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.
Highlights
Most environmental studies aim to understand the mobility of the chemical elements and their environmental and human health risk
The modeled results of the sequential extractions in this study are in good agreement with the experimental results for the elements of interest which are both the major and the trace elements (U and 226 Ra)
This model could be applied to gain additional information on the behavior of the elements and minerals during the sequential extraction procedure. This sequential extraction protocol is well designed for the study of uranium in such samples, as the possible minerals which U can be associated with, and their reactivity, were well identified
Summary
Most environmental studies aim to understand the mobility of the chemical elements and their environmental and human health risk. Such studies require (i) the analysis of the chemistry of solutions. The sequential extraction technique allows for the characterization of the binding phases and the retention capacity (solubility of phase, sorption, coprecipitation) of an element of interest in a solid sample. This is achieved through the successive use of specific reactants. The main question about sequential extractions is whether they give information on the reactivity of the species under a certain chemical environment or allow the identification of the binding phases
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