Incorporation of organic tritium (3H) by marine organisms and sediment in the severn estuary/Bristol channel (UK).     

D McCubbin  K S Leonard  T A Bailey  J Williams  P Tossell 《Marine pollution bulletin》2001,42(10):852-863

Discharges of tritium (3H) into the Severn estuary/Bristol Channel (UK) arise from the authorized release of wastes from nuclear power plants at Hinkley Point and Berkley/Oldbury and from the Nycomed-Amersham radiochemical plant, via the sewer system, at Cardiff. The wastes from the nuclear power plants probably consist almost entirely of 3H2O, whereas those from the radiochemical plant also include uncharacterized 3H labelled organic compounds. The total 3H concentrations in demersal fish and other benthic organisms in the vicinity of the Cardiff Eastern sewer outfall are significantly elevated compared to those observed around other UK nuclear establishments. Concentrations in filtered seawater were approximately 10 Bq kg(-1) whilst levels in surface sediment, seaweed (Fucus vesiculosis) and mussels (Mytilus edulis)/flounder (Platichthys flesus) were in the order of 6 x 10(2), 2 x 10(3), and 10(5) Bq kg(-1) (dry weight), respectively. Almost all the 3H found in sediment and biota were organically bound tritium (OBT). The high concentration in these materials, relative to that in seawater, is due to the presence of bioavailable organic 3H labelled compounds in the radiochemical waste. It is suggested that bioaccumulation of 3H by benthic organisms and demersal fish occurs primarily via a pathway of physico-chemical sorption/bacterial transformation of dissolved 3H labelled organic compounds into particulate organic matter, and subsequent transfer up a web of sediment dwelling microbes and meiofauna. Variations in 3H accumulation between individual organisms have been interpreted in terms of their different feeding behaviour. Relatively low concentrations were observed in the herbivorous winkle (Littorina littorea) and the pelagic Sprat (Spratus spratus) compared with other benthic organisms and demersal fish. The elevated 3H concentrations in seafood, due to bioaccumulation of OBT, have low radiological significance even for the local critical group of seafood consumers.  相似文献   

A felsic dyke swarm formed under the sea: the Archean Hunter Mine Group,south-central Abitibi Belt,Quebec, Canada     

W Mueller  JA Donaldson 《Bulletin of Volcanology》1992,54(7):602-610

An extensive rhyolitic dyke swarm has intruded subaqueous pyroclastic deposits, iron-formations, hyaloclastite breccias and lava flows of the 2730 Ma Hunter Mine Group (HMG) in the south-central part of the Archean Abitibi belt, Quebec. The dyke swarm has a minimum width of 500 m and can be traced perpendicular to the section for 2.4 km. Based on crosscutting relationships, chilled margins, quartz content and colour, five distinct dyke generations have been established. Each dyke generation has several magmatic pulses as indicated by parallel rows of columnar joints. Absence of brecciation between parallel rows suggests extremely brief intervals between magma pulses. The central parts of most dykes display inverted V-shaped patterns of columnar-joint convergence, inferred to indicate differential cooling during the late stages of dyke propagation. The dykes commonly display delicate spherulites suggesting rapid cooling, solidification temperatures between 400 and 600°C and penecontemporaneous devitrification. Quartz-feldspar aggregates in the groundmass have locally developed microgranophyric textures. Large spherulites near the chilled margins probably formed at temperatures below 400°C. Percolation of abundant water throughout the dyke complex is suggested by ubiquitous prominent chilled dyke margins. Development of a chilled margin 500 m along one dyke suggests that water percolated at least 500 m below the water/rock interface. Because the dykes intruded subaqueous pyroclastic deposits of similar composition, dyke emplacement below the sea floor is inferred. Interstratification of pillowed flows and brecciated pillowed flows containing rhyolite fragments at the top of the 4–5-km-thick sequence indicates that the central felsic complex probably never emerged during its evolutionary history, supporting the contention that the felsic dyke complex was emplaced beneath the Archean sea floor.  相似文献   

Theoretical studies of the electronic structure of copper in tetrahedral and triangular coordination with sulfur     

John A. Tossell 《Physics and Chemistry of Minerals》1978,2(3):225-236

Molecular orbital calculations are presented for the copper-sulfur polyhedral clusters CuS ₄ ^7? , CuS ₄ ^6? , CuS ₃ ^5? and CuS ₃ ^4? , which occur in many minerals. Calculated and experimental optical and X-ray energies are found to be in good agreement. The crystal field orbitals of Cu⁺ in tetrahedrally coordinated sulfides are found to be less tightly bound than the S3p nonbonding orbitals by about 2–3 eV whereas the e and t ₂ crystal field orbitals are split by about 1 eV. The crystal field splitting of Cu²⁺ in tetrahedral coordination is about 0.7–0.8 eV while the separation of the S3p nonbonding orbitals and the partially filled t ₂ crystal field orbital is about 2 eV. In triangular coordination both the Cu⁺ and Cu²⁺ crystal field orbitals are more stable than in tetrahedral coordination, more widely split and more strongly mixed with the S3p orbitals. CuS is shown to be unstable as the mixed oxidation state compound Cu^2+III (Cu^+IV)₂S^2?(S ₂ ^2? ); rather each Cu is predicted to have a fractional oxidation state and partially-empty crystal field orbitals.  相似文献   

A reinterpretation of the electronic structures of FeAs2 and related minerals     

J. A. Tossell 《Physics and Chemistry of Minerals》1984,11(2):75-80

Several additional pieces of evidence are offered in support of the recent reinterpretation (Tossell et al. 1981a, b) of the electronic and geometric structure of lollingite, FeAs₂, and related minerals. This evidence consists of: (1) the experimental electron affinities of diatomic molecules such as P₂ and S₂, (2) an analysis of the structure and spectra of TiP₂, (3) quantum mechanical calculations on FeAs₆, CoAs₆ and As₄ polyhedra which allow prediction of the photoelectron spectra of FeAs₂ and CoAs₃, (4) an analysis of bimetal cluster and band calculations suggesting metallic behavior for a hypothetical Fe⁴⁺As ₄ ^4? species and (5) qualitative molecular orbital (MO) interpretations of the stability of As ₄ ^4? polyhedra, the interaction between adjacent As ₂ ^2? anions in FeAs₂, and the variations in <M-S-S in pyrites and in R(As-As) in diarsenides. It is suggested that previous theories of sulfide and arsenide electronic structure have put too much emphasis upon metal-metal interaction and too little upon anion-anion interaction.  相似文献   

Calculation of the NMR shielding of O in MgO at high temperature     

J. A. Tossell 《Physics and Chemistry of Minerals》1998,25(6):453-456

¹⁷O NMR studies indicate that the O in crystalline MgO becomes less shielded magnetically as the temperature is increased. Over a range of about 1300?°C the shielding decreases by about 10?ppm. Increasing the temperature in MgO also leads to a small increase in the average Mg-O bond length and a large increase in the isotropic temperature factor, corresponding to a large increase in the root mean square deviation of the Mg-O distance from its equilibrium value. Using the gauge-including-atomic-orbital (GIAO) version of coupled Hartree-Fock perturbation theory, implemented at the self-consistent-field (SCF) level using a polarized split valence basis set, we have calculated the NMR shielding at the central O of a cluster OMg₆(OH)₁₂ ^?2, O_h symmetry, for several values of the Mg-O distance within a range of 30% about our calculated equilibrium value. We have also considered some distorted geometries for this cluster. We find that the O NMR shielding decreases nonlinearly as the central Mg-O distance is decreased. For the shortest Mg-O distance studied (1.391 Å) in the symmetric geometry the calculated O NMR shielding was 229.4?ppm, while at the equilibrium Mg-O distance calculated for the cluster (1.987?Å) the calculated shielding was 317.8?ppm. If the average ¹⁷O shielding is estimated by weighting the calculated NMR shieldings at various Mg-O distances by the probability of that distance, as obtained from the experimental isotropic temperature factors, at least a part of the observed deshielding at high temperatures can be reproduced. Calculations on a C_4v symmetry OMg₆(OH)₁₂ ^?2 cluster, with the central O displaced from the centre, show even stronger deshielding. Therefore, consideration of such asymmetric motions within the central OMg₆ unit could further reduce the calculated ¹⁷O shielding.  相似文献   

Calculation of the energetics for the oxidation of Sb(III) sulfides by elemental S and polysulfides in aqueous solution     

J.A. Tossell 《Geochimica et cosmochimica acta》2003,67(18):3347-3354

Recent experimental studies have reported the existence of two new Sb sulfide species, Sb₂S₅²⁻ and Sb₂S₆²⁻, in alkaline sulfidic solutions in equilibrium with stibnite, Sb₂S₃, and orthorhombic S. These species contain Sb(V), which has also recently been identified in similar solutions using EXAFS by other researchers. This represents a significant change from the consensus a decade ago that sulfidic solutions of Sb contained only Sb(III) species. I have calculated from first principles of quantum mechanics the energetics for the oxidation of the Sb(III) sulfide dimer Sb₂S₄²⁻ to the mixed Sb(III,V) dimer Sb₂S₅²⁻ and then to the all Sb(V) dimer, Sb₂S₆²⁻. Gas-phase reaction energies have been evaluated using polarized valence double zeta effective core potential basis sets and Moller-Plesset second order treatments of electron correlation. All translational, rotational and vibrational contributions to the gas-phase reaction free energy have been calculated. Hydration energies have been obtained using the COSMO version of the self-consistent reaction field polarizable continuum method. Negative free energy changes are calculated for the oxidation of the dianion of the III,III dimer to the III,V dimer by both small polysulfides, like S₄H⁻, and elemental S, modeled as S₈. For the further oxidation of the III,V dimer to the V,V dimer the reaction free energies are calculated to be close to zero. The partially protonated Sb III,III dimer monoanion HSb₂S₄⁻ can also be oxidized, but the reaction is not so favorable as for the dianion. Comparison of the calculated aqueous deprotonation energies of H₂Sb₂S₄, H₂Sb₂S₅ and H₂Sb₂S₆ and their dianions with values calculated for various oxyacids indicates that the III,V and V,V dimers will have pK_a2 values <5, so that their dianions will be the dominant species in alkaline solutions. These results are thus consistent with the recent identification of Sb₂S₅²⁻ and Sb₂S₆²⁻ species. I have also calculated the Raman spectra of Sb₂S₅²⁻ and Sb₂S₆²⁻ to assist in their identification. The calculated vibrational frequencies of the III,V and V,V dimers are characteristically higher than those of the III,III dimer I previously studied. The III,V dimer may contribute shoulders to the Raman spectrum.  相似文献   

Calculation of the effect of deprotonation on the Si NMR shielding for the series Si(OH)4 to SiO 4 4−     

J. A. Tossell 《Physics and Chemistry of Minerals》1991,17(7):654-660

²⁹Si NMR shieldings have been calculated by ab initio coupled Hartree-Fock perturbation theory for the orthosilicate species Si(OH)₄, Si(OH)₃O^-, Si(OH)₂O ₂ ^2- , Si(OH)O ₃ ^3- and SiO ₄ ^4- using energy optimized geometries from ab initio Hartree-Fock calculations. The shielding of Si(OH)₃O^- is smaller than that of Si(OH)₄ by 2.7 ppm and the shielding trend along the Si(OH)₄ to SiO ₄ ^4- series is decidedly nonlinear. The unprotonated O in Si(OH)₃O^- is more shielded and has a much smaller electric field gradient than do the protonated oxygens. Calculated anisotropies show the largest components of the shielding to lie along or near the short Si-O bonds. Calculations employing point charges in place of H reproduce the shielding trends in T _d and S ₄ symmetry Si(OH)₄ semiquantitatively. The calculated trends in shielding with < si-o-h=" can=" also=" be=" correlated=" with=" the=" energies=" of=" the=" si3p,o2p="> bonding orbitals.  相似文献