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

Abstract

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 UO 2 2+) in seawater, focusing on the solution equilibria leading to the formation of the stable complexes, M m (UO 2 )(CO 3 ) 3 (2m−4) (aq) (M = Ca or Mg, m = 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 UO 2 2+ in seawater leading to the formation of the known minerals, including Liebigite, Ca 2 (UO 2 ), and Andersonite, Na 2 Ca(UO 2 )(CO 3 ) 3 •6H 2 O(cr), are also critically reviewed. Newly calculated values of the solubility products (log K 0 s ) 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 UO 2 2+. 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.a Total Inorganic Carbon. b Dissolved Inorganic Phosphorus. c From ref 1, p 224.

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.…”

“…6 Nevertheless, the activation and functionalization of UO 2 2+ still remain challenging and elusive. In the condensed phase, a few examples of apparent activation and functionalization of the highly stable U=O bonds have been proposed recently, with notably the reductive silylation and coordination by a Lewis acid such as B(C 6 F 5 ) 3 or an alkali metal. [7][8][9] However, with these approaches, the uranyl oxo bond (U=O) undergoes reduction due to the instability of the hypothetical UO Bu-calix [6]arene and La(OTf) 3 .…”

“…In the condensed phase, a few examples of apparent activation and functionalization of the highly stable U=O bonds have been proposed recently, with notably the reductive silylation and coordination by a Lewis acid such as B(C 6 F 5 ) 3 or an alkali metal. [7][8][9] However, with these approaches, the uranyl oxo bond (U=O) undergoes reduction due to the instability of the hypothetical UO Bu-calix [6]arene and La(OTf) 3 . 13 While characterization of this reaction's product revealed an octahedral U(VI) atom ligated by three phenolate oxygens from each of two calixarene ligands, the mechanism of activation was not understood.…”

Development of the application of speciation in chemistry

“…Table 5 Application areas for speciation trace analysis [74] Element Application area for speciation analysis Aluminum Al Aggregates [75] Antimony Sb Redox forms and organoantimony compounds in the environment [76,77] Arsenic As Redox forms and organoarsenic compounds in the environment [77] Arsenic in food products [78] Forms of arsine in air [79] Cadmium Cd Cadmium in food products [80] Chromium Cr Redox forms of chromium, Cr(VI) in the environment [81,82] Lead Pb Forms of lead compounds in the environment [83] Mercury Hg Forms of mercury compounds in the environment [84][85][86] Selenium Se Inorganic and organometallic selenium compounds in the environment [87] Thallium Tl Thallium compounds in river water [88] Tellurium Te Tellurium compounds in the environment [89] Uranium U Forms of uranium compounds in seawater [90] Zinc Zn Forms of zinc compounds in the environment and food [91] The usual analytical procedure comprises filtration (to obtain the soluble and insoluble fractions), separation from the matrix and fractionation (e.g., by high-performance liquid chromatography (HPLC), size exclusion chromatography (SEC), gel permeation chromatography, anion/cation exchange chromatography, CZE, ultrafiltration, dialysis or gas chromatography in the case of volatile species). The isolated species are then determined by atomic absorption spectrometry, flame atomic absorption spectrometry, ICP-optical emission spectrometry, ICP-MS, MS, fluorimetry, atomic fluorescence spectrometry, UV-Vis, or electrochemical methods depending on the type of analyte [74].…”

Development of electrospun polymer nanofiber mats for removal of uranium from aqueous systems

“…Previous studies indicated that amidoximation 21,77,[108][109][110][111][112][113][114][115][116] 33-43 of PAN led to uranyl uptake that was dependent on pH, and additional gains in U extraction efficiency were achieved using high surface area to volume nanomaterials 117,118 and electrospun fibers. 21,119,120 Here, high , and spectra are similar to previous literature reports.…”