e - T e r r a http://e-terra.geopor.pt

ISSN 1645-0388 Volume 5 – nº 2

2008

R e v i s t a E l e c t r ó n i c a d e C i ê n c i a s d a T e r r a G e o s c i e n c e s O n - l i n e J o u r n a l GEOTIC – Sociedade Geológica de Portugal

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Evaluation of geochemical conditions favourable for the colloid-mediated uranium migration in a granite fracture NAIROBY ALBARRAN – nairoby.albarran@ciemat.es (Environmental Department, CIEMAT, Avda. Complutense 22, Edif. 19, 28040 Madrid, Spain) ÚRSULA ALONSO – ursula.alonso@ciemat.es (same address as N. Albarran) TIZIANA MISSANA – tiziana.missana@ciemat.es (same address as N. Albarran) MIGUEL GARCÍA-GUTIÉRREZ – miguel.garcia@ciemat.es (same address as N. Albarran) MANUEL MINGARRO – m.mingarro@ciemat.es (same address as N. Albarran) ABSTRACT: The presence of colloids in groundwater can significantly affect contaminant migration. Therefore, the evaluation of the conditions in which the presence of colloids can favour contaminant migration is one of the most critical tasks for the assessment of the security of radioactive waste repositories. Colloids will be important for radionuclide migration towards the geosphere if they exist in non negligible concentration and if they are stable and mobile. Those aspects are strictly related to the geochemical conditions of the medium, which are of primary importance. In this work, we centred our attention on the uranium migration in a granitic fractured environment. Colloids were shown to be stable over months in low mineralized and alkaline waters (pH=9.7 and ionic strength 1·10-3 M). The transport of uranium in a granite fracture was studied, using a natural groundwater from the Grimsel Test Site (Switzerland) located in a granitic massive, in absence and presence of bentonite colloids. Results showed that the uranium migration was significantly affected by the presence of colloid in the medium. KEYWORDS: uranium, colloids, transport, radioactive waste repository, granite. 1. INTRODUCTION

Colloids are particles suspended in a fluid with a size smaller than 1 µm and that present high sorption capability for many solutes because of their high surface area and electric charge.

If colloids are mobile in groundwaters, they can play an important role on the contaminant transport. As reported in Kersting et al. (1999), colloids contributed to plutonium migration along few kilometres far from nuclear test sites (Nevada Test Site, USA).

To be relevant in contaminants transport, colloid must be present in non-negligible concentration, to be stable and mobile in the medium and they must be able to adsorb contaminant in an irreversible form (Miller et al. 1994).

All these condition are strictly dependent on the geochemistry of the medium. One of the options taken into account for isolating high level radioactive waste from the

biosphere is the deep geological disposal (DGP) in a granite host rock. This kind of disposal consists of a geological barrier (host rock) and engineering barriers that aim to contain the radionuclides until they have decayed below harmful levels.

The waste is generally contained in steel canisters surrounded by a layer of compacted clay such as bentonite. Bentonite has a very low permeability, which provides a hydrological barrier

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and good swelling properties and high plasticity that allow sealing rock fractures. In addition, bentonite has a very high sorption capability for most radionuclides.

For the evaluation of the security such kind of disposal many studies have to be undertaken and, in particular, the analysis of the possible contribution of colloids to radionuclide transport has to be considered.

In a DGP, different types of colloids can be present. The radionuclides themselves may be present in a colloidal form (radio-colloids) above all, those radio-elements with very small solubility. On the other hand, radionuclides may be adsorbed onto natural colloids or colloids generated from the engineered barriers of the DGP and form pseudo-colloids.

It has been recently shown that the compacted bentonite barrier can generate bentonite colloids (Missana et al. 2003). Since bentonite has a high sorption capability for many radionuclides, these colloids could be enhance their migration to the geosphere. For this reason it is of interest to analyse the problem.

Colloid mediated radionuclide transport is a process that presents many uncertainties mainly because the shortage of experimental data obtained in realistic conditions (Alonso et al. 2006).

The main objectives of this paper are: (1) to analyse the geochemical conditions in which bentonite colloids would be stable in a granitic medium and (2) to evaluate their effect on uranium transport in a granite fracture.

The stability of the colloids was studied by the analysis of aggregation processes as a function of the time in different geochemical conditions and measuring their surface charge.

Uranium transport in granite was analysed, in presence and absence of bentonite colloids, in a granite column presenting a longitudinal fracture. 2. EXPERIMENTAL 2.1 Water and bentonite colloids

The water used in all the experiments was the groundwater from the Grimsel Test Site (GTS,

Switzerland). Grimsel water is a very low mineralised water (0.001 M), NaHCO3 type, with Eh (- 200 ± 50 mV) and pH (9.5 ± 0.2).

The bentonite (FEBEX bentonite) comes from the Cortijo de Archidona deposit (Almeria, Spain). This clay has a high smectite content (93±2%), with quartz (2±1%), plagioclase (3±1%), cristobalite (2±1%), potassic feldspar, calcite and tridymite as accessory minerals. Further details on this clay can be found elsewhere (Huertas et al., 2000).

Bentonite colloids were obtained after purification and conditioning of the clay to Grimsel water as detailed in Missana et al. (2003). 2.2 Evaluation of the favourable geochemical conditions for colloid-mediated transport: stability studies

To evaluate the charge of bentonite colloids, their zeta-potential (Hunter 1988) was experimentally measured. The zeta-potential is always negative and almost independent on pH (-20 -30 mV).

The kinetic of aggregation of colloids was studied by Photon Correlation Spectrometry, PCS (Holthoff et al., 1996). The main parameters studied were the pH and the ionic strength of the solution.

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2.3 Column experiments

A natural granite bore core with a longitudinal fracture was used for migration experiments. The core came from the Grimsel Test Site (GTS, Switzerland). The effects of bentonite colloids on the migration of relevant actinides and fission products in granite fractures were studied, in the framework of an international project (Colloids and Radionuclide Retardation, [CRR] Project, Möri et al., 2003) at the GTS. In this project, several in-situ migration experiments with colloids and/or radionuclides were performed to evaluate their mobility.

Figure 1 shows the details of the core used in the experiments. In the left part of figure 1 the as-received column can be seen, with a detail of the fracture, whether in the right part the column already mounted and located in an anoxic chamber is visible.

Figure 1 – Pictures of the granite column from the GTS used for transport experiments.

Transport experiments were carried out injecting the tracer or the colloidal suspension (2 mL)

as a “pulse”. The Grimsel water was introduced at the top of the column with a HLPC pump. Before the starting of the experiment, the stability flow rate was monitored during 7 days. The eluted water was periodically collected in previously weighted polyethylene tubes with a fraction collector. The water flow used (Q) was 5.56 mL/h.

The evolution of the tracer or colloid concentration at the column outlet (breakthrough curve) allows the analysis of their transport in comparison to the transport of water.

Tracer concentration is determined by measuring their activity while in the case of colloids, their concentration is measured by PCS.

2.4 Hydrodynamic characterization of the fracture

The hydrodynamic characterization of the fracture was made with tritiated water (HTO). The

initial HTO concentration was C0=0.21µCi/mL. HTO presents negligible interactions with the solids, it is no-sorbing (conservative) thus its transport behaviour representing the water transport. The dynamic porosity according the position of the peak obtained in the HTO breakthrough curve (1.8 h or 10.05 mL) was found. In a first approximation, the porosity of the system corresponds to the volume of the fracture. Supposing that the fracture has a uniform width and it is vertical, the surface exposed to the water flow is 228 cm2. Therefore the width of the fracture is 0.0441 cm and the water velocity 12.61 cm/h.

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The HTO elution curve was fitted using an analytical solution of the transport equation for a fractured media.

2.5 Uranium migration

After the HTO injection, a washing was performed and afterwards 233U (4.4E-5 M, in Grimsel water) was injected as a pulse at the same flow rate. The uranium concentration evolution was analysed by liquid scintillation counting with a Packard 2700 TR detector.

2.6 Uranium migration in the presence of bentonite colloids

To study the effects of the bentonite colloids on the uranium migration in the fracture, 2 mL of bentonite colloids (150 ppm), where previously uranium had adsorbed, were injected in the column at the same water flow as the previous tests with HTO or uranium alone.

The colloids and uranium were maintained in contact during 5 days to ensure the sorption equilibrium before injecting the suspension in the column. The distribution coefficient (KD) of uranium on the bentonite colloids is:

KD =

€

UADS VUFIN m

where UADS is the uranium adsorbed onto colloids (Bq), UFIN is the uranium in solution at the equilibrium (Bq), V the volume of the liquid (mL) and m the mass of colloids (g). The KD value for uranium onto bentonite colloids in Grimsel water conditions was 761 mL/g. Approximately a 11 % of the uranium was adsorbed onto colloids.

3. RESULTS AND DISCUSSION 3.1 Stability studies

The stability of bentonite colloids was analysed in detail in Missana et al. (2003). Results showed that bentonite colloids are very stable in alkaline (>8) and low mineralised waters (I~1·10-3 M) as Grimsel water. It was also showed that divalent ions Ca2+, Mg2+ are much more effective than monovalent ones (Na+, K+) to produce colloid aggregation.

To make clear which is the dependence on the main chemical characteristic of the water on colloid stability (pH and ionic strength), figure 2 shows the probability of silica colloid aggregation. The probability is calculated from the values of their surface potential and calculations are experimental validated by PCS.

If the aggregation probability is near 1 (red region in the graph), the suspension is very instable. Probability 1 means that every collision between two colloids gives rise to aggregation. On the contrary, lower aggregation probabilities indicate higher degree of colloid stability.

In Grimsel water conditions (light blue region) the predicted colloid stability is high, in agreement with experimental results.

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Figure 2 – Aggregation probability calculated as a function of pH and ionic strength.

3.2 Stability studies Figure 3 shows the breakthrough curves of HTO and uranium. The eluted concentration is

normalised to the initial one (C/C0). The recovery of HTO was higher than 90%.

Figure 3 – Comparison between the HTO and uranium breakthrough curves. The position of the peaks allows calculating the uranium retardation factor.

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Uranium presents no negligible sorption onto granite, therefore it is a no conservative element. The uranium recovery at the end of the experiment was approximately 37 %. No conservative elements are expected to present certain retardation in respect to the water in their migration, which can be easily observed by comparing its elution curve with that of HTO.

The retardation factor (Rf) defined by the following expression:

Rf =

€

ν ΗΤΟν Uranium

where νHTO and νUranium are the velocities of HTO and uranium respectively, is 5.58. Therefore, results clearly show that uranium elution is retarded in respect to water.

3.3 Uranium migration in the fracture in the presence of bentonite colloids

Figure 4 show the breakthrough curve of uranium in the presence of bentonite colloids. It is

interesting noticing that the uranium peak, this time, is only slightly retarded with respect to the HTO peak whereas the uranium retardation factor for uranium alone was 5.58.

Figure 4 – Breakthrough curve of uranium in the presence of 150 ppm of bentonite colloids (Q = 5.56 mL/h). In this case, the recovery of the uranium was very low (around 2 %). This indicated that a

small fraction of uranium was moving with colloids and without retardation but that the rest of uranium was immobilised in the fracture, at least during the experimental time span.

Recent experiments (Missana et al., in press) showed that when 150 ppm of bentonite colloids are used with a flow rate of 5.5 mL/h, only approximately a 5% is mobile in the system whereas the rest remains “filtered” the column.

Filtration of the colloids inevitably affects the mobility of uranium. It is very interesting noticing that the migration of uranium in the presence of colloids is totally different from that observed without colloids.

Therefore, the presence of colloids in a DGP may play a relevant role in the transport of radionuclides.

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4. CONCLUSIONS

Stability studies showed that colloids are stable in low mineralised and alkaline waters. In these favourable geochemical conditions bentonite colloids could play a role in the enhancement of transport of sorbing radionuclides towards the geosphere.

This study aimed to point out the difference in the migration behaviour of uranium in absence and in presence of colloids. Results showed that the breakthrough curve of uranium in presence of colloids was totally different from the one of uranium alone. In the presence of colloids, a very small quantity of uranium was eluted, moving un-retarded, but the rest was immobilised in the system. This is an interesting result because shows that even in favourable geochemical conditions stable colloids can be retained / filtrated in the system.

Many factors might affect the filtration of colloids in the fracture but, in particular, the size of colloids, their concentration and the water flow rate can be of greater importance.

Acknowledgements

This work has been partially supported by the EC 6th Framework IP-FUNMIG (Ref. FP6-516514) and by the Spanish Ministry of Education and Science PROMICOL (CGL 2005-01482/BTE). References Alonso, U.; Missana, T.; Geckeis, H.; García-Gutiérrez, M.; Turrero, M.J.; Möri, R.; Schäfer, T.;

Patelli, A. & Rigato, V. (2006) – Role of inorganic colloids generated in a high-level deep geological repository in the migration of radionuclides: Open questions. Journal of Iberian Geology 32, 79-94.

Holthoff, H.; Egelhaaf, S.U.; Borkovec, M.; Shurtenberger, P. & Sticher, H. (1996) – Coagulation rates of colloidal particles by simultaneous static and dynamic light scattering. Langmuir 12, 5541-5549.

Huertas, F.; Fuentes-Santillana, J.L.; Jullien, F.; Rivas, P.; Linares, J.; Fariña, P.; Ghoreychi, M.; Jockwer, N.; Kickmaier, W.; Martínez, M.A.; Samper, J.; Alonso, E. & Elorza, F.J. (2000) –Full scale engineered barriers experiment for a deep geological repository for high-level radioactive waste in crystalline host rock. in EC Final REPORT EUR 19147.

Hunter, R.H. (1988) – Zeta Potential in Colloid Science. Academic Press London. Kersting, A.B.; Efurt, D.W.; Finnegan, D.L.; Rokop, D.J.; Smith, D.K. & Thompson, J.L. (1999)

– Migration of Plutonium in groundwater at the Nevada Test Site. Nature 397, 56-59. Miller, W.; Alexander, R.; Chapman, N.; McKinley, J. & Smellie, J. (1994) – Natural analogue

studies in the geological disposal of radioactive waste. Studies in Environmental Science 57. Missana, T.; Alonso, U. & Turrero, M.J. (2003) – Generation and stability of bentonite colloids

at the bentonite /granite interface of a deep geological radioactive waste repository. Journal of Contaminant Hydrology 61,17-31.

Missana, T.; García-Gutiérrez, M. & Alonso, U. (2004) – Kinetics and irreversibility of cesium and uranium sorption onto bentonite colloids in a deep granitic environment. Applied Clay Science 26, 137-150.

Missana, T.; Alonso, U.; Garcia-Gutiérrez, M. & Mingarro, M. (In press) – The role of colloids in the plutonium and europium migration in a granite fracture. Appl. Geochem.

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Möri, A.; Alexander, W.R.; Geckeis, H.; Hauser, W.; Schäfer, T.; Eikenberg, J.; Fierz, T.; Degueldre, C. & Missana T. (2003) – Colloid and radionuclide retardation experiment at the Grimsel Test Site: influence of bentonite colloids on radionuclide migration in fractured rock. Colloids and Surfaces A 217, 33-47.

Presented during the VI Congresso Ibérico de Geoquímica / XV Semana de Geoquímica, Universidade de Trás-os-Montes e Alto Douro, Vila Real, Portugal, 16-21 July 2007. Received 12 January 2008 Revised 23 January 2008 Published 6 May 2008