Batch sorption experiments of cesium and strontium on crushed rock and biotite for the estimation of distribution coefficients on intact crystalline rock

Abstract

The distribution coefficient (K d) of radionuclides on bedrock is one of the key parameters used in the safety analysis of spent nuclear fuel repositories. Typically, distribution coefficients have been determined using crushed rock. However, recent studies have shown that crushing of the rock increases considerably the distribution coefficient compared with the values of intact rock. This study aimed to test if batch sorption experiments using different grain sizes (i.e. mean diameter of grains) can be used to evaluate the K d of strontium (Sr) and cesium (Cs) on intact crystalline rock, which would decrease the needed experimental time compared with transport experiments. Here we report the results of the batch sorption experiments with crushed rocks and compare the results with those from a recent study performed using electromigration experiments with intact drill core samples (Puukko et al., 2018). The batch sorption experiments were done for rock samples from Olkiluoto, Finland, as a function of grain size and of Cs and Sr concentration. Furthermore, the specific surface areas of the same rock samples with different grain sizes were determined. It was shown that Cs distribution coefficients correlate with specific surface areas of the studied rocks and biotite, the correlation coefficient being 0.95. The Cs distribution coefficient was highest for biotite at about 0.1 m 3 /kg at 10 À4 M cesium concentration and increased systematically to about 1 m 3 /kg at 10 À8 M. Distribution coefficients for rocks were up to about two orders of magnitude lower, being lowest with the rock with the lowest biotite content (3.3%). The distribution coefficient of Sr varied from 0.04 m 3 /kg to 0.007 m 3 /kg and behaved in a different manner: it remained constant in two out of three studied rocks in the concentration range of 10 À8-10 À4 M and only in the case of one rock a decreasing trend was seen at the higher concentration range. It was also shown that batch sorption experiments overestimate the distribution coefficient in respect to intact rock. The decrease of the distribution coefficient as a function of grain size can be estimated using a power law function. It was also shown that estimation of distribution coefficients of Cs and Sr for intact rock by extrapolation of distribution coefficients determined for different grain sizes is not possible without increasing grain size, but in that case diffusion into the grains would also affect the results. A new method was developed for estimating the fraction of the inner surface area of the total surface area of crushed grains. For the mean grain sizes of 0.25 mm and 0.75 mm the fraction of the inner surface was found to be 35-70% and 60-90%, respectively. The inner specific surface area was highest with biotite at 1.2 m 2 /g and lowest with the rock with lowest biotite content (3.3%) at 0.07 m 2 /g. The surface area analysis revealed that crushing creates and/or allows access to additional inner surface area that is not measured in intact rock. Furthermore, it was demonstr...

Phyllite/bentonite mixture—an alternative effective buffer material for a geological disposal of radioactive waste

“…The laboratory batch sorption equilibrium test allowed the estimation of Eu(III) distribution coefficients ( K d ). This is an important parameter in transport modelling and one of the key parameters used in the safety analysis of spent fuel repositories (Lehto et al 2019 ). Distribution coefficients are relevant to the specific geochemical conditions of each system under study and give an indication of the extent of Eu(III) partitioning to the solid phase in different systems and are useful in assessing the migration of Eu(III) in the barrier.…”

A literature review on thermodynamic sorption models of radionuclides with some selected granitic minerals

“…Freundlich? 7 0.01 M NaCl B4 (Lehto, et al, 2019) Cs(I) (10 -8 -10 -4 M) −2 to 1 -3 6 and 8 0.1M NaCl, KCl, CaCl 2 B7 (Kyllönen, et al, 2014) Cs(I) (10 -9 -10 -3 M) −2 to 0 -3 6.9 (7.9) synthetic groundwater B10 (Muuri, et al, 2017) Cs(I) (10 -9 -10 -3 M) −2 to 0 -3 9.7 (8.9) synthetic groundwater B11 (Muuri, et al, 2016) Cs(I) (5•10 -6 -5•10 -3 M) −1.5 to −0.3 Langmuir (single site) Langmuir (single site) 7-8 0.01M Na lactate B14 (Brookshaw, et al, 2016) Eu(III) (9•10 −8 M) −1 to 3 1 1 3-9.5 0.05, 0.5 M NaClO 4 B1 (Puukko, et al, 2007) Eu(III) (10 -5 M) Not given -1 2 -6 0.01, 0.1 and 1M NaCl B9 (Fukushi, et al, 2013) Ni(II) (10 −6 M) −1 to 1 1 1 3-9.5 0.05, 0.5 M NaClO 4 B1 (Puukko, et al, 2007) Pu(VI) (10 −11 M) Not given --3-11 0.1M NaCl B12 (Furuya, et al, 1997) Ra(II) (10 -10 -10 −7 M) −1 to 1 -3 8 0.01, 0.1 and 1 M B2 (Söderlund, et al, 2019) Ra(II) (5•10 -9 -5• 10 -7 M) 0 to 0.8 Freundlich Freundlich 7 0.01 M NaCl B3 (Ames, et al, 1983) Se(IV) −2 to 0.08 3(4)** -3-11 0.01M KClO 4 B13 (Li, et al, 2020) Sr(II) (10 −8 -10 -2 M) −2 to 0.5 -3 8 0.01, 0.1 and 1 M B2 (Söderlund, et al, 2019) Sr(II) (10 −9 -10 −4 M) −2 to −1 Freundlich? Freundlich?…”

“…Freundlich? 7 0.01 M NaCl B4 (Lehto, et al, 2019) Sr(II) (5•10 -6 -5•10 -3 M) −1.5 Langmuir (single site) Langmuir (single site) 7-8 0.01M Na lactate B14 (Brookshaw, et al, 2016) Frontiers in Nuclear Engineering frontiersin.org…”

Diffusion and Sorption Studies of Cs, Sr and Co in Intact Crystalline Rock

“…However, the strength of the effect will differ based on the affinity of the tracer for minerals. For example, Lehto et al [34] studied the sorption of Cs and Sr with batch experiments in crushed crystalline rock and biotite. Their findings indicate that Cs sorbed more strongly than Sr in crystalline rock.…”