Study of leaching mechanisms of caesium ions incorporated in Ordinary Portland Cement

Journal of Hazardous Materials 171 (2009) 1024–1031

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Study of leaching mechanisms of caesium ions incorporated in Ordinary Portland Cement Kyriaki G. Papadokostaki a,∗ , Anastasia Savidou b a b

Institute of Physical Chemistry, National Centre for Scientific Research “Demokritos”, 153 10 Ag. Paraskevi, Athens, Greece Institute of Nuclear Technology–Radiation Protection, National Centre for Scientific Research “Demokritos”, 153 10 Ag. Paraskevi, Athens, Greece

a r t i c l e

i n f o

Article history: Received 28 November 2008 Received in revised form 19 June 2009 Accepted 19 June 2009 Available online 27 June 2009 Keywords: Leaching Diffusion Radioactive Waste Caesium Portland Cement

a b s t r a c t In this work, a study of the leaching kinetics of Cs+ ions from cement paste solids, containing inactive Cs2 SO4 , is presented, involving (i) the parallel performance of leaching experiments at two temperatures (30 ◦ C and 70 ◦ C); (ii) the performance of leaching tests with intermediate changes in temperature between 30 ◦ C and 70 ◦ C; (iii) the use of specimens of two different thicknesses and (iv) the determination of the distribution of Cs+ in the cement specimen at various stages of the leaching test. The results of leaching studies at 30 ◦ C with cement solids simulating the composition of real radioactive wastes, containing NaNO3 , small amounts of inactive CsNO3 and traces of 137 Cs+ are also reported. Concentration profiles of Cs+ in inactive specimens showed that part of the Cs+ (20–30%) tends to be immobilized in the matrix, while elution of the readily leachable portion follows Fick’s law reasonably well. No immobilized Cs+ was detected in the samples containing considerable amounts of NaNO3 . Long-term leaching experiments (up to 8 years) revealed acceleration of the elution process (not detectable in short-term tests), attributable to increase in porosity caused by erosion of the cement matrix. Sorption experiments of Cs+ ions by cement granules indicated that adsorption on cement pore surfaces is not significant. On the other hand, the leaching tests at two different temperatures or with intermediate changes in temperature between 30 ◦ C and 70 ◦ C, yielded activation energies that indicated a more complicated kinetic behavior. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The main purpose of the leaching studies of radioactive or other toxic concentrates that are incorporated in blocks of suitable embedding materials such as asphalt, cement, ceramics, plastics, glass etc. is to assess their potential hazard to the environment, when these blocks come into contact with water during long-term storage or disposal. More specifically, one must first assess the maximum possible rate at which radioactive ions can find their way out of the block. The said maximum rate will be obtained in leaching tests (i) under conditions where the block approximates a semi-infinite medium which, in practice, means that no material is leached out of locations farthest away from the exposed surfaces and (ii) for practically zero boundary concentration, i.e. for very small concentrations of leachate in the leachant. One must, secondly, rely on the results of the leaching tests, which necessarily extend over relatively short testing periods, to predict the amounts likely to be eluted at much longer times after actual disposal. This is done by extrapolation of the data either (i) on the basis of semi-

∗ Corresponding author. Tel.: +30 210 650 3661; fax: +30 210 6511766. E-mail addresses: [email protected] (K.G. Papadokostaki), [email protected] (A. Savidou). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.06.118

empirical curve-fitting equations [1–3] or (ii) through rate laws related to the fundamental mechanisms governing the transport of leachate ions being eluted within the solid [4–8]. In our view, there is no doubt about the superiority of the second approach. The standard leaching test methods mostly used to measure the release of radionuclides from waste forms include the IAEA [9] and American Nuclear Society [10] test. The method used in this work has adopted the experimental set up prescribed in the original IAEA proposal but the leaching tests were performed at two temperatures 30 ◦ C and 70 ◦ C. The period of leaching was in some cases as long as 8 years and sampling of the leachate was initially performed daily and then at progressively longer intervals (namely twice, then once a week and finally once a fortnight or monthly). The leaching process consists of physico-chemical phenomena, in which diffusion is anticipated to play an important role. Accordingly the IAEA suggested that the diffusion coefficient (leach coefficient) might be used for inter-comparison of leaching data [11]. For the determination of the effective or apparent diffusion coefficient, Fick’s law of diffusion in the form of the linear relation √ of the amount of ion leached at time t vs. t has been used [12–14]. However, as pointed out by previous work at the “Demokritos” Centre [4,15–18] as well as by the works of others [5,7,8,19,20], Fick’s law alone is not always sufficient for a reasonably full description of the fundamental processes involved in the kinetics of leaching.

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Among factors, that have an important effect on the elution process, the heterogeneity of the structure (including the pore structure) of the block, as well as the possible changes in structure of the block under the influence of the environment, are particularly worth noting. In our previous work [17,18], as well as in other works [21,22] the change in the porosity of Portland Cement specimens under the influence of the leachant was noted. Recently Haga et al. [19,23] investigated the alteration of Portland Cement associated with dissolution. It is evident that the phenomenon of alteration of the materials where the waste is embedded leads to discrepancies from the Fick’s law for long-term prediction of leaching rates. It is obviously of great importance to study physico-chemical mechanisms through which the retention properties of the block of waste might be impaired under the influence of prolonged contact with leachant. Also the influence of the dissolution rate of the embedded materials on the kinetics of leaching is an important issue for investigation. Since 1972 it was shown [4,24], that in the case of sparingly soluble salts, a low rate of salt dissolution in the embedding hydrophobic matrix leads to a completely different release kinetics (linear vs. time at the initial stages of the elution process). Another significant factor is the immobilization of the leachate in the block structure [17,25–27]. With the above points in mind, the leaching kinetics of Cs+ from Ordinary Portland Cement has been examined by members of our group in previous years. Many of the findings of these studies were included in the reports [15,17] disseminated among Nuclear Research Centres but not published in the open literature. Accordingly the results of these investigations are presented here complemented with the results of other unpublished relevant studies conducted by our group. The studies of our group on this subject comprise the investigation of the elution kinetic behaviour of Cs+ in cement solids (containing 3.4% by wt Cs2 SO4 ) at two temperatures (30 ◦ C and 70 ◦ C) and two thicknesses. Also the partition coefficient between cement and water was determined for various caesium concentrations in the external solution. In a series of experiments, the leaching temperature was changed for certain periods during the leaching tests from 30 ◦ C to 70 ◦ C and reversely and the corresponding activation energies for diffusion were estimated. Likewise the elution kinetic behaviour of Cs+ at 30 ◦ C in cement solids, simulating the composition of real waste (containing NaNO3 , small amounts of inactive CsNO3 and 137 Cs+ ) was examined. In addition to the above, two important aspects of this work, usually not found in similar studies, are (a) determination of concentration profiles of Cs in the cement matrix at various stages of leaching, to get further insight into the leaching mechanism and confirm or correct the values of apparent diffusion coefficient deduced from elution curves and (b) long-time leaching tests which are useful in practice, in order to avoid underestimation of the predicted long-term Cs+ leaching [23].

2. Materials and methods 2.1. Materials For the preparation of the cement specimens all the salts used i.e. NaNO3 , Cs2 SO4 and CsNO3 were of analytical grade. The cement used in all cases was Ordinary Portland Cement from Heracles General Cement Company, Greece. Its composition by weight was: SiO2 : 20.35%, Al2 O3 : 4.90%, Fe2 O3 : 3.20%, CaO: 62.95%, MgO: 3.00%, SO3 : 2.70%, K2 O: 0.33%, Na2 O: 0.46%; and the loss on ignition (1000 ◦ C) was 1.95% with insoluble residue: 0.30%. The heat of hydration and compressive strength were 83.1 cal g−1 and 44.9 MPa respectively, after 28 days of curing.

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2.2. Preparation of samples Cylindrical cement specimens of 4 cm diameter and of two thicknesses, 4.0 cm and 1.2 cm, containing either (i) inactive Cs2 SO4 or (ii) NaNO3 , CsNO3 and 137 Cs+ (370 KBq specimen−1 ), in the case of specimens simulating real radioactive waste, were prepared. In all cases water to cement ratio was ∼0.42. All the specimens were poured in plastic moulds and were cured in a humid atmosphere (90–100% relative humidity) at room temperature. The main characteristics of six series of the cement specimens are shown in Table 1. Initially, it was thought that long curing might be beneficial from the point of view of greater stability of the cement specimens during long leaching tests, but later experience showed that little advantage could be gained thereby. Series C was obtained by slicing two of the specimens of series A into three slices. Near the end of the curing period all outer surfaces except one flat surface of each specimen were covered with epoxy glue. Blank cement specimens i.e. specimens that contained no salt were also prepared, cured and leached exactly in the same way as salt-containing specimens.

2.3. Leaching tests This leaching process involved exposure to 170 ml of stagnant distilled water in cylindrical vessels at 30 ◦ C or/and at 70 ◦ C as indicated in Table 1. In all cases one flat surface of the specimens was exposed to leachant. The leachant was renewed periodically, daily at the beginning and then at progressively longer intervals (namely twice, then once a week and finally once a fortnight or monthly). For the leaching tests, 2–4 specimens of the same series were examined each time under the same conditions. 2.4. Distribution of Cs+ ions along the diffusion axis In the course of leaching, duplicate or triplicate specimens of series A, B or D were withdrawn at appropriate times and sliced usually into seven slices of thickness ∼0.5 cm. The amount of noneluted Cs+ (series A and B) was determined by powdering each slice and extracting with hot water. 137 Cs+ (series D) was extracted by treatment with acid. The use of acid in series A or B was avoided because of ensuing complications in the determination of Cs by atomic absorption. 2.5. Sorption of Cs+ ions by cement The sorption of Cs+ ions by the cement used was studied by equilibrating 4 g samples of cement granules (0.18–1.0 mm), obtained from blank specimens, with 25 ml of aqueous Cs2 SO4 solutions in the concentration range 10−3 –1 M Cs+ ions. The cement granules were obtained from blank specimens prepared together with, and subjected for 900 days to the same leach treatment as those of series A (Table 1). To obtain homogeneous samples, a surface slice about 4 mm thick was taken away (to eliminate the effect of erosion) and the rest of the specimen was crushed and sieved to give granules in the range 0.18–1.0 mm. The granules and the Cs2 SO4 solutions were allowed to equilibrate at room temperature in sealed plastic containers for more than 15 days. Checks showed that the concentration of the equilibrating solution became constant within about a week. At the end of the equilibrium experiment, the cement granules were quickly washed in distilled water and extracted in a Soxhlet apparatus for one day. It was shown that extraction under these conditions was complete. The concentration of the extract was further concentrated by evaporation where necessary.

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Table 1 Characteristics of cement specimens. Series

Thickness (cm)

Composition

Curing time (days)

Elution temperature (◦ C)

A B C D E F

4 4 1.2 4 4 4

Cement + Cs2 SO4 (3.4% by wt, inactive) Cement + Cs2 SO4 (3.4% by wt, inactive) Cement + Cs2 SO4 (3.4% by wt, inactive) Cement + 137 Cs 370 KBq per specimen + CsNO3 (0.017% wt) + NaNO3 (8% wt) Cement + NaNO3 (8% wt) Cement + Cs2 SO4 (3.4% by wt, inactive)

330 330 330 49 54 57

30 70 30 30 30 30/70/30

For ˙˛n /A0 > 0.5, Eq. (1) must be replaced by:

2.6. Measurement of ions in leachates or extracts The concentrations of ions in the leachates or extracts from cement samples were determined as follows: (i) inactive Cs+ and Na+ , by atomic absorption (PerkinElmer 2380), (ii) 137 Cs by a high resolution gamma-spectroscopy system with an HpGe detector of 20% relative efficiency. Reference and standards were prepared using leachates or extracts from blank cement specimens which had undergone exactly the same treatment (leaching, powdering and extraction) as the Cs-containing specimens. The relative errors of measurements were less than 2%. 3. Results and discussion 3.1. Cement specimens containing inactive Cs+ (series A–C) 3.1.1. Leaching kinetics Typical elution curves for series A and B specimens are presented √ in Fig. 1, in the form of plots of ˙˛n /A0 vs. t. ˙˛n is the total amount of ion eluted from the sample at time t and A0 is the amount originally incorporated therein. The fractional amount eluted at 30 ◦ C was ∼58% in 1050 days for series A and in cases where the leaching experiments had run for 8 years ∼85% of Cs+ had been eluted. At 70 ◦ C, leaching tests lasted 247 days i.e. until ∼90% of + the plotted оn a √ Cs load had been eluted. In Fig. 2 elution curves t basis relating to “thin” cement specimens at 30 ◦ C (series C) are shown. The fractional amount eluted was higher than 85% over a √ period of 770 days. The t plots in Figs. 1 and 2 are initially linear (at least approximately) as required by Fick’s law for a semi-infinite medium (i.e. in the range ˙˛n /A0 < 0.5) [28], namely: ˙˛n /A0 = 2

 Dt 1/2 L2

(1)

Fig. 1. Elution curves of Cs+ from duplicate or triplicate cement specimens con√ taining inactive Cs2 SO4 (series A and B, Table 1) plotted on a t scale. A0 = 2.34 g of caesium. Solid lines correspond to Fickian diffusion with Dapp = 3.9 × 10−8 cm2 s−1 and 3.9 × 10−7 cm2 s−1 for 30 ◦ C and 70 ◦ C respectively.

˙˛n /A0 = 1 −

8 2



exp

−

D2 t 4L2



(2)

where L = V/S (V = sample volume, S = exposed surface) is the thickness of the specimen, assumed to be in the form of a slab with one surface exposed; and D is the diffusion coefficient. Application of Eq. (1) to the initial approximately linear portions √ of the t plots of Figs. 1 and 2 yields apparent diffusion coefficients, Dapp , presented in Table 2. The values of the diffusion coefficient for Cs+ ions determined in this study, at 30 ◦ C for Ordinary Portland Cement are about double those found previously [15] (the differences are presumably due to the use of different batches of Ordinary Portland Cement). Although diffusion of Cs+ in cement depends strongly on various factors (e.g. the chemical composition and porosity of cement, the water to cement ratio at fabrication, the presence of additives), estimated Dapp values are in reasonable agreement to the values reported in literature for Portland Cement [27,29–31]. In Figs. 1 and 2, the theoretical Fickian curves, calculated with the aid of Eqs. (1) and (2), using Dapp values shown in the legends, are also presented. In the case of thicker specimens (series A and B in Fig. 1), √ it is clear that while the process of leaching is developing, the t plots exhibit a distinct tendency to turn upward which means acceleration of the elution process, possibly due to increased porosity caused by the erosion of the cement matrix in the form of dissolution of solid phases, mainly portlandite (Ca(OH)2 ) as reported in the literature [19,23]. This explanation is supported by the fact that the said acceleration effect is more prominent for specimens of series B, leached at 70 ◦ C (Fig. 1) and by measurements of the imbibed water content of slices of partially eluted specimens of series A and B (see examples in Table 3). The latter results show substantial increases in water content and hence in

Fig. 2. Elution curves of Cs+ from triplicate cement specimens of series C (thin spec√ imens), plotted on a t scale. A0 = 0.70 g of caesium. The solid line corresponds to Fickian diffusion with Dapp = 3.7 × 10−8 cm2 s−1 .

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Table 2 Apparent diffusion coefficients (from leaching curves on the basis of Eq. (1)), corrected diffusion coefficients (from leaching curves as above applicable to easily leachable fraction) and real diffusion coefficients (applicable to easily leachable fraction) calculated from concentration profiles. Series A and C B D *

Elution temperature (◦ C) 30 70 30

Apparent diffusion coefficient (cm2 s−1 )

Corrected diffusion coefficient (cm2 s−1 )

−8

−8

2.4–4.7 × 10 3.6–4.7 × 10−7 0.7–1.0 × 10−8

Real diffusion coefficient (cm2 s−1 ) 5.8–8.6 × 10−8 7.5–8.9 × 10−7 0.8–1.3 × 10−8

6.0–7.8 × 10 7.6–8.6 × 10−7 0.7–1.0 × 10−8 *

No correction is needed in this case because no immobilized Cs+ was detected.

Table 3 Results of measurements of imbibed water content in slices of partially eluted Cs2 SO4 -containing cement specimens as a function of leaching temperature and distance from surface exposed to eluent. Series

Elution temperature (◦ C)

Period of leaching (days)

Location of the slice in the specimen*

Total imbibed water content as measured (cm3 /100 g dry cement)

Imbibed water content net of volume of eluted Cs2 SO4 (cm3 /100 g dry cement)

A

30

900

Surface slice Middle slice Bottom slice

23.4 20.9 20.7

22.7 20.4 20.3

B

70

250

Surface slice Middle slice Bottom slice

29.9 22.8 22.2

29.0 22.0 21.4

*

Cement specimen sliced into seven slices of thickness ∼0.5 cm.

porosity (after allowing for the pertinent small volume of eluted Cs2 SO4 ), near the exposed surface of the specimen (where maximum erosion effects are expected), which are more prominent at 70 ◦ C (again as expected). For thin specimens (series C), the experimental data follow quite well the theoretical curves (Fig. 2). This is in keeping with the fact that the duration of leaching experiments was much shorter for thin than for thick specimens; hence the increase of porosity associated with erosion was not so visible. Thus, longtime leaching tests should prove to be very useful in practice, since the normal duration of practical leaching tests is unlikely to show up the erosion of cement; hence the long-term leaching of Cs+ may be underestimated [23]. High temperature leaching also should prove useful as an accelerated test, which can show up deviations due to erosion on a shorter time scale. 3.1.2. Concentration profiles The distribution of Cs+ in series A and B cement samples at various stages of the elution process are presented in Figs. 3 and 4 respectively; where C is the measured concentration of Cs+ in a slice of the specimen located at distance X from the surface exposed to leachant, C0 is the initial concentration and L is the thickness of the specimen. A striking feature of the plots of series A and B specimens (which contain relatively large amounts of Cs+ ) is

that they extrapolate to a concentration C1 at the exposed surface (X = 0) of the specimen well above zero (see Figs. 3 and 4) indicating that part of the Cs+ is immobilized in some way in the cement matrix, possibly due to formation of Cs aluminosilicates [25,26]. An alternative explanation is by the formation of silicate hydrates which through the closure of available pores may entrap caesium ions, although this mechanism is more possible in the presence of pozzolanic additives, like silica fume or fly ash [32–34]. Following the√above findings, the values of diffusion coefficient estimated from t plots of Figs. 1 and 2 are subject to correction, because of the fact that part of the Cs+ is “immobilized” in the cement matrix as shown by the relevant Cs+ distribution profiles. The fact that the experimental elution curves of Fig. 2, that concern the thinner samples (series C), fall, at longer times, below the theoretical curve can be attributed to the “immobilized” part of Cs+ . The concentration profile at time t in a slab with C = C0 at t = 0 and concentration at the exposed surface, X = 0, maintained constant at C = Cl , according to Fick’s law for a semi-infinite medium or a medium of finite thickness are respectively [28]: (C − C1 ) = erf (C0 − C1 )

 X  √ 2 Dt

(3)

Fig. 3. Distribution of Cs+ in duplicate cement specimens of series A eluted at 30 ◦ C, at various stages of the elution process. The corresponding fraction of Cs+ eluted and the surface concentrations C1 /C0 are given in the legend. C0 = 0.025 g g−1 of cement. The theoretical concentration profiles from Eq. (3) or (4) correspond to the given in the legend diffusion coefficients.

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Fig. 4. Distribution of Cs+ in duplicate cement specimens of series B eluted at 70 ◦ C, at various stages of the elution process. The corresponding fraction of Cs+ eluted and the surface concentrations C1 /C0 are given in the legend. C0 = 0.025 g g−1 of cement. The theoretical concentration profiles from Eq. (3) or (4) correspond to the given in the legend diffusion coefficients.

or C − C1 4  (−1)n (2n + 1)(L − X) D(2n + 1)2 2 t = cos exp  2n + 1 2L C0 − C1 4L2 ∞

n=0

(4)

Figs. 3 and 4 show that in spite of the noticeable spread of the values of Cl from one specimen to another, it is possible to say definitely that there is a significant fraction of Cs+ which is released at a rate much lower than the main fraction. This means that: (i) even though Eqs. (3) and (4) may not be strictly applicable (because they assume perfectly constant C1 ), the results obtained by application of these equations are useful for practical purposes as shown below and (ii) corrected values of apparent diffusion coefficient (Dcor ) (applicable to the main readily leachable Cs+ fraction) can be determined from Eq. (1) by replacing A0 therein by A0 (1 − C1 /C0 ). Accordingly, in Figs. 3 and 4 the experimental results have been fitted by theoretical curves calculated with the aid of Eq. (3) or (4) using the values of D and C1 as indicated in the Figures. The different values of D required at 30 ◦ C (Fig. 3) reflect chiefly the variability of D among duplicate specimens; whereas at 70 ◦ C (Fig. 4) there is a clear tendency for D to be higher at longer times, in keeping with the conclusions concerning increase of porosity, previously drawn from the corresponding elution curves. The degree of fit achieved in Figs. 3 and 4 is good and, as shown in Table 2, the values of D deduced from Figs. 3 and 4 are in good agreement with Dcor obtained for the same specimens from Eq. (1) when corrected for immobilized Cs+ as indicated above.

Fig. 5. Elution curves of 137 Cs+ (30 ◦ C) from triplicate cement specimens simulating √ real waste (series D), containing 370 KBq, plotted on a t scale.

Fig. 5 yields apparent diffusion coefficients for series D specimens (which contained only trace amounts of Cs+ in conjunction with rather large amounts of NaNO3 ), much lower than the apparent diffusion coefficients for series A (Table 2).

3.2. Cement specimens simulating real radioactive wastes (Series D) 3.2.1. Leaching kinetics of 137 Cs+ ions The elution curves for cement specimens, simulating real radioactive wastes (series D) is presented in Fig. 5 in the form of √ plots of ˙˛n /A0 vs. t. The fractional amount eluted at 30 ◦ C was ∼25% in 850 days and ∼60% in 8 years. The acceleration effect is more prominent in series D than in series A and B specimens (Fig. 1). This is due to the fact that, in series D, apart from the increase in porosity due to the dissolution of solid phases (mainly portlandite), the loss of NaNO3 (not present in series A and B) entails a corresponding increase of imbibed water. The elution of Na+ from blank cement specimens of series E (curing and leaching conditions were similar to those of series D), containing the same amount of NaNO3 as in series D, was followed and found to parallel closely that of Cs+ (see Fig. 6). Application of Eq. √ (1) to the initial approximately linear portions of the t plots of

Fig. 6. Elution curves of Na+ from cemented specimens containing NaNO3 (8% wt) at 30 ◦ C, (series E, filled points) compared with elution of 137 Cs+ from specimens of series D (open points) containing the same amount of NaNO3 .

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Fig. 7. Distribution of 137 Cs+ in cement specimens of series D eluted at 30 ◦ C, at various stages of the elution process. C0 = 4.1 KBq g−1 of cement. The corresponding fraction of 137 Cs eluted is given in the legend. The theoretical concentration profiles from Eq. (3) or (4) correspond to the given in the legend diffusion coefficients and C1 /C0 = 0.

3.2.2. Concentration profiles The distribution of Cs+ in series D samples at various stages of the elution process is presented in Fig. 7; where C is the measured concentration of 137 Cs+ in a slice of the specimen located at distance X from the surface exposed to leachant, C0 is the initial concentration and L is the thickness of the specimen. The curves for series D specimens, which contain much smaller amounts of Cs+ than series A and B specimens, together with rather large amounts of Na+ , extrapolate smoothly to C1 = 0 at the exposed surface (X = 0) of the specimen (see Fig. 7). This is possibly due to the elevated concentrations of Na+ able to compete with Cs+ for fixation sites [25,35]. The values of D deduced with the aid of Eq. (3) or (4) from the distribution curves (Fig. 7) and Dapp values from Eq. (1) are also in good agreement (see Table 2). 3.3. Temperature dependence of the diffusion coefficient An important parameter in the study of leaching kinetics is the dependence of the diffusion coefficient (D) on temperature (T), usually expressed in terms of the Arrhenius energy of activation (Ea ) given by [36]: Ea = −R

ln D1 − ln D2 1/T1 − 1/T2

(5)

where Dl and D2 are the diffusion coefficients at T1 and T2 respectively. Using the values of diffusion coefficient for Cs+ at T1 = 303 K (series A, 30 ◦ C) and T2 = 343 K (series B, 70 ◦ C), Ea was found to be 12–14 Kcal mol−1 , a value which is well above what would be expected for simple aqueous diffusion of caesium that is 4–5 Kcal mol−1 . The effect of temperature was also examined by changing the temperature for certain periods during the test (Fig. 8). Samples of series F were used. The test was carried out in duplicate and the leaching procedure consisted of an initial period at 30 ◦ C (∼260 days), a second period at 70 ◦ C (∼70 days) before reducing to 30 ◦ C. Raising the temperature results in considerable enhancement of the leaching rate. If it is assumed that the Arrhenius energy of activation can be given by the relation (5), where the diffusion coefficients Dl and D2 were replaced by the leaching rates Rl and R2 respectively, then the break in the elution curves occurring at the point where the temperature was raised corresponds to apparent activation energies at the range of 11–16 Kcal mol−1 . The high activation energy at the first break (which is similar with that found from tests in series A and B samples), that is well above what might be expected for simple unhindered aqueous diffusion [29], indicates the presence of a second more highly activated process. This result can be attributed

Fig. 8. Elution curves of Cs+ from duplicate cemented specimens containing inactive Cs2 SO4 (series F; Table 1) plotted on a t scale. Leaching tests were performed with intermediate changes in temperature from 30 ◦ C to 70 ◦ C and vice-versa.

(i) to the release of “immobilized” part of Cs2 SO4 located within less accessible regions and (ii) to enhancement of the value of D at 70 ◦ C due to the increase in porosity of the cement matrix at that temperature. The considerably lower value of activation energy at the second break (where the temperature is reduced), amounting to 5–9 Kcal mol−1 , indicates a largely irreversible process [29]. 3.4. Sorption of Cs+ ions by cement from aqueous solution The main aim of these measurements is to determine the amounts of Cs+ which can be sorbed by the cement specimens in a fully reversible manner from aqueous solution. The equilibrium sorptive capacity of cement for Cs+ ions, when exposed to an equilibrating solution of concentration Cs may be expressed in terms of an empirical partition coefficient Kp defined by (e.g. [31]) Kp =

Cc Cs

(6)

where Cs and Cc are the caesium concentrations in external solution and in the water-saturated cement specimen respectively, expressed in mol per unit volume of solution and of cement specimen respectively. In a porous medium, ions are sorbed (a) by dissolution in the pore water at practically the same concentration as the external solution and (b) by adsorption on the pore surfaces. For a linear sorption isotherm, Eq. (6) can be written as Kp = Vp + Kads

(7)

where Vp is the pore volume fraction of the solid medium, and Kads is the concentration of adsorbed ions per unit volume of the porous medium divided by the concentration of ions per unit volume of solution. Values of Kp ≈ Vp (hence Kads → 0) correspond to concentration of Cs+ within the pores practically equal to that of the bulk solution, while values of Kp > Vp (hence Kads > 0) denote the presence of Cs+ ions adsorbed on the pore surfaces. Equilibrium sorption results are presented in Fig. 9, where Cc is expressed in mol per unit volume of cement specimen (the volume of solid was calculated from the mass of solid and the density of the hydrated cement specimen ∼2 g cm−3 ). A linear log–log plot of unit slope is obtained, indicating a constant partition coefficient Kp = 0.22 over the range Cs = 10−3 –1 M Cs+ ions. This value of Kp is quite close to the fractional pore volume found for blank cement specimens by means of mercury porosimetry which amounted to

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aqueous solution in the pores, thus confirming the applicability of Fick’s equation to the elution process. • Finally, it was pointed out that although simple diffusion plays an important role in the leaching process, it is not sufficient for the description of leaching kinetics of Cs+ incorporated in Ordinary Portland Cement. Other factors that should be taken into account include erosion of cement and immobilization of a part of caesium in the block structure. Acknowledgement Thanks are due to Dr J.H. Petropoulos and Dr S.G. Amarantos for valuable discussions. Part of this work was performed under Contract Nos. FI1W/0026/00 and FI1W/0174/GR(TT) as part of the European Union’s Research Program on Management and Disposal of Radioactive Waste. References Fig. 9. Sorption of Cs+ by cement at room temperature from aqueous solutions of Cs2 SO4 . Cs and Cc are the caesium concentrations in external solution and in the cement specimen respectively.

0.18–0.21 [17]. Our results show negligible adsorption of Cs+ in agreement with [31,37]. We conclude that (i) the Cs+ concentration in the pore fluid does not differ markedly from that of the equilibrating solution and (ii) partial immobilization of Cs+ (indicated by concentration profiles) can occur only if Cs+ ions are present at the preparation stage of cement samples. The above value of Kp , implies that even the higher loads of Cs+ (3.4% Cs2 SO4 by wt) incorporated in samples used in the elution tests, do not exceed the reversible sorption limit corresponding to a saturated aqueous pore solution (solubility of Cs2 SO4 : 2 g g−1 of water), thus confirming the applicability of Fick’s rather than Higuchi’s equation [38] to the elution process. 4. Conclusion The mechanism of leaching kinetics of caesium from Portland Cement was studied using (a) specimens containing relatively large quantities of inactive Cs+ (Cs2 SO4 ∼3.4% by wt) and (b) cement specimens simulating radioactive waste, containing small quantities of Cs+ (137 Cs as a tracer) and NaNO3 (8% by wt). The main conclusions drawn from the present work are the following: • Concentration profiles of Cs+ ions at various stages of the leaching experiments, can give significant information for leaching mechanisms. In case (a), concentration profiles showed that part of Cs+ (20–30%), present at the preparation stage of the cement matrix, is partially immobilized, most probably due to the formation of insoluble complexes (e.g. caesium aluminium silicates). Elution of the readily leachable portion appears to obey Fick’s law reasonably well. No immobilized Cs+ was detected in samples of case (b). • Long-term leaching experiments revealed acceleration of the elution process (not detectable in short-term tests), attributable to increase in porosity resulting from erosion of the cement matrix which is most prominent at high temperatures (70 ◦ C). This is also consistent with the estimated high values of activation energy for diffusion which cannot be reconciled with a simple aqueous diffusion mechanism. • Sorption studies of Cs+ ions by cement granules from aqueous solution indicated that (i) Cs+ is taken up by dissolution in waterfilled pores without significant adsorption on the pore surfaces and (ii) the highest loads of Cs+ used in the elution tests do not exceed the reversible sorption limit corresponding to a saturated

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