Photoredox processes in the Cr(VI)–Cr(III)–oxalate system and their environmental relevance

Irradiation of the [Cr(C₂O₄)₃]³⁻ complex or the chromate(VI)–oxalate mixture, or the ternary system composed of Cr(III), Cr(VI) and oxalate, leads to chromium photoreductions in consequence of the ligand to metal charge transfer (LMCT) excitations induced by artificial solar radiation. In the case of the Cr(III) complex, the photoreduction involves the innersphere electron transfer leading to the formation of the Cr(II) species and the C₂O₄⁻ radicals. On reacting with molecular oxygen, Cr(II) is oxidised to Cr(III) catalysing thereby the oxalate substitution reaction. Moreover, under specific conditions, Cr(II) can be also oxidised to Cr(VI). Chromate(VI) is not photoreducible, but in the presence of oxalate, or other sacrificial electron donor, the outersphere photoinduced electron transfer (PET) produces Cr(V) species and the C₂O₄⁻ radicals. This initiates a series of thermal reactions leading to the formation of Cr(III) and oxidized oxalate (CO₂). In the system composed of [Cr(C₂O₄)₃]³⁻ and chromate(VI), the acidic medium and anoxic conditions favour the Cr(VI) photoreduction, whereas alkaline oxygenated solutions assist the Cr(VI) photoproduction. When an approximately neutral solution equilibrated with the ambient air is irradiated intermittently, Cr(VI) is consumed and/or produced, accordingly to the time sequence of exposure and dark periods. The oscillations of Cr(VI) concentrations are accompanied by continuous oxidation of oxalate, playing the role of the sacrificial electron donor. The effects of solution pH, molecular oxygen, concentrations of reagents and cations on the reaction rates were investigated. The results of this paper revealed that the Cr(III)/Cr(VI) system under environmental conditions behaves as the photocatalytic one catalysing the oxidation of oxalate or other organic matter by molecular oxygen, contributing thereby to the abatement of pollution.     

Poly(sodium 4-styrenesulfonate)–metal ion interactions

The metal ion-binding properties of poly(sodium 4-styrenesulfonate) in conjunction with membrane filtration were investigated for Cu(II), Cd(II), Co(II), Cr(III), Hg(II), Ni(II), Pb(II), Zn(II), and Fe(II). Different experiments were carried out at different pH's, metal ion concentrations, polymer concentrations, and molecular weight fractions. Only Fe(II) and Cr(III) were retained at pH 1, which allows a selective separation of these metals from all the other metal ions. At pH 3 the retention ability of this polymer increased for all the metal ions. On the other hand, the metal ion-retention properties are dependent on the polymer/metal ratio. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 219–225, 1998     

Uranium (VI) Complexes of Some Tridentate Schiff Bases

Protonation constants (log K^H₁ and log K^H₂) of N-acetylacetoneanthranilic acid (abbr. H₂ AA), N-salicylideneanthranilic acid (abbr. H₂SA) and N, (2-hydroxy-1-naphthalidene) anthranilic acid (abbr. H₂ NA), and formation constants of their uranium (VI) complexes have been determined potentiometrically in 50% dioxan (vol/vol) solutions (μ = 0.1 M NaClO₄) at 30 ± 0.1°C. The stabilities of these complexes were found to be in the order: UO₂⁺²-H₂NA > UO₂⁺²-H₂SA > UO₂⁺²-H₂AA.     

Extraction Behavior of Uranium(VI) with Polyurethane Foam

Abstract

The extraction of uranium(VI) from aqueous solution with polyether-based polyurethane (PU) foam was studied. The effects of the kinds and concentrations of nitrate salts, uranium(VI) concentration, temperature, nitric acid concentration, pH, the content of poly(ethylene oxide) in the polyurethane foam, and the ratio of PU foam weight and solution volume on the extraction of uranium(VI) were investigated. The interferences of fluoride and carbonate ions on the extraction of uranium(VI) were also examined, and methods to overcome both interferences were suggested. It was found that no uranium was extracted in the absence of a nitrate salting-out agent, and the extraction behaviors of uranium(VI) with polyurethane foam could be explained in terms of an etherlike solvent extraction mechanism. In addition, the percentage extraction of a multiple stage was also estimated theoretically.     

Uranium(VI) and Ruthenium Extraction by Dialkyldithio-Phosphoric Acids

Abstract

Oxygen donors like dialkylphosphoric acids are good extractants for actinide ions, but little is known about their sulfur homologs. In this paper investigations of U(VI) and Ru extraction from various aqueous media are reported. This includes extraction of U(VI) from nitric, perchloric, and phosphoric acids by solutions of dialkyldithiophosphoric acids in dodecane or benzene. Extraction of U(VI) by synergistic mixtures, of which at least one of the components is a sulfur donor, has been investigated. The extracted species have been identified, and a comparison with the complexes obtained by extraction with the homologous oxygen donors is made. The sulful-actinide bond is weaker than the oxygen-actinide one, but in some synergistic extractions the dialkyldithlophosphonates are more efficient than the oxygen donors. In addition to size effects, this behavior could be attributed to the weakness of the hydrogen bonds of the SH groups, which allows a greater variety of the ligands to enter the coordination sphere of the metal. Ruthenium, like the d-transitiori elements, gives strong bonds with the sulfur donors. However, its extraction from nitric acid is slow. We investigated the influence of several parameters on the distribution coefficients and found that the presence of a reagent which destroys nitrous ions is necessary to achieve quantitative extraction. The role of RuNO groups is also discussed.     

A study of Cu(I)‐ethylene complexation for olefin–paraffin separation

The current cryogenic distillation technology used for olefin–paraffin separation incurs extensive capital and operating costs. An alternative olefin–paraffin separation process, based on reactive absorption, could yield significant cost reductions. The research efforts described herein explored the structural characteristics of an NMP‐CuCl‐aniline absorption solution with ethylene to aid future development of olefin–paraffin separation systems. Solution IR and ¹H NMR spectroscopy suggested weak and labile Cu(I)‐ethylene and Cu(I)‐aniline coordination, which point to the coexistence of multiple structures in solution. Experiments also revealed solvent‐dependent and temperature‐dependent coordination. The agreement of the collected spectral data with literature implied single ethylene coordination, whereas the Cl^? ion likely remained coordinated with Cu(I). Solvent interference prohibited detailed investigation of IR spectra, but ¹H NMR spectroscopy showed more promise as an analytical technique for the NMP‐CuCl‐aniline‐ethylene system. Finally, a tradeoff appears to exist between ethylene capacity and complex stability, and thus, an optimal ligand must be found that balances these two competing needs. © 2010 American Institute of Chemical Engineers AIChE J, 2011