Trace Level Uranyl Complexation with Phenylphosphonic Acid in Aqueous Solution: Direct Speciation by High Resolution Mass Spectrometry

Abstract

The complexation of U(VI) by organic P-containing ligands in humic substances (HS) is an important issue of uranyl mobility in soil. We have investigated the complexation of uranyl by a model ligand for aromatic phosphorus functionalities in HS, phenylphosphonic acid, by using ultrahigh resolution electrospray ionization-mass spectrometry (ESI-MS). The high sensitivity permitted to investigate the complexation of trace level uranyl and to explore directly in the native aqueous solutions the nature of the uranyl-phenylphosphonate complexes. Positive identification of the complexes coexisting in solutions with low pH and varying ligand-to-metal ratio was achieved thanks to the high resolving power, high mass accuracy, and reliability of ion abundance of the technique. The positively charged and neutral uranyl species were detected simultaneously on negative ion mass spectra, evidencing formation of three types of U(VI)-phenylphosphonate complexes. Two complexes with a metal-to-ligand stoichiometry of 1:1 (in the monoprotonated and nonprotonated forms) existed in solutions at pH 3-5, and a 1:2 complex was additionally formed at relatively high ligand-to-metal ratio. A strategy based on the use of uranyl-phosphate solution complexes as internal standards was developed to determine from the ESI(-)MS results the stability constants of the complexes, which were calculated to be log K111 = 3.4 ± 0.2 for UO2(HPhPO3)(+), log K101 = 7.1 ± 0.1 for UO2PhPO3, and log K112 = 7.2 ± 0.2 for UO2(HPhPO3)2. The speciation model presented here suggests that organic P existing at low concentration in HS is involved significantly in binding by humic and fulvic acids of trace level uranyl in soil.

Evaluating Best Practices in Raman Spectral Analysis for Uranium Speciation and Relative Abundance in Aqueous Solutions

“…A wide range of spectroscopic techniques are employed for the identification of hexavalent uranium (U(VI)) in solution 1−4 and solid phase, 5,6 but Raman spectroscopy is increasingly being utilized for both species identification 7 and quantitative analysis. 8,9 Alpha spectrometry and mass spectrometry 10 are considered the gold standard for quantitative analysis of U 11 and provide information on isotopic signatures for nuclear forensics applications. 12−15 Pretreatment steps for both methods can be time-consuming as the typical sample preparation includes dissolution of the solid phase in acid, followed by ion exchange, 16 liquid−liquid extraction, 17 or extraction chromatography.…”

“…A wide range of spectroscopic techniques are employed for the identification of hexavalent uranium (U­(VI)) in solution − and solid phase, , but Raman spectroscopy is increasingly being utilized for both species identification and quantitative analysis. , Alpha spectrometry and mass spectrometry are considered the gold standard for quantitative analysis of U and provide information on isotopic signatures for nuclear forensics applications. − Pretreatment steps for both methods can be time-consuming as the typical sample preparation includes dissolution of the solid phase in acid, followed by ion exchange, liquid–liquid extraction, or extraction chromatography. , Optical spectroscopy methods including fluorescence, FTIR, and UV, do provide some additional chemical information; − however, the resulting spectra contain broad overlapping features from various sample components. This limitation prevents positive identification of U species using UV spectroscopy and results in species validation if species also exhibit unique fluorescence spectra .…”

Speciation of uranium‐mandelic acid complexes using electrospray ionization mass spectrometry and density functional theory

“…[24] [UO 2 ] + (9) can successively combine with MA leading to the formation of UO 2 (MA) 1 (10), UO 2 (MA) 2 (11) and UO 2 (MA) 3 (12). DFT calculations ( Supplementary Table S3, Supporting Information) predict UO 2 (MA) 3 (-52.72 kcal mol -1 ) to be more stable than UO 2 (MA) 2 (-32.20 kcal mol -1 ), as there are more MA molecules in the primary coordination shell of uranium thus satisfying the equatorial coordination number (5)(6). The higher stability of the [UO 2 (MA) 3 ] + species is similar to the observation in the negative ion mode.…”

“…[3,4] Due to the complicated and variable composition of humic substances, small molecules containing carboxylic, hydroxyl and phosphoryl (P = O, P-OH) functional groups with similar physiochemical properties to humic substances are frequently used as model compounds for humic substances. [5] Hydroxycarboxylic acid-based ligands, such as mandelic acid (MA) and α-hydroxy-isobutyric acid (HIBA), are extensively used for the separation of actinides and lanthanides in liquid chromatography and capillary electrophoresis. [6][7][8][9] Although MA and HIBA contain the same functional groups, the presence of an aromatic ring in MA is speculated to be responsible for the change in elution behavior of uranyl and thorium ions in liquid chromatography.…”

Collision‐induced dissociation of [UVIO2(ClO4)]+ revisited: Production of [UVIO2(Cl)]+ and subsequent hydrolysis to create [UVIO2(OH)]+

“…outside the influence of solvent or other condensed phase effects) in a species‐specific fashion . Since then, ESI has been used to create gas‐phase uranyl species for a range of tandem mass spectrometry (MS/MS), anion photoelectron spectroscopy, and ion mobility studies . ESI and MS/MS have also been used to study and compare the intrinsic dissociation and ion‐molecule reactions of transuranic species …”