Sources and toxicity of hexavalent chromium

Chromium exists in oxidation states ranging from ?IV to +VI, inclusively. The compounds exhibit a wide range of geometries including square planar, tetrahedral, octahedral, and various distorted geometries. Ore refining, chemical and refractory processing, cement-producing plants, automobile brake lining, catalytic converters for automobiles, leather tanneries, and chrome pigments contribute to the atmospheric burden of chromium. Hexavalent chromium is known to have 100-fold more toxicity than trivalent chromium, for both acute and chronic exposures because of its high water solubility and mobility, as well as easy reduction. The respiratory tract is the major target organ for hexavalent chromium following the inhalation exposure in humans. Chronic inhalation exposure to hexavalent chromium results in effects on the respiratory tract, with perforations and ulcerations of the septum, bronchitis, decreased pulmonary function, pneumonia, and nasal itching and soreness as reported. Chronic human exposure to high levels of hexavalent chromium by inhalation or oral exposure may produce effects on the liver, kidney, gastrointestinal, and immune systems, and possibly the blood. Dermal exposure to hexavalent chromium may cause contact dermatitis, sensitivity, and ulceration of the skin.     

The determination of trace metals and fluoride in mineralogical and biological samples from the marine environment

Résumé Par l'analyse par activation, on recherche des traces de métaux et de fluor dans la poussière atmosphérique récoltée au-dessus des différents océans. On a utilisé l'activation neutronique pour l'analyse des métaux, l'activation aux photons pour évaluer la teneur en fluor. On discute de fa?on détaillée l'importance des résultats obtenus pour Hg et F en fonction du milieu. L'étude du rendement d'extraction du mercure a été faite au laboratoire au moyen d'algues marines, phaeodactylum tricornutum. Au cours d'expériences préliminaires faites en utilisant le radiotraceur (¹⁹⁷Hg), on a évalué différents paramètres expérimentaux. On compare les résultats finaux pour les concentrations de mercure dans les algues avec ceux obtenus par une méthode d'analyse par absorption atomique.      

Pharmaceuticals and their metabolites in the marine environment: Sources,analytical methods and occurrence

The occurrence of pharmaceuticals in the environmental waters is a global concern. There is little research conducted on the monitoring of pharmaceuticals in the marine environment. In this article, the occurrence of pharmaceuticals and their metabolites in the coastal waters as well as associated risks related to their uptake by marine organisms are critically reviewed. The literature showed antibiotics as the most plentiful pharmaceuticals in the marine environment. Other therapeutic classes of pharmaceuticals appeared prominently in the marine environment are non-steroidal anti-inflammatory drugs and β-blockers, while gemfibrozil and carbamazepine were singled-out as the most studied lipid regulator and antiepileptic, respectively. Some pharmaceuticals have been found present in the marine organisms that are regarded as important food sources for humans. We reviewed the negative effects associated with the presence of pharmaceuticals in the marine environment. This article is concluded by deliberating on the possible future studies in this research niche area.     

Microplastics in Mediterranean coastal area: toxicity and impact for the environment and human health

The so-called marine litter, and in particular microplastics (MPs) and nanoplastics (NPs), are ubiquitously distributed and recognised as an emerging risk for the environment and human health. It is known that marine environments are one of the most impacted areas and among them; coastal zones are the most contaminated ones. They are subjected to population pressure, tourism, harbours, desalination plants, marine traffic and fish farms.This review is focused on the Mediterranean Sea, currently considered one hot spot of microplastics pollution in the world, as a consequence of the high number of plastic marine litter generating activities and its characteristic morphology of semi-enclosed sea. MPs and NPs have been detected not only in surface water and water columns but also in sediments, deep seafloor, and biota including fish and seafood for human consumption. Because of this, different European legislation initiatives have been launched during the last years in order to prevent MPs and NPs contamination and to face derived problems. Finally, this review summarises the main problems and shortcomings associated to MPs and NPs analyses such as their identification and quantification or the necessity of standardised protocols.     

Local structural analyses on molten terbium fluoride in lithium fluoride and lithium-calcium fluoride mixtures

X-ray absorption fine structure (XAFS) measurements on terbium fluoride in molten lithium fluoride and in molten lithium-calcium fluoride mixtures, (e.g. 0.20TbF₃-0.80LiF, 0.20TbF₃-0.62LiF-0.18CaF₂, 0.20TbF₃-0.48LiF-0.32CaF₂, 0.50TbF₃-0.50LiF, and 0.50TbF₃-0.38LiF-0.12CaF₂), have been carried out. In the solid state, coordination number of terbium (N_i) and inter ionic distances between terbium and fluorine in the first neighbor (r_i) are nearly constant in all mixtures. In 0.20TbF₃-0.80LiF, 0.20TbF₃-0.62LiF-0.18CaF₂ and 0.50TbF₃-0.50LiF mixtures, N_i's decrease from ca. 8 to 6 and r_i's also decrease from ca. 2.29 to 2.26 Å on melting. On the other hands, in molten 0.20TbF₃-0.48LiF-0.32CaF₂ and 0.50TbF₃-0.38LiF-0.12CaF₂ mixtures, N_i's are slightly larger than 6 and r_i's do not change. These facts correspond to the amount of F⁻ supplied by solvent melts, i.e. the effect of CaF₂ becomes predominant at bCaF₂ > 0.32 in ternary 0.20TbF₃-aLiF-bCaF₂ mixtures and at bCaF₂ > 0.12 in ternary 0.50TbF₃-aLiF-bCaF₂ mixtures.     

Thiophosphoryl fluoride and phosphoryl fluoride as initiators for the polymerisation of tetrahydrofuran

Thiophosphoryl fluoride and phosphoryl fluoride have been found to initiate the polymerisation of tetrahydrofuran. The living polymer formed has a high molecular weight of the order of a million and the density is found to be between 0.98 - 1.02 g/cc. A cationic mechanism for the polymerisation has been proposed.     

Sodium fluoroaluminates formed in the reaction between aluminum fluoride solution and crystalline sodium fluoride

The reaction of aluminum fluoride solution with crystalline sodium fluoride was investigated. Conditions for the formation of Na₃AlF₆ (cryolite), Na₅Al₃F₁₄ (chiolite) and NaAlF₄.H₂O were established. The hitherto presumed to be unstable NaAlF₄.H₂O was isolated and its X-ray diffraction data as well as thermal behavior were determined. The possibility to convert these compounds one into the other was outlined.     

Theoretical calculation of the singlet-triplet electronic transition and photochemical reactivity in carbonyl fluoride,trifluoroacetyl fluoride and perfluoromethyl perfluoroacetate

Molecular properties following electronic excitation were studied and calculations were performed for the singlet-triplet electronic transitions of carbonyl fluoride, perfluoroacetyl fluoride and perfluoromethyl perfluoroacetate by means of the CNDO/S-CI method. Various approximations were used to evaluate the core Hamiltonian matrix elements.     

Tetramethylphosphonium fluoride: "naked" fluoride and phosphorane

Me(4)PF was investigated in the solid state, in the gas phase, and in solutions. Vibrational spectra of the solid and a single-crystal structure show an ionic tetramethylphosphonium fluoride. The compound crystallizes in the space group Pbca with a = 1016.0(1), b = 1018.0(1), c = 1205.8(4) pm, and Z = 8. The fluoride ion is nearly trigonal planar surrounded by three Me(4)P+ cations forming six H...F contacts between 218 and 240 pm. The compound is stable below 120 degrees C and sublimes in a vacuum. It possesses a phosphorane structure in the gas phase that was studied by electron diffraction and vibrational spectra, and additionally by theoretical calculations. The Me(4)PF molecule has a trigonal bipyramidal structure with one methyl group and the fluorine atom in axial positions and bond lengths of d(PC(eq)) = 182.6(4) pm, d(PC(ax)) = 188.4(8) pm, and d(PF) = 175.3(6) pm. The compound is remarkably soluble in acetonitrile, water, and alcohols, and slightly soluble in benzene, dimethyl ether, and diethyl ether. The solutions were studied by (1)H, (13)C, (19)F, and (31)P NMR spectroscopy. The hygroscopic Me(4)PF forms a tetrahydrate which crystallizes in the space group I4(1)/a with a = 1106.1(1) pm, c = 816.3(1) pm, and Z = 4. The fluoride ion in Me(4)PF.4 H(2)O is surrounded by four water molecules. These units form a three-dimensional network in which the Me(4)P+ cations are embedded without any contacts.     

Theoretical studies on the elimination of hydrogen fluoride from alkyl fluoride and its substituent effect

The reaction mechanism of the α, α and α, β elimination of hydrogen fluorides from alkyl fluorides has been studied theoretically. For fluoroethane as a reactant, the transition state (TS) optimized at the level of the 6-31G** basis set shows that the α, β elimination proceeds via a four membered-ring TS with a barrier height 64.6 kcal/mol, while the α, α elimination, via a three-membered ring TS with a 83.7 kcal/mol barrier. Four substituents, CH₃, CN, F, and NH₂, were used to investigate the substituent effect of elimination by using the 3-21G basis set. The calculated barriers show that NH₂-substituted alkyl fluorides favor both the α, α and α, β elimination and these two reactions would be expected to proceed simultaneously. © 1996 John Wiley & Sons, Inc.     

Vibrational excitation of deuterium fluoride and hydrogen fluoride by atomic and diatomic species

The vibrational excitation of HF and DF and the energy transfer efficiencies for various collision partners were investigated over the temperature and pressure ranges of 1400°K to 4100°K and 0.1 to 0.3 atm, respectively. The extent of excitation was determined as a function of time by continuously monitoring the infrared emission intensity at the center of the 1–0 vibration-rotation band of the molecule. Collisional efficiencies of HF, N₂, O₂, F, Cl, and DF in relaxing HF and of DF, HF, and N₂ in relaxing DF are reported. A comparison with relaxation data for pure HF taken at lower temperature suggests that long-range attractive forces are mechanistically of major importance in the relaxation process. The relatively high efficiency of atomic chlorine in relaxing HF, i.e., (τP)_HF–HF/(τP)_HF–C1 ≥ 5 at 3000°K is discussed in terms of our previous result for atomic fluorine, i.e., (τP)_HF–HF/(τP)_HF–F = 18.     

Mechanism of the formation of trifluoroacetyl fluoride and carbonyl fluoride in the low temperature liquid phase oxidation of hexafluoropropene with molecular oxygen

The known reactions of fluorine substituted alkoxy radicals cannot explain the mechanism of formation of low-molecular acid fluorides. The direct interaction of peroxy radicals, which agrees well with the experimental results, is proposed as an alternative mechanism.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 3, pp. 211–215, May-June, 1992.     

The potentiometric titration of fluoride without fluoride ion-selective electrodes

Graphite and platinum sensors were investigated as indicators in the potentiometric titration of fluoride vs lanthanum(III) and thorium(IV). In every case a partially nonaqueous medium yielded larger breaks than in aqueous solution, similar to the fluoride electrode. All the sensors yielded endpoint breaks which were smaller than those obtained with a fluoride ion-selective electrode. The largest breaks were obtained with the vitreous carbon, pyrolytic graphite, and platinum sensors. Conditioning in neutral permanganate solution significantly enhanced the breaks for all types of graphite, except vitreous carbon. The break obtained with the platinum electrode can be enhanced by application of a polarizing current of 2.0 μA in the reducing direction.