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Iron-bearing nanominerals such as ferrihydrite, schwertmannite, and green rust behave as metastable precursors leading to the formation of more thermodynamically stable iron mineral phases (e.g., jarosite, goethite, hematite, and magnetite). However, this transformation may last from days to tens or even hundreds of years, making them the most predominant iron-bearing minerals at environmental conditions and at the human time scale. The present review characterizes ferrihydrite, schwertmannite, and green rust nanominerals according to their main physical and chemical properties, and at both nano- and mesoscales. It also presents a comprehensive review of the multiple past and present Earth environments where these nanominerals have played, and still play, a pivotal role in the geochemistry, mineralogy and environmental nanogeosciences of these environments. Finally, the present and future technological applications of these nanominerals as well as their role in the generation of a more sustainable human- Earth relationship is discussed, with a special emphasis on their use in new circular economies and green based technologies., Advanced Mining Technology Center (AMTC) from Universidad de Chile, National Agency for Research and Development, ANID, Ministry of Science and Innovation, Spain (MICINN) Spanish Government AFB180004, European Union's Horizon 2020 MarieSklodowska-Curie Innovative Training Network RYC2019-026496-I, Helmholtz Recruiting Initiative 675219, Junta de Andalucia, Universidad de Huelva / CBUA

The travertines of the Fitero thermal springs, with more than 98% of aragonite in most of the samples, are studied in this paper. The main objective is to improve the general understanding of aragonite precipitation, since the deposits of almost pure aragonite are very scarce. This study presents a complete mineralogical and isotopic characterisation, including the evaluation of the δ18O and δ13C fractionation during precipitation, as valuable information for paleoclimate and paleoenvironmental studies. Samples of a laminated travertine deposit constituted by almost pure aragonite were taken from a pipe discharging water at 40 °C. Waters suffered an important CO2 outgassing, as suggested by the geochemical calculations and the δ13C values of travertines and waters. This outgassing triggers the oversaturation and precipitation of carbonate phases. Temperature seems to be the main factor controlling the precipitation of aragonite or calcite, as checked by studying another travertine sample with higher calcite content. Various δ18O isotope fractionation equations for aragonite and calcite were used. The results indicate that precipitation took place close to equilibrium according to some of these equations. The fact that the equilibrium is maintained in a natural system with an important CO2 loss is surprising. However, it can be explained by an HCO3−–water oxygen isotopic equilibrium and a direct transfer of the HCO3− isotope signal to the carbonate without fractionation due to the fast CO2 loss and precipitation. Finally, considering other natural aragonite samples, a fractionation equation is defined for natural aragonite in the temperature range between 23 and 80 °C.

International audience; The reactivity of iron(II/III) oxide surfaces may be influenced by their interaction with silica, which is ubiquitous in aquatic systems. Understanding the structure–reactivity relationships of Si-coated mineral surfaces is necessary to describe the complex surface behavior of nanoscale iron oxides. Here, we use Si-adsorption isotherms and Fourier transform infrared spectroscopy to analyze the sorption and polymerization of silica on slightly oxidized magnetite nanoparticles (15% maghemite and 85% magnetite, i.e., ∼2 maghemite surface layers), showing that Si adsorption follows a Langmuir isotherm up to 2 mM dissolved Si, where surface polymerization occurs. Furthermore, the effects of silica surface coatings on the redox-catalytic ability of magnetite are analyzed using selenium as a molecular probe. The results show that for partially oxidized nanoparticles and even under different Si surface coverages, electron transfer is still occurring. The results indicate anion exchange between silicate and the sorbed SeIV and SeVI. X-ray absorption near-edge structure analyses of the reacted Se indicate the formation of a mixed selenite/Se0 surface phase. We conclude that neither partial oxidation nor silica surface coatings block the sorption and redox-catalytic properties of magnetite nanoparticles, a result with important implications to assess the reactivity of mixed-valence phases in environmental settings.

Crystal growth & design 20(10), 6831 - 6846 (2020). doi:10.1021/acs.cgd.0c00912, Amorphous calcium carbonate (ACC) is an important precursor in the biomineralization of crystalline CaCO$_{3}$. In nature, it serves as a storage material or as a permanent structural element, whose lifetime is regulated by an organic matrix. The relevance of ACC in materials science is primarily related to our understanding of CaCO3 crystallization pathways and CaCO$_{3}$/(bio)polymer nanocomposites. ACC can be synthesized by liquid���liquid phase separation, and it is typically stabilized with macromolecules. We have prepared ACC by milling calcite in a planetary ball mill. Phosphate ���impurities��� were added in the form of monetite (CaHPO$_{4}$) to substitute the carbonate anions, thereby stabilizing ACC by substitutional disorder. The phosphate anions do not simply replace the carbonate anions. They undergo shear-driven acid/base and condensation reactions, where stoichiometric (10%) phosphate contents are required for the amorphization to be complete. The phosphate anions generate a strained network that hinders ACC recrystallization kinetically. The amorphization reaction and the structure of BM-ACC were studied by quantitative Fourier transform infrared spectroscopy and solid state $^{31}$P, $^{13}$C, and $^{1}$H magic angle spinning nuclear magnetic resonance spectroscopy, which are highly sensitive to symmetry changes of the local environment. In the first���and fast���reaction step, the CO$_{3}$$^{2���}$ anions are protonated by the HPO$_{4}$$^{2���}$ groups. The formation of unprecedented hydrogen carbonate (HC$_{3}$ $^{-}$ ) and orthophosphate anions appears to be the driving force of the reaction, because the phosphate group has a higher Coulomb energy and the tetrahedral PO43��� unit can fill space more efficiently. In a competing second���and slow���reaction step, pyrophosphate anions are formed in a condensation reaction. No pyrophosphates are formed at higher carbonate contents. High strain leads to such a large energy barrier that any reaction is suppressed. Our findings aid in the understanding of the mechanochemical amorphization of calcium carbonate and emphasize the effect of impurities for the stabilization of the amorphous phases in general. Our approach allowed the synthesis of new amorphous alkaline earth defect variants containing the unique HCO$_{3}$ $^{-}$ anion. Our approach outlines a general strategy to obtain new amorphous solids for a variety of carbonate/phosphate systems that offer promise as biomaterials for bone regeneration., Published by ACS Publ., Washington, DC

Some landslides around the world that have low-angle failure planes show exceptionally poor mechanical properties. In some cases, an extraordinarily pure clay layer has been detected on the rupture surface. In this work, a complex landslide, the so-called Diezma landslide, is investigated in a low- to moderate-relief region of Southeast Spain. In this landslide, movement was concentrated on several surfaces that developed on a centimeter-thick layer of smectite (montmorillonite-beidellite) clay-rich level. Since these clayey levels have a very low permeability, high plasticity, and low friction angle, they control the stability of the entire slide mass. Specifically, the triggering factor of this landslide seems to be linked to the infiltration of water from a karstic aquifer located in the head area. The circulation of water through old failure planes could have promoted the active hydrolysis of marly soils to produce new smectite clay minerals. Here, by using geophysical, mineralogical, and geochemical modelling methods, we reveal that the formation and dissolution of carbonates, sulfates, and clay minerals in the Diezma landslide could explain the elevated concentrations of highly plastic secondary clays in its slip surface. This study may help in the understanding of landslides that show secondary clay layers coinciding to their low-angle failure planes., Spanish Government PID2019-107138RB-I00, Junta de Andalucía P18-RT-3632

The long-term stability of U(IV) solid phases in anaerobic aquifers depends upon their reactivity in the presence of oxidizing chemical species and microbial catalysts. We performed flow-through column experiments under anaerobic conditions to investigate the mechanisms and dissolution rates of biogenic, noncrystalline UO2(s) by chemical oxidants (nitrate and/or nitrite) or by Thiobacillus denitrificans, a widespread, denitrifying, chemolithoautotrophic model bacterium. Dissolution rates of UO2(s) with dissolved nitrite were approximately 5 to 10 times greater than with nitrate alone. In the presence of wild-type T. denitrificans and nitrate, UO2(s) dissolution rates were similar to those of abiotic experiments with nitrite (from 1.15 × 10-14 to 4.94 × 10-13 mol m-2 s-1). Experiments with a T. dentrificans mutant strain defective in U(IV) oxidation supported microbially mediated U(IV) oxidation. X-ray absorption spectroscopy (XAS) analysis of post-reaction solids showed the presence of mononuclear U(VI) species rather than a solid U(VI) phase. At steady-state U release, kinetic and spectroscopic results suggest detachment of oxidized U(VI) from the UO2(s) surface as the rate-determining step rather than electron transfer or ion diffusion. Under anaerobic conditions, production of nitrite by nitrate-reducing microorganisms and enzymatically catalyzed, nitrate-dependent U(IV) oxidation are likely dual processes by which reduced U solids may be oxidized and mobilized in the aqueous phase.

Sulfate reduction and its associated contaminant immobilization in marsh soils supporting a phosphogypsum stack was examined by pore-water and solid analysis, selective extractions, microscopy and sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The negative impact of this stack on estuarine environments is a concerning problem. In the weathering profile, total concentrations of most pollutants increase with depth; instead, dissolved contents in pore-waters increase to middle of the saturated zone but then decrease drastically down to reach the marsh due to sulfide precipitation. Excess of acid-volatile sulfide plus pyritic sulfur over metals bound to the oxidizable fraction indicates that sulfide precipitation is the main mechanism responsible for metal removal in the marsh. Thus, abundant pyrite occurred as framboidal grains, in addition to other minor sulfides of As, Zn and Cu as isolated particles. Moreover, high contents of elemental sulfur were found, which suggest partial sulfide oxidation, but marsh may have capacity to buffer potential release of contaminants. The importance of sulfur species was quantitatively confirmed by XANES, which also supports the accuracy of selective extraction schemes. Accordingly, managing pore-water quality through organic carbon-rich amendments over phosphogypsum stacks could lead to a decrease in contaminant loading of leakages resulting from weathering.

Arsenic contamination in groundwater is pervasive throughout deltaic regions of Southeast Asia and threatens the health of millions. The speciation of As in sediments overlying contaminated aquifers is poorly constrained. Here, we investigate the chemical and mineralogical compositions of sediment cores collected from the Mekong Delta in Vietnam, elucidate the speciation of iron and arsenic, and relate them to the sediment depositional environment. Gradual dissolution of ferric (oxyhydr)oxides with depth is observed down to 7 m, corresponding to the establishment of reducing conditions. Within the reduced sediment, layers originating from marine, coastal or alluvial depositional environments are identified and their age is consistent with a late Holocene transgression in the Mekong Delta. In the organic matter- and sulfur-rich layers, arsenic is present in association with organic matter through thiol-bonding and in the form of arsenian pyrite. The highest arsenic concentration (34-69 ppm) is found in the peat layer at 16 m and suggests the accumulation of arsenic due to the formation of thiol-bound trivalent arsenic (40-55%) and arsenian pyrite (15-30%) in a paleo-mangrove depositional environment (∼8079 yr BP). Where sulfur is limited, siderite is identified, and oxygen- and thiol-bound trivalent arsenic are the predominant forms. It is also worth noting that pentavalent arsenic coordinated to oxygen is ubiquitous in the sediment profile, even in reduced sediment layers. But the identity of the oxygen-bound arsenic species remains unknown. This work shows direct evidence of thiol-bound trivalent arsenic in the Mekong Delta sediments and provides insight to refine the current model of the origin, deposition, and release of arsenic in the alluvial aquifers of the Mekong Delta.

The journal of physical chemistry / C 124(46), 25645 - 25656 (2020). doi:10.1021/acs.jpcc.0c08670, Amorphous calcium carbonate (ACC) is an essential component present during the early stages of biomineralization of many calcifying organisms, which is used as a precursor of crystalline calcium carbonate phases. Here, we performed X-ray photon correlation spectroscopy experiments on ACC which show that the amount of adsorbed water has a strong control over its diffusive dynamics. Results of crystallization experiments under conditions of controlled humidity indicate that the adsorbed water is enough to spark crystallization of calcite. A direct proportionality is found between the relative humidity of the environment and the final size of the calcite crystals. Different hypotheses are made to explain this result, with the confinement within the amorphous matrix being the main suspect of controlling the size of the crystallites. Control of water activity and water content is, therefore, a possible way to regulate biomineral crystallinity both in nature and for the design of functional biomimetic materials., Published by Soc., Washington, DC

Achromatium is a long known uncultured giant gammaproteobacterium forming intracellular CaCO3 that impacts C and S geochemical cycles functioning in some anoxic sediments and at oxic-anoxic boundaries. While intracellular CaCO3 granules have first been described as Ca oxalate then colloidal CaCO3 more than one century ago, they have often been referred to as crystalline solids and more specifically calcite over the last 25 years. Such a crystallographic distinction is important since the respective chemical reactivities of amorphous calcium carbonate (ACC) and calcite, hence their potential physiological role and conditions of formation, are significantly different. Here, we analyzed the intracellular CaCO3 granules of Achromatium cells from Lake Pavin using a combination of Raman microspectroscopy and scanning electron microscopy. Granules in intact Achromatium cells were unequivocally composed of ACC. Moreover, ACC spontaneously transformed into calcite when irradiated at high laser irradiance during Raman analyses. Few ACC granules also transformed spontaneously into calcite in lysed cells upon cell death and/or sample preparation. Overall, the present study supports the original claims that intracellular Ca-carbonates in Achromatium are amorphous and not crystalline. In that sense, Achromatium is similar to a diverse group of Cyanobacteria and a recently discovered magnetotactic alphaproteobacterium, which all form intracellular ACC. The implications for the physiology and ecology of Achromatium are discussed. Whether the mechanisms responsible for the preservation of such unstable compounds in these bacteria are similar to those involved in numerous ACC-forming eukaryotes remains to be discovered. Last, we recommend to future studies addressing the crystallinity of CaCO3 granules in Achromatium cells recovered from diverse environments all over the world to take care of the potential pitfalls evidenced by the present study.

The alteration of chromian spinel is a key process during serpentinization and metamorphism of ultramafic rocks controlled by oxygen fugacity (fO) and Fe ↔ Fe exchange during fluid–rock interaction. Chromian spinel alteration is better recorded in less permeable chromitite than in peridotites where extensive fluid–rock interaction frequently overprints the record of earlier stages of alteration. To shed light on that process we have studied the distribution of Fe and Fe in variably altered chromian spinel grains from a set of chromitite samples from the same mining district using synchrotron-based microscopic chemical imaging and spatially resolved X-ray absorption near edge structure spectroscopy. Our results show that early stages of alteration do not involve changes in Cr and Fe contents or in Fe speciation but only depletion in Al and Mg resulting in the formation of porous chromite. With ongoing alteration Fe migrates into porous chromite mainly along fracture walls and fracture zones as well as along grain boundaries. Sheared-type chromitites record the maximum rates of fluid–rock interaction because in these chromitite-types the accommodation of deformation on porous chromite allowed higher rates of diffusion of Fe and Fe (a magnetite component with Fe/Fe = 0.66) into the newly formed neoblasts. In porous chromite-type texture (all the original chromite grains fully transformed to porous chromite) the deformation and accompanying diffusion processes result in the formation of homogenous ferrian chromite grains. In contrast, in partly altered-type texture (chromite grains with original cores surrounded by porous chromite), such processes are only restricted to the porous rims, giving rise to zoned chromian spinel-ferrian chromite grains., The authors acknowledge the Paul Scherrer Institut, Villigen, Switzerland for provision of synchroton radiation beamtime (proposals 20131062 and 20141107) at beamline microXAS (X05LA) of the SLS. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant agreement no. 312284 (CALIPSO) and from Spanish ministerio de Ciencia, Innovación y Universidades (project RTI2018-099157-A-I00). Maria P. Asta has received financial support from the European Comission through the Marie Skłodowska-Curie Research Fellowship Programme. J. M. González-Jiménez thanks the support of a Ramón y Cajal Fellowship (RYC-2015-17596) granted by the Spanish “Ministerio de Economía y Competitividad (MINECO)”.

There is growing concern regarding human dietary exposure to arsenic (As) via consumption of rice. The concentration and speciation of As in rice are highly variable, and models describing rice As speciation as a function of environmental covariates remain elusive. We conducted a survey of paddy rice and soil in the Mekong Delta with the objective of linking patterns in rice As content to soil chemical variables or hydrogeological parameters. The sum of As species (ΣAs) in husked rice averaged 243 μg/kg and the average inorganic As (iAs) content was 84%. There was no relationship found between rice As concentration or speciation and As levels in soil. However, mean As concentrations in groundwater near rice sampling locations were strongly correlated with grain ΣAs and iAs over a large part of the study region, despite the fact that groundwater is not commonly used for rice paddy irrigation in this region. We hypothesize that surficial sediments with high concentrations of soluble and plant-available As also serve as sources of arsenic to downgradient shallow aquifers, explaining the observed associations between rice and groundwater As. This study suggests that shallow groundwater As concentrations may serve as a useful indicator for locations at risk of elevated iAs concentrations in rice.

The Ebro Valley in the outskirts of Zaragoza (NE Spain) is severely affected by evaporite karstification, leading to multiple problems related to subsidence and sinkhole formation. In this work, a combination of inverse (mixing + mass-balance) and forward (reaction-path) geochemical calculations is applied for the quantification of the main karstification processes and seasonal variations in this area. The obtained results prove the suitability of the applied methodology for the characterization of similar problems in other areas with scarce geological and hydrogeological information. The hydrogeology and hydrochemistry of the system can be mainly attributed to the mixing of variable proportions of concentrated groundwater from the evaporitic aquifer and more dilute water from the overlying alluvial aquifer. The existence of a good connection between these aquifers is supported by: (1) the fast changes in the hydrochemistry of the karst aquifer related to recharge by irrigation, and (2) the deduced input of evaporitic groundwater in the alluvial materials. The evolution in some parts of the alluvial/evaporitic aquifer system is clearly dominated by the seasonal variations in the recharge by dilute irrigation waters (up to 95% of water volume in some sinkhole ponds), whereas other points seem to be clearly determined by the hydrochemistry of the concentrated evaporitic aquifer groundwater (up to 50% of the water volume in some springs). The following reactions, previous or superimposed to mixing processes, explain the observed hydrochemistry in the studied area: dissolution of halite (NaCI), gypsum (CaSO4.2H(2)O)/anhydrite (CaSO4) and dolomite (CaMg(CO3)(2)), CO2(B) input and degassing and calcite (CaCO3) dissolution/precipitation. The modeling results suggest the existence of a large spatial variability in the composition of the evaporitic groundwater, mainly caused by large differences in the availability of halite in contact with the groundwater. Active subsidence associated with halite dissolution is expected to continue in the study area, together with the episodic increase of gypsum dissolution associated with the input of dilute irrigation waters. (C) 2015 Elsevier B.V. All rights reserved.

Geothermometrical characterisation of low-temperature, carbonate-evaporitic geothermal systems is usually hampered by the lack of appropriate mineral equilibria to successfully use most of the classical geothermometers and/or by the thermodynamic uncertainties affecting some of the most probable mineral equilibria in low temperature conditions. This situation is further hindered if the thermal waters are additionally affected by secondary processes (e.g., CO2 loss) during their ascent to surface. All these problems cluster together in the low-temperature Alhama-Jaraba thermal system, hosted in carbonate rocks, with spring temperatures about 30 °C and waters of Ca-Mg−HCO3/SO4 type. This system, one of the largest naturally flowing (600 L/s) low temperature thermal systems in Europe, is used in this paper as a suitable frame to assess the problems in the application of chemical geothermometrical techniques (classical geothermometers and geothermometrical modelling) and to provide a methodology that could be used in this type of geothermal system or in potential CO2 storage sites in similar aquifers. The results obtained have shown that the effects of the secondary processes can be avoided by selecting the samples unaffected by such processes and, therefore, representative of the conditions at depth, or by applying existing methodologies to reconstruct the original composition, as is usually done for medium to high temperature systems. The effective mineral equilibria at depth depend on the temperature, the residence time and the specific lithological/mineralogical characteristics of the system studied. In the present case, the mineral equilibria on which classical cation geothermometers are based have not been attained. The low proportion of evaporitic minerals in the hosting aquifer prevents the system from reaching anhydrite equilibrium, otherwise common in carbonate-evaporitic systems and necessary for the specific SO4-F geothermometer or the specially reliable quartz (or chalcedony) – anhydrite equilibrium in the geothermometrical modelling of these geothermal systems. Under these circumstances, the temperature estimation must rely on quartz (or chalcedony), clay minerals and, especially, calcite and dolomite. However, clay minerals and dolomite present important thermodynamic uncertainties related to possible variations in composition or crystallinity degree for clays and order/disorder degree for dolomite. To deal with these problems, a sensitivity analysis to the thermodynamic data for clay minerals has been carried out, comparing the results obtained when considering different solubility data. The uncertainties associated with dolomite have been addressed by reviewing the solubility data available for dolomites with different order degrees and performing specific calculations for the order degree of the dolomite in the aquifer. This approach can be used to find the most adequate dolomite thermodynamic data for the system under consideration, including medium-high temperature geothermal systems. Finally, the temperature estimation of the Alhama-Jaraba waters in the deep reservoir has been obtained from simultaneous equilibria of quartz, calcite, partially disordered dolomite and some aluminosilicate phases. The obtained value of 51 ± 14 °C is within the uncertainty range normally affecting this type of estimations and is coherent with independent estimations from geophysical data.

Land snail shell δ13C value is often used as a paleovegetation proxy assuming that snails ingest all plants in relation to their abundance, and that plants are the only source of carbon. However, carbonate ingestion and variable metabolic rates complicate these relationships. We evaluate if live-collected snails from Lanzarote (Canary Islands) reflect the abundance of C3 and CAM plants. Snails were collected on either CAM or C3 plants for isotope analysis of shell and body, and shell size. Respective shell and body δ13C values of snails collected on CAM plants averaged − 8.5 ± 1.7‰ and − 22.8 ± 1.6‰, whereas specimens from C3 plants averaged − 10.1 ± 0.7‰ and − 24.9 ± 1.1‰. A flux balance model suggests snails experienced comparable metabolic rates. A two-source mass balance equation implies that snails consumed ~ 10% of CAM, which agrees with their abundance in the landscape. Snails collected on CAM plant were smaller than those on C3 plants. Conclusively: 1) snails consume CAM plants when they are available; 2) migration of snails among C3 and CAM plants is a common phenomenon; and 3) C3 plants may be a more energetic food for growth than CAM plants. This study shows that shell δ13C values offer approximate estimates of plants in C3–CAM mixed environments.

Aragonitic and calcitic land snails from carbonate-rich paleosols in northwestern Lanzarote (Canary Islands) were analyzed for 13C/12C and 18O/16O ratios to deduce the Pleistocene–Holocene transition in the westernmost Sahara zone. Modern, mid-late Holocene (~ 2.1–5.5 cal ka BP) and late Pleistocene (~ 23.3–24.0 cal ka BP) aragonitic shells exhibited respective values of − 9.5 ± 1.6‰, − 7.7 ± 1.5‰, and − 2.3 ± 2.8‰ for δ13C; and + 0.3 ± 0.3‰, + 0.1 ± 0.7‰, and + 2.5 ± 0.4‰ for δ18O. Holocene and Pleistocene calcitic shells of the endemic slug Cryptella canariensis showed respective values of − 0.7 ± 2.6‰ and − 8.5 ± 2.5‰ for δ13C; and + 0.8 ± 1.5 and + 3.6 ± 0.4‰ for δ18O. Both aragonitic and calcitic shells showed equivalent temporal isotopic trends. Higher δ13C values during ~ 23.3–24.0 cal ka BP suggest higher abundance of C4 and/or CAM plants, likely associated with drier conditions and/or lower atmospheric CO2 concentration. Maximum shell δ18O values during ~ 23.3–24.0 cal ka BP opposes minimal values of Greenland ice cores and probably reflect the combined effects of (1) higher rain δ18O values linked to higher glacial seawater δ18O values and/or larger snail activity during summer seasons; (2) relative humidity values similar or slightly lower than at present; (3) higher evaporation rates; and (4) cooler temperatures. Bone remains of the extinct Dune Shearwater Puffinus holeae were only recovered from the Holocene bed. Collagen δ13C and δ15N values (− 13.5 ± 0.2‰[PDB] and + 13.7 ± 1.0‰[air], respectively) match with the signature of a low trophic level Macaronesian seabird that fed upon local fish. Bone carbonate δ13C (− 7.4 ± 1.0‰[PDB]) and phosphate δ18O (+ 18.2 ± 0.4‰[SMOW]) values exhibited pristine signals denoting their potential value in future paleoenvironmental studies in the region. The age of P. holeae (~ 2.1–2.7 cal ka BP) supports that the aboriginal population possibly caused its extinction. In contrast, the extinction of the endemic helicid Theba sp. (~ 23.3–24.0 cal ka BP) was likely caused by environmental change.

The Tiermas low temperature geothermal system, hosted in the Paleocene-Eocene carbonates of the Jaca-Pamplona basin, has been studied to evaluate the geochemistry and the temperature of the waters in the deep reservoir. These waters are of chloride-sodium type and emerge with a temperature of about 37 degrees C. Two hydrogeochemical groups of waters have been distinguished: one with lower sulphate concentration and lower TDS (about 7500 ppm) and the other with higher sulphate content and TDS values (close to 11,000 ppm). There are also slight differences in the reservoir temperature estimated for each group. These temperatures have been determined by combining several geothermometrical techniques: (1) classical chemical geothermometers (SiO2-quartz, Na-K, K-Mg and Na-K-Ca), (2) specific geothermometers for carbonate systems (Ca-Mg), (3) isotopic geothermometers and, (4) geothermometrical modelling. The good agreement in the temperature obtained by these techniques, including the cationic geothermometers which are not usually considered suitable for this type of systems, allows establishing a reliable range of temperature of 90 +/- 20 degrees C for the low-sulphate waters and 82 +/- 15 degrees C for the high-sulphate waters. The mineral assemblage in equilibrium in the reservoir is assumed to be the same for both groups of waters (calcite, dolomite, quartz, anhydrite, albite, K-feldspar and other aluminosilicate phases); therefore, the differences found in the reservoir temperature and, mostly, in the geochemical characteristics of each group of waters must be due to the existence of two flow paths, with slightly different temperatures and intensity of water-rock interaction. Anhydrite is at equilibrium in the reservoir suggesting that, although this system is hosted in carbonates, evaporites may also be present. The dissolution of halite (and the consequent increase in the chloride concentration) conditions the chemical characteristics of the waters and the equilibrium situations in the reservoir and waters acquire their chloride-sodium affinity at depth and not during their ascent to the surface. Finally, a favourable tectonic structure for CO2 storage has been recognised in the Paleocene-Eocene carbonates of this area. Therefore, considering the characteristics of these waters (in equilibrium with calcite, dolomite and anhydrite in the reservoir), the results of this work are useful to understand some of the geochemical processes that might take place during the CO2 injection: 1) precipitation of carbonates and sulphates in the vicinity of the injection well due to desiccation of the waters and, 2) carbonate dissolution and sulphate precipitation in the long term.

Freshwater carbonates are interesting archives in palaeoenvironmental reconstructions. However, more studies of those systems are needed to fully understand past environments. In this work the actively-forming travertines of the Alhama-Jaraba thermal system were studied for the first time in order to evaluate the relationship between the geochemical and mineralogical composition and the environmental conditions during their formation. With that aim, a combination of petrographical, mineralogical, geochemical and stable isotope analyses were carried out. These carbonates provide a natural laboratory for the study of the effect of different variables (natural and anthropogenic) on carbonate precipitation. The results showed that there is a close relationship between the mineralogy of the solid precipitates and the formation temperature, and only the samples formed from overheated waters (40-60 degrees C) show significant concentrations of aragonite. Aragonite-bearing samples show higher concentrations in Sr, Ba and U while calcitic solids are enriched in Mg. These differences could be attributed to mineralogy, temperature or different precipitation rates. The geochemical evaluation of the chemistry of both the solids and their parental waters suggests that differences in the rate of CO2-degassing and, in some cases, evaporation are the primary environmental controls on isotopic compositions. In addition, the results show that, if strong evaporation and CO2-degassing are involved, calcite precipitation occurs under conditions of isotopic disequilibrium with its parental water. The results of our study are useful to interpret old depositional environments and palaeotemperatures. (C) 2016 Elsevier B.V. All rights reserved.

Phosphogypsum is the main waste generated by the phosphate fertilizer industry. Despite the high level of pollutants found in phosphogypsum and the proximity of stacks to cities, there are no specific regulations for the management of this waste. This study addresses this issue by applying to phosphogypsum, from a fertilizer plant in Huelva (SW Spain), the leaching tests proposed by the current European and US environmental regulations for wastes management and classification. Two main conclusions were obtained: 1) the anomalous metal and metalloid concentrations (e.g. As, Fe, Pb, Sb, Mn, V and Cu) and higher mobility observed in the Huelva phosphogypsum compared to other stacks worldwide, and 2) the discrepancies observed between EU and US regulations dealing with hazardousness classification of these materials. This latter finding suggests the need to use complementary assessment protocols to obtain a better characterization and classification of these wastes. An evaluation of the potential risk to the aquatic life according to the US EPA regulation is proposed in this study. The results warn about the acute and chronic effects on the aquatic life of this waste and suggest the adoption of more strict measures for a safe disposal of phosphogypsum stacks. (C) 2017 Elsevier B.V. All rights reserved.

The chemical characteristics of the low-temperature geothermal system of Panticosa (Spain) were investigated in order to determine the water temperature at the reservoir and to identify the main geochemical processes that affect the water composition during the ascent of the thermal waters. In general, the studied waters are similar to other geothermal systems in the Pyrenees, belonging to the group of granite-related alkaline thermal waters (high pH, low total dissolved solids, very low magnesium concentration, and sodium as the dominant cation). According to the alkaline pH of these waters, they have a very low CO 2 partial pressure, bicarbonate is the dominant anion and silica is partially ionized as H 3 SiO 4 − . The unusually active acid–base pairs (HCO 3 − /CO 3 2 − and, mainly, H 4 SiO 4 /H 3 SiO 4 − ) act as homogeneous pH buffers and contribute to the total alkalinity in these alkaline waters. On the basis of the study of the conservative elements, a mixing process between a hot and a cold end-member has been identified. Additionally, in order to determinate the water temperature at the reservoir, several geothermometric techniques have been applied, including both geothermometrical modeling and classical geothermometrical calculations. The geothermometrical modeling seems to indicate that thermal waters re-equilibrate with respect to calcite and kaolinite during their ascent to the surface. Modeling results suggest that these thermal waters would be in equilibrium with respect to albite, K-feldspar, quartz, calcite, kaolinite and zoisite at a similar temperature of 90 ± 20 °C in the reservoir, which is in good agreement with the results obtained by applying the classical geothermometers.

Arsenic and Sb are common mine-water pollutants and their toxicity and fate are strongly influenced by redox processes. In this study, simultaneous Fe(II), As(III) and Sb(III) oxidation experiments were conducted to obtain rates under laboratory conditions similar to those found in the field for mine waters of both low and circumneutral pH. Additional experiments were performed under abiotic sterile conditions to determine the biotic and abiotic contributions to the oxidation processes. The results showed that under abiotic conditions in aerated Fe(III)–H2SO4 solutions, Sb(III) oxidizes slightly faster than As(III). The oxidation rates of both elements were accelerated by increasing As(III), Sb(III), Fe(III), and Cl− concentrations in the presence of light. For unfiltered circumneutral water from the Giant Mine (Yellowknife, NWT, Canada), As(III) oxidized at 15–78 μmol/L/h whereas Sb(III) oxidized at 0.03–0.05 μmol/L/h during microbial exponential growth. In contrast, As(III) and Sb(III) oxidation rates of 0.01–0.03 and 0.01–0.02 μmol/L/h, respectively, were obtained in experiments performed with acid unfiltered mine waters from the Iberian Pyritic Belt (SW Spain). These results suggest that the Fe(III) formed from microbial oxidation abiotically oxidized As(III) and Sb(III). After sterile filtration of both mine water samples, neither As(III), Sb(III), nor Fe(II) oxidation was observed. Hence, under the experimental conditions, bacteria were catalyzing As and Sb oxidation in the Giant Mine waters and Fe oxidation in the acid waters of the Iberian Pyrite Belt.

Simultaneous analysis of micro-X-ray diffraction (μ-XRD) and micro-X-ray fluorescence (μ-XRF) based on synchrotron light sources, and electron microprobe (EMP) analyses, were performed on iron terrace samples taken from Tinto-Odiel river system from the Iberian Pyrite Belt (IBP, SW Iberian Peninsula). Iron terraces are formed during the oxidation and precipitation of dissolved iron along the riverbeds impacted by acid mine drainage (AMD). This paper includes the study of actively-forming current terraces and fossil terraces isolated from the stream courses due to the river migration over time. The results of the study of current terrace samples from AMD-affected streams of two IPB abandoned mines (Tinto Santa Rosa and Cueva de la Mora) showed that fresh precipitates at the surface are composed primarily of metastable schwertmannite, which is gradually transformed at depth over short-time scales into goethite. Sediments of ancient terraces are composed mainly of goethite, which most likely originated from the re-crystallization of a precursor schwertmannite. However, at century-time scale, goethite partially re-crystallizes to hematite due to diagenetic processes. The transformation rate of goethite into hematite is negatively correlated with grain size and the crystallinity of goethite. Moreover, this transformation is accompanied by an increase in grain size and a decrease in surface area of hematite, and a concomitant decrease in arsenic trapped in the solid. This increase in the arsenic mobility during the diagenetic maturation should be considered in the development of conceptual and analytical models describing long-term fate, transport and bioavailability of arsenic in environmental systems.

The understanding of the main hydrogeochemical processes controlling the features and behaviour of crystalline rock environments considered for the deep geological storage of spent nuclear fuel is a key issue for the improvement of our knowledge and management of these systems and for a more accurate prediction of their future evolution. In this paper, some of the most important irreversible processes (cooling, mixing and CO 2 exchange) are studied and their effects quantified by combining classical geochemical calculations and geochemical modelling. This methodology is applied to an alkaline thermal water system developed in crystalline rocks (the Caldas de Boi groundwater system, NE Spain) analogous to the deep geological environments expected for nuclear waste storage. One of the most remarkable results obtained in this study is related to the high pH-buffering capacity exhibited by these alkaline-thermal waters during the mixing processes, with colder and shallower waters, taking place during the uprising of the hot waters from the deep reservoir. Even for high mixing proportions of cold water (up to 60%), the final pH remains as alkaline as in the thermal and deep end member, although the rest of the physicochemical parameters (including temperature) change consistently with mixing. Another important secondary pH-determining process detected in this study is the conductive cooling of the thermal waters suffered during their rise to surface conditions, which is clearly responsible for a pH increase which enhances the typical alkaline character of these waters. Finally, the existence of external inputs of edaphic or biogenic CO 2 to the groundwater system, producing a pH decrease in some of the springs, has been proven to play a key role in their behaviour and evolution.

Arsenopyrite dissolution was studied by means of long-term, stirred and non-stirred flow-through experiments in the pH range of 1 to 9 at 25, 50 and 70 °C and at different input dissolved-O2 concentrations (from 0.2 to 8.7 mg L− 1). At pH lower than 4, aqueous iron, which is mainly in the ferrous form, and arsenic are stoichiometrically released. Sulphur concentrations released were lower than stoichiometrically expected (S/As In the light of these results, the steady-state dissolution rates were estimated by the release of arsenic at pH R arsenopyrite mol m − 2 s − 1 25 ° C = 10 − 7.41 ± 0.47 ⋅ a O 2 0.76 ± 0.11 ⋅ a H + − 0.12 ± 0.07 where aO2 and aH+ are the activities of hydrogen ions and dissolved oxygen, respectively and their exponents were estimated from multiple linear regression of the dissolution rates. Temperature increase from 25 to 70 °C yields an apparent activation energy for the arsenopyrite oxidation by dissolved oxygen of 18.5 ± 1.6 kJ mol− 1. At pH > 6, aqueous iron is mainly in the ferric form and is depleted as it precipitates as Fe-oxyhydroxide onto arsenopyrite surfaces, yielding Fe/As and Fe/S less than one; between pHs 7 and 9, iron depletion is complete, and sulphur released is more abundant than arsenic released, which is precipitated as As–O phases, as confirmed by MicroRaman spectroscopy. At pHs 6–9, iron-oxyhydroxide phases and arsenic oxide phases upon the arsenopyrite surface provide an effective layer that reduces diffusion of dissolved oxygen and arsenopyrite dissolution. As coating on the arsenopyrite surface becomes the rate-limiting step, the Shrinking Core Model (SCM) allows quantification of the surface dissolution rate, especially from data obtained where the effect of coating was still negligible. The SCM also allowed us to calculate the effective coefficient for oxygen diffusion through the coating, which can vary from 10− 17 to 1.5 · 10− 16 m2 s− 1. The formation of such a coating produced a decrease in arsenic and sulphur release over time and a final surface passivation.

Marcasite (FeS2), which is a polymorph of pyrite and isostructural with arsenopyrite, is common in Acid Rock Drainage and Acid Mine Drainage (ARD & AMD) and its oxidative dissolution increases the acidity of water. The effects of pH (1–3), input dissolved oxygen (6.3 × 10−6 − 2.7 × 10−4 mol L−1), and temperature (25–70 °C) on marcasite dissolution were studied by means of stirred and non-stirred flow-through experiments. Under these experimental conditions, marcasite dissolution was characterized by a systematic deficit in released sulphur ( e.g. , Fe/S ratio > 0.5). Consequently, dissolution rates were only based on the release of iron at steady state. At this pH range, the pH effect on dissolution rates is negligible. However, an increase in the dissolved oxygen or in temperature results in an increase in the dissolution rate. A rise in temperature yields an apparent activation energy of 14.7 ± 8.5 kJ mol−1. Under these conditions an empirical rate law that accounts for the effects of pH, dissolved oxygen and temperature on marcasite dissolution is proposed as: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathit{R}\_{marcasite}\ =\ 10^{{-}5.9\ {\pm}\ 1.5}\ \mathit{a}\_{O\_{2}(aq)}^{0.45\ {\pm}\ 0.11}\ \mathit{a}\_{H^{+}}^{{-}0.02\ {\pm}\ 0.06}e^{\frac{14.7\ {\pm}\ 8.5}{\mathit{RT}}}\] \end{document} where R marcasite is the marcasite dissolution rate (mol m −2 s−1), R is the gas constant (kJ mol−1 K−1), T is the temperature (K), and a O2(aq) and a H + are the activities of dissolved oxygen and hydrogen ions in solution, respectively. At high oxygen concentrations ( e.g. , atmospheric concentration) the reaction rate appears to be independent of dissolved oxygen (DO), becoming constant when DO ≥ 1 × 10−4 M. This behaviour, consistent with a mechanism involving equilibrium adsorption–desorption for DO can be described by a non-linear rate dependence on DO concentration. X-ray photoelectron spectroscopic (XPS) analyses of the marcasite surface before and after dissolution showed an enrichment of the reacted marcasite surfaces in sulphur (polysulphides and sulphates). These products, which formed during marcasite dissolution, do not exert a passivation effect on long-term marcasite dissolution rate.

Acid waters and sediments of the Tinto Santa Rosa acid stream (Iberian Pyritic Belt; SW, Spain) were analysed to determine the role of sedimentary phases in the behaviour of arsenic. Aqueous arsenic and iron concentrations decreased markedly from the adit mouth to 300 m downstream indicating iron minerals precipitation as well as arsenic sorption onto these newly-formed phases. This was confirmed by the high arsenic concentrations observed in bed-stream precipitates, which play a major role in controlling arsenic mobility. To unravel the complex nature of the AMD sediments a combination of techniques including X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), total solid digestions, X-Ray Fluorescence (XRF) and X-Ray Absorption Spectroscopy (XAS) were employed. Results showed that (1) arsenic was present predominantly in its pentavalent state; (2) upstream arsenic was sorbed onto the main phase, schwertmannite, whereas downstream it was chiefly associated with goethite and jarosite; and (3) changes in arsenic speciation with depth were observed in the consolidated terrace sediments, where arsenic appeared primarily associated with schwertmannite in the upper part of the terraces, but with goethite at depth. Arsenic mobilization was controlled by sorption onto newly formed precipitates (schwertmannite, goethite and jarosite), causing natural arsenic attenuation.

Geological storage of carbon dioxide is presently considered to be one of the main strategies to mitigate the impact of the emissions of this gas on global warming. Among the various alternatives considered for CO 2 geological storage, one of the main geological candidates for hosting injected CO 2 in the long term are deep porous reservoir rock formations saturated with brackish or saline solutions. Although valuable information on the expected hydrogeochemical processes involved in the CO 2 storage in such deep saline aquifers can be obtained in laboratory or modeling studies, the only direct source of information about the long-term behavior of geological storages for CO 2 in deep aquifers is natural analogues. In this work, a classical and simple geochemical methodology is successfully applied to the study of the features and hydrogeochemical processes determining the evolution of a Spanish thermal system (the Alhama–Jaraba complex), which can be considered as a natural analogue for deep geological CO 2 storage in carbonate rocks. The geology, structure, depth and hydrogeochemistry of the Alhama–Jaraba thermal system are very similar to the expected features of a potential CO 2 reservoir in carbonate materials. The processes determining the hydrogeochemical evolution in the Alhama–Jaraba thermal system have been successfully identified and quantified with the assistance of ion–ion plots, speciation–solubility calculations and mass-balance calculations. Furthermore, the feasibility of the proposed conceptual hydrogeochemical model for this system has been verified by using reaction-path calculations. Mass-balance calculation results have indicated that the observed hydrogeochemical evolution between springs is mainly due to halite dissolution and dedolomitization triggered by gypsum or anhydrite dissolution. CO 2 (g) mass transfer has been estimated to be negligible, which suggests that the main processes responsible for the variation in the TIC and the CO 2 (g) pressure during deep circulation are dissolution and precipitation reactions for carbonate minerals. All the processes identified in the Alhama–Jaraba thermal system are relevant for the long-term evolution of a deep CO 2 storage site hosted by carbonate rocks. As shown in this study, the application of classical geochemical tools provides an excellent starting point for understanding the behavior of prospective storage systems. Moreover, the existence of dedolomitization is very relevant for the hydraulic properties of carbonate aquifers potentially used for CO 2 geological storage because of the effects on porosity and, therefore, permeability during the long-term evolution of such systems. Furthermore, dedolomitization may represent a mechanism of mineral trapping for CO 2 sequestration under certain conditions.

Schwertmannite (Fe(8)O(8)(OH)(5.5)(SO(4))(1.25)), jarosite (KFe(3)(SO(4))(2)(OH)(6)) and goethite (FeOOH) control natural attenuation of arsenic in acid mine drainage (AMD) impacted areas. Batch experiments were conducted to examine the sorption capacity of synthetic goethite and synthetic jarosite at highly acidic pH (1.5-2.5), at two ionic strengths (0.02-0.15 mol dm(-3), NaCl) and at sulphate concentrations in the range of 5 x 10(-3) to 2.8 x 10(-1) mol dm(-3). In the absence of competitive effects of other anions, K-jarosite presents better removal efficiency than goethite for As(V). The maximum sorption capacity is estimated to be 1.2 x 10(-4) and 7.0 x 10(-6)mol m(-2) for jarosite and goethite, respectively, under similar experimental conditions. The variation of arsenic sorbed on goethite as a function of the equilibrium arsenic concentration in solution fits a non-competitive Langmuir isotherm. In the case of K-jarosite, sorption data could not fit a Langmuir or Freundlich isotherm since sulphate-arsenate anion exchange is probably the sorption mechanism. Ionic strength and pH have little effect on the sorption capacity of goethite and jarosite in the small range of pH studied. The presence of sulphate, which is the main anion in AMD natural systems, has a negative effect on arsenic removal since sulphate competes with arsenate for surface sorption sites. Moreover, mobilization of arsenic in the transformation of schwertmannite to jarosite or goethite at pH 2-3 is proposed since the sorption capacities of goethite and K-jarosite are considerably lower than those reported for schwertmannite.

In this project, we combined molecular genetic, spectroscopic, and microscopic techniques with kinetic and reactive transport studies to describe and quantify biotic and abiotic mechanisms underlying anaerobic, nitrate-dependent U(IV) and Fe(II) oxidation, which influences the long-term efficacy of in situ reductive immobilization of uranium at DOE sites. In these studies, Thiobacillus denitrificans, an autotrophic bacterium that catalyzes anaerobic U(IV) and Fe(II) oxidation, was used to examine coupled oxidation-reduction processes under either biotic (enzymatic) or abiotic conditions in batch and column experiments with biogenically produced UIVO2(s). Synthesis and quantitative analysis of coupled chemical and transport processes were done with the reactive transport modeling code Crunchflow. Research focused on identifying the primary redox proteins that catalyze metal oxidation, environmental factors that influence protein expression, and molecular-scale geochemical factors that control the rates of biotic and abiotic oxidation.

This study provides geochemical data with the aim of identifying and quantifying the main processes occurring in an Acid Mine Drainage (AMD) affected estuary. With that purpose, water samples of the Huelva estuary were collected during a tidal half-cycle and ion–ion plots and geochemical modeling were performed to obtain a general conceptual model. Modeling results indicated that the main processes responsible for the hydrochemical evolution of the waters are: (i) the mixing of acid fluvial water with alkaline ocean water; (ii) precipitation of Fe oxyhydroxysulfates (schwertmannite) and hydroxides (ferrihydrite); (iii) precipitation of Al hydroxysulfates (jurbanite) and hydroxides (amorphous Al(OH) 3 ); (iv) dissolution of calcite; and (v) dissolution of gypsum. All these processes, thermodynamically feasible in the light of their calculated saturation states, were quantified by mass-balance calculations and validated by reaction-path calculations. In addition, sorption processes were deduced by the non-conservative behavior of some elements (e.g., Cu and Zn).

Reactive transport modelling of the main processes related to the arsenic natural attenuation observed in the acid mine drainage (AMD) impacted stream of Tinto Santa Rosa (SW Spain) was performed. Despite the simplicity of the kinetic expressions used to deal with arsenic attenuation processes, the model reproduced succesfully the major chemical trends observed along the acid discharge. Results indicated that the rate of ferrous iron oxidation was similar to the one obtained in earlier field studies in which microbial catalysis is reported to occur. With regard to the scaled arsenic oxidation rate, it is one order of magnitude faster than the values obtained under laboratory conditions suggesting the existence of a catalytic agent in the natural system. Schwertmannite precipitation rate, which was represented by a simple kinetic expression relying on Fe(III) and pH, was in the range calculated for other AMD impacted sites. Finally, the obtained distribution coefficients used for representing arsenic sorption onto Fe(III) precipitates were lower than those deduced from reported laboratory data. This discrepancy is attributed to a decrease in the schwertmannite arsenate sorption capacity as sulphate increases in the solution.

Peat layers within alluvial sediments are considered effective arsenic (As) sinks under reducing conditions due to the binding of As(III) to thiol groups in natural organic matter (NOM) and the formation of As-bearing sulfide phases. However, their possible role as sources of As for anoxic groundwaters remains unexplored. Here, we perform laboratory experiments to provide evidence for the role of a sediment peat layer in releasing As. Our results show that the peat layer, deposited about 8,000 years ago in a paleomangrove environment in the nascent Mekong Delta, could be a source of As to porewater under reducing conditions. X-ray absorption spectroscopy (XAS) analysis of the peat confirmed that As was bound to NOM thiol groups and incorporated into pyrite. Nitrate was detected in peat layer porewater, and flow-through and batch experiments evidenced the release of As from NOM and pyrite in the presence of nitrate. Based on poisoning experiments, we propose that the microbially mediated oxidation of arsenic-rich pyrite and organic matter coupled to nitrate reduction releases arsenic from this peat. Although peat layers have been proposed as As sinks in earlier studies, we show here their potential to release depositional- and/or diagenetically-accumulated As.