The ionization of uranyl triperoxide monomer, [(UO 2 )(O 2 ) 3 ] 4− (UT), and uranyl peroxide cage cluster, [(UO 2 ) 28 (O 2 ) 42 − x (OH) 2x ] 28− (U 28 ), was studied with electrospray ionization mass spectrometry (ESI-MS). Experiments including tandem mass spectrometry with collision-induced dissociation (MS/CID/ MS), use of natural water and D 2 O as solvent, and use of N 2 and SF 6 as nebulizer gases, provide insight into the mechanisms of ionization. The U 28 nanocluster under MS/CID/MS with collision energies ranging from 0 to 25 eV produced the monomeric units UO x − (x = 3−8) and UO x H y − (x = 4−8, y = 1, 2). UT under ESI conditions yielded the gas-phase ions UO x − (x = 4−6) and UO x H y − (x = 4−8, y = 1−3). Mechanisms that produce the observed anions in the UT and U 28 systems are: (a) gas-phase combinations of uranyl monomers in the collision cell upon fragmentation of U 28 , (b) reduction−oxidation resulting from the electrospray process, and (c) ionization of surrounding analytes, creating reactive oxygen species that then coordinate to uranyl ions. The electronic structures of anions UO x − (x = 6−8) were investigated using density functional theory (DFT).

Two uranyl vanadate heteropolyoxometalates (h-POMs) have been synthesized by ionothermal methods using the ionic liquid 1ethyl-3-methylimidazolium diethyl phosphate (EMIm-Et 2 PO 4 ). The hybrid actinide−transition metal shell structures have cores of (UO 2 ) 8 (V 6 O 22 ) and (UO 2 ) 6 (V 3 O 12 ), which we designate as {U 8 V 6 } and {U 6 V 3 }, respectively. The diethyl phosphate anions of the ionic liquids in some cases terminate the core structures to form actinyl oxide clusters, and in other cases the diethyl phosphate oxyanions link these cluster cores into extended structures. Three compounds exist for the {U 8 V 6 } cluster core: {U 8 V 6 }monomer, {U 8 V 6 }-dimer, and {U 8 V 6 }-chain. Tungsten atoms can partially substitute for vanadium in the {U 6 V 3 } cluster, which results in a chainbased structure designated as {U 6 V 3 }-W. Each of these compounds contains charge-balancing EMIm cations from the ionic liquid. These compounds were characterized crystallographically, spectroscopically, and by mass spectrometry.

We investigated the aqueous solubility and thermodynamic properties of two meta-autunite group uranyl arsenate solids (UAs). The measured solubility products (log K sp ) obtained in dissolution and precipitation experiments at equilibrium pH 2 and 3 for NaUAs and KUAs ranged from −23.50 to −22.96 and −23.87 to −23.38, respectively. The secondary phases (UO 2 )(H 2 AsO 4 ) 2 (H 2 O) (s) and trogerite, (UO 2 ) 3 (AsO 4 ) 2 • 12H 2 O (s) , were identified by powder X-ray diffraction in the reacted solids of KUA precipitation experiments (pH 2) and NaUAs dissolution and precipitation experiments (pH 3), respectively. The identification of these secondary phases in reacted solids suggest that H 3 O + co-occurring with Na or K in the interlayer region can influence the solubilities of uranyl arsenate solids. The standard-state enthalpy of formation from the elements (ΔH f-el ) of NaUAs is −3025 ± 22 kJ mol −1 and for KUAs is −3000 ± 28 kJ mol −1 derived from measurements by drop solution calorimetry, consistent with values reported in other studies for uranyl phosphate solids. This work provides novel thermodynamic information for reactive transport models to interpret and predict the influence of uranyl arsenate solids on soluble concentrations of U and As in contaminated waters affected by mining legacy and other anthropogenic activities.

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Electrospray ionization tandem mass spectrometry with collision-induced dissociation (ESI-MS/MS) was utilized to study the gas phase fragmentation of uranyl peroxide nanoclusters with hydroxo, peroxo, oxalate, and pyrophosphate bridging ligands. These nanoclusters fragment into uranium monomers and dimers with mass-to-charge (m/z) ratios in the 280-380 region. The gas phase fragmentation of each cluster studied yields a distinct UO 6À anion attributed to the cleavage of a uranyl ion bound to 2 peroxide groups, along with other anions that can be attributed to the initial composition of the nanoclusters.