Aqueous speciation and sorption properties of Be(II) in Aptian Sands

Beryllium is a chemotoxic, silver-gray, light-weight metal. Beryllium-bearing waste is present in the Aube Disposal Facility (CSA). This surface facility, managed by Andra since 1992, is specifically designed for low- and intermediate-level, short-lived radioactive waste. As for other chemo-toxic elements, the evaluation of Be release towards the biosphere is required for safety and environment impact assessment. This study aims a better characterization of Be migration in the sandy aquifers (Aptian sands) below the waste cells at CSA. Be(II) is characterized by a strong hydrolysis and the formation of sparingly soluble hydroxide phases [1]. In the presence of carbonate, Be(II) forms stable binary/ternary complexes Be(II)-OH-CO3(aq), although the aqueous species prevailing in near-neutral to alkaline conditions remain ill-defined. Groundwater in Aptian sands is characterized by a near-neutral pH and carbonate concentrations ~ 1 mM. Be(II) uptake by Aptian sands has been quantified associated to the characterization of the aqueous speciation in carbonate-containing porewaters.
All experiments were conducted under Ar atmosphere with PCO2= 0.01 bar. ... mehrSolubility was investigated from undersaturation conditions with α-Be(OH) 2(cr) at 5.5 ≤ pHm ≤ 7. Sorption experiments were conducted at pH = (6.4 ± 0.3) using a combination of 9Be and active 7Be isotopes. The short-lived 7Be isotope (t1/2 = 53.22 days) was generated by irradiation of a LiF probe in order to reach lower detection limits.
A systematic decrease in the solubility of Be was observed with increasing pH, in line with the solubility of α-Be(OH)2(cr) in the absence of carbonate [2]. The experimentally measured Be concentrations are significantly lower than the calculated solubility, reflecting that the stability of Be(II)-OH-CO3(aq) species is possibly overestimated in the available thermodynamic studies. Sorption isotherms obtained for the Aptian sands demonstrate a linear trend in the uptake of Be(II) (slope of ≈ +1) within –8.5 ≤ log [Be(II)]aq ≤ –6. A decrease in the slope of the sorption isotherm is observed at log [Be(II)]aq > –6, likely due to the saturation of the sorption sites. Sorption data were modelled with a Langmuir isotherm, considering also the surface precipitation of Be(II). The presence of low concentrations of fluoride (~ 10–5 M) induces a slight decrease in the uptake, which is attributed to the formation of stable Be(II)-F aqueous complexes.

Acknowledgements: This work was partially funded by the National agency for the management of radioactive waste in France (Andra).

References
[1] Brown, P. L., Ekberg, C., 2016, Wiley VCH.
[2] Cevirim-Papaioannou et al., 2020, Appl. Geochem., 98, 237-246