Elizabeth H. Bailey

Uranium and thorium solubilities in subduction zone fluids

Uranium is enriched in depleted island arc magmas more than would be anticipated, due to the overall enrichment in large ion lithophile elements. To attempt to understand this enrichment calculations were performed to establish concentrations of uranium and thorium in fluids under the pressure and temperatures experienced in a subduction zone. The solubility and speciation of uranium(VI), uranium(IV) and thorium(IV) were determined along the pressure-temperature profile of a subduction slab, using a density model for water. The presence of hydroxide, carbonate, fluoride and phosphate ligands were investigated. Results suggest that only hydroxide and carbonate complexing are important. The uranium(VI) carbonate species has a higher solubility, by over 10 orders of magnitude, than all other hydroxide and carbonate species. Thorium(IV) solubility is extremely low at all pressures and temperatures. With increasing pressure and temperature the uranium(IV) hydroxide species become more soluble than the uranium(VI) carbonate species, when only molecular species are considered. Calculations assuming mantle wedge enrichment of a 1 km high column over 107 yr, indicate that enrichment from pure H2O fluids could be of the order of 1 part per billion (ppb) for uranium(IV) and 1000 ppb for uranium(VI). Average primitive mantle contains 18 ppb uranium and 64 ppb thorium. Fluid addition of uranium can, therefore, increase U/Th ratios into the range observed in island arcs. It therefore appears most probable that uranium is enriched by the action of H2O-rich fluids, derived from the dehydration of minerals in the subduction slab. Carbonate-rich fluids can enrich uranium over thorium but the enrichment produced in the fluids by molecular species is too small to account for the observed uranium concentrations in island arc magmas, unless ionic species or very high water/rock ratios are anticipated.

Role for lichen melanins in uranium remediation

Lichens are successful colonizers in extreme terrestrial habitats world-wide, including metalliferous environments. Their ability to accumulate metals has led to their use in monitoring radionuclide fall-out from Chernobyl and uranium uptake from dust resulting from mining. Here we report for the first time a lichen growing directly on uranium minerals and uranium being concentrated within its tissues. Our study suggests that melanin-like pigments, substances previously unreported within lichens, are involved.

Uranium Biosorption by the Lichen Trapelia involuta at a Uranium Mine

Metal localisation was investigated in the lichenised ascomycete Trapelia involuta growing on a range of uraniferous minerals including metazeunerite [Cu(UO2)2(AsO4)2·8H2O], metatorbernite [Cu(UO2)2(PO4)2·8H2O], autunite [Ca(UO2)2(PO4)2·10H2O] and uranium-enriched iron oxide and hydroxide minerals at the abandoned South Terras mine site, Cornwall, UK. Apothecia from samples collected from waste dumps at the mine have an unusually dark colour that decolorized with NaOCl, an observation which together with Fourier Transform Infrared Spectroscopy of apothecial extracts, suggested the presence of melanin-like pigments. X-ray element mapping and probe traverses across the lichen-rock interface identify the highest U, Fe, and Cu concentrations in the outer parts of melanised apothecia. Accumulation of mineral particulates and complexing with lichen acids are not considered responsible for this since element ratios in the traverses do not correspond with those of likely mineral phases and lichen metabolites are localised in different tissues. Metal biosorption by melanin-like pigments are likely to be responsible for the observed metal fixation. No detectable U or Cu was observed in control samples although Fe showed a similar localisation in some specimens. The high concentrations of mucopolysaccharides and P recorded inside apothecia (within asci containing reproductive spores and hypothecium) suggests that the formation of melanised tissues may help protect vital reproductive tissues from the toxic effects of U and other metals, since the uranyl ion complexes strongly with phosphate species.

Fractionation of lead in soil by isotopic dilution and sequential extraction

Environmental context The chemical reactivity of lead in soil is difficult to assess and depends on both soil conditions and the origins of the lead. This paper tests the combined application of lead isotopic techniques and chemical extraction against our understanding of lead fractionation in soils. Possibly against expectation, it appears that the ‘reactivity’ of lead can be high and yet there is tentative evidence that the original source of the metal affects its fractionation in soil, even after long contact times. Abstract ‘Reactivity’ or ‘lability’ of lead is difficult to measure using traditional methods. We investigated the use of isotopic dilution with 204Pb to determine metal reactivity in four soils historically contaminated with contrasting sources of Pb, including (i) petrol-derived Pb, (ii) Pb/Zn minespoil, (iii) long-term sewage sludge application and (iv) 19th century urban waste disposal; total soil Pb concentrations ranged from 217 to 13 600 mg kg–1. A post-spike equilibration period of 3 days and suspension in 5.0 × 10–4 M ethylenediaminetetraacetic acid provided reasonably robust conditions for measuring isotopically exchangeable Pb. However, in acidic organic soils a dilute Ca(NO3)2 electrolyte may be preferable to avoid mobilisation of ‘non-labile’ Pb. Results showed that the reactive pool of soil Pb can be a large proportion of the total soil lead content but varies with the original Pb source. A comparison of isotopic exchangeability with the results of a sequential extraction procedure showed that (isotopically) ‘non-labile’ Pb may be broadly equated with ‘residual’ Pb in organic soils. However, in mineral soils the ‘carbonate’ and ‘oxide-bound’ Pb fractions included non-labile forms of Pb. The individual isotopic signatures of labile and non-labile Pb pools suggested that, despite prolonged contact with soil, differences between the lability of the original contaminant and the native soil Pb may remain.

Determination of uranium and thorium in basalts and uranium in aqueous solution by inductively coupled plasma mass spectrometry

Methods are described for the simultaneous determination of uranium and thorium in basaltic rocks at concentrations from mg kg–1(ppm) to µg kg–1(ppb) levels and for uranium in solution at femtogram levels. The technique for basalts does not require preconcentration and samples are taken into solution using a conventional rapid acid digestion method. The results obtained agree, in general, within 10% of published values for eight basalt reference materials. In addition, optimum conditions with respect to acidic media and storage vessel were ascertained as 5% HNO3 and high-density polyethylene, respectively. The method is applicable to a wide range of silicate rocks and other geological materials. For the determination of uranium at sub-ng ml–1 concentrations, such as occur in natural waters, sample introduction by electrothermal vaporization was investigated using several matrices. Complexation of uranium with ethylenediaminetetraacetic acid (EDTA) prior to analysis proved successful at levels down to 0.5 fg ml–1. Routine determination of such low concentrations is thus feasible.

Quantification of changes in zero valent iron morphology using X-ray computed tomography.

Morphological changes within the porous architecture of laboratory scale zero valent iron (ZVI) permeable reactive barriers (PRBs), after exposure to different groundwater conditions, have been quantified experimentally for different ZVI/sand ratios (10%, 50% and 100%, W/W) with the aim of inferring porosity changes in field barriers. Column studies were conducted to simulate interaction with different water chemistries, a synthetic groundwater, acidic drainage and deionised (DI) water as control. Morphological changes, in terms of pore size and distribution, were measured using X-ray computed tomography (CT). CT image analysis revealed significant morphological changes in columns treated with different water chemistries. For example, 100% ZVI (W/W) columns had a higher frequency of small pores (0.6 mm) was observed in ZVI grains reacted with typical groundwater, resulting in a porosity of 27%, compared to 32% when exposed to DI water. In comparison, ZVI grains treated with the acidic drainage had higher porosity (44%) and larger average pore size (2.8 mm). 10% ZVI PRB barrier material had the highest mean porosity (56%) after exposure to any water chemistry whilst 100% ZVI (W/W) columns always had the lowest (34%) with the 50% ZVI (W/W) in between (40%). These results agree with previously published PRB field data and simultaneously conducted geochemical monitoring and mass balance calculation, indicating that both the geochemical and hydraulic environment of the PRB play an important role in determining barrier lifespan. This study suggests that X-ray CT image analysis is a powerful tool for studying the detailed inter pores between ZVI grains within PRBs.

Time-dependent surface reactivity of Cd sorbed on calcite, hydroxylapatite and humic acid

Abstract Changes in the lability and local environment of Cd sorbed onto calcite, hydroxylapatite and humic acid have been investigated as a function of time (30 min - 9 months) using a combination of X-ray adsorption spectroscopy (XAS) and isotopic dilution techniques with Cd concentrations between 1 and 1000 mmol kg-1. Enhanced X-ray absorption fine structure (EXAFS) shows that precipitation of Cd carbonate was initially observed on calcite. Ageing of a 10 mmol kg-1 sample showed replacement of Cd in the fourth shell with Ca, indicating diffusion between the two phases had occurred. This diffusion appeared to be ongoing for a 10 mmol kg-1 sample even after 9 months of reaction. Only 20% of the Cd in a sample with 10 mmol kg-1 Cd remained labile after 6 months of reaction, suggesting a high degree of incorporation into the solid phase, in agreement with the XAS results. The XAS results for hydroxylapatite indicated that Cd is not fixed in the mineral lattice and Cd lability in a sample loaded with 10 mmol kg-1 Cd was ~55% after 6 months of reaction. This indicates that a relatively high proportion of the Cd sorbed to hydroxylapatite remained as kinetically active surface species. Little change was observed in the local environment of Cd adsorbed on flocculated Ca-humate samples as a function of time and ~75% of the Cd remained radio-labile after 6 months of reaction regardless of initial Cd loading. High lability is expected because incorporation into a solid matrix or surface precipitation of a Cd phase is less likely than with the mineral phases.

Kinetic study of time-dependent fixation of UVI on biochar

Biochar, a by-product from the production of biofuel and syngas by gasification, was tested as a material for adsorption and fixation of UVI from aqueous solutions. A batch experiment was conducted to study the factors that influence the adsorption and time-dependent fixation on biochar at 20°C, including pH, initial concentration of UVI and contact time. Uranium (UVI) adsorption was highly dependent on pH but adsorption on biochar was high over a wide range of pH values, from 4.5 to 9.0, and adsorption strength was time-dependent over several days. The experimental data for pH>7 were most effectively modelled using a Freundlich adsorption isotherm coupled to a reversible first order kinetic equation to describe the time-dependent fixation of UVI within the biochar structure. Desorption experiments showed that UVI was only sparingly desorbable from the biochar with time and isotopic dilution with 233UVI confirmed the low, or time-dependent, lability of adsorbed 238UVI. Below pH 7 the adsorption isotherm trend suggested precipitation, rather than true adsorption, may occur. However, across all pH values (4.5-9) measured saturation indices suggested precipitation was possible: autunite below pH 6.5 and either swartzite, liebigite or bayleyite above pH 6.5.

Lability of Pb in soil: effects of soil properties and contaminant source

Environmental Context: There is growing concern that lead (Pb) in the environment may cause adverse health effects in human populations. We investigated the combined use of isotopic abundance and isotopic dilution to show how the origins of soil Pb and soil characteristics affect lability. Soil pH and soil Pb content are the dominant controls on Pb lability; the lability of recent petrol-derived Pb is similar to that of other sources in urban soils but greater than geogenic Pb in rural roadside topsoils. Lability of lead (Pb) in soils is influenced by both soil properties and source(s) of contamination. We investigated factors controlling Pb lability in soils from (i) land adjacent to a major rural road, (ii) a sewage processing farm and (iii) an archive of the geochemical survey of London. We measured isotopically exchangeable Pb (E-values; PbE), phase fractionation of Pb by a sequential extraction procedure (SEP) and inferred source apportionment from measured Pb isotopic ratios. Isotopic ratios (206Pb/207Pb and 208Pb/207Pb) of total soil Pb fell on a mixing line between those of petrol and UK coal or Pb ore. The main determinant of the isotopically exchangeable Pb fraction (%E-value) was soil pH: %E-values decreased with increasing pH. In rural roadside topsoils and there was also evidence that, petrol-derived Pb remained more labile (35%) than Pb from soil parent material (27%). However, in biosolid-amended and London soils %E-values were low (c. 25%), covered a restricted range, and showed no clear evidence of source-dependent lability.

Selenium in commercial beer and losses in the brewing process from wheat to beer

There is increasing interest in enhancing the micronutrient composition of cereals through fertilization. The aims of this study were (1) to determine the Se concentration of commercial beers retailing in the UK, and (2) to test if the transfer of Se, from biofortified grain to final beer product, is <10% under UK cultivation conditions, as seen previously under Mediterranean conditions. The Se concentration of 128 commercial beers was measured, using inductively coupled plasma-mass spectrometry (ICP-MS). The selenium content of commercial beers varied 6.5-fold, with beers originating from America having higher Se concentrations than those from Europe. Laboratory-scale brewing trials with isotopically-enriched (77)Se wheat, sampled from UK field-sites, showed that most (77)Se losses in the brewing process occurred during mashing (54%), with fermented beer containing ∼ 10% of the (77)Se initially present in the wheat grain. Total N values in wort and malt were positively correlated with the (77)Se content of the wheat grain.