Uptake of oxo-anions by cements through solid ...

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Hasset,D.F.: Oxyanion substituted ettringites: synthesis and characterisation and their potential role in immobilisation of As,. B, Cr, Se and V. Mat. Res. Soc.
Radiochim. Acta 90, 639–646 (2002)  by Oldenbourg Wissenschaftsverlag, München

Uptake of oxo-anions by cements through solid-solution formation: experimental evidence and modelling By Michael Ochs1 , ∗, Barbara Lothenbach1 and Eric Giffaut2 1 2

BMG Engineering Ltd, Ifangstrasse 11, CH-8952 Schlieren-Zürich, Switzerland ANDRA, Parc de la Croix Blanche, 1/7 rue Jean Monnet, F-92298 Châtenay-Malabry, France

(Received September 3, 2001; accepted April 11, 2002)

Uptake / Solid-solution / Selenium / Chromium / Cement / Ettringite Summary. Uptake experiments were carried out with selenate and chromate on fresh and leached Portland and high-alumina cements; and in addition with selenate and selenite on synthetic ettringite. In all experiments with cements, exceptionally high uptake could be observed under conditions where significant amounts of secondary ettringite were formed. Experimental data obtained for pure ettringite corroborated the important role of this mineral phase. However, uptake kinetics show opposite trends in these two systems, which can be viewed as end-members of the same process: Where a fast precipitation of secondary ettringite occurred, initial uptake was high, with K d values in the range of ≈ 1–5 m3 /kg for Se(VI), but decreased with time. Uptake by pre-formed (primary) ettringite initially gave lower K d values (≈ 0.1 m3 /kg), which increased with time. After prolonged equilibration times, the two systems started to approach each other. Concurrent measurements of sulphate concentrations allowed to extract a mean partition coefficient for the solid-solution formation of selenite and selenate with ettringite. Based on this simple solid-solution model, a pragmatic quantitative relation was developed that permits to estimate K d for Se(VI) on whole cement as a function of the concentrations of sulphate in the cement and in the solution. A test against experimental data shows reasonably good agreement between measurements and calculations. This relation can also be directly applied to estimate K d values of chromate (and perrhenate) by different cements, indicating the same uptake mechanism. The approach may be less well suited for Se(IV), whose uptake on cement appears to be related mainly to minerals other than ettringite.

1. Introduction Preliminary French repository designs for several types of radioactive wastes rely on cements for the retention of radionuclides, similar to concepts used in several other countries. While many elements exhibit low solubilities and high sorptive uptake in cementitious systems, very high solubilities are observed for oxo-anions formed by e.g. Se, Mo, Tc, and Cr, depending on redox conditions (see e.g. [1]). *Author for correspondence (E-mail: [email protected]).

Therefore, the performance of engineered barriers for the retention of such oxo-anions rests on uptake by the solid cement matrix. However, K d values also appear to be small, based on the limited information available. For Se, a safety-relevant fission product, practically no K d values for cement have been measured [2]. Only recently, [3] published K d values for Se(IV) on Portland cement (PC), but no data for Se(VI) are available, to our knowledge. Sorption and spectroscopic data for Se(VI) and Se(IV) on other minerals, including aluminium oxide [4, 5], iron oxide [6, 7] and calcite [8, 9], leave open questions regarding the exact sorption mechanisms, but clearly show that Se(IV) and Se(VI) sorb appreciably in the acidic-neutral pH range, while uptake decreases drastically with increasing pH. This is a typical behaviour of anions sorbing via surface complexation reactions [10] and is consistent with low K d values of oxo-anions on cement. Chromate is an important chemical toxicant, and a chemical analogue for other hexavalent oxo-anions; K d values for chromate on Portland cement [11], [12] also show the tendency discussed for Se(VI): At circumneutral pH, Cr(VI) uptake is considerable, but decreases sharply at pH > 11. In comparison, the K d values reported by [3] are surprisingly high, which may be an indication of different or additional uptake mechanisms. For such mobile species, relatively small uncertainties in K d can make a significant difference with regard to the performance assessment. In recent K d databases [2], little or no sorption of oxo-anions has been assumed, based on the scarce data and circumstantial evidence. Therefore, it is important to (i) assess the uptake of oxo-anions by cement and (ii) to develop quantitative means that allow to relate it to repository conditions and predicted cementitious buffer evolution. To this end, we measured the uptake of Se and Cr by PC, and on high-alumina cements (HAC) for comparison to evaluate the influence of Ca-aluminates. Additional experiments on pure ettringite were done to gain insight into the underlying mechanisms as a basis for model development.

2. Experimental Cement samples included a Portland cement and two highalumina cements: The Portland cement used (henceforth

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termed CEM-HS) is a CEM I 42.5 HS sulphate resistant cement, obtained from HCB. It contains ca. 1.8% C3 A, 64.5% C3 S, 20.0% C2 S, 5.3% C4 AF, and 6%–7% CaSO4 . As a typical high-alumina cement, “Ciment Fondu Lafarge” (henceforth termed CFL) was obtained from Lafarge, it contains CA as major and C12 A7 , C2 S, and C4 AF as minor constituents, plus 16% Fe- and 4% Si-oxides. As a pure Caaluminate, “Secar 71” (henceforth termed S71) by Lafarge was used, it contains CA and CA2 as major, and C12 A7 plus α-Al2 O3 as trace constituents. Hydrated cement paste (HCP) was prepared from each cement using a cement/water ratio of 0.40 and cured for 28 days, initially under water and then at 90% humidity. The CEM-HS sample was cured at 25 ◦ C, the two HAC samples at 5 ◦ C, using pre-cooled materials, to avoid conversion (cf. [13]). Simulated porewater solutions corresponding to fresh and leached cement were prepared for each cement type by leaching crushed HCP with artificial groundwater (AGW), following the procedure outlined in [14]. AGW was prepared from soluble salts, its composition is given in Table 1. Simulated fresh cement porewater (FPW) was prepared by equilibrating finely crushed HCP and AGW in a closed bottle for 72 h on an overhead shaker (1 rpm, solid : water ratio 1 : 1). In case of CEM-HS, separate samples were kept for 42 days, to ensure constancy of solution composition with time. The resulting compositions after 72 h are given in Table 1. Simulated aged cement porewater (APW) was prepared for each cement type by repeatedly reacting crushed and sieved HCP (0.25–0.5 mm) and AGW for 24 h in a closed bottle on an overhead shaker (1 rpm, solid : water ratio 1 : 5). After each reaction period, ≈ 82% of the supernatant was replaced with fresh AGW and the pH was determined. The final composition of the resulting solutions is given in Table 1. The final pH and Ca concentration of the leached PC-solution correspond to equilibrium with portlandite, and soluble alkalis have Table 1. Composition of artificial groundwater AGW and all simulated fresh and leached cement porewaters as used for the uptake measurements. Total background concentrations of Se and Cr are also given. Background concentrations in the fresh solid hydrated cement pastes had been determined for Se as < 1.3 × 10−6 mol/kg in all samples, and for Cr as < 4.8 × 10−7 mol/kg in the HAC samples and 3.9 × 10−5 mol/kg in the PC sample. AGW contained 5.7 × 10−4 M dissolved carbonate (calculated).

parameter

been leached, which is consistent with cement degradation models (e.g., [15, 16]) and data from other leaching studies [17]. The mineralogy of the fresh and leached CEM-HS samples was assumed to correspond to the phases identified by [16–18]: Mainly calcium-silicate-hydrate (CSH) gel with a high Ca/Si ratio plus portlandite for the fresh, and CSH gel with a low Ca/Si ratio plus portlandite for the leached sample, with minor amounts of hydrogarnet, ettringite, and hydrotalcite present in each. HAC samples were investigated with XRD (carried out at the Swiss Federal Institute of Technology Zürich), results for fresh and leached samples are given in Table 2. Leaching of the HAC samples with AGW lead to the precipitation of a white gel during the first 20 exchange cycles. This gel was removed together with the exchanged solutions. After filtration and drying at 40 ◦ C, ettringite was identified by XRD as the main constituent. Ettringite (Ca6 Al2 (SO4 )3 (OH)12 ·26H2 O) was synthesised as proposed by [19] with the difference that Al2 (SO4 )3 was used instead of Na2 Al2 O4 and Na2 SO4 . All solutions and suspensions were constantly purged with N2 to exclude CO2 . The 0.5 M Al2 (SO4 )3 solution was added at a rate of 40 mL/min to the CaC4 H6 O4 under constant stirring. After a white precipitate formed, approx. 5.5 moles NaOH were added to bring the suspension to a pH of 12.5. The resulting suspension was stirred for 24 hours. The precipitate was filtered (0.45 µm) and washed once with H2 O to remove excess salts. The solid was dried in a vacuum desiccator at room temperature and identified as well-crystallised ettringite by XRD. Uptake of Se and Cr on HCP samples was measured in batch experiments, using 50 mL HDPE bottles. Crushed, size-fractionated (0.25–0.5 mm) HCP was weighted into the bottles, then the corresponding solutions were added. These solutions had been pre-reacted for several days with separate HCP samples and then spiked with Se(VI) and Cr(VI)

AGW

CFL

S71

CEM-HS

AGW, simulated porewaters for fresh hydrated cements (1 equilibration step, 72 h) pH 8.0 12.66 12.79 13.24 1.2 × 10−3 1.1 × 10−3 n.d. Ca (M) 5.8 × 10−3 < 2.1 × 10−4 < 2.1 × 10−4 < 4.1 × 10−4 Mg (M) 5.2 × 10−3 Na (M) 4.0 × 10−2 4.2 × 10−2 6.8 × 10−2 1.0 × 10−1 K (M) 2.5 × 10−4 4.8 × 10−3 4.5 × 10−4 9.1 × 10−2 Al (M) n.d. 7.5 × 10−3 3.7 × 10−3 n.d. 1.4 × 10−2 3.5 × 10−4 7.2 × 10−5 n.d. SO4 (M) 3.3 × 10−3 2.6 × 10−3 n.d. Cl (M) 3.3 × 10−2 n.d. Se (M) n.d. n.d. < 6.7 × 10−7 < 1.9 × 10−7 < 1.9 × 10−7 1.7 × 10−5 Cr (M) < 1.9 × 10−7 simulated porewaters for leached hydrated cement number of exchange cycles 30 18 pH 10.4 12.0 2.2 × 10−3 Ca (M) 8.2 × 10−3 −4 < 2.1 × 10−4 Mg (M) < 4.1 × 10 Na (M) 3.4 × 10−2 3.8 × 10−2 K (M) 2.2 × 10−4 2.3 × 10−4 Al (M) 5.6 × 10−6 5.5 × 10−4 1.3 × 10−2 4.2 × 10−4 SO4 (M) −2 Cl (M) 3.3 × 10 4.1 × 10−2 Se (M) < 6.3 × 10−8 < 6.3 × 10−7 < 2.9 × 10−7 Cr (M) 1.2 × 10−6

12 12.5 3.0 × 10−2. < 4.1 × 10−4 4.1 × 10−2 2.5 × 10−4 n.d. 1.5 × 10−2 3.6 × 10−2 n.d. n.d.

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Uptake of oxo-anions by cements through solid-solution formation Table 2. Minerals that could be identified by XRD analysis in fresh, hydrated HAC pastes, and in the corresponding leached samples.

Sample

Un-reacted clinker phases a

Hydration products a

fresh samples (curing time = 28 days) CFL Ca-aluminates (not identified) S71 CA2 , C3 A5

CAH10 , Ca-Al-hydrates (not identified) C3 AH6 , gibbsite

leached samples CFL S71

ettringite, CAH10 , C3 AH6 ettringite, gibbsite

CA, C3 A −

a: In this notation, C = CaO, S = SiO2 , A = Al2 O3 , F = Fe2 O3 , and H = H2 O (see, for example, [18]).

to give initial concentrations of 8 × 10−5 M and 1 × 10−3 M, respectively. These concentrations were stable under the respective conditions; they had been selected based on solubility considerations and background concentrations of Se and Cr in HCP samples and corresponding equilibrium solutions (Tables 1 and 2). To avoid introduction of CO2 , the headspace in the bottles was minimised, and separate bottles were set up for each desired equilibration time. Set-up, sampling, and storage of samples was done under Ar. Agitation was done periodically by hand, to avoid abrasion of HCP. For each system, a corresponding blank was set up, and initial Cr and Se concentrations were analytically verified. Phase separation was done by filtration through 0.45 µm membranes. It had been previously verified that the use of 10 000 MWCO filters did not lead to different results. Assessment of overall experimental uncertainties is based on experiments with CEM-HS (fresh and aged HCP with 5, 50 g/L) carried out in quadruplicate, and are expressed as ± the standard deviation of the sample. Because sample standard deviations are very similar for each dataset, average values are reported. Uptake by ettringite was measured in batch experiments, using 100 mL PET bottles to which 100 mL of solutions containing Na2 SO4 as well as Na2 SeO4 or Na2 SeO3 (only Na2 SO4 for experimental blanks), 0.25 g of ettringite (corresponding to a maximal SO4 2− contribution from the ettringite of ≈ 6 mM), and 0.5 mM NaOH (initial pH ≈ 11) were added. The sum of sulphate and selenate/selenite added always equalled 6 mM. Suspensions were handled under Ar to exclude CO2 , and solutions were prepared from CO2 -free bi-destilled water (boiled, cooled under Ar). Vessels were stored in tightly closing plastic boxes that were filled with Ar and gently shaken. Aliquots were removed from the batch vessels after 13, 20, 41, 325, and 552 days, filtered with a 0.45 µm filter and analysed for SO4 , Ca, Al and Se; the pH was also controlled. For each reaction period, data for all SO4 2− /SeO4 2− ratios were pooled to calculate mean values. Dissolved Se was measured by cold vapour-atomic absorption spectrometry (NaBH4 hydride system), Se(IV) directly, Se(VI) after pre-reduction with HCl. This was used to verify the oxidation state of Se in samples with long equilibration time. Cr(VI) was measured photometrically (diphenylcarbazide method), major ions (Ca2+ , Mg2+ , Na+ , K+ , Cl− , SO4 2− ) by capillary electrophoresis. pH was determined with a Metrohm combined glass and Ag/AgCl/3 M KCl reference electrode which was calibrated for each measurement series with the help of Merck Titrisol standard buffer solutions with pH values of 4, 7, 10, 12, and 13.

3. Uptake of Se and Cr by hydrated cements and ettringite 3.1 K d values for fresh and leached hydrated cements The uptake of Se(VI) and Cr(VI) by fresh and leached PC and HAC samples is shown in Fig. 1 as a function of equilibration time. Both SeO4 2− and CrO4 2− sorb only weakly on PC, resulting in similar K d values of ≈ 0.01 m3 /kg. The measured sorption on aged HAC (CFL, S71) samples is in the same range for both elements. In these cases, K d does not vary very much with time, and ca. 30–50 d seem to be sufficient to reach maximum uptake. The moderate uptake of Cr(VI) by PC is also in good agreement with the findings of [11] and [12]. The uptake of Se(VI) and Cr(VI) by fresh HAC, however, was much stronger, resulting in high apparent K d values of ≈ 1–5 m3 /kg after 81 days, which decrease again to ≈ 0.1–1 m3 /kg after 500–700 days (Fig. 1). This very strong uptake can be explained by the formation of solid solutions (substitution of SO4 2− by CrO4 2− and SeO4 2− ) with secondary ettringite formed during the uptake experiments by the interaction of fresh HAC and the sulphate-containing APW-solution (Table 1). Ettringite had already been identified as the main reaction product of fresh HAC and AGW in the preparation of leached HAC samples (Table 2). Ettringite can be considered as being made up of columns of the composition {Ca6 [Al(OH)6 ]2 ·24H2 O}6+ and channels of the composition {(SO4 )3 ·nH2 O}6− , with n variable but typically ≈ 2 [18]. The formation of chromateand selenate-substituted ettringite under alkaline conditions has been demonstrated in co-precipitation experiments [19, 20] where fast precipitation of ettringite-type minerals resulted in extended isomorphous substitution of SO4 2− by CrO4 2− and SeO4 2− . In comparison to SO4 2− , CrO4 2− and SeO4 2− have slightly larger ionic radii and the resulting partially substituted ettringites have somewhat larger a0 but smaller c0 unit cell parameters [19]. For an ideal solid solution, the distribution of the different ions between solution and solid can be expressed by Eq. (1), where {} denote activities; X MO4 (s) and X SO4 (s) are the mole fractions of chromate/selenate and sulphate in the solid, respectively; K S0(S-ettringite) and K S0(M-ettringite) are the solubility products of pure and substituted ettringite, respectively; and D is the homogeneous partitioning coefficient (cf. [21]): X MO4 (s) K S0(S-ettringite) {MO4 2− } {MO4 2− } = · = D· . 2− X SO4 (s) K S0(M-ettringite) {SO4 } {SO4 2− }

(1)

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Fig. 1. K d values for selenate and chromate on fresh and leached samples of hydrated Portland cement (CEM-HS, panels (a), (b)) and high-alumina cements (CFL and S71, panels (c)–(f)), as a function of time and solid:water ratio. Error bars represent two times the overall experimental standard deviation (see text for explanations).

Fast precipitation reactions may trap ions indiscriminately, leading to homogeneous but non-ideal solid solutions, where the coefficient D is time-dependent and not equal to the ratio K S0(S-ettringite) /K S0(M-ettringite) as D in Eq. (1): [MO4 2− ] X MO4 (s) = D · . X SO4 (s) [SO4 2− ]

(2)

Calculated partition coefficients are discussed together with data for ettringite further below (see Fig. 3). Note that K d values for the leached CFL and S71 samples are much lower than for the fresh ones (Fig. 1), although the leached samples also contain large amounts of ettringite. This can be explained by much slower uptake

kinetics in case of the structurally well-defined ettringite formed prior to the uptake experiments (primary ettringite) in comparison to the secondary ettringite formed during the uptake experiments, whose structure is less defined and displays less selectivity. In fact, the K d values obtained for the fresh samples start to decrease at very long equilibration times, indicating a re-crystallisation process leading to a more defined mineral structure, whereas the K d values obtained for the leached samples remain constant or increase slightly. In the case of PC, the amount of available Al-ions is probably limiting ettringite formation, so that there is no significant difference between fresh and leached samples.

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Uptake of oxo-anions by cements through solid-solution formation

3.2 Solid solution formation with synthetic ettringite To corroborate the presumed importance of ettringite for the uptake of oxo-anions, further experiments were conducted with synthetic, well-defined ettringite. Our goal was to (i) compare extent and kinetics of solid solution formation of oxo-anions with primary and secondary ettringite, and (ii) to obtain partition coefficients that could be used in uptake models for oxo-anions on whole cements based on a solid solution formalism. Since selenate and chromate showed almost identical behaviour, these studies were done with selenate as VI-valent and selenite as IV-valent species. Pure ettringite (containing only sulphate) was equilibrated with different solutions containing selenate or selenite, varying the initial Se/S ratio in the solutions from 0 to 1. If solid solution formation is operative, a (slow) recrystallisation of pure to partially substituted ettringite, e.g. Ca6 Al2 (SO4 )3−x (SeO4 )x (OH)12 ·26H2 O (s), will take place. In this process, the exchange of selenate/selenite vs. sulphate will occur first in the surface layers of the solid, leading to a heterogeneous distribution of the selenate/selenite and sulphate ions in the beginning. To indicate heterogeneous partitioning, D in Eqs. (1), (2) can be replaced with λ (the heterogeneous partition coefficient in the classical Doerner–Hoskins law; [22]), although the form of the equation remains the same (cf. [21]). Note that the composition of partially substituted ettringite is influenced not only by the Se/S ratio, but also by the concentration of OH, Ca, Al and other anions and cations present in the solution. Therefore, care was taken to keep their concentrations constant in the experiments. The results are given in Table 3 in the form of partition coefficients as a function of equilibration time and added Table 3. Partition coefficients (D, λ) obtained as a function of equilibration time and added selenate or selenite. Data corresponding to the precipitation of CaSeO3 (s) are indicated in italics. Negative values are caused by having to compare nearly equal numbers in the case of little Se uptake, in combination with analytical uncertainties.

Se(VI) and Se(IV). Measured values of D, λ for both Se(VI) and Se(IV) are small, in case of Se(VI) over its entire concentration range, in case of Se(IV) only at low concentrations, whereas more Se(IV) is removed from solution at higher initial concentrations. Measured Ca and Al concentrations as well as pH remained more or less constant with increasing amounts of Se(VI) added, while for Se(IV) a distinct increase of pH and dissolved Al and a decrease of dissolved Ca was observed with increasing initial Se(IV) concentrations (data not shown). From these data we conclude that solid CaSeO3 (s) was precipitated in most samples (Table 3). Solubility calculations using a formation constant for CaSeO3 (s) of log K = 5.44 (based on data in [23]) indicated a solubility more than a factor 10 higher. Using the measured concentrations of Se(IV), Ca and pH after 41 days, a formation constant for Ca2+ + SeO3 2− = CaSeO3 (s) of log K = 6.5 was calculated at I = 0 using the Davies equation. Measured Ca, Al, and SO4 concentrations and pH in experimental blanks also allowed to calculate a solubility product for pure ettringite of log K sp◦ = 56.2 at I = 0 using the Davies equation. This is in excellent agreement with the log K sp◦ = 57 calculated by [16]. The addition of Se resulted in a slight increase of pH and an increased solubility of ettringite. Measured partition coefficients for Se(VI) and Se(IV) on ettringite are quite scattered, ranging from 0 to 1 with a mean value of 0.2 (Table 3, Fig. 2), which indicates a preference of ettringite for SO4 2− over SeO4 2− and SeO3 2− . This is in good agreement with the observations of [4, 5, 8] on the competition between Se and sulphate for uptake by different solids. The influence of the ratio of sulphate to selenate present in the system could be observed in our experiments with ettringite (Fig. 2). The fraction of Se

13 days

20 days

Equilibration time 41 days

325 days

552 days

Se(VI) 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005 0.0055 0.006

0.494 0.063 0.132 0.351 0.986 0.265 0.406 0.599 0.024 0.049 0.025 0.016

0.200 −0.134 −0.068 0.126 0.017 0.066 −0.122 0.094 −0.037 0.195 0.111 −0.023

n.d. 0.036 0.070 0.051 −0.163 −0.008 −0.040 −0.015 −0.035 0.118 0.084 0.086

1.055 −0.058 0.176 0.516 −0.034 0.238 0.683 −0.240 0.121 0.154 0.224 0.212

0.615 0.174 0.038 0.418 0.086 0.266 0.292 0.032 0.007 0.045 0.051 n.d.

Se(IV) 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005 0.0055 0.006

−0.016 −0.068 −0.194 0.515 0.705 0.736 1.202 1.076 1.060 2.370 1.592 4.025

−0.155 −0.033 −0.081 0.863 1.365 1.578 1.048 1.599 0.835 2.463 3.777 6.871

0.646 0.061 0.526 1.652 2.411 2.290 2.751 2.913 4.510 2.665 2.469 6.013

−0.501 −0.492 3.216 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

Se added (M)

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Fig. 2. Measured sulfate concentrations and corresponding mole fraction of Se(VI) in ettringite (X Se ) as a function of time and of the concentration of Se initially added. Theoretical values of X Se calculated with D, λ = 0.2 ± 0.1 are represented by solid and dashed lines.

incorporated in the solid (X Se = Sesolid /(Sesolid + Ssolid )) increases with a decrease of the sulphate concentration, which agrees well with the formation of a solid solution as defined in Eq. (1). A similar influence of sulphate concentration has been observed for Se(IV) uptake by calcite where a decrease of SeO3 2− uptake upon addition of SO4 2− was found by [8].

4. An uptake model based on solid solution-formation with ettringite Fig. 3 gives an overview of K d values and partition coefficients obtained for Se(VI) on fresh HAC and ettringite as a function of equilibration time. At short to intermediate times, uptake is much higher in the cement systems than in case of synthetic ettringite, resulting in K d values in the range ≈ 1–5 m3 /kg and partition coefficients ≥ 1. At very long equilibration times, the two systems start to approach each other; K d values are in the range ≈ 0.01–0.1 m3 /kg, and partition coefficients range from ca. 0.1–0.3. Based on the different uptake kinetics in the two systems, the following mechanism is proposed: (i) a fast precipitation of secondary ettringite occurred in the presence of fresh HAC, leading to an extensive incorporation of selenate with no or little preference for sulphate; (ii) when selenate solutions are equilibrated with primary (pre-formed) ettringite, incorporation of selenate is slower and less extensive, indicating that sulphate is preferred. These two cases can be viewed as kinetic end members of the same process. In the long run, the re-crystallisation of ettringite is expected to lead to an increase of Se uptake in the case of primary ettringite and to a decrease in the case of secondary ettringite. Thermodynamically, the uptake is expected to be of the same magnitude for each case. Based on the data in Table 3 as well as Figs. 2 and 3b, a mean value of D , λ = 0.2 is selected for the incorporation of selenate into ettringite. Using this value, the simple solid solution model based on Eq. (2) can be incorporated in the usual K d -expression to yield a relation for estimating K d in whole cement systems,

Fig. 3. A comparison of K d values and partition coefficients (D, λ) for selenate on fresh high-alumina cements (error bars for K d are from Fig. 1, error bars for D, λ are based on the same data) and on ettringite (arithmetic means of the data given in Table 3 ± their standard deviation) as a function of equilibration time (D, λ = 0.2 is indicated).

without prior knowledge of the amount of ettringite present: Kd = D

{SeO4 2− }{SO4 solid } V {SO4 solid } V = 0.2 , {SO4 2− }Sedissolved m {SO4 2− } m

(3)

where V is the volume of the solution [m3 ] and m is the mass of the solid phase [kg]. This simple model relates K d to only two chemical parameters, the sulphate concentrations

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in the solid (hydrated cement) and in the corresponding solution. For dissolved sulphate concentrations measured values are typically available, but for {SO4 solid }, the sulphate content of the clinker used for HCP preparation (as specified by the supplier) had to be used as an approximate value. As a first application, this model was tested against our K d measurements for Se(VI) on different cements (Fig. 1). Of particular interest was whether the approximation of {SO4 solid } would give reasonable results, and whether the model would be applicable to PC, since it was developed based on data for ettringite (corroborated by high-alumina cement). Next, the model was also applied to K d measurements for chromate, selenite, and perrhenate (as analogue for pertechnetate; [24]) to see whether a uniform approach could be used for different oxo-anions. The results are shown in Figs. 4a and 4b. In general, the calculated and measured values show reasonably good agreement, considering uncertainties and the approximation of {SO4 solid }. Notably, the simple model developed for the Se(VI)-ettringite system (i) can be applied to fresh and leached PC, and (ii) gives also good estimates for K d values of Cr(VI) on cements and ettringite, as well as for Re(VII) on PC. The K d values for the uptake of Se(VI) and Cr(VI) by fresh HAC samples are underestimated for 50 g/L, probably because the slower re-crystallisation of the substituted

secondary ettringite at higher solid:water ratios. For Se(IV), only the data by [3] were available. There are some uncertainties regarding the sulphate concentrations in their experiments, but based on typical values for Portland cements and porewaters (Table 1), their K d values would be drastically underestimated by our model. On-going, not yet completed uptake studies in our laboratory [25] indicate that this is not related to the ettringite model, which appears to be able to explain Se(IV) uptake by ettringite itself. In contrast to Se(VI) and Cr(VI), Se(IV) uptake by PC seems to be controlled by mineral phases other than ettringite.

5. Conclusions and implications A comparison of K d measurements on different fresh and leached hydrated cement samples and on pure ettringite strongly suggests that selenate is taken up by cements through solid solution-formation with ettringite. Uptake kinetics show opposite trends for primary and secondary ettringite, and can be viewed as end-members of the same process: A fast precipitation of secondary ettringite in the presence of Se lead to high initial uptake that decreased with time, whereas the reaction of Se with pure or pre-formed ettringite is slow, leading to low initial uptake that increases with time. It is pointed out that very long equilibration times were required to observe these effects. A solid-solution model was developed for Se(VI) uptake by ettringite, which allowed to define a simple relation between K d of Se(VI) and dissolved/solid sulphate concentrations. This K d model was able to estimate uptake of different oxo-anions on a variety of hydrated cements under different conditions. Based on the limited data available, the model appears to underpredict uptake of Se(IV) however, even though there are no indications that solid solution-formation with ettringite differs significantly between selenite and selenate. Tentatively, this is interpreted through additional important contributions to Se(IV) uptake by other hydrated cement phases; this is the subject of an on-going investigation. With regard to performance assessment calculations, it is important to note that the solid solution-processes presented here are expected to become relevant at (much) lower concentrations of oxo-anions than the formation of pure solids, such as the respective Ca-phases. This incorporation process is therefore not only important for estimating K d , but may also be taken into account in assessments of maximum radionuclide solubility (see e.g. [1]). Since typical cement degradation models (e.g. [16]) include sulphate as parameter, the model developed in the present investigation can help to link overall retention (solubility and uptake) of some oxoanions with the chemical degradation of cements. Acknowledgment. M. O. and B. L. gratefully acknowledge the financial support by ANDRA, France. The preparation of the cement specimen by MBT (Schweiz) AG, R&D Laboratory, is also appreciated.

Fig. 4. Comparison of calculated (according to Eq. (3)) and measured K d values for Se(VI) and Cr(VI) sorption on fresh and leached hydrated cements (from Fig. 1, two years equilibration time), and of Re(VII) on leached CEM-HS (5, 50 g/L; 480 h reaction time; data from [24]). The error bars indicate the value calculated for D, λ ± 0.1. Values measured for 50 g/L fresh CFL and S71, where the kinetically slowest re-crystallization of secondary ettringite is expected, are indicated. Data by [19] for ettringite are also included in panel (b).

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