Scientific Basis for Efficient Extraction of Uranium from Seawater. I: Understanding the Chemical Speciation of Uranium under Seawater Conditions

In recent years, the prospective recovery of uranium from seawater has become a topic of interest owing to the increasing demand for nuclear fuel worldwide and because of efforts to find sustainable alternatives to terrestrial mining for uranium. To date, the most advanced and promising method of extracting and concentrating uranium from seawater involves the use of polymeric sorbents containing the amidoxime binding moiety. Among a number of different moieties investigated, glutaroimide-dioxime is the most promising one, forming very stable complexes with U­(VI) even in the presence of carbonate. To properly assess the affinity of uranium toward the amidoxime substrates, a comprehensive knowledge of the aqueous chemical equilibria of uranium is required. With this aim, in this paper we review the chemical equilibria of uranium (as UO₂²⁺) in seawater, focusing on the solution equilibria leading to the formation of the stable complexes, M_<i>m</i>(UO₂)­(CO₃)₃^{(2<i>m</i>–4)}(aq) (M = Ca or Mg, <i>m</i> = 0–2). These binary and ternary species dominate the chemistry of uranium in seawater and have recently been the object of study in several papers in the literature. The solubility equilibria of UO₂²⁺ in seawater leading to the formation of the known minerals, including Liebigite, Ca₂(UO₂)­(CO₃)₃·10H₂O­(cr), Swartzite, CaMg­(UO₂)­(CO₃)₃·12H₂O­(cr), Bayleyite Mg₂(UO₂)­(CO₃)₃·18H₂O­(cr), and Andersonite, Na₂Ca­(UO₂)­(CO₃)₃·6H₂O­(cr), are also critically reviewed. Newly calculated values of the solubility products (log <i>K</i>⁰_<i>s</i>) for these solid compounds are presented based on the currently proposed speciation model that includes the most recent aforementioned data for the aqueous speciation of UO₂²⁺. Based on these data, simulated speciation diagrams are calculated, both at zero ionic strength and in seawater-like media. In combination with the speciation data for uranium with glutaroimide-dioxime, these models provide a better, more comprehensive picture of the chemical equilibria of U­(VI) in seawater while also providing useful tools to help assess the feasibility of its recovery through amidoxime-based collection systems