Uranium mobility in organic matter-rich sediments: A review of geological and geochemical processes

Uranium (U) is of enormous global importance because of its use in energy generation, albeit with potential environmental legacies. While naturally occurring U is widespread in the Earth's crust at concentrations of ~1 to 3 ppm, higher concentrations can be found, includingwithin organicmatter (OM)-rich sediments, leading to economic extraction opportunities. The primary determinants of U behaviour in ore systems are pH, Eh, U oxidation state (U(IV), U(VI)) and the abundance of CO3 2– ions. The concentration/availability and interrelationships among such determinants vary, and the solubility and mobility of ions (e.g. OH-, CO3 2–, PO4 3-, SiO4 4-, SO4 2-) that compete for U (primarily as U(VI)) will also influence the mobility of U. In addition, the presence of OM can influence U mobility and fate by the degree of OMsorption to mineral surfaces (e.g. Fe- and Si- oxides and hydroxides). Within solid-phase OM, microbes can influence U oxidation state and U stability through direct enzymatic reduction, biosorption, biomineralisation and bioaccumulation. The biogenic UO2 product is, however, reported to be readily susceptible to reoxidation and therefore more likely remobilised over longer time periods. Thus several areas of uncertainty remain with respect to factors contributing to U accumulation, stability and/or (re)mobilisation. To address these uncertainties, this paper reviews U dynamics at both geological and molecular scales. Here we identify U-OMbond values that are in agreement, relatively strong, independent from ionic strength and which may facilitate either U mobilisation or immobilisation, depending on environmental conditions. We also examine knowledge gaps in the literature, with U-OM solubility data generally lacking in comparison to data for U sorption and dissolution, and little information available on multi-component relationships, such as UOM-V (V as vanadate). Furthermore, the capability ofOMto influence the oxidation state of U at near surface conditions remains unclear, as it can be postulated that electron shuttling by OM may contribute to changes in U redox state otherwise mediated by bacteria. Geochemical modelling of the environmental mobility of U will require incorporation of data from multi-corporation studies, as well as from studies of U-OM microbial interactions, all of which are considered in this review

Environmental geochemical research for the levels and the sources of toxic metals in the agricultural soils of Dimitra-Eleftherion and Platycampos region, Thessaly, Greece

The contents of eleven elements (Cu, Pb, Zn, Ni, Co, Mn, Fe, As, V, Cr and Mg) have been assayed in the agricultural soils of Dimitra-Eleftherion and Platycampos region (Thessaly, Greece). One hundred twenty two top soil samples were collected and analysed by ICP-AES after digestion with a mixture of HClO4-HNO3-HCl-HF Mean concentration of elements (1g g(-1)) in soil samples were: Cu,43; Pb,16; Zn,78; Ni,216; Co,27; Mn,1032; Fe 4.1%; As,5; V,109; Cr,254; Mg,2.5%. The contamination of the soils was assessed on the basis of comparison with average soil composition, phytotoxic levels and GLC guidelines. All the elements analysed except Pb and As have mean concentrations above the average soil comparison. The metals Zn, Ni and Cr show mean concentrations clearly higher than the phytotoxic levels. The metals Ni, Mn, V and Cr show concentrations that according to G.L.C guidelines the Dimitra-Eleftherion and Platycampos soils are classified as slightly contaminated to contaminated. Correlation analysis (CA) and principal component analysis (PCA) were applied to the data matrix to determine which elements have a common origin. PCA revealed that the studied area was mainly influenced from four sources, namely: natural (factors 1 and 2), traffic (factor 3), agricultural sources (factor 4). Cu, Zn, Co, Mn, Fe and V are associated together in iron and manganese oxideshydroxides; so grouping of these variables in the first factor reflects the adsorption of Cu,Zn, Co and V by the Fe-Mn oxides-hydroxides, as well as their weathering products. The existence of Ni,Co,Cr and Mg in the second factor explains the weathering products of ultrabasic rocks. The third factor is dominated by Pb and is related to the traffic emissions. Finally, the fourth factor is dominated by As and is related to the use of phosphate and other fertilizers

Diagenesis of the sedimentary rocks enclosing coaly layers in Gavatha area, Lesvos Island, based on silica polymorph's transformations

A Tertiary non-marine stratigraphic sequence composed of carbonates (limestone), siliceous carbonates, coaly layers overlain by pyroclastic rocks and lavas, outcrops in the Gavatha area of northwestern Lesvos Island. Pure carbonates consist almost completely of calcite, the siliceous carbonate sediments of quartz, opal-CT and calcite, the shales of quartz, opal-CT, K-feldspar, smectite-illite and calcite, and the coaly layers of organic matter, quartz, opal-CT, feldspars and pyrite. Geochemical data indicate that smectite-illite, feldspars and associated elements (La, Zr, Y, Ba, Ce) are the products of alteration of volcanic rocks in a subtropical area. A combination of sources is suggested for the formation of silica polymorphs: (a) biogenic or non-biogenic silica (opal-A) that was originally present in the form of diatom frustules or in the form of inorganically precipitated silica; (b) transformation of opal-A to opal-CT and quartz opal-C from alteration of volcanic glass of intercalated tuffites and overlying volcanics; and (c) opal-CT deposited primarily from hydrothermal solutions. © 1996 Institute of Geochemistry, Chinese Academy of Sciences

Soil contamination by toxic metals in the cultivated region of Agia, Thessaly, Greece. Identification of sources of contamination

A soil geochemical survey was undertaken in the cultivated region of Agia in Thessaly area, Central Greece. The objectives of the study were to assess the levels of soil contamination in respect to average concentrations of toxic metals in the region, to determine the associations between the different toxic elements and their spatial distribution and to identify possible sources of contamination that can explain the spatial patterns of soil pollution in the area. One hundred seventy three soil samples were collected and analysed by ICP-AES after digestion with a mixture of HClO4-HNO 3-H2O. The study focused on eleven elements (Cu, Pb, Zn, Ni, Co, Mn, As, V, Cr, Fe and Mg) and all of them except Pb have mean concentrations above the average global soil composition. The elements Ni, Cr, Mn and V show concentrations that according to G.L.C guidelines the Agia soils are classified as slightly contaminated to contaminated. Factor analysis explained 84.02% of the total variance of the data through four factors. Combined with spatial interpretation of its output, the method successfully grouped the elements according to their sources and provided evidence about their natural or anthropogenic origin. © Springer-Verlag 2005

Oligocene swamp sediments of Lesvos Island, Greece (geochemistry and mineralogy)

The Tertiary Lapsarna swamp sedimentary rocks include thin horizons (organic-rich sediments, up to 22% Corg). Analyses of major elements of carbonates, shaly and siliceous rocks, and a thin, intensively silicified coaly horizon were taken (10 representative samples). Pure carbonates consistalmost completely of calcite, the siliceous sediments of quartz and calcite, and the shales of montmorillonite, feldspar, quartz, dolomite and calcite. The shales contain montmorillonite as the only clay component. The presence of montmorillonite in all shales suggests that the chemistry of the clay-forming solutions lies within the stability field of montmorillonite. Geochemical data indicate that montmorillonite and associated immobile elements (La, Zr, Y, Nb) are the products of hydrolysis of volcanic material in a subtropical region. The intense silification of the coaly horizon is due to a contact with silica-rich solutions that formed during the formation of montmorillonite. Quartz is produced diagenetically by transformation of amorphous opal-A after burial. © 1992 Institut für Paläontologie, Universität Erlangen

Environmental impact of heavy metals (Fe, Ni, Cr, Co) in soils waters and plants of triada in Euboea from ultrabasic rocks and nickeliferous mineralisation

Soil, water and vegetation samples were collected from the Triada area of Central Euboea and analysed for heavy metals in order to evaluate their environmental impact. The geology of the are studied includes ultrabasic rocks that are overlaid by Upper Cretaceous limestones whereas Fe-Ni mineralisation is intercalated between either the ultrabasic parent rocks or the karstified Jurassic/Triassic carbonates and the transgressive Upper Cretaceous limestones. All the samples were analysed for heavy metals by using atomic absorption spectroscopy. The heavy metal ranges (in μg g-1) for soils samples are: Ni 480-4000, Cr 240-2720, Co 40-208, Fe 24,000-380,000, Mn 46-1680, Pb 16-56, Zn 40-144, Cu 2-82. The values of soil samples of the Triada area are much higher than the values found for Ni, Cr, Co and Fe, in normal soils of the world. The heavy metal ranges (in μg L-1) for water samples are: Ni 19-24, Cr 19-476, Co <5, Fe <100, Mn <100, Mg 5.7-220.5, As 30-69, Cd <2, Pd <10, Zn 5-11, Cu 2-7. The water samples of the Triada area have Cr and Mg concentrations higher than the permittable values. The heavy metal ranges (in μg g-1) for vegetation samples are: Ni 1-135, Cr 0-24, Co 1-21.5, Fe 20-680, Mn 10-206, Cd 0-10, Pb 0-14, Zn 14-70, Cu 0-10.5. The vegetation samples of the Triada area have so high values of Ni, Cr and Co that are considered toxic. The intercorrelated elements Fe, Ni, Cr, Co of the Triada soils, waters and vegetation reflect their association with the ultrabasic rocks and with the Fe-Ni mineralisation

Oligocene swamp sediments of Lesvos Island, Greece (geochemistry and mineralogy)

The Tertiary Lapsarna swamp sedimentary rocks include thin horizons (organic-rich sediments, up to 22% Corg). Analyses of major elements of carbonates, shaly and siliceous rocks, and a thin, intensively silicified coaly horizon were taken (10 representative samples). Pure carbonates consistalmost completely of calcite, the siliceous sediments of quartz and calcite, and the shales of montmorillonite, feldspar, quartz, dolomite and calcite. The shales contain montmorillonite as the only clay component. The presence of montmorillonite in all shales suggests that the chemistry of the clay-forming solutions lies within the stability field of montmorillonite. Geochemical data indicate that montmorillonite and associated immobile elements (La, Zr, Y, Nb) are the products of hydrolysis of volcanic material in a subtropical region. The intense silification of the coaly horizon is due to a contact with silica-rich solutions that formed during the formation of montmorillonite. Quartz is produced diagenetically by transformation of amorphous opal-A after burial. © 1992 Institut für Paläontologie, Universität Erlangen

Heavy metal contamination of soils at old mining sites on Thasos Island, Greece

Thasos Island has a long history of metalliferous mining, the first mining activities having been initiated by the Phoenicians during the seventh century. The mineralogy of the mineralisation includes primary minerals (galena, sphalerite) and secondary oxidised minerals (smithsonite, cerussite). In the soils studied only secondary minerals were found. Clay minerals (kaolinite, illite, montmorillonite, mixed layer clays), plagioclase, calcite and dolomite are also present in the soils. Contamination derived from the old mining sites results in extremely high levels of Pb, Zn, Mn, Fe, As, Sb, Ag, Cd in soils in the vicinity of the old workings. Since many of the Thasos mining sites are in, or adjacent to, areas of agricultural land, plants growing on the polluted sous have increased concentrations of heavy metals. This may well have a possible effect on livestock. © 1991 Sciences and Technology Letters