Partitioning and Mobility of Chromium in Iron-Rich Laterites from an Optimized Sequential Extraction Procedure

Chromium (Cr) leached from iron (Fe) (oxyhydr)oxide-rich tropical laterites can substantially impact downstream groundwater, ecosystems, and human health. However, its partitioning into mineral hosts, its binding, oxidation state, and potential release are poorly defined. This is in part due to the current lack of well-designed and validated Cr-specific sequential extraction procedures (SEPs) for laterites. To fill this gap, we have (i) first optimized a Cr SEP for Fe (oxyhydr)oxide-rich laterites using synthetic and natural Cr-bearing minerals and laterite references, (ii) used a complementary suite of techniques and critically evaluated existing non-laterite and non-Cr-optimized SEPs, compared to our optimized SEP, and (iii) confirmed the efficiency of our new SEP through analyses of laterites from the Philippines. Our results show that other SEPs inadequately leach Cr host phases and underestimate the Cr fractions. Our SEP recovered up to seven times higher Cr contents because it (a) more efficiently dissolves metal-substituted Fe phases, (b) quantitatively extracts adsorbed Cr, and (c) prevents overestimation of organic Cr in laterites. With this new SEP, we can estimate the mineral-specific Cr fractionation in Fe-rich tropical soils more quantitatively and thus improve our knowledge of the potential environmental impacts of Cr from lateritic areas

Formation and modern state of ecosystem in Tolmachevskoye reservoir (Kamchatka) and the acclimatized there population of kokanee ( <i>Oncorhynchus nerka kennerlyi</i>)

Evolution of local ecosystem in Tolmachevskoye reservoir and changes in its artificial population of kokanee salmon are traced on the data of authors’ observations in 2009-2013 and previous archival and cited data. Decreasing of inorganic phosphorous, nitrogen, and bioavailable iron is detected in the water against a background of water warming. As the result, phytoplankton production and abundance decrease, its species composition becomes simpler, chlorophyll а concentration becomes lower. Zooplankton abundance is stable (1-2 g/m3), as before the reservoir appearance, but species structure of plankton crustaceans is changed, and the copepods Cyclops scutifer , small cladocerans Holopedium gibberum, Bosmina longirostris , and Daphnia ( Daphnia ) cristata group prevail recently. The salmon food spectrum is wide and includes a lot of unedible fractions as wood chips and plant detritus; coefficient of consumption is high - these factors indicate a deficiency of forage resources for fish. The fish condition indicates a chronic malnutrition. The state of female gonads is satisfactory, without any visible pathology, as in 2003-2007, but the oocytes resorption goes concurrently with maturation of gonads that is a sign of hard competition for the food within the kokanee population. The current stable state of the population differs from preceded stages of its development by lower stock add smaller size of fish, the age of spawning is now 4-7 years. Among other freshwater fish, the kokanee salmon is distinguished by high content of ω-3 polyunsaturated fatty acids. Artificial populations in Kamchatka could be considered as a resource for both amateur or sport fishing and commercial aquaculture

Natural hydrogen the fuel of the 21

Much has been learned about natural hydrogen (H2) seepages and accumulation, but present knowledge of hydrogen behavior in the crust is so limited that it is not yet possible to consider exploitation of this resources. Hydrogen targeting requires a shift in the long-standing paradigms that drive oil and gas exploration. This paper describes the foundation of an integrated source-to-sink view of the hydrogen cycle, and propose preliminary practical guidelines for hydrogen exploration

Structure, stability and geochemical role of palladium chloride complexes in hydrothermal fluids

International audienceIn situ X-ray absorption spectroscopy (XANES and EXAFS) was applied to study palladium speciation in model (Na,Li)Cl-HCl-HNO3-HClO4-H2O solutions and to measure the solubility of PdO(s) and PdS(s) in NaCl-HCl-H2O solutions up to 450 degrees C and 600 bar. In HNO3 and HClO4 solutions, the square planar Pd(H2O)(4)(2+) cation with an average Pd-O distance of 2.00 +/- 0.01 angstrom is the dominant form at 30-100 degrees C. At T > 100 degrees C, this cation undergoes hydrolysis resulting in Pd precipitation. In (Na,Li)Cl-HCl solutions Pd forms square planar PdCln(H2O)(4-n)(2-n) complexes with Pd-O and Pd-Cl distances of 2.00-2.10 and 2.26-2.31 angstrom, respectively. At 30-250 degrees C our data are consistent with a mixture of PdCl(H2O)(3)(+), PdCl2(H2O)(2 (aq))(0) , PdCl3(H2O)(-), and PdCl42-, but at T > 250 degrees C PdCl3(H2O)(-) and PdCl42- become dominant over a wide range of chloride concentration, from similar to 0.03 to at least 9 mol/kg H2O. XANES and EXAFS analyses of these species distribution allowed derivation of the stability constant of the reaction PdCl3- + Cl- = PdCl42- from 300 to 450 degrees C and 600 bar. These results, coupled with in situ PdO(s) and PdS(s) solubility measurements of this study and a revision of thermodynamic data for these solid phases, allowed generation of a self-consistent thermodynamic data set of the system PdS(s)-PdO(s) PdCl3―PdCl42-. Our data indicate that Pd can be significantly mobilized as chloride complexes only in sulfide-free oxidizing geological settings (e. g., Chudnoe deposits in Russia, Waterberg deposits in South Africa, Jacutinga-type deposits in Brazil). By contrast, at typical pH (4-8), chlorinity (<10-15 wt% NaCl) and H2S contents (0.001-0.1 wt%) of most hydrothermal fluids, the absolute concentration of Pd-Cl complexes is too low to explain Pd enrichment in volcanogenic massive sulfide, modern seafloor sulfide, and porphyry Cu-Au-Mo deposits. Complexes with S-bearing ligands, very likely other than H2S/HS-, such as S-3(-) and other polysulfide anions, are required to account for Pd hydrothermal mobility in these geological settings

Tungsten (VI) speciation in hydrothermal solutions up to 400°C as revealed by in-situ spectroscopy

International audienceTungsten (VI) speciation in hydrothermal solutions is investigated through in-situ Raman spectroscopy coupled with the fused silica glass capillary technique at temperatures up to 400 °C. The effect of temperature, pH, chlorinity and carbonate speciation are evaluated. At all investigated temperatures, the tungstate ion WO4 2-(927 cm-1) is the only W species in solution at pH > 10. At a given pH, the presence of dissolved carbonates and chloride does not affect the tungsten speciation. Tungsten polymers remain stable up to 400 °C under acidic to circum-neutral pH conditions and total tungsten concentration above 0.01 mol kgH2O-1. Among the three observed polymers, namely [W7O24] 6-(paratungstate-A, ~ 960 cm-1), [W10O32] 4-(tungstate-Y, ~ 970 cm-1), and α-[H2W12O40] 6-(α-metatungstate, ~ 990 cm-1), only the hepta-and dodeca-tungstate are stable at elevated temperature. Combined with revised literature data, these results allow the thermodynamic stability constants of these W polymers to be constrained, enabling quantitative predictions of their relative abundance at temperatures up to 300 °C. These predictions suggest that W polymerization occurs under hydrothermal conditions even at low W concentration (down to 10-5 mol•kgH2O-1) under acidic conditions. These observations imply that the currently available geochemical models on W transport and deposition in deep and hot geological fluids need to be revised

In situ X-ray absorption spectroscopy study of zinc and cadmium transport by S-rich fluids

This experimental and theoretical study is aimed at better quantifying the transport of Zn and Cd by hydrothermal fluids rich in sulfur by using in-situ X-ray absorption spectroscopy (XAS). We measured simultaneously fluorescence and transmission spectra of dissolved metal in NaHS-H2S, K2S2O3, Na2S2O3, K2S2O3-HCl aqueous solutions at 250-350°C and 600 bar. Here the first experimental results are presented. Possible sulfur speciation models for Zn and Cd are briefly discussed

Direct measurement of CO<SUB>2</SUB> solubility and pH in NaCl hydrothermal solutions by combining in-situ potentiometry and Raman spectroscopy up to 280 °C and 150 bar

International audienceThe in-situ monitoring of aqueous solution chemistry at elevated temperatures and pressures is a major challenge in geochemistry. Here, we combined for the first time in-situ Raman spectroscopy for concentration measurements and potentiometry for pH measurement in a single hydrothermal cell equipped with sampling systems and operating under controlled conditions of temperature and pressure. Dissolved CO2 concentration and pH were measured at temperatures up to 280 °C and pressures up to 150 bar in the H2O-CO2 and H2O-CO2-NaCl systems. A Pitzer specific-ion-interaction aqueous model was developed and confirmed the accuracy and consistency of the measurements, at least up to 250 °C. The revised Pitzer parameters for the H2O-CO2-NaCl system were formatted for the Phreeqc geochemical software. Significant changes with respect to the Pitzer.dat database currently associated with Phreeqc were observed. The new model parameters are now available for further applications. The Raman and pH probes tested here may also be applied to field monitoring of hydrothermal springs, geothermal wells, and oil and gas boreholes

The role of S-3(-) ion in thermochemical sulphate reduction: Geological and geochemical implications

International audienceThermochemical sulphate reduction (TSR) plays a crucial role in the global sulphur cycle in the Earth's crust, and may affect current and past sulphur isotopic records. However, the extrapolation of experimental reaction rates measured at high temperature (above 200 degrees C) towards lower temperatures, as well as the interpretation of the sulphur isotopic fractionation recorded in natural samples, require an accurate description of the elementary steps controlling these reactions. We addressed this question through dedicated experiments. Based on in situ Raman spectroscopy measurements, we show that the trisulphur ion S-3(-) is the dominant intermediate sulphur valence species involved in abiogenic sulphate reduction processes initiated by H2S, over a wide range of temperature (100-350 degrees C) and solution compositions, whatever the electron donor considered. The in situ spectroscopic data reported here unambiguously demonstrate the presence of S-3(-) at temperatures as low as 100 degrees C. The presence of S-3(-) is critical to achieve rapid sulphate reduction, especially at low temperature. We propose that any dissolved constituent which decreases the dielectric constant of water, or which yields favourable S-3(-) coordination, will stabilise the trisulphur ion (thus promoting TSR) at T and pH conditions that are less extreme than previously thought. The importance of S in these processes should also be taken into account when discussing the mass-independent sulphur isotopic compositions recorded in natural and/or experimental TSR-related samples

Platinum group elements and sulfur in hydrothermal fluids: a love story told by in situ spectroscopy, molecular dynamics, and thermodynamics

International audienceKnowledge of Platinum Group Elements (PGE) speciation in hydrothermal fluids is essential to better understanding the transport of these metals in the Earth's crust and to identifying potential hydrothermal deposits where PGE may be present in economic grades. Existing data on aqueous chloride, sulfate, and hydroxide complexes of PGEs indicate extremely low metal contents (< ppt to ppb) in fluids from most geological settings [1-3] that cannot explain multiple instances of PGE concentration and mobilization in hydrothermal systems, thus appealing to an important role of sulfide (HS–) and, potentially, trisulfur (S3–) ligands [4] in PGE transport. To quantify the effect of sulfur on the solubility of platinum and palladium in hydrothermal fluids, we combined in situ solubility (Fig. 1A) and X-ray absorption spectroscopy (XAS; Fig. 1B) measurements with molecular dynamics (MD; Fig. 1C) and thermodynamic (TD) simulations [5]. Our results show that two main complexes transport these metals in hydrothermal fluids across a wide pH range (4–8), temperature and pressure (up to at least 350 °C and 1000 bar): Pt(HS)42– and Pd(HS)42– in H2S/HS– solutions, Pt(HS)2(S3)22– and Pd(HS)2(S3)22– in H2S/SO42–/S3– solutions. The role of the trisulfur ion in PGE hydrothermal transport thus appears to be particularly crucial [5,6], with solubilities (10s ppm Pt, Pd) up to 10,000 times higher than those of the ‘traditional’ complexes with H2S/HS–. Our results offer perspectives for the exploration of new PGE resources, their extraction and recycling, and hydrothermal synthesis of PGE-based nanomaterials. Future research on other metals in fluid-mineral systems will benefit from the combined approach implemented in this study, as well as an open-access database of XAS spectra of Pt reference compounds acquired in our work [7].[1] Bazarkina et al. (2014), GCA 146, 107–131; [2] Kokh et al. (2017), GCA 197, 433–466; [3] Tagirov et al. (2019), GCA 254, 86–101; [4] Pokrovski and Dubessy (2015), EPSL 411, 298–309; [5] Laskar et al. (2022), GCA 336, 407–422; [6] Pokrovski et al. (2021), PNAS 118, e2109768118; [7] Laskar et al. (2022), Minerals 12, 1602