Nanoscale calcium uranyl carbonate clusters in water

Abstract

Aqueous calcium uranyl carbonate complexes are the predominant form of U(VI) in neutral to alkaline water in the environment, including seawater. We report a robust and repeatable synthesis of a nanoscale [(UO2)24O4(OH)12(CO3)36]44− cage cluster that we co-crystallized with Ca and Na countercations and determine its structure using single-crystal X-ray diffraction. Crystals were harvested and dissolved in water, followed by collection of electrospray ionization-mass spectra (ESI-MS). Introduction of a pH 8.1 solution containing 1.19 ± 0.04 mmol·L−1 U into the spectrometer produced broad m/z peak envelopes with −7 and 6 charge states at 1375, 1430, 1530, 1600, and 1675 m/z corresponding to mass 9625, 10,010, 10,710, 9600 and 10,080 amu, respectively. The spectra are consistent with [(UO2)24O4(OH)12(CO3)36]44− in solution (8909.07 amu), together with charge-balancing cations and H2O. Intentional fragmentation of the clusters yielded uranyl hydroxide and uranyl carbonate species containing Na. The [(UO2)24O4(OH)12(CO3)36]44− cluster is similar to that found in the mineral ewingite, which has been found to form in mine waters of an abandoned uranium mine. Demonstration of persistence of a nanoscale Ca uranyl carbonate cage cluster in aqueous solution reveals the need to re-examine aspects of environmental metal speciation to evaluate the extent to which nanoscale clusters may be important. SynopsisThis study demonstrates the existence of a nanoscale aqueous uranyl carbonate macroion at environmentally relevant pH conditions using mass spectrometry.