Occurrence, Geochemistry and Speciation of Elevated Arsenic Concentrations in a Fractured Bedrock Aquifer System

The presence of elevated arsenic concentrations (≥10 µg L-1) in groundwaters has been widely reported in areas of south east Asia with recent studies showing its detection in fractured bedrock aquifers mainly in regions of north-eastern United States. Data within Europe remains limited; therefore, the objective of this work was to understand the geochemical mobilisation mechanism of arsenic in this geologic setting. Physiochemical (pH, Eh, d-O2), trace metals, major ion and arsenic speciation samples were collected and analysed using a variety of field and laboratory-based techniques and evaluated using statistical analysis including multivariate analysis. Elevated dissolved arsenic concentrations (up to 73.95 µg L-1) were observed in oxic-alkali groundwaters with the co-occurrence of other oxyanions (e.g. Mo, Se, Sb and U), low dissolved concentrations of Fe and Mn and low Na/Ca ratios indicating that arsenic was mobilised through alkali desorption of Fe oxyhydroxides. Arsenic speciation using a solid-phase extraction methodology (n=20) showed that the dominant species of arsenic present in groundwater was arsenate, with pH being a major controlling factor. The expected source of arsenic is sulfide minerals within fractures of the bedrock aquifer with transportation of arsenic and other oxyanion-forming elements facilitated by secondary Fe mineral phases. However, the presence of methylarsenical compounds detected in the groundwaters illustrates that microbially mediated mobilisation processes were also (co)-occurring. This study demonstrates how field speciation of arsenic can be utilised to overcome analytical limitations of conventional laboratory speciation and to facilitate in the interpretation of the environmental mobility of arsenic within groundwaters.

Tracing gold nuggets back to the source: a microchemical analysis of tertiary gold placers in Central Spain.

<p>Gold placers are abundant and intensively surveyed in western Iberia since antiquity. Three Cenozoic gold placers covering Neoproterozoic-Lower Paleozoic basement rocks have recently been revealed, which stand out for the number and size of the samples recovered: Salvatierra de Tormes (ST), Santibáñez el Alto (SA) and Casas de Don Pedro-Talarrubias (CSDP). To date, primary sources remain undiscovered. We have combined microchemical, inclusion analysis and morphology of gold nuggets to define the placer gold signature and its relationship with bedrock primary sources and infer mineralization styles. Coarse gold particles prevent secondary resetting of source signature and increase the chances to investigate mineral assemblages. Nuggets morphology analysis have reveled that ST and SA deposit are fluvial "trunk" placers, while CSDP represents an autochthon or colluvial placer type. Four different types of gold have been defined in nuggets: core gold (T1), rim gold (T2) fine grained gold in Fe-oxyhydroxides aggregates (T3) and "mustard" gold (Au+Sb-Pb-Fe-oxides) (T4).</p><p>Based on those categories we have explored primary and secondary signatures in the deposits. Lode signature is observed in the core of nuggets (T1) with a fineness between 800 and 1000. Alloy composition ranges from binary (Au:Ag) in SA to ternary (Au:Ag:Cu) in ST and CSDP. Sulphides and sulfarsenides dominates inclusions association in ST, while Sb- and Sb-Pb-Fe phases appear in ST and CSDP respectively. CSDP primary gold shows a distinct Hg content. The identification of mineral phases non-compatible with supergene conditions in gold and textural remnants of annealing microstructures, point to an hypogenic origin of T1 in all deposits and coul be compatible with a mesothermal system (<400ºC) in which, CSDP represents the higher T and SA the lower T end. A hybrid hydrothermal-magmatic system is proposed.</p><p>Secondary signature is complex and reveals several stages. Older evidence of in-situ modification of primary gold was found in CSDP gold-bearing quart fragments, with pervasive alteration under oxidizing and alkaline conditions. This process liberated gold from T1 and primary phases (e.g., aurostibite), leading to the formation of auroatimonades and "mustard" gold (T4), showing a complex textural pattern. Gold particles were subsequently modified during fluvial transport and deposition through the interaction with fluids, which activated Ag-leaching processes, resulting in the development of gold-rich rims (T2). Partial dissolution and re-precipitation of gold in reduction conditions formed very fineness gold particles embedded in Fe oxy-hydroxides (T3).</p>

Effect of pedogenic iron-oxyhydroxide removal on the metal sorption by soil clay minerals

Purpose The close association of Fe-oxyhydroxides and clay minerals might influence the sorption properties of these components. We aimed to study the effect of removing the pedogenic Fe-oxyhydroxides on the sorption of Cd, Cu, Pb, and Zn by the clay mineral particles in soils with contrasting pH. Methods Competitive batch sorption experiments before and after Fe-oxyhydroxide extraction in soils were carried out together with the direct analysis of the metal sorption on individual particles of ferrihydrite, smectite, and illite/smectite by TEM. Results Ferrihydrite was a more effective metal sorbent than clay minerals, although its removal resulted in decreased sorption only for Cd, Cu, and Zn. Ferrhydrite coating blocked metals’ access for certain sorption sites on clay surfaces, which were only accessible for Pb as the most efficient competitor after removing the coating. This observation was the most remarkable for the smectite particles in the alkaline soil. Mineral surfaces sorbed higher Cu than Pb concentrations and higher Zn than Cd concentrations despite the former metals’ lower bulk sorption. Thus, organic surfaces and precipitation contributed to Pb and Cd’s retention to a greater extent than for Cu and Zn. The structural Fe of smectite also promoted the metal sorption in both soils. Conclusion Removal of iron-oxyhydroxide coatings from the soil affects metal sorption selectively. Direct study of metal sorption on individual soil particles enables us to gain a more in-depth insight into soil minerals’ role in this process.

Environmental Impact Assessment in the Former Mining Area of Regoufe (Arouca, Portugal): Contributions to Future Remediation Measures

The W-Sn Regoufe mine, closed since the 1970s, was once intensively exploited for tungsten concentrates. Throughout its activity, considerable amounts of arsenopyrite-rich mine wastes were produced and, to this day, are still exposed to weathering conditions. Thus, this work aims at assessing soil contamination, using a combination of chemical, physicochemical and mineralogical analyses and sequential selective chemical extraction of the main potentially toxic elements (PTEs) in topsoils. Results show that Regoufe soils are enriched in most of the PTEs associated with the ore assemblage, but As and Cd contents far outstrip both international and national guidelines. The estimated contamination factor reveals that 67% of soil samples are classified as highly to ultra-highly contaminated. Similar distribution patterns, with the main focus around the unsealed mine adits, are observed when spatially projecting the modified degree of contamination (mCd) and arsenic contents. Fe-oxyhydroxides and organic matter demonstrate to have a preponderant role in the retention of Cd and As. In fact, despite the high PTE contents in soils, local surface waters are characterised by low metal(loid) contents and nearly neutral pH, with PTE concentrations below national thresholds for irrigation waters.

Water oxidation kinetics of nanoporous BiVO4 photoanodes functionalised with nickel/iron oxyhydroxide electrocatalysts

Elucidating the role of charge accumulation and reaction kinetics in governing the performance of Ni/Fe oxyhydroxides as electrocatalysts and as co-catalysts on BiVO4 photoanodes water oxidation.

Gold and tin mineralisation in the ultramafic-hosted Cheoeum vent field, Central Indian Ridge

The Cheoeum vent field (CVF) is the first example of an inactive ultramafic-hosted seafloor massive sulphide (SMS) deposit identified in the middle part of the Central Indian Ridge. Here, we report on the detailed mineralogy and geochemistry of ultramafic-hosted sulphide sample atop a chimney, together with a few small fragments. Hydrothermal chimneys are characterised by high concentrations of Au (up to 17.8 ppm) and Sn (up to 1720 ppm). The sulphide mineralisation in the CVF shows (1) early precipitation of anhedral sphalerite and pyrite–marcasite aggregates under relatively low-temperature (< 250 °C) fluid conditions; (2) intensive deposition of subhedral pyrrhotite, isocubanite, chalcopyrite, Fe-rich sphalerite (Sp-III), and electrum from high-temperature (250–365 °C) and reduced fluids in the main mineralisation stage; and (3) a seawater alteration stage distinguished by the mineral assemblage of marcasite pseudomorphs, altered isocubanite phase, covellite, amorphous silica, and Fe-oxyhydroxides. Electrum (< 2 μm in size) is the principal form of Au mineralisation and is mainly associated with the main mineralisation stage. The consistently high fineness of electrum (801 to 909‰) is indicative of the selective saturation of Au over Ag in the fluid during high-temperature mineralisation, which differs from the Au mineralisation associated with typical basaltic-hosted hydrothermal systems on mid-ocean ridges. Tin is mainly substituted in structures of sphalerite, isocubanite, and chalcopyrite as a solid solution, and not as mineral inclusions. The continuously ascending hydrothermal fluids enable the early formed Sn-bearing sulphide to be dissolved and reprecipitated, producing significantly Sn-enriched replacement boundaries between isocubanite and Sp-III. This study suggests that Au–Sn mineralisation could be facilitated by the low redox potential of ultramafic-hosted hydrothermal systems such as in the CVF, which may be a common occurrence along slow-spreading mid-ocean ridges.

The effect of organic matter (OM) quality on the redox stability of OM-Fe association in freshwater sediments

Redox sensitive iron (Fe) oxyhydroxides participate in the biogeochemical cycling of nutrients and trace metals. Their co-precipitation with organic matter (OM) leads to environmentally relevant OM-Fe associations. The stability of OM in such associations is still uncertain. It has been proposed that OM either increases the stability of the complex against reductive dissolution or enhance the dissolution (both abiotic and biotically) of Fe oxyhydroxides. The OM character, in terms of specific functional groups binding to Fe, could be a critical factor determining the fate and stability of OM-Fe. Here, sediment samples from a vertical profile of a mesotrophic freshwater reservoir were treated using a sequential extraction scheme designed to distinguish Fe oxyhydroxides of different redox reactivity based on dissolution kinetics. To assess the reactivity and stability of the complexes, special attention was payed to the determination of redox active vs. stable fractions of Fe and the corresponding dissolved organic matter (DOM) components sequentially extracted. The quality of the extracted DOM was evaluated using UV-VIS and fluorescence spectroscopy together with the PARAFAC model. A selectivity model was used to ascertain the quality of OM preferentially associated with the different redox stable Fe oxyhydroxides. Accordingly, we found that humic-like substances render the OM-Fe associations redox labile, while non-humic substances enhance the stability of the associations. These findings improve the understanding required for predicting the fate of OM-Fe associations in freshwater sediments with different organic matter sources and characters.

Characterization of Weathering Processes of the Giant Copper Deposit of Tizert (Igherm Inlier, Anti-Atlas, Morocco)

The giant Tizert copper deposit is considered as the largest copper resource in the western Anti-Atlas (Morocco). The site is characterized by Cu mineralization carried by malachite, chalcocite, covellite, bornite and chalcopyrite; azurite is not observed. The host rocks are mainly limestones (Formation of Tamjout Dolomite) and sandstones/siltstones (Basal Series) of the Ediacaran/Cambrian transition. The supergene enrichment is most likely related to episodes of uplift/doming (last event since 30 Ma), which triggered the exhumation of primary/hypogene mineralization (chalcopyrite, pyrite, galena, chalcocite I and bornite I), generating their oxidation and the precipitation of secondary/supergene sulfides, carbonates and Fe-oxyhydroxides. The Tizert supergene deposit mainly consists of (i) a residual patchwork of laterite rich in Fe-oxyhydroxides; (ii) a saprolite rich in malachite, or “green oxide zone” where primary structures such as stratification are preserved; (iii) a cementation zone containing secondary sulfides (covellite, chalcocite II and bornite II). The abundance of Cu carbonates results from the rapid neutralization of acidic meteoric fluids, due to oxidation of primary sulfides, by carbonate host rocks. Chlorite is also involved in the neutralization processes in the sandstones/siltstones of the Basal Series, in which supergene clays, such as kaolinite and smectites, subsequently precipitated. At Tizert, as can be highlighted in other supergene Cu-deposits around the world, azurite is absent due to low pCO2 and relatively high pH conditions. In addition to copper, Ag enrichment is also observed in weathered rocks; Fe-oxyhydroxides contain high Zn, As, and Pb contents. However, these secondary enrichments are quite low compared to Cu in the whole Tizert site, which is therefore, considered as relatively homogeneous.