Mobility and retention mechanisms of 238U, 230Th, 226Ra, and 210Pb in soils from the waste dump of an abandoned Cu mine

The interactions of uranium-series radionuclides with soils allow for the assessment of their mobility and retention in the environment. However, waste dumps from abandoned mines, such as the one studied in this work, present complex conditions due to the presence of high concentrations of heavy metals. The studied waste dump contains high concentrations of Fe, Cu, and As, mainly due to the presence of chalcopyrite (CuFeS₂), arsenopyrite (FeAsS), malachite (Cu₂CO₃(OH)₂), azurite (Cu₃(CO₃)₂(OH)₂), and pyrite (FeS₂), together with the presence of torbernite (Cu₂(UO₂)₂(PO₄)₂·8 H₂O) and zeunerite (Cu₂(UO₂)₂(AsO₄)₂·10 H₂O) of hydrothermal geological origin. Samples were collected from three profiles in three zones selected according to the radioactive content of the study area. The correlations obtained between Fe and organic matter, both in the principal component analysis and in the parameter weights of the multiple regression analysis, revealed that the main retention mechanism was the formation of inner-sphere complexes. On the other hand, the sequential analysis of ²³⁵U, ²³⁴Th, ²²⁶Ra, and ²¹⁰Pb using the Schultz method allowed for the evaluation of the evolution of the site, as the associations of these radionuclides evolved towards a significant increase in the carbonate fraction.