Migration of 75Se(IV) in crushed Beishan granite: Effects of the iron content
Graphical abstract
Introduction
Deep geological repositories generally rely on a multi-barrier system to permanently isolate nuclear waste from the biosphere at depths of hundreds of meters. This multi-barrier system typically comprises an engineered barrier system and the natural geological barrier provided by the host rock and its surroundings [1]. In China, Beishan granite (BsG), an abundant, tough, and low porosity igneous rock, is a preferred host rock at the potential repository site called the Beishan area [2]. Therefore, a better understanding of the migration behaviors of important radionuclides in granite is important for a pre-safety assessment of the site [3].
79Se (t1/2 = 3.77 × 105 y) is considered one of the few radionuclides with a long half-life that can be transported into the biosphere and contribute to the ultimate exposure dose over the geological time scale according to a scenario analysis [3], [4], [5]. It can exist in various oxidation states, which determine its solubility [6], [7] and migration behavior. Anionic selenite and selenate are more soluble and prefer to diffuse, while selenium with lower oxidation states (0, −I, and −II), such as elemental Se or Fe selenides, are present in the solid state [6], [7] and have low solubility. For redox-sensitive selenium, pH and Eh are two important factors that influence the chemical behaviors of the selenium in the repository. Thus, it is important to investigate the migration of the dominant aqueous species of 79Se [8] in the host rock for China’s potential geologic repository.
Our previous work [9] and a recent study [10] investigated the effects of some factors (e.g., pH, oxygen conditions, and ionic strength) on sorption and diffusion of selenium oxyanions in several types of crushed granitic rock. The results show that the sorption behavior is closely related to pH and oxygen conditions. Crushed granite has always attracted attention [11], [12], due to its controllable loading density for diffusion experiments and favorable diffusion access in the ultimate barrier. Apart from selenium oxyanions, some other mobile anionic radionuclides (e.g., 125I− [13], and 99TcO4− [14]) in granite rocks were also studied via a through-diffusion experiment. The in-diffusion capillary method, first used by Wang, X.K. et al. [15], was widely applied to study the migration behaviors of the radionuclide in compacted bentonite [1], [16] and crushed granite [12]. It is a powerful tool for obtaining diffusion coefficients (Da) and distribution coefficients (Kd), which enables a more efficient study of nuclide migration behavior.
The objective of this work is to study selenite migration behavior in BsG under oxic and anoxic conditions as a function of pH. Considerable research [17] has shown that the oxides and oxyhydroxides of Al, Fe, Ca, and Mg compounds, which exist in BsG [13], have a good affinity for selenite and the strength varies as pH changes [19], [20], [21]. Especially, Fe compounds on the BsG surface should be paid more attention due to its reactivity towards oxygen. The oxygen concentration changes the Fe(II)/Fe(III) compounds’ ratio, which strongly affects selenite migration behavior, rather than Al(III), Ca(II), and Mg(II) compounds [18], [19], [20]. Furthermore, reducibility of Fe(II)-bearing minerals [7], [21], [22] and the sorption capacity of Fe(III)-oxyhydroxide products [23], [24] for selenite are always attracting researchers’ attention, and several sorption processes, such as surface complexation, anionic exchange, and reductive inhibition, are involved simultaneously. Further study is required to clarify how and to what extent Fe influences the diffusion of selenite in BsG.
Section snippets
Materials and chemicals
The employed BsG powder was obtained by grinding the granite cores, which were drilled out from the No. BS03 drill hole in the Beishan area (Gansu, China) at a depth of approximately 600 m. The composition of the Beishan groundwater (pH ∼8.03, Eh ∼120 mV, ionic strength ∼0.04 M) from the same drill hole and at a similar depth (∼550 m) was determined using ion chromatography (IC, DIONEX ICS 3000) (Text S.1, Table S.1). The carrier-free H275SeO3 liquid source used for the capillary diffusion and
Characterization of fresh BsG
The mineralogical composition of fresh BsG was determined using PXRD, and Rietveld refinement was performed using BGMN [30] (Fig. S.5, Text S.6). According to the refinement results, the granite was mainly composed of quartz, annite, plagioclase feldspars, and microcline (Table S.2). The F-rich analogue of annite (K0.88Na0.07Ca0.03)(Fe2+0.82Ti0.07Mg0.04)3(Al1.22Si2.84O10)(F0.11OH0.86Cl0.03)2, which was calculated by BGMN, is an Fe(II)-bearing mineral and was found (∼5.24 wt.%, ∼8.71°2θ) in the
Conclusions
This study indicates that the diffusion of selenite in compacted BsG is influenced by Fe compounds, oxygen and pH conditions. With all of the influences of inner-sphere complexation, surface charge effect and reductive precipitation, selenite always diffused slowest in BsG at approximately pH 5. The Da values under anoxic conditions are nearly one order of magnitude larger than those under oxic conditions. This difference results from larger amount of Fe(III)O compounds on the granite’s surface
Acknowledgments
Nathaniel FINDLING (XRD engineer at ISTerre) is acknowledged for providing BGMN refinement instruction. Great gratitude to Prof. Jinglin XIE for the scientific advice on XPS analysis. Funding for this research was provided by the Special Foundation for High-level Radioactive Waste Disposal (2007-840, 2012-851) and the National Natural Science Foundation of China (NSFC, No. 11075006, 91026010). In addition, we are grateful to the 1W2B beamline at the Beijing Synchrotron Radiation Facility
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Cited by (0)
- 1
These authors contributed equally to this work.
- 2
Present address: ISTerre, Université Grenoble Alpes, F-38041 Grenoble, France.
- 3
Present address: Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, China.