berger (1998) and further in our previous work (Huittinen et al., 2010). Results from these characterizations are compiled in Table 1. No crystalline phase impurities such as illite, anatase, quartz or iron oxides found in e.g. the natural source-clay kaolinite minerals KGa-1 and KGa-2 (Chipera and Bish, 2001; Pruett and Webb, 1993) could be detected in the XRD survey (Huittinen et al., 2010). The tabulated Hinckley index (HI) (Hinckley, 1963) reflects the degree of crystallinity in kaolinite minerals. Kaolinite is considered to be well-ordered when the HI exceeds 0.9 (Aparicio and Galan, 1999) implying that our kaolinite material is highly ordered. Before sample preparation for the NMR experiments the kaolinite mineral was washed with ultrapure water (MilliQ) over night under constant stirring. After mineral sedimentation the water was discarded and the mineral was allowed to dry. 2.2. 1H and
27
Al NMR investigations
2.2.1. Sample preparation NMR samples were prepared by weighing 300 mg of kaolinite in HDPE bottles. 50–55 ml of MilliQ water and varying concentrations of the metal ions (either Y 3+ or Eu 3+) were added to the dispersions. Due to pH-dependent protonation and dissociation reactions occurring at the mineral surface, the pH of all samples was adjusted to 8.0 ± 0.1, a typical pH prevailing in deep bedrock, by addition of NaOH. Furthermore, the Na + concentration was kept at 1 mM by adjusting the amount of added Na + after NaOH additions by addition of an appropriate amount of 0.1 M NaClO4 to the samples. The final sample volume of 60 ml, resulting in a solid content of 5 g/l, and a metal ion concentration ranging from 4 × 10 −6 to 8 × 10 −5 M was adjusted with addition of MilliQ water. All samples were allowed to equilibrate for at least 6 days before they were separated by centrifugation at 20,000 rpm. An aliquot of the solution phase after centrifugation at 80,000 rpm was subjected to ICP-MS analysis of the remaining, unsorbed europium and yttrium concentrations, to confirm that the majority (~ 100%) of the metal ions are attached on the mineral surface. The separated minerals were dried in an oven at
Table 1 Characteristics of kaolinite. Kaolinite property (method)
Determined value
Specific surface area (N2-BET)a Grain size (SEM)a
11.72 m2/g Thickness 0.2 μm Diameter 0.4–1.0 μm O — 68.27 ± 3.40 (atom) % Al — 15.26 ± 1.50% Si — 16.47 ± 1.90% Traces (b1%) of K, Na, Sb, Fe 1.34
Elemental composition (SEM-EDX)b
Hinckley index (XRD)b a b
Bauer and Berger (1998). Huittinen et al. (2010).
2.2.2. 1H and 27Al NMR experimental set-up The 1H and 27Al single-pulse MAS NMR experiments were performed on a Bruker BioSpin Avance III NMR spectrometer using a Bruker triple-resonance MAS probehead. Parameters used in the experiments for the two nuclei, 1H and 27Al, are compiled in Table 2. Solid adamantane and alum were used as secondary references because of easier handling in comparison to the liquid standards TMS and Al(NO3)3. All the samples were spun with dry air at 18,000 ± 1 Hz. The spinning-speed was stabilized by a Bruker MAS controller. For our quantitative 27Al experiments a small nonselective pulse (10°) was needed to excite the Al sites with different quadrupolar interactions to the same extent (Lippmaa et al., 1986). Considering our experimental conditions (high magnetic field, fast MAS spinning) and modest (less than 6 MHz) 27Al quadrupolar coupling constant of kaolinite (Ashbrook et al., 2000), we may assume, that all the intensity from the central transition of 27Al is in the central band and spinning-sidebands (SSB) belong to the satellite transitions (Massiot et al., 1990). Subtracting the intensity of one SSB from the central band, we obtain the pure central transition intensity, which was used to calculate the aluminum concentration. The 1H MAS NMR spectrum of the empty rotor (the background) was subtracted from the sample spectra to get the 1H MAS NMR spectra of the pure samples. For such and other manipulations (discussed in the following section) it is important, that the SSBs are positioned at the same frequency throughout the spectra which is why we needed the spinning-speed stabilization. In quantitative calculations the intensities from the SSBs, if present, were taken into account. To ensure compatibility between acquired 27Al and 1H data for a given sample measurements both nuclei were probed in succession, i.e. without stopping the sample rotation between the measurements. 2.2.3. 1H NMR spectral treatment In order to enable direct comparison of the background corrected proton spectra all recorded spectra had to be phase and baseline corrected. In addition, as the proton signal is proportional to the amount (mass) of sample, the slightly different sample amounts (arising from uneven rotor filling) require that all spectra are normalized before they are comparable. Zero-order and first order phase corrections were done to each acquired spectrum. The first order, frequency dependent phase, was acquired from the measured reference material, i.e. adamantane, and the same phase parameter was used for all measured mineral samples. Zero order phase correction is frequency independent and this correction was done manually by adjusting the phase a few degrees for each sample spectrum. Thus, we aimed at obtaining proton peaks with precisely same shapes and phases with one another to allow for direct comparison of the spectra. The baseline correction of the proton spectra was done using a cubic spline function over the 80–(−80) ppm region. Approximately 10–14 points were selected as true baseline points for the spline correction. After the above mentioned spectral treatments a perfect match between all spectra in the yttrium and europium series could unfortunately not be acquired, and these spectra had to be left out from the data interpretations. Omitted spectra are those without any metal
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Table 2 NMR parameters used in the experiments. Nucleus
Resonance frequency (MHz)
Magnetic field (T)
Exciting pulse
Relaxation delay (s)
Number of scans
Chemical shift reference
1
800.13 208.49
18.8 18.8
90°(3.7 μs) 10°(0.5 μs)
40 2
160 240
Adamantanea C10H16 Alumb KAl(SO4)2 × 12H2O
H 27 Al a b
Chemical shifts given relative to TMS. Chemical shifts given relative to alum, which itself is −0.35 ppm relative to 3 M Al(NO3)3 at the magnetic field 18.8 T (this work).
ion addition as well as the spectrum of kaolinite containing 8 × 10−6 M yttrium. Finally, for a direct comparison of the spectra acquired from slightly different sample amounts, the spectra had to be normalized. Gravimetric normalization was not satisfactory due to the very small differences in the proton spectra induced by the adsorption of the trivalent metal to the kaolinite surface. Therefore, proton spectra acquired from the kaolinite samples were normalized with respect to the amount of aluminum in the samples, assuming that the aluminum concentration per unit mass is constant in all samples. By integrating the phase and baseline corrected aluminum spectra and setting one sample integral to 100% and comparing the other integrals to this sample, normalization factors were obtained that were further used to normalize the proton spectra. Normalized spectra could thereafter be compared to one another to acquire information on the different surface protons and the exchange of these due to trivalent metal ion attachment to the surface. The validity of our normalization procedure was evaluated by calculating the number of aluminum and hydrogen atoms in the samples after normalization. According to the chemical formula of kaolinite, Al2Si2O5(OH)4, the hydrogen to aluminum ratio should be 2:1. Values obtained from our normalized spectra yielded a ratio of 1.97 ± 0.08 hydrogens per one aluminum atom, thus, confirming the validity of our normalization procedure. 3. Results and discussion 3.1.
27
Al experiments
27 Al spectra were recorded for all NMR samples. However, no differences between spectra of pure kaolinite and samples including Eu 3+/Y 3+ could be detected. Thus, no information on metal sorption onto kaolinite could be gained from the 27Al NMR spectra. The aluminum data, however, serves two very important purposes. The possible alteration of the mineral due to the drying treatment was monitored from hydrated and dehydrated samples by measuring some kaolinite samples before and after drying treatment. Secondly, the phase, and baseline corrected aluminum data was used to normalize the recorded
proton spectra as explained previously. In Fig. 1 (left) two of the collected spectra with and without Eu(III) are presented. The spectra show one peak with a peak position at 6.15 ppm, corresponding to octahedrally coordinated aluminum in the gibbsite-like aluminum hydroxide layers of kaolinite. This peak position falls within the range of reported values found in literature e.g. ranging from 2.4 ppm (Temuujin et al., 1998) to 8.64 ppm (Crosson et al., 2006). The magnetic field strength will affect the position of the aluminum signal, implying that spectra collected at varying field strengths result in slightly different chemical shift values. Zhou et al. (2009) showed how the peak position of the 27 Al signal changes as a function of magnetic field and performed ab initio calculations to support their findings. Based on their theoretical ab initio quantum mechanical calculations, the δiso for the 27Al center transition in kaolinite is located at 6.25 ppm which is very close to their experimentally obtained value of 6.2 ppm at a magnetic field of 21.06 T. At this magnetic field quadrupolar interactions become insignificant and the δiso is only moved very little by quadrupole induced shifts. Our experiments were performed at 18.8 T leading to a slight quadrupolar shift that moves the peak position of the kaolinite 27Al signal upfield in comparison to the results obtained by Zhou et al. (2009). In Fig. 1 (right) aluminum spectra before and after two different drying treatments are presented. The spinning speed of the hydrated aluminum sample was slightly faster at 20 kHz in comparison to the dehydrated samples that were spun at 18 kHz, which is why the spinning sidebands (SSBs) are located at different chemical shifts. Spinning sidebands are separated by a chemical shift equal to the rate of spinning. The sample dehydrated at 180 °C (0.2 mTorr) i.e. the drying temperature we subjected all samples to, show no evidence of dehydroxylation. To illustrate the effect of a too harsh drying treatment we subjected one kaolinite sample to a much higher drying temperature of 450 °C. At this temperature substantial mineral dehydroxylation occurs that shows up as two peaks around 30, and 57 ppm. In literature aluminum bearing minerals with 27Al chemical shift values in the 30 ppm region, e.g. 28 ppm (Fernandez et al., 2011; Rocha and Klinowski, 1990), 35 ppm (Alemany and Kirker, 1986), 36–37 ppm (Fitzgerald et al., 1997), have been assigned to five-coordinated aluminum. Therefore, in accordance
Fig. 1. Left — 27Al spectra of kaolinite (black) and kaolinite with adsorbed Eu(III) (blue). The asterisks denote spinning sidebands. Right — 27Al spectra of a hydrated kaolinite sample and two dehydrated samples subjected to drying temperatures of 180 °C and 450 °C, respectively.
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Fig. 2. Left — normalized 1H NMR spectra of kaolinite samples containing varying concentrations of yttrium (red colors) and europium (blue colors). Proton spectra of yttrium containing samples were divided by a factor of 1.25 to allow for better visualization in the graph. Asterisks denote spinning sidebands. Right — magnification of the 10–(− 4) ppm region.
to these studies we assign the peak at 30 ppm to AlO5 forming as a result of kaolinite dehydroxylation. Based on chemical shift data for fourcoordinated aluminum (AlO4), e.g. 53 ppm (Temuujin et al., 1998), 55.3 ppm (Lambert et al., 1989), 56 ppm (Fernandez et al., 2011), 57 ppm (Rocha and Klinowski, 1990) the broad peak appearing between 50 and 57 ppm is assigned to AlO4. 3.2. 1H experiments The acquired and normalized proton spectra of kaolinite with yttrium (red colors) and europium (blue colors) are presented in Fig. 2 (left). To separate the two different series of metal ion containing sample spectra in the figure and, thus, allow for better visualization all proton spectra containing yttrium were divided by a factor of 1.25. Due to the very small differences induced by the metal ion addition the spectra that could not be matched and phased perfectly (as explained in Section 2.2.3) in comparison to the others were not considered for data interpretations. Such spectra include those of the pure kaolinite mineral and kaolinite with 8 × 10−6 M Y(III). Thus, the results and discussions are based on data acquired from kaolinite samples containing 4 × 10−6, 4 × 10−5, and 8 × 10−5 M yttrium and 4 × 10−6, 8 × 10−6, 4 × 10−5, and 8 × 10−5 M europium. In the magnification of the proton spectra Fig. 2 (right) a peak at 1.9 ppm and a shoulder at approximately 2.7 ppm can be distinguished. The yttrium containing spectra are close to identical with no apparent loss of proton signal with increasing yttrium concentration in the samples, while a small loss of proton signal is visible in the sample containing 8 × 10−5 M Eu(III). Therefore, to be able to see the possible effect induced by the metal ion addition, difference spectra were plotted in which the spectra containing higher metal ion concentrations (8 × 10−6 M–8 × 10−5 M) were subtracted from the spectrum with the least added amount of metal ion (4 × 10 −6 M). These difference spectra are shown in Fig. 3. The relative differences of the Y(III) and Eu(III) containing difference spectra are illustrated in Fig. 4. The Eu(III)-containing spectra indicate removal of a greater amount of protons than Y(III) from the kaolinite surface (Fig. 4), similarly to what we observed in our γ-alumina study (Huittinen et al., 2011), but the overall effect on the total proton signal is very small. The difference between the spectra with lowest and highest concentrations of added metal ion is only 0.72% in case of Y(III) and three times higher, i.e. 2.16% in case of Eu(III). Using a total surface site density of 12.0 sites/nm 2 (Peacock and Sherman, 2005) the surface protons would amount to 1.5% of the total number of protons in the Al2Si2O5(OH)4 structure. It is clear that Eu(III) removes more protons
than theoretically available at the surface. We believe that this is a combined effect of the paramagnetism of the europium ion and the normalization procedure that we used. Europium is paramagnetic which means that the ion produces a new magnetic field that the probed hydrogen or aluminum nucleus feels in addition to the magnetic field produced by the spectrometer. This induced magnetization causes an isotropic shift of atoms in the vicinity of the paramagnetic substance. Thus, not only protons exchanged in the surface complexation reaction at the kaolinite surface will be absent in the spectra containing europium, but also protons in the close vicinity of the adsorbed ion. In addition, it is also possible that aluminum atoms close to the europium ion are influenced by the paramagnetism. In that case our normalization procedure, that we based on the assumption that the aluminum concentration per unit mass in the samples is constant, will introduce a small additional error to the actual number of protons that are being removed in the surface complexation reaction. In Fig. 4 the relative intensities of the yttrium containing and europium containing difference spectra are shown. Europium can be seen to remove approximately three times more of the proton signal than the corresponding yttrium concentration at the kaolinite surface does. In both sets of difference spectra, Fig. 3, a broad peak in the chemical shift region 2.3–3.5 ppm is observable. The yttrium spectra also show peaks at 1.6, 1.3, 0.9 and 0.7–0.3 ppm. The corresponding chemical shifts of the europium spectra are located at similar chemical shift values but the spectra are less featured than the yttrium ones. In addition, the greater removal of proton signal in the europium containing spectra results in a much larger contribution of the 2.3–3.5 ppm broad peak in the 4 × 10−6–4 × 10−5 M difference spectrum than what is observed for the corresponding Y(III) difference spectrum. The difference spectra of 4 × 10−6–8 × 10−6 M Eu(III) and 4 × 10−6–4 × 10−5 M Y(III) on the other hand are very much alike, both with respect to their relative intensities as well as the chemical shifts of the observed proton signals. To enable the assignment of these proton signals to various hydroxyl groups on the kaolinite surface we will discuss the different hydroxyl groups available in the kaolinite structure. The majority of OH-groups in the kaolinite mineral are confined to the gibbsite-like aluminum hydroxide layers. The exposed aluminum hydroxide basal surface contains doubly coordinated hydroxyls (Al–OH–Al), referred to as inner surface hydroxyls (Frost, 1998). The kaolinite edge surfaces feature three different hydroxyl groups, singly coordinated Al–OH and Si–OH groups and bridged Al–OH–Si groups. These edge hydroxyls are considered to be the major sorbing hydroxyl sites (Morris et al., 1990). Whether or not these surface sites are protonated or not is highly dependent on the
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Fig. 3. 1H difference spectra produced by subtracting the spectra containing higher metal ion concentrations from the spectrum with the lowest added metal ion concentration.
solution pH. Silanol groups of silicate minerals are known to deprotonate already in the acidic pH range (pH 2–4) yielding negatively charged Si–O − groups (Franks, 2002; Schwarz et al., 2000), while the majority of the aluminol groups exist as either protonated positively charged Al–OH2 groups or Al–OH groups up to pH values of 8–11 (Huittinen et al., 2009; Rabung et al., 2000). Newman (1987) showed that the kaolinite edges carry a positive double layer in acidic solution and a negative double layer in alkaline solution, with a point of zero charge close to pH 7. A similar edge isoelectric point (IEP) of 7.3 ± 0.2 has been reported in Rand and Melton (1977). Braggs et al. (1994) modeled the edge IEPs for two different kaolinites obtaining values at pH 5.25 and 6.75. They showed how the deprotonation of the edge hydroxyls depends on the edge iso-electric point (IEP) of the kaolinite mineral, Fig. 5. We prepared our NMR samples at pH 8.0, clearly above any reported edge IEP for kaolinite. Thus, the majority of edge proton signal in the recorded 1H NMR spectra stems from Al–OH groups. All of the silanol groups can be assumed to occur as Si–O−, thus, not contributing to any proton signal in the recorded NMR spectra. Similarly the majority of Al–OH–Si groups can be assumed to exist at deprotonated Al–O−½–Si groups. Thus, our experimental set-up in terms of sample pH does not allow for the detection of silanol or Al–OH–Si groups and very limited or no information on Y(III) or Eu(III) attachment onto these surface sites can be obtained. However, according to the Pauling bond valence principle (Pauling, 1929), aluminol groups are stronger Lewis bases
than silanol groups and, thus, have a stronger affinity towards Lewis acids such as the trivalent metal ions investigated in the present study. Therefore, aluminol groups can be assumed to be the preferential sorption sites on the kaolinite surface. This assumption is supported by spectroscopic findings on Eu(III) or Cm(III) sorption onto kaolinite by Tertre et al. (2006) and Stumpf et al. (2001), respectively. In these investigations the authors compared TRLFS emission spectra and lifetimes of sorbed europium or curium in aluminum (hydr)oxide or silica suspensions with data obtained for Eu(III)/Cm(III) in kaolinite suspensions and they could conclude that the metal ion complex on the clay surface is mainly bound to aluminol sites. Data on kaolinite proton signals in literature occur at a rather broad range of chemical shifts. Wang et al. (2002) assigned signals at 2.4–3.0 ppm to the inner surface hydroxyls on kaolinite. Fitzgerald et al. (1996) made a similar assignment of their pyrophyllite mineral where the 2.4 ppm peak was assigned to OH groups in the gibbsitelike AlOH layer. This would indicate that the broad peak in our kaolinite spectra at 2.3–3.5 ppm could stem from the inner surface hydroxyl proton Al–OH–Al on the exposed aluminum hydroxide basal surface of kaolinite. The Al–OH–Si groups have been reported at higher chemical shifts e.g. 4.0 ± 0.2 ppm and 5.0 ± 0.2 ppm (Hunger et al., 1991) and 5–6 ppm (Fitzgerald et al., 1996). We do not see any proton signals at these chemical shifts but the dissociation of the hydroxyl proton at the sample dispersion pH value is likely and we will, thus, not observe these groups in the acquired proton spectra. Water remaining on the mineral surface after drying was detected in our previous study on Eu3+/Y3+ attachment on γ-alumina at 1.3 and 0.9 ppm (Huittinen et al., 2011). In addition physisorbed water on the exposed siloxane plane of silica or silica-aluminas has been found at >3 ppm (Bronnimann et al., 1987, 1988) and at ≥4 ppm (Bronnimann et al., 1987) on the aluminum oxide plane of silica-aluminas and alumina. Our proton spectra of
Fig. 4. Illustration of the relative intensities between all obtained difference spectra. Solid lines (yttrium series), dashed lines (europium series).
Fig. 5. Edge hydroxyls in kaolinite and their deprotonation as a function of pH. Figure adapted from Braggs et al. (1994).
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kaolinite indeed show a small shoulder at 0.9 ppm that we, in analogy to our previous study, assign to physisorbed water. The proton peak at 1.3 ppm stems most likely from surface hydroxyls but the presence of physisorbed water in this chemical shift region cannot be excluded. Proton shifts at >3 or >4 ppm, corresponding to physisorbed water on the basal planes are not present in our spectra. To facilitate the assignment of peaks at 1.6, 1.3 ppm and b 0.7 ppm we plotted a difference spectrum obtained for Y(III) sorption on kaolinite together with yttrium difference spectra on γ-alumina (Huittinen et al., 2011), Fig. 6. The kaolinite difference spectrum shares identical peak positions at 1.3, 0.9, 0.7 ppm and around 0.5 ppm with γ-alumina. In our γ-alumina study we assigned the signals with chemical shifts below 0.0 ppm to singly coordinated hydroxyls of either four-coordinated Al (AlIV-OH) or six-coordinated Al (AlVI-OH). Peaks between 2 and 0 ppm were assigned to (AlVI)2–OH or AlVI–OH–AlIV groups. We cannot distinguish any signals below 0.0 ppm from the background in any of our kaolinite difference spectra. However, it has to be noted that in contrast to the γ-alumina spectra we could not compare the proton spectra with metal ion attachment to the spectrum of a pure kaolinite surface due to phasing problems. The difference spectra show very similar features, i.e. very similar sorption sites at 0.7 and 0.5 ppm, which cannot be ascribed to background scattering only. Due to the low concentration of these sites and the small shift in reference to TMS, typical for basic protons, these signals are likely to stem from singly coordinated AlVI–OH groups on the kaolinite edges. Proton signals in the higher chemical shift region, i.e. 1 ppm and above are assigned, in accordance to our previous study, to doubly coordinated hydroxyls, (AlVI)2–OH. Whether these groups are located on the gibbsite-like basal plane or at the edges faces of the kaolinite mineral due to e.g. condensation of singly coordinated surface groups as a consequence of surface relaxation is unclear. Both kaolinite and γ-alumina show substantial 1H resonance peak intensities roughly between 1 and 2 ppm. As γ-alumina does not have a surface plane comparable to the gibbsite-like sheet in kaolinite, the 1.3 and 1.6 ppm peaks in the kaolinite difference spectrum are likely to stem from hydroxyl protons located on the kaolinite edge planes.
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Structural proton signals from the mineral gibbsite have been identified over a large range of chemical shifts between 1.8 and 5.8 ppm (Piedra et al., 1996; Xue and Kanzaki, 2007). Together with the assignment by Wang et al. (2002) discussed above, where signals appearing at 2.4– 3.0 ppm were assigned to the inner surface hydroxyls on kaolinite, it is possible to ascribe the rather broad peak at 2.3–3.5 ppm in the kaolinite difference spectrum to doubly coordinated hydroxyls with varying acid strengths on the gibbsite-like basal plane of kaolinite. A definite assignment of these hydroxyls, however, would require simulation of NMR spectra. 4. Conclusions Our study has demonstrated that the loss of surface proton signal upon trivalent yttrium or europium attachment on the kaolinite surface can be obtained in 1H NMR experiments, even thought the vast majority of the recorded proton signal stems from the hydrogen atoms in the kaolinite bulk structure. The extremely small differences between spectra containing varying amounts of metal ion, however, require very precise manipulation and normalization of the recorded spectra. Normalization by sample mass was found to be inadequate due to the insufficient precision of the analytical scale used to weigh the samples. Therefore, available aluminum data had to be applied for normalization purposes. This is the first time, to our knowledge, that aluminum data has been used in this manner to accurately normalize acquired proton spectra. By employing this normalization method differences in the scale of 0.72% and below of the total recorded proton signal could be discerned. The loss of proton signal occurs at chemical shift values of 2.3–3.5 ppm, 1.6 ppm, 1.3 ppm and 0.7–0.3 ppm. By comparing these signals to what we previously obtained for Y3+ sorption on γ-alumina (Huittinen et al., 2011) and building on our assignment of hydroxyl groups on the aluminum oxide surface we can state that singly coordinated Al–OH groups on the kaolinite edge surfaces participate in the sorption reaction of the trivalent metals. In addition to edge surface adsorption our results indicate that the gibbsite-like basal plane is available for trivalent metal ion attachment. This surface is in many studies considered inert when it comes to proton or metal ion sorption reactions (Hiemstra et al., 1999; Weerasooriya et al., 2002). According to the difference spectra the lower metal ion concentrations favor attachment to sites below 2 ppm, while protons with shifts at 2.3–3.5 ppm are removed only for the higher metal ion concentrations. The silanol and mixed Al–OH–Si sites are not protonated under the experimental conditions and, thus, do not show in recorded 1H spectra. Therefore, we cannot rule out the adsorption of Y3+ or Eu3+ to Si–O− or Al–O−½–Si sites, even though we believe that the aluminol groups are the preferential sorption sites for the trivalent metal ion complexes. The potential involvement of these surface sites is planned to be investigated in a series of experiments conducted at lower pH values where the majority of all available surface groups are protonated. Finally, the comparison of difference spectra shows the presence of very similar sorption sites on the different mineral phases of kaolinite and γ-alumina, thus, confirming that aluminum oxides can to some extent be used as model phases for the more complex aluminosilicate minerals. Acknowledgments We want to thank Dr. Juhani Virkanen for his assistance in ICP-MS analyses. This work was funded by the Finnish Research Programme on Nuclear Waste Management (KYT 2014).
Fig. 6. Difference spectra obtained for yttrium sorption on kaolinite (this work) and γ-alumina (Huittinen et al., 2011). The pale yellow (right) and pale red regions (middle) indicate the chemical shift range of singly coordinated, and doubly coordinated hydroxyl protons on the mineral surfaces, respectively. Hydroxyl protons with chemical shifts in the pale blue region (left) are assigned to hydrogen atoms in the gibbsite like basal plane of kaolinite. The spectra are normalized to the highest intensity and do not represent relative loss of proton signal.
References Alemany, L.B., Kirker, G.W., 1986. First observation of 5-coordinate aluminum by MAS 27Al NMR in well-characterized solids. Journal of the American Chemical Society 108, 6158–6162. Aparicio, P., Galan, E., 1999. Mineralogical interference on kaolinite crystallinity index measurements. Clays and Clay Minerals 47, 12–27.
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Ashbrook, S.E., McManus, J., MacKenzie, K.J.D., Wimperis, S., 2000. Multiple-quantum and cross-polarized 27Al MAS NMR of mechanically treated mixtures of kaolinite and gibbsite. The Journal of Physical Chemistry. B 104, 6408–6416. Bauer, A., Berger, G., 1998. Kaolinite and smectite dissolution rate in high molar KOH solutions at 35 and 80 °C. Applied Geochemistry 13, 905–916. Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.), 2006. Handbook of Clay Science, first ed. Elsevier Science, The Netherlands. Braggs, B., Fornasiero, D., Ralston, J., St. Smart, R., 1994. The effect of surface modification by an organosiliane on the electrochemical properties of kaolinite. Clays and Clay Minerals 42, 123–136. Bronnimann, C.E., Chuang, I.-S., Hawkins, B.L., Maciel, G.M., 1987. Dehydration of silicaaluminas monitored by high-resolution solid-state proton NMR. Journal of the American Chemical Society 109, 1562–1564. Bronnimann, C.E., Zeigler, R.C., Maciel, G.M., 1988. Proton NMR study of dehydration of the silica gel surface. Journal of the American Chemical Society 110, 2023–2026. Cheremisinoff, N.P., 1997. Groundwater Remediation and Treatment Technologies. William Andrew, New Jersey. Chipera, S.J., Bish, D.J., 2001. Baseline studies of the clay minerals society source clays: powder X-ray diffraction analyses. Clays and Clay Minerals 49, 398–409. Crosson, G.S., Choi, S., Chorover, J., Amistadi, M.K., O’Day, P.A., Mueller, K.T., 2006. Solidstate NMR identification and quantification of newly formed aluminosilicate phases in weathered kaolinite systems. The Journal of Physical Chemistry. B 110, 723–732. Fernandez, R., Martirena, F., Scrivener, K., 2011. The origin of the pozzolanic activity of calcined clay minerals: a comparison between kaolinite, illite and montmorillonite. Cement and Concrete Research 41, 113–122. Fitzgerald, J.J., Hamza, A.I., Dec, S.F., Bronnimann, C.E., 1996. Solid-state 27Al and 29Si NMR and 1H CRAMPS studies of the thermal transformation of the 2:1 phyllosilicate pyrophyllite. Journal of Physical Chemistry 100, 17351–17360. Fitzgerald, J.J., Piedra, G., Dec, S.F., Seger, M., Maciel, G.E., 1997. Dehydration studies of a high-surface-area alumina (pseudo-boehmite) using solid-state 1H and 27Al NMR. Journal of the American Chemical Society 119, 7832–7842. Franks, G.V., 2002. Zeta potentials and yield stresses of silica suspensions in concentrated monovalent electrolytes: isoelectric point shift and additional attraction. Journal of Colloid and Interface Science 249, 44–51. Frost, R.L., 1998. Hydroxyl deformation in kaolins. Clays and Clay Minerals 46, 280–289. Geckeis, H., Rabung, Th., 2008. Actinide geochemistry: from the molecular level to the real system. Journal of Contaminant Hydrology 102, 187–195. Hiemstra, T., Yong, Han, Van Riemsdijk, W.H., 1999. Interfacial charging phenomena of aluminum (hydr)oxides. Langmuir 15, 5942–5955. Hinckley, D.N., 1963. Variability in “crystallinity” values among the kaolin deposits of the coastal plain of Georgia and South Carolina. Clays and Clay Minerals 11, 229–235. Huittinen, N., Rabung, Th., Lützenkirchen, J., Mitchell, S.C., Bickmore, B.R., Lehto, J., Geckeis, H., 2009. Sorption of Cm(III) and Gd(III) onto gibbsite, α-Al(OH)3: a batch and TRLFS study. Journal of Colloid and Interface Science 332, 158–164. Huittinen, N., Rabung, Th., Andrieux, P., Lehto, J., Geckeis, H., 2010. A comparative batch sorption and time-resolved laser fluorescence spectroscopy study on the sorption of Eu(III) and Cm(III) on synthetic and natural kaolinite. Radiochimica Acta 98, 613–620. Huittinen, N., Sarv, P., Lehto, J., 2011. A proton NMR study on the specific sorption of yttrium(III) and europium(III) on gamma-alumina [γ-Al2O3]. Journal of Colloid and Interface Science 361, 252–258. Huittinen, N., Rabung, Th., Schnurr, A., Hakanen, M., Lehto, J., Geckeis, H., 2012. New insight into Cm(III) interaction with kaolinite — influence of mineral dissolution. Geochimica et Cosmochimica Acta 99, 100–109. Hunger, M., Freude, D., Pfeifer, H., 1991. Magic-angle nuclear magnetic resonance studies of water molecules adsorbed on Brønstedt- and Lewis-acid sites in zeolites and amorphous silica-aluminas. Journal of the Chemical Society, Faraday Transactions 87, 657–662.
Lambert, J.F., Millman, W.S., Fripiat, J.J., 1989. Revisiting kaolinite dehydroxylation: a 29 Si and 27Al MAS NMR study. Journal of the American Chemical Society 111, 3517–3522. Lippmaa, E., Samoson, A., Mägi, M., 1986. High-resolution 27Al NMR of aluminosilicates. Journal of the American Chemical Society 108, 1730–1735. Massiot, D., Bessada, C., Coutures, J.P., Taulelle, T., 1990. A quantitative study of 27Al MAS NMR in crystalline YAG. Journal of Magnetic Resonance 90, 231–242. Miranda-Trevino, J.C., Coles, C.A., 2003. Kaolinite properties, structure and influence of metal retention on pH. Applied Clay Science 23, 133–139. Morris, H.D., Shelton, B., Ellis, P.D., 1990. 27Al NMR spectroscopy of iron-bearing montmorillonite clays. Journal of Physical Chemistry 94, 3121–3129. Newman, A.C.D., 1987. Chemistry of Clays and Clay Minerals. Longman Group UK Limited, London. Pauling, L., 1929. The principles determining the structure of complex ionic crystals. Journal of the American Chemical Society 51, 1010–1026. Peacock, C.L., Sherman, D.M., 2005. Surface complexation model for multisite adsorption of copper(II) onto kaolinite. Geochimica et Cosmochimica Acta 69, 3733–3745. Piedra, G., Fitzgerald, J.J., Dando, N., Dec, S.F., Maciel, G.E., 1996. Solid-state 1H NMR studies of aluminum oxide hydroxides and hydroxides. Inorganic Chemistry 35, 3474–3478. Pruett, R.J., Webb, H.L., 1993. Sampling and analysis of KGa-1b well-crystallized kaolin source clay. Clays and Clay Minerals 41, 514–519. Rabung, Th., Stumpf, Th., Geckeis, H., Klenze, R., Kim, J.I., 2000. Sorption of Am(III) and Eu(III) onto γ-alumina: experiment and modelling. Radiochimica Acta 88, 711–716. Rand, B., Melton, I.E., 1977. Particle interactions in aqueous kaolinite suspensions: I. Effect of pH and electrolyte upon the mode of particle interaction in homoionic sodium kaolinite suspensions. Journal of Colloid and Interface Science 60, 308–320. Rocha, J., Klinowski, J., 1990. 29Si and 27Al magic-angle spinning NMR studies of the thermal transformation of kaolinite. Physics and Chemistry of Minerals 17, 179–186. Schwarz, S., Lunkwitz, K., Kessler, B., Spiegler, U., Killmann, E., Jaeger, W., 2000. Adsorption and stability of colloidal silica. Colloids and Surfaces A: Physicochemical and Engineering Aspects 163, 17–27. Stumpf, Th., Bauer, A., Coppin, F., Kim, J.I., 2001. Time-resolved laser fluorescence spectroscopy study of the sorption of Cm(III) onto smectite and kaolinite. Environmental Science and Technology 35, 3691–3694. Sutheimer, S.H., Maurice, P.A., Zhou, Q., 1999. Dissolution of well and poorly crystallized kaolinites: Al speciation and effects of surface characteristics. American Mineralogist 84, 620–628. Temuujin, J., Okada, K., MacKenzie, K.J.D., Jadambaa, T., 1998. The effect of water vapour atmospheres on the thermal transformation of kaolinite investigated by XRD, FTIR and solid state MAS NMR. Journal of the European Ceramic Society 19, 105–112. Tertre, E., Berger, G., Simoni, E., Castet, S., Giffaut, E., Loubet, M., Catalette, H., 2006. Europium retention onto clay minerals from 25 to 150 °C: experimental measurements, spectroscopic features and sorption modelling. Geochimica et Cosmochimica Acta 70, 4563–4578. Wang, L., Wu, D., Yuan, P., Chen, Z., Chen, Z., 2002. 1H MAS NMR spectra of kaolinite/ formamide intercalation compound. Chinese Science Bulletin 47, 504–508. Weerasooriya, R., Wijesekara, H.K.D.K., Bandara, A., 2002. Surface complexation modeling of cadmium adsorption on gibbsite. Colloids and Surfaces A: Physicochemical and Engineering Aspects 207, 13–24. Xue, X., Kanzaki, M., 2007. High-pressure δ-Al(OH)3 and δ-AlOOH phases and isostructural hydroxides/oxyhydroxides: new structural insights from high-resolution 1H and 27Al NMR. The Journal of Physical Chemistry. B 111, 13156–13166. Zhou, B., Sherriff, B.L., Wang, T., 2009. 27Al NMR spectroscopy at multiple magnetic fields and ab initio quantum modeling for kaolinite. American Mineralogist 94, 865–871.