Sorption of U(VI) at the TiO2–water interface: An in situ vibrational spectroscopic study

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Geochimica et Cosmochimica Acta 76 (2012) 191–205 www.elsevier.com/locate/gca

Sorption of U(VI) at the TiO2–water interface: An in situ vibrational spectroscopic study Katharina Mu¨ller a,⇑, Harald Foerstendorf a, Tilmann Meusel a, Vinzenz Brendler a, Gre´gory Lefe`vre b, M. Josick Comarmond c, Timothy E. Payne c a

Helmholtz-Zentrum Dresden-Rossendorf e.V., Institute of Radiochemistry, P.O. Box 510119, D-01314 Dresden, Germany b CNRS-ENSCP, LECIME-UMR7575, 11, Rue Pierre et Marie Curie, 75231 Paris Cedex 05, France c Australian Nuclear Science and Technology Organisation, Institute for Environmental Research, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia Received 16 February 2011; accepted in revised form 27 September 2011; available online 10 October 2011

Abstract Molecular-scale knowledge of sorption reactions at the water–mineral interface is important for predicting U(VI) transport processes in the environment. In this work, in situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy was used in a comprehensive investigation of the sorption processes of U(VI) onto TiO2. The high sensitivity of the in situ ATR FT-IR technique allows the study of U(VI) concentrations down to the low micromolar range, which is relevant to most environmental scenarios. A set of highly purified and well characterized TiO2 phases differing in their origin, the ratio of the most stable polymorphs (anatase and rutile), in specific surface area, isoelectric points and in particle size distribution was investigated. Irrespective of the composition of the mineral phase, it was shown that U(VI) forms similar surface complexes, which was derived from the antisymmetric stretching mode m3(UO2) showing a characteristic shift to lower wavenumbers compared to the respective aqueous species. The availability of a fast scanning IR device makes it feasible to perform time-resolved experiments of the sorption processes with a time resolution in the sub-minute range. It is shown that during the early stages of the U(VI) uptake, a surface species on the mineral phase is formed, characterized by a significantly redshifted absorption maximum which is interpreted as a bidendate inner-sphere complex. After prolonged sorption, the IR spectra indicate the formation of a second surface species showing a smaller shift compared to the aqueous species. These findings were verified by a series of spectroscopic experiments performed on a U(VI)-saturated surface, at different U(VI) concentrations, pH values and in the absence of atmospheric-derived carbonate. This work provides new molecular insights into the sorption processes of U(VI) on TiO2. Basic thermodynamic ideas of surface complexation are substantiated by in situ infrared spectroscopy. Ó 2011 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

⇑ Corresponding author. Tel.: +49 351 260 2438; fax: +49 351 260 3553. E-mail addresses: [email protected] (K. Mu¨ller), foersten@ hzdr.de (H. Foerstendorf), [email protected] (T. Meusel), [email protected] (V. Brendler), [email protected] (G. Lefe`vre), [email protected] (M.J. Comarmond), [email protected] (T.E. Payne).

0016-7037/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.10.004

The migration behavior of heavy metal contaminants in water bearing rock formations is mainly controlled by numerous hydro-geochemical reactions. Under the prevailing conditions, the concentration of the contaminant is restricted to an upper limit by the contaminant’s solubility in the aqueous phase and is considered as a basic limitation on transport. The retention of metals at the mineral/ rock–water interface constitutes a second constraint. In particular, when the metal concentrations are far below

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the solubility limits, the interfacial processes dominate their retardation (Choppin and Stout, 1989). Various processes can occur at the solid–water interface, including physical adsorption, ion exchange, chemisorption, surface precipitation, sorption of colloidal phases, electron transfer (oxidation state changes), coordination changes (number and/or type of ligands around the sorbed species), modification of the surface structure and dissolution of the substrate (Silva and Nitsche, 1995; O’Day, 1999; Blesa et al., 2000). Physisorption or formation of an outer-sphere complex occurs when a positively or negatively charged complex retains its water of hydration and is hydrogen bonded or attracted to the surface via long-range forces, i.e. electrostatic and van der Waals attraction. Electrostatic adsorption is frequently rapid and reversible, e.g. ion exchange reactions (Parks, 1990; Silva and Nitsche, 1995). Chemisorption, or formation of an inner-sphere complex, involves loss of water molecules from the hydration shell and direct shortrange interactions with the surface, e.g. covalent bonding or hydrophobic forces (Parks, 1990). Because of the tightened bond strengths, inner-sphere complexes are generally more strongly bound to the surface than outer-sphere complexes (Brown, 1990). The sorption behavior and reactivity of the metal ions at the solid–water interface is controlled by the physico-chemical properties of the surface, that is the composition of the functional groups, topology and purity of the solid phase, in combination with aqueous solution parameters, namely metal concentration, pH, and ionic strength (Brown, 1990). Among various contaminants and their exposure paths in the environment, uranium is of special interest because of its chemotoxicity combined with radiotoxicity. Understanding its migration behavior is of primary environmental concern to estimate water contamination of facilities of former uranium mining and milling sites in Saxony and Thuringia (Germany) and of subsurface dumps and sites with radioactive and/or heavy metal inventories. Under oxidizing environmental conditions, the hexavalent form is the most prominent. The sorption of U(VI) on a multiplicity of naturally occurring and artificial substrates has been intensively studied in the last decades, using macroscopic batch and column experiments, spectroscopic and microscopic analysis, theoretical calculations and modeling (Waite et al., 1994; Bargar et al., 2000; Duff et al., 2002; Catalano et al., 2005; Arai et al., 2006; Perron et al., 2006; Ulrich et al., 2006; Chardon et al., 2008; Pasilis and Pemberton, 2008; Sherman et al., 2008; Hattori et al., 2009; Rossberg et al., 2009). In air (p(CO2)  37.5 Pa), U(VI) sorption onto mineral surfaces generally is at a maximum at near neutral pH and sharply decreases towards more acidic or more alkaline conditions (Prikryl et al., 1994; Waite et al., 1994; Payne et al., 2004; Thakur et al., 2005; Arai et al., 2006; Pandey, 2006; Drot et al., 2007). Under acidic conditions, U(VI) adsorption is limited by electrostatic repulsion between positively charged uranyl units (UO2þ 2 ) and the protonated oxide surfaces, whereas at alkaline pH values, the sorption is hampered because of the formation of negatively charged U(VI) carbonate complexes in solution and the negative surface charges which become predominant above the zero

point of charge (PZC) of the solid phases (Yamaguchi et al., 2004). Under near neutral conditions, where several uranyl hydroxo species dominate the speciation, deprotonation of hydroxyl groups of oxide surfaces potentially provides an increased number of adsorption sites. These processes may be accompanied by stronger inner-shell surface adsorption and by the formation of aqueous polynuclear complexes which can be sorbed onto the surfaces or might induce surface precipitation (Prikryl et al., 1994; Sylwester et al., 2000; Hattori et al., 2009; Rossberg et al., 2009). Titanium dioxide is ubiquitous in the environment, with an additional anthropogenic influx because of its industrial application as white pigment in construction materials, cosmetics, and food. In the investigation of sorption phenomena, TiO2 was found to be important as a trace constituent in some minerals (Payne et al., 2004). Moreover, it is often taken as a model oxide due to its high stability, its low solubility over a wide pH range, and its well-known structure and surface properties (Dixon and Weed, 1989; Tochiyama et al., 1996; Jakobsson and Albinsson, 1998; Lefe`vre et al., 2008). TiO2 polymorphs occur as the tetragonal forms anatase and rutile and as orthorhombic brookite. In nature, the most common modifications are anatase and rutile. Anatase is built up from TiO6 2 octahedra linked by their vertices, whereas in rutile the edges are connected. (Milnes and Fitzpatrick, 1989; Carp et al., 2004). In recent spectroscopic investigations of U(VI) sorption onto TiO2, mainly single crystals of rutile have been used (Den Auwer et al., 2003; Vandenborre et al., 2007). The formation of bidentate inner-sphere and outer-sphere complexes on rutile (1 1 0) and (0 0 1) planes has been suggested. Furthermore, the type of dominant complexes for U(VI) sorption onto rutile are reported to depend on the U(VI) surface concentration. At lower concentrations, a bidentate inner-sphere complex onto two bridging oxygen atoms has been proposed, with another complex at higher U(VI) surface concentration bound onto one top and one bridging oxygen atom (Perron et al., 2006; Drot et al., 2007; Vandenborre et al., 2007). Recently, the formation of only one type of surface complex, possibly a trimer was proposed for a mixed anatase and rutile sample with a higher percentage of sorbed U(VI) (Lefe`vre et al., 2008). However, the sorption mechanisms of U(VI) on anatase and rutile have not been comparatively studied in detail up to now. Furthermore, data derived from systematic studies across a wide range of sorption conditions, i.e. variation of pH, ionic strength, loading capacity, the presence of ligands, are still rare. Vibrational spectroscopy is a valuable tool to distinguish between different sorption processes on a molecular level (Lefe`vre, 2004; Elzinga and Sparks, 2007). Previous investigations have shown the sensitivity of the UO2 symmetric and asymmetric stretching modes (m1 and m3) to changes in the coordination environment of the cation (Maya, 1982; Tsushima et al., 1998; Lefe`vre et al., 2006b, 2008; Mu¨ller et al., 2008, 2009b). Complexation of aqueous UO2þ 2 ions with organic or inorganic ligands in solution or at interfaces weakens the OAUAO bonds, i.e. a reduction of the force constants of the O@U@O bonds. The replacement of the first shell water results in a shift to lower

Sorption of U(VI) onto TiO2

frequencies of the m(UO2) stretching modes. Although the origin of the bond weakening was investigated by several groups in the past, no overall consensus has been reached so far. Possibly, both r and p donating abilities of the ligands account for the uranyl bond weakening (Tsushima, 2011). The extent of this shift is found to be correlated with the number of ligands in aqueous complexes whereas for sorption complexes it can be related to the coordination of the uranyl unit at the surface. Because of this correlation the stretching modes can be used as a marker for specific molecular information on surface coordination of the sorbed UO2þ 2 unit (Pasilis and Pemberton, 2008). To further constrain uncertainties associated with U(VI) surface chemistry, attenuated total reflection (ATR) FT-IR spectroscopy is a promising technique which provides molecular information from the interface between a stationary phase coated on the ATR crystal, i.e. the mineral oxide TiO2, and the sorptive aqueous U(VI) solution. A detailed analysis of the in situ spectroscopic data may lead to a more complete understanding of the reactions occurring at the interface at a micromolar concentration range, relevant to environmental concern. The variation of distinct parameters of the mobile and/or the solid phase potentially provokes altered spectral properties which can be unequivocally interpreted. The aim of this in situ ATR FT-IR spectroscopic work is the systematic study of the sorption mechanism of U(VI) onto TiO2 surfaces on a molecular level. For this purpose, this study focuses on the variation of a multiplicity of sorbent and solution properties. Several TiO2 samples, providing differences in their composition of anatase and rutile, in specific surface area, particle size and isoelectric point are

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chosen. In addition, the impact of contact time, pH, U(VI) concentration, ionic strength and presence of atmospheric-derived carbonate are investigated in situ. 2. EXPERIMENTAL SECTION 2.1. Preparation and characterization of U(VI) solutions The diluted uranium(VI) solutions were freshly prepared using a 0.05 M UO2Cl2 stock solution in 1 M HCl and Milli-Q water with a resistivity of 18.2 MX/cm. All chemicals were of analytical grade. Ionic strength was controlled by the use of 0.1 M NaCl, pH was adjusted by adding aliquots of NaOH and HCl. All sample preparations and analysis were done under normal atmosphere and at room temperature (25 °C). To study the impact of atmosphericderived carbonate one experiment was performed under a N2 atmosphere in a glove box using CO2-free solutions. Using photon correlation spectroscopy (PCS, Brookhaven Instr. 90), the solutions were monitored for precipitation. No formation of insoluble uranium phases or colloidal solutions were obtained in freshly prepared 20 lM uranyl solutions at pH 6 8.5 (Mu¨ller et al., 2008). 2.2. Titanium oxides A set of highly purified (P99.9%) and well characterized TiO2 phases from different origins was investigated. The samples vary in the ratio of the most stable polymorphs, i.e. anatase and rutile, in specific surface area and in particle size distribution (Table 1). Of the studied samples, S4 and S5 are pure rutile and S6 is pure anatase. Sample S5 was

Table 1 Characteristics of the TiO2 samples. Sample Origin

Composition

BET/m2 g–1 IEP Particle sizea

S1

Alfa Aesar No. 40458

Mixture of 80–90% anatase and rutile

223

5.2

S2

Degussa P-25 Mixture of 86% anatase and 14% rutile Nippon Aerosil Co., Ltd, supplied by Degussa Australia Pty Ltd Melbourne No. 4166120498 Cerac Inc., Milwaukee, WI, USA Mixture of 90% rutile and 10% anatase

57

5.8

5

2.5

100% rutile

7

3.6

100% rutile

2

2.9

305

6.0

7

4.0

S3

S4

S5

S6 S7

a

Tronox Pigments GmbH (Krefeld), via Kerr McGee Pigments (USA); No. TR-HP-2 S3 calcined (6 h at 1000 °C as in (Hippel et al., 1946)) MTI Corporation Richmond, CA, USA; No. NP-TiO2-A-10 Aldrich No. 232033

100% anatase Mixture of 91% anatase and 9% rutile

14% < 6 nm 38% 6–20 nm 34% 20–80 nm 14% >80 nm (Lefe`vre et al., 2008) 10–50 nm

90% <2.50 lm 50% <0.98 lm 10% <0.22 lm 90% <1.49 lm 50% <0.71 lm 10% <0.19 lm 90% <4.49 lm 50% <1.84 lm 10% <0.63 lm 5–10 nm 90% <1.19 lm 50% <0.54 lm 10% <0.17 lm

The particle size distribution for the samples S3–S5 and S7 was obtained by laser diffraction. The range of particle sizes for S2 and S6 was estimated by TEM (cf. Fig. 1). Because of the very small sample amount of S1, the distribution is taken from the literature, where the equal Alfa Aesar batch was studied. For further information see text.

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synthesized from sample S3 according to the procedure described elsewhere (Hippel et al., 1946). The surface areas of the sorbent materials were determined by the N2-BET method using a Beckman Coulter analyzator SA 3100. Prior to the measurements, the TiO2 samples were heated at 300 °C for 480 min for elimination of any bound water. Generally, the samples containing a predominant fraction of anatase (S1, S2, S6, S7) have higher surface areas compared to the rutile samples (S3–S5). The particle size ranges from the nm to lm scale. In particular, the TiO2 samples with high surface areas provide very small particle sizes. Particle size distributions for the samples S3–S5 and S7 were determined by laser diffraction (HELOS H0735) (Table 1). For the samples S2 and S6, it was found that the particle size was below the applicable minimum particle size of approximately 0.05 lm. For further characterization, the morphologies of all TiO2 samples were examined by electron microscopy, namely SEM (Hitachi, S-4800) for samples S1–S5, S7 and TEM (FEI, Titan 80-300) for samples S1, S2 and S6. The micrographs show distinct differences in morphology and grain sizes between the applied samples (Fig. 1). S6 shows

the smallest grain size (5–10 nm), followed by the anatase materials S1, S2 and S7. The rutile samples S3 and S4 are similar in grain size, whereas S5 presents increased particle size. In particular, the TEM micrographs of samples S1 and S2 show well defined characteristics of anatase (Fig. 1b). In Table 1 the range of particle sizes for S2 and S6 which were estimated from TEM analysis are listed. The zeta potential measurements of TiO2 suspensions with a concentration of 0.15 g/L in 0.1 M NaCl were carried out in the pH range from 1.5 to 9 using a Malvern Zetasizer Nano-ZS. Before analysis, the samples were suspended using an ultrasonic finger for 5 min. The temperature was kept constant at 25 °C and equilibration time was set to 120 s. The isoelectric points (IEP) vary significantly between pH 2.5 and 6.0. The IEP at pH 5.2 of sample S1 is in agreement with literature data (Lefe`vre et al., 2008). Two different washing procedures were applied to the TiO2 samples, namely a mild acid-base wash and a strong acid-base wash. The mild wash was performed similar to the procedure described by Lefe`vre et al. (2006a), using S2, S5, and S6. First, a solution of 0.1 M NaOH was used for the removal of anions. Then, a solution of 0.1 M HCl

Fig. 1. Micrographs of (a) SEM of samples S1–S5 and S7 (0.5kV-D x 50k) and (b) TEM of samples S1, S2 and S6. Note the TEM micrograph of S6 is in different magnification.

Sorption of U(VI) onto TiO2

was applied twice to eliminate cations and carbonate. In a third step, the samples were shaken five or six times with Milli-Q water until the conductivity of the supernatant solution after centrifugation did not change significantly. Each step was conducted with a solid liquid ratio of 2.5 g of TiO2 in 50 mL reagent in an overhead shaker at 4 rpm for 24 h with subsequent centrifugation for the separation of solid and liquid. The solids were then dried at 50 °C for three days and ball-milled for approximately 10 min. The more aggressive wash was performed on TiO2 samples S2 and S7 according to a previously described procedure using 5 M NaOH, 5 M HNO3 and Milli-Q water (Kosmulski and Matijevic, 1992). The solid–liquid ratio was much higher relative to the mild wash, with 1:10 for S2 and 1:5 for S7. Samples were shaken with the strong base solution for several hours, followed by several rinses with Milli-Q water. Then, the samples were contacted with the strong acid solution for several hours and subsequently rinsed with Milli-Q water until the conductivity reached a low and constant value (typically <10 lS/cm). The solid was recovered by centrifugation and dried at 80 °C, and ball-milled back to powder form. A more detailed characterization of the washed samples is given in (Comarmond et al., 2011). Digestion and ICP-MS (ELAN 6000, Perkin Elmer) and AAS-GF (Analysentechnik Jena) analysis of the sorbent samples were performed to assess the amount of the following cations: Na, Mg, Al, Si, K, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Rb, Sr, Zr, Nb, Mo, Ag, Sn, Ba, La, Ta, W, Tl, Pb before and after the washing procedure. The microwave digestion was performed using 50 mg solid sample and 5 mL of a solvent mixture, containing HNO3, HF and HCl (3:1:1). 2.3. In situ ATR FT-IR spectroscopic sorption studies In this work, in situ vibrational spectroscopic experiments are based on the principle of reaction-induced difference spectroscopy, in which explicitly spectral changes related to selectively induced changes of the investigated sample are detected. IR single beam spectra of a mineral film, prepared as a stationary phase, are continuously recorded while it is flushed by a mobile phase, comprising aqueous solutions for equilibration and for induced sorption. The progress of the sorption process is monitored with a time resolution in the sub-minute time range, since the acquisition time of each single beam spectrum is about

195

30 s. The selective change of only one experimental parameter, i.e. the presence of actinide ions, allows the detection of spectral features showing very small absorption changes (optical density P105) depicted as difference spectra which are calculated from the single beam spectra recorded before and after the selectively induced change of the sample. Infrared spectra were measured on a Bruker Vertex 80/v vacuum spectrometer equipped with a Mercury Cadmium Telluride (MCT) detector with a low frequency cut-off at 600 cm1. Spectral resolution was 4 cm1 and spectra were averaged from 256 scans. The in situ sorption experiments of this work are described by three stages, as illustrated in Scheme 1. In a first step, the mineral film is conditioned to an appropriate pH and ionic strength for 60 min. The difference spectrum, herein after referred to as “conditioning”, is calculated from single beam spectra recorded after 30 and 60 min of rinsing the mineral film with blank solution. This spectrum reflects the stability of the film under the chosen conditions for a time range of 30 min and serves as a measure of quality for the experimental setup. In a second step, the U(VI) sorption process onto the mineral film is performed for the next 90 min. Difference spectra, referred to as “sorption”, are calculated from the last single beam spectra obtained at the conditioning, and from spectra obtained at different time intervals after starting the induced sorption process. In a last step, the film is again flushed with the blank solution for further 30 min (“flushing”). The respective difference spectra are calculated from single beam spectra recorded at the end of the sorption stage of the experiment and at distinct time intervals of this second blank flushing. This stage of the experiment provides additional information on the reversibility of the sorption process and weakly bound metal species are predominantly identified. The TiO2 films were prepared directly on the surface of the ATR crystal as previously described (Lefe`vre et al., 2008). The ATR accessory used in this study was a DURA SamplIR II (Smiths Inc.) with a horizontal diamond crystal (A = 12.57 mm2) with nine internal reflections on the upper surface and an angle of incidence of 45°. An aliquot of 1 lL of the TiO2 suspension (2.5 g/L in 1:1 MilliQ water and pure ethanol) was pipetted onto the crystal with subsequent drying under a gentle N2 flow. This procedure was repeated five times. The final deposited density was calculated to be approximately 0.1 mg/cm2.

Scheme 1. Illustration of the experimental sorption protocol and the calculation of difference spectra for the in situ sorption experiments under standard conditions (upper time scale) and for the sorption stage of the time-resolved (TR) experiment (lower time scale). Single beam spectra were continuously recorded throughout the whole experimental time (approx. one spectrum per minute, including spectral acquisition, processing and delay time). The difference spectra shown in the figures were calculated from the single beam spectra recorded at two different points indicated by arrowheads (sample) and by full circles (reference) connected by solid or dotted lines. For details see text.

K. Mu¨ller et al. / Geochimica et Cosmochimica Acta 76 (2012) 191–205

3.1. Sorption of U(VI) on TiO2 An overview of the TiO2 samples and their respective physico-chemical parameters is given in Table 1. In situ U(VI) sorption experiments monitored by ATR FT-IR spectroscopy were conducted on all samples. In our standard spectroscopic sorption experiment, the U(VI) concentration of the sorptive solution was set to 20 lM at ionic strength of 0.1 M NaCl, pH 5 and ambient atmosphere (cf. Scheme 1). At this low concentration level, monomeric hydroxo species of U(VI) are dominant in aqueous solution and the impact of polymeric species on the sorption processes is excluded (Mu¨ller et al., 2008). The respective spectra presented in Fig. 2 (lower panel) represent difference spectra calculated from spectra recorded before and after 90 min of induced sorption (see also Scheme 1). All spectra exhibit absorption bands in the spectral ranges between 1550–1350 and 1100–880 cm1. A comparison of the spectral data shows only minor differences between all TiO2 samples (Fig. 2, lower panel). Generally, all spectra are characterized by a significant band around 915 cm1 which can be assigned to the antisymmetric stretching mode m3 of the uranyl(VI) unit, m3(UO2). Because the frequency of this mode reflects the molecular environment of the UO2þ 2 ion, we have to consider the frequency of this vibrational mode in aqueous solution as a reference. The spectra of aqueous U(VI) solutions are presented in Fig. 2 (upper panel). The m3(UO2) mode of the fully hydrated UO2þ 2 species, present in highly acidic aqueous solution, shows an absorption maximum at 961 cm–1 (Jones and Penneman, 1953; Quile`s and Burneau, 2000). With increasing pH, hydrolysis reactions occur, monomeric and polymeric species are formed which, above all, strongly depend on the prevailing uranyl(VI) concentration (Grenthe et al., 1992; Guillaumont et al., 2003). The reference spectrum of a 20 lM U(VI) aqueous solution under almost identical conditions, that is pH 5.5 instead of pH 5 shows the uranyl band at 923 cm–1 (Fig. 2, second trace). From our recent studies, this band has been assigned to monomeric hydroxo species formed under the prevailing conditions (Mu¨ller et al., 2008). Moreover, the spectrum shows further bands at 1527 and 1456 cm–1 (Fig. 2). Although a detailed assignment of these bands cannot be given, they apparently are due to an intrinsic spectral property of the hydrolysis products representing vibrational

961

10 mM U(VI), pH 2 923 20 µM U(VI), pH 5.5 U(VI) loading

Sorption on TiO2

917 [mg U / g TiO ] 2 S1: 160.7 908 S2: 179.4 S3: 81.6 S4: 29.2

1527

3. RESULTS AND DISCUSSION

Aq. solution

S5: 98.6 S6: 193.2 1456 1390

A flow cell (total volume 200 lL) was used to deliver the aqueous solutions required for conditioning the prepared mineral film and subsequent sorption of U(VI) at the mineral film surface. To avoid a possible photo-induced reaction all experiments are performed in the absence of light. A constant velocity of 0.2 mL/min was maintained by a peristaltic pump (Ismatec S.A.) and experiments were carried out at room temperature. The cleaning procedure of the diamond surface between sorption experiments included washing with 1 M HCl, NaOH and ethanol, respectively, followed by rinsing several times with MilliQ water, and rinsing with acetone as a last step.

Absorption / a.u.

196

1600

1400

S7: 16.3 1100 1000 900

Wavenumber / cm

800

–1

Fig. 2. Mid-IR spectra of U(VI) aqueous solutions and of U(VI) sorption onto different TiO2 samples (20 lM initial U(VI), 0.1 M NaCl, pH 5, 90 min of induced sorption). Indicated values on the IR spectra are in cm1. Additionally, U(VI) loadings of the removed TiO2 films are given (as mg U/g TiO2).

modes of water molecules coordinated to the uranyl-hydroxo species. The comparison of aqueous uranyl-hydroxo species and sorption complexes at the TiO2–water interface shows significant frequency shifts of the m3(UO2) mode when the uranyl ion interacts with the surface. This shift of 53–44 cm–1 can be ascribed to an intense decrease of the U@O force constant as a result of complexation with TiO units of the mineral phase. This is in accordance with previous vibrational spectroscopic studies of U(VI) sorption onto different mineral phases, where considerable downshifts (up to 50 cm–1) of the m3(UO2) mode of the free ion UO2þ 2 have been observed (Duff et al., 2002; Wazne et al., 2003; Lefe`vre et al., 2006b, 2008; Ulrich et al., 2006; Pasilis and Pemberton, 2008). This band is further shifted to lower frequencies of about 6–15 cm1 compared to the spectrum of the aqueous UO2þ 2 solution and the formation of inner-sphere surface complexes at the mineral phases can be suggested. In contrast, coordination via electrostatic attraction, namely outer-sphere complexation, would modify the antisymmetric stretching mode to a considerably lesser extent of only a few wavenumbers (Lefe`vre, 2004). All spectra obtained for U(VI) sorption on TiO2 (Fig. 2, lower panel) show similar, but slightly different frequencies of the m3(UO2) mode ranging from 917 to 908 cm1. The significantly shifted frequencies of the m3(UO2) modes of the surface species, relative to the spectrum of the aqueous species, strongly suggest that uranyl surface complexes are formed at all titania surfaces investigated in this work.

Sorption of U(VI) onto TiO2

3.2. In situ monitoring of the sorption process of U(VI) on TiO2 The course of an in situ sorption experiment according to the setup explained in Scheme 1 (top) is depicted by the IR spectra shown in Fig. 3. During the “conditioning” stage (see Scheme 1) no significant absorption changes are observed in the respective spectrum in the region from 1600 to 800 cm1 within 30 min (Fig. 31, red trace). In particular, below 900 cm1, absorption bands of TiO2 can be expected (Milnes and Fitzpatrick, 1989). Thus, the absence of bands in this spectrum indicates a sufficient stability of 1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.

908

0.05

Sorption min 90 60 40 20 10 5

1061

1532 1455 1399

Absorption / a.u.

However, the observed variation suggests the formation of more than one specific surface complex. A comparison of the spectral data of the different TiO2 samples indicates that there is no correlation of the band maxima neither with the crystallographic form, nor the particle size, morphology, the specific surface area, nor the IEP (see Table 1), which have been expected to be relevant for the sorption processes. An explanation for the observed spectral alterations will be provided in detail in the following sections by the results of further experiments where a variety of different sorption conditions, e.g. contact time, flow velocity, pH, ionic strength and uranyl(VI) concentration have been applied. For those experiments, the pure anatase sample S6 was chosen as a reference, since it provides high U(VI) retention capability and very satisfactory signal-to-noise ratio. The spectra show broad and overlapping bands between 1200 and 1000 cm1. An unequivocal assignment to distinct vibrational modes is challenging. Because the infrared spectrum of pure TiO2 does not show strong intrinsic absorption bands above 900 cm1 (Milnes and Fitzpatrick, 1989; Mu¨ller et al., 2009a), the bands must be due to reactions occurring at the solid–water interface. In our recent study of Np(V) sorption on titania (sample S1), we demonstrated that these absorption changes are not due to specific interactions between the solid phase and the background electrolyte (NaCl), or the solvent water (Mu¨ller et al., 2009a). Nevertheless, because of their high reproducibility in the different TiO2 samples, these bands are likely to represent vibrational modes of the TiO2-surface, most probably dOH torsion modes, reflecting changes of the chemical and physical properties of the solid phases during the sorption of the uranyl cations. Furthermore, bands at 1527, 1456, and 1390 cm1 are detected with different intensities in each spectrum, being particularly conspicuous in the spectra representing a high U(VI) surface loading and, hence, a high signal-to-noise ratio. In this spectral region, the appearance of both modes from atmospheric-derived carbonate coordinated to the solid phase, as well as modes of the aqueous hydroxo uranyl species (Fig. 2, top) can be expected. These spectral features will be discussed in Section 3.4.1 where spectroscopic results of the impact of the atmospheric conditions on the sorption processes are presented.

197

Conditioning

899 5 10 20

Flushing 1600

1400

915 1200

1000

Wavenumber / cm

800

–1

Fig. 3. In situ time-resolved mid-IR spectra of U(VI) sorption on TiO2 (S6, 20 lM initial U(VI), 0.1 M NaCl, pH 5, flow rate 0.2 mL min1). The spectra of the sorption process are recorded at different times after induction as given. See text for details. Ordinate scaling is given by the bar in units of optical density. Other values indicated are in cm–1.

the mineral film on the ATR crystal’s surface under the chosen experimental conditions. The difference spectra referred to as “sorption” were calculated from spectra recorded at the late stage of the conditioning and at 5, 10, 20, 40, 60 and 90 min after the induction of U(VI) sorption, respectively, and are shown in Fig. 3 (black traces). These spectra exhibit absorption bands in the spectral ranges from 1550 to 1350 cm1 and from 1100 to 880 cm1. The progress of U(VI) sorption on the TiO2 surface can be monitored online by the time-dependent increase of the absorption band at 899 cm1, corresponding to the m3(UO2) mode. After around 90 min, only a slight intensity increase was observed in comparison to the calculated spectrum after 80 min of sorption (data not shown), indicating the achievement of steady state conditions under the prevailing conditions. Interestingly, the m3(UO2) band of the sorbed U(VI) species shows different maxima and half-band widths throughout the time interval of induced U(VI) sorption. At a low surface loading (up to 10 min contact time), a broad band with maximum at 899 cm1 is observed. Upon prolonged U(VI) accumulation the band is hypsochromically shifted to 908 cm1. The observed spectral characteristics evidence the formation of different types of surface complexes. By systematic variation of well defined experimental parameters during the time-resolved measurements (e.g. U(VI) concentration and pH), such phenomena can be clarified and will be discussed in detail in Section 3.4. The spectral data obtained after 5, 10, and 20 min during the “flushing” stage (see Scheme 1) exhibit a distinct

K. Mu¨ller et al. / Geochimica et Cosmochimica Acta 76 (2012) 191–205

negative band at 915 cm1 (Fig. 3, blue traces). Again, this band is assigned to the m3(UO2) mode. It represents a release of a U(VI) species from the TiO2 film. It is suggested that this species slightly adheres in the pores of the oxide film and is weakly bound to the titanol groups. Since the m3(UO2) mode of the aqueous monomeric species present in the sorptive solution is observed at a similar frequency (923 cm1; see Fig. 2), it can be assumed that the band at 915 cm1 represents a species weakly coordinated to the TiO2 surface. A complexation with ligands dissolved from the mineral into the aqueous phase can be ruled out according to the results of the washing procedures, where no significant ion concentrations were detected in the leachates (see Section 3.4.2). In the spectral region >1350 cm1, three bands at 1532, 1455 and 1399 cm1 are distinct during the sorption process. In the spectra of the subsequent flushing step, only the latter two bands are observed. Again, in this spectral region, the impact of atmospheric-derived carbonate anions interacting with the solid phase can not be ruled out (considered in Section 3.4.1).

surface coverage or higher initial U(VI) concentrations has been reported for several surfaces, e.g. albite (Walter et al., 2005), alumina and silica (Sylwester et al., 2000) and montmorillonite (Chisholm-Brause et al., 1994). Therefore, we performed IR spectroscopic measurements with an enhanced time resolution of the U(VI) sorption processes on TiO2 in order to elucidate the evolution of different species possibly formed at different stages of the proceeding sorption processes. To evaluate the transformation of surface species as a function of surface loading with a sufficient time resolution, a sorption experiment was performed under standard conditions but at a reduced flow rate (0.053 mL min1) and an extended time of induced sorption (360 min.). The difference spectra, shown in Fig. 4 (left), were calculated from spectra recorded at subsequent time intervals of 60 min during the stage of sorption (see Scheme 1, bottom). These difference spectra represent the evolving sorption process within different time intervals after induced sorption, e.g. 0–60, 60–120 min etc. Different frequencies of the m3(UO2) mode are observed as a function of U(VI) loading: an absorption maximum at 899 cm1 at very low surface loading (Fig. 4, red trace), shifted to 908 cm1 at an advanced sorption stage, from 60 to 180 min (Fig. 4, blue trace) and finally to 915 cm1 after a prolonged contact time, i.e. P240 min (Fig. 4, green trace). The calculated sum of the spectra representing the early (60 min.) and late (240 min.) stage of sorption shows a band at 908 cm1 (Fig. 3, right panel, dotted trace). This band is in excellent agreement with the band observed in the spectra from the medium stage of sorption (e.g. 120 min., Fig. 3, blue trace). Consequently, at this stage

3.3. Identification of different U(VI) surface species on TiO2 by time-resolved (TR) IR spectroscopy

calculated sum

300 → 360 240 → 300 180 → 240 120 → 180 60 → 120 908 0 → 60

1600

915

1383

1455

Δ min

1400 1000

Wavenumber / cm

908

Absorption / a.u.

0.005

1522

The sorption mechanisms occurring at the water–mineral interface strongly depend on the contact time of U(VI) with the mineral. Furthermore, the formation of different species may involve various thermodynamically stable and a series of metastable phases (O’Day, 1999). The formation of different U(VI) species depending on the

923

198

899 900 –1

950

900

850

Wavenumber / cm

–1

Fig. 4. Mid-IR spectra showing the elucidation of U(VI) surface species on TiO2. Left: long-time sorption experiment (S6, 20 lM initial U(VI), 0.1 M NaCl, pH 5, flow rate 0.053 mL min1). Right: comparison of single species spectra and calculated spectra (the red, blue, and green spectra are experimental data; the gray dotted spectrum is the calculated sum of the red and the green spectra). Ordinate scaling is given by the bar in units of optical density (left panel only). Other values indicated are in cm–1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Sorption of U(VI) onto TiO2

of sorption, two species with absorption maxima at 899 and 915 cm1 are simultaneously present. A comparison of the observed frequencies of the m3(UO2) mode with those of the aqueous species obtained at 20 lM U(VI), 0.1 M ionic strength and pH 5.5 (Fig. 1, second upper trace), allows a preliminary assignment to a specific complexation behavior. At low surface loading, the formation of a surface complex with an inner-sphere character is suggested. The considerable spectral shift of the m3(UO2) mode to 899 cm1 compared to the aqueous species which was found at 923 cm1 can only be explained by an extensive reduction of the hydration sphere of the actinide ion upon coordination to TiO groups. In contrast, at higher surface loadings, the reduced shift of the m3(UO2) mode to 915 cm1 reflects a surface species showing a hydration sphere structurally similar to the aqueous species. As a consequence, the binding of such a surface species can be described by electrostatic interactions, i.e. the formation of outer-sphere complexes (Table 2). For verification of these assumptions and for the identification of the outer-sphere complex at high U(VI) surface loading, in situ IR experiments were performed where the TiO2 phase was initially saturated with a 20 lM U(VI) solution for 16 h. This extensive sorption step was followed by alternating steps of flushing with blank solution and sorption steps with 20 lM U(VI). Because outer-sphere complexes are expected to be released easily from saturated solid phases, the spectra obtained from these experiments should mainly show contributions from such complexes. In contrast, inner-sphere bound species are not expected to be released to a significant extent during the time range

199

of these reaction cycles. Chemisorbed species are of higher thermodynamic stability and, thus, the reaction rate of the desorption processes is considerably reduced. The spectrum obtained from the prolonged sorption procedure (16 h) is referred to as “1st sorption” (Fig. 5, lower trace) and shows three bands at 1520, 1386 and 915 cm1. The spectrum is in good agreement with the spectra obtained at a late stage in the previous experiment representing a high U(VI) surface accumulation (cf. Fig. 4, upper traces). After flushing the solid phase with 0.1 M NaCl for 60 min (“1st flushing”), the resulting spectrum is inverted with respect to the sorption spectrum, again with (negative) bands at 1520, 1386, and 915 cm1 indicating that the same surface species is released from the TiO2 phase upon flushing with a blank solution under constant conditions (Fig. 5). In addition, this spectrum is in agreement with the flushing spectra of the online monitored sorption experiment (cf. Fig. 3, blue traces). Subsequently, a 2nd sorption step for 90 min was induced with 20 lM U(VI) solution, as had previously been used for the 1st sorption stage. No differences between the sorption spectra obtained at the end of the 1st and during the 2nd stage of sorption were detected, indicating that the same U(VI) surface species of high surface loading was attached to the TiO2 (Fig. 5). After the 2nd flushing with the blank solution for 30 min, the same spectral characteristics as already observed for the previous flushing processes are evident (Fig. 5, upper trace). The high reproducibility of these spectra indicates the formation of the same U(VI) surface species. Such desorption and re-sorption processes within the sub-hour time

Table 2 Observed frequencies of the m3(UO2) mode and their assignments to aqueous U(VI) species and suggested surface structures. Conditions

m3(UO2)/cm1

Assignment and suggested structures

Aqueous solution 0.1 M U(VI), pH 1.55 20 lM U(VI), 0.1 M NaCl, pH 5.5 5 mM, 0.1 M HCO 3 , pH 8.8

961 923 893

Free uranyl(VI) aquo ion (Quile`s and Burneau, 2000) Monomeric uranyl(VI) hydroxo complex (Mu¨ller et al., 2008) Uranyl(VI) tricarbonato complex (Mu¨ller et al., 2008)

899

Inner-sphere bidentate complexa

915

Outer-sphere complexa

TiO2–water interface 1st surface complex favored at low surface loading, low pH, high ionic strength

2nd surface complex favored at higher surface loading, neutral pH, low ionic strength

a

The gray shading for the two surface complex structures represents a partial hydration sphere (inner-sphere) and an intact hydration sphere (outer-sphere).

200

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on thermodynamic considerations where the formation of the more stable surface species is expected to occur prior to the formation of the outer-sphere complexes.

nd

1044

1386

3.4. Selective modifications of the sorption system

nd

2 sorption

1000

–1

800

Fig. 5. Mid-IR spectra of a U(VI) saturated TiO2 film (S6, 20 lM initial U(VI), 0.1 M NaCl, pH 5). From bottom to top: the 1st sorption step was performed at 53 lL min1 for 18 h to saturate the TiO2 with U(VI), the spectrum represents absorption changes between 360 and 300 min of induced sorption (cf. Fig. 4). The 1st flushing was performed for 60 min, the 2nd sorption for 90 min, and the 2nd flushing for a further 30 min. The last steps were done at 0.2 mL min1. Indicated values are in cm1.

range support the assumption of a predominant electrostatic surface complexation of the actinide species at high surface loading. Hence, the previously suggested assignment of the 915 cm1 band of m3(UO2) to outer-sphere U(VI) surface complexation is supported. In contrast, the removal of the species represented by the absorption maximum at 899 cm1 is not evident from these spectra. From TRLFS experiments reported recently, it was concluded that two U(VI) complexes are formed on the TiO2 surface (Vandenborre et al., 2007). The molecular structure of monocrystalline rutile surfaces provides oxygen atoms which are coordinated 3-fold, 2-fold (bridging oxygens), or single-fold (top oxygens) to titanium atoms. From the spectroscopic analysis of batch samples of U(VI) sorption onto these surfaces, it was inferred that one surface complex arises from the sorption of UO2þ 2 on two bridging oxygen atoms, while the formation of the second complex is preferred as the sorption rate increases, involving the reaction of UO2þ 2 onto one bridging and one top oxygen atoms (Vandenborre et al., 2007). Furthermore, results from grazing incidence EXAFS experiments of the (1 1 0) plane of monocrystalline TiO2 and from isotropic EXAFS on polycrystalline TiO2 reveal a similar sorption behavior, namely the formation of a bidentate complex. Additionally, grazing incidence EXAFS on the (0 0 1) plane suggests the formation of an outer-sphere uranium complex (Den Auwer et al., 2003). Our spectroscopic analysis of the U(VI) sorption on TiO2 provides strong evidence for the presence of the two postulated surface species. Moreover, from the evolution of the m3(UO2) mode with time, the spectra demonstrate that these two surface species are formed successively in situ with increasing surface loading, that is, the contact time of U(VI) at the TiO2–water interface. This is in accordance with predictions of the molecular mechanisms based

pH 5

a 893

1200

5 µM

b

c [U(VI)]init: 20 µM

d e

g

[U(VI)]init: 20 µM pH 5

[U(VI)]init: 20 µM pH 5 900

Wavenumber / cm

50 µM pH

f

1600 1400 1200 1000

[U(VI)]init

20 µM

908

1400

Wavenumber / cm

1123 1059

1600

1523 1455 1399

st

1 sorption

3.4.1. Influence of the U(VI) solution properties on sorption onto TiO2 In a first series of experiments, the influence of the initial U(VI) concentration in the range from 5 to 50 lM at a fixed pH of 5 was studied. The spectra obtained after 90 min of induced sorption are comparatively shown in Fig. 6a–c. At an initial U(VI) concentration of 5 lM, and consequently only sparsely covered surface, the m3(UO2) mode is exhibited as a weak, broad absorption band with a maximum at 893 cm1 (Fig. 6a). Upon increasing the initial U(VI) concentration in the freshly prepared sorptive solution, this band becomes hypsochromically shifted to 908 cm1 for 20 lM U(VI) (Fig. 6b) and to 910 cm1 for 50 lM U(VI) (Fig. 6c). These results support the findings that the type of surface complex formed strongly depends on the U(VI) saturation level on TiO2.

Absorption / a.u.

st

1 flushing

Time

915

1520

Absorption / a.u.

2 flushing

3 4 7

[NaCl] 3M

h

10 M

i

N2

-4

800 –1

Fig. 6. Mid-IR spectra of U(VI) sorption onto TiO2(S6). Influence of U(VI) concentration at pH 5 (a–c). Influence of pH conditions at 20 lM U(VI) (d and f). Influence of ionic strength at 20 lM U(VI), pH 5 (g and h). Influence of the absence of atmospheric CO2 (i) (20 lM initial U(VI), 0.1 M NaCl, pH 5, 90 min of induced sorption). Indicated values are in cm1.

Sorption of U(VI) onto TiO2

In a second series of experiments, the sorption processes at different pH values ranging from 3 to 7 were investigated. The IR spectra are shown in Fig. 6d–f for pH 3, 4 and 7, respectively. At pH 3, a less intense band representing m3(UO2) is observed at 893 cm1 (Fig. 6d). Upon increasing the pH to 4, this band is slightly shifted to 900 cm1 (Fig. 6e). At pH 5, the respective band is further hypsochromically shifted to 908 cm1 and shows a significantly increased relative intensity (Fig. 6b). No further spectral changes are observed when pH 7 is reached (Fig. 6f). In the pH range from 3 to 7, the spectroscopic data indicate increasing U(VI) uptake by TiO2. This is in good agreement to previous spectroscopic and macroscopic data (Guo et al., 2004; Pandey, 2006; Lefe`vre et al., 2008; Comarmond et al., 2011). Moreover, the isoelectric point (IEP) of sample S6 was determined to be at pH 6 (cf. Table 1), indicating a positively charged mineral surface below this pH. In this pH range, the presence of cationic U(VI) species is inferred under the prevailing conditions of this study (cf. Fig. 1 in (Mu¨ller et al., 2008)). Hence, electrostatic interactions, i.e. formation of outer-sphere surface complexes at the TiO2 interface can be ruled out in an acidic medium, but might become favorable under neutral conditions. Because the formation of inner-sphere complexes occurs also under electrostatic repulsion, the predominance of such complexes is assumed in the acidic pH range. Moreover, the spectroscopic data arising from the pH series (Fig. 6d–f) show similarities to those from the concentration series (Fig. 6a–c). In both series, the spectra representing low amounts of sorbed U(VI) on the solid phase due to low initial U(VI) concentration (Fig. 6a) or low pH (Fig. 6d), exhibit an absorption maximum of the m3(UO2) mode below 900 cm1. With increasing surface loading of the solid phase either due to increasing U(VI) concentrations (Fig. 6b and c) or due to increasing pH (Fig. 6e and f), the uranyl absorption band is shifted to higher wavenumbers. The increasing U(VI) surface accumulation was also confirmed by ICP-MS analysis of the U/TiO2 ratios of the removed films after the sorption experiments which were found to be less than 100 mg U/g TiO2 (0.83 U/nm2) for sorption at pH 6 4 or 5 lM initial U(VI) concentration. Significant higher ratios (>150 mg U/g TiO2, 1.24 U/nm2) were found after sorption experiments performed at increased pH levels or with initial U(VI) concentration P 20 lM with absorption maximum at 910 cm1, respectively. In summary, both series of experiments support the formation of distinct uranyl surface species as it was assumed from time-resolved experiments (Section 3.3). One species is represented by a considerable red-shifted band around 893 cm1 whereas the second species is represented by the absorption band at 908 cm1 as observed before (Figs. 3, 4, and 6b, c, f, i). As it was already shown by results of time-resolved experiments (Section 3.3), the band observed at 908 cm1 has to be considered as a superposition of bands representing two species with absorption maxima around 893 and 915 cm1 simultaneously formed under the prevailing conditions. In a previous IR study of U(VI) sorption on TiO2, using the sample S1 (see Table 1), Lefe`vre et al. performed a series

201

of measurements in the pH range from pH 4.1 to 7 using a circulating flow system (Lefe`vre et al., 2008). The m3(UO2) peak is increased in intensity upon increasing pH conditions, which is in accordance with the observations of this study (Fig. 6d–f). Further on, deconvolution of the absorption band of m3(UO2) evidenced the presence of two peaks at 905 and 920 cm1 with almost constant contribution ratios along the measured pH range. The peaks were assigned to one type of surface complex, a trimer linked by two uranyl ions to the TiO2 surface. The spectra obtained in our study show bands at 895 and 915 cm1 (Figs. 3 and 4). These spectral differences can be explained by the reduced concentration level (20 lM U(VI)) in this study compared to the much higher level used before (Lefe`vre et al., 2008). From previously performed ATR FT-IR spectroscopic studies of U(VI) speciation in aqueous solution, it is known that polymerization of hydrolysis products may occur in the submillimolar range, whereas in the micromolar range, monomeric species are predominant (Mu¨ller et al., 2008). Additionally, the different experimental protocol and set-up, e.g. ATR crystal’s dimensions, resulting in different solid–liquid ratios, flow velocities and U(VI) saturation states may explain the spectral deviations and the different conclusions reached. Further indication for the presence of two surface species is provided by the results of experiments at different ionic strengths. In Fig. 6 the spectra obtained at 3 and 0.0001 M are compared. Whereas no spectral shift of the m3(UO2) mode are observed for ionic strengths between 0.1 and 0.0001 M NaCl (Fig. 6b and h), the mode shows a considerable red-shift from 908 to 899 cm1 for a higher value of 3 M (Fig. 6g). This shift can be explained by the hampered formation of outer-sphere complexes at increased ionic strengths. Furthermore, the relative intensity of this vibrational mode is decreased at higher ionic strength, indicating a reduced sorption capability. Carbonate species are abundant in soils and soil solutions and are known to adsorb on mineral surfaces. In turn, the adsorbed carbonate affects surface chemical properties such as surface charge and protonation and the sorption of other ions (Su and Suarez, 1997; Wijnja and Schulthess, 2001). Additionally, a change of the surface properties due to the presence of carbonate species might influence the sorption processes. Carbonate is present in aqueous phases equilibrated with air due to dissolved carbon dioxide. Hence, a standard in situ sorption experiment under an inert gas atmosphere (CO2-free N2) was performed (Fig. 6i). The comparison of the 90 min sorption spectra obtained under normal air and the inert gas atmosphere shows no distinct differences in the position of the band of m3(UO2) (cf. Fig. 6b and i) suggesting the formation of similar surface species as those observed under normal atmospheric conditions. However, the relative intensities of the bands above 1000 cm1 in both spectra are significantly altered compared to the intensity of the band representing the m3(UO2) mode. Under inert gas conditions, the broad band around 1040 cm1 is considerably increased. In contrast, the intensities of the bands at 1523 and 1455 cm1 are decreased and the band at 1399 cm1 is not observed under inert gas conditions. The former band is attributed to

K. Mu¨ller et al. / Geochimica et Cosmochimica Acta 76 (2012) 191–205

1529 1454 1393

Sample S6

a

MQ HCl NaOH

1600

1400

908

pure 1200

1000

Wavenumber / cm

1600

800

–1

Sample S2

aggressively washed 1529 1454 1393

b

slightly washed pure

1400

1200

908

3.4.2. Sorption of U(VI) on purified TiO2 phases As shown in Section 3.1 and Table 1, this study involves a number of TiO2 samples of different origins and manufacturing procedures, resulting in potentially significant differences of chemical composition and surface structure which may cause minor spectral changes. In particular, surface contamination from strong complexing ions might influence the sorption behavior at the solid–liquid interface to a significant extent. Hence, microwave digestion of the TiO2 samples and subsequent analyses of minor constituents were performed. Although the results from ICP-MS analysis indicate a very low weight percentage of impurities, i.e. <0.1%, their distribution in the solid could directly influence the surface reactivity. Several possibilities of solid contamination can be suggested: (1) the impurities are in the bulk of the solid and their impact on surface properties is negligible, (2) they are present as a distinct impurity phase, e.g. Na2SO4 and washing can purify the solid, or (3) they are present as a surface contamination and may

provide highly reactive additional surface binding sites. In the latter case, purification of the solid is often difficult to achieve (Lefe`vre et al., 2006a). Comparing the success of both washing procedures, according to the protocols given in Section 2.2, no significant reduction of the concentrations of most of the impurities was observed for the solids after washing (data not shown). This suggests that the impurities are bound within the mineral structure, i.e. the bulk solid, and not on the TiO2 surface. In consequence, the impurities are not removed by the procedures performed, but are detected after the digestion and subsequent analysis by ICP-MS. In contrast to the problematic determination of impurities by ICP-MS, changes in the IR spectra allow one to draw precise conclusions concerning the impact of impurities on the surface reactivity. Standard in situ sorption studies of U(VI) were conducted with TiO2 obtained after each washing step to monitor the progress of the procedure and to assign possible spectroscopic changes to the removed contaminants. Fig. 7 exemplarily shows the resulting spectra for the sample S6. No significant changes in the spectral position of band maxima and in the relative intensities were observed upon each single step of the mild washing procedure. Furthermore, the type of TiO2 washing, that is irrespective of using strong (5 M) or weak (0.1 M) agents, does not influence significantly the sorption process of

Absorption / a.u.

unspecific surface modes of the solid titania phase. Its increased relative intensity can be explained by retarded sorption kinetics under a N2 atmosphere, derived from the reduced U/TiO2 ratio after 90 min of sorption determined by ICP-MS. A reduction by a factor of 2 was found compared to ambient atmosphere conditions (data not shown). Consequently, this 1040 cm1 band represents non-specific structural alterations of the mineral phase induced by the sorption processes. Under inert gas conditions, the overall uptake of UO2þ 2 ions is reduced and the relative intensity of this band is increased in the spectrum as shown in Fig. 5i. The spectral features observed above 1350 cm1 in nearly all spectra related to sorption and flushing of the solid TiO2 phases presented in this work are attributed to uranyl carbonate species. These complexes result from dissolution of atmospheric CO2 in the aqueous U(VI) solution at near-neutral pH conditions prior to sorption at the TiO2–water interface. The assignment is confirmed by the absence of these bands in the spectra recorded in inert gas atmosphere (Fig. 6i) and at pH 3 (Fig. 6d). Under these conditions, the presence of the atmospheric-derived carbonate ion can be neglected. In this spectral range, the band patterns generally consist of three bands around 1532-22, 1455 and below 1400 cm1 showing strong variations of their relative intensities. In addition, contributions from modes of the uranyl hydroxo species should be considered as was shown in Section 3.1 (see also Fig. 2). This complicates the interpretation of the spectral data. Finally, an accurate assignment to distinct surface species can only be achieved by extensive investigations of the carbonate surface complexes as it was demonstrated for hematite (Bargar et al., 2005). Therefore, the data presented here will be helpful for further investigations of the sorption complexes of U(VI) on titania under consideration of the formation of ternary carbonate complexes. Nevertheless, with respect to the observed frequencies of the uranyl band in the spectra of this work, we suggest that the absence of carbonate ions only hampers the kinetics of the uranyl surface complexation, but does not influence the U(VI) speciation at the titania phase under the prevailing conditions.

Absorption / a.u.

202

1000

Wavenumber / cm

–1

800

Fig. 7. Mid-IR spectra showing the influence of TiO2 washing on U(VI) sorption (20 lM initial U(VI), 0.1 M NaCl, pH 5, 90 min of induced sorption). (a) S6, sorption performed on the pure sample and after each washing step (from bottom to top). (b) S2, comparison of the effect of two different washing protocols. For more details see Section 2.2. Indicated values are in cm1.

Sorption of U(VI) onto TiO2

U(VI), as it is exemplarily shown for the sample S2 in Fig. 7b. Thus, it can be stated that impurities, possibly removed by the mentioned washing protocol, have no impact on the U(VI) surface speciation on TiO2. Additionally, the spectra provide evidence that the absorption bands observed in the U(VI) sorption spectra between 1200 and 1000 cm1 are not due to removable impurities at the TiO2 surface. In summary, the different procedures for purification of the various TiO2 samples show no relevant differences in the complexation behavior of the uranyl ion as reflected by IR spectroscopy. 4. CONCLUSIONS The sorption of U(VI) from micromolar solutions onto TiO2 was comprehensively investigated by application of surface sensitive in situ ATR FT-IR spectroscopy. The systematic study considered the impact of various anatase and rutile samples with differences in specific surface area, particle size, and IEP. Furthermore, variations in the aqueous U(VI) system, e.g. pH, U(VI) concentration, ionic strength and the absence of atmospheric-derived carbonate, as well as the reaction time of the U(VI) at the TiO2 interface were considered. From the results of this work, the formation of two different U(VI) surface complexes on TiO2 can be derived as a function of accumulated U(VI) concentration on the TiO2 surface. At low surface loading, that is during the first stage of sorption, the formation of an inner-sphere complex, probably with a bidentate coordination occurs. In comparison to the aqueous species, the significantly lowered frequency of the absorption maximum of this species (895 cm1) can only be explained by strong interactions of the U(VI) ion with functional surface groups. From aqueous complexes of the U(VI) ion it is known that such strong bathochromic shifts are observed when strong ligands bidentately coordinate in the equatorial plane of the UO2þ 2 unit. For instance, the frequency of the uranyl mode of the UO2 ðCO3 Þ4 complex is found at 893 cm1 3 (Mu¨ller et al., 2008). Therefore, the surface species formed during the first sorption steps is assigned to a bidendate surface complex (Table 2). With increasing loading of the TiO2 phase, a second surface species is preferentially formed showing a less shifted absorption maximum at 915 cm1. This species can be rapidly and reversibly removed and re-sorbed by subsequently flushing of the saturated TiO2 phase with blank and uranyl solutions (Fig. 5). Therefore, the second species represents a less stable surface complex than the first species. From the IR data, an assignment to a monodentate or bidendate coordination cannot be given (Table 2). Recently, U(VI) sorption species on single crystal TiO2 surfaces were investigated with fluorescence (Vandenborre et al., 2007) and X-ray absorption spectroscopic techniques (Drot et al., 2007). From these results, two different bidentate surface complexes were derived with pH dependent stability ranges. A surface complex at strong surface sites was postulated at lower pH values whereas weaker surface sites become relevant at higher pH values. This is in very good

203

agreement with results presented in this work. However, it has to be noted that the initial concentration of the U(VI) stock solutions in our work was significantly reduced by a factor of at least 10. This is of great importance for the aqueous U(VI) speciation particularly in this concentration range (Mu¨ller et al., 2008), and thus, it might become relevant for the formation of the sorption complexes. Further structural information of the two surface species may be derived from quantum chemical calculations based on the available spectroscopic results in future times. Sorption parameters, such as concentration, pH, and contact time significantly influence the U(VI) uptake and, hence, determine the type of U(VI) complexation. In contrast, the crystallographic modification of the TiO2 phase plays a minor role. Surface specific parameters, such as surface area, porosity, and changes in PZC have been shown to be decisive for the retention capability of the respective phase, but the impact on the type of surface complexes seems to be of relatively low significance on a molecular level. The identification of the subsequent formation of the two different sorption species provided in this work substantiate the ideas of surface complexation modeling, which are generally derived from investigations based on thermodynamic approaches. Consequently, this work presents valuable reference data for further in situ spectroscopic analysis of sorption and complexation reactions of hexavalent actinide ions at the solid–liquid interface. Furthermore, the data may act as a model for the investigation of surface complexation on more complex, naturally occurring mineral phases showing numerous functional groups. ACKNOWLEDGEMENTS The presented work was supported by the Deutsche Forschungsgemeinschaft (Grant FO 619/1-2). The authors are grateful to K. Heim, N. Baumann, U. Schaefer, A. Ritter, C. Eckardt, E. Christalle, A. Mu¨cklich, S. Weiß and C. Fro¨hlich for the preparation of the U(VI) stock solution, TiO2 sample S5, and for ICP-MS, BET, SEM, TEM, PCS and Zeta potential measurements, respectively. Particle size distribution measurements at TU BA Freiberg (B. Kubier) and visiting scientist fellowships for J. Harrison, MJC and GL from the HZDR are acknowledged.

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