Chemical Speciation of Uranium(VI) in Marine Environments: Complexation of Calcium and Magnesium Ions with [(UO₂)(CO₃)₃]⁴⁻ and the Effect on the Extraction of Uranium from Seawater

Abstract

The interactions of Ca(2+) and Mg(2+) with [UO2 (CO3 )3 ](4-) were studied by calcium ion selective electrode potentiometry and spectrophotometry. The stability constants of ternary Ca-UO2 -CO3 and Mg-UO2 -CO3 complexes were determined with calcium ion selective electrode potentiometry and optical absorption spectrophotometry, respectively. The enthalpies of complexation for two successive complexes, [CaUO2 (CO3 )3 ](2-) and [Ca2 UO2 (CO3 )3 ](aq), were determined for the first time by microcalorimetry. The data help to revise the speciation of uranium(VI) species under seawater conditions. In contrast to the previously accepted assumption that the highly negatively charged [UO2 (CO3 )3 ](4-) is the dominant species, the revised speciation indicates that the dominant aqueous uranium(VI) species under seawater conditions is the neutral [Ca2 UO2 (CO3 )3 ](aq). The results have a significant impact on the strategies for developing efficient sorption processes to extract uranium from seawater.

XAS and TRLIF spectroscopy of uranium and neptunium in seawater

“…24 The same applies to neptunium and the expected complexes are both a carbonato complex and the pentavalent aquo cation. 24 The same applies to neptunium and the expected complexes are both a carbonato complex and the pentavalent aquo cation.…”

“…52 The results of theoretical speciation calculations of uranium(VI) and neptunium(V) at 5 × 10 −5 M in seawater taking into account the different anion and cation concentrations from Table 1 are shown in Fig. 24 The same calculations were done at 10 −5 M but are not shown here. The speciation of uranium(VI) (Fig.…”

Experimental determination of factors controlling U/Ca of aragonite precipitated from seawater: Implications for interpreting coral skeleton

“…Uranyl speciation is potentially influenced by other factors in addition to carbonate chemistry. In freshwater at low ionic strength (I = 0.1, whereas for seawater I = $0.7), UO 2þ 2 forms complexes with both Ca 2+ and CO 2À 3 (CaUO 2 ðCO 3 Þ 2À 3 and Ca 2 UO 2 ðCO 3 Þ 0 3 ) (Dong and Brooks, 2006;Endrizzi and Rao, 2014), and -at pH less than $8 -with dissolved organic matter (DOM) (Gustafsson et al, 2009). Nevertheless, several studies have found that the dominant uranyl complex in seawater at pH $8 is uranyl tricarbonate (UO 2 ðCO 3 Þ 4À…”

Complexation of Manganese with Glutarimidedioxime: Implication for Extraction Uranium from Seawater

“…Uranium is present at an extremely low concentration (3.3 μg·L −1 ) in the forms of very stable triscarbonato complex, [UO 2 (CO 3 ) 3 ] 4− 1. More recent studies suggest that this complex is further stabilized by the formation of ternary complexes with calcium and magnesium due to the overwhelmingly high concentrations of calcium, magnesium, and carbonate in seawater23. Many other cations including transition metal ions also exist in seawater, some of which are in concentrations higher than or comparable to that of uranium4.…”