Browsing by Subject "Uranium"

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  • Jokinen, Sami A.; Koho, Karoliina; Virtasalo, Joonas J.; Jilbert, Tom (2020)
    Molybdenum (Mo) and uranium (U) contents in sedimentary archives are often used to reconstruct past changes in seafloor oxygenation. However, their sequestration processes are as yet poorly constrained in low-salinity coastal waters, which often suffer from anthropogenic eutrophication but only mild oxygen depletion. Due to the consequent lack of robust long-term paleo-redox reconstructions in such settings often characterized by a shallow front of dissolved sulfide accumulation within the sediment pore waters, inadequate understanding of the long-term drivers behind oxygen loss impedes cost-effective mitigation of this environmental problem. Here, we investigate the mechanisms of Mo and U sequestration in an oxic, low-salinity coastal setting in the northern Baltic Sea where anthropogenic eutrophication over the 20th century has resulted in formation of a shallow sulfate-methane transition zone (SMTZ) in the sediment column of this brackish-water basin. Our results demonstrate remarkably similar patterns for authigenic Mo and U sequestration, whereby the depth and intensity of the SMTZ exerts a first-order control on their solid-phase uptake. Sequential extraction analysis suggests that a large part of the authigenic Mo pool is hosted by refractory Fe-S phases such as pyrite and nanoscale FeMoS4, implying that the Fe-sulfide pathway is the dominating process of authigenic Mo scavenging. However, we also observe a pool of extremely labile Mo deep within the SMTZ, which might record an intermediate phase in authigenic Mo sequestration and/or partial switch to the organic matter (OM) pathway at low dissolved Fe levels. Authigenic U resides in acid-extractable and refractory phases, likely reflecting uptake into poorly crystalline monomeric U(IV) and crystalline uraninite, respectively. Similarly to Mo, authigenic U uptake is active at two fronts within the SMTZ, paralleled by increases in dissolved sulfide levels, suggesting coupling between sulfide production and U reduction. Our results imply that both Mo and U could provide viable proxies for mild bottom water deoxygenation in these settings, through the indirect link between seafloor oxygen conditions and the depth of SMTZ. Of these, Mo appears to more robustly capture variations in seafloor oxygen levels due to the significantly higher share of the authigenic pool. However, temporal resolution of these proxies is limited by the vertical offset between seafloor and the zone of authigenic uptake, and the superimposed character of the signal at a given depth due to vertical migrations of the SMTZ. These results have important implications for the use of Mo and U as paleo-redox proxies in other low-salinity coastal settings exposed to eutrophication.
  • Suikkanen, E.; Rämö, Tapani (2019)
    Episyenites are sub-solidus, quartz-depleted alkali-feldspar-rich rocks. They form veins and lenticular bodies in granitoid rocks and migmatites in a late- to post-orogenic or anorogenic setting. Leaching of quartz is usually a response to a flux of weakly saline hydrothermal solution in circulation cells above cooling intrusions, where sufficient fluid-rock ratios and thermal gradients are achieved. Fluid Si-undersaturation is achieved by rapid cooling within the field of retrograde Si solubility or by temperature and pressure increase outside retrograde conditions. Some quartz may also be consumed in metasomatic reactions and in response to pressure fluctuation in sealed episyenite bodies. The small size and overall rarity of episyenites imply that conditions for episyenite formation are not commonly encountered in the crust. In addition to quartz depletion, episyenites record complex histories of metasomatic alteration and hydrothermal mineral growth. Nearly all episyenites have undergone Na-metasomatism, which may have led to the formation of nearly monomineralic albitite, and which is occasionally followed by late K-metasomatism, phyllic alteration, and argillization. Depending on the effectiveness of later compaction, recrystallization and vug-filling episyenites may preserve the macroscopic porosity formed by quartz dissolution and brittle deformation. Vuggy episyenites can act as significant sinks for metals carried by crustal fluids and host many significant U, Sn, and Au deposits worldwide. Rare earth-critical syenitic fenites around alkaline intrusions share mineralogical and genetic traits with episyenites.
  • Veikkolainen, Toni Henri Kristian; Kukkonen, Ilmo Tapio (2019)
    Radiogenic heat production in Finland has been previously studied using airborne gamma-ray surveys and glacial till measurements alike. For the first time, this paper presents a detailed survey on the spatial variation in radiogenic heat production determined using outcrop samples obtained from all important lithologies of the country. The dataset of 6465 samples represents mostly Mesoarchean (about 2.7 Ga), Paleoproterozoic (ca. 2.2-1.8 Ga) and Mesoproterozoic (ca. 1.6-1.3 Ga) rocks. Nearly all data are from Precambrian Fennoscandian Shield area, but heat production appears to be highly variable, and above global Archean and Proterozoic averages. Spot readings show an arithmetic average of 1.34 +/- 1.19 mu Wm(-3), and a range from 0.02 to 19.4 mu Wm(-3). The interpolated areal average of the whole area is 1.42 +/- 1.41 mu Wm(-3). The high standard deviation of data is related to the geochemical characteristics of uranium (U), thorium (Th) and potassium (K) resulting in a skewed distribution of heat production. Mesoproterozoic anorogenic rapakivi granites, and late Paleoproterozoic Svecofennian granitoids show the highest heat production values in the range of 3-5 mu Wm(-3). The results show no distinct dependencies of heat production with geological age, metamorphic grade nor seismic P-wave velocity, but an increasing trend of heat production with SiO2 content and decreasing trends of heat production with Fe2O3 content and with rock density are evident. Surface heat flow (44 borehole data values) correlates weakly with heat production (r = 0.35). The general heterogeneity of heat production calls for supporting information from other geophysical methods for better understanding of the thermal state of the lithosphere in Finland and beyond.
  • van Helmond, Niels A. G. M.; Jilbert, Tom; Slomp, Caroline P. (2018)
    Anthropogenic nutrient input has caused a rapid expansion of bottom water hypoxia in the Baltic Sea over the past century. Two earlier intervals of widespread hypoxia, coinciding with the Holocene Thermal Maximum (HTMHI; 8-4 ka before present; BP) and the Medieval Climate Anomaly (MCA(HI); similar to 1200-750 years BP), have been identified from Baltic Sea sediments. Here we present sediment records from two sites in the Baltic Sea, and compare the trace metal (As, Ba, Cd, Cu, Mo, Ni, Pb, Re, Sb, Tl, U, V, Zn) enrichments during all three hypoxic intervals. Distinct differences are observed between the intervals and the various elements, highlighting the much stronger perturbation of trace metal cycles during the modern hypoxic interval. Both Mo and U show a strong correlation with C-org and very high absolute concentrations, indicative of frequently euxinic bottom waters during hypoxic intervals. During the modern hypoxic interval (Modern(HI)) comparatively less Mo is sequestered relative to C-org than in earlier intervals. This suggests partial drawdown of the water column Mo inventory in the modern water column due to persistent euxinia and only partial replenishment of Mo through North Sea inflows. Molybdenum contents in modern sediments are likely also affected by the recent slowdown in input of Mo in association with deposition of Fe and Mn oxides. Strong enrichments of U in recent sediments confirm that the Modern(HI) is more intense than past intervals. These results suggest that U is a more reliable indicator for the intensity of bottom water deoxygenation in the Baltic Sea than Mo. Sedimentary Re enrichment commences under mildly reducing conditions, but this element is not further enriched under more reducing conditions. Enrichments of V are relatively minor for the MCA(HI) and Modern(HI), possibly due to strong reservoir effects on V in the water column, indicating that V is unreliable as an indicator for the intensity of bottom water hypoxia in this setting. Furthermore, Ba profiles are strongly influenced by post-depositional remobilization throughout the Holocene. The strong relationship between C-org and Ni, Tl and particularly Cu suggests that these trace metals can be used to reconstruct the C-org flux into the sediments. Profiles of As, Sb and Cd and especially Pb and Zn are strongly influenced by anthropogenic pollution.
  • Angileri, Axel; Sardini, Paul; Donnard, Jerome; Duval, Samuel; Lefeuvre, Hugo; Oger, Tugdual; Patrier, Patricia; Rividi, Nicolas; Siitari-Kauppi, Marja; Toubon, Herve; Descostes, Michael (2018)
    A new approach is proposed in order to spatially localize and determine the equilibrium state of natural decay chains on hand-scale geological samples, thanks to a combination of three techniques: 1) Elementary chemical mapping by microprobe; 2) Alpha autoradiograph by gaseous detectors and 3) bulk alpha particle spectrometry. The quantitative nature of alpha autoradiograph and its comparison with U chemical maps allows to locate radioactive equilibrium state in four samples. This equilibrium state was confirmed by alpha spectrometry analysis.
  • Suominen, Topi (Helsingin yliopisto, 2020)
    Naturally occurring radioactive material (NORM) is defined by the IAEA as “Radioactive material containing no significant amounts of radionuclides other than naturally occurring radionuclides”. In the EC directive 2013/58/Euratom several industry sectors are listed that are known to often deal with NORM, either in residues, wastes, by-products, or products, the mining industry is one of these. The risk posed by NORM is defined by exposure potential and concentration. In the wastes created by the mining industry these are tied to the management of wastes and concentration of radionuclides in the exploited mineral resource. Wastes created by the mining industry are often of environmental concern as they are in many cases piled on the mining site after closure. The tailings of a mine may contain pyrite, which when oxidized creates acid mine drainage. The acidic waters in such sites can enhance the mobility of radionuclides and other harmful elements. In this thesis two mine waste sites were selected for study, which were known to have had issues with natural radioactivity. These two sites were the old Zn, Pb, Cu mine of Vihanti and the Pb, REE mine of Korsnäs. The current state of these two sites was studied. Possible transport of radionuclides or other harmful elements and the dose to a member of the public on the sites was also studied. Soil, waste, sediment, and water samples were collected from both sites and analyzed. Solid samples were analyzed using gamma spectrometry and the radionuclides of interest were: 238U, 226Ra, 210Pb, 232Th, 228Th, 228Ra. Water samples were analyzed with ICP-MS and the elements measured with this method were: Al, Si, P, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Mo, Cd, Pb and U. In addition, some mine waste samples were studied further using XRF and SR-XRPD methods. The results of this thesis indicate that in Vihanti, the wastes have been adequately covered, reducing the external radiation dose to near background levels. The gamma spectrometry results showed no concerning activity concentrations in soil, sediment or waste samples. Two locations were found where the ICP-MS analyses yielded high concentrations of nearly all measured elements, the pH of these sites was low as well. Signs of acid mine drainage were found in these locations, but the effects seem to be localized and no evidence of large-scale transport of contaminants through waterways was found. In Korsnäs the wastes are split into two piles, one containing tailings, and the other enriched lanthanide that was never sold. The results indicate that the lanthanide pile has been adequately covered and the external radiation dose around the pile is near background levels. While the tailings have not been covered like the lanthanide pile has, the results showed that a member of the public is unlikely to receive a dose exceeding 0.1 mSv/a from spending time on the site. Activity concentrations exceeding 1 Bq/g were detected in samples collected from the lanthanide pile, with some evidence of uranium mobilization also seen. In addition, uranium concentrations in the waters of the old open pit mine were relatively high.
  • Fuller, Adam J.; Leary, Peter; Gray, Neil D.; Davies, Helena S.; Mosselmans, J. Frederick W.; Cox, Filipa; Robinson, Clare H.; Pittman, Jon K.; McCann, Clare M.; Muir, Michael; Graham, Margaret C.; Utsunomiya, Satoshi; Bower, William R.; Morris, Katherine; Shaw, Samuel; Bots, Pieter; Livens, Francis R.; Law, Gareth T. W. (2020)
    Understanding the long-term fate, stability, and bioavailability of uranium (U) in the environment is important for the management of nuclear legacy sites and radioactive wastes. Analysis of U behavior at natural analogue sites permits evaluation of U biogeochemistry under conditions more representative of long-term equilibrium. Here, we have used bulk geochemical and microbial community analysis of soils, coupled with X-ray absorption spectroscopy and mu-focus X-ray fluorescence mapping, to gain a mechanistic understanding of the fate of U transported into an organic-rich soil from a pitchblende vein at the UK Needle's Eye Natural Analogue site. U is highly enriched in the Needle's Eye soils (similar to 1600 mg kg(-1)). We show that this enrichment is largely controlled by U(VI) complexation with soil organic matter and not U(VI) bioreduction. Instead, organic-associated U(VI) seems to remain stable under microbially-mediated Fe(III)-reducing conditions. U(IV) (as non-crystalline U(IV)) was only observed at greater depths at the site (>25 cm); the soil here was comparatively mineral-rich, organic-poor, and sulfate-reducing/methanogenic. Furthermore, nanocrystalline UO2, an alternative product of U(VI) reduction in soils, was not observed at the site, and U did not appear to be associated with Fe-bearing minerals. Organicrich soils appear to have the potential to impede U groundwater transport, irrespective of ambient redox conditions. (C) 2020 The Authors. Published by Elsevier Ltd.
  • Kurihara, Eitaro; Takehara, Masato; Suetake, Mizuki; Ikehara, Ryohei; Komiya, Tatsuki; Morooka, Kazuya; Takami, Ryu; Yamasaki, Shinya; Ohnuki, Toshihiko; Horie, Kenji; Takehara, Mami; Law, Gareth T.W.; Bower, William; Mosselmans, J. Frederick W.; Warnicke, Peter; Grambow, Bernd; Ewing, Rodney C.; Utsunomiya, Satoshi (2020)
    Traces of Pu have been detected in material released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March of 2011; however, to date the physical and chemical form of the Pu have remained unknown. Here we report the discovery of particulate Pu associated with cesium-rich microparticles (CsMPs) that formed in and were released from the reactors during the FDNPP meltdowns. The Cs-pollucite-based CsMP contained discrete U(IV)O2 nanoparticles,
  • Matara-aho, Minja (Helsingin yliopisto, 2020)
    Uranium and thorium are naturally occurring radionuclides with trace concentrations found in seawater. The growing use of nuclear energy has increased the risk of non-negligible releases of radioactivity into the environment. Most of the radioactivity is derived from uranium, which is why it is essential to understand more about the speciation of uranium in seawater and evaluate its possible impact on living organisms. The significance of thorium speciation studies rises from the use of plutonium, a more toxic actinide used in nuclear activities. Thorium is chemically similar to plutonium but less toxic and with simpler solution chemistry, so it can be used as an analogue for plutonium speciation studies. In this thesis, the role of earth alkaline cations in the structure of uranyl carbonate complex was investigated. The Ca2UO2(CO3)3 complex was previously identified as the main uranyl species in seawater but the role of calcium cations in the structure was not yet established. We prepared artificial seawater free of Mg2+ and Ca2+ using Sr2+ as a spectroscopic probe, spiked it with uranium ([U]=5·10-5 M) and measured with EXAFS spectroscopy. Together with FTIR spectroscopy and DFT calculations, the formation of Sr2UO2(CO3)3 complex was identified. Bioaccumulation of uranium was studied in sea urchins in a marine-like environment exposing the sea urchins to uranium for at least 200 hours. The U concentration in the seawater was followed throughout the experiment and at the end of the experiment the sea urchins were sacrificed and measured with ICP-OES. Accumulation of uranium was found mostly in the entrails of the sea urchins and in the excrement. The measured U from the seawater settled to about 10 % of the added uranium suggesting that there might be an equilibrium between seawater, sea urchins and sand. Th stability in seawater was studied by using Xylenol Orange as a colored complexing agent and UV-vis spectroscopy to follow the change in thorium concentration in time. Results show that thorium is rather stable in seawater if it is examined with Teflon covered instruments to prevent the thorium adsorption onto surfaces.