Behaviour of the uranyl (UO₂²⁺) ion in different strongly acidic media: Characterisation of UO₂²⁺ in common acids by electronic absorption spectroscopy

Abstract

The linear uranyl (UO 2 2+ ) ion has been characterised by electronic absorption spectroscopy in the media of four highly concentrated acids: sulfuric, nitric, acetic, and hydrochloric acids. In sulfuric acid, the magnitude of splitting (cm -1 ) of the vibronic peaks in the UO 2 2+ spectra increased linearly as molar concentration of the acid increased from zero to 18 M. Such an unprecedented spectral progression has been attributed to the high concentrations of sulfuric acid reducing the extent of coordination of water molecules to the central uranium atom of UO 2 2+ . This is believed to reduce the electron density in the uranium valence shell and decrease the electrostatic repulsion between the uranium and oxygen valence shells, making the U=O bonds stronger. It results in the observed increase in splitting of the vibronic peaks. In nitric acid, a completely different spectral feature of uranyl was observed. First, as molar concentration of nitric acid increased, the vibronic structure in the UO 2 2+ spectra disappeared gradually and the absorption band became continuous at 6 M. As the acid concentration further increased, vibronic lines were resolved in the lower-energy side of the spectra. The higher-energy side remained continuous with no vibronic structure observed. The convergence limit (cm -1 ), the borderline between vibronic peaks in the lower-energy side and a continuous band in the higher-energy side, decreased linearly as the molarity of nitric acid increased from 8 M to 16 M. All this shows that nitric acid, with involvement of its conjugate nitrate anion, has made UO 2 2+ dissociative beyond the convergence limit of its excited state. In acetic acid, as the molar concentration of the acid increased from zero to 17.5 M, the UO 2 2+ spectra exhibited three-stages of progression due to a predominating coordination of a carboxyl group to UO 2 2+ ([HOAc] = 0-9 M), dissociation of UO 2 2+ at the convergence limit of its excited state ([HOAc] = 10-14 M), and formation of an undissociative excited state for UO 2 2+ ([HOAc] = 15-17.5 M). In hydrochloric acid, UO 2 2+ exhibited a comparable spectral feature to that observed in nitric and acetic acids due to a predominating coordination of the chloride anion to UO 2 2+ .

Structural Correspondence between Uranyl Chloride Complexes in Solution and Their Stability Constants

“…As illustrated in Figure the total average equatorial coordination number, which is slightly less than 5 waters in a chlorine free solution, is slightly increased in the 2.5 m Cl solutions, after which it decreases with increasing Cl concentration until it levels off at about 4.4, corresponding to 1.7 O and 2.7 Cl ligands per U. This result supports a previously published optical spectroscopy study in which the number of coordinating chloride ions was seen to saturate at concentrations above 6 M Cl – . This result rules out the simple chloride substitution mechanism modeled for this reaction, at least under the conditions of our experiments, and is inconsistent with one favored model chosen to fit EXAFS data on a similar series of solutions but consistent with another …”

f‐block MOFs: A Pathway to Heterometallic Transuranics

“…In particular, we monitored the rate of the U→Th conversion spectrophotometrically through analysis of the supernatant solution. As shown in Figure 2, the intensity of the absorption bands at λ max =424 nm, associated with the UO 2 2+ π–π* transitions, [59–61] increased over time, which is consistent with the increased concentration of UO 2 2+ as transmetallation progresses.…”

Investigation of charge-transfer absorptions in the uranyl UO22+(VI) ion and related chemical reduction of UO22+(VI) to UO2+(V) by UV–Vis and electron paramagnetic resonance spectroscopies

“…Charge-transfer absorption transition between uranyl(VI) and halides (Br À and I À ) p-p ⁄ transition [11,18]). As the molar concentration of bromide in solution increased gradually to 6.0 M (from bottom to top), the intensity of the absorption in the higher-energy side (350-400 nm) of the spectra was enhanced dramatically.…”

“…Of these oxidation states, U 4+ can readily precipitate out as U(OH) 4 in neutral or weakly basic aqueous media [10], facilitating removal of radioactive waste (uranium) from the water system. Studying chemical speciation of UO 2 2+ in both aqueous and non-aqueous solutions by electronic and molecular spectroscopies and theoretical calculations has received much attention [11][12][13][14][15][16][17][18][19]. As part of the efforts made in this area of (VI) reduction.…”