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Immobilization of trivalent actinides by sorption onto quartz and incorporation into siliceous bulk: Investigations by TRLFS
Journal of Colloid and Interface Science 318 (2008) 5–14 www.elsevier.com/locate/jcis
Immobilization of trivalent actinides by sorption onto quartz and incorporation into siliceous bulk: Investigations by TRLFS S. Stumpf a,∗ , Th. Stumpf b , J. Lützenkirchen b , C. Walther b , Th. Fanghänel a,c a European Commission, Joint Research Centre, Institute for Transuranium Elements, Karlsruhe, Germany b Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung, P.O. Box 3640, 76021 Karlsruhe, Germany c Ruprecht-Karls-Universität Heidelberg, Physikalisch-Chemisches Institut, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany
Received 29 June 2007; accepted 28 September 2007 Available online 3 October 2007
Abstract The adsorption of Cm(III) on quartz is studied by time resolved laser fluorescence spectroscopy (TRLFS) in the pH range from 3.75 to 9.45. The raw spectra are deconvoluted into three single components. The first one has a peak maximum at 593.8 nm and can be attributed to the Cm(III) aquo ion with an emission lifetime of 68 ± 3 µs. The second one corresponds to an adsorbed species and has a peak maximum at 601.4 nm and an emission lifetime of 123 ± 10 µs. The peak maximum of the third component is shifted to higher wavelength (603.6 nm) while the lifetime remains constant. Additionally, the adsorption of Am(III) on quartz is investigated in batch experiments. Based on the spectroscopic data a sorption mechanism is suggested. In addition, the obtained Am uptake data and the Cm-TRLFS data are modeled simultaneously using a single site Basic Stern model in combination with the charge distribution concept of Pauling. The finally suggested model consists of two bidentate surface complexes where the second one is the product of hydrolysis of the first sorption species. In a separate set of experiments the influence of silicic acid at different concentrations on the Cm(III) speciation in a quartz system is investigated by TRLFS. In suspension silicic acid at low concentration (3.5 × 10−4 mol/L) has no influence on the Cm(III) speciation. At high concentration (3.5 × 10−2 mol/L) the Cm(III) speciation is definitely influenced. The results at higher concentration indicate the formation of Cm(III)/silicic acid complexes and the incorporation of Cm(III) into siliceous bulk. This is confirmed by measurements at a quartz single crystal surface. Moreover, these measurements indicate the formation of quartz/Cm(III)/silicic acid ternary complexes at the mineral surface. © 2007 Elsevier Inc. All rights reserved. Keywords: Curium; Quartz; Silicic acid; Surface complexation; Sorption; Ternary complexes; TRLFS; Single crystals; Adsorption model
1. Introduction Radionuclide migration in natural aqueous systems is an ongoing concern in environmental research in particular in the context of the long term performance of nuclear waste repositories. The transport of actinides is strongly influenced by adsorption onto mineral surfaces and interaction with organic and inorganic ligands. Fundamental insight into sorption and complexation mechanisms such as identification of dissolved and adsorbed species is of cardinal importance for reliable prediction of actinide reactions in natural systems. Therefore it is * Corresponding author.
E-mail address: [email protected] (S. Stumpf). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.09.080
necessary to characterize the different actinide species and to elucidate the reaction mechanisms involved. Time resolved laser fluorescence spectroscopy (TRLFS) enables the speciation of lanthanides like Eu(III) and actinides like U(VI), Am(III) and Cm(III) [1] in aqueous solution and on the water/mineral interface. Due to the high fluorescence yield of Cm(III) TRLFS allows speciation studies in the nanomolar concentration range corresponding to a surface loading far below a monolayer. Up to now the interaction of Cm(III) with mineral surfaces like γ -alumina [2], clay minerals [3], feldspars [4], CSH phases [5], cement [6], calcite [7] and α-alumina single crystal surfaces [8] has been investigated. The characterization of the adsorbed species is deduced from excitation and emission spectra and from the fluorescence emission lifetime of Cm(III).
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TRLFS investigations of the interaction of Eu(III) with amorphous silica led to the conclusion that the trivalent lanthanide is not adsorbed but incorporated into the bulk structure [9]. Comparable results have been found for the interaction of Cm(III) with amorphous silica colloids [10]. These results indicate different mechanisms for the interaction of trivalent lanthanides/actinides with amorphous silica and with the afore mentioned minerals. One possible explanation is the presence of dissolved silica from weathering of silicate minerals as described in literature [11–13]. The solubility of amorphous silica is known to be 2 × 10−3 M at pH < 9, dominated by monosilicic acid and increases rapidly with pH via deprotonation and through formation of polysilicic acid [14–16]. The different Si species may be involved in various reactions like complexation [17], formation of colloids [18,19] and precipitation [13]. These reactions have to be taken into account when the speciation of Cm(III) is investigated in the presence of silicate minerals. The present TRLFS investigation is part of a study, which intends to give a complete Cm(III) speciation in a natural system mainly composed of silicates like quartz. Hereby, Cm(III) was selected as a representative of a trivalent actinide ion. Because of their omnipresence in nature silicates can have a dominating influence on the aqueous chemistry and hence on the migration behavior of actinides in the natural environment. In this study the adsorption of Cm(III) on quartz and the influence of silicic acid, which can be generated by the dissolution of the silicate mineral, at different concentrations on the Cm(III) speciation was investigated in dependence on pH. The studies of the Cm(III) speciation were performed on quartz particles in suspension as well as on quartz single crystal surfaces. In addition to TRLFS measurements the adsorption of Cm(III) and Am(III) was investigated by batch experiments and α-spectrometry. The batch sorption and TRLFS data for the quartz particles in absence of silicic acid are described by a surface complexation model. 1.1. Surface complexation model Adsorption of cations on oxides has been the subject of many studies. A number of different models have been used to describe the adsorption behavior [20]. Most models interpret the adsorption as an interaction of metal ions with functional groups at the surface. Those models usually consider competitive adsorption of protons and metal ions in proton–metal ion– adsorbent systems. Before interacting with the surface functional groups the metal ions have to overcome a potential difference between the bulk solution and the interface. A surface complexation model is therefore composed of two parts: a discription of the electric double layer (e.g., that by Gouy and Chapman [21]) and conventional mass law equations and balances for the reactions at the surface. With this, the overall reaction for the absorption of a metal ion to a surface functional group can be separated into a chemical (intrinsic) and a variable electrostatic part: ◦
K = e−Gads /RT = e−(Gr +Gel )/RT = Kin e−Gel /RT = Kin Kel ,
(1)
Fig. 1. Electrostatic double layer model for the interpretation of the sorption data.
where K: overall reaction constant, Kin : intrinsic reaction constant, Kel : electrostatic reaction constant, Gads : Gibbs free energy for the adsorption, G◦r : standard Gibbs free energy, Gel : electrostatic energy. The electrostatic energy change, Gel , is variable due to the change in charge upon adsorption of ions and is determined by the electrostatic potential Ψ which is calculated by the use of an electrostatic model, in which the charge distribution (CD) of ions is involved [22]. Models may differ in the formulation of the surface chemical reactions (stoichiometries) and the assumed structure of the electrostatic double layer. In this study the basic Stern model was used as the electrostatic double layer model for the interpretation of the adsorption data (Fig. 1). Two electrostatic planes are considered that are separated from each other by a charge free layer in between called the Stern layer. The charges of protons are allocated to the surface (σ0 ). Adsorption of background electrolyte ions is considered by treating them as point charges and placing that charge at the head end of the diffuse part of the double layer (σ1 ). The sum of σ0 and σ1 is compensated by the charge σd of the diffuse part of the electrostatic double layer, which is calculated from the Gouy–Chapman equation. The Stern layer is characterized by a capacitance. This capacitance is usually determined from model fits to acid base titration data [23]. As demonstrated in Eq. (2) the interface of an oxide may be composed of several functional groups, which are coordinated to one or more metal ions of the solid. O−2 + 2H+ ⇔ OH−1 + H+ ⇔ OH02 .
(2)
This picture leads to a discrete surface heterogeneity because each group has its own proton affinity and charge characteristics, which may even vary with the crystallographic plane. For the determination of the overall charge σ0 at the surface one must take into account that the surface oxygen is not only partially neutralized by protons but also by the metal ions in the mineral structure. Vice versa, the charge of the cation is compensated by the charge of the surrounding oxygens. As it was introduced by Pauling [24] for neutralization, the charge is distributed over the surrounding ligands, which can be expressed per bond. With this, the bond valence v is defined as the charge
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z of the cation divided by its coordination number CN: v = z/CN.
(3)
A charge distribution (CD) concept like this is also applied for cations adsorbed to the mineral surface. Whereas electrolyte ions are still considered as point charges as stated above (true outer-sphere surface complexes), the charge of sorbed cations like Cm and Am is distributed between the surface plane and the head end of the diffuse layer thus rationalizing their size compared to that of a proton. The simple Pauling bond valence concept can be used as a first order estimate in relating the charge distribution needed in the model to the structure of the adsorbing ion. In case of a structural change of the cation adsorbed to the surface a change of the charge distribution and with this of the surface potential Ψ results. The structure of the surface, the structure of the adsorbed species, and the electrostatic potential profile are all essential features of an adsorption model using physically realistic surface species.
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the system. Thus in the present study, for the first time experimentally obtained solution and surface speciation (for Cm from TRLFS) could be directly used in the modeling exercise in simultaneous combination with classical Am uptake data (assuming that both exhibit equal behavior). 3. Materials All used chemicals were of analytical grade and carbonate free. 3.1. Quartz particles The used quartz was obtained from Redon/Bretagne, France. The quartz was crushed in a mortar and sieved to get fractions <20 µm. A <5 µm fraction (BET surface >1 m2 /g) that was used for the sorption experiments was selectively obtained via centrifugation. Purity and crystallinity of the quartz powder were verified by ICP-MS and XRD measurements.
2. Experimental
3.2. Quartz single crystal
2.1. Methods
The quartz single crystal was obtained from Belo Horizonte, Brazil and was cut (X-cut) in 10 × 10 × 1 mm single crystal slices. XPS measurements indicated a carbon impurity on the surface that was removed by cleaning the surface with methanol.
2.1.1. TRLFS (time resolved laser fluorescence spectroscopy) TRLFS measurements were performed using a dye laser (Lambda Physics, scanmate) pumped by a XeCl-excimer laser (Lambda Physics, EMG, 308 nm, 24 ns) for excitation. Emission spectra were recorded from 580 to 620 nm. The fluorescence emission is detected by an optical multichannel analyzer consisting of a polychromator (Jobin Yvon, HR 320) with a 300/600/1200 lines/mm grating and a photodiode array (Spectroscopy instruments, ST 180, IRY 700G). The Cm(III) was excited at 396.6 nm. The emission spectra of Cm(III) were recorded in the 580–620 nm range (1200 lines/mm grating). For measuring the time dependent emission decay (300 lines/mm grating), the delay time between laser pulse and camera gating was scanned with time intervals between 5 and 10 µs. For measuring the decay time of the fluorescence emission the delay time was shifted in steps between 5 and 10 µs using a constant time window of 1 ms. 2.1.2. Data modeling All sorption data were modeled as surface charge densities simultaneously using the FITEQL 2.0 program [25] in combination with the optimization code UCODE [26]. The Davies equation was used to correct for ionic strength effects. This allows fitting of all parameters, including capacitance values, which can only be adjusted manually in FITEQL. The linear correlation coefficient (between the capacitance value and the electrolyte binding constant) which can be obtained with this code combination has an absolute value of 0.91. This indicates a relatively strong correlation between the two adjustable parameters. This information would not be obtained using FITEQL in optimization mode. Another advantage of using UCODE for parameter estimation is that a high degree of flexibility is obtained in the combination of experimental data pertaining to
3.3. Silicic acid The silicic acid solution was prepared by diluting a silicon standard for ICP (1 mg/ml Si in 2% NaOH, ACROS Fisher Scientific GmbH) with HClO4 to a desired concentration of 3.5 × 10−4 mol/L (undersaturation) and 3.5 × 10−2 mol/L (oversaturation) [14,15,27,28]. 3.4. Experimental set-up A stock solution of the long-lived curium isotope Cm-248 (t 1 = 3.4 × 105 years) with the isotopic composition 97.3% 2 Cm-248, 2.6% Cm-246, 0.04% Cm-245, 0.02% Cm-247 and 0.009% Cm-244 in 1.0 M HClO4 was used for the TRLFS experiments. The initial curium concentration determined by ICP-mass spectroscopy was adjusted to 2.0 × 10−7 mol/L. To avoid complexation by carbonate all experiments were made in a glove box under argon atmosphere at 25 ± 1 ◦ C. The Cm(III) adsorption was investigated in batch experiments with a quartz suspension of 1 g/L quartz particles <5 µm in size. The pH of the batch samples was varied by adding NaOH and HClO4 . During the curium/quartz contact time the samples were shaken periodically. All samples had an electrolytic background of 0.1 mol/L NaClO4 . Additionally batch sorption experiments with Am(III) were performed. The samples were prepared analogous to the Cm(III) samples. The pH edges were obtained by analyzing the Am(III) concentration of the solution by α-spectrometry after equilibration (1 day) and solid liquid separation.
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The influence of silicic acid was investigated by adding the silicon standard to the quartz suspension at acidic pH (≈3.5) before adding the Cm(III). The concentration of silicic acid was 3.5 × 10−4 mol/L (undersaturation) and 3.5 × 10−2 mol/L (oversaturation), respectively. The sorption of Cm(III) onto single crystals was obtained contacting the single crystal with a Cm(III) solution (2 × 10−7 mol/L) at different pH values. An additional set of experiments was done contacting the single crystal with a Cm(III)/quartz suspension (1 g/L quartz; <5 µm; 2 × 10−7 mol/L Cm(III)) in presence of 3.5 × 10−2 mol/L silicic acid at different pH values. 4. Results and discussion 4.1. Adsorption of Cm(III) on quartz-TRLFS measurements Fluorescence emission spectra of 2 × 10−7 mol/L Cm(III) in a quartz suspension, recorded at different pH values ranging from 3.75 to 9.45, are shown in Fig. 2. At low pH, the emission band with a peak maximum at 593.8 nm can be attributed to the Cm3+ aquo ion [29]. With increasing pH, the intensity of this peak decreases and a red-shift of the fluorescence emission up to 603.6 nm at pH 9.45 appears. Under the present experimental conditions hydrolysis of Cm(III) can be ruled out and the change of fluorescence spectra can be solely explained by a stepwise complexation of the curium aquo ion at the mineral surface and the formation of inner-sphere complexes. The mixed spectra were deconvoluted using a factoranalysis program [30]. The species identified by the deconvolution procedure are plotted in Fig. 3. Moreover, from the peak deconvolution data and the respective FI values (fluorescence intensity) the mole fractions of the three Cm-species are determined and plotted in Fig. 3 as a function of pH (speciation plot). The aquo ion dominates the system up to a pH value of 4.5. Then the formation of the first surface complex (601.4 nm) starts with a maximum fraction of 30% at pH 5.5. With increasing pH
(a)
(b) Fig. 3. (a) Calculated single components and (b) speciation plot for the sorption of Cm(III) onto quartz.
Fig. 2. Fluorescence emission spectra of 2 × 10−7 mol/L Cm(III) in a suspension of 1 g/L quartz at different pH values.
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Fig. 4. Calculated slope for the formation reaction of sorption Species 2.
the second surface complex (603.6 nm) is formed. This species dominates the system up to pH 9. The fluorescence emission of the Cm3+ aquo ion decays with a fluorescence emission lifetime of 68 µs, while the first as well as the second Cm complex emitting at 601.4 nm and 603.6 nm both have an emission lifetime of 123 ± 10 µs. The increase in lifetime by complexation reflects the exclusion of water molecules from the first coordination sphere of Cm(III). In the first coordination sphere H2 O acts as a fluorescence quencher by energy transfer and excitation of H2 O vibrations and, therefore, causes a decrease of the emission lifetime. The short lifetime of the Cm3+ aquo ion (68 ± 3 µs) compared to a lifetime of 1300 µs in D2 O [29] is a result of these quenching processes. A mathematical expression for the correlation of lifetime and number of H2 O quenchers is given in literature by Kimura et al. [29]. Complexes 1 and 2 are characterized by the same fluorescence lifetime, implying that the number of quenching ligands in the first Cm(III) coordination sphere remains unchanged. Applying the correlation of Kimura et al. the number of water molecules in the first coordination shell for both surface complexes can be estimated to be 5 ± 0.2. However, the change in the emission spectra clearly indicates that there are two different species with different composition or structure. With increasing pH a stepwise deprotonation according to Eq. (4) is very likely. Species 1 → Species 2 + nH+ .
(4)
From this equation it follows the correlation for the law of mass action: n K = [Species 2]/[Species 1] × H+ .
(5)
A plot of the ratio log(Species 2/Species 1) with pH allows the determination of the proton stoichiometry for the given reaction. Here, the plot gives a slope of 1 (Fig. 4), indicating the exchange of one proton during the formation reaction. The increase of the error bar value together with an increase of pH is a result of the change of concentration ratio [Species 2]/[Species 1] during the deprotonation reaction.
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This result is in good accordance with spectroscopic investigations of the adsorption of Cm(III) on alumina [2], clay minerals [3] and feldspar surfaces [4]. Relying on the cited investigations and on the basis of the actual study a mechanism for the Cm adsorption on a quartz surface can be deduced. In a first reaction step a surface complex is formed by exchange of approximately half of the hydration shell from the first Cm coordination sphere. According to the determined proton stoichiometry in Eq. (5) in a second reaction step the release of a proton is proposed. Whether this proton is released by the hydrolysis of the surface complex or by a deprotonation of the surface itself is not clear at this point. Moreover, the spectroscopic data give no evidence how many oxygen of the surface are involved into the bonding to the sorption species. To get more structural insight the adsorption of Am(III) on quartz at different concentrations of the actinide was investigated in an analogue experiment. The Cm and Am sorption data were then modeled and a sorption mechanism is proposed. 4.2. Adsorption of Cm(III) and Am(III) on quartz-surface complexation modeling As mentioned above, a surface complexation model is composed of two parts—one part describes the electric double layer whereas the other one gives conventional mass law equations for the reactions at the surface. Here, the Basic Stern Model was used as the electrostatic double layer model for the interpretation of the adsorption data. With regard to the mass law equations two types of surface groups can be considered for the quartz surface. The log K value for the protonation of the doubly coordinated Si2 –O group is extremely low and can therefore be regarded as inert [31]. Only the singly coordinated groups are reactive and the protonation reactions can be formulated as: ≡SiOH ↔ ≡SiO− + H+ ,
(6)
+ ≡SiOH+ 2 ↔ ≡SiOH + H .
(7)
Due to the very low log K value for Eq. (7) the protonation of this surface group is not very likely. Therefore, in this study a potentially complex situation is simplified by the use of only one charging reaction (Eq. (6); 1-pK approach) [32,33]. At the point of zero charge of quartz (around pH 2–3) all surface groups are protonated and are uncharged (SiOH0 ). Therefore, in the pH range that is relevant for the sorption experiments the surface charge of quartz is negative and the quartz surface is supposed to be homogeneous [34]. With this, the reactions at the quartz surface within the 1-pK, 1-site basic Stern model are the following: SiOH ↔ SiO− + H+ ,
(6)
SiOH + Na+ ↔ SiO− · · ·Na+ + H+ .
(9)
The log K value of reaction (6) is −7.9 for I = 0 according to previous work [35] and is calculated to be −7.79 for I = 0.1 (i.e., for the background electrolyte used in the is study: 0.1 M NaClO4 ). The log K for reaction (9) was optimized numerically (log K = −6.93) [31]. Based on previous work [31], the
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Fig. 5. pH edges of the sorption of 248 Cm and 243 Am onto quartz. Fig. 7. Slope analysis of the sorption data in consideration of different assumptions for log K.
Fig. 6. Sorption of trivalent actinides onto a quartz surface: modeled mechanism and sorption species.
number of sites at the quartz surface was set to 4.6 nm−2 . Moreover the capacitance of the Stern layer was set to a value of 1.44 F/m2 according to acid base titration data [23]. All sorption data were used in one UCODE set-up file and fitted using the Basic Stern Model outlined above and considering ideal charge distribution. The experimental data were all used as the fraction of species with respect to the total concentration of Cm (Cm3+ , Species 1, Species 2) or Am (Amads ), respectively. Fig. 5 shows pH sorption edges of Am(III) at three different Am(III) concentrations (2 × 10−7 /10−6 /10−5 mol/L). Moreover, the Cm sorption data ([Cm(III)] = 2 × 10−7 M), obtained by the calculation of the speciation from the TRLFS data (Fig. 3), are added to the plot in Fig. 5. The pH edges of both actinides fit very well for nearly identical conditions. This result is in good agreement with the assumption of equal chemical behavior for americium and curium. As expected, with increasing actinide concentration the pH edges are shifted to higher pH values. The fit of the sorption data is also given in Fig. 5 (black line). The proposed structure model for the two different sorption species is shown in Fig. 6. The structure of the surface complexes determines the charge distribution at the mineral surface and with this also determines the electrostatic part of the equation constant which finally describes the given sorption data. Here, the trivalent actinide ion is attached to the surface in a bidentate conformation. Once the location of the complex has been defined one has to consider the position of charge for both surface species. Part of the charge of the surface complex is shared with the surface itself. The remaining part is attributed to the other ligands of the complex which are oriented towards the solution. By application of the Pauling bond valence concept (Eq. (3)) and assuming a coordination number of 8 for the actinide a value of v = 3/8 can be attributed to each metal bond. It follows for the charge of the actinide in the first complex
+3 − 2 × 3/8 = 2.25. The surface oxygen has a bond valence of v = −2/2 = −1 whereas a value of v = −2/2 + 1 (which comes from H+ ) can be attributed to the hydroxid group. Taking the contribution of the actinide valence bond into account the charge at the surface becomes −1 + 0 + 2 × 3/8 = −0.25. As a result of a proposed twofold hydrolysis (v(OH–) = −1) for the second complex, the charge of the actinide changes to a value of +3 − 2 × 3/8 − 1 = 1.25 and for the surface to a value of −2 + 2 × 3/8 = −1.25. It is obvious that in the proposed model a change of surface complex structure is associated with a change of charge at the solid solution interface. Finally, the electrostatic part in the reaction equation of the sorption process is changed. This means, that the classical plot for the estimation of proton stoichiometries (i.e., log[Species 2]/[Species 1] vs pH) is not applicable, since this involves the assumption of one invariant overall stability constant. As it was already shown, by application of a surface complexation model the stability constant K is separated into an intrinsic and electrostatic part (K = Kin × Kel ; Eq. (1)). A plot taking Kin as overall stability constant results in a value of 2 for the slope (Fig. 7). It follows that two protons are released during the surface reaction (Fig. 6) and not one as inferred from the classical slope determination that was applied for the interpretation of the TRLFS data. Additionally, taking the electrostatic part into account as given in Eq. (1) and applying K as overall stability constant a plot of log[Species 2]/[Species 1] vs pH results a line with a slope of 1 (Fig. 7). With the separation of K into an intrinsic and electrostatic part the stoichiometry of the formation reaction of Species 2 from Species 1 as well as the slope determination are very well reflected. With the proposed structure model it is possible to fit the sorption data of the Cm(III) sorption as well as of the Am(III) sorption onto quartz (Fig. 5). At the highest Am concentration (2 × 10−5 mol/L) Am is present in excess of the surface sites. Therefore, the observed uptake data beyond site saturation may be attributed to the formation of a solid Am-phase which is either amorphous or crystalline or a mixture of both. Therefore, two fitting curves from two
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solubility products, namely log KL0 = −25.1 (amorphous Amhydroxide) and log KL0 = −26.4 (crystalline Am-hydroxide), have been calculated for the Am sorption at higher concentrations. As the experimental data are located in between the two fitting lines (Fig. 5) obviously a mixture of an amorphous and crystalline Am-phase is formed. The log K values for the sorption of Cm(III) and Am(III) at the quartz surface are: log K = 5.05, log K = −6.15,
2SiOH ↔ Species 1 + H+ , Species 1 ↔ Species 2 + 2H+ .
The interpretation of TRLFS data by application of the slope analysis in the simplest form gives an idea for the adsorption of trivalent actinides on a quartz surface. The modeling of sorption data by application of a surface complexation model confirms the initial assumption. Moreover, it results in the formulation of an unique sorption mechanism, that is, the formation of a bidentate at the mineral surface in a first step (sorption Species 1) and the twofold hydrolysis of the bidentate in a second step (sorption Species 2). 4.3. Adsorption of Cm(III) on quartz-influence of silicic acid The dissolution of quartz results in the formation of silicic acid that tends to act as inorganic ligand for cations. The question arises weather silicic acid has an influence on the Cm(III) speciation in a quartz system. The interaction of Cm(III) with silicic acid at low concentrations (undersaturation) were investigated by Panak et al. [36]. Two different Cm(III)/silicic acid complexes together with their stability constants were determined. Moreover, the influence of silicic acid at higher concentrations (oversaturation) was investigated [36]. At these concentrations silicic acid polymerizes and forms colloids. As a result of colloid formation the trivalent actinide is incorporated into the amorphous silica phase resulting in the total loss of its hydration sphere. According to the investigations performed by Panak et al., in this study, the influence of silicic acid at different concentrations (3.5 × 10−4 and 3.5 × 10−2 mol/L) on the Cm(III) speciation in the presence of quartz was investigated by TRLFS. The emission spectra were recorded in a pH range from 3.48 to 9.25 and deconvoluted as described above. In Fig. 8 the single components together with the speciation are shown for the interaction of Cm(III) with a quartz surface in absence and presence of 3.5×10−4 mol/L (undersaturated) silicic acid. For both systems the same species with the same fluorescence emission lifetimes are observed. Moreover, silicic acid at this concentration has no influence on the Cm speciation. However, the speciation changes significantly when increasing the silicic acid concentration to a value of 3.5 × 10−2 mol/L (oversaturated). A comparative presentation of the single components together with the speciation for Cm(III) in the presence of quartz with and without addition of silicic acid at high concentrations is given in Fig. 9. The influence of silicic acid results in a shift of emission maximum for the first surface complex from 601.4 nm without silicic acid to 602.6 nm in the presence of the complexing ligand. The emission band for the second surface complex
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(604.0 nm) is broadened indicating that this peak must be attributed to more than one surface species. Up to a pH of 6 the speciation plots of the two systems coincide with each other. At higher pH the Cm speciation is obviously influenced by the silicic acid. Panak et al. report on the formation of silicic acid colloids by formation of polymers at pH values >6 [36]. Furthermore, these colloids complex Cm(III) in an undefined stoichiometry. Taking these investigations into account, the influence of silicic acid at high concentration on the Cm(III) speciation in a quartz system can be explained by the formation of such polysilicic acid colloids. The complexation of Cm(III) by these colloids results in the formation of Cm species with an unknown stoichiometry. As reported by Panak et al. the incorporation of Cm(III) into silica colloids is associated with a total loss of the Cm hydration sphere [36] which is indicated by lifetimes of 310 ± 19 µs. Here, the measured lifetimes at pH values <6 are in accordance with the lifetimes of the silicic acid free system. But, with increasing pH, when silicic acid colloids are formed, also the lifetimes increase up to a value of 750 ± 30 µs. This increase of lifetimes indicates the loss of the Cm(III) hydration sphere (i.e., incorporation) which is in good accordance with the investigations performed by Panak et al. The question arises if adsorbed Cm(III) is desorbed from the quartz surface and preferentially incorporated into the formed silica colloids in solution instead which would result in a mobilization of Cm(III). Another possibility is the formation of polysilicic gel-like layers and incorporation of Cm(III) in such layers at the quartz surface (ternary quartz/Cm(III)/silicic acid complexes) which could result in an immobilization of Cm(III). To solve this issue, Cm sorption experiments in the presence of silicic acid at high concentration were performed at a single crystal quartz surface. 4.4. Adsorption of Cm(III) on quartz-single crystal measurements The adsorption of Cm(III) on a quartz single crystal in the pH range from 2.5 to 6.2 was observed by autoradiographic measurements (Fig. 10). With increasing pH adsorption at the surface increases. Simultaneously, the intensity of the Cm(III) fluorescence emission signal increases, indicating the formation of a surface complex (Fig. 11). Because of the small single crystal surface area compared to the one provided in suspension the adsorption process is shifted to higher pH values and only the first sorption species is observed in the selected pH range. The fluorescence emission maximum of this species is at 600.3 nm and shows a shoulder at lower wavelength. The peak position differs slightly from the fluorescence emission maxima of the first surface complex given in a quartz suspension (601.4 nm). In suspension Cm(III) adsorbs on different crystal planes. Here, only the x-plane is available for adsorption. Recently it was found that with changing the crystal plane of a single crystal the maximum of Cm fluorescence emission changes [8]. With this, the determined fluorescence emission maxima in suspension can be attributed to an averaged wavelength over all planes whereas the maximum at 600.3 nm corresponds to one species adsorbed at one designated plane. The lifetime of
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(a)
(b) Fig. 8. (a) Calculated single components and (b) speciation plot for the sorption of Cm(III) onto quartz in presence of silicic acid at low concentration.
the adsorbed species was measured at pH 5.7 and 6.2. For both pH values a biexponential decay behavior is observed whereas the rate of the species with the shorter lifetime becomes less at higher pH. The quartz surface is covered by a thin liquid film of curium solution as a result of preparation. Therefore, the shorter lifetime with a value of 68 ± 3 µs can be attributed to the nonsurface complexed Cm3+ aquo ion. Moreover, the shoulder at lower wavelength in the emission spectrum can be explained by the fact, that with increasing pH the ratio of the aquo ion decreases because of increasing adsorption. The longer lifetime has a value of 125 ± 10 µs. This value corresponds to a surface species with approximately five water molecules in the first coordination sphere of curium which is in good accordance with the lifetimes that can be attributed to the afore determined Cm sorption species in a quartz suspension. We conclude that the same surface complex is formed at the single crystal surface and in suspension. Fig. 12 presents the Cm(III) emission spectra after adding silicic acid to the system. The peak maximum
is shifted to 602.7 nm indicating a change of the ligand field of the adsorbed Cm(III) by complexation with silicic acid. Beside the spectral shift, the emission signal is broadened indicating the formation of more than one species as it was found for the measurements in suspension in the presence of high silicic acid concentrations. The measured lifetimes in presence of silicic acid increase up to a value of 234 µs at pH 6.7 and 323 µs at pH 9.1. The observed increase can be attributed to the reduction of the first Cm(III) hydration sphere to one water molecule as a result of the incorporation of Cm(III) into the bulk material. The Cm(III) emission spectrum at pH 9.1 was measured a second time after 78 days. The spectrum with a maximum at 602.7 nm was again broadened. The decay behavior that could be attributed to the spectrum was fitted with a function of higher order. This again indicates that the emission spectrum is composed of several complexation species. The determined lifetime increased to a value >500 µs which can be attributed to the complete loss of the first Cm(III) hydration sphere. As it was
S. Stumpf et al. / Journal of Colloid and Interface Science 318 (2008) 5–14
13
(a) Fig. 11. Fluorescence emission spectra of 2 × 10−7 mol/L Cm(III) at a quartz surface (X-cut) at pH 4.0 and 6.2.
(b) Fig. 9. (a) Calculated single components and (b) speciation plot for the sorption of Cm(III) onto quartz in presence of silicic acid at high concentration. Fig. 12. Fluorescence emission spectra of 2 × 10−7 mol/L Cm(III) at a quartz surface (X-cut) in presence of 3.5 × 10−2 mol/L silicic acid at pH 6.7, 9.1 and pH 9.1 after 78 days contact time.
Fig. 10. Autoradiographic images of the Cm(III) sorption onto a quartz surface (X-cut) in the pH range 2.5 to 6.2.
additionally confirmed by autoradiographic measurements the incorporation of Cm in the bulk structure does not result in the desorption of Cm from the crystal surface. The measurements show a constant Cm(III) occupancy of the single crystal surface instead of a reduction caused by desorption processes. The influence of silica colloids solely results in the formation of ternary quartz/Cm(III)/silicic acid complexes at the quartz surface. Hence, we conclude that higher concentrations of silicic acid do not cause any mobilization of the already sorbed actinide. This result is of great importance in view of the as-
sessment of the actinide migration in natural systems and with this the safety assessment of nuclear waste repositories. 5. Summary Depending on pH Cm(III) adsorbs on quartz by formation of two sorption species. The modeling of the sorption data suggests the following sorption mechanism. Cm(III) adsorbs in a bidentate fashion in a first step and is hydrolyzed in a second step. Silicic acid at low concentration (3.5 × 10−4 mol/L) has no influence on the Cm(III) speciation. At higher silicic acid concentration (3.5 × 10−2 mol/L) and pH values >6 Cm(III) is incorporated into the gel-like bulk. This complexation results in the total loss of the first Cm(III) hydration sphere. The TRLFS measurements and α-spectrometric measurements show that
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Cm(III) is not desorbed from the surface and mobilized by this “incorporation” process. But, the spectroscopic results can be interpreted by the formation of quartz/Cm(III)/silicic acid ternary complexes at the surface. Acknowledgment We thank Dr. André Rossberg for providing the factoranalysis program code designed for the analysis of EXAFS, UV–vis and TRLFS spectra. References [1] H. Geckeis, R. Klenze, J.I. Kim, Radiochim. Acta 87 (1999) 13. [2] Th. Stumpf, Th. Rabung, R. Klenze, H. Geckeis, J.I. Kim, J. Colloid Interface Sci. 238 (2001). [3] Th. Stumpf, A. Bauer, F. Coppin, J.I. Kim, Environ. Sci. Technol. 35 (2001) 3691. [4] S. Stumpf, Th. Stumpf, C. Walther, D. Bosbach, Th. Fanghänel, Radiochim. Acta 94 (2006) 243. [5] J. Tits, Th. Stumpf, Th. Rabung, E. Wieland, Th. Fanghänel, Environ. Sci. Technol. 37 (2003) 3568. [6] Th. Stumpf, J. Tits, C. Walther, E. Wieland, Th. Fanghänel, J. Colloid Interface Sci. 276 (2004) 118. [7] Th. Stumpf, Th. Fanghänel, J. Colloid Interface Sci. 249 (2002) 119. [8] Th. Rabung, D. Schild, H. Geckeis, R. Klenze, Th. Fanghänel, J. Phys. Chem. B 108 (2004) 17160. [9] Y. Takahashi, T. Kimura, Y. Kato, Y. Minai, T. Tominaga, Radiochim. Acta 82 (1998) 227. [10] K.H. Chung, R. Klenze, K.K. Park, P. Paviet-Hartmann, J.I. Kim, Radiochim. Acta 82 (1998) 215. [11] R.K. Iler, in: The Chemistry of Silica, Wiley–Interscience, New York, 1997. [12] M. Dietzel, Geochim. Cosmochim. Acta 64/19 (2000) 3275. [13] W. Stumm, J.J. Morgan, in: Aquatic Chemistry, Wiley–Interscience, New York, 1981, pp. 540–541.
[14] G. Lagerstrom, Acta Chem. Scand. 13 (1959) 722. [15] N. Ingri, Chem. Scand. 13 (1959) 758. [16] M. Dietzel, in: I. Stober, K. Bucher (Eds.), Water–Rock Interaction, Kluwer Academic, Netherlands, 2002, pp. 207–235. [17] W.J. Weber, W. Stumm, J. Inorg. Nucl. Chem. 27 (1965) 237. [18] M.A. Kim, P.J. Panak, J.I. Yun, J.I. Kim, R. Klenze, K. Köhler, Colloids Surf. A 216 (2003) 97. [19] P.J. Panak, M.A. Kim, J.I. Yun, J.I. Kim, Colloids Surf. A 227 (2003) 93. [20] P. Venema, T. Hiemstra, W.H. Van Riemsdijk, J. Colloid Interface Sci. 181 (1996) 45. [21] D.C. Grahame, Chem. Rev. 41 (1947) 441. [22] T. Hiemstra, W.H. Van Riemsdijk, J. Colloid Interface Sci. 179 (1996) 488. [23] G.H. Bolt, J. Phys. Chem. 61 (1957) 1166. [24] L. Pauling, J. Am. Chem. Soc. 51 (1929) 1010. [25] J. Westall, FITEQL: A Computer Program for Determination of Chemical Equilibrium Constants from Experimental Data, Version 2.0’, Report 82-02, Dept. of Chemistry, Oregon State University, Corvallis, OR, USA, 1982. [26] E.P. Poeter, M.C. Hill, Documentation of UCODE, a computer code for universal inverse modeling, US Geological Survey Water-Resources Investigations Report 98-4080, 1998, 116 pp. [27] R. Siever, Am. Mineral. 42 (1957) 821. [28] S.M. Elgawhary, L. Lindsay, Soil Sci. Soc. Am. Proc. 36 (1972) 439. [29] T. Kimura, G.R. Choppin, J. Alloys Compd. 213/214 (1994) 313. [30] A. Roßberg, T. Reich, G. Bernhard, Anal. Bioanal. Chem. 376 (2003) 631. [31] T. Hiemstra, J.C.M. De Wit, W.H. Van Riemsdijk, J. Colloid Interface Sci. 133/1 (1989) 105. [32] W.H. Van Rimsdijk, J.C.M. de Wit, L.K. Koopal, G.H. Bolt, J. Colloid Interface Sci. 116 (1987) 511. [33] T. Hiemstra, W.H. Van Rimsdijk, M.G.M. Bruggenwert, Neth. J. Agric. Sci. 35 (1987) 281. [34] T. Hiemstra, W.H. Van Rimsdijk, G.H. Bolt, J. Colloid Interface Sci. 133 (1989) 91. [35] T. Hiemstra, P. Venema, W.H. Van Rimsdijk, J. Colloid Interface Sci. 184 (1996) 680. [36] P.J. Panak, M.A. Kim, R. Klenze, J.I. Kim, Th. Fanghänel, Radiochim. Acta 93 (3) (2005) 133.
Immobilization of trivalent actinides by sorption onto quartz and incorporation into siliceous bulk: Investigations by TRLFS S. Stumpf a,∗ , Th. Stumpf b , J. Lützenkirchen b , C. Walther b , Th. Fanghänel a,c a European Commission, Joint Research Centre, Institute for Transuranium Elements, Karlsruhe, Germany b Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung, P.O. Box 3640, 76021 Karlsruhe, Germany c Ruprecht-Karls-Universität Heidelberg, Physikalisch-Chemisches Institut, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany
Received 29 June 2007; accepted 28 September 2007 Available online 3 October 2007
Abstract The adsorption of Cm(III) on quartz is studied by time resolved laser fluorescence spectroscopy (TRLFS) in the pH range from 3.75 to 9.45. The raw spectra are deconvoluted into three single components. The first one has a peak maximum at 593.8 nm and can be attributed to the Cm(III) aquo ion with an emission lifetime of 68 ± 3 µs. The second one corresponds to an adsorbed species and has a peak maximum at 601.4 nm and an emission lifetime of 123 ± 10 µs. The peak maximum of the third component is shifted to higher wavelength (603.6 nm) while the lifetime remains constant. Additionally, the adsorption of Am(III) on quartz is investigated in batch experiments. Based on the spectroscopic data a sorption mechanism is suggested. In addition, the obtained Am uptake data and the Cm-TRLFS data are modeled simultaneously using a single site Basic Stern model in combination with the charge distribution concept of Pauling. The finally suggested model consists of two bidentate surface complexes where the second one is the product of hydrolysis of the first sorption species. In a separate set of experiments the influence of silicic acid at different concentrations on the Cm(III) speciation in a quartz system is investigated by TRLFS. In suspension silicic acid at low concentration (3.5 × 10−4 mol/L) has no influence on the Cm(III) speciation. At high concentration (3.5 × 10−2 mol/L) the Cm(III) speciation is definitely influenced. The results at higher concentration indicate the formation of Cm(III)/silicic acid complexes and the incorporation of Cm(III) into siliceous bulk. This is confirmed by measurements at a quartz single crystal surface. Moreover, these measurements indicate the formation of quartz/Cm(III)/silicic acid ternary complexes at the mineral surface. © 2007 Elsevier Inc. All rights reserved. Keywords: Curium; Quartz; Silicic acid; Surface complexation; Sorption; Ternary complexes; TRLFS; Single crystals; Adsorption model
1. Introduction Radionuclide migration in natural aqueous systems is an ongoing concern in environmental research in particular in the context of the long term performance of nuclear waste repositories. The transport of actinides is strongly influenced by adsorption onto mineral surfaces and interaction with organic and inorganic ligands. Fundamental insight into sorption and complexation mechanisms such as identification of dissolved and adsorbed species is of cardinal importance for reliable prediction of actinide reactions in natural systems. Therefore it is * Corresponding author.
E-mail address: [email protected] (S. Stumpf). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.09.080
necessary to characterize the different actinide species and to elucidate the reaction mechanisms involved. Time resolved laser fluorescence spectroscopy (TRLFS) enables the speciation of lanthanides like Eu(III) and actinides like U(VI), Am(III) and Cm(III) [1] in aqueous solution and on the water/mineral interface. Due to the high fluorescence yield of Cm(III) TRLFS allows speciation studies in the nanomolar concentration range corresponding to a surface loading far below a monolayer. Up to now the interaction of Cm(III) with mineral surfaces like γ -alumina [2], clay minerals [3], feldspars [4], CSH phases [5], cement [6], calcite [7] and α-alumina single crystal surfaces [8] has been investigated. The characterization of the adsorbed species is deduced from excitation and emission spectra and from the fluorescence emission lifetime of Cm(III).
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TRLFS investigations of the interaction of Eu(III) with amorphous silica led to the conclusion that the trivalent lanthanide is not adsorbed but incorporated into the bulk structure [9]. Comparable results have been found for the interaction of Cm(III) with amorphous silica colloids [10]. These results indicate different mechanisms for the interaction of trivalent lanthanides/actinides with amorphous silica and with the afore mentioned minerals. One possible explanation is the presence of dissolved silica from weathering of silicate minerals as described in literature [11–13]. The solubility of amorphous silica is known to be 2 × 10−3 M at pH < 9, dominated by monosilicic acid and increases rapidly with pH via deprotonation and through formation of polysilicic acid [14–16]. The different Si species may be involved in various reactions like complexation [17], formation of colloids [18,19] and precipitation [13]. These reactions have to be taken into account when the speciation of Cm(III) is investigated in the presence of silicate minerals. The present TRLFS investigation is part of a study, which intends to give a complete Cm(III) speciation in a natural system mainly composed of silicates like quartz. Hereby, Cm(III) was selected as a representative of a trivalent actinide ion. Because of their omnipresence in nature silicates can have a dominating influence on the aqueous chemistry and hence on the migration behavior of actinides in the natural environment. In this study the adsorption of Cm(III) on quartz and the influence of silicic acid, which can be generated by the dissolution of the silicate mineral, at different concentrations on the Cm(III) speciation was investigated in dependence on pH. The studies of the Cm(III) speciation were performed on quartz particles in suspension as well as on quartz single crystal surfaces. In addition to TRLFS measurements the adsorption of Cm(III) and Am(III) was investigated by batch experiments and α-spectrometry. The batch sorption and TRLFS data for the quartz particles in absence of silicic acid are described by a surface complexation model. 1.1. Surface complexation model Adsorption of cations on oxides has been the subject of many studies. A number of different models have been used to describe the adsorption behavior [20]. Most models interpret the adsorption as an interaction of metal ions with functional groups at the surface. Those models usually consider competitive adsorption of protons and metal ions in proton–metal ion– adsorbent systems. Before interacting with the surface functional groups the metal ions have to overcome a potential difference between the bulk solution and the interface. A surface complexation model is therefore composed of two parts: a discription of the electric double layer (e.g., that by Gouy and Chapman [21]) and conventional mass law equations and balances for the reactions at the surface. With this, the overall reaction for the absorption of a metal ion to a surface functional group can be separated into a chemical (intrinsic) and a variable electrostatic part: ◦
K = e−Gads /RT = e−(Gr +Gel )/RT = Kin e−Gel /RT = Kin Kel ,
(1)
Fig. 1. Electrostatic double layer model for the interpretation of the sorption data.
where K: overall reaction constant, Kin : intrinsic reaction constant, Kel : electrostatic reaction constant, Gads : Gibbs free energy for the adsorption, G◦r : standard Gibbs free energy, Gel : electrostatic energy. The electrostatic energy change, Gel , is variable due to the change in charge upon adsorption of ions and is determined by the electrostatic potential Ψ which is calculated by the use of an electrostatic model, in which the charge distribution (CD) of ions is involved [22]. Models may differ in the formulation of the surface chemical reactions (stoichiometries) and the assumed structure of the electrostatic double layer. In this study the basic Stern model was used as the electrostatic double layer model for the interpretation of the adsorption data (Fig. 1). Two electrostatic planes are considered that are separated from each other by a charge free layer in between called the Stern layer. The charges of protons are allocated to the surface (σ0 ). Adsorption of background electrolyte ions is considered by treating them as point charges and placing that charge at the head end of the diffuse part of the double layer (σ1 ). The sum of σ0 and σ1 is compensated by the charge σd of the diffuse part of the electrostatic double layer, which is calculated from the Gouy–Chapman equation. The Stern layer is characterized by a capacitance. This capacitance is usually determined from model fits to acid base titration data [23]. As demonstrated in Eq. (2) the interface of an oxide may be composed of several functional groups, which are coordinated to one or more metal ions of the solid. O−2 + 2H+ ⇔ OH−1 + H+ ⇔ OH02 .
(2)
This picture leads to a discrete surface heterogeneity because each group has its own proton affinity and charge characteristics, which may even vary with the crystallographic plane. For the determination of the overall charge σ0 at the surface one must take into account that the surface oxygen is not only partially neutralized by protons but also by the metal ions in the mineral structure. Vice versa, the charge of the cation is compensated by the charge of the surrounding oxygens. As it was introduced by Pauling [24] for neutralization, the charge is distributed over the surrounding ligands, which can be expressed per bond. With this, the bond valence v is defined as the charge
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z of the cation divided by its coordination number CN: v = z/CN.
(3)
A charge distribution (CD) concept like this is also applied for cations adsorbed to the mineral surface. Whereas electrolyte ions are still considered as point charges as stated above (true outer-sphere surface complexes), the charge of sorbed cations like Cm and Am is distributed between the surface plane and the head end of the diffuse layer thus rationalizing their size compared to that of a proton. The simple Pauling bond valence concept can be used as a first order estimate in relating the charge distribution needed in the model to the structure of the adsorbing ion. In case of a structural change of the cation adsorbed to the surface a change of the charge distribution and with this of the surface potential Ψ results. The structure of the surface, the structure of the adsorbed species, and the electrostatic potential profile are all essential features of an adsorption model using physically realistic surface species.
7
the system. Thus in the present study, for the first time experimentally obtained solution and surface speciation (for Cm from TRLFS) could be directly used in the modeling exercise in simultaneous combination with classical Am uptake data (assuming that both exhibit equal behavior). 3. Materials All used chemicals were of analytical grade and carbonate free. 3.1. Quartz particles The used quartz was obtained from Redon/Bretagne, France. The quartz was crushed in a mortar and sieved to get fractions <20 µm. A <5 µm fraction (BET surface >1 m2 /g) that was used for the sorption experiments was selectively obtained via centrifugation. Purity and crystallinity of the quartz powder were verified by ICP-MS and XRD measurements.
2. Experimental
3.2. Quartz single crystal
2.1. Methods
The quartz single crystal was obtained from Belo Horizonte, Brazil and was cut (X-cut) in 10 × 10 × 1 mm single crystal slices. XPS measurements indicated a carbon impurity on the surface that was removed by cleaning the surface with methanol.
2.1.1. TRLFS (time resolved laser fluorescence spectroscopy) TRLFS measurements were performed using a dye laser (Lambda Physics, scanmate) pumped by a XeCl-excimer laser (Lambda Physics, EMG, 308 nm, 24 ns) for excitation. Emission spectra were recorded from 580 to 620 nm. The fluorescence emission is detected by an optical multichannel analyzer consisting of a polychromator (Jobin Yvon, HR 320) with a 300/600/1200 lines/mm grating and a photodiode array (Spectroscopy instruments, ST 180, IRY 700G). The Cm(III) was excited at 396.6 nm. The emission spectra of Cm(III) were recorded in the 580–620 nm range (1200 lines/mm grating). For measuring the time dependent emission decay (300 lines/mm grating), the delay time between laser pulse and camera gating was scanned with time intervals between 5 and 10 µs. For measuring the decay time of the fluorescence emission the delay time was shifted in steps between 5 and 10 µs using a constant time window of 1 ms. 2.1.2. Data modeling All sorption data were modeled as surface charge densities simultaneously using the FITEQL 2.0 program [25] in combination with the optimization code UCODE [26]. The Davies equation was used to correct for ionic strength effects. This allows fitting of all parameters, including capacitance values, which can only be adjusted manually in FITEQL. The linear correlation coefficient (between the capacitance value and the electrolyte binding constant) which can be obtained with this code combination has an absolute value of 0.91. This indicates a relatively strong correlation between the two adjustable parameters. This information would not be obtained using FITEQL in optimization mode. Another advantage of using UCODE for parameter estimation is that a high degree of flexibility is obtained in the combination of experimental data pertaining to
3.3. Silicic acid The silicic acid solution was prepared by diluting a silicon standard for ICP (1 mg/ml Si in 2% NaOH, ACROS Fisher Scientific GmbH) with HClO4 to a desired concentration of 3.5 × 10−4 mol/L (undersaturation) and 3.5 × 10−2 mol/L (oversaturation) [14,15,27,28]. 3.4. Experimental set-up A stock solution of the long-lived curium isotope Cm-248 (t 1 = 3.4 × 105 years) with the isotopic composition 97.3% 2 Cm-248, 2.6% Cm-246, 0.04% Cm-245, 0.02% Cm-247 and 0.009% Cm-244 in 1.0 M HClO4 was used for the TRLFS experiments. The initial curium concentration determined by ICP-mass spectroscopy was adjusted to 2.0 × 10−7 mol/L. To avoid complexation by carbonate all experiments were made in a glove box under argon atmosphere at 25 ± 1 ◦ C. The Cm(III) adsorption was investigated in batch experiments with a quartz suspension of 1 g/L quartz particles <5 µm in size. The pH of the batch samples was varied by adding NaOH and HClO4 . During the curium/quartz contact time the samples were shaken periodically. All samples had an electrolytic background of 0.1 mol/L NaClO4 . Additionally batch sorption experiments with Am(III) were performed. The samples were prepared analogous to the Cm(III) samples. The pH edges were obtained by analyzing the Am(III) concentration of the solution by α-spectrometry after equilibration (1 day) and solid liquid separation.
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The influence of silicic acid was investigated by adding the silicon standard to the quartz suspension at acidic pH (≈3.5) before adding the Cm(III). The concentration of silicic acid was 3.5 × 10−4 mol/L (undersaturation) and 3.5 × 10−2 mol/L (oversaturation), respectively. The sorption of Cm(III) onto single crystals was obtained contacting the single crystal with a Cm(III) solution (2 × 10−7 mol/L) at different pH values. An additional set of experiments was done contacting the single crystal with a Cm(III)/quartz suspension (1 g/L quartz; <5 µm; 2 × 10−7 mol/L Cm(III)) in presence of 3.5 × 10−2 mol/L silicic acid at different pH values. 4. Results and discussion 4.1. Adsorption of Cm(III) on quartz-TRLFS measurements Fluorescence emission spectra of 2 × 10−7 mol/L Cm(III) in a quartz suspension, recorded at different pH values ranging from 3.75 to 9.45, are shown in Fig. 2. At low pH, the emission band with a peak maximum at 593.8 nm can be attributed to the Cm3+ aquo ion [29]. With increasing pH, the intensity of this peak decreases and a red-shift of the fluorescence emission up to 603.6 nm at pH 9.45 appears. Under the present experimental conditions hydrolysis of Cm(III) can be ruled out and the change of fluorescence spectra can be solely explained by a stepwise complexation of the curium aquo ion at the mineral surface and the formation of inner-sphere complexes. The mixed spectra were deconvoluted using a factoranalysis program [30]. The species identified by the deconvolution procedure are plotted in Fig. 3. Moreover, from the peak deconvolution data and the respective FI values (fluorescence intensity) the mole fractions of the three Cm-species are determined and plotted in Fig. 3 as a function of pH (speciation plot). The aquo ion dominates the system up to a pH value of 4.5. Then the formation of the first surface complex (601.4 nm) starts with a maximum fraction of 30% at pH 5.5. With increasing pH
(a)
(b) Fig. 3. (a) Calculated single components and (b) speciation plot for the sorption of Cm(III) onto quartz.
Fig. 2. Fluorescence emission spectra of 2 × 10−7 mol/L Cm(III) in a suspension of 1 g/L quartz at different pH values.
S. Stumpf et al. / Journal of Colloid and Interface Science 318 (2008) 5–14
Fig. 4. Calculated slope for the formation reaction of sorption Species 2.
the second surface complex (603.6 nm) is formed. This species dominates the system up to pH 9. The fluorescence emission of the Cm3+ aquo ion decays with a fluorescence emission lifetime of 68 µs, while the first as well as the second Cm complex emitting at 601.4 nm and 603.6 nm both have an emission lifetime of 123 ± 10 µs. The increase in lifetime by complexation reflects the exclusion of water molecules from the first coordination sphere of Cm(III). In the first coordination sphere H2 O acts as a fluorescence quencher by energy transfer and excitation of H2 O vibrations and, therefore, causes a decrease of the emission lifetime. The short lifetime of the Cm3+ aquo ion (68 ± 3 µs) compared to a lifetime of 1300 µs in D2 O [29] is a result of these quenching processes. A mathematical expression for the correlation of lifetime and number of H2 O quenchers is given in literature by Kimura et al. [29]. Complexes 1 and 2 are characterized by the same fluorescence lifetime, implying that the number of quenching ligands in the first Cm(III) coordination sphere remains unchanged. Applying the correlation of Kimura et al. the number of water molecules in the first coordination shell for both surface complexes can be estimated to be 5 ± 0.2. However, the change in the emission spectra clearly indicates that there are two different species with different composition or structure. With increasing pH a stepwise deprotonation according to Eq. (4) is very likely. Species 1 → Species 2 + nH+ .
(4)
From this equation it follows the correlation for the law of mass action: n K = [Species 2]/[Species 1] × H+ .
(5)
A plot of the ratio log(Species 2/Species 1) with pH allows the determination of the proton stoichiometry for the given reaction. Here, the plot gives a slope of 1 (Fig. 4), indicating the exchange of one proton during the formation reaction. The increase of the error bar value together with an increase of pH is a result of the change of concentration ratio [Species 2]/[Species 1] during the deprotonation reaction.
9
This result is in good accordance with spectroscopic investigations of the adsorption of Cm(III) on alumina [2], clay minerals [3] and feldspar surfaces [4]. Relying on the cited investigations and on the basis of the actual study a mechanism for the Cm adsorption on a quartz surface can be deduced. In a first reaction step a surface complex is formed by exchange of approximately half of the hydration shell from the first Cm coordination sphere. According to the determined proton stoichiometry in Eq. (5) in a second reaction step the release of a proton is proposed. Whether this proton is released by the hydrolysis of the surface complex or by a deprotonation of the surface itself is not clear at this point. Moreover, the spectroscopic data give no evidence how many oxygen of the surface are involved into the bonding to the sorption species. To get more structural insight the adsorption of Am(III) on quartz at different concentrations of the actinide was investigated in an analogue experiment. The Cm and Am sorption data were then modeled and a sorption mechanism is proposed. 4.2. Adsorption of Cm(III) and Am(III) on quartz-surface complexation modeling As mentioned above, a surface complexation model is composed of two parts—one part describes the electric double layer whereas the other one gives conventional mass law equations for the reactions at the surface. Here, the Basic Stern Model was used as the electrostatic double layer model for the interpretation of the adsorption data. With regard to the mass law equations two types of surface groups can be considered for the quartz surface. The log K value for the protonation of the doubly coordinated Si2 –O group is extremely low and can therefore be regarded as inert [31]. Only the singly coordinated groups are reactive and the protonation reactions can be formulated as: ≡SiOH ↔ ≡SiO− + H+ ,
(6)
+ ≡SiOH+ 2 ↔ ≡SiOH + H .
(7)
Due to the very low log K value for Eq. (7) the protonation of this surface group is not very likely. Therefore, in this study a potentially complex situation is simplified by the use of only one charging reaction (Eq. (6); 1-pK approach) [32,33]. At the point of zero charge of quartz (around pH 2–3) all surface groups are protonated and are uncharged (SiOH0 ). Therefore, in the pH range that is relevant for the sorption experiments the surface charge of quartz is negative and the quartz surface is supposed to be homogeneous [34]. With this, the reactions at the quartz surface within the 1-pK, 1-site basic Stern model are the following: SiOH ↔ SiO− + H+ ,
(6)
SiOH + Na+ ↔ SiO− · · ·Na+ + H+ .
(9)
The log K value of reaction (6) is −7.9 for I = 0 according to previous work [35] and is calculated to be −7.79 for I = 0.1 (i.e., for the background electrolyte used in the is study: 0.1 M NaClO4 ). The log K for reaction (9) was optimized numerically (log K = −6.93) [31]. Based on previous work [31], the
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Fig. 5. pH edges of the sorption of 248 Cm and 243 Am onto quartz. Fig. 7. Slope analysis of the sorption data in consideration of different assumptions for log K.
Fig. 6. Sorption of trivalent actinides onto a quartz surface: modeled mechanism and sorption species.
number of sites at the quartz surface was set to 4.6 nm−2 . Moreover the capacitance of the Stern layer was set to a value of 1.44 F/m2 according to acid base titration data [23]. All sorption data were used in one UCODE set-up file and fitted using the Basic Stern Model outlined above and considering ideal charge distribution. The experimental data were all used as the fraction of species with respect to the total concentration of Cm (Cm3+ , Species 1, Species 2) or Am (Amads ), respectively. Fig. 5 shows pH sorption edges of Am(III) at three different Am(III) concentrations (2 × 10−7 /10−6 /10−5 mol/L). Moreover, the Cm sorption data ([Cm(III)] = 2 × 10−7 M), obtained by the calculation of the speciation from the TRLFS data (Fig. 3), are added to the plot in Fig. 5. The pH edges of both actinides fit very well for nearly identical conditions. This result is in good agreement with the assumption of equal chemical behavior for americium and curium. As expected, with increasing actinide concentration the pH edges are shifted to higher pH values. The fit of the sorption data is also given in Fig. 5 (black line). The proposed structure model for the two different sorption species is shown in Fig. 6. The structure of the surface complexes determines the charge distribution at the mineral surface and with this also determines the electrostatic part of the equation constant which finally describes the given sorption data. Here, the trivalent actinide ion is attached to the surface in a bidentate conformation. Once the location of the complex has been defined one has to consider the position of charge for both surface species. Part of the charge of the surface complex is shared with the surface itself. The remaining part is attributed to the other ligands of the complex which are oriented towards the solution. By application of the Pauling bond valence concept (Eq. (3)) and assuming a coordination number of 8 for the actinide a value of v = 3/8 can be attributed to each metal bond. It follows for the charge of the actinide in the first complex
+3 − 2 × 3/8 = 2.25. The surface oxygen has a bond valence of v = −2/2 = −1 whereas a value of v = −2/2 + 1 (which comes from H+ ) can be attributed to the hydroxid group. Taking the contribution of the actinide valence bond into account the charge at the surface becomes −1 + 0 + 2 × 3/8 = −0.25. As a result of a proposed twofold hydrolysis (v(OH–) = −1) for the second complex, the charge of the actinide changes to a value of +3 − 2 × 3/8 − 1 = 1.25 and for the surface to a value of −2 + 2 × 3/8 = −1.25. It is obvious that in the proposed model a change of surface complex structure is associated with a change of charge at the solid solution interface. Finally, the electrostatic part in the reaction equation of the sorption process is changed. This means, that the classical plot for the estimation of proton stoichiometries (i.e., log[Species 2]/[Species 1] vs pH) is not applicable, since this involves the assumption of one invariant overall stability constant. As it was already shown, by application of a surface complexation model the stability constant K is separated into an intrinsic and electrostatic part (K = Kin × Kel ; Eq. (1)). A plot taking Kin as overall stability constant results in a value of 2 for the slope (Fig. 7). It follows that two protons are released during the surface reaction (Fig. 6) and not one as inferred from the classical slope determination that was applied for the interpretation of the TRLFS data. Additionally, taking the electrostatic part into account as given in Eq. (1) and applying K as overall stability constant a plot of log[Species 2]/[Species 1] vs pH results a line with a slope of 1 (Fig. 7). With the separation of K into an intrinsic and electrostatic part the stoichiometry of the formation reaction of Species 2 from Species 1 as well as the slope determination are very well reflected. With the proposed structure model it is possible to fit the sorption data of the Cm(III) sorption as well as of the Am(III) sorption onto quartz (Fig. 5). At the highest Am concentration (2 × 10−5 mol/L) Am is present in excess of the surface sites. Therefore, the observed uptake data beyond site saturation may be attributed to the formation of a solid Am-phase which is either amorphous or crystalline or a mixture of both. Therefore, two fitting curves from two
S. Stumpf et al. / Journal of Colloid and Interface Science 318 (2008) 5–14
solubility products, namely log KL0 = −25.1 (amorphous Amhydroxide) and log KL0 = −26.4 (crystalline Am-hydroxide), have been calculated for the Am sorption at higher concentrations. As the experimental data are located in between the two fitting lines (Fig. 5) obviously a mixture of an amorphous and crystalline Am-phase is formed. The log K values for the sorption of Cm(III) and Am(III) at the quartz surface are: log K = 5.05, log K = −6.15,
2SiOH ↔ Species 1 + H+ , Species 1 ↔ Species 2 + 2H+ .
The interpretation of TRLFS data by application of the slope analysis in the simplest form gives an idea for the adsorption of trivalent actinides on a quartz surface. The modeling of sorption data by application of a surface complexation model confirms the initial assumption. Moreover, it results in the formulation of an unique sorption mechanism, that is, the formation of a bidentate at the mineral surface in a first step (sorption Species 1) and the twofold hydrolysis of the bidentate in a second step (sorption Species 2). 4.3. Adsorption of Cm(III) on quartz-influence of silicic acid The dissolution of quartz results in the formation of silicic acid that tends to act as inorganic ligand for cations. The question arises weather silicic acid has an influence on the Cm(III) speciation in a quartz system. The interaction of Cm(III) with silicic acid at low concentrations (undersaturation) were investigated by Panak et al. [36]. Two different Cm(III)/silicic acid complexes together with their stability constants were determined. Moreover, the influence of silicic acid at higher concentrations (oversaturation) was investigated [36]. At these concentrations silicic acid polymerizes and forms colloids. As a result of colloid formation the trivalent actinide is incorporated into the amorphous silica phase resulting in the total loss of its hydration sphere. According to the investigations performed by Panak et al., in this study, the influence of silicic acid at different concentrations (3.5 × 10−4 and 3.5 × 10−2 mol/L) on the Cm(III) speciation in the presence of quartz was investigated by TRLFS. The emission spectra were recorded in a pH range from 3.48 to 9.25 and deconvoluted as described above. In Fig. 8 the single components together with the speciation are shown for the interaction of Cm(III) with a quartz surface in absence and presence of 3.5×10−4 mol/L (undersaturated) silicic acid. For both systems the same species with the same fluorescence emission lifetimes are observed. Moreover, silicic acid at this concentration has no influence on the Cm speciation. However, the speciation changes significantly when increasing the silicic acid concentration to a value of 3.5 × 10−2 mol/L (oversaturated). A comparative presentation of the single components together with the speciation for Cm(III) in the presence of quartz with and without addition of silicic acid at high concentrations is given in Fig. 9. The influence of silicic acid results in a shift of emission maximum for the first surface complex from 601.4 nm without silicic acid to 602.6 nm in the presence of the complexing ligand. The emission band for the second surface complex
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(604.0 nm) is broadened indicating that this peak must be attributed to more than one surface species. Up to a pH of 6 the speciation plots of the two systems coincide with each other. At higher pH the Cm speciation is obviously influenced by the silicic acid. Panak et al. report on the formation of silicic acid colloids by formation of polymers at pH values >6 [36]. Furthermore, these colloids complex Cm(III) in an undefined stoichiometry. Taking these investigations into account, the influence of silicic acid at high concentration on the Cm(III) speciation in a quartz system can be explained by the formation of such polysilicic acid colloids. The complexation of Cm(III) by these colloids results in the formation of Cm species with an unknown stoichiometry. As reported by Panak et al. the incorporation of Cm(III) into silica colloids is associated with a total loss of the Cm hydration sphere [36] which is indicated by lifetimes of 310 ± 19 µs. Here, the measured lifetimes at pH values <6 are in accordance with the lifetimes of the silicic acid free system. But, with increasing pH, when silicic acid colloids are formed, also the lifetimes increase up to a value of 750 ± 30 µs. This increase of lifetimes indicates the loss of the Cm(III) hydration sphere (i.e., incorporation) which is in good accordance with the investigations performed by Panak et al. The question arises if adsorbed Cm(III) is desorbed from the quartz surface and preferentially incorporated into the formed silica colloids in solution instead which would result in a mobilization of Cm(III). Another possibility is the formation of polysilicic gel-like layers and incorporation of Cm(III) in such layers at the quartz surface (ternary quartz/Cm(III)/silicic acid complexes) which could result in an immobilization of Cm(III). To solve this issue, Cm sorption experiments in the presence of silicic acid at high concentration were performed at a single crystal quartz surface. 4.4. Adsorption of Cm(III) on quartz-single crystal measurements The adsorption of Cm(III) on a quartz single crystal in the pH range from 2.5 to 6.2 was observed by autoradiographic measurements (Fig. 10). With increasing pH adsorption at the surface increases. Simultaneously, the intensity of the Cm(III) fluorescence emission signal increases, indicating the formation of a surface complex (Fig. 11). Because of the small single crystal surface area compared to the one provided in suspension the adsorption process is shifted to higher pH values and only the first sorption species is observed in the selected pH range. The fluorescence emission maximum of this species is at 600.3 nm and shows a shoulder at lower wavelength. The peak position differs slightly from the fluorescence emission maxima of the first surface complex given in a quartz suspension (601.4 nm). In suspension Cm(III) adsorbs on different crystal planes. Here, only the x-plane is available for adsorption. Recently it was found that with changing the crystal plane of a single crystal the maximum of Cm fluorescence emission changes [8]. With this, the determined fluorescence emission maxima in suspension can be attributed to an averaged wavelength over all planes whereas the maximum at 600.3 nm corresponds to one species adsorbed at one designated plane. The lifetime of
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(a)
(b) Fig. 8. (a) Calculated single components and (b) speciation plot for the sorption of Cm(III) onto quartz in presence of silicic acid at low concentration.
the adsorbed species was measured at pH 5.7 and 6.2. For both pH values a biexponential decay behavior is observed whereas the rate of the species with the shorter lifetime becomes less at higher pH. The quartz surface is covered by a thin liquid film of curium solution as a result of preparation. Therefore, the shorter lifetime with a value of 68 ± 3 µs can be attributed to the nonsurface complexed Cm3+ aquo ion. Moreover, the shoulder at lower wavelength in the emission spectrum can be explained by the fact, that with increasing pH the ratio of the aquo ion decreases because of increasing adsorption. The longer lifetime has a value of 125 ± 10 µs. This value corresponds to a surface species with approximately five water molecules in the first coordination sphere of curium which is in good accordance with the lifetimes that can be attributed to the afore determined Cm sorption species in a quartz suspension. We conclude that the same surface complex is formed at the single crystal surface and in suspension. Fig. 12 presents the Cm(III) emission spectra after adding silicic acid to the system. The peak maximum
is shifted to 602.7 nm indicating a change of the ligand field of the adsorbed Cm(III) by complexation with silicic acid. Beside the spectral shift, the emission signal is broadened indicating the formation of more than one species as it was found for the measurements in suspension in the presence of high silicic acid concentrations. The measured lifetimes in presence of silicic acid increase up to a value of 234 µs at pH 6.7 and 323 µs at pH 9.1. The observed increase can be attributed to the reduction of the first Cm(III) hydration sphere to one water molecule as a result of the incorporation of Cm(III) into the bulk material. The Cm(III) emission spectrum at pH 9.1 was measured a second time after 78 days. The spectrum with a maximum at 602.7 nm was again broadened. The decay behavior that could be attributed to the spectrum was fitted with a function of higher order. This again indicates that the emission spectrum is composed of several complexation species. The determined lifetime increased to a value >500 µs which can be attributed to the complete loss of the first Cm(III) hydration sphere. As it was
S. Stumpf et al. / Journal of Colloid and Interface Science 318 (2008) 5–14
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(a) Fig. 11. Fluorescence emission spectra of 2 × 10−7 mol/L Cm(III) at a quartz surface (X-cut) at pH 4.0 and 6.2.
(b) Fig. 9. (a) Calculated single components and (b) speciation plot for the sorption of Cm(III) onto quartz in presence of silicic acid at high concentration. Fig. 12. Fluorescence emission spectra of 2 × 10−7 mol/L Cm(III) at a quartz surface (X-cut) in presence of 3.5 × 10−2 mol/L silicic acid at pH 6.7, 9.1 and pH 9.1 after 78 days contact time.
Fig. 10. Autoradiographic images of the Cm(III) sorption onto a quartz surface (X-cut) in the pH range 2.5 to 6.2.
additionally confirmed by autoradiographic measurements the incorporation of Cm in the bulk structure does not result in the desorption of Cm from the crystal surface. The measurements show a constant Cm(III) occupancy of the single crystal surface instead of a reduction caused by desorption processes. The influence of silica colloids solely results in the formation of ternary quartz/Cm(III)/silicic acid complexes at the quartz surface. Hence, we conclude that higher concentrations of silicic acid do not cause any mobilization of the already sorbed actinide. This result is of great importance in view of the as-
sessment of the actinide migration in natural systems and with this the safety assessment of nuclear waste repositories. 5. Summary Depending on pH Cm(III) adsorbs on quartz by formation of two sorption species. The modeling of the sorption data suggests the following sorption mechanism. Cm(III) adsorbs in a bidentate fashion in a first step and is hydrolyzed in a second step. Silicic acid at low concentration (3.5 × 10−4 mol/L) has no influence on the Cm(III) speciation. At higher silicic acid concentration (3.5 × 10−2 mol/L) and pH values >6 Cm(III) is incorporated into the gel-like bulk. This complexation results in the total loss of the first Cm(III) hydration sphere. The TRLFS measurements and α-spectrometric measurements show that
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Cm(III) is not desorbed from the surface and mobilized by this “incorporation” process. But, the spectroscopic results can be interpreted by the formation of quartz/Cm(III)/silicic acid ternary complexes at the surface. Acknowledgment We thank Dr. André Rossberg for providing the factoranalysis program code designed for the analysis of EXAFS, UV–vis and TRLFS spectra. References [1] H. Geckeis, R. Klenze, J.I. Kim, Radiochim. Acta 87 (1999) 13. [2] Th. Stumpf, Th. Rabung, R. Klenze, H. Geckeis, J.I. Kim, J. Colloid Interface Sci. 238 (2001). [3] Th. Stumpf, A. Bauer, F. Coppin, J.I. Kim, Environ. Sci. Technol. 35 (2001) 3691. [4] S. Stumpf, Th. Stumpf, C. Walther, D. Bosbach, Th. Fanghänel, Radiochim. Acta 94 (2006) 243. [5] J. Tits, Th. Stumpf, Th. Rabung, E. Wieland, Th. Fanghänel, Environ. Sci. Technol. 37 (2003) 3568. [6] Th. Stumpf, J. Tits, C. Walther, E. Wieland, Th. Fanghänel, J. Colloid Interface Sci. 276 (2004) 118. [7] Th. Stumpf, Th. Fanghänel, J. Colloid Interface Sci. 249 (2002) 119. [8] Th. Rabung, D. Schild, H. Geckeis, R. Klenze, Th. Fanghänel, J. Phys. Chem. B 108 (2004) 17160. [9] Y. Takahashi, T. Kimura, Y. Kato, Y. Minai, T. Tominaga, Radiochim. Acta 82 (1998) 227. [10] K.H. Chung, R. Klenze, K.K. Park, P. Paviet-Hartmann, J.I. Kim, Radiochim. Acta 82 (1998) 215. [11] R.K. Iler, in: The Chemistry of Silica, Wiley–Interscience, New York, 1997. [12] M. Dietzel, Geochim. Cosmochim. Acta 64/19 (2000) 3275. [13] W. Stumm, J.J. Morgan, in: Aquatic Chemistry, Wiley–Interscience, New York, 1981, pp. 540–541.
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