Macroscopic and spectroscopic characterization of uranium(VI) sorption onto orthoclase and muscovite and the influence of competing Ca2+

Available online at www.sciencedirect.com

ScienceDirect Geochimica et Cosmochimica Acta 189 (2016) 143–157 www.elsevier.com/locate/gca

Macroscopic and spectroscopic characterization of uranium(VI) sorption onto orthoclase and muscovite and the influence of competing Ca2+ Constanze Richter, Katharina Mu¨ller, Bjo¨rn Drobot, Robin Steudtner, Kay Großmann, Madlen Stockmann, Vinzenz Brendler ⇑ Helmholtz-Zentrum Dresden – Rossendorf e.V., Institute of Resource Ecology, Bautzner Landstr. 400, D-01328 Dresden, Germany Received 21 August 2015; accepted in revised form 30 May 2016; available online 4 June 2016

Abstract The uranium(VI) sorption onto orthoclase and muscovite, representing the mineral groups of feldspars and micas as important components of the earth crust, was investigated in the presence and absence of Ca2+ under aerobic conditions. Batch experiments were accompanied by time-resolved laser-induced fluorescence spectroscopy (TRLFS) as well as in situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy. The results indicate that the U(VI) sorption is reduced by Ca2+ at pH P 8 up to 30% due to the formation of the neutral aqueous Ca2UO2(CO3)3 (aq) complex. TRLFS measurements on the supernatant confirmed the predominance of this Ca2UO2(CO3)3 (aq) complex in accordance with thermodynamic calculations. Furthermore, TRLFS measurements on the mineral suspension as a function of pH (4–9) and Ca2+ revealed the existence of several species. Parallel factor analysis (PARAFAC) indicated the formation of three
surface species totally. In the absence of Ca2+, the „XOAUO+ 2 and „XOAUO2CO3 surface complexes were formed, 2+ + whereas the presence of Ca leads to the formation of „XOAUO2 and „XOAUO2OH as the formation of the aqueous Ca2UO2(CO3)3 (aq) complex reduces the free UO2+ 2 concentration in the solution. In addition, ATR FT-IR spectroscopy confirmed an outer-sphere surface species in the absence of Ca2+. These experimental results were used for the assessment of surface complexation parameters to improve the basis for a mechanistic modeling of the sorption processes of U(VI) onto orthoclase and muscovite including the influence of Ca2+. Namely, log K BXOAUOþ2 ¼ 1:69 and log K BXOAUO2 CO
3 ¼ 8:96 were determined for sorption onto orthoclase, whereas log K BXOAUOþ2 ¼ 0:41 and log K BXOAUO2 CO
3 ¼ 8:71 best describe sorption onto muscovite in the absence of Ca2+. Ó 2016 Elsevier Ltd. All rights reserved. Keywords: Sorption; Uranium; Orthoclase; Muscovite; Calcium; TRLFS; ATR FT-IR; PARAFAC; SCM

1. INTRODUCTION

⇑ Corresponding author. Tel.: +49 351 260 2430; fax: +49 351

260 3553. E-mail addresses: [email protected] (C. Richter), [email protected] (K. Mu¨ller), [email protected] (B. Drobot), [email protected] (R. Steudtner), [email protected] (K. Großmann), [email protected] (M. Stockmann), [email protected] (V. Brendler). http://dx.doi.org/10.1016/j.gca.2016.05.045 0016-7037/Ó 2016 Elsevier Ltd. All rights reserved.

In general minerals show a potential to sorb ions from the surrounding solution. The sorption potential of minerals is different for each mineral due to chemical and structural differences. A comprehensive understanding of them as well as the related chemical processes (hydrolysis, complexation, sorption, ion exchange, dissolution/precipitation, redox processes – to name only the most prominent

144

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157

ones) is substantial for predicting the distribution and transport of elements in nature. In the last decades many studies (Lu¨tzenkirchen, 2006; Tan et al., 2010; Shaheen et al., 2013) focused on mechanistic models, e.g. surface complexation and ion exchange models, which are able to describe sorption processes in a proper way. Uranium (U) is a chemo- and radiotoxic environmental contaminant. It occurs in different oxidation states. The most common ones are U(VI) under oxic conditions and U(IV) under anoxic conditions. The first one is more mobile, and thus shows a higher affinity to be transported in the environment. Major sources of U contaminations are U mining, effluents from phosphate and rare earth production, ammunition based on depleted U, and incidents from nuclear installations (civilian and military). U(VI) occurs in acid mine drainage waters in concentrations of 4.4  10
5 mol L
1, e.g. in Ko¨nigstein (Germany) (Krawczyk-Ba¨rsch et al., 2011). Baumann et al. (2012) detected concentrations between 0.3  10
6 mol L
1 and 3.7  10
6 mol L
1 in seepage water and pore water near Ronneburg (Germany). Furthermore, the natural uranium content in granite in the Erzgebirge (Germany) is on average 11 ppm (OECD/NEA, 2002). Moreover, U is a potential contaminant from final disposal of nuclear waste (Choppin et al., 2002). Representatives of the feldspar and mica groups are main rock forming minerals and consequently, ubiquitous in nature. Nevertheless, sorption of U(VI) onto these minerals has been rarely investigated. This also holds for orthoclase (K-feldspar) and muscovite as prominent members of feldspars and mica, respectively. Ames et al. (1983) has observed excellent sorption efficiency on muscovite as a function of temperature and initial U(VI) concentration. Adsorption at the mica interface has been observed by Ilton et al. (2006) and Saslow Gomez et al. (2012). A closer look to the surface of muscovite by Moyes et al. (2000) and Arnold et al. (2006) has been performed using TRLFS and X-ray absorption spectroscopy, respectively. These investigations determined the adsorbed species to be a bidentate surface complex and an amorphous U(VI) condensate/precipitate. Some studies have focused on surface complexation modeling (SCM), where in most cases a combination of minerals was used to predict the sorption capacity of a rock containing several mineral components, or the socalled ‘‘Component Additivity” approach (Davis et al., 1998; Arnold et al., 2001; Nebelung and Brendler, 2010). Nebelung and Brendler (2010) have indirectly studied the sorption of U(VI) onto the feldspar group as component of granite, described by albite sorption data, that showed maximum sorption between pH 6 and 7. Furthermore, Kerisit and Liu (2012) have closely evaluated the diffusion and adsorption of uranium(VI) carbonate species in nanosized mineral fractures of feldspars. They stated, that the presence of feldspar surface diminishes the diffusion coefficients of aqueous species such as Ca2UO2(CO3)3, UO2+ 2 , CO2
3 , and UO2CO3. Furthermore, they indicated a large favorable free energy of adsorption for UO2+ and 2 UO2CO3, which directly bond with their uranium atom to a surface hydroxyl oxygen, in contrast to an only slightly favorable or unfavorable adsorption of CO2
and 3

Ca2UO2(CO3)3, which bond over a carbonate oxygen to a surface hydroxyl group. For albite, a Na-feldspar, Walter et al. (2005) observed an inner-sphere, mononuclear, bidentate uranium(VI) surface complex („SiO2AUO2). Additionally, Arnold et al. (2001) and Nebelung and Brendler (2010) modeled the U(VI) sorption on phyllite and granite („XOAUO+ 2 , „XO2AUO2) but no SCM of U(VI) on orthoclase has been performed up to now. For the uranium(VI) sorption onto minerals of the feldspar and mica groups in carbonate systems only few data and rare spectroscopic evidence for surface complexes are available. When considering silicates, Al(hydr)oxides and aluminosilicates, a variety of ternary surface complexes with carbonate were postulated: „SiO2AUO2OHCO3
3 and „(SiO)2AUO2CO3
were proposed by Gabriel 3 (1998) and Gabriel et al. (2001) for amorphous silica gel based on TRLFS measurements. The latter was also favored by Kar et al. (2012). Davis and Kohler (2001) proposed „SiOAUO2CO
3 for quartz. A more complicated structure, i.e. „AlOA(UO2)2CO3(OH)
3 was suggested by Prikryl et al. (1994) for corundum. Not distinguishing between aluminol and silanol binding sites, Jung et al. (1999) assumed „XOAUO2CO
3 for kaolinite, whereas the same structure was used by Fernandes et al. (2012) to describe sorption onto montmorillonite. A proper characterization of the U(VI) interaction with muscovite and orthoclase is still missing, including quantitative parameters as well as basic information like the stoichiometry of the predominant surface species and the bond denticity. A more thorough description of these specific systems would allow a better understanding of the underlying aspects behind the sorption behavior of heavy metal cations on feldspar and mica. The investigation of U(VI) interactions with these minerals should take into account boundary conditions implied by typical environmental scenarios. This includes the consideration of more complex systems, such as alkaline earths. They play a major role in nature due to interactions in the form of: (1) influence on the aquatic speciation (e.g., formation of ternary complexes – which in some cases may sorb on mineral surfaces), (2) sorption on the mineral (competing for binding sites), or (3) (co-)precipitation (Scheffer and Schachtschabel, 2010). Calcium, namely the Ca2+ cation, is ubiquitous in natural (ground, surface, and rain) waters as well as in clay pore waters. It is a component in soil solutions, e.g., forest soil: <2.5  10
5–4.5  10
3 mol L
1 Ca2+ and farmland: 1.25  10
4–1.5  10
2 mol L
1 Ca2+ (Blume et al., 2010). It may also be leached from backfill materials such as bentonites often used in geotechnical barriers in waste repositories. Waters related to host rocks of potential final repositories for radioactive waste contain Ca2+: pore waters from Opalinus Clay (1.86  10
3–1.05  10
2 mol kg
1 H2O) (Nagra, 2002), water at the pit Konrad (5  10
2 mol L
1–1.3  10
1 ¨ spo¨ groundwater mol L
1) (Brewitz, 1982), the A (5.7  10
2 mol L
1–8  10
2 mol L
1) (Hallbeck and Pedersen, 2008), and the groundwater in Gorleben (2.4  10
4 mol L
1–2.2  10
2 mol L
1) (Klinge et al., 2007; Noseck et al., 2012). Thus, its presence and therefore, the formation of ternary complexes with U(VI) and

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157

carbonate changes U(VI) retardation in ways that are difficult to predict. According to most environmental scenarios, the concentrations of U(VI) and Ca2+ investigated in this work were selected to be in the micromolar range for U and in the millimolar range for Ca2+. Further constraints are the solubility of U(VI) solid phases and the detection limit of the applied spectroscopic methods. The aim of this work is the quantitative description of the retention capacity of orthoclase and muscovite towards U(VI) under environmentally relevant conditions. Concerning this, batch experiments were performed, and furthermore, the influence of Ca2+ as competing cation was investigated. 0.01 mol L
1 NaClO4 was used as a background electrolyte, and the influence of various parameters on the U(VI) sorption was studied, such as pH and initial U (VI) concentration. Uranium species in solution as well as sorbed species on the mineral surface were identified by spectroscopic methods, such as TRLFS and ATR FT-IR spectroscopy. Auxiliary experiments aimed at the better understanding of the dissolution behavior of the minerals. Finally, surface complexation parameters for both minerals were assessed by coupling the geochemical speciation code PhreeqC (Parkhurst and Appelo, 2013) with the parameter estimation code UCODE (Poeter et al., 2014). 2. EXPERIMENTAL SECTION 2.1. Analytics Element concentrations were determined using Inductively coupled plasma-mass spectrometry (ICP-MS; mod. ELAN 9000, Perkin Elmer, Boston, USA) and Atomic absorption spectroscopy (AAS) (mod. AAS-4100, Perkin Elmer). ICP-MS was applied for Si, Al, Fe, Ni, Nd, Eu, Gd, Dy, and U, whereas AAS was used for K, Na, Ca, Fe, Mg, Sr, and Ba. 2.2. Materials 2.2.1. Minerals For the experiments, Norflot Kali 600 orthoclase from Sibelco (Germany – mineral mined in Scandinavia) and muscovite from Normag (Germany – mineral mined in China) were used without further treatment. The minerals were sieved to a grain size of 63–200 lm for sorption experiments and to <63 lm for spectroscopic investigations. The N2-BET specific surface area (SSA) of the minerals was determined using a surface area and pore size analyzer (mod. Coulter SA 3100, Beckman Coulter, Fullerton, USA). The main components of the minerals were determined by ICP-MS and AAS after digestion of the mineral samples with HNO3 (p.a. Merck, Darmstadt, Germany; distilled by sub-boiling), HCl (suprapur, Merck), and HF (suprapur, Merck) in a microwave oven (mod. Multiwave, Anton Paar, Perkin Elmer). Electrophoretic mobility measurements of mineral suspensions (solid-to-liquid ratio (SLR) 0.1 g L
1, 0.01 mol L
1 NaClO4) were performed with laser Doppler electrophoresis using a Zetasizer Nano ZS (Malvern Instruments, Malvern, U.K.) with disposable

145

capillary cells. From these measurements zeta potential was derived by using the Henry equation with Smoluchowski approximation. The zeta potential measurements were repeated tenfold at a temperature of 298 ± 0.1 K. 2.2.2. Solutions U(VI) solutions for batch sorption and TRLFS experiments were prepared using a U(VI) stock solution of 0.1 mol L
1 UO2+ in 0.5 mol L
1 HClO4. For ATR FT2 IR spectroscopic measurements, a 0.2 mol L
1 UO2+ 2 stock solution in 1 mol L
1 HCl was used. All chemicals were of analytical grade. Solutions were prepared using ultrapure water with a resistivity of 18.2 MX cm
1, and the pH was adjusted by means of HClO4, HCl, or NaOH, respectively. Sample preparations, experiments, and analysis were performed under ambient conditions. Batch sorption was investigated at room temperature (298 K) whereas the TRLFS measurements were run at 153 K, and at 274 K for validation. 2.3. Leaching experiments: batch and flow-through The leaching of the minerals was investigated in batch and flow-through experiments to monitor the difference of the dissolution behavior under static conditions (batch) and in the presence of fresh solution under dynamic conditions (flow-through). Batch experiments were performed with 20 g of the mineral suspended in 400 mL 0.01 mol L
1 NaClO4 (equals a SLR of 1/20 g mL
1) in a 500 mL polypropylene bottle. The samples were prepared as duplicates at pH 7 and stored in an overhead shaker for three months. Once a week a sample of 3 mL of the solution was centrifuged at 4000g for 30 min (Sigma 3K18, Laboratory Centrifuges) and the supernatant was analyzed by ICP-MS and AAS for Si, Al, K, Mg, Ca, Fe, Ni, Sr, Ba, Nd, Eu, Gd, Dy, and U. Flow-through experiments were performed at pH 7 (muscovite) and pH 6 (orthoclase) with 0.01 mol L
1 NaClO4 as background electrolyte. The mineral 0.5 g, placed in the reactor (flow rate 0.3 mL/min, volume of 46 mL), was continuously contacted by a fresh 0.01 mol L
1 NaClO4 solution and mixed with a stirrer. The output solution was sampled for analysis at different time steps. The temperature, pH, and conductivity were continuously monitored. 2.4. Sorption experiments Orthoclase and muscovite sorption experiments were performed as triplicates in 0.01 mol L
1 NaClO4 in the pH range from 5 to 8. After achieving stable pH values (pH ± 0.1) for the SLRs 1/20, and 1/80 g/mL U(VI) was added to get a final uranium concentration in solution of 10
5 mol L
1 and 10
6 mol L
1. SLRs were chosen from preliminary experiments shown in Fig. S1 (Supplementary material). The conditioning of the minerals is a long-term process which took up to three months. After a sorption time of one week (selected according to a preliminary experiment see Fig. S2 in supplementary material), the

146

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157

supernatant was centrifuged (1 h at 6800g – Avanti J-20XP, Beckman Coulter) and analyzed with ICP-MS. Furthermore, the influence of Ca2+ as competing cation on the sorption of U(VI) was investigated. Therefore, batch samples from pH 5–8 at an SLR of 1/20 g/mL were prepared. After reaching stable pH values, a Ca(ClO4)2 stock solution was added to reach a final concentration for Ca2+ of 1.5  10
3 mol L
1 which sorbed for one week. Subsequently, 10
6 mol L
1 U(VI) was added and sorbed for an additional week, following which the samples were centrifuged and the supernatant was analyzed by ICP-MS for uranium and by AAS for calcium. The experimental error from the ICP-MS analysis were assumed as 5% for uranium concentrations of 10
5 mol L
1 and 7.5% for 10
6 mol L
1 following Arnold et al. (2001). 2.5. TRLFS measurements For molecular information of the sorption process, TRLFS was applied. Batch samples of the minerals (grain size <63 lm, SLR of 1/20 g/mL) were prepared, and pH values from 4 to 9 in 0.5 step size were adjusted. After equilibration, U(VI) was added to an initial concentration of 10
5 mol L
1. For each experiment, two kinds of TRLFS samples were prepared. First, the aqueous U(VI) species were investigated in the supernatant in a quartz glass cuvette at 153 K. Second, speciation of the mineral suspension was measured after centrifugation and twice washed with 0.01 M NaClO4. The slurry was filled into quartz glass tubes of 4 cm length and 4.35 ± 0.2 mm in outer diameter and a wall thickness of 1 ± 0.2 mm (Ilmasil PN, QSIL GmbH, Ilmenau, Germany) and was attached to a specifically designed sample holder. As described in Steudtner et al. (2011), these samples were cooled down to 153 K by using a cryogenic cooling system (mod. TG-KKK, KGWIsotherm, Karlsruhe, Germany). The laser system used to determine all U(VI) luminescence spectra is described in detail in Steudtner et al. (2011). The fourth harmonic of the Nd:YAG laser system (266 nm, Inlite laser system, Continuum, Santa Clara, CA, USA), was used. The gate width was set to 0.5 ms. Time-resolved spectra were recorded by measuring at dynamic delay times ti using a formula (ti = 0.1 + (i4)/700) where i stands for the number of steps in series. This ensures equal variance of fast and slow decays in the data. An average of 50 measurements was used for each delay time. Furthermore, the influence of Ca2+ on the sorption of U(VI) was studied by TRLFS. Therefore, batch sorption samples with 1.5  10
3 mol L
1 Ca2+ and 10
5 mol L
1 U(VI) in contact with orthoclase (grain size <63 lm SLR of 1/20 g/mL) at pH 4–9 were prepared and processed in the same way as the samples without Ca2+. Only the validation of the Ca2UO2(CO3)3 (aq) complex was done in addition at 274 K. Baseline corrected TRLFS data were deconvoluted with Parallel factor analysis (PARAFAC) from the N-way toolbox for Matlab (Andersson and Bro, 2000) as previously described by Drobot et al. (2015). Monoexponential (for individual luminescence decays) and nonnegative (emission spectra and distribution) constraints were used.

2.6. ATR FT-IR spectroscopy measurements For complementary molecular identification of U(VI) species on orthoclase and muscovite surfaces, ATR FT-IR spectroscopy was applied. Infrared spectra were measured from 1800 to 800 cm
1 on a Bruker Vertex 80/v vacuum spectrometer equipped with a Mercury Cadmium Telluride detector. Spectral resolution was 4 cm
1, and spectra were averaged over 256 scans. A horizontal diamond crystal with nine internal reflections (DURA SamplIR II, Smiths Inc.) was used. Further details on the experimental ATR FT-IR spectroscopy setup are compiled in Mu¨ller et al. (2012). The performance of in situ sorption experiments requires a thin mineral film prepared directly on the surface of the ATR diamond crystal as stationary phase. This was accomplished by pipetting aliquots of 5 lL of 2.5 g L
1 suspensions of orthoclase and muscovite on the crystal and subsequent drying with a gentle stream of N2. This procedure was repeated for an average mass density per area of 0.08 mg cm
2. The mineral film was conditioned by flushing with the blank solution (0.01 mol L
1 NaCl, pH 6) for 60 min using a flow cell (V = 200 lL) at a rate of 100 lL min
1. Subsequently, the sorption reactions were induced by rinsing the mineral film with 20 lmol L
1 U (VI) solution (0.01 mol L
1 NaCl, pH 6) for 90 min. Finally, the loaded mineral phase was flushed again with the blank solution (30 min) in order to gain more information on the reversibility of the sorbed species. NaCl was used as background electrolyte for the IR spectroscopic work because it shows no absorption throughout the frequency range of interest. The background electrolyte should not significantly affect uranium surface speciation since the samples were prepared at pH 6 where chloride is not a strong complexant of uranium (Mu¨ller et al., 2008). The principle of reaction-induced difference spectroscopy was applied for the detection of very small absorption changes provoked by the sorption process in comparison to the very strong absorbing background, i.e., water, mineral film. Further details on the calculation of difference spectra are given in Mu¨ller et al. (2012, 2013). 2.7. Speciation modeling and thermodynamic database The speciation of U(VI) in 0.01 mol L
1 NaClO4 as well as in mineral supernatants was calculated using the geochemical speciation code PhreeqC version 3.1.7-9213 (Parkhurst and Appelo, 2013). The thermodynamic database used was project-specific, mainly built on the Nagra/ PSI database (Hummel et al., 2002). Concerning uranium it was updated with thermodynamic data from the respective volume of the OECD/NEA Thermochemical Database (Guillaumont et al., 2003). For some minerals not contained in these databases, solubility data was taken from the original literature (see Supplementary material for an extensive discussion). In the case of orthoclase, however, no original experimental solubility constants were accessible. Thus, microcline, the triclinic form of K-feldspar, was used as an analogue. It is chemically very similar to orthoclase (the monoclinic version of

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157

147

free uranyl(VI) ion which works as ion exchanger dominates at acidic pH values (64). The 2pK model has been selected concerning the surface protolysis. Generally, the protolysis constants (pK1/pK2) and site density (SSD) were taken from studies of muscovite and albite described in Arnold et al. (2000) and Arnold et al. (2001). The pK-values were corrected to infinite dilution using Davies equation (Davies, 1962). The values (SSA, SSD, pK1, pK2) taken for the fitting procedure were given in Table 4. The SSA was determined experimentally by BET-measurements, see Section 3.1. In a preliminary step, all experimental data from the Cafree batch sorption experiments were fit to either the
„XOAUO+ 2 or „XOAUO2CO3 surface complex to derive robust starting values for the fit of the combined chemical model comprising of both surface complexes. A weighted residuals model was used to fit the data. Fit quality was judged using the correlation coefficient R. Any fits to more complex chemical models also including further surface species such as „XOAUO2(OH) or „XOA(UO2)3(OH)5 were not successful. Species with two or more carbonate units were not considered as the experimental conditions in the present work would not allow their formation. Finally, the obtained log K values were used for predictive modeling of the experimental data of the system containing calcium.

K-feldspar), and thus its values can be used as approximation for orthoclase (Hochleitner, 1996). Namely, the solubility of microcline with log Ksp =
0.12 was taken from Stefa´nsson and Arno´rsson (2000). Log Ksp calculations based on Gibbs free energies of formation confirm that the value is in the expected dimension. For muscovite, only calculations based on formation data are possible. Averaging the respective solubility constants yields a log Ksp = 14.15 ± 0.74 (the error representing 2r). Finally, complex formation constants for the aqueous complexes Ca2UO2(CO3)3 (aq) (log K =
0.92) and CaUO2(CO3)2
(log K =
3.80) as first described by 3 Bernhard et al. (1996) were taken from the THEREDA database, release 9 (www.thereda.de). 2.8. Surface complexation modeling The pH-dependent U(VI) sorption onto orthoclase and muscovite and the estimation of the surface complexation parameters were performed with the geochemical speciation code PhreeqC (details see Section 2.7) coupled with the parameter estimation code UCODE, version 1.024 (Poeter et al., 2014). The project-specific thermodynamic database (see Section 2.7) was used for all reactions in aqueous solution and for precipitation/dissolution. The Diffuse Double Layer Model (DDLM) described by Dzombak (1990) was applied. Ion exchange processes are not relevant in the investigated pH range (5–8), since the

3. RESULTS 3.1. Mineral characterization

Table 1 Mineral composition determined by ICP-MS (error: ±10%) and AAS (error: ±2%) after complete digestion. Compound

Method

Orthoclase wt.%

Muscovite wt.%

SiO2 Al2O3 K2O Fe2O3 Na2O CaO

ICP-MS ICP-MS AAS ICP-MS AAS AAS

62.24 ± 6.22 17.13 ± 1.71 10.33 ± 0.20 0.07 ± 0.01 3.53 ± 0.07 1.20 ± 0.02

39.68 ± 3.97 23.24 ± 2.32 8.59 ± 0.17 4.47 ± 0.45 0.39 ± 0.01 0.66 ± 0.01

The minerals with a grain size of 63–200 lm (used for the sorption & leaching experiments) showed a rather small SSA: 0.66 ± 0.01 m2 g
1 in the case of muscovite and 0.083 ± 0.003 m2 g
1 in the case of orthoclase. As expected, the grain size fraction <63 lm (used for TRLFS) exhibited larger values for SSA of 1.8 ± 0.06 m2 g
1 and 0.25 ± 0.01 m2 g
1 for muscovite and orthoclase, respectively. Determination of the zeta potential indicated that both minerals possess a negative surface charge in the studied

t [d] 20

40

60

100 0

20

40

60

80

flow-through batch

flow-through batch

4.0x10-4

Si (mol L-1)

80

100 5.0x10-4 4.0x10-4

3.0x10-4

3.0x10-4

2.0x10-4

2.0x10-4

1.0x10-4

1.0x10-4

0.0 0

20

40

60

80

100

0

50

100

150

-1

0

Si (mol L )

5.0x10-4

0.0 200

t [h] Fig. 1. Si concentrations leached from orthoclase (left) and muscovite (right) by flow-through experiments (shown as black squares) and batch experiments (shown as red triangle). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

148

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157

pH range. When U(VI) sorbs onto the minerals, the zeta potential is shifted to less negative or in some cases even to positive values. The highest shift was observed in the pH range from 6 to 7 corresponding to the highest amount of U(VI) sorption expected at these values (see Fig. S3 in supplementary material). The main composition of the minerals is given in Table 1. In addition, U contents of 3.2 mg kg
1 and 0.26 mg kg
1 in orthoclase and muscovite, respectively, were determined by ICP-MS. Therefore, the leached uranium content was also monitored, but turned out to be not critical, i.e. well below the experimental values (see Table S1). 3.2. Leaching experiments: batch and flow-through Leaching experiments of orthoclase and muscovite were performed to investigate their dissolution behavior. The leached elements with their corresponding concentrations are shown in Table S1 for the final measurement of the batch and flow-through experiments. The dissolution of Si from orthoclase and muscovite over time in batch and flow-through experiment is exemplarily shown in Fig. 1. These results indicate that even after three months the dissolution has not reached a steady state. More details to the leaching of orthoclase and muscovite are given in the Supplementary material. In the case of orthoclase, the molar ratio of K:Al:Si obtained after the batch dissolution experiment was 118:2.2:340, which translates into 1:0.02:2.88. Moreover, the pH increased by 1.0 pH unit during the equilibration time. Thus, the first dissolution reaction as proposed by Okrusch and Matthes (2005) can be ruled out. Conversely, the leaching-based analyses indicated a saturation index close to 1 for quartz and amorphous Al(OH)3. The other solids (i.e. kaolinite, muscovite, and boehmite, see Table S3) assumed to be products of orthoclase dissolution were strongly supersaturated and probably did not form. Thus, the following dissolution reaction is proposed: KAlSi3 O8 þ Hþ þ H2 O ! Kþ þ AlðOHÞ3 ðamÞ þ 3 SiO2 Concerning muscovite, the molar ratio of K:Al:Si obtained after the batch dissolution experiment was 269:4.91:233, which equals 1:0.02:0.87. Moreover, the pH increased by 1.8 pH unit during the equilibration time. Geochemical speciation calculations based on the leachate analyses indicated (as for orthoclase) that the solution in

contact with amorphous quartz and gibbsite phases had reached equilibrium. This eventually leads to the following dissolution reaction: KAl3 Si3 O10 ðOHÞ2 þ Hþ þ 3 H2 O ! Kþ þ 3 AlðOHÞ3 ðamÞ þ 3 SiO2

Assuming an incongruent dissolution for the two minerals with the formation of amorphous silica gel and gibbsite phases, the theoretically derived equilibrium concentrations for K+ and Al3+ coincide within less than one order of magnitude with the experimental findings from the batch leaching experiments (cf Table 2 with Table S1 from the supplementary material). This is expected since the steady-state of dissolution was not reached. Moreover, the analytical Si concentrations are between the values corresponding to pure crystalline quartz (log Ksp taken from Helgeson et al. (1978)) and the one obtained by assuming an amorphous phase as used by Lindsay and Walthall (1996). 3.3. Aqueous U(VI) speciation The aqueous speciation of 10
5 mol L
1 U(VI) in 0.01 mol L
1 NaClO4 in the absence and presence of 1.5  10
3 mol L
1 Ca2+ was calculated with PhreeqC taking into account also the elements leached from either orthoclase or muscovite. The respective speciation patterns were very similar for both minerals, thus only the speciation of orthoclase is shown in Fig. 2. At acidic pH values, the free uranyl(VI) ion dominates the speciation in all calculations. At pH values 3 6 7 the complex UO2OH+ (and to a lesser degree (UO2)2(OH)2+ 2 ) is present. The species (UO2)3(OH)+ 5 is predominant in the pH range from pH 5.5 to 6.25. At higher pH values the carbonate complexes ((UO2)2CO3(OH)
UO2(CO3)2
Ca2UO2(CO3)3 (aq), 3; 2 ; and UO2(CO3)4
) are present. At 1.5  10
3 mol L
1 3 Ca2+, UO2(CO3)2
is suppressed by CaUO2(CO3)2
2 3 and Ca2UO2(CO3)3 (aq) dominating between pH 7.75 and 9.75. To verify the calculated U(VI) species distribution in the absence and in the presence of Ca2+, respective TRLFS measurements were performed at 153 K. Additionally, the Ca2UO2(CO3)3 (aq) complex was validated at 274 K. This was done because the quenching of the U(VI) luminescence by carbonate (Balzani et al., 1978) restrains the observation of the luminescence spectra for (UO2)2CO3(OH)
3, 4
UO2(CO3)2
2 , and UO2(CO3)3 at room temperature, while only Ca2UO2(CO3)3 (aq) is still detectable (Bernhard et al.,

Table 2 Elemental concentration (lmol L
1) after congruent or incongruent (formation of SiO2 and amorphous gibbsite) dissolution of orthoclase and muscovite in 0.01 mol L
1 NaClO4 calculated with PhreeqC. Orthoclase

Al K Si pH a

Muscovite

Congruent dissolution

With secondary phase formation

Congruent dissolution

With secondary phase formation

14.16 14.16 42.48 7.17

0.74 22 100–1936a 6.92

2.22 0.74 2.22 6.96

0.54 35 100–1935a 6.72

The lower value corresponds to crystalline quartz, the higher one to amorphous silica gel.

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157 100

UO22+ UO2OH+

80

80

UO2SiO(OH)3+ 2+ 2 + 5

(UO2)2(OH)

60

60

(UO2)3(OH) 40

(UO2)2CO3(OH)3

20

UO2(CO3)22-

-

40

UO2(CO3)34-

20

Ca2UO2(CO3)3 23 3

CaUO2(CO )

0 3

4

5

6

7

8

9

10

3

4

5

6

pH

Fig. 2. Aquatic U(VI) speciation of orthoclase supernatant with 10 and with 1.5  10
3 mol L
1 Ca2+ (right).

1996). Thus, in the solutions without Ca2+, no U(VI) luminescence signals were detected at higher pH values. Whereas in the presence of Ca2+, a luminescence signal can be observed indicating the presence of Ca2UO2(CO3)3 (aq). Its deconvoluted steady state emission spectrum measured with TRLFS in the presence of Ca2+ is shown in Fig. 3. Six emission bands were identified at 464, 483, 503, 525, 550, and 577 nm. These band positions are in very good agreement with the literature values for the Ca2UO2(CO3)3 (aq) complex (Krawczyk-Ba¨rsch et al., 2012; Schmeide et al., 2014). The respective luminescence lifetime calculation indicated the presence of one single species with a lifetime of 80.3 ± 3.0 ns. 3.4. U(VI) batch sorption 3.4.1. Influence of U(VI) concentration and SLR on the sorption Sorption of U(VI) onto both orthoclase and muscovite yields typical sorption trends with a maximum sorption in the neutral pH range (see Figs. 4 and 5). As expected, decreasing SLR from 1/20 to 1/80 g/mL reduced the sorption slightly due to the reduction of available binding sites on the mineral surfaces. Depending on the concentration of the sorbed U(VI) the maximum sorption for orthoclase (SLR 1/20) was around 70% at 10
5 mol L
1 U(VI) and

Intensity (a.u.)

0.10

0.05

460

480

500

520

7

8

9

0 10

pH
5

0.00 440

U(VI) species (%)

U(VI) species (%)

100

149

540

560

580

600

Wavelength (nm) Fig. 3. Deconvoluted luminescence spectrum of aqueous U(VI) in the presence of Ca2+ by PARAFAC analysis. (10
5 mol L
1 U(VI), 1.5  10
3 mol L
1 Ca2+, 0.01 mol L
1 NaClO4, T = 274 K).


1

mol L


1

U(VI) in 0.01 mol L

NaClO4 without addition of Ca2+ (left)

around 85% at 10
6 mol L
1 U(VI). For muscovite (SLR 1/20), the sorption reached around 85% and 90% at a U (VI) concentration of 10
5 mol L
1 and 10
6 mol L
1, respectively. 3.4.2. Influence of Ca2+ as competing cation upon the sorption of U(VI) Sorption of 10
6 mol L
1 U(VI) onto orthoclase and muscovite in the absence and presence of 1.5  10
3 mol L
1 Ca2+ yielded for both cases typical sorption curves except at pH 8 where a difference in the sorption is observed (see Fig. 6). At this pH value, the presence of Ca2+ reduces the U(VI) sorption strongly. For orthoclase, the sorption dropped at pH 8 from approximately 25% to almost no sorption, and muscovite also showed a decrease of sorption from around 65% to less than 30%. 3.5. U(VI) surface speciation 3.5.1. TRLFS under cryogenic conditions TRLFS measurements were performed at 153 K to identify the surface speciation of U(VI) on orthoclase and muscovite in the absence and presence of Ca2+. The obtained TRLFS data (0.01 mol L
1 NaClO4, pH 4–9, grain size 663 lm, 10
5 mol L
1 U(VI), 1.5  10
3 mol L
1 Ca2+) were analyzed using PARAFAC. The samples with muscovite showed no signal for U(VI). This might be due to a high iron content (4.47 wt.% compared to 0.07 wt.% in orthoclase) in the sample being responsible for quenching (Taha and Morawetz, 1971). In the orthoclase system four luminescence U(VI) species (A ? D) could be detected from PARAFAC analysis. Luminescence lifetimes s as well as the emission bands are given in Table 3. The comparison of our observed emissions bands with the band positions of dominant aqueous U(VI) species relevant under the chosen conditions (Bernhard et al., 2001; Go¨tz et al., 2011; Drobot et al., 2015) excluded the presence of aquatic U(VI) species. For the sorption of U(VI) onto orthoclase in the absence of Ca2+, two species (A, B) were determined whereas in the presence of Ca2+ species (C, D) were observed. The normalized intensity profiles of these species are shown in Fig. 7 and their emission spectra as well as the ones of free uranyl(VI) (Billard et al., 2003) are given in Fig. 8. In the acidic pH region, species A is the most prominent, whereas species

150

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157 100

100

SLR 1/20 1/80

U(VI) sorption (%)

80

90 80

70

70

60

60

50

50

40

40

30

30

20

U(VI) sorption (%)

90

20

10

-6

-1

-5

10

-1

10 mol L U(VI)

10 mol L U(VI)

0

0

5.0

5.5

6.0

6.5

7.0

7.5

8.0

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

pH

pH

Fig. 4. Sorption onto orthoclase at 10
6 M U(VI) (left) and 10
5 mol L
1 U(VI) (right) at SLRs of 1/20 g/mL and 1/80 g/mL.

U(VI) sorption (%)

100

SLR 1/20 1/80

90 80

90 80

70

70

60

60

50

50

40

40

30

30

U(VI) sorption (%)

100

20

20 10

-6

-1

-5

10 mol L U(VI)

10 mol L U(VI)

0 5.0

5.5

6.0

6.5

7.0

10

-1

7.5

8.0

5.0

5.5

6.0

pH

6.5

7.0

0 7.5

8.0

pH

Fig. 5. Sorption onto muscovite at 10
6 mol L
1 U(VI) (left) and 10
5 mol L
1 U(VI) (right) at SLRs of 1/20 g/mL and 1/80 g/mL.

1.5 10-3 mol L-1 Ca2+ no Ca2+

100

90

90

80

80

70

70

60

60

50

50

40

40

30

30

20

20

10

orthoclase

0 5.0

5.5

6.0

6.5

7.0

10

muscovite 7.5

8.0

pH

5.0

5.5

6.0

6.5

7.0

U(VI) sorption (%)

U(VI) sorption (%)

100

0 7.5

8.0

pH

Fig. 6. Effect of Ca2+ on the sorption of 10
6 mol L
1 U(VI) onto orthoclase (left) and muscovite (right) at a SLR of 1/20 g/mL and ambient atmosphere.

B dominates at pH values P6 and started to form at pH 5. TRLFS measurements of U(VI) sorbed on orthoclase in the presence of Ca2+ showed again a species that is dominant at lower pH values (C) and one that is more common over the whole pH range (D). The main difference (gray area in Fig. 7) between the speciation on the surface of orthoclase in the absence and

presence of Ca2+ is the suppression of species B when Ca2+ is part of the system. 3.5.2. In situ ATR FT-IR spectroscopy ATR FT-IR spectroscopy is a very useful tool for the investigation of reactions of heavy metals at the mineral– water interface (Lefe`vre, 2004). The sensitivity of the UO2

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157

151

Table 3 Luminescence decay lifetimes s and main emission bands (peak maxima positions, rounded to full nm) for the species obtained from a series of TRLFS data of U(VI) sorbed on orthoclase compared to literature values. PARAFAC species or Reference

Surface species

Emission bands (nm)

s (ls)

T (K)

A; C B D

„SiO2UO02 „SiO2UO2CO
3 „SiO2UO2OH


490–494; 511–515; 535–538; 561–565 479; 500; 521; 544; 570 481; 502; 523; 546; 573

328.25 ± 23.75 644 ± 6.6 334.7 ± 3.1

153 153 153

Ilton et al. (2012) Gabriel et al. (2001) Drot et al. (2007) Ilton et al. (2012) Gabriel et al. (2001) Drot et al. (2007) Krepelova (2007) Baumann et al. (2005) Froideval et al. (2006)

„SiO2UO02

494–496; 514–516 – – 501–503; 522–526 – 506; 528 487; 501; 520; 542; 567 480; 497; 519; 542; 564; 586 499; 519

– 170 ± 25 180 ± 20 – 360 ± 50 400 ± 30 42.5 ± 3.3 5.6 ± 1.6 106–141 233–247

4 RT

523.0; 542.3; 561.3 480; 499; 520; 542; 566 481; 500; 521; 544 480; 499; 520; 543; 566

144 883 887 820.4 ± 11.8

(UO2)x(OH)y(2x
y)+ on SiO2 UO2+ 2 on Kaolinite (UO2)x(OH)y(2x
y)+ on Al(OH)3 UO2+ 2 on Al(OH)3

(UO2)2(OH)3CO
3 UO2(CO3)4
3

Table 4 Recommended surface complexation parameters for orthoclase and muscovite in the absence of Ca2+. SSA [m2 g
1] SSD [sites nm
2] pK1 pK2 log K „XOAUO+ 2 log K „XOAUO2CO
3

Orthoclase

Muscovite

0.083 3.1 6.47
7.85 1.69 ± 0.58 8.96 ± 0.62

0.66 2.61 6.01
7.86 0.41 ± 0.31 8.71 ± 0.12

1.0

Normalized Intensity

Aquatic species Wang et al. (2004) Wang et al. (2004) Go¨tz et al. (2011) Steudtner et al. (2011)

UO2+ 2 on SiO2 „SiO2UO2OH


4 RT RT RT RT

4 6 153 153

A B

(a)

0.8 0.6 0.4 0.2 0.0 4

5

6

7

8

9

pH 1.0

Normalized Intensity

antisymmetric stretching mode m3, occurring at 961 cm
1 for the fully hydrated UO2+ 2 , to changes in the coordination environment of the cation has previously been shown on different mineral oxide surfaces (Mu¨ller et al., 2012, 2013; Schmeide et al., 2014). The evaluation of the U(VI) sorption process on orthoclase and muscovite requires information on the distribution of the metal ion species in solution and on the surface. In the ATR FT-IR spectrum of an aqueous 20 lmol L
1 U(VI) solution in 0.1 mol L
1 NaCl at pH 5.5, the m3 mode of the U(VI) moiety is observed at 923 cm
1 (Fig. S4, green trace) (Mu¨ller et al., 2008). This spectrum of U(VI) hydrolysis serves as a reference for the elucidation of the surface complex. Coordination of aqueous UO2+ 2 ions on surfaces generally reduces the force constants of the O@U@O bonds. Thus, a displacement of water molecules from the first shell lowers the frequency of the m3(UO2) stretching mode. The extent of this shift is found to be correlated with the type of surface bonding and coordination. Whereas chemical bonding, namely the formation of inner-sphere sorption complexes results in a considerable red-shift of m3(UO2), physical interaction, i.e. outer-sphere complexation, reveals only very small m3(UO2) shifts because of the remaining intact first hydration sphere (Lefe`vre, 2004).

C D

(b)

0.8 0.6 0.4 0.2 0.0 4

5

6

7

8

9

pH Fig. 7. Normalized intensity profiles of 10
5 mol L
1 U(VI) sorbed onto orthoclase in 0.01 mol L
1 NaClO4 in the absence of Ca2+ referred to the amount of sorbed uranium (a) and in the presence of 1.5  10
3 mol L
1 Ca2+ (b). The capital letters in the plots indicate the species determined with PARAFAC and the gray region represents the area with most differences.

The course of in situ U(VI) sorption experiments onto orthoclase (I) and muscovite (II), namely time-resolved infrared spectra obtained during conditioning, sorption, and flushing steps, is illustrated in Fig. 9.

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157

process seems to be confirmed by the band at 982 cm
1, which is absent in the spectrum of the aqueous solution and can be assigned to surface modes that undergo significant alterations during the sorption reactions (Fig. 9). Following sorption, the U(VI) loaded mineral films were flushed again with blank solution. The difference spectrum obtained (Fig. 9, blue trace) shows negative bands at equal frequencies to those observed during the sorption stage.

D C

(b)

(a)

B A

4. DISCUSSION 4.1. Leaching experiments

free UO22+

450

475

500

525

550

575

600

Wavelength (nm) Fig. 8. Band position of the luminescence signals of the species A ? D (of 10
5 mol L
1 U(VI) sorbed onto orthoclase in 0.01 mol L
1 NaClO4 in the absence of Ca2+ referred to the amount of sorbed uranium (a) and in the presence of 1.5  10
3 mol L
1 Ca2+ (b)) compared to the band positions of free uranyl (Billard et al., 2003).

The spectra of the conditioning (Fig. 9, red traces) show no significant bands indicating stable stationary phases of orthoclase (I) and muscovite (II), which is indispensable for the detection of sorbed species during the following sorption process. The ATR FT-IR difference spectra calculated between the conditioning and at several time steps after induction of U(VI) sorption are shown in Fig. 9 (black traces). These spectra exhibit the same absorption bands with maxima at 982 and 917 cm
1. The intensities of the bands increase with time of sorption reflecting U(VI) accumulation on the mineral surfaces. The forming U(VI) surface structures seem to be very similar. A comparison with the aqueous solution reference spectrum (Fig. S4, green trace) reveals a very small shift of the m3 mode of the U(VI) moiety by 6 cm
1 occurring due to U(VI) sorption. The sorption

At the beginning of the flow-through experiment, the solution reaching the mineral surface preferentially leached weak bound atoms. Here, because pure material was used, mainly sorbed components (e.g., from the technologic process of mineral preparation) have to be considered, but in the case of mica ions from the interlayer are also exchanged. This is a rather fast process and leads to comparatively high concentrations. Afterwards, the leaching of the surface only concerns the remaining rather stable bound atoms of the crystal lattice, thus the kinetics are slower and the concentration drops significantly. The observed dissolution of Al to a lesser extent than Si and K shows that Al is strongly bound within the lattice structure and other elements (such as Ca and Mg and to a lesser degree Ni, Sr, Ba, and U) are more exchangeable. Moreover, the two types of experiments imply different kinetics, because the concentration gradient in flow-through mode is always steeper (due to the fresh solution influx) than in batch mode. Both methods indeed show different kinetics for the described process. The comparison of the two minerals showed that the amount of K dissolved from muscovite was nearly twice as high as from orthoclase, and more Mg also dissolved from muscovite. This is due to the release of these elements from the interlayer of muscovite where these elements can be located. Conversely, Ca2+ dissolved in the batch experiments from both minerals at the same order of magnitude, although no dissolution of Ca2+ was measured for muscovite at the end of the flow-through

0.001

Normalized luminescence intensity (a.u.)

152

II

Sorption

917

982

time / min 30

917

I 982

Absorption / a.u.

Flushing

90 60 30 20 10 5

Condition. 30 1200

1100

1000

900 −1

Wavenumber / cm

800

1200

1100

1000

900

800

−1

Wavenumber / cm

Fig. 9. In situ time-resolved ATR FT-IR spectra of U(VI) sorption on orthoclase (I) and muscovite (II) (20 lmol L
1 U(VI), 0.01 mol L
1 NaCl, pH 6, air, 0.08 mg cm
2). The spectra of the conditioning, sorption and flushing process are recorded at different times as given (from bottom to top). For more details, the reader is referred to the text. Indicated values are in cm
1.

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157

experiment. Therefore, all exchangeable Ca2+ had prior already been leached, as muscovite contains less Ca2+ than orthoclase (see Table 1). The orthoclase batch leaching leads to 10
8 mol L
1 U in solution whereas U was not detected in muscovite. Interestingly, the flow-through experiments indicated a U release, but an order of magnitude lower than in the flow-through experiments for orthoclase. This is in disagreement to the digestion of the minerals and cannot be explained. 4.2. Aquatic speciation and sorption The TRLFS measurement of the U(VI) speciation as a function of the pH confirmed the speciation calculated with PhreeqC. In the absence of Ca2+, carbonate complexes exist at this pH range (4–9) that are not detectable by conventional TRLFS at 274 K, and consequently no TRLFS signal was observed at this temperature. In contrast, when Ca2+ was added to the system the luminescence emitting Ca2UO2(CO3)3 (aq) complex was observed. The sorption curves of U(VI) onto muscovite and orthoclase are as expected for metal cation sorption onto aluminosilicates. The mineral surface carries a negative charge, as do the U(VI) species in solution at higher pH, where carbonates dominate. Consequently U(VI) sorption decreases at pH above 6.5 for orthoclase and 7 for muscovite. In general, the relative sorption is lower at higher U(VI) concentrations due to increasing competition of the aqueous U(VI) species for the same amount of sites at the mineral surface. The observed decrease of uranium sorption at pH 8 in the presence of Ca2+ exists because of the formation of the Ca2UO2(CO3)3 (aq) complex as observed in the aquatic speciation. This complex shows only a weak tendency to sorb on the mineral surface. In addition, Ca2+ could also sorb on the surface and thus reduce the available binding sites for U(VI). These two effects combined inhibit U(VI) sorption onto both muscovite and orthoclase. This is in accordance to results for U(VI) sorption onto Hanford Subsurface Sediment, Opalinus Clay, ferrihydrite, and quartz (Dong et al., 2005; Fox et al., 2006; Joseph et al., 2013). 4.3. Speciation at the mineral surface Experimentally obtained spectroscopic fingerprints, i.e. major peak maxima and luminescence decay constants, were compared to values published previously and summarized in Table 3. Therefore, three mineral groups were considered: aluminosilicates, SiO2 polymorphs, and Al (hydr) oxides. With respect to alumosilicates, direct spectroscopic results are published for uranyl sorption on montmorillonite (Chisholm-Brause et al., 1994; Kowal-Fouchard et al., 2004), albite (Walter et al., 2005) or muscovite (Arnold et al., 2006). With respect to the interactions of uranyl units with silanol surface groups, there are few papers related to respective TRLFS investigations on SiO2 samples (Glinka et al., 1998; Gabriel et al., 2001; Brendler et al., 2004). Detailed spectroscopic information about UO2+ 2 bound onto aluminol groups is available from

153

various publications (Baumann et al., 2005; Wang et al., 2005; Chang et al., 2006; Froideval et al., 2006). However, the fluorescence parameters derived from these works differ significantly or are not separately given at all. Four surface species were obtained with PARAFAC for U(VI) sorption on orthoclase. A shift of the main emission band (compared to the free uranyl) to smaller wavelength indicates a carbonate species (Wang et al., 2004). Within Fig. 8 it is obvious that species B is the only one showing a shift in this direction and thus is referred to as uranium–carbonate surface species. In contrast a shift to higher wavelength indicates a surface complexation of pure uranium or uranium hydroxide (Froideval et al., 2006; Drot et al., 2007; Ilton et al., 2012). Species A and C showed comparable positions of their emission bands indicating them to be the same surface complex. At lower pH values, the peak positions indicated a „SiO2AUO02 surface complex being in accordance with the results for SiO2. This surface complex was also found to be the dominating species for the sorption of U(VI) onto pure albite (Walter et al., 2005). Species D in the presence of Ca2+ is comparable to the „SiO2AUO2OH
surface complex described by Ilton et al. (2012). Thus, the binding on the surface of orthoclase tends to be through the SiO2 sites of this mineral. Comparing the results of the emission spectrum of species B with the peak position of an aqueous U(VI) carbonate species (Wang et al., 2004), the species B most probably is a ternary U(VI) carbonate surface complex, like „XOAUO2CO
3 postulated by Davis and Kohler (2001) onto quartz or „SiO2AUO2OHCO3
3 postulated by Gabriel et al. (2001). The absence of species B when Ca2+ was present in the system is due to the formation of Ca2UO2(CO3)3 (aq) in the solution that traps almost all the uranium preventing any significant formation of a U (VI) carbonate surface species. This results in the drop of sorption at pH 8 observed in the batch experiments when Ca2+ is present. The U(VI) surface structures observed by ATR FT-IR spectroscopy showed only a small shift of the m3 mode. Hence, similar complex structures in solution and at the interface, i.e., the formation of outer-sphere complexes, can be assumed. The assignment of m3 peak positions is in accordance with the findings from a previous study of U(VI) sorption onto anatase (TiO2). Here, two different U(VI) surface species could be identified, one innersphere complex with m3 at 895 cm
1 occurring at low coverage and one outer-sphere complex with m3 at 917 cm
1 dominating the U(VI) speciation at high surface coverage (Mu¨ller et al., 2012). At higher frequencies, the slight appearance of intrinsic bands at 1525 and 1460 cm
1 may provide further evidence of the presence of the hydroxide complex from solution physically interacting with the surface (Fig. S4). The negative values of the difference spectrum obtained by flushing can only represent desorbed uranium complexes released from the orthoclase and muscovite film by the flushing. This suggests that the sorbed U(VI) species was only weakly bound to the surface and can be easily released. This is in accordance with previous observations of outersphere sorption complexes (Jordan et al., 2011; Mu¨ller et al., 2012).

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157

U(VI) sorption (%)

100

100

measurement prediction

80

80

60

60

40

40

20

20

orthoclase

U(VI) sorption (%)

154

muscovite

0

0 5.0

5.5

6.0

6.5

7.0

7.5

8.0

5.0

5.5

6.0

6.5

pH
5

7.0

7.5

8.0

pH


1


3

Fig. 10. Prediction of sorption of 10 mol L U(VI) in the presence of 1.5  10 using the log K values obtained in this work (cf. Table 4).

4.4. SCM fitting The predominant aquatic U(VI) species in the absence of Ca2+, independent of the solid being present, between the investigated pH 5 and pH 8 are the free uranyl-ion at lower pH values, and the hydroxide and carbonate complexes at higher pH values. However, TRLFS results indicated only the simple uranyl surface complex as well as a uranium carbonate complex for orthoclase in the absence of Ca2+. Due to difficulties in distinction whether the sorption takes place at aluminol or silanol surface sites it was decided to use a generic surface species („X). Consequently, the following surface complexation reactions were considered in the present model, additionally assuming that the surface speciation of uranium is similar for orthoclase and muscovite. þ þ BXOH þ UO2þ 2 () BXO-UO2 þ H

ð1Þ

2

þ BXOH þ UO2þ 2 þ CO3 () BXO-UO2 CO3 þ H

ð2Þ

Both surface complexes describe the experimental data quite well (see Supplementary material Figs. S5 and S6) with the exception of the combination for the highest U (VI) concentration and lowest SLR (for orthoclase and to a lesser extent also for muscovite). An explanation for the underestimation of the observed sorption at this special system setup is still missing, unfortunately. The obtained final data sets including the specific surface site parameter and the optimized formation constants log K are provided in Table 4. In general the fitted log K BXOAUOþ2 (1.69 for orthoclase and 0.41 for muscovite, respectively) are in good agreement with published values, e.g. log K = 1.67 for albite (Arnold et al., 2000) and log K =
0.5 for muscovite (Arnold et al., 2001). Concerning uranium carbonate surface complexes, no values for the mineral groups of feldspar and mica are available to compare with. But the fitted log K BXOAUO2 CO
3 for orthoclase and muscovite (8.96 and 8.71, respectively) show a good agreement with values published for quartz with log K = 10.18 (Davis and Kohler, 2001) and kaolinite with log K = 10.85 (Jung et al., 1999).


1

mol L

Ca2+ for orthoclase (left) and muscovite (right)

The predictive modeling for the experiments with Ca2+ as shown in Fig. 10 indicated for orthoclase an overprediction of the sorption. This is in accordance with the spectroscopic results, where the uranium carbonate complex was not observed. Thus, this complex leads to an overestimation of the experimental data. In contrast the predictive modeling of muscovite is in very good agreement with the experimental data. 5. CONCLUSIONS U(VI) sorption onto orthoclase and muscovite as well as the influence of Ca2+ on this sorption process was studied by sorption experiments. Further characterizations of the sorption processes and identification of the respective species were performed with TRLFS and ATR FT-IR spectroscopy measurements. All the prior mentioned investigations were used to fit log K values for the surface complexes of U(VI) onto orthoclase and muscovite. It was demonstrated that the sorption of U(VI) reaches its maximum at circumneutral pH values. In the presence of Ca2+ the U(VI) sorption decreases at pH 8. Speciation calculations and TRLFS measurements show that this is due to the formation of the neutral Ca2UO2(CO3)3 (aq) complex. With ATR FT-IR spectroscopy the sorption onto orthoclase was identified at pH 6 as outer-sphere sorption complex that shows only weak bounding of U(VI) to the surface. TRLFS studies on this sorption of U(VI) onto orthoclase indicate that there are two surface species present. With ATR FT-IR spectroscopy, only one species could be identified, but it cannot be excluded that to a lesser amount other complexes are present. In the presence of Ca2+, the surface complexes were attributed to the species „XOAUO+ 2 and „XOAUO2OH. In the absence of Ca2+, a uranium carbonate complex was identified in addition to the „XOAUO+ 2 surface complex. Moreover, the investigation shows that U(VI) TRLFS and ATR FT-IR spectroscopy are good tools to clarify the sorption onto orthoclase. A coupling of PhreeqC and UCODE was applied to fit log K values for the sorbed surface

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157

complexes (log K BXOAUOþ2 ¼ 1:69 and log K BXOAUO2 CO
3 ¼ 8:96 for orthoclase, log K BXOAUOþ2 ¼ 0:41 and log K BXOAUO2 CO
3 ¼ 8:71 for muscovite). With regard to the U(VI) sorption ability onto orthoclase and muscovite, not only the binary system needs to be understood, but also more complex systems, such as ones that occur in nature where a multitude of components can be present, must be studied. For Ca2+, this work showed that there is an influence on the speciation and thus on the potential transport of U(VI), e.g. by groundwater. In addition, many other elements leached from surrounding minerals or contained in the groundwater may affect the sorption behavior of orthoclase and muscovite. Thus, future work has to focus on more complex systems as well as on the creation of their surface complexation parameters in order to estimate the mobility of U(VI) and serve as a basis for long-term safety analysis. However, this work provides for the first time reasonable data for describing predictive modeling of uranium(VI) sorption onto orthoclase and muscovite. ACKNOWLEDGEMENTS This work was supported by the BMWi (WEIMAR project with grant 02 E 11072B). The authors are grateful to S. Weiß and C. Eckardt for the measurement of zeta potential and BET, to A. Ritter, S. Gurlit and I. Kappler for ICP-MS and AAS measurements, K. Heim for ATR FT-IR measurements, and F. Bok for help with speciation modeling.

APPENDIX A. SUPPLEMENTARY DATA Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.gca.2016.05.045. REFERENCES Ames L. L., McGarrah J. E. and Walker B. A. (1983) Sorption of uranium and radium by biotite, muscovite, and phlogopite. Clays Clay Miner. 31, 343–351. Andersson C. A. and Bro R. (2000) The N-way toolbox for MATLAB. Chemometr. Intell. Lab. Syst. 52, 1–4. Arnold T., Zorn T., Bernhard G. and Nitsche H. (2000) Modeling the Sorption of Uranium(VI) onto Albite Feldspar, Annual Report 1999. Forschungszentrum Rossendorf e.V., Institute of Radiochemistry, Dresden (Germany). Arnold T., Zorn T., Za¨nker H., Bernhard G. and Nitsche H. (2001) Sorption behavior of U(VI) on phyllite: experiments and modeling. J. Contam. Hydrol. 47, 219–231. Arnold T., Utsunomiya S., Geipel G., Ewing R. C., Baumann N. and Brendler V. (2006) Adsorbed U(VI) surface species on muscovite identified by laser fluorescence spectroscopy and transmission electron microscopy. Environ. Sci. Technol. 40, 4646–4652. Balzani V., Bolletta F., Gandolfi M. and Maestri M. (1978) Bimolecular Electron Transfer Reactions of the Excited States of Transition Metal Complexes, Organic Chemistry and Theory. Springer, Berlin Heidelberg, pp. 1–64. Baumann N., Brendler V., Arnold T., Geipel G. and Bernhard G. (2005) Uranyl sorption onto gibbsite studied by time-resolved

155

laser-induced fluorescence spectroscopy (TRLFS). J. Colloid Interface Sci. 290, 318–324. Baumann N., Arnold T. and Lonschinski M. (2012) TRLFS study on the speciation of uranium in seepage water and pore water of heavy metal contaminated soil. J. Radioanal. Nucl. Chem. 291, 673–679. Bernhard G., Geipel G., Brendler V. and Nitsche H. (1996) Speciation of uranium in seepage waters of a mine tailing pile studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Radiochim. Acta 74, 87–91. Bernhard G., Geipel G., Reich T., Brendler V., Amayri S. and Nitsche H. (2001) Uranyl(VI) carbonate complex formation: validation of the Ca2UO2(CO3)3(aq.) species. Radiochim. Acta 89, 511–518. Billard I., Ansoborlo E., Apperson K., Arpigny S., Azenha M. E., Birch D., Bros P., Burrows H. D., Choppin G., Couston L., Dubois V., Fangha¨nel T., Geipel G., Hubert S., Kim J. I., Kimura T., Klenze R., Kronenberg A., Kumke M., Lagarde G., Lamarque G., Lis S., Madic C., Meinrath G., Moulin C., Nagaishi R., Parker D., Plancque G., Scherbaum F., Simoni E., Sinkov S. and Viallesoubranne C. (2003) Aqueous solutions of uranium(VI) as studied by time-resolved emission spectroscopy: a round-robin test. Appl. Spectrosc. 57, 1027–1038. Blume H.-P., Bru¨mmer G., Horn R., Kandeler E., Ko¨gel-Knabner I., Kretzschmar R., Stahr K., Wilke B.-M., Thiele-Bruhn S. and Welp G. (2010) Chemische Eigenschaften und Prozesse, Lehrbuch der Bodenkunde. Spektrum Akademischer Verlag, Heidelberg, pp. 121–170. Brendler V., Baumann N., Arnold T. and Geipel G. (2004) Uranium(VI) Surface Speciation on Quartz, Report FZR-419, Dresden, p. 52. Brewitz, W., 1982. Eignungspru¨fung der Schachtanlage Konrad fu¨r die Endlagerung radioaktiver Abfa¨lle., Abschlussbericht (GSF – T 136). Gesellschaft fu¨r Strahlen- und Umweltforschung mbH (GSF), Inst. fu¨r Tieflagerung in Zusammenarbeit mit Kernforschungszentrum Karlsruhe GmbH, Inst. fu¨r Nukleare Entsorgungstechnik, Neuherberg. Chang H. S., Korshin G. V., Wang Z. M. and Zachara J. M. (2006) Adsorption of uranyl on gibbsite: A time-resolved laser-induced fluorescence spectroscopy study. Environ. Sci. Technol. 40, 1244–1249. Chisholm-Brause C., Conradson S. D., Buscher C. T., Eller P. G. and Morris D. E. (1994) Speciation of uranyl sorbed at multiple binding sites on montmorillonite. Geochim. Cosmochim. Acta 58, 3625–3631. Choppin G., Liljenzin J. O. and Rydberg J. (2002) Radiochemistry and Nuclear Chemistry, third ed. Butterworth-Heinemann, Woburn. Davies C. W. (1962) Ion Association. Butterworths, Washington DC, USA. Davis J. A. and Kohler M. (2001) Surface Complexation Modeling of Uranium(VI) Adsorption on Quartz, Surface Complexation Modeling of Uranium (VI) Adsorption on Natural Mineral Assemblages. Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, Washington, DC, pp. 145– 150. Davis J. A., Coston J. A., Kent D. B. and Fuller C. C. (1998) Application of the surface complexation concept to complex mineral assemblages. Environ. Sci. Technol. 32, 2820–2828. Dong W., Ball W. P., Liu C., Wang Z., Stone A. T., Bai J. and Zachara J. M. (2005) Influence of calcite and dissolved calcium on uranium(VI) sorption to a Hanford subsurface sediment. Environ. Sci. Technol. 39, 7949–7955. Drobot B., Steudtner R., Raff J., Geipel G., Brendler V. and Tsushima S. (2015) Combining luminescence spectroscopy, parallel factor analysis and quantum chemistry to reveal metal

156

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157

speciation – a case study of uranyl(VI) hydrolysis. Chem. Sci. 6, 964–972. Drot R., Roques J. and Simoni E. (2007) Molecular approach of the uranyl/mineral interfacial phenomena. C. R. Chim. 10, 1078–1091. Dzombak D. A. (1990) Surface Complexation Modeling: Hydrous Ferric Oxide. John Wiley & Sons. Fernandes M. M., Baeyens B., Daehn R., Scheinost A. C. and Bradbury M. H. (2012) U(VI) sorption on montmorillonite in the absence and presence of carbonate: A macroscopic and microscopic study. Geochim. Cosmochim. Acta 93, 262–277. Fox P. M., Davis J. A. and Zachara J. M. (2006) The effect of calcium on aqueous uranium(VI) speciation and adsorption to ferrihydrite and quartz. Geochim. Cosmochim. Acta 70, 1379– 1387. Froideval A., Del Nero M., Gaillard C., Barillon R., Rossini I. and Hazemann J. L. (2006) Uranyl sorption species at low coverage on Al-hydroxide: TRLFS and XAFS studies. Geochim. Cosmochim. Acta 70, 5270–5284. Gabriel U. (1998) Transport reactif de l’uranyle: mode de fixation sur la silice et la goethite; experiences en colonne et reacteur ferme; simulations, p. 161. Gabriel U., Charlet L., Schlapfer C. W., Vial J. C., Brachmann A. and Geipel G. (2001) Uranyl surface speciation on silica particles studied by time-resolved laser-induced fluorescence spectroscopy. J. Colloid Interface Sci. 239, 358–368. Glinka Y. D., Jaroniec C. P. and Jaroniec M. (1998) Studies of surface properties of disperse silica and alumina by luminescence measurements and nitrogen adsorption. J. Colloid Interface Sci. 201, 210–219. Go¨tz C., Geipel G. and Bernhard G. (2011) The influence of the temperature on the carbonate complexation of uranium(VI): a spectroscopic study. J. Radioanal. Nucl. Chem. 287, 961–969. Guillaumont R., Fangha¨nel T., Neck V., Fuger J., Palmer D. A., Grenthe I. and Rand M. H. (2003) Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium. North Holland Elsevier Science Publisher B. V, Amsterdam. Hallbeck L. and Pedersen K. (2008) Characterization of microbial processes in deep aquifers of the Fennoscandian Shield. Appl. Geochem. 23, 1796–1819. Helgeson H. C., Delany J. M., Nesbitt H. W. and Bird D. K. (1978) Summary and critique of the thermodynamic properties of rock-forming minerals. Am. J. Sci. 278, 1–229. Hochleitner R. (1996) GU-Naturfu¨hrer Mineralien und Kristalle: Mineralien nach Strichfarben bestimmen. Gra¨fe und Unzer, Mu¨nchen. Hummel W., Berner U., Curti E., Pearson F. J. and Thoenen T. (2002) Nagra/PSI chemical thermodynamic data base 01/01. Radiochim. Acta 90, 805–813. Ilton E. S., Heald S. M., Smith S. C., Elbert D. and Liu C. (2006) Reduction of uranyl in the interlayer region of low iron micas under anoxic and aerobic conditions. Environ. Sci. Technol. 40, 5003–5009. Ilton E. S., Wang Z., Boily J.-F., Qafoku O., Rosso K. M. and Smith S. C. (2012) The effect of pH and time on the extractability and speciation of uranium(VI) sorbed to SiO2. Environ. Sci. Technol. 46, 6604–6611. Jordan N., Foerstendorf H., Weiss S., Heim K., Schild D. and Brendler V. (2011) Sorption of selenium(VI) onto anatase: macroscopic and microscopic characterization. Geochim. Cosmochim. Acta 75, 1519–1530. Joseph C., Stockmann M., Schmeide K., Sachs S., Brendler V. and Bernhard G. (2013) Sorption of U(VI) onto opalinus clay: effects of pH and humic acid. Appl. Geochem. 36, 104–117.

Jung J. H., Hyun S. P., Lee J. K., Cho Y. H. and Hahn P. (1999) Adsorption of UO2+ on natural composite materials. J. 2 Radioanal. Nucl. Chem. 242, 405–412. Kar A. S., Kumar S. and Tomar B. S. (2012) U(VI) sorption by silica: effect of complexing anions. Colloids Surf. A Physicochem. Eng. Asp. 395, 240–247. Kerisit S. and Liu C. (2012) Diffusion and adsorption of uranyl carbonate species in nanosized mineral fractures. Environ. Sci. Technol. 46, 1632–1640. Klinge H., Boehme J., Grissemann C., Houben G., Ludwig R.-R., Ru¨bel A., Schelkes K., Schildknecht F. and Suckow A. (2007) Description of the Gorleben site, Part 1: Hydrogeology of the Overburden of the Gorleben Salt Dome. Bundesanstalt fu¨r Geowissenschaften und Rohstoffe (BGR), pp. 1–153. Kowal-Fouchard A., Drot R., Simoni E. and Ehrhardt J. J. (2004) Use of spectroscopic techniques for uranium(VI)/montmorillonite interaction modeling. Environ. Sci. Technol. 38, 1399– 1407. Krawczyk-Ba¨rsch E., Lu¨nsdorf H., Arnold T., Brendler V., Eisbein E., Jenk U. and Zimmermann U. (2011) The influence of biofilms on the migration of uranium in acid mine drainage (AMD) waters. Sci. Total Environ. 409, 3059–3065. Krawczyk-Ba¨rsch E., Lu¨nsdorf H., Pedersen K., Arnold T., Bok F., Steudtner R., Lehtinen A. and Brendler V. (2012) Immobilization of uranium in biofilm microorganisms exposed to groundwater seeps over granitic rock tunnel walls in Olkiluoto, Finland. Geochim. Cosmochim. Acta 96, 94–104. Krepelova A. (2007) Influence of Humic Acid on the Sorption of Uranium(VI) and Americium(III) onto Kaolinite. Forschungszentrum Dresden Rossendorf, TU Dresden, p. 163. Lefe`vre G. (2004) In situ Fourier-transform infrared spectroscopy studies of inorganic ions adsorption on metal oxides and hydroxides. Adv. Colloid Interface Sci. 107, 109–123. Lindsay W. D. and Walthall W. M. (1996) The solubility of aluminum in soils. In The Environmental Chemistry of Aluminum (ed. G. Sposito), second ed. Lewis Publishers, Boca Raton, pp. 333–361. Lu¨tzenkirchen J. (2006) Surface Complexation Modeling. Elsevier, Amsterdam. Moyes L. N., Parkman R. H., Charnock J. M., Vaughan D. J., Livens F. R., Hughes C. R. and Braithwaite A. (2000) Uranium uptake from aqueous solution by interaction with goethite, lepidocrocite, muscovite, and mackinawite: an X-ray absorption spectroscopy study. Environ. Sci. Technol. 34, 1062–1068. Mu¨ller K., Brendler V. and Foerstendorf H. (2008) Aqueous uranium(VI) hydrolysis species characterized by attenuated total reflection Fourier-transform infrared spectroscopy. Inorg. Chem. 47, 10127–10134. Mu¨ller K., Foerstendorf H., Meusel T., Brendler V., Lefe`vre G., Comarmond M. J. and Payne T. E. (2012) Sorption of U(VI) at the TiO2–water interface: an in situ vibrational spectroscopic study. Geochim. Cosmochim. Acta 76, 191–205. Mu¨ller K., Foerstendorf H., Brendler V., Rossberg A., Stolze K. and Gro¨schel A. (2013) The surface reactions of U(VI) on gamma-Al2O3 – in situ spectroscopic evaluation of the transition from sorption complexation to surface precipitation. Chem. Geol. 357, 75–84. Nagra (2002) Projekt Opalinuston: Synthese der geowissenschaftlichen Untersuchungsergebnisse: Entsorgungsnachweis fu¨r abgebrannte Brennelemente, verglaste hochaktive sowie langlebige mittelaktive Abfa¨lle. Nagra, Wettingen. Nebelung C. and Brendler V. (2010) U(VI) sorption on granite: prediction and experiments. Radiochim. Acta 98, 621–625. Noseck, U., Brendler, V., Flu¨gge, J., Stockmann, M., Britz, S., Lampe, M., Schikora, J., Schneider, A., 2012. Realistic Integration of Sorption Processes in Transport Codes for

C. Richter et al. / Geochimica et Cosmochimica Acta 189 (2016) 143–157 Long-Term Safety Assessments. In: GRS (Ed.). GRS/HZDR, Braunschweig, pp. 1–293. OECD/NEA (2002) Radionuclide Retention in Geologic Media. Nuclear Energy Agency, Paris, pp. 1–269. Okrusch M. and Matthes S. (2005) Mineralogie – Eine Einfu¨hrung in die spezielle Mineralogie, Petrologie und Lagersta¨ttenkunde, seventh ed. Springer, Berlin. Parkhurst D. L. and Appelo C. (2013) Description of Input and Examples for PHREEQC Version 3—A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport, and Inverse Geochemical Calculations. U.S. Geological Survey Techniques and Methods, pp. 1–497. Poeter, E.P., Hill, M.C., Lu, D., Tiedeman, C.R., Mehl, S., 2014. UCODE_2014, with New Capabilities to Define Parameters Unique to Predictions, Calculate Weights using Simulated Values, Estimate Parameters with SVD, Evaluate Uncertainty with MCMC, and More. In: Energy, U.U.S.D.o. (Ed.), Integrated Groundwater Modeling Center Report Number GWMI 2014-02. Prikryl J. D., Pabalan R. T., Turner D. R. and Leslie B. W. (1994) Uranium sorption on a-alumina: effects of pH and surfacearea/solution-volume ratio. Radiochim. Acta, 291. Saslow Gomez S. A., Jordan D. S., Troiano J. M. and Geiger F. M. (2012) Uranyl adsorption at the muscovite (mica)/water interface studied by second harmonic generation. Environ. Sci. Technol. 46, 11154–11161. Scheffer F. and Schachtschabel P. (2010) Lehrbuch der Bodenkunde. Spektrum Akademischer Verlag, Heidelberg. Schmeide K., Gu¨rtler S., Mu¨ller K., Steudtner R., Joseph C., Bok ¨ spo¨ F. and Brendler V. (2014) Interaction of U(VI) with A diorite: a batch and in situ ATR FT-IR sorption study. Appl. Geochem. 49, 116–125.

157

Shaheen S. M., Tsadilas C. D. and Rinklebe J. (2013) A review of the distribution coefficients of trace elements in soils: Influence of sorption system, element characteristics, and soil colloidal properties. Adv. Colloid Interface Sci. 201–202, 43–56. Stefa´nsson A. and Arno´rsson S. (2000) Feldspar saturation state in natural waters. Geochim. Cosmochim. Acta 64, 2567–2584. Steudtner R., Sachs S., Schmeide K., Brendler V. and Bernhard G. (2011) Ternary uranium(VI) carbonato humate complex studied by cryo-TRLFS. Radiochim. Acta 99, 687–692. Taha I. A. and Morawetz H. (1971) Catalysis of Ionic reactions by polyelectrolytes. 3. Quenching of uranyl ion fluorescence by iron(II) ions in poly(vinylsulfonic acid) solution. J. Am. Chem. Soc. 93, 829–833. Tan X. L., Fang M. and Wang X. K. (2010) Sorption speciation of lanthanides/actinides on minerals by TRLFS, EXAFS and DFT studies: a review. Molecules 15, 8431–8468. Walter M., Arnold T., Geipel G., Scheinost A. and Bernhard G. (2005) An EXAFS and TRLFS investigation on uranium(VI) sorption to pristine and leached albite surfaces. J. Colloid Interface Sci. 282, 293–305. Wang Z., Zachara J. M., Yantasee W., Gassman P. L., Liu C. and Joly A. G. (2004) Cryogenic laser induced fluorescence characterization of U(VI) in Hanford vadose zone pore waters. Environ. Sci. Technol. 38, 5591–5597. Wang Z. M., Zachara J. M., Gassman P. L., Liu C. X., Qafoku O., Yantasee W. and Catalano J. G. (2005) Fluorescence spectroscopy of U(VI)-silicates and U(VI)-contaminated Hanford sediment. Geochim. Cosmochim. Acta 69, 1391–1403. Associate editor: Annie B. Kersting