Mobilization of radionuclides and trace metals in tailings at the Rautuvaara mining site

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Title: Mobilization of radionuclides and trace metals in tailings at the Rautuvaara mining site
Author: Pelkonen, Mila Kristiina
Contributor: University of Helsinki, Faculty of Science, Department of Chemistry
Publisher: Helsingin yliopisto
Date: 2018
Language: eng
Thesis level: master's thesis
Discipline: Radiochemistry
Abstract: The old Rautuvaara mining area is located in North-West Finland, in the municipality of Kolari. Rautuvaara mine was operational between the years of 1962 and 1988 and the tailings field, located in the Niesajoki valley, was used to dispose of mine tailings from several different mines until 1995. Currently there are no mining activities at the Rautuvaara mining area, but it has become an object of interest, since an on-going mining prospect in Hannukainen is planning on transporting their produced tailings to Rautuvaara and depositing them above the existing ones. Sulphide-bearing ores and mine waste are known to form acid mine drainage (AMD) when sulphuric acid forms in the oxidative dissolution of sulphide minerals. The decreased pH in the area affected by AMD can further dissolve and mobilize harmful elements from the surroundings minerals, which may contaminate the natural water systems. The aim of this study was to evaluate the occurrence and possible mobilization of the base metals Ni, Zn, Cu, Co, Fe, and Mn, potentially toxic metals As, Pb, Cr, and Cd, as well as the radioactive elements U-238 and Th-232, in samples collected from the Rautuvaara mining area. The collected samples included water samples from the mining area and its surroundings and four types of solid samples: enrichment sand samples from the tailings, sediment samples from the old settling pond, waste rock samples from the old mining area and acidic pond sand samples from the area of the tailings affected by AMD. Measuring the total concentrations of metals and radionuclides in solid sample materials is not enough to understand the mobility and bioavailability of different elements and for this purpose a six-step sequential extraction procedure, with progressively increasing leaching reagents, was applied for all the solid samples. This was done in order to access the geochemical association of metals and radionuclides in the solid samples. The studied fractions were the exchangeable fraction I (pH 7,8), exchangeable fraction II (pH 5,0), mild acid-soluble fraction (pH 4,1), reducible fraction (pH 2,0), oxidizable fraction (pH 1,5) and strong acid-soluble fraction (pH 0). Gamma ray spectrometry was used to determine the total concentrations of radionuclides in the solid samples. The metal concentrations in the leaching supernatants and water samples from the mining area were determined by Inductively Coupled Plasma Mass Spectrometry. Furthermore, the behaviour of the mineral composition in the samples during extraction was studied by using Synchrotron Radiation X-ray Powder Diffraction (SR-XRPD) and by applying the Rietveld method for the acquired data. Water analysis showed the metal concentrations in the settling pond and downstream river samples are comparable to the typical levels of the area, aside from slightly elevated base metal and uranium concentrations. The highest metal and radionuclide concentrations were seen in the acidic pond waters. Gamma ray spectrometry showed that the highest activity concentrations of U-238, Ra-226 and Pb-210 were found in the sediment samples. In the sequential extractions the total amounts of leached metals were in order of: enrichment sand, sediment, acidic pond sand and waste rock. The overall trend is that the highest amounts of metals are leached out in the oxidizable and strong acid-soluble fractions, indicating that the elements are tightly bound to the sample matrix. The dissolution of thorium and uranium was different: thorium mainly dissolved in the strong acid-soluble fraction, whereas uranium started dissolving in the mild acid-soluble fraction as the pH of the solution decreased. The sequential extraction test showed that the mobilization between the enrichment sand and acidic pond sand had been altered, possibly as an effect of AMD. The SR-XRPD data showed that the samples mainly consisted of albite, dolomite and quartz. A complete dissolution of dolomite, bassanite and calcite was seen, as well as a partial attack on pyrrhotite and pyrite, during the extractions. Also, gypsum formation happened in the mild acid-soluble leaching step as the pH decreased during the extractions. The results can be used to evaluate the possible hazards the occurring metals and radionuclides pose to the surrounding environment and they also give a reference point of contamination for the future, if the use of the old tailings area continues.

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