Retention of 226Ra by illite and montmorillonite: an adsoption study

The fate of important radionuclides must be assessed in geochemical scenarios to ensure the long-term safety of deep nuclear waste geological repositories. Depending on the host-rock, some of these scenarios have examined the consequences of contact with the interstitial “in situ” porewaters with spent nuclear fuel, indicating that 226Ra can become a main contributor to the total dose after 104 - 105 years of waste disposal [1]. The fate of 226Ra is also relevant from the environmental perspective as it is also a main contributor to naturally occurring radioactive materials (NORM) and the environmental hazards associated with NORM e.g. in the surroundings of mining operations or geothermal operations [2]. Argillaceous rocks are considered suitable for the construction of deep geological waste repositories for high-level nuclear waste in several waste management programs e.g. in Switzerland [3], Belgium [4], or France [5]. Clay minerals are major constituents of these formations and due to their low permeability and high adsorption properties they considerably retard the migration of potentially released positively charged radionuclides in the biosphere. To account for these retardation properties in safety studies, thermodynamic models capable of predicting the retention of radionuclides over a wide range of physico-chemical conditions are crucial. Illite and illite/smectite mixed layers are relevant constituents of many argillaceous host rocks. In addition, bentonites are envisaged as engineered barriers in underground waste disposal concepts (e.g. [1], [2]). For example, in the Swiss concept, bentonite is foreseen as a buffer material to seal waste canisters, while bentonite-sand mixtures are foreseen as backfill material for access tunnels [7]. Montmorillonite, the major constituent of bentonite (up to 90 wt.-%) is an important sink for potentially released radionuclides and a common mineral in soils and sediments. Adsorption data for 226Ra onto illite and montmorillonite are sparse in the open literature. Because of this knowledge gap, the adsorption and diffusion of 226Ra in clay minerals are assumed to be like that for Ba, based on chemical analogy. In order to verify this assumption and improve the quantitative understanding of the retention of 226Ra by 2:1 clay minerals relevant for radioactive waste disposal, a combined adsorption and modelling study was carried out. Parallel adsorption experiments were carried out onto the homo-ionic Na form of Wyoming montmorillonite (SWy-2) and illite (Na-illite, IdP-2; Illite de Puy, Le Puy-en-Velay, France) in diluted systems and compared to the adsorption of Ba under the same conditions. 226Ra and Ba adsorption edges in the pH range 5 to 10 at different ionic strengths (0.01 - 0.3 M NaCl) and an isotherm (pH 7, 10-9 M 10-2 M) were carried out on Na-SWy and IdP. For montmorillonite, the 2-site Protolysis Non Electrostatic Surface Complexation and Cation Exchange adsorption model (2SPNE SC/CE, [8]) reproduces the experimental data for both elements quite well [9]. The Kc values for Ba2+-Na+ and 226Ra2+-Na+ exchange equilibria are in good agreement at low ionic strength (log Kc = 0.70 - 0.84), whereas at high ionic strength, the Kc (Ba-Na) is slightly higher (log Kc of 0.90 (0.3 M) vs 0.70 (≤ 0.02 M)). In contrast, the Kc (Ra-Na) exhibits a clear dependency on ionic strength, with log Kc values of 0.7, 1.14 and 1.34 at NaCl background concentrations of 0.01/0.02 M, 0.14 M and 0.3 M, respectively. For illite, the 2SPNE SC/CE model derived in earlier studies (?) reproduces the experimental Ba adsorption data quite well. In comparison to Ba, pH edges at varying ionic strengths consistently reveal a higher selectivity of illite for 226Ra. The 226Ra pH edges are well described by an adjusted 2SPNE SC/CE model for illite. Additional 226Ra-Ba adsorption experiments with Ba intended as a carrier for 226Ra again showed a higher affinity of 226Ra. The distinct differences in the behavior of 226Ra compared to Ba can only be described by a more complex model, which includes two additional high affinity sites for 226Ra. These sites are characterized by a low capacity and high selectivity for 226Ra. These may be sub-sites of the Ba high affinity sites, leading to a partial competition between Ba and 226Ra.The results presented here indicate that Ba can be used as a good analogue for 226Ra regarding the adsorption on montmorillonite at ionic strengths < 0.1 M and pH < 8 but deviates in its behaviour at higher ionic strength. Different from the adsorption on montmorillonite, for the adsorption on illite Ba cannot be considered a very good analogue for 226Ra but needs to be described by a more complex adsorption model.[1] SKB (2011). Long-term Safety for the Final Repository of Spent Nuclear Fuel at Forsmark. Main Report of the Sr-Site Project I-III. Stockholm, Sweden.[2] Fisher, R.S. (1998). Geologic and geochemical controls on naturally occurring radioactive materials (NORM) in produced water from oil, gas, and geothermal operations. Environ. Geosci. 5, 139–150.[3] Nagra (2002). Project Opalinus Clay: Safety Report. Demonstration of Disposal Feasibility (Entsorgungsnachweis) for Spent Fuel, Vitrified High-Level Waste and Long-Lived Intermediate-Level Waste. Technical Report NTB 02-05. Wettingen, Switzerland.[4] ONDRAF/NIRAS (2001). SAFIR 2: Safety assessment and feasibility report 2. Technical report NIROND 2001-05 E. ONDRAF/NIRAS, Brussels, Belgium.[5] Andra (2001). Référentiel géologique du site de Meuse/Haute Marne. Rapp. A RP ADS 99-005 de l’Agence nationale pour la gestion des déchets radioactifs, Châtenay-Malabry, France[6] Nagra (2014). Modellhaftes Inventar für radioaktive Materialien MIRAM 14. Wettingen, Switzerland[7] Jenni, A., Wersin, P., Thoenen, T., Baeyens, B., Ferrari, A., Gimmi, T., Mäder, U., Marschall, P., Hummel, W., Leupin, O. (2019). Bentonite Backfill Performance in a High-Level Waste Repository: a Geochemical Perspective (Nagra: Technical Report, Report No.: NTB 19-03).[8] Bradbury, M.H., Baeyens, B. (1997). A mechanistic description of Ni and Zn sorption on Na-montmorillonite Part II: modelling. J. Contam. Hydrol. 27, 223–248. [9] Klinkenberg, M., Brandt, F., Baeyens, B., Bosbach, D., Fernandes, M.M., 2021. Adsorption of barium and radium on montmorillonite: A comparative experimental and modelling study. Applied Geochemistry 135, 105117