The Role of Barite in the Post-Mining Stabilization of Radium-226: A Modeling Contribution for Sequential Extractions

Abstract

Barite is ubiquitous and known to incorporate 226Ra through the formation of a solid-solution. In U mining mill tailings, barite is one of the dominant sulfate-binding minerals. In such environments, sequential extractions are generally used to identify the U- and 226Ra-binding phases and their associated reactivity. To better decipher the main processes governing the behavior of 226Ra during such sequential extractions, a geochemical model was developed with PHREEQC mimicking the sequential extraction of U and 226Ra from Bois-Noirs Limouzat U mine tailings, France. The model results were compared with a dataset produced by an experimental sequential extraction from the same mine tailings and including data on the solids and selective extraction results with the major elements, U and 226Ra. The simulations reproduced the results of the experimental chemical extractions accurately, with iron oxyhydroxides being the major U binding phase. However, the modeling indicated rather that barite would be the main 226Ra binding phase, instead of the iron oxyhydroxides identified by the experimental extractions. This is consistent with the 226Ra concentration measured in pore water, but in disagreement with the direct interpretation of the sequential extractions. The direct interpretation disregarded the role of barite in the geochemical behavior of 226Ra because barite was not specifically targeted by any of the extraction steps. However, the modeling showed that the dissolution of 226Ra-binding barite by reactants would lead to a 226Ra redistribution among the clay minerals, resulting in a skew in the experimental results. Similar results were achieved by referring simply to the bulk mineralogy of the tailings. This study highlights the importance of considering the mineralogy, mineral reactivity and retention capacity for more realistic interpretation of sequential extractions. Moreover, this paper provides new perspectives on the long-term consequences of these mill tailings in which barite controls the geochemical behavior of the 226Ra.

Barite precipitation in porous media: Impact of pore structure and surface charge on ionic diffusion

“…Barite was chosen as its formation is of concern in geothermal reservoirs and hydraulic fracturing (Mundhenk et al, 2013;Vengosh et al, 2014) and as it is capable of trapping radium in mill tailings of former uranium mines (Besançon et al, 2020;Chautard et al, 2020) and in nuclear waste disposal facilities (Brandt et al, 2018;Vinograd et al, 2018). The second objective is to quantify the impact of barite precipitates on diffusive transport properties.…”

Assessment of 226Ra and U colloidal transport in a mining environment

“…Its mobility is also controlled by sorption reactions on the surface of clay minerals (Reinoso-Maset and Ly, 2016;Robin et al, 2017) or manganese-iron (oxy-)hydroxides (Ames et al, 1983;Nirdosh et al, 1990;Sajih et al, 2014). Furthermore, radium can be incorporated into the structure of other alkaline earth sulfate or carbonate minerals occurring in mining environments (Curie, 1911;Doerner and Hoskins, 1925;Langmuir and Reise, 1985;Gnanapragasam and Lewis, 1995;Curti, 1999;Grandia et al, 2008;Yoshida et al, 2008;McDevitt et al, 2019), such as barite, through the formation of a (Ba,Ra)SO4 solid solution (Doerner and Hoskins, 1925;Brandt et al, 2015;Ouyang et al, 2019;Besançon et al, 2020;Brandt et al, 2020). Sorption of U and 226 Ra on natural organic matter has also been documented (see Bordelet et al (2018) and references therein).…”

A lab-on-a-chip approach integrating in-situ characterization and reactive transport modelling diagnostics to unravel (Ba,Sr)SO4 oscillatory zoning

“…Solid solutions including barium sulphate (BaSO 4 ) as a major component are widely present in various natural and anthropogenic systems. They are commonly formed during hydraulic fracturing in geothermal systems for energy production 1 , uranium mining tailings 2 and treatment of industrial waste and contaminated water 3 – 6 . The low solubility of BaSO 4 enables the removal of structure-compatible trace contaminants and radionuclides such as 226 Ra and 90 Sr.…”