Mobility Control Of Uranium And Other Potentially Toxic Elements In Mine Waters By Ochre-Precipitates
Valente, Teresa Maria (1); Antunes, Isabel Margarida (1); Neiva, Ana (2); Santos, António (2); Moreno, Filipa (1)
1: Universidade do Minho, Portugal; 2: Geobiotec, University of Coimbra, Portugal
Mineral-water interaction processes in mine drainage often result in the formation of iron-rich precipitates, typically called ochre products. The strong reactivity of these ochres, often with very small size, comprising particles in the nanoscale, made them very important in controlling fate and transport of potentially toxic elements (PTE), such as uranium, and thorium. The streams that receive these mine waters are typically covered by this red-yellowish material, usually with low crystallinity or amorphous nature. The present study is focused on the ochre products precipitated from mine waters that pose major environmental concern, such as the uranium-rich mine waters.
Hydrogeochemical and mineralogical studies were performed in a uranium mine, located in the Reboleiro area - Guarda, Portugal. The local geology is characterized by smoky and zoned quartz veins and basic rocks with pitchblende, sulfides, and secondary uranium minerals (Cameron, 1982). Underground exploitation of U3O8 resulted in waste rock dumps with high concentration of radionuclides. High levels of radiation have been reported in the surrounding water and soil. Surface runoff and mine water are discharged into a small creek (Paul creek), which displays typical features of mine contamination, such as sediment coatings and deposition of ochre sludge. Therefore, the present work intends to elucidate the role of the nanoparticles that compose these ochres on the retention or dispersion of uranium, thorium, and other PTE.
For that, samples of water and ochre material were collected along the creek to be analyzed by inductively coupled plasma optical emission spectroscopy. Water samples were filtered in the field with syringe filters of 0.45 mm pore sizes and acidified to pH <2.0 for the determination of elements such as U, Th, As, Co, Cd, Pb, Cu, Zn, Fe, Mn. The same elements were analyzed in the ochre samples after a digestion with aqua regia. Blanks, replicates and stock solutions were used to assess quality control. In addition, x-ray diffraction, and electron microscopy (scanning and transmission) were applied in order to obtain data about composition, morphology, crystallinity, and relationships between minerals and PTE.
The results indicate the ability to retain PTE, particularly by the smallest size and lowest crystalline ochre products. Moreover, they gave indication about the sorption mechanisms that control the association between the ochres, other small size minerals such as clay minerals, and the PTE. Thus, iron-rich nanoparticles with specific morphology control dispersion of uranium, thorium and other PTE mobilized from U-ores and from the sulfides.