Dissolution of poorly soluble uranyl phosphate phases in the Metaautunite Subgroup under uranyl peroxide cage cluster forming conditions

Abstract

Uranyl phosphate minerals are widespread in uranium deposits and normally exhibit very low solubility in aqueous systems. Uranyl phosphates of the autunite group and metaautunite subgroup impact the mobility of uranium in the environment and have inspired groundwater remediation strategies that emphasize their low solubility. The importance of soluble uranium-bearing macro-anions, including nanoscale uranyl peroxide cage clusters, is largely unexplored relative to solubilization of normally low-solubility uranium minerals. Eight synthetic analogs of metaautunite subgroup minerals have been prepared and placed in various alkaline aqueous solutions containing hydrogen peroxide and tetraethylammonium hydroxide. Each uranyl phosphate studied has a topologically identical anionic sheet of uranyl square bipyramids and phosphate tetrahedra combined with various cations (Li+, Na+, K +, Rb+, Cs+, Mg2+, Ca2+, Ba2+) and water in the interlayer. Uranyl peroxides formed under many of the experimental conditions examined, including solid studtite [(UO2)(O2)(H2O)2](H2O)2 and soluble uranyl peroxide cage clusters containing as many as 28 uranyl ions. Uranyl phosphate solids in contact with solutions in which uranyl peroxide cage clusters formed dissolved extensively or completely. The greatest dissolution of uranyl phosphates occurred in systems that contained cations with larger hydrated radii, Li+ and Na+. The details of the uranium speciation in solution depended on the pH and counter cations provided from the interlayers of the uranyl phosphate solids.

Molecular Mechanisms Underlying Bacterial Uranium Resistance

“…For instance, if long-term immobilization is preferential or if the removal of uranium without recovery is the main goal, uranium-phosphate precipitation could be the best option. Uranium-phosphate minerals, such as autunite or meta-autunite, have generally low aqueous solubility ( Lobeck et al, 2020 ), are stable over a wide temperature and pH range ( Dzik et al, 2017 ; Wufuer et al, 2017 ; Gudavalli et al, 2018 ) and are not prone to remobilization through reoxidation ( Williamson et al, 2014 ; Romanchuk et al, 2020 ). Abiotic remediation with Pi has been tested, but resulted rapidly in phosphate mineral precipitation not linked with uranium and clogged pore spaces that inhibited further diffusion, which was alleviated by using microbial activity to release Pi continuously from polyphosphates or phytate ( Wellman et al, 2006 ).…”

Organic Functionalization of Uranyl Peroxide Clusters to Impact Solubility

“…Cluster species were obtained with tetraethylammonium counter cations and are highly soluble in water. 18,21,22 Following a previously reported surfactant encapsulation method that afforded the transfer of UPCs from an aqueous phase to a mixture of kerosene/ hexanol, 23 we hypothesize that grafting organic ligands to UPCs may enable their dissolution in organic solvents. Organic-ligand-bearing UPCs have been obtained by employing organic phosphonate ligands.…”

“…The role of the counter cations in stabilizing different rings of uranyl ions (tetramers, pentamers, and hexamers) has been debated in the context of a templating effect that is related to the structure of the formed clusters. − Counter cations also contribute to the generally high solubility of UPCs in water, as most UPCs are charge-balanced by alkali and alkaline earth cations. ,, Whereas many transition metal POMs can be synthesized with tetraalkylammonium that renders them soluble in organic solvents, , attempts to synthesize UPCs with tetraalkylammonium have not yet been successful. Cluster species were obtained with tetraethylammonium counter cations and are highly soluble in water. ,, Following a previously reported surfactant encapsulation method that afforded the transfer of UPCs from an aqueous phase to a mixture of kerosene/hexanol, we hypothesize that grafting organic ligands to UPCs may enable their dissolution in organic solvents. Organic-ligand-bearing UPCs have been obtained by employing organic phosphonate ligands. − However, these UPCs have not been studied in organic media but were demonstrated to be readily dissolved in water.…”

Structure, Stability, and Acidity of the Uranyl Arsenate Dimer in Aqueous Solution

“…Autunite-group minerals are the most abundant in nature and are considered effective scavengers for U in arsenic-bearing mine drainage or phosphate-injection nuclear wastes. − The autunite-group minerals can be further divided into two subgroups: autunite subgroup and meta -autunite subgroup. − The structural difference between these subgroups is that each neighbor sheet is offset by [1/2, 1/2, 0] in the autunite subgroup, while there is no offset in the meta -autunite subgroup. Both subgroups have the chemical formula of A n + (UO 2 BO 4 )­(H 2 O) x , where A n + represents a mono-, di-, or trivalent cation, B stands for As/P, and x is the number of water molecules.…”