Natural speciation of nickel at the micrometer scale in serpentine (ultramafic) topsoils using microfocused X-ray fluorescence,diffraction, and absorption

Serpentine soils and ultramafic laterites develop over ultramafic bedrock and are important geological materials from environmental, geochemical, and industrial standpoints. They have naturally elevated concentrations of trace metals, such as Ni, Cr, and Co, and also high levels of Fe and Mg. Minerals host these trace metals and influence metal mobility. Ni in particular is an important trace metal in these soils, and the objective of this research was to use microscale (µ) techniques to identify naturally occurring minerals that contain Ni and Ni correlations with other trace metals, such as Fe, Mn, and Cr. Synchrotron based µ-XRF, µ-XRD, and µ-XAS were used. Ni was often located in the octahedral layer of serpentine minerals, such as lizardite, and in other layered phyllosilicate minerals with similar octahedral structure, such as chlorite group minerals including clinochlore and chamosite. Ni was also present in goethite, hematite, magnetite, and ferrihydrite. Goethite was present with lizardite and antigorite on the micrometer scale. Lizardite integrated both Ni and Mn simultaneously in its octahedral layer. Enstatite, pargasite, chamosite, phlogopite, and forsterite incorporated various amounts of Ni and Fe over the micrometer spatial scale. Ni content increased six to seven times within the same 500 µm µ-XRD transect on chamosite and phlogopite. Data are shown down to an 8 µm spatial scale. Ni was not associated with chromite or zincochromite particles. Ni often correlated with Fe and Mn, and generally did not correlate with Cr, Zn, Ca, or K in µ-XRF maps. A split shoulder feature in the µ-XAS data at 8400 eV (3.7 Å^?1 in k-space) is highly correlated (94% of averaged LCF results) to Ni located in the octahedral sheet of layered phyllosilicate minerals, such as serpentine and chlorite-group minerals. A comparison of bulk-XAS LCF to averaged µ-XAS LCF results showed good representation of the bulk soil via the µ-XAS technique for two of the three soils. In the locations analyzed by µ-XAS, average Ni speciation was dominated by layered phyllosilicate and serpentine minerals (76%), iron oxides (18%), and manganese oxides (9%). In the locations analyzed by µ-XRD, average Ni speciation was dominated by layered phyllosilicate, serpentine, and ultramafic-related minerals (71%) and iron oxides (17%), illustrating the complementary nature of these two methods.

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New insights into mineralogy and geochemistry of allanite-bearing Mediterranean coastal sands from Northern Greece

Allanite-bearing black coastal sands of Kavala (N. Greece) were studied using a combination of single-crystal XRD, EMPA, μ-XRF, bulk ICP-MS, LA-ICP-MS, μ-XANES and γ-ray spectrometry. The sands are rich in REE (ΣREE + Y: 4010 to 10,810 mg/kg), Th (236–1205 mg/kg) and other critical metals such as Nb, Ta and Co. The main REE- and Th-hosts are allanite and titanite. The allanite unit cell parameters were calculated whereas its formula was found to be (REE_0.470Ca_1.499Th_0.031) (Ti_0.031Fe⁺³_1.089Al_1.880) (Si_2.906Al_0.094O₁₂) (OH). The μ-XANES spectra showed that LREE are present in trivalent oxidation state. Analyses of the non-magnetic sand fractions showed higher LREE (12,470 mg/kg) due to accumulation of allanite. The materials showed elevated radioactivity ranging from 885 ± 13 to 3467 ± 20 Bq/kg. The obtained results provide new insights on the provenance of the sands, the abundance of REE, Th, and other immobile elements, and offer new clues for potential exploration and exploitation.     

Microheterogeneity of element distribution and sulfur speciation in an organic surface horizon of a forested Histosol as revealed by synchrotron-based X-ray spectromicroscopy

In recent years, the relevance of physico-chemical heterogeneity patterns in soils at the micron and submicron scale for the regulation of biogeochemical processes has become increasingly evident. For an organic surface soil horizon from a forested Histosol in Germany, microspatial patterns of element distribution (sulfur, phosphorus, aluminium, silicon) and S speciation were investigated by synchrotron-based X-ray spectromicroscopy. Microspatial patterns of S, P, Al and Si contents in the organic topsoil were assessed for a sample region of 50 μm × 30 μm by spatially resolving μ-XRF. Sulfur speciation at four microsites was investigated by focused X-ray absorption near edge structure (μ-XANES) spectroscopy at the S K-edge. The results show a heterogeneous distribution of the investigated elements on the (sub)micron scale, allowing the identification of diatoms, aluminosilicate mineral particles and sulfide minerals in the organic soil matrix. Evaluation of the S K-edge μ-XANES spectra acquired at four different microsites by linear combination fitting revealed a substantial microspatial heterogeneity of S speciation, characterized by the presence of distinct enrichment zones of inorganic sulfide and zones with dominant organic disulfide S within a few micrometers distance, and coexistence of different S species (e.g. reduced inorganic and organic S compounds) at a spatial scale below the resolution of the instrument (60 nm × 60 nm; X-ray penetration depth: 30 μm).     

Arsenic attenuation in tailings at a former Cu–W–As mine,SW Finland

Nearly half a century after mine closure, release of As from the Ylöjärvi Cu–W–As mine tailings in groundwater and surface water run-off was observed. Investigations by scanning electron microscopy (SEM), electron microprobe analysis (EMPA), synchrotron-based micro-X-ray diffraction (μ-XRD), micro-X-ray absorption near edge structure (μ-XANES) and micro-extended X-ray absorption fine structure (μ-EXAFS) spectroscopy, and a sequential extraction procedure were performed to assess As attenuation mechanisms in the vadose zone of this tailings deposit. Results of SEM, EMPA, and sequential extractions indicated that the precipitation of As bearing Fe(III) (oxy)hydroxides (up to 18.4 wt.% As₂O₅) and Fe(III) arsenates were important secondary controls on As mobility. The μ-XRD, μ-XANES and μ-EXAFS analyses suggested that these phases correspond to poorly crystalline and disordered As-bearing precipitates, including arsenical ferrihydrite, scorodite, kaňkite, and hydrous ferric arsenate (HFA). The pH within 200 cm of the tailings surface averaged 5.7, conditions which favor the precipitation of ferrihydrite. Poorly crystalline Fe(III) arsenates are potentially unstable over time, and their transformation to ferrihydrite, which contributes to As uptake, has potential to increase the As adsorption capacity of the tailings. Arsenic mobility in tailings pore water at the Ylöjärvi mine will depend on continued arsenopyrite oxidation, dissolution or transformation of secondary Fe(III) arsenates, and the As adsorption capacity of Fe(III) (oxy)hydroxides within this tailings deposit.     

Identifying calcium-containing mineral species in the JEB Tailings Management Facility at McClean Lake,Saskatchewan

The JEB Tailings Management Facility (TMF) is central to reducing the environmental impact of the McClean Lake uranium mill facility that is operated by AREVA Resources Canada. This facility has been designed around the idea that elements of concern (e.g., U, As, Ni, Se, Mo) will be controlled through equilibrium with precipitants. Confirming the presence of calcium-containing carbonates in the JEB TMF is the first step in determining if gypsum (CaSO₄·2H₂O) controls the concentration of HCO₃⁻ (aq), limiting the formation of soluble uranyl bicarbonate complexes. A combination of X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), and microprobe X-ray fluorescence (XRF) mapping was used to analyze a series of tailings samples from the JEB TMF. Calcium carbonate in the form of calcite (CaCO₃), aragonite (CaCO₃), and dolomite (CaMg(CO₃)₂) were identified by analysing Ca K-edge μ-XANES spectra coupled with microprobe XRF mapping. This is the first observation of these phases in the JEB TMF. The combination of μ-XANES and XRF mapping provided a greater sensitivity to low concentration calcium species compared to the other techniques used, which were only sensitive to the major species present (e.g., gypsum).     

The role of Al in the formation of secondary Ni precipitates on pyrophyllite,gibbsite, talc,and amorphous silica: a DRS study

Formation of secondary Ni precipitates is an important mechanism of Ni retention in neutral and alkaline clay/water systems. However, the structure and composition of these secondary phases, and their stability is still disputable. Using existing structure refinement data and new ab-initio FEFF 7 calculations we show that Ni-edge X-ray absorption fine structure spectroscopy alone may not be able to unequivocally discriminate four possible candidate compounds: α-Ni(OH)₂, the isostructural but Al-substituted layered double hydroxide (Ni-Al LDH), and 1:1 and 2:1 Ni-containing phyllosilicates. Hence, we investigated the potential of diffuse reflectance spectroscopy (DRS) in determining in situ the Ni phase forming in the presence of four sorbents, pyrophyllite, talc, gibbsite, and amorphous silica. The ³A_2g → ³T_1g(F) band (ν2) of octahedrally coordinated Ni²⁺ could be reliably extracted from the reflectance spectra of wet pastes. In the presence of the Al-free talc and amorphous silica, the ν2 band was at ≈14,900 cm⁻¹, but shifted to 15,300 cm⁻¹ in the presence of Al-containing pyrophyllite and gibbsite. This shift suggests that Al is dissolved from the sorbent and substitutes for Ni in brucite-like hydroxide layers of the newly forming precipitate phase, causing a decrease of the Ni-O distances and, in turn, an increase of the crystal-field splitting energy. Comparison with Ni model compounds showed that the band at 14,900 cm⁻¹ is a unique fingerprint of α-Ni(OH)₂, and the band at 15,300 cm⁻¹ of Ni-Al LDH. Although the complete transformation of α-Ni(OH)₂ into a Ni phyllosilicate causes a significant contraction of the Ni hydroxide sheet as indicated by band positions intermediate to those of α-Ni(OH)₂ and Ni-Al LDH, incipient states of silication do not influence Ni-O distances and cannot be detected by DRS. The first evidence for the formation of a precipitate was obtained after 5 min (pyrophyllite), 7 hr (talc), 24 hr (gibbsite), and 3 days (amorphous silica). For both pyrophyllite and talc, where sufficiently long time series were available, the ν2 energy slightly increased as long as the Ni uptake from solution continued (3 days for pyrophyllite, 30 days for talc). This may be explained by a relative decrease of relaxed surface sites due to the growth of crystallites. Our study shows that the formation of both α-Ni(OH)₂ and Ni-Al LDH may effectively decrease aqueous Ni concentrations in soils and sediments. However, Ni-Al LDH seems to be thermodynamically favored when Al is available.     

Speciation and weathering of copper in “copper red ruby” medieval flashed glasses from the Tours cathedral (XIII century)

Three “copper ruby red” (or “flashed”) glasses from the St Gatien cathedral in Tours (windows from the XIII century) were investigated at the Cu K-edge by synchrotron X-ray fluorescence microscopy and μ-XANES/EXAFS spectroscopies. The spectra are compared to XANES/EXAFS spectra collected for modern glasses synthesized at various O₂ fugacities. Two main types of red glasses are present in Tours, which show a distinct Cu speciation. In the first type (“plaqués”), Cu is present as sub-micron metallic nucleates, which coexist with monovalent Cu linear moieties (30:70 at.%). In contrast, the glasses of the other type (“feuilletés”) show mostly monovalent Cu (together with some evidence for metallic Cu in the edge region of the XANES).The plaqués glasses appear to have been synthesized at lower O₂ fugacity and higher temperatures to promote homogeneous nucleation of metallic Cu. A relative enrichment in Al (provided by the addition of orthoclase in the melt) greatly enhances the glass durability by decreasing the melt peralkalinity by a factor of ∼4. In contrast, the feuilletés were equilibrated at much higher O₂ fugacities. Hence, metallic Cu, despite being scarce in that glass, is not at equilibrium and could have well been added on purpose during the melt cooling. It is proposed that a technique called ‘tourage’ could have been used to help create the micron scale greenish and red layers. The feuilleté glasses are also weathered at their surface, promoting the formation of amorphous Cu(II) species related to a Cu sulphate.     

The effect of phytostabilization on Zn speciation in a dredged contaminated sediment using scanning electron microscopy, X-ray fluorescence, EXAFS spectroscopy, and principal components analysis

The maintenance of waterways generates large amounts of dredged sediments, which are deposited on adjacent land surfaces. These sediments are often rich in metal contaminants and present a risk to the local environment. Understanding how the metals are immobilized at the molecular level is critical for formulating effective metal containment strategies such as phytoremediation. In the present work, the mineralogical transformations of Zn-containing phases induced by two graminaceous plants (Agrostis tenuis and Festuca rubra) in a contaminated sediment ([Zn] = 4700 mg kg⁻¹, [P₂O₅] = 7000 mg kg⁻¹, pH = 7.8), untreated or amended with hydroxylapatite (AP) or Thomas basic slag (TS), were investigated after two yr of pot experiment by scanning electron microscopy coupled with energy-dispersive spectrometry (SEM-EDS), synchrotron-based X-ray microfluorescence (μ-SXRF), and powder and laterally resolved extended X-ray absorption fine structure (μ-EXAFS) spectroscopy. The number and nature of Zn species were evaluated by principal component (PCA) and least-squares fitting (LSF) analysis of the entire set of μ-EXAFS spectra, which included up to 32 individual spectra from regions of interest varying in chemical composition. Seven Zn species were identified at the micrometer scale: sphalerite, gahnite, franklinite, Zn-containing ferrihydrite and phosphate, (Zn-Al)-hydrotalcite, and Zn-substituted kerolite-like trioctahedral phyllosilicate. Bulk fractions of each species were quantified by LSF of the powder EXAFS spectra to linear combinations of the identified Zn species spectra.In the untreated and unvegetated sediment, Zn was distributed as ∼50% (mole ratio of total Zn) sphalerite, ∼40% Zn-ferrihydrite, and ∼10 to 20% (Zn-Al)-hydrotalcite plus Zn-phyllosilicate. In unvegetated but amended sediments (AP and TS), ZnS and Zn-ferrihydrite each decreased by 10 to 20% and were replaced by Zn-phosphate (∼30∼40%). In the presence of plants, ZnS was almost completely dissolved, and the released Zn bound to phosphate (∼40-60%) and to Zn phyllosilicate plus (Zn,Al)-hydrotalcite (∼20-40%). Neither the plant species nor the coaddition of mineral amendment affected the Zn speciation in the vegetated sediment. The sediment pore waters were supersaturated with respect to Zn-containing trioctahedral phyllosilicate, near saturation with respect to Zn-phosphate, and strongly undersaturated with respect to (Zn,Al)-hydrotalcite. Therefore, the formation of (Zn,Al)-hydrotalcite in slightly alkaline conditions ought to result from heterogeneous precipitation on mineral surface.     

Theoretical studies of the alteration of spodumene,petalite, eucryptite and pollucite in granitic pegmatites: exchange reactions with alkali feldspars

The ratios Na/Li, K/Li, Na/Cs and K/Cs have been calculated for exchange equilibria among the Li and Cs silicates spodumene, petalite, eucryptite, and pollucite, and the alkali feldspars albite and K-feldspar plus quartz, in pure water and in chloride solutions at temperatures from 100° to 700°C and pressures from 0.5 to 4 kbar, using available thermodynamic data for minerals and the modified HKF equation of state for aqueous species. For exchange equilibria between Li-bearing aluminosilicates and the alkali feldspars, the activities of the alkali metals in solution under most of the conditions investigated follow the order Li>Na>K, and Na/Li and K/Li decrease with decreasing temperature. For exchange equilibria between pollucite and the alkali feldspars the order is Na>K>Cs in solution; Na/Cs and K/Cs increase strongly with decreasing temperature. The absolute values of these alkali metal ratios are in good agreement with the few available experimental data. The effect of chloride ion pairing on the calculated ratios is slight and does not consistently improve agreement between theory and experiment. These results suggest that the alteration of eucryptite, petalite or spodumene to albite and/or K-feldspar should be a normal consequence of the closed system evolution of rare element pegmatites upon cooling, in agreement with the ubiquity of such phenomena world-wide. On the other hand, alteration of pollucite to albite or K-feldspar upon cooling is only likely to occur if external fluids, with very high Na/Cs and/or K/Cs ratios, gain access to the pegmatite. Owing to the heterogeneity of rare element pegmatites, the fluid need not be external to the entire pegmatite, but could be simply external to the particular zone containing pollucite. Fluids in equilibrium with typical subsolidus rare metal pegmatite assemblages will invariably have high Li contents, thus explaining the common occurrence of Li-metasomatic halos about pegmatites. These same fluids are predicted to have relatively low Cs contents, in apparent agreement with the lesser role of Cs relative to Li in metasomatic halos. However, preferential formation of complexes of the alkali metals with fluoride, borate or aluminosilicate components potentially could alter the calculated alkali metal behaviors.     

Dolomite effect on borosilicate glass alteration

Dolomite (CaMg(CO₃)₂) is one of the common rock-forming minerals in many geological media, in particular in clayey layers that are currently considered as potential host formations for a deep radioactive waste disposal facility. Magnesium in solution is one of the elements known to potentially enhance the alteration of nuclear glasses. The alteration of borosilicate glasses with dolomite as a Mg-bearing mineral source was investigated for 8 months in batch tests at 90 °C. Glass composition effects were investigated through two compositions (SiBNaAlCaZrO and SiBNaAlZrO) differing in their Ca content. The Ca-rich glass alteration is slightly enhanced in the presence of dolomite compared to the alteration observed in pure water. This greater alteration is explained by the precipitation of Mg silicate phases on the dolomite and glass surfaces. In contrast, the Ca-free glass alteration decreases in the presence of dolomite compared to the alteration observed in pure water. This behavior is explained by Ca incorporation in the amorphous layer (formed during glass alteration) coming from dolomite dissolution. Calcium acts as a layer reorganizer and limits glass alteration by reducing the diffusion of reactive species through the altered layer. Modeling was performed using the GRAAL model implemented within the CHESS/HYTEC geochemical code to discriminate and interpret the mechanisms involved in glass/dolomite interactions. Magnesium released by dolomite dissolution reacts with silica provided by glass alteration to form Mg silicates. This reaction leads to a pH decrease. The main mechanism controlling glass alteration is the ability of dolomite to dissolve. During the experiment the quantities of secondary phases formed were very small, but for longer time scales, this mechanism could supply sufficient Mg in solution to form large amounts of Mg silicates and sustain glass alteration. The ability of the GRAAL model to reproduce the concentrations of elements in solution and solid phases regardless of the amount of dolomite and the glass composition strongly supports the basic modeling hypothesis.     

Cs-dominant polylithionite in the Koktokay#3 pegmatite,Altai, NW China: in situ micro-characterization and implication for the storage of radioactive cesium

Except forming two Cs-rich minerals like pollucite and londonite, cesium generally occurs in trace amounts in potassium-bearing minerals owing to its very low Clarke value and large ionic radius. However, in the Koktokay#3 pegmatite (Altai, NW China), lepidolite is extremely enriched in cesium (typically 21–26 wt% Cs₂O). Cs-enriched lepidolite is restricted to the inner of the pegmatite, where four types of occurrence are characterized by using in situ techniques (EMP: electron microprobe, micro-XRD: micro-area X-ray diffractometer and Raman probe: micro-area Raman spectrometer) as: (1) outer zones on Cs-poor lepidolite cores; (2) fine overgrowths on muscovite veinlets; (3) veinlets in Cs-poor lepidolite and (4) veinlets in elbaite. There is an inverse correlation between K and Cs and in terms of ^VIAl versus Cs/(K+Cs), the Cs-dominant micas are an analog of the Li-rich mica polylithionite. Micro-XRD patterns and micro-Raman spectra indicate that Cs-dominant polylithionite structurally corresponds to minerals of the lepidolite series. According to its distribution and compositional characteristics, the Cs-dominant polylithionite seems to have formed at the magmatic to hydrothermal transition stage of pegmatitic magma evolution as a result of marginal replacement of early-formed lepidolite by Cs-rich fluids. Alternatively, it may have formed through direct precipitation from Cs-rich fluids. The results of the present study have important implications for the storage of nuclear waste in that Li mica such as polylithionite is a good candidate for immobilizing high-level radioactive cesium waste.     

Effects of alteration on element distributions in archean tholeiites from the Barberton greenstone belt,South Africa

Major and trace element and modal analyses are presented for unaltered, epidotized, and carbonated tholeiite flows from the Barberton greenstone belt. Au, As, Sb, Sr, Fe⁺³, Ca, Br, Ga, and U are enriched and H₂O, Na, Mg, Fe⁺², K, Rb, Ba, Si, Ti, P, Ni, Cs, Zn, Nb, Cu, Zr, and Co are depleted during epidotization. CO₂, H₂O, Fe⁺², Ti, Zn, Y, Nb, Ga, Ta, and light REE are enriched and Na, Sr, Cr, Ba, Fe⁺³, Ca, Cs, Sb, Au, Mn, and U are depleted during carbonization-chloritization. The elements least affected by epidotization are Hf, Ta, Sc, Cr, Th, and REE; those least affected by carbonization-chloritization are Hf, Ni, Co, Zr, Th, and heavy REE. Both alteration processes can significantly change major element concentrations (and ratios) and hence caution should be used in distinguishing tholeiites from komatiites based on major elements alone. The amount of variation of many of the least mobile trace elements in the altered flows is approximately the same as allowed by magma model calculations. Hence, up to about 10% carbonization and 60% epidotization of tholeiite do not appreciably affect the interpretation of trace-element models for magma generation.     

Alteration of basaltic glass: Mechanisms and significance for the oceanic crust-seawater budget

Alteration of basaltic glass to palagonite is characterized by a nearly isomolar exchange of SiO₂, Al₂O₃, MnO, MgO, CaO, Na₂O, P₂O₅, Zn, Cu, Ni, Cr, Hf, Sc, Co and REE for H₂O and K₂O, whilst TiO₂ and FeO are passively accumulated during removal of the remaining cations. The network forming cations Al and Si are removed from the glass in proportion to the gain in Ti and Fe, whilst the other cations do not show a significant relationship to the amount of Ti and Fe accumulation. Sr isotopic data show that during palagonite formation approximately 85% of the basaltic Sr is lost to the hydrous solutions and 40% of seawater Sr is added to the glass, yielding an average loss of the same order of magnitude as of the network forming cations. Losses and gains of oxides yield an average increase of +105% TiO₂.K, Rb, and Cs show high increases, but $\text{K}\text{Rb}$ and $\text{K}\text{Cs}$ ratios indicate two different alteration processes: (1) formation of palagonite involves a drastic decrease in these ratios, indicating structural similarities between palagonite and smectite; (2) surface alteration of glass is characterized by an increase in $\text{K}\text{Rb}$ and $\text{K}\text{Cs}$ ratios, probably best interpreted as sorption of alkalies in ratios approximating those of seawater.The total fluxes involved in alteration of glass in the upper portion of the oceanic crust are estimated from the modal abundance of palagonite in the oceanic crust and the abundance of the vein materials smectite and carbonate. Smectite and carbonates act as a sink for a significant portion of the elements liberated up during alteration of basaltic glass except for Na and Al, which are probably taken up by zeolites and/or albite, possibly hidden in the macroscopic estimate of carbonate. Formation of the observed quantity of secondary phases requires additional sources for Si, Fe. Ca and K. K is provided in excess from the inflowing seawater at reasonable water/rock ratios. The remaining excess Ca, Si and Fe required may be derived by alteration of interstitial glass and breakdown of anorthite rich plagioclase and titano-magnetite, and/or by supply of deeper seated metamorphic reactions.     

Geological and anthropogenic factors influencing mercury speciation in mine wastes: an EXAFS spectroscopy study

The speciation of Hg is a critical determinant of its mobility, reactivity, and potential bioavailability in mine-impacted regions. Furthermore, Hg speciation in these complex natural systems is influenced by a number of physical, geological, and anthropogenic variables. In order to investigate the degree to which several of these variables may affect Hg speciation, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the Hg phases and relative proportions of these phases present in Hg-bearing wastes from selected mine-impacted regions in California and Nevada. The geological origin of Hg ore has a significant effect on Hg speciation in mine wastes. Specifically, samples collected from hot-spring Hg deposits were found to contain soluble Hg-chloride phases, while such phases were largely absent in samples from silica-carbonate Hg deposits; in both deposit types, however, Hg-sulfides in the form of cinnabar (HgS, hex.) and metacinnabar (HgS, cub.) dominate. Calcined wastes in which Hg ore was crushed and roasted in excess of 600 °C, contain high proportions of metacinnabar while the main Hg-containing phase in unroasted waste rock samples from the same mines is cinnabar. The calcining process is thought to promote the reconstructive phase transformation of cinnabar to metacinnabar, which typically occurs at 345 °C. The total Hg concentration in calcines is strongly correlated with particle size, with increases of nearly an order of magnitude in total Hg concentration between the 500–2000 μm and <45 μm size fractions (e.g., from 97–810 mg/kg Hg in calcines from the Sulphur Bank Mine, CA). The proportion of Hg-sulfides present also increased by 8–18% as particle size decreased over the same size range. This finding suggests that insoluble yet soft Hg-sulfides are subject to preferential mechanical weathering and become enriched in the fine-grained fraction, while soluble Hg phases are leached out more readily as particle size decreases. The speciation of Hg in mine wastes is similar to that in distributed sediments located downstream from the same waste piles, indicating that the transport of Hg from mine waste piles does not significantly impact Hg speciation. Hg L_III-EXAFS analysis of samples from Au mining regions, where elemental Hg(0) was introduced to aid in the Au recovery process, identified the presence of Hg-sulfides and schuetteite (Hg₃O₂SO₄), which may have formed as a result of long-term Hg(0) burial in reducing high-sulfide sediments.     

Hydrothermal alteration of basalt by seawater under seawater-dominated conditions

Fresh mid-ocean ridge basalt glass and diabase have been reacted with seawater at 150–300°C, 500 bar, and water/rock mass ratios of 50, 62, and 125, using experimental apparatus which allowed on-line sampling of solution to monitor reaction progress. These experiments characterize reaction under what we have called “seawater-dominated” conditions of hydrothermal alteration.In an experiment at 300°C, basalt glass undergoing alteration removed nearly all Mg²⁺ from an amount of seawater 50 times its own mass. In the process, the glass was converted entirely to mixed-layer smectite-chlorite, anhydrite, and minor hematite. Removal of Mg from seawater occurred as a Mg(OH)₂ component incorporated into the secondary clay. This produced a precipitous drop in solution pH early in the experiment, accompanied by a dramatic increase in the concentrations of Fe, Mn, and Zn in solution. As Mg removal neared completion and the glass was hydrolyzed, pH rose again and heavy metal concentrations dropped.At water/rock ratios of 62 and 125 and 150–300°C, the mineral assemblage produced was similar to that at a water/rock ratio of 50. Solution chemistry, however, contrasted with the earlier experiment in that Mg concentrations in solution were greater and pH lower. This caused significant leaching of heavy metals. At 300°C nearly all of the Na, Ca, Cu, Zn, and CO₂ and most of the K, Ba, Sr, and Mn were leached from the silicates. H₂S, Al, Si, and possibly Co were also significantly mobilized, whereas V, Cr, and Ni were not. Little or no seawater sulfate was reduced.Although submarine hot spring solutions sampled to date along mid-ocean ridges clearly come from rock-dominated hydrothermal systems, evidence from ocean floor metabasalts and from heat flow studies indicates that seawater-dominated conditions of alteration prevail at least locally both in axial hightemperature systems and in ridge flank systems at lower temperatures.     

Characterization of uranium-contaminated sediments from beneath a nuclear waste storage tank from Hanford, Washington: Implications for contaminant transport and fate

The concentration and distribution of uranium (U) in sediment samples from three boreholes recovered near radioactive waste storage tanks at Hanford, Washington, USA, were determined in detail using bulk and micro-analytical techniques. The source of contamination was a plume that contained an estimated 7000 kg of dissolved U that seeped into the subsurface as a result of an accident that occurred during filling of tank BX-102. The desorption character and kinetics of U were also determined by experiment in order to assess the mobility of U in the vadose zone. Most samples contained too little moisture to obtain quantitative information on pore water compositions. Concentrations of U (and contaminant phosphate—P) in pore waters were therefore estimated by performing 1:1 sediment-to-water extractions and the data indicated concentrations of these elements were above that of uncontaminated “background” sediments. Further extraction of U by 8 N nitric acid indicated that a significant fraction of the total U is relatively immobile and may be sequestered in mobilization-resistant phases. Fine- and coarse-grained samples in sharp contact with one another were sub-sampled for further scrutiny and identification of U reservoirs. Segregation of the samples into their constituent size fractions coupled with microwave-assisted digestion of bulk samples showed that most of the U contamination was sequestered within the fine-grained fraction. Isotope exchange (²³³U) tests revealed that ∼51% to 63% of the U is labile, indicating that the remaining fund of U is locked up in mobilization-resistant phases. Analysis by Micro-X-ray Fluorescence and Micro-X-ray Absorption Near-Edge Spectroscopy (μ-XRF and μ-XANES) showed that U is primarily associated with Ca and is predominately U(VI). The spectra obtained on U-enriched “hot spots” using Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLIFS) provide strong evidence for uranophane-type [Ca(UO₂)₂(SiO₃OH)₂(H₂O)₅] and uranyl phosphate [Ca(UO₂)₂(PO₄)₂(H₂O)_10-12] phases. These data show that disseminated micro-precipitates can form in narrow pore spaces within the finer-grained matrix and that these objects are likely not restricted to lithic fragment environments. Uranium mobility may therefore be curtailed by precipitation of uranyl silicate and phosphate phases, with additional possible influence exerted by capillary barriers. Consequently, equilibrium-based desorption models that predict the concentrations and mobility of U in the subsurface matrix at Hanford are unnecessarily conservative.     

Early diagenetic vivianite [Fe3(PO4)2 · 8H2O] in a contaminated freshwater sediment and insights into zinc uptake: A μ-EXAFS, μ-XANES and Raman study

The sediments in the Salford Quays, a heavily-modified urban water body, contain high levels of organic matter, Fe, Zn and nutrients as a result of past contaminant inputs. Vivianite [Fe₃(PO₄)₂ · 8H₂O] has been observed to have precipitated within these sediments during early diagenesis as a result of the release of Fe and P to porewaters. These mineral grains are small (<100 μm) and micron-scale analysis techniques (SEM, electron microprobe, μ-EXAFS, μ-XANES and Raman) have been applied in this study to obtain information upon the structure of this vivianite and the nature of Zn uptake in the mineral. Petrographic observations, and elemental, X-ray diffraction and Raman spectroscopic analysis confirms the presence of vivianite. EXAFS model fitting of the FeK-edge spectra for individual vivianite grains produces Fe–O and Fe–P co-ordination numbers and bond lengths consistent with previous structural studies of vivianite (4O atoms at 1.99–2.05 Å; 2P atoms at 3.17–3.25 Å). One analysed grain displays evidence of a significant Fe³⁺ component, which is interpreted to have resulted from oxidation during sample handling and/or analysis. EXAFS modelling of the Zn K-edge data, together with linear combination XANES fitting of model compounds, indicates that Zn may be incorporated into the crystal structure of vivianite (4O atoms at 1.97 Å; 2P atoms at 3.17 Å). Low levels of Zn sulphate or Zn-sorbed goethite are also indicated from linear combination XANES fitting and to a limited extent, the EXAFS fitting, the origin of which may either be an oxidation artifact or the inclusion of Zn sulphate into the vivianite grains during precipitation. This study confirms that early diagenetic vivianite may act as a sink for Zn, and potentially other contaminants (e.g. As) during its formation and, therefore, forms an important component of metal cycling in contaminated sediments and waters. Furthermore, for the case of Zn, the EXAFS fits for Zn phosphate suggest this uptake is structural and not via surface adsorption.     

Distribution and bioavailability of Cr in central Euboea, Greece

Plants and soils from central Euboea, were analyzed for Cr_(totai), Cr(VI), Ni, Mn, Fe and Zn. The range of metal concentrations in soils is typical to those developed on Fe-Ni laterites and ultramafic rocks. Their bioavailability was expressed in terms of concentrations extractable with EDTA and 1 M HNO₃, with EDTA having a limited effect on metal recovery. Cr(VI) concentrations in soils evaluated by alkaline digestion solution were lower than phytotoxic levels. Chromium and Ni — and occasionally Zn — in the majority of plants were near or above toxicity levels. Cr(VI) concentrations in plants were extremely low compared to total chromium concentrations. Cr_(total) in ground waters ranged from <1 μg.L^?1 to 130 μg.L^?1, with almost all chromium present as Cr(VI). With the exception of Cr_(total) and in some cases Zn, all elements were below regulatory limits for drinking water. On the basis of Ca, Mg, Cr_(total) and Si ground waters were classified into three groups: Group(I) with Cr concentrations less than 1 μg.L^?1 from a karstic aquifer; Group(II) with average concentrations of 24 μg.L^?1 of Cr and relatively high Si associated with ophiolites; and Group(III) with Cr concentrations of up to 130 μg.L^?1, likely due to anthropogenic activity. Group(III) is comparable to ground waters from Assopos basin, characterized by high Cr(VI) concentrations, probably due to industrial actrivities.     

Incorporation of chitosan and glass substrate for improvement in adsorption,separation, and stability of TiO<Subscript>2</Subscript> photodegradation

It is demonstrated that single titanium dioxide (TiO₂) has high potential for photodegradation of pollutants. However, it is still far from becoming an effective photocatalyst system, due to issues of adsorption process, separation, as well as dissolution. Therefore, this study highlights the high adsorption capacity, simplified separation, and the promising stability of TiO_2(SY) (synthesized via sol–gel method) photocatalyst, fabricated using chitosan–TiO_2(SY) and supported by glass substrate (Cs–TiO_2(SY)/glass substrate) photocatalysts. Chitosan (Cs), with abundant –R–NH and NH₂ groups, promotes the adsorption sites of methyl orange (MO) and OH groups for major attachment to TiO_2(SY). Meanwhile, the glass substrate increases stability and assists separation of the photocatalysts. Initially, nano-TiO_2(SY) has been characterized using high-resolution transmission electron microscope. Cs–TiO_2(SY)/glass substrate was fabricated via dip-coating. The distribution and interface between the photocatalytic components were characterized by Fourier transform infrared absorption spectroscopy, UV–Vis diffuse reflectance spectroscopy, field emission scanning electron microscopy, and energy-dispersive spectrometer. UV–Vis analysis of the multilayer photocatalyst (2, 4, 6, and 8 layers) was further carried out by the adsorption–photodegradation, with MO as model of pollutant. Seventy percent of the total removal of MO via optimized eight layers of photocatalyst was achieved within 1 h of UV irradiation. The adsorption photocatalyst achieved 50 % with no exposure to UV light for 15 min of irradiation. It is concluded that suitable photocatalytic conditions and sample parameters possessing the multilayer photocatalyst of Cs–TiO_2(SY) are beneficial toward the adsorption–photodegradation process in wastewater treatment.