Projects & Contracts
Junta de Andalucía FEDER 14/20 2018 | UHU-1255876 | 01/01/2020 - 30/06/2022
The quantification of the non-secular equilibrium between pairs of natural radionuclides (father-daughter), provides very valuable information about the geological and environmental processes, as the rate or the time scale at which they take place. Given the wide range of half-lives for the different natural radionuclides, especially the U and Th series, it is possible to study processes that take place from scales Earth age to minutes. In surface environments, the different chemical behavior of radioelements as U, Th, Ra, Pb, Po, causes their fractionation and the initial secular equilibrium is broken. It is known that various mechanisms produce fractionation between isotopes of the same chemical element (234U-238U, 230Th-234U, etc.), which are linked to physical processes related to the radioactive decay of the radionuclides that precede its daughter, or because they are associated to different primary minerals.
Few studies have been carried out to study the fractionation and enrichment of natural radionuclides under Acid Mine Drainage (AMD). Preliminary results of our research group show that the contents of both U- and Th-isotopes in the dissolved phase (0.45 μm) of the acid waters from the Tinto and Odiel rivers are 2-3 orders of magnitude higher than those ones measured in undisturbed rivers, demonstrating this fact that under acidic conditions (pH < 4), as is the case of environments subjected to AMD, the geochemical behavior of radioelements such as U, Th or Ra is very different from the shown one under neutral conditions (pH = 6-8). Likewise, high activity ratios 234U/238U (up to 15), have been measured in surface waters affected by AMD of the Sierra de Huelva, or in other systems from Japan. In addition, unpublished preliminary results of the research team of this project suggest that colloids play a central role in the generation of these fractionations and enrichments, since the 234U/238U activity ratio increases as the size of the colloidal fraction decreases.
On the other hand, the knowledge of the mechanisms that regulate the mobility from both U- and Th-series radionuclides is very relevant for studies on the behavior of analogous anthropogenic radionuclides derived from the nuclear industry, since the majority of transuranic elements (Am, Pu, Np, etc.), have similar geochemical properties to some U- and Th-series radionuclides, which allows to improve the evaluation of their potential migration into the environment from potential deep geological repositories.
Taking into account the above considerations, the main objective of this project is to find the mechanisms of fractionation and enrichment of long-lived natural radionuclides from the U and Th-series in extreme conditions of superficial environments, such as those produced in acid mining drains. Based on the fact that the geochemical and isotopic composition of the basins of the Tinto and Odiel rivers has already been determined in other studies, and considering that the radionuclides of greatest interest are U-isotopes, Th-isotopes and Ra-isotopes, laboratory leaching experiments will be carried to study the variables involved in the mobility of these radionuclides under different environmental conditions, similar to those of these rivers, which will allow to evaluate the mechanisms governing the fractionation of the isotopes of these radionuclides. And specially it will be study the role of the colloids on these enrichments will be analyzed through the use of ultrafiltration techniques.
RESUMEN DE LA PROPUESTA / SUMMARY OF THE PROPOSAL