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

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.