Browsing by Subject "Mediterranean Sea"

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.