XAS and TRLIF spectroscopy of uranium and neptunium in seawater

Abstract

Seawater contains radionuclides at environmental levels; some are naturally present and others come from anthropogenic nuclear activity. In this report, the molecular speciation in seawater of uranium(VI) and neptunium(V) at a concentration of 5 × 10(-5) M has been investigated for the first time using a combination of two spectroscopic techniques: Time-resolved laser-induced fluorescence (TRLIF) for U and extended X-ray absorption fine structure (EXAFS) for U and Np at the LIII edge. In parallel, the theoretical speciation of uranium and neptunium in seawater at the same concentration is also discussed and compared to spectroscopic data. The uranium complex was identified as the neutral carbonato calcic complex UO2(CO3)3Ca2, which has been previously described in other natural systems. In the case of neptunium, the complex identified is mainly a carbonato complex whose exact stoichiometry is more difficult to assess. The knowledge of the actinide molecular speciation and reactivity in seawater is of fundamental interest in the particular case of uranium recovery and more generally regarding the actinide life cycle within the biosphere in the case of accidental release. This is the first report of actinide direct speciation in seawater medium that can complement inventory data.

Experimental Speciation of Plutonium(IV) in Natural Seawater

“…Previously, this method was successfully applied to the study of uranium, neptunium and americium in natural seawater. X‐ray Absorption Spectroscopy (XAS) was performed together with Time Resolved Laser Fluorescence Spectroscopy for uranium(VI) ,…”

Cleaving Off Uranyl Oxygens through Chelation: A Mechanistic Study in the Gas Phase

“…[1][2][3][4][5] In spite of its apparent chemical inertness, UO 2 2+ has recently caught growing attention for the tantalizing prospect of activating its remarkably stable metal-oxygen double bonds. 6 Nevertheless, the activation and functionalization of UO 2 2+ still remain challenging and elusive.…”

The uranium isotopic record of shales and carbonates through geologic time

“…In the modern oxic ocean, U is highly soluble as uranyl carbonate complexes (Langmuir 1978;Dong and Brooks 2006;Endrizzi and Rao 2014;Maloubier et al 2015). Its residence time of ~400…”

“…Indeed, uranium is a redox-sensitive element, and its isotopic composition in modern seawater ( 238 U sw ) reflects the mass balance between riverine input of U (assumed to be the major source) and removal into several sinks (Tissot and Dauphas 2015;Andersen et al 2016). In the modern ocean, U occurs in two main oxidation states: highly soluble U(VI), which exists as uranyl carbonate complexes (Langmuir 1978;Dong and Brooks 2006;Endrizzi and Rao 2014;Maloubier et al 2015), and insoluble U(IV).…”