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The oxidation states of dissolved iron (Fe) and sulfur (S) in silicate melts were studied experimentally by heating Hawaiian olivines containing S-bearing melt inclusions (MIs) in a 1 atm gas-mixing furnace. The MIs are closed with respect to S and C, but equilibrate with the oxygen fugacity (ƒO 2) and water fugacity (ƒH 2 O) of the furnace. We also conducted experiments using a S-free synthetic Hawaiian MI composition held on Pt-loops. Experiments were run at 1225 °C for 8–96 hr in H 2 -CO 2 gas mixtures corresponding to ƒO 2 values that varied over seven orders of magnitude and then drop-quenched into water. Time-series experiments show that MI Fe3+/Fe2+ and S6+/S2− ratios reached constant values within ∼24 hr. The H 2 O contents of the Pt-loop glasses and the dehydrated MIs overlap and are proportional to the f H 2 O in the furnace gas. The slope of log 10 (Fe3+/Fe2+) vs. log 10 ƒO 2 is 0.25±0.02(2σ) for the Pt-loop glasses and 0.23±0.01(2σ) for the quenched MIs; the slopes of the two sets of experiments are consistent with equilibrium having been reached via the reaction FeO + ¼ O 2 = FeO 1.5. S-bearing quenched MI glasses have Fe3+/Fe2+ ratios that are systematically low compared to ratios measured in the S-free Pt-loop glasses run at the same T and ƒO 2 ; this may reflect an effect of S on Fe3+/Fe2+, although further experiments are needed to explore this possibility. S6+/S2− ratios in the experimental melt inclusions vary systematically as a function of ƒO 2 , and the measured ratios agree with independent calculations of S oxidation state in basaltic melts. After being held at 1225 °C for 24 or 48 hr, a subset of olivine-hosted MIs were cooled at rates of 1700–40,000 °C/hr and quenched at temperatures between 900 °C and 1150 °C to explore whether Fe and S speciation are modified during cooling. Fe and S speciation in isothermal and cooled MIs held initially at the same ƒO 2 overlap within uncertainty, suggesting that the T dependence of the homogeneous reaction 8Fe3+ + S2− = 8Fe2+ + S6+ is small and/or its kinetics are sluggish relative to cooling rates required to quench basaltic liquids to glasses. The centers of unheated, naturally quenched MIs from Papakōlea, Hawaii with syneruptive cooling rates ranging from ∼50 to 12,000 °C/hr were also measured for Fe and S speciation. MIs that experienced the lowest cooling rates have higher Fe3+/Fe2+ and S6+/S2− ratios relative to rapidly cooled inclusions of comparable size. The increase in Fe3+/Fe2+ is due to olivine crystallization on the inclusion walls during cooling and diffusion of the resulting oxidized boundary layer into MI interiors. Diffusive modification is minimized in MIs that have been rapidly cooled and/or are sufficiently large such that the oxidized boundary layer is restricted to a narrow region adjacent to the inclusion walls. Our experiments and their comparison to naturally quenched MIs show that measurements of Fe3+/Fe2+ and S6+/S2− ratios in quenched basaltic glasses and in the centers of rapidly cooled and/or large MIs are likely to be representative of the redox state in the melt at magmatic temperatures prior to quenching to glass. [ABSTRACT FROM AUTHOR]

Hydration by H 2 O clusters has been shown to be an important means of transporting metal complexes in hydrothermal vapours and low-density supercritical aqueous fluids. However, the effect of hydration on the transport of relatively volatile metal complexes such as those involving bismuth and chloride has not been evaluated. This effect is important to evaluate because bismuth is a key pathfinder element for gold in many intrusion-related gold ore-forming systems, where low-density supercritical aqueous fluids and vapour may play an important role in metal transport. Here, we present the results of experiments investigating the solubility of BiOCl(s) and the speciation of bismuth in HCl-bearing aqueous vapour and low-density supercritical aqueous fluids at temperatures between 250 and 400 °C and pressures of 5 to 296 bar. Two gaseous bismuth species formed in the HCl-bearing fluids (X (HCl) > 0.0005), namely BiCl 3 (g) and BiCl 3 (H 2 O) n (g). Our data clearly show that BiCl 3 (g) is highly volatile at low water fugacity (between 30 and 110 bar) and acts as an unhydrated gas molecule. At higher water fugacity, the gaseous BiCl 3 (g) reacts with water to form hydrated BiCl 3 (H 2 O) n (g) species. Significantly, the stability of hydrated bismuth chloride species, BiCl 3 (H 2 O) n (g), with low hydration numbers is lower than that of the unhydrated species, BiCl 3 (g), resulting in a decrease in bismuth solubility with increasing f (H 2 O). However, at a water fugacity great than 30–110 bar, that increases with increasing temperature, the hydration number and the solubility of hydrated species BiCl 3 (H 2 O) n (g) increase with increasing water fugacity or the density of the fluid. Consequently, unhydrated gaseous bismuth species increase in importance with increasing temperature and become dominant at temperatures > 450 °C in fluids that have low density (<0.10 g/cm3). Hydrated bismuth chloride species are dominant in fluids having higher density (0.10–0.34 g/cm3). The overarching conclusion of this study is that aqueous low-density supercritical fluids, which exsolve from magmas to form intrusion-related gold deposits, can transport bismuth in concentrations (up to hundreds ppm) sufficient to form economic deposits. [ABSTRACT FROM AUTHOR]

Understanding the effects of water on the geochemical behavior of carbonate melts in the Earth's interior requires thorough knowledge of their speciation and dynamical properties. However, the mechanisms of water dissolution in carbonate melts are unknown due to experimental challenges. Here we used ab-initio molecular dynamics simulations as a complementary approach to study the speciation and element diffusivities of MgCO 3 melt containing 10 wt.% water at conditions up to 1765 K and ∼3.9 GPa. The simulations revealed a diverse COH speciation including CO 3 2 − , HCO 3 − , H 2 O, OH−, and CO 2. The structural analysis was complemented by the calculation of vibrational density of states, and anharmonic infrared and Raman spectra. Good agreement between theoretical Raman spectra and experimental spectra from the literature supports the finding that HCO 3 − is an important species in hydrous MgCO 3 melt. Mean square displacement analysis revealed rapid diffusional motion of hydrogen. However, the contribution of hydrogen to the total ionic conductivity is estimated to be relatively low compared to the already very high conductivity of anhydrous carbonate melts. In addition to fundamental insights into molecular structure and transport properties, our results provide a valuable basis for the development of thermodynamic speciation and solubility models, which are essential for a quantitative description of hydrous carbonate melts in the deep Earth. [ABSTRACT FROM AUTHOR]

Organic complexation of iron plays a crucial role in preventing its precipitation, facilitating its transport, and modulating its reactivity and bioavailability in natural waters. Although humic substances (HS) complexes serve as the primary source of terrestrial iron reaching ocean waters, the transition from Fe-HS species to other forms of organic complexation with ocean autochthonous ligands has not yet been properly described. Taking advantage of the electrolability of Fe-HS complexes, we monitored the ligand exchange of iron-saturated Suwannee River fulvic and humic acids (SRFA and SRHA) after the addition of desferrioxamine B (DFOB) and other ligands for comparison. We observed that Fe-HS concentrations gradually decreased until reaching an apparent steady state, typical of a reversible reaction within 1 to 15 h. The dissociation kinetics and species partitioning of Fe-SRHS complexes at the equilibrium showed some features contrary to the current paradigm of HS iron complexation. The affinity of SRFA to bind iron is close to that of DFOB and for SRHA is even higher. The heterogeneity of the HS iron binding groups was confirmed, although experiments in NaCl solutions revealed that in seawater it is substantially caused by the interaction of major divalent ions. The different dissociation kinetics of Fe-SRHS complexes obtained with different competing ligands and the absence of Fe-DFOB dissociation in the presence of iron-free SRFA indicate an intimate associative mechanism of ligand exchange, with the presence of a ternary complex (SRHS-Fe-DFOB) that does not form if the departing complex is Fe-DFOB. We hypothesize that at the concentrations of HS and siderophores found in the open ocean, iron ligand exchange is limited and organic iron speciation will be close to being regulated on a "first come, first served" basis. Experiments conducted under saturation of all iron complexes support the formation of a permanent concentration of a ternary complex. Our findings show the real complexity of cation-ligand interactions in seawater, with implications for the interpretation of recent chromatographic and electrochemical measurements and for the understanding of iron partitioning in the presence of ubiquitous HS. [ABSTRACT FROM AUTHOR]

As critical transition zones between the land and the sea, estuaries are not only hotspots of hydrogeochemical and microbial processes/reactions, but also play a vital role in processing and transferring terrestrial fluxes of metals and nutrients to the sea. This study focused on three estuaries in the Gulf of Bothnia. All of them experience frequent inputs of acidic and Mn/metal-rich creek waters due to flushing of acid sulfate soils that are widespread in the creekś catchments. Analyzing existing long-term water chemistry data revealed a strong seasonal variation of Mn loads, with the highest values in spring (after snow melt) and autumn (after heavy rains). We sampled surface waters, suspended particulate matter (SPM), and sediments from the estuarine mixing zones and determined the loads and solid-phase speciation of Mn as well as the composition and metabolic potentials of microbial communities. The results showed that the removal, cycling, and lateral transport of Mn were governed by similar phases and processes in the three estuaries. Manganese X-ray absorption spectroscopy data of the SPM suggested that the removal of Mn was regulated by silicates (e.g., biotite), organically complexed Mn(II), and MnOx (dominated by groutite and phyllomanganates). While the fractional amounts of silicate-bound Mn(II) were overall low and constant throughout the estuaries, MnOx was strongly correlated with the Mn loadings of the SPM and thus the main vector for the removal of Mn in the central and outer parts of the estuaries, along with organically complexed Mn(II). Down estuary, both the fractional amounts and average Mn oxidation state of the MnOx phases increased with (i) the total Mn loads on the SPM samples and (ii) the relative abundances of several potential Mn-oxidizing bacteria (Flavobacterium, Caulobacter, Mycobacterium, and Pedobacter) in the surface waters. These features collectively suggested that the oxidation of Mn, probably mediated by the potential Mn-oxidizing microorganisms, became more extensive and complete towards the central and outer parts of the estuaries. At two sites in the central parts of one estuary, abundant phyllomanganates occurred in the surface sediments, but were converted to surface-sorbed Mn(II) phases at deeper layers (>3–4 cm). The occurrence of phyllomanganates may have suppressed the reduction of sulfate in the surface sediments, pushing down the methane sulfate transition zone that is typically shallow in estuarine sediments. At the outermost site in the estuary, deposited MnOx were reduced immediately at the water–sediment interface and converted most likely to Mn carbonate. The mobile Mn species produced by the Mn reduction processes (e.g., aqueous Mn(II) and ligand complexed Mn(III)) could partly diffuse into the overlying waters and, together with the estuarine Mn loads carried by the surface waters, transfer large amounts of reactive Mn into open coastal areas and subsequently contribute to Mn shuttling and inter-linked biogeochemical processes over the seafloor. Given the widespread occurrence of acid sulfate soils and other sulfidic geological materials on many coastal plains worldwide, the identified Mn attenuation and transport mechanisms are relevant for many estuaries globally. [ABSTRACT FROM AUTHOR]

Chromium is an important redox-active element, with significant Cr isotope fractionations driven by redox transformations in the natural environment and quantified through laboratory experiments. Recent work has demonstrated that non-redox-driven transformations can also produce small variations in Cr isotopes, but the role of these processes in controlling environmental Cr isotope variations remains unclear. The highly acidic but biologically diverse acid rock drainage system in Río Tinto, Spain (and affiliated rivers) provides a unique opportunity to examine how non-redox-related processes can substantially affect the isotopic composition of Cr in a natural system. Our results suggest that under conditions where Cr redox transformations are obviated, Cr cycling is largely controlled by formation of Fe-oxyhydroxides and Fe-hydroxysulfates, Cr adsorption, and Cr speciation, with little apparent Cr isotope fractionation between source materials, waters, and sediments. Moderate Cr isotope shifts, however, may be the result of non-redox processes, including adsorption to organic ligands and potentially biological uptake. These results add to a growing body of evidence for the importance of non-redox transformations in contributing to environmental Cr isotope variations, with implications for the use of chromium isotopes in studies of sedimentary Cr isotope records. In particular, this study is a validation of the potential for sulfide weathering in the Proterozoic to deliver relatively unfractionated Cr isotopes to the ocean, despite rising and sustained high oxygen levels. [ABSTRACT FROM AUTHOR]

The distribution of nitrogen in geologic systems is modulated by its partitioning between silicate (mineral and melt) and fluid phases. Under geologically applicable oxygen fugacity, pressure, and temperatures, nitrogen can be multiply-speciated, with N 2 coexisting with reduced nitride (N−3) species. Non-polar, neutral species, including N 2 , tend to concentrate in fluids, while charged nitride species have a greater propensity to concentrate in silicate phases. The stoichiometry of converting N 2 to single N atom nitride species implies that nitrogen speciation may depend on its concentration, and this leads to the hypothesis that the partitioning of nitrogen between silicate and fluid phases also depends on concentration, potentially biasing prior experimental work in doped systems and influencing the behavior of nitrogen in geologic systems. To test this hypothesis, we have completed a series high pressure (∼1.75 GPa, 800 °C) experiments that react minerals, melts, and fluids with variable nitrogen concentrations (3.1–17.1 wt% N). Our results imply order-of-magnitude-scale increases in mineral/melt and melt/fluid partitioning as nitrogen concentrations decrease within natural ranges. For example, decreasing the N concentration from 2500 to 2 ppm increases predicted D melt / f l u i d N values by over an order of magnitude at constant PT conditions. This means that loss of nitrogen from a degassing magma or dehydrating slab is a self-limiting process that becomes increasingly inefficient as nitrogen concentration falls. Despite this, nitrogen remains highly concentrated in the atmosphere, which receives N from fluids exsolved from slabs and magmas. To maintain a nitrogen-rich atmosphere we therefore suggest that warm and oxidizing conditions have prevailed over subduction zones because warm slabs dehydrate under lower pressures where nitrogen is more easily partitioned into fluids, and oxidizing conditions also promote nitrogen partitioning into fluids. Concentration-dependent partitioning of nitrogen will also serve to moderate any initial variations of N/K in slab materials upon dehydration, and this may help to explain the relatively uniform N/K ratio of MORB mantle. We supplement our nitrogen concentration experiments with a temperature series (1.5–2 GPa, 750–950 °C). Our temperature series data reveal that at high temperature nitrogen favors melts over fluids, while temperature has no resolvable effect of biotite-fluid partitioning. [ABSTRACT FROM AUTHOR]

Niobium (Nb) is one of the most immobile elements during supergene weathering, widely used for mass-balance calculations, despite elusive information on the mechanisms controlling its dynamics in the critical zone. Here, a multiscale approach, from weathering profile to atomic-scale, is developed to monitor Nb speciation along a thick (about 50 m) lateritic regolith formed over the Pitinga pluton (Amazonas, Brazil). In the A-type parent granite, Nb is mainly hosted in ilmenite (1.5–2 wt% Nb 2 O 5) and, to a lesser extent, in rutile (2–3 wt% Nb 2 O 5). A quantitative assessment of the average Nb speciation from the parent rock to the upper horizons has been carried out by combining spectroscopic and spatially-resolved chemical techniques. The contribution of Nb-bearing Ti oxides (2–6 wt% Nb 2 O 5) and Fe oxides (0.1–0.3 wt% Nb 2 O 5) in the average Nb speciation increases with the degree of weathering and reaches 80% in the most altered horizon. This unusual Nb speciation results from the nature of the primary Nb carriers, less resistant to weathering than common Nb ore minerals such as pyrochlore. X-ray absorption spectroscopy demonstrates that Nb released from the weathering of ilmenite substitutes for Fe and Ti in goethite and Ti oxides, respectively, providing atomic-scale evidence of the high affinity of Nb for these phases. Elemental mobility of Nb, Ta and Sn followed by geochemical mass-transfer calculations evidences lateral transport from Nb-Sn-enriched laterites developed over the surrounding Nb-enriched facies of the Pitinga granite to the studied profile. Elements such as Ti, Zr and Hf, often considered immobile, are leached or redistributed in the studied profile. Our results demonstrate Nb remobilization during intense weathering. Although Nb scavenging by secondary Fe and Ti oxides limits Nb mobility at the mineral scale, this work questions the unrestricted use of Nb as a chemical invariant during surficial alteration processes. [ABSTRACT FROM AUTHOR]

In the last several decades, microbial reduction of U(VI) to U(IV) has been applied as a method to remediate uranium contamination in situ. The U(VI) bioreduction kinetics and resulting U(IV) speciation are affected by various environmental factors, such as humic substances (HS) and Fe-bearing clay minerals. Previous studies have largely focused on their individual effects on U(VI) bioreduction. However, HS and Fe(III)-bearing clay minerals often co-exist at uranium contaminated sites, their joint effects may be different from the sum of their individual ones, due to either synergistic or antagonistic interactions between the two. In this work, we investigated the combined effects of HS and Fe(III)-bearing clay minerals (Fe-rich nontronite and Fe-poor montmorillonite) on U(VI) reduction by Shewanella putrefaciens and the resulting U(IV) speciation. HS alone exhibited an inhibitory effect on U(VI) bioreduction, likely because HS retained a fraction of electrons from microbial oxidation of lactate. The resulting U(IV) was in the form of aqueous U(IV)-HS complex. Clay minerals alone also inhibited U(VI) bioreduction because a fraction of reduced U(IV) was likely re-oxidized by structural Fe(III) and thus electrons were partially diverged from U(VI) to structural Fe(III). Bioreduced U(IV) was in a solid phase. However, in the presence of both HS and clay minerals, their combined effect of inhibition on U(VI) bioreduction was not a simple addition of the two individual ones. In the co-presence of clay minerals and a low concentration of HS (10 and 20 mg C/L), their combined inhibition effect on U(VI) bioreduction was stronger than that of HS but weaker than that of clays. The stronger inhibition than that of HS alone was due to electron diversion by clay minerals. The weaker inhibition than that of clays alone was because HS and/or Fe(III)-HS/Fe(II)-HS served as electron shuttles to accelerate the rate of Fe(III) reduction. These double shuttles decreased the ability of structural Fe(III) in clays to diverge electrons from U(VI) and thus partially negated the inhibitory effect of clay minerals. In the co-presence of clay minerals and a high concentration of HS (100 mg C/L), their combined effect on U(VI) bioreduction was initially similar to that in the low concentration of HS scenario. However, over extended time, clay minerals partially negated the inhibitory effect of HS, likely because clay minerals adsorbed a fraction of HS and thus lowered the ability of residual aqueous HS to retain electrons. Biogenic U(IV) was sequestered in solid in the presence of Fe-rich nontronite and HS but partially remained soluble in the presence of Fe-poor montmorillonite and HS. This study highlights the importance of the complex interaction between HS and Fe-bearing clays in bioreduction of U(VI) and speciation of the resulting U(IV). [ABSTRACT FROM AUTHOR]

Manganese speciation is a key to understanding the fate of contaminants, nutrients, and organic matter in soils. To date, quantification of Mn species in bulk soils has been performed mainly by sequential extraction methods and rarely supported by spectroscopic analysis. In order to obtain quantitative information on the Mn species inventory of soils, we investigated 46 soil horizons (<2-mm fraction, 45.1–2,280 mg/kg Mn) of nine typical Central European soils (Cambisols, Chernozems, Luvisols, Podzol, Stagnosol) by chemical Mn analyses and Mn K-edge X-ray absorption spectroscopy, and related speciation results to major soil properties. Amounts of Mn2+, Mn3+, and Mn4+ and the average oxidation state of Mn were evaluated by linear combination fitting (LCF) of X-ray absorption near edge structure (XANES) spectra. Additionally, we used extended X-ray absorption fine structure (EXAFS) spectroscopy to identify and quantify major Mn species. For this, EXAFS spectra of 20 organic and mineral soil samples from five soils (Cambisols, Chernozem, Luvisol) were analyzed by LCF and shell fitting. XANES analyses revealed a high Mn redox variability in organic surface layers, with Mn2+ being most abundant (≤100%, x ¯ = 54%), followed by Mn3+ (≤80%, x ¯ = 32%) and Mn4+ (≤55%, x ¯ = 14%). Mineral soil horizons contained significantly less Mn2+ (≤56%, x ¯ = 23%), about equal quantities of Mn3+ (≤68%, x ¯ = 31%), and were enriched in Mn4+ (≤89%, x ¯ = 46%). EXAFS analyses implied the presence of six major Mn species groups: manganates, organically complexed Mn, Mn(III) oxyhydroxides, silicate-bound Mn, Mn oxides without tunnel- or layer structure, and physisorbed Mn. In litter horizons, Mn was mainly present in organic complexes (58–91%, x ¯ = 78%) and as physisorbed Mn (≤15%), but individual horizons also comprised manganates, Mn(III) oxyhydroxides, and silicate-bound Mn. Manganates, likely mixtures of phyllomanganates with hexagonal layer symmetry and tectomanganates, dominated in all mineral soil horizons (37–94%, x ¯ = 67%). Correlation analysis showed that manganates dissolve completely during dithionite-citrate and acid ammonium oxalate extractions, and suggested that Mn4+-rich manganates preferentially form under less acidic soil conditions, partly by oxidation of organically complexed Mn(II), and that they are enriched in the soil clay fraction. Mineral soil horizons also contained minor quantities of organically complexed Mn (≤39%, x ¯ = 11%), silicate-bound Mn (≤30%, x ¯ = 8%), Mn(III) oxyhydroxides (≤37%, x ¯ = 7%), Mn oxides without tunnel- or layer structure (≤18%, x ¯ = 5%), and physisorbed Mn (≤14%, x ¯ <1%). The detection of Mn(III) oxyhydroxides such as feitknechtite (β-MnOOH) or groutite (α-MnOOH) as well as the spinel hausmannite (Mn 3 O 4) in acidic soils is remarkable, since their formation is normally linked to neutral or alkaline pH conditions. Minor contributions of silicate-bound Mn indicate the release of Mn from primary minerals already at early stages of soil formation, and low concentrations of physisorbed Mn suggest that exchangeable Mn is rapidly converted into manganates in oxic soils. The predominance of manganates in mineral soils has far-reaching implications for the functioning of soils and biogeochemical element cycles, as these minerals play an important role in metal binding, plant nutrition, and redox-related processes. [ABSTRACT FROM AUTHOR]

Examining vanadium (V) geochemistry in organic-rich rocks might provide new insights into the redox evolution of paleo ocean and atmosphere through Earth's history. Previous work, mainly based on experiments and thermodynamic predictions, suggested that V is mainly bound to oxygen (O) atoms and/or O N (nitrogen) groups with oxidation states ranging from +III to +V, when removed from the water column. This conceptual model represented our understanding of the burial processes of V in sedimentary archives. However, until our study, the speciation of V had not been investigated in ancient sediments. For the first time, micro-focused X-ray Absorption Near Edge Structure (µ-XANES) analysis is applied to characterize V speciation in the late Cambrian – Early Ordovician, organic-rich Alum Shale of the Scandinavian region. The result shows the presence of a new V(+IV)–S structure that largely dominates the V speciation (>80%), subordinated by a V(+III)–O structure (<20%) in our samples deposited under euxinic conditions. The V(+IV)–S/∑V ratio seems to be influenced by the intensity of euxinic conditions. We suggest that the new V(+IV)–S structure may have been formed under strongly sulfidic conditions. Organic matter is most likely to be the dominant host phase for this newly identified species. Additionally, we propose an updated model describing the processes involved during the burial of V under a wide range of redox conditions. This model shows that the V speciation has the potential to provide a more nuanced picture of the redox conditions that prevailed at the time of deposition. [ABSTRACT FROM AUTHOR]

Knowledge of the chemical speciation of platinum and the solubility of Pt-bearing minerals in hydrothermal fluids is required to assess Pt transport, remobilization and concentration in the Earth's crust. In this study, we combined PtS(s) solubility measurements in a hydrothermal reactor allowing fluid sampling, in situ X-ray absorption spectroscopy, and first-principles molecular dynamics simulations to systematically investigate the structure, composition and stability of Pt sulfide complexes in model aqueous H 2 S-bearing solutions up to 300 °C and 600 bar. The results demonstrate that tetrahydrosulfide, PtII(HS) 4 2–, is the major Pt-bearing complex in aqueous solutions saturated with PtS(s) over a wide range of dissolved hydrogen sulfide concentrations, from < 0.2 to ∼ 2 molal. The equilibrium constants of the dissolution reaction of PtS(s) generated in this study, PtS (s) + 3 H 2 S (aq) = Pt(HS) 4 2– + 2 H+ (β 4), are described by the equation log 10 β 4 = 0.9 × 1000/ T (K) – 19.7 (± 0.5) over the temperature range 25–300 °C and pressure range P sat –600 bar. Furthermore, the stepwise formation constants of four PtII-HS complexes, Pt(HS)+, Pt(HS) 2 0, Pt(HS) 3 –, and Pt(HS) 4 2–, were estimated, for the first time, from molecular dynamics simulations. The generated constants indicate that the maximum solubility of platinum in the form of Pt(HS) 4 2– in reduced H 2 S-dominated hydrothermal fluids at moderate temperatures (≤ 350 °C) is close to 1 ppb Pt at near-neutral pH of 6–8 and hydrogen sulfide concentrations of 0.1 molal. Although this solubility is much greater than that of Pt-Cl, Pt-OH and Pt-SO 4 complexes at such conditions, it is yet too low to account for significant Pt transport in most shallow-crust hydrothermal settings, characterized by the presence of both sulfide and sulfate. Complexes with S-bearing ligands, very likely other than H 2 S/HS–, such as S 3 –, would be required to account for Pt hydrothermal mobility. Our results provide a basis for more systematic future studies, using combined approaches, of the role of hydrothermal fluids in the behavior of platinum group elements in nature. [ABSTRACT FROM AUTHOR]

The speciation of iron in sediments and sedimentary rocks is a widely used proxy for the chemistry and oxidation state of ancient water bodies. Specifically, the fraction of reactive iron out of the total iron (Fe HR /Fe T) and the fraction of pyrite iron out of the reactive iron pool (Fe PYR /Fe HR) are thought to constrain the oxidation state and the presence of sulfide in the water column, respectively. This approach was developed and tested against modern core-top sediments, but application to sedimentary rocks requires consideration of the effects of diagenesis and lithification on iron speciation. Furthermore, the effects of deep burial, metamorphism, and late-stage alteration during exhumation or sampling (e.g., oxidative weathering) have not been systematically explored. To bridge this gap, we combined new data from four sediment cores (n = 54) with an extensive literature compilation of modern sediments (2936 measurements from 316 cores) and ancient sedimentary rocks (12,173 measurements spanning the Neoarchean to Quaternary). The modern data include both surface and buried sediments, allowing an investigation of the effects of diagenesis on iron speciation. Depending on the thresholds used to distinguish oxic from anoxic environments and ferruginous from euxinic environments, interpretation of the modern sedimentary iron speciation data within the existing framework yields incorrect environmental classifications up to ≈70% of the time. In modern sediments, diagenesis is the main reason that iron speciation does not represent the chemistry and oxidation state of the water column. We find that iron speciation correlates with porewater chemistry and that it changes with progressive burial along three distinctive Fe HR /Fe T –Fe PYR /Fe HR arrays, each of which represents a different set of diagenetic processes. We suggest that similarly to modern sediments, stratigraphic variation in iron speciation in sedimentary rocks primarily reflects progressive burial diagenesis or variation in depositional conditions rather than temporal variation in water-column chemistry and oxidation state. Indeed, analysis of the geologic iron speciation data reveals no statistically significant trends in either Fe HR /Fe T or Fe PYR /Fe HR from the Archean to the present day. The diagenetic Fe HR /Fe T –Fe PYR /Fe HR arrays that we identified in modern marine sediments suggest that under certain conditions, iron speciation analyses may be used to constrain Fe HR /Fe T in the local sediment source(s). Hence, we suggest that iron speciation data, together with complementary petrographic, mineralogical and geochemical constraints, may be used to constrain the local iron source(s) and early and late diagenetic processes, but rarely the chemistry or oxidation state of ancient water columns. [ABSTRACT FROM AUTHOR]

Among the mechanisms controlling elevated arsenic (As) speciation in groundwater, dissolution-precipitation of As-bearing solids, possibly as colloids, has not been systematically evaluated even though reported groundwater saturation states often indicate super- or near-saturation with respect to multiple solids. In this contribution, a detailed geochemical analysis was performed on well-constrained groundwater quality data collected through (a) sustained sampling (n = 84) over 2.5 y at a newly-identified site in the middle Gangetic plain of India; and (b) metadata analysis on studies conducted worldwide (n = 414). Groundwater saturation indices, speciation (E H -pH), and mineral solubilities (logC-pH) were calculated, consistent with a carefully-selected and updated thermodynamic database. Results suggest that under oxidizing conditions, secondary precipitation of solids similar to scorodite [FeAsO 4 ·2H 2 O (s) ] and pharmacolite [CaHAsO 4 ·2H 2 O (s) ] influences the As(V) concentrations in groundwater. In addition, groundwater at the investigated site was saturated with calcite [CaCO 3(s) ] and rhodochrosite [MnCO 3(s) ]. Evidence of colloidal forms of As-containing and As-free solids was found from SEM-EDS characterization of solids collected on 0.2 μm filter membranes used to sample groundwater. XPS analysis showed that the relative As(V) and As(III) signatures in these solids were consistent with the prevalence of dissolved As(V) and As(III) in groundwater, independently quantified using IC-ICP-MS. HR-TEM-SAED characterization of these solids indicated the possible presence of poorly crystalline scorodite- and pharmacolite-like phases along with calcite and lepidocrocite [γ–FeOOH (s) ] in a predominantly amorphous matrix. Also, a possible role of Mn in inducing As immobilization in calcite was suggested with the identification of ∼ 1:1 Mn:As atomic ratios in these solids, consistent with significant (p < 0.05) correlation of dissolved total As and total Mn. These findings imply that solubility-driven secondary processes may exert additional controls on the eventual fate and transport of arsenic in mixed-redox state shallow groundwaters, apart from the known primary mobilization mechanisms. To the best of our knowledge, this is the first study that has characterized colloidal arsenic in groundwater and related it to prevailing mechanisms. [ABSTRACT FROM AUTHOR]

Magma-derived fluids containing chlorine and sulfur are critical for the transport of ore metals to porphyry ore-forming environments. However, experimental data on the speciation and the solubility of ore metals in these fluids at conditions relevant to the arc magmatism are scarce. In particular, the effect of redox conditions on ore metal speciation and solubilities in sulfur-bearing fluids has not yet been experimentally constrained. We performed experiments to determine the effect of oxygen fugacity (f O 2) on the solubility of Cu, Ag and Au in high-temperature, low- density, low-salinity fluids and hypersaline brine. The experiments were conducted at T = 900 °C, P = 2000 bar varying f O 2 in 7 steps between 0.5 log units below the Ni-NiO buffer (NNO − 0.5) to NNO + 2.5. A prototype rapid-quench Molybdenum-Hafnium Carbide (MHC) externally heated pressure vessel assembly was used, which was fitted with a Shaw membrane for precise control of f O 2. The fluid phase was sampled as synthetic fluid inclusions (SFI) by in situ fracturing of quartz chips during the experiments. As capsule material, Au 97 Ag 2 Cu 1 alloy was used, which imposed activities of 0.962, 0.0082 and 0.0097 for Au, Ag and Cu, respectively. The apparent solubility of Cu and Ag at the imposed metal activities increases by a factor of 7 in the H 2 O-NaCl-KCl-S low-salinity fluid with f O 2 increasing from NNO − 0.5 to NNO + 2.5. The addition of 0.198 m HCl increases the overall solubility of Cu by a factor of 1.2–2.4. The apparent solubility of Au decreases by a factor of 9 as f O 2 changes from NNO − 0.5 to NNO + 2.5. The relationship between the logarithms of the apparent Cu and Ag solubilities and f O 2 is linear and the slope of the fitted line corresponds to 1+ oxidation state of these metals indicating that they are dominantly complexed by ligands that are S-free (e.g., chloride). Thermodynamic model calculations indicate that the dominant species for Cu and Ag are NaCuCl 2 and NaAgCl 2 , respectively. For Au, the dominant species is predicted to be NaAu(HS) 2 at low and intermediate f O 2 conditions, and AuCl and NaAuCl 2 at oxidizing conditions. The measured gold solubilities at intermediate f O 2 do not indicate significant Au complexation with S species containing S in intermediate oxidation states. Considering previous studies on silicate melts and our experimental data for volatiles, we conclude that Cu and Ag likely have constant fluid/melt partition coefficients in the typical f O 2 range of arc magmatism because they dissolve in the same oxidation state in the fluid and the melt (1+) and are dominantly chloride complexed in the fluid. [ABSTRACT FROM AUTHOR]

Rare Earth elements (REE) are gaining importance due to their increasing industrial applications and usefulness as petrogenetic indicators. REE-sulfate complexes are some of the most stable REE aqueous species in hydrothermal fluids, and may be responsible for REE transport and deposition in a wide variety of geological environments, ranging from sedimentary basins to magmatic hydrothermal settings. However, the thermodynamic properties of most REE-sulfate complexes are derived from extrapolation of ambient temperature data, since direct information on REE-sulfate complexing under hydrothermal conditions is only available for Nd, Sm and Er to 250 °C (Migdisov and William-Jones, 2008, 2016). We employed ab initio molecular dynamics (MD) simulations to calculate the speciation and thermodynamic properties of yttrium(III) in sulfate and sulfate-chloride solutions at temperatures and pressures up to 500 °C and 800 bar. The MD results were complemented by in situ X-Ray Absorption Spectroscopy (XAS) measurements. Both MD and XAS show that yttrium(III) sulfate complexes form and become increasingly stable with increasing temperature (≥200 °C). The MD results also suggest that mixed yttrium-sulfate-chloride complexes (that cannot be distinguished from mixtures of chloride and sulfate complexes in XAS experiments) form at ≥ 350 °C. Two structures with two different Y(III)-S distances (monodentate and bidentate) are observed for Y(III)-sulfate bonding. The formation constants, derived via thermodynamic integration, for the Y(III) mono- and di-sulfate complexes parallel the trends for those of Nd, Sm and Er determined experimentally to 250 °C. The derived formation constants were used to fit revised Helgeson-Kirkham-Flowers equation-of-state parameters that enabled calculation of formation constants for Y(SO 4)+ and Y(SO 4) 2 − over a wide range of temperatures and pressures. The presence of sulfate increases the solubility of Y(III) under specific conditions. Since the stability of sulfate is redox sensitive, Y(III) solubility becomes highly redox-sensitive, with rapid precipitation of Y minerals upon destabilisation of aqueous sulfate. [ABSTRACT FROM AUTHOR]

[Display omitted] Tungsten (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 in systems with highly soluble salts Na 2 WO 4 and Na 6 W 12 O 39. At all investigated temperatures, the tungstate ion WO 4 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⋅kg H2O −1. Among the three observed polymers, namely [W 7 O 24 ]6− (paratungstate-A, ∼960 cm−1), [W 10 O 32 ]4− (tungstate-Y, ∼970 cm−1), and α-[H 2 W 12 O 40 ]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 10-3 mol·kg H2O −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. [ABSTRACT FROM AUTHOR]

Trace metal are essential for microbially-mediated biogeochemical processes occurring in anoxic wetland soils and stream bed sediments, but low availability of these elements may inhibit anaerobic element cycling and transformations. Solid-phase speciation is likely a critical control on trace metal availability but has seen limited study in anoxic systems having concentrations similar to geological background levels, where metal limitations may be most prevalent. We have investigated trace metal concentrations and solid-phase speciation in three freshwater subsurface aquatic systems: marsh wetland soils, riparian wetland soils, and the sediments of a streambed. These systems displayed low solid-phase trace metal concentrations, generally at or below geological background levels, which generally followed the trend Zn > Cu ≈ Ni > Co and showed no correlation with major element compositions. All soils and sediments were dominated by quartz but varied in clay mineralogy as well as the organic matter, total sulfur, and total iron contents. X-ray absorption near-edge structure (XANES) spectroscopy shows that sulfur speciation in both wetlands is dominated by organic sulfur. Elemental sulfur and iron sulfides together made up <25% of the sulfur in the wetland soils, but the distribution between inorganic and organic forms was reversed in the stream sediments. Ferrous and ferric iron in clay minerals were common species identified by both XANES and extended X-ray absorption fine structure (EXAFS) spectroscopies at all sites. Iron(III) oxides were substantial components in all but the marsh wetland soils. Quantitative analysis of copper, nickel, and zinc XANES spectra revealed similar metal speciation across all sites. Copper speciation was dominated by sulfides, adsorbed species, and minor amounts of copper bound to organic matter; no metallic copper was detected. Nickel speciation also varied little and was dominated by nickel in clay mineral octahedral sheets and nickel sulfide, with adsorbed species also present. Zinc speciation was slightly more varied, with the marsh wetland soils and stream bed sediments containing adsorbed species, zinc associated with clay mineral structures, and zinc bound to reduced sulfur groups on organic matter, whereas the riparian wetland soils lacked clay-associated zinc but contained zinc sulfide. Trace metals bound to reduced sulfur occurred at every site, with a greater sulfur-bound fraction for copper. The fractional abundance of sulfur-bound species showed no relationship with soil or sediment total sulfur content, which varied by two orders of magnitude. More broadly, the observations in this study suggest that trace metal speciation in freshwater wetland soils and stream sediments is consistently dominated by a small set of recurring components which are distinct for each metal. This may represent a general geochemical phenomenon in anoxic soils and sediments containing trace metals at background concentrations (as low as 3 µg g−1) that was not predicted from systems that are contaminated with or naturally-enriched in copper, nickel, or zinc. [ABSTRACT FROM AUTHOR]

Sorption of Mo to goethite and related iron (hydr)oxides plays an important role in controlling the mobility of Mo in soil and aquatic environments. We first performed ab-initio molecular dynamics (MD) simulations of Mo complexation at the goethite {1 0 1}-water interface. We find that molybdenum adsorbed on goethite mainly exists as the inner-sphere tetrahedral bidentate and monodentate corner-sharing complex, instead of the previous argued bidentate edge-sharing or outer-sphere complex. Different from previous literature, we didn't find any distorted octahedral structure of Mo on goethite. The predicted Mo-Fe distances of bidentate and monodentate corner-sharing complex based on ab-initio MD is 3.53 and 3.38 Å, respectively, which are consistent with the fitted Mo-Fe distances: 3.40 ± 0.15 Å and 3.41 ± 0.1 Å from EXAFS. We measured the adsorption of Mo (VI) on goethite (α-FeOOH) at different initial concentration and pH from 3 to 11 under N 2 atmosphere and constructed a surface complexation model that is consistent with the molecular speciation based on ab-initio MD, XANES, EXAFS and reported in-situ ATR-IR analysis (Davantes and Lefevre, 2015, 2016). Application of SCM for Mo on goethite assuming iron oxides as the principal component of ferromanganese crust (containing average 276 mg/kg Mo) will have ∼0.3 μg/l Mo in seawater in equilibrium. The observed ∼10 μg/l Mo in seawater (c a. 400–800 yr) is between 0.3 (adsorption equilibrium by marine goethite) and 23 μg/l (adsorption equilibrium by marine birnessite, Kashiwabara et al., 2011), suggesting that molybdenum in seawater has reached equilibrium with ferromanganese crust, and goethite (57%) probably plays a more important role than birnessite (43%) in regulating molybdenum abundance in the ocean, which has not been fully recognized in past. Observed molybdenum isotopic fractionation between seawater and ferromanganese crust excesses the experimental molybdenum isotopic fractionation range during adsorption by birnessite and goethite (Goldberg et al., 2009). Based on reported experimental molybdenum isotopic fractionation values and our SCM prediction, the molybdenum isotopic fractionation δ98/95Mo value should vary between 1.6 and 2.3‰, which is lower than the observed range between seawater and ferromanganese crust: 2.7–3.2‰ about 1‰, suggesting the disequilibrium of molybdenum isotopic fractionation in ocean. To clarify the discrepancy, this study highlights the necessity of clarifying the molybdenum source in hydrogenic and hydrothermal ferromanganese crusts before understanding molybdenum isotopic fractionation between seawater and ferromanganese crust. [ABSTRACT FROM AUTHOR]

Molybdenum (Mo) and tungsten (W) concentration patterns and isotope signatures are widely used to study changes in ocean redox conditions. This is in part due to the differential adsorption of Mo and W onto various types of minerals that are abundant under different redox regimes. The speciation of Mo and W, which also varies under different redox conditions, greatly controls their particle reactivities in the environment. Manganese (Mn) oxide minerals can not only provide surfaces for Mo and W adsorption, but also catalyze Mo and W speciation change. Thus, the Mn oxides that are often present at redox interfaces can play an important role in Mo and W speciation change. However, we lack knowledge of how rates for Mn oxide catalyzed Mo and W speciation transformation compare to rates for their adsorption and isotope fractionation processes, which creates uncertainty in our ability to interpret Mo and W concentration and isotope patterns in sediments and sedimentary rocks. Here, we present data that constrain rates, environmental controls, and mechanisms for tetrathiomolybdate (MoS 4 2–) and tetrathiotungstate (WS 4 2–) hydrolysis on Mn oxide surfaces, which transforms the thioanion species to oxyanion species. The experimental results show that hydrolysis rate constants for these Mo and W thioanions are first order with respect to Mn oxide surface areas. Hence, high abundance of Mn oxides in aquatic environments greatly catalyzes MoS 4 2– and WS 4 2– hydrolysis. In addition, experimental results indicate that crystal structure differences of Mn oxides contribute to differences in rates for Mo and W thioanion hydrolysis reactions. Solution pH and ionic strength have limited impacts on Mo and W thioanion hydrolysis rates catalyzed by Mn oxides, while sulfate has a moderate impact. Geochemical modeling results indicate that equilibrium times for MoS 4 2– and WS 4 2– hydrolysis reactions in the presence of Mn oxides at abundances found in the modern ocean are three orders of magnitude longer than those for adsorption and isotope fractionation of Mo and W on Mn oxide surfaces. This implies that Mn oxides play a larger role in affecting adsorption and isotope fractionation processes than Mo and W speciation transformation in the modern, well-oxygenated ocean. In contrast, the potentially high levels of dissolved Mn(II) in the Archean ocean would have shortened the equilibrium time for Mo or W speciation transformation such that this process could compete with their adsorption and isotope fractionation processes, and subsequently affect sedimentary Mo and W isotope records. The impact of dissolved Mn(II) catalyzed Mo and W speciation change should be considered in future studies of Archean Mo and W isotope records. [ABSTRACT FROM AUTHOR]

Phosphorus (P) availability in soils controls critical functions and properties of terrestrial ecosystems. Vertical distribution patterns of P concentration and speciation in soil profiles provide historical records of how pedogenic processes redistribute and transform P and thus change its availability in soils, which, however, remain poorly understood. We determined the patterns in three forest soil profiles of contrasting climate, using fine sampling intervals, P K -edge X-ray absorption near edge (XANES) spectroscopy and chemical extractions. The major features of the patterns persist under the contrasting climate. The total P concentration decreases from A to B horizons, reaches a minimum in the B horizons, and then increases towards the upper C horizons, but with little variations with depth in the lower C horizons. Both calcium-bound inorganic P (Ca–P i) and organic P (P o) decrease and Fe- and Al-bound P i [(Fe + Al)–P i ] increases in proportion downward in the A horizons because dust inputs and accumulation of organic matter both decline with increasing depth. Ca–P i is negligible and (Fe + Al)–P i is dominant in the B horizons due to strong weathering. There is a strong downward increase in Ca–P i proportion and decrease in (Fe + Al)–P i proportion from the lower B to the upper C horizons. New Ca–P i seems to form in the upper C horizons where downward leaching Ca2+ and phosphate accumulate due to the low water permeability of the soils. In the lower C horizons, Ca–P i increases and (Fe + Al)–P i decreases with increasing depth due to decreasing chemical weathering. Regarding P bioavailability, the proportion of occluded P (P occ) shows an increasing and decreasing trend with increasing depth, being the highest in the B horizons; however, there are no consistent trends for non-occluded P (P n-occ). While the P vertical patterns can be understood by considering the relative importance of different pedogenic processes, climate affects the intensities of these processes and thus the details of the patterns. When depth-integrated, warmer/wetter climate results in decreases in the proportions of both Ca–P i and P n-occ but increases in the P loss and the proportions of P o , (Fe + Al)–P i , and P occ. Regardless of soil depth and climate, the P i speciation, i.e., the relative proportions of Ca–P i and (Fe + Al)–P i over total P i , correlates well with soil pH and weathering degree (Chemical Index of Alteration, CIA), and the P o concentration correlates with pedogenic Fe and Al and organic carbon concentration. The correlations suggest that the P i speciation is primarily controlled by soil geochemistry/mineralogy, and the P o concentration by both soil geochemistry/mineralogy and biological activities. P occ correlates with CIA, and thus is mainly controlled by soil mineralogy; but P n-occ correlates weakly with soil properties, probably due to its susceptibility to combined influences of dust inputs, leaching, biological activities, and adsorption on minerals. The above quantitative relationships may help predict P speciation and availability in diverse soils. We further show that soil profiles, and climate and CIA gradients are useful tools for studying P transformations, particularly for the P i pool, during pedogenesis. This study provides an integration and synthesis of controls of climatic and edaphic variables on P dynamics in forest soils. [ABSTRACT FROM AUTHOR]

• First investigation of Ga-Al systematics by HR-ICP-MS in marine Fe-Mn crusts. • New concentration data for reference materials NOD-P-1, NOD-A-1 and JMn-1. • Ga/Al ratios of Fe-Mn crusts significantly lower than those of ambient seawater. • Fe-Mn crusts record inter-oceanic differences between Ga/Al of seawater. • Significant impact of organic complexes on Ga-Al behavior in seawater. Despite its status as a critical metal in the EU and the U.S., the geochemical behavior of gallium (Ga) in the Earth's surface environment is not yet well constrained, which is in marked contrast to its geochemical partner aluminum (Al). Especially hydrogenetic ferromanganese crusts which may represent a potential future unconventional resource of critical metals, are surprisingly underexplored with regard to their Ga-Al systematics. This study presents the first data on the Ga-Al pair in hydrogenetic Fe-Mn crusts from various locations in the Pacific Ocean and from the Rio Grande Rise in the SW Atlantic. We also provide Ga-Al data for certified reference materials for Fe-Mn nodules NOD-P-1, NOD-A-1, and JMn-1. Our results also show that marine hydrogenetic Fe-Mn crusts cannot be considered an unconventional Ga resource, in marked contrast to several other critical metals. Aluminum and Ga concentrations in these precipitates are not controlled by detrital components. Both elements show a geochemical behavior comparable to that of the hydroxide-dominated elements Zr and Hf during scavenging from ambient seawater by hydrogenetic Fe-Mn crusts. The Ga/Al mass ratios of Atlantic (0.10–0.16 g/kg) and Pacific crusts (0.18–1.2 g/kg) are about one and a half orders of magnitude lower than those of respective ambient seawater and mimic the inter-oceanic fractionation of the Ga-Al pair between these two ocean basins. Without exception, the Atlantic Fe-Mn crusts show lower Ga/Al ratios than the Pacific crusts. The systematically lower Ga/Al ratios of the crusts relative to those of seawater reveal preferential scavenging of Al relative to Ga during the formation of Fe-Mn crusts. This is in line with previous studies that reported a higher particle reactivity of Al compared to Ga in the marine environment. However, this fractionation trend contradicts predictions based on a simplified model commonly used to approximate trace element scavenging by marine metal (hydr)oxides based on hydrolysis constants, which favors Ga enrichment relative to Al as the hydrolysis constant of Ga is higher than that of Al. The observed Ga-Al fractionation may be related to the preferential retention of Al compared to Ga on the surface of the Fe-Mn (oxyhydr)oxides because in contrast to Ga, desorption of Al is characterized by very slow reaction kinetics or is even irreversible. Alternatively, stronger organic complexation of dissolved Ga than Al in seawater (which is supported by the stability constants of various organic Ga and Al complexes) and the resulting preferential retention of Ga over Al in solution by dissolved organic ligands may be another viable mechanism to explain the observed Ga-Al fractionation between hydrogenetic Fe-Mn crusts and seawater. While more experimentally determined thermodynamic data are necessary, the rather consistent difference between hydrogenetic Fe-Mn crusts and ambient seawater supports an important role of organic ligands in the chemical speciation of Ga in seawater. [ABSTRACT FROM AUTHOR]

• Peatlands with similar geology, climate and Hg deposition differed in MeHg formation. • Vascular plant removal did not affect net MeHg formation or porewater geochemistry. • Electron donors, acceptors and bioavailable Hg correlated with net MeHg formation. • Short-term resource supply responses deviated from those caused by natural processes. A geographically constrained chronosequence of peatlands divided into three age classes (young, intermediate and old) was used to explore the role of biogeochemical influences, including electron donors and acceptors as well as chemical speciation of inorganic mercury (Hg(II)), on net formation of methylmercury (MeHg) as approximated by the fraction of MeHg to total mercury (THg) in the peat soil. We hypothesized that removing vascular plants would reduce availability of electron donors and thus net MeHg formation. However, we found no effect of the vascular plant removal. The sum of the potential electron donors (acetate, lactate, propionate and oxalate), the electron donation proxy organic C/Organic N, and the potential electron acceptors (Fe(III), Mn and sulfate) in porewater all showed significant correlations with the net MeHg formation proxies in peat soil (MeHg concentration and %MeHg of THg). Thus differences in both electron donor and acceptor availability may be contributing to the pattern of net MeHg formation along the chronosequence. In contrast, Hg(II) concentrations in peat porewater showed small differences along the gradient. A chemical speciation model successfully predicted the solubility of Hg and MeHg in the porewater. The modeling pointed to an enhanced concentration of Hg-polysulfide species in the younger peatlands as a potential factor behind increased Hg(II) solubility and methylation in the more nutrient-rich peatlands. This work contributes to the understanding of Hg and MeHg cycling in peatlands which can help guide mitigation measures to reduce aquatic MeHg biomagnification in peatland dominated landscapes. [ABSTRACT FROM AUTHOR]

Knowledge of Sn(II) speciation in aqueous and gaseous phases and the corresponding isotope effects are critical for understanding the transport and deposition of Sn in various geological and cosmochemical processes. In this study, we use first principles method to investigate the speciation of stannous (Sn(II)) chloride in fluids under hydrothermal and supercritical conditions. The results show that SnCl 3 −, SnCl 2 (H 2 O) and SnCl(H 2 O) 2 + are stable in hydrothermal solutions at temperatures of up to 300 °C, with SnCl 3 − being the dominant species, whereas SnCl 2 and SnCl 2 (H 2 O) are the stable species in vapor phases. Notably, SnCl 2 is found to be stable under supercritical conditions. The reduced partition function ratios (β factors) for the stable Sn(II) species and three major Sn minerals (cassiterite, megawite, and romarchite) are also calculated by first principles methods. The calculation results show that under equilibrium, heavy Sn isotopes are preferentially partitioned into stannic (Sn(IV)) species, and gaseous species enrich heavy Sn isotopes relative to aqueous species. Based on the equilibrium Sn isotope fractionation factors derived in this study, we use a transport-precipitation model to evaluate the Sn isotope response to cassiterite precipitation in hydrothermal fluids. The modeling results show that significant Sn isotope variability could be produced during cassiterite precipitation, with temperature and Sn speciation being the primary controlling factors. Furthermore, by comparing the Sn isotope variability in natural cassiterite and those derived from the model, we argue that Sn should occur predominantly as Sn(IV) species in hydrothermal fluids during cassiterite precipitation in tin mineralizing systems. [ABSTRACT FROM AUTHOR]

The chemical speciation of mercury (Hg), methyl mercury (MeHg), sulfur and iron was investigated in the sediment and porewater of Lake Ängessjön, a boreal, shallow (maximum depth 2.5 m), oligo-/dystrophic lake in northern Sweden. The lake receives terrestrial stream runoff from surrounding coniferous forest soils and peatlands having a low pH (4.6) and high concentrations of dissolved organic matter (DOM, annual average: 45 mg L−1), Fe (60 µM), sulfate (105 µM), inorganic Hg (1200 pM) and MeHg (250 pM). Sulfur K-edge XANES and Hg L III -edge EXAFS spectroscopic measurements were used to characterize and quantify the sulfur speciation in the lake sediment at nine occasions, covering different seasons in the years of 2007 and 2009. In the surface sediment (0–3 cm) sulfate is reduced to zero-valent S and inorganic sulfide, that in turn reacts with Fe to form FeS m (mackinawite) and FeS 2 (framboidal pyrite). The latter mineral becomes increasingly dominant by depth in the sediment. Thermodynamic modeling successfully predicted measured porewater concentrations of Hg in the sediment. Metacinnabar (β- HgS) and Hg(NOM-RS) 2 complexes (the latter formed as a reaction between Hg(II) and thiol groups associated with natural organic matter, NOM-RSH) were the dominant forms of Hg(II) in the solid phase of sediments and Hg(II)-polysulfides (aq) dominated in the porewater. We argue that FeS m is a key component that indirectly controlled the Hg(II) speciation in the sediment by keeping the aqueous phase concentration of inorganic sulfide in the 0.5–2 µM range throughout the year. Besides providing a pool of readily soluble inorganic sulfide for formation of β- HgS(s), as demonstrated by previously reported EXAFS experiments, we further suggest FeS m may serve as a precursor for the formation of a more crystalline (less soluble) β- HgS(s) phase than present in environments devoid of FeS m. Support for this was provided by comparing our results with previously reported thermodynamic modelling results of Hg(II) and MeHg solubility in organic soils devoid of FeS m. In more general terms, we suggest the presence or absence of FeS m , through its influence on the chemical speciation of Hg and MeHg, may be a key factor behind the variability in rates of Hg(II) and MeHg transformation processes, such as methylation, reduction and demethylation, reported for different environmental settings. [ABSTRACT FROM AUTHOR]

• We present the characteristics of Fe speciation and ligands in hydrothermal plumes over low- and high-temperature vents and over diffuse venting fields. • Total Fe in plumes over the low-temperature hydrothermal vent contained the highest proportions of dissolved Fe and labile Fe. • Diffuse flow had the highest proportion of dissolved Fe present as labile Fe. • The linear relationships between Fe species and total Mn in the buoyant plume over Burke suggest a simple conservative mixing with ambient seawater. • Organic Fe ligands in hydrothermal plumes constituted a significant portion of dissolved Fe and labile Fe. We investigated the speciation of Fe and distributions of Fe-binding ligands in the hydrothermal plumes over high- and low-temperature vents and over diffuse venting fields above the Mariana back-arc spreading center. The concentrations of ligands ([L]) and conditional stability constants (K' FeL) of the natural Fe ligand pool were measured by a reverse titration-competitive ligand exchange-adsorption cathodic stripping voltammetry (RT-CLE-ACSV). The results showed that the buoyant and non-buoyant plume samples over the low-temperature Burke had the highest dissolved Fe/total Fe (DFe/TFe) of 82.2 ± 8.8% and labile Fe/total Fe (Fe Lab /TFe) of 43.3 ± 5.6%. In contrast, in the plume samples above the high-temperature Perseverance field, TFe had the lowest proportions of DFe (48.8 ± 12.2%) and Fe Lab (19.4 ± 8.8%). The linear relationships between Fe species and total Mn (TMn) in the buoyant plume over Burke suggest a simple conservative mixing with ambient seawater, which resulted in constant values of DFe/TFe. However, in the non-buoyant plume over Burke, DFe/TFe decreased with plume dilution. The plume samples sourced from the diffuse flow over newly erupted lava had the highest proportion of DFe present as Fe Lab (74.6 ± 4.0%). The [L] in the buoyant plumes over low-temperature Burke vent were up to 113.6 nM, and the logK' FeL decreased with increasing DFe and [L], suggesting the importance of weaker ligands in stabilizing Fe. For all the plume samples, the organically complexed Fe (FeL) constituted significant proportions of the DFe (29.0 ± 9.3%) and Fe Lab (57.5 ± 15.6%) fractions. These Fe-binding ligands were likely sourced from diffuse venting fluids adjacent to the venting sites, and may be also produced by microbes within the hydrothermal plumes. [ABSTRACT FROM AUTHOR]

The kinetics of tungstate (i.e., WO 4 2−) thiolation were investigated in experimental solutions buffered at different pH values and as a function of varying dissolved sulfide concentrations. Similar to molybdate (MoO 4 2−), tungstate undergoes stepwise thiolation to tetrathiotungstate according to: W O x S 4 - x 2 - + H 2 S (a q) ↔ W O x - 1 S 5 - x 2 - + H 2 S ; 1 ≤ x ≤ 4. Under equilibrium conditions at standard conditions (298.15 K, 105 Pa) WO 4 2− also converts to tetrathiotungate (WS 4 2−) around 1.0 mM H 2 S (aq) , reminiscent of the chemical switch for the MoO 4 2− → MoS 4 2− transition reported in the literature, but at nearly 100-fold higher H 2 S (aq) concentrations. The laboratory experiments show that thiotungstate formation is first order with respect to H 2 S concentration, and that the thiolation reactions are catalyzed by general Brønsted acids. Therefore, the high NH 4 + and HCO 3 − concentrations employed in the experiments both favored WO 4 2− thiolation. The experimental data were used to develop Brønsted relationships for the successive thiolation reactions (i.e., WO 3 S2− → WO 2 S 2 2−, WO 2 S 2 2− → WOS 3 2−, WOS 3 2− → WS 4 2−) that allow the acid-catalyzed thiolation rates of WO 4 2− to be estimated as a function of pH in natural waters. Reaction modeling of tungstate thiolation kinetics indicates that di- to trithiotungstate (WO 2 S 2 2− → WOS 3 2−) conversion and tri- to tetrathiomolybdate (MoOS 3 2− → MoS 4 2−) conversion may not be achieved in temporally variable sulfidic waters. In such environments, intermediate thioanions of W and Mo dominate with Mo exhibiting a higher degree of thiolation than that of W. As the partition coefficient of W into minerals is affected by its speciation in solution, its enrichment in minerals may change due to the level and changes in dissolved sulfide concentrations. Specifically, the partition coefficient of W decreases from oxic environments to sulfidic environments, suggesting W enrichment in sediment is likely to be a good tracer of redox conditions (i.e., oxic and sulfidic conditions) in the overlying waters. In comparison, the partition coefficient of Mo increases from seasonally sulfidic environments to permanently sulfidic environments, indicating that Mo enrichment in sediments is a good tracer of sulfidic conditions. In addition, the Mo/W concentration ratio in black shales shows the potential for identifying fluctuation of redox conditions from ca. 2500 Ma to ca. 0.1 Ma and is a potential proxy for tracking deep time sulfidic conditions. Thus, even though the geochemical behavior of W is different from Mo, W can nevertheless be a potential proxy for tracking changing redox conditions in the modern and ancient ocean. [ABSTRACT FROM AUTHOR]

• At low ƒO 2 conditions sulfur acts as an important anion. • In very reduced basaltic melts (

Particulate and dissolved metal(loid) release from mine tailings is of concern in (semi-)arid environments where tailings can remain barren of vegetation for decades and, therefore, become highly susceptible to dispersion by wind and water. Erosive weathering of metalliferous tailings can lead to arsenic contamination of adjacent ecosystems and increased risk to public health. Management via phytostabilization with the establishment of a vegetative cap using organic amendments to enhance plant growth has been employed to reduce both physical erosion and leaching. However, prior research suggests that addition of organic matter into the oxic weathering zone of sulfide tailings has the potential to promote the mobilization of arsenate. Therefore, the objective of the current work was to assess the impacts of phytostabilization on the molecular-scale mechanisms controlling arsenic speciation and lability. These impacts, which remain poorly understood, limit our ability to mitigate environmental and human health risks. Here we report on subsurface biogeochemical transformations of arsenic and iron from a three-year phytostabilization field study conducted at a Superfund site in Arizona, USA. Legacy pyritic tailings at this site contain up to 3 g kg−1 arsenic originating from arsenopyrite that has undergone oxidation to form arsenate-ferrihydrite complexes in the top 1 m. Tailings were amended in the top 20 cm with 100, 150, or 200 g kg−1 (300–600 t ha−1) of composted organic matter and seeded with native halotolerant plant species. Treatments and an unamended control received irrigation of 360 ± 30 mm y−1 in addition to 250 ± 160 mm y−1 of precipitation. Cores to 1 m depth were collected annually for three years and sectioned into 20 cm increments for analysis by synchrotron iron and arsenic X-ray absorption spectroscopy (XAS) coupled with quantitative wet chemical and mass balance methods. Results revealed that >80% of arsenic exists in ammonium oxalate-extractable and non-extractable phases, including dominantly ferrihydrite and jarosite. Arsenic release during arsenopyrite oxidation resulted in both downward translocation and As(V) attenuation by stable Fe(III)(oxyhydr)oxide and Fe(III) (hydroxy)sulfate minerals over time, highlighting the need for sampling at multiple depths and time points for accurate interpretation of arsenic speciation, lability, and translocation in weathering profiles. Less than 1% of total arsenic was highly-labile, i.e. water-extractable, from all treatments, depths, and years, and more than 99% of arsenate released by arsenopyrite weathering was attenuated by association with secondary minerals. Although downward translocation of both arsenic and iron was detected during phytostabilization by temporal enrichment analysis, a similar trend was measured for the uncomposted control, indicating that organic amendment associated with phytostabilization practices did not significantly increase arsenic mobilization over non-amended controls. [ABSTRACT FROM AUTHOR]

The speciation of water dissolved into and reacted with hydrous alumino-silicate glasses (of NaAlSi 3 O 8 and "rhyolitic" compositions) quenched from high temperature is re-investigated where the predominant species are expected to be "X(Si and Al)-OH" and "H 2 O". Only, two analytical methods are capable of assessing such speciation: Near InfraRed (NIR) spectroscopy and solid-state 1H Nuclear Magnetic Resonance (NMR) spectroscopy. It is observed that the apparent water speciation, as a function of total water content, as determined by NIR spectroscopy is nearly the opposite from what the 1H NMR data reveal. Deuterium (2H) NMR and silicon (29Si) NMR report consistent trends in apparent speciation (depolymerization) with those indicated by the 1H NMR data. Compared with four previous NMR studies of hydrous NaAlSi 3 O 8 glasses it is shown that whereas NIR data always report the same apparent systematic variation in the intensity of the 4500 ("X-OH") and 5200 ("H 2 O") cm−1 bands with total water content, multiple 1H NMR studies of hydrous NaAlSi 3 O 8 report a wide range in OH/H 2 O. The discrepancy between the various NMR studies likely reflects differences in how the various glasses were made. Specifically, quench rate (fast or slow) and synthesis pressure (higher or lower), might impose a strong effect on observed water speciation in glasses via 1H NMR. It is concluded that the application of NIR spectroscopy, specifically the use of the intensities of the 4500 ("X-OH") and 5200 ("H 2 O") cm−1 NIR bands, does not provide an accurate assessment of water speciation in hydrous alumino-silicate glasses. NIR spectroscopy does remain a very valuable analytical tool for determination of total water content. [ABSTRACT FROM AUTHOR]

• C-O-N-H speciation in reduced silicate melts varies systematically as a function of f O 2 at any given P - T. • At a given f O 2 , f H 2 controls whether C and N dissolve as anhydrous or hydrous species. • D N alloy/silicate is comparable for hydrous and anhydrous magma oceans, while D C alloy/silicate is not. • The compositions of proto-atmospheres formed by magma ocean degassing depends on the f O 2 and f H 2 of the accreting material. The composition of atmospheres and the resulting potential for planetary habitability in the rocky bodies of our Solar System and beyond is strongly controlled by the volatile exchange between their silicate reservoirs and exospheres. The initial budget and speciation of major volatiles, like carbon (C), nitrogen (N) and water (H 2 O), in the silicate reservoirs and atmospheres was set during the formation stages of rocky bodies. However, the speciation of these major volatiles in reduced silicate melts prevalent during the differentiation stages of rocky bodies and its effect on the partitioning of volatiles between major rocky body reservoirs is poorly known. Here we present SIMS and vibrational spectroscopy (FTIR and Raman) data, determining C solubility, H content, and speciation of mixed C-O-N-H volatiles in graphite saturated silicate glasses from high P (1–7 GPa)- T (1500–2200 °C) experiments reported in Grewal et al. (2019b, 2019a). The experiments recorded oxygen fugacity (log f O 2) between IW–4.3 and IW–0.8. C-O-N-H speciation varied systematically as function of f O 2 at any given P - T. We find out that C-N−, C O 3 2 - , N 2 , and OH− are the dominant species in the oxidized range (>IW–1.5), along with some contributions from C-H, N-H, and C-O bearing species. Between IW–3.0 and IW–1.5, C is bonded as C-O either in the form of isolated C-O molecules or Fe-carbonyl complexes, or as C-H in hydrocarbons, or as combination of both in esters, while almost all of the H is bonded with the dominant N species, i.e., NH2− or N H 2 -. At the most reduced conditions (

The diffusion of water through silicate melts is a key process in volcanic systems. Diffusion controls the growth of the bubbles that drive volcanic eruptions and determines the evolution of the spatial distribution of dissolved water during and after magma mingling, crystal growth, fracturing and fragmentation, and welding of pyroclasts. Accurate models for water diffusion are therefore essential for forward modelling of eruptive behaviour, and for inverse modelling to reconstruct eruptive and post-eruptive history from the spatial distribution of water in eruptive products. Existing models do not include the kinetics of the homogeneous species reaction that interconverts molecular (H 2 O m) and hydroxyl (OH) water; reaction kinetics are important because final species distribution depends on cooling history. Here we develop a flexible 1D numerical model for diffusion and speciation of water in silicate melts. We validate the model against FTIR transects of the spatial distribution of molecular, hydroxyl, and total water across diffusion-couple experiments of haplogranite composition, run at 800–1200 °C and 5 kbar. We adopt a stepwise approach to analysing and modelling the data. First, we use the analytical Sauer-Freise method to determine the effective diffusivity of total water D H 2 O t as a function of dissolved water concentration C H 2 O t and temperature T for each experiment and find that the dependence of D H 2 O t on C H 2 O t is linear for C H 2 O t ≲ 1.8 wt.% and exponential for C H 2 O t ≳ 1.8 wt.%. Second, we develop a 1D numerical forward model, using the method of lines, to determine a piece-wise function for D H 2 O t C H 2 O t , T that is globally-minimized against the entire experimental dataset. Third, we extend this numerical model to account for speciation of water and determine globally-minimized functions for diffusivity of molecular water D H 2 O m C H 2 O t , T and the equilibrium constant K for the speciation reaction. Our approach includes three key novelties: (1) functions for diffusivities of H 2 O t and H 2 O m , and the speciation reaction, are minimized simultaneously against a large experimental dataset, covering a wide range of water concentration (0.25 ≤ C H 2 O t ≤ 7 wt.%) and temperature (800 ° C ≤ T ≤ 1200 ° C), such that the resulting functions are both mutually-consistent and broadly applicable; (2) the minimization allows rigorous and robust analysis of uncertainties such that the accuracy of the functions is quantified; (3) the model can be straightforwardly used to determine functions for diffusivity and speciation for other melt compositions pending suitable diffusion-couple experiments. The modelling approach is suitable for both forward and inverse modelling of diffusion processes in silicate melts; the model is available as a MATLAB script from the electronic supplementary material. [ABSTRACT FROM AUTHOR]

Fluids released by subducting slabs are important to the geochemical evolution of Earth's crust-mantle system. However, it is still not well constrained for the mobility of redox sensitive elements (like Fe, C or S) in subduction zone fluids, and its effect on the redox property of the overlying mantle wedge and its derived arc magmas. Iron isotope fractionation is sensitive to the redox state and speciation of Fe in fluids, which can potentially provide clues on the oxygen fugacity (f O 2) of slab-derived fluids. Whiteschists at the Dora-Maira Massif in the Western Alps have experienced ultrahigh-pressure metamorphism at subarc depths. They are rich in both SiO 2 (mostly >65 wt.%) and MgO (4–10 wt.%), and mainly composed of pyrope, quartz/coesite, talc, phengite and kyanite. They were demonstrated to have a granite protolith, but metasomatized by serpentinite-derived Mg-rich fluids at the slab-mantle interface in a subduction channel. Thus these rocks provide an excellent target to explore the fluid-rock interaction by Fe isotopes. The whiteschists show extremely high δ56Fe values of +0.32 to +1.22‰, being much higher than those of +0.10 to +0.38‰ for the surrounding metagranites. The Fe isotope composition of whiteschists significantly deviates from the igneous differentiation trend defined by rocks with basaltic to rhyolitic compositions. The heavy Fe isotope compositions must be produced by a metasomatic process. The much lower Fe 2 O 3 and FeO t contents and Fe3+/∑Fe ratios but higher δ56Fe values for the whiteschists relative to their protolith suggest the reduction of Fe3+ to Fe2+ and the loss of isotopically light Fe, probably in the form of Fe(II)-Cl and/or Fe(II)-(HS) complexes, through saline fluids and/or HS- bearing fluids. The reduction of Fe3+ to Fe2+ and the possible presence of HS- suggests the occurrence of relatively reduced fluids. Such reduced fluids were probably derived from serpentinite dehydration at the slab-mantle interface in the subduction channel. Therefore, the Fe isotope results demonstrate that the f O 2 at the slab-mantle interface can be locally highly heterogeneous, leading to various oxidation states in the mantle wedge and arc magmas. [ABSTRACT FROM AUTHOR]

Phosphorus (P) in corals has shown potential as a proxy for dissolved inorganic phosphate (DIP) in seawater, but additional investigations are needed to understand its incorporation mechanism, down-core applicability, and sensitivity, especially in oligotrophic environments with low DIP variability. In this study, we used a new method to distinguish between inorganic and organic P in the skeletons of two Porites sp. cores from Singapore and Bali. We found that around 50% of the total P in the corals is organic P, and both inorganic and organic P in the skeleton can correlate with seawater DIP variability. The Bali core was collected offshore of a major agricultural area in which artificial fertilizer use began in the mid-1970s. Total P/Ca in this core shows a large increase in 1974, and is thereafter strongly related to precipitation. This suggests that P/Ca in this coral records the history of agricultural fertilizer run-off. We used the Singapore coral to directly relate skeletal P/Ca to a contemporaneous seawater DIP record. Despite the overall low DIP concentrations and modest seasonal variability at this site, we found a significant correlation between total P/Ca and seawater DIP (r 2 = 0.42, P = 0.04, N = 10) after excluding highly oligotrophic periods (DIP < 0.050 µmol/L, N = 4). Based on the global P/Ca–DIP calibration from multiple coral cores, seawater DIP reconstructions probably have an uncertainty of around ±0.115 µmol/L, which is likely to decrease if more studies are undertaken. Considering just the propagated errors from analytical uncertainty and skeletal heterogeneity suggests that the precision within a coral core could be as low as ±0.056 µmol/L, supporting the downcore application of the P/Ca proxy to track relative changes in DIP. We also estimate that the proxy is valid down to a lower boundary of coral P/Ca of roughly 6.5 µmol/mol, indicating limitations to this proxy only under consistently oligotrophic conditions. Finally, we speculate that organic P in coral skeletons may derive partly from the passive inclusion of dissolved organic P molecules from seawater and partly from the coral holobiont, providing a possible explanation for why total P/Ca can correlate with dissolved phosphate concentration. [ABSTRACT FROM AUTHOR]

Micro-analytical determination of copper (Cu) speciation in natural magmatic glasses, equilibrated below the nickel – nickel oxide (NNO) buffer, reveals that two copper species are commonly stabilized in such basaltic melts. X-ray absorption fine structure (XAFS) spectroscopic analysis of basaltic matrix glasses and melt inclusions (MI) from samples of mid-ocean ridge basalt (MORB), and from Nyamuragira, Etna and Kīlauea volcanoes shows that Cu complexes as both Cu(I)-sulfide and Cu(I)-oxide species in silicate melts. The proportion of each species correlates with the measured sulfur (S) abundance of the glass. In glasses with S abundances greater than ∼1000 ppm, Cu(I)-sulfide species are dominant, whereas in glasses with S abundances between 500 and 1000 ppm, both species are found to coexist. The Cu(I)-oxide species dominate at S concentrations below 500 ppm. In 1 atm S-free experimental glasses of basaltic composition that we analyzed, only Cu(I)-oxide species are detectable, regardless of the oxygen fugacity (f O 2), even at relatively high f O 2 values well above the NNO buffer. Our results demonstrate that XAFS techniques are highly sensitive in measuring Cu speciation in reduced (below NNO) basaltic glasses and that both oxide and sulfide complexes can be stabilized. The relative proportion of these two species is highly dependent on the concentration of S in the melt, and thus the Cu speciation in natural melts changes as S is lost from the melt by low pressure degassing. [ABSTRACT FROM AUTHOR]

The solubility of tungsten trioxide solid and the speciation of tungsten in NaCl-bearing solutions have been investigated through experiments conducted at 250, 300, and 350 °C under vapour-saturated water pressure. Based on the results of these experiments, the solubility of tungsten trioxide was controlled by temperature and pH, whereas the NaCl concentration did not affect the solubility except through its influence on the ionic strength of the solution. Two tungsten species were found to be present in the solutions, namely H 2 WO 4 0 at low pH and HWO 4 − at higher pH. These two species formed via the reactions WO 3 + H 2 O = H 2 WO 4 0 and WO 3 + H 2 O = HWO 4 − + H+, respectively. The logarithms of the equilibrium constants for these reactions are −5.18 ± 0.26, −4.97 ± 0.25, −4.69 ± 0.10, and −7.91 ± 0.30, −7.67 ± 0.29, −7.52 ± 0.18 for 250, 300, and 350 °C, respectively. In addition, the logarithms of the first and second association constants of H 2 WO 4 0 were determined to be 2.72, 2.71, 2.83, and 5.59, 6.49, 8.07 for 250, 300, and 350 °C, respectively. These values indicate that H 2 WO 4 0 is only important at low pH values (<2.8), and that HWO 4 − is the dominant tungsten species at pH conditions commonly encountered in nature. The data obtained in this study were used to model the solubility of scheelite and ferberite. This modeling indicates that tungsten concentrations are highest at high temperature in solutions with high salinity, low contents of calcium and iron, and either very low or high pH. The opposite is true for tungsten mineral precipitation from a fluid. [ABSTRACT FROM AUTHOR]

To better understand the solution of volatile species in a reduced magma ocean, we identify via Raman spectroscopy the nature of C-O-H-N volatile species dissolved in a series of reduced basaltic glasses. The oxygen fugacity (ƒ O2) during synthesis varied from highly reduced at two log units below the iron-wustite buffer (IW-2.1) to moderately reduced (IW-0.4), spanning much of the magmatic ƒ O2 conditions during late stages of terrestrial accretion. Raman vibrational modes for H 2 , NH 2 –, NH 3 , CH 4 , CO, CN–, N 2 , and OH– species are inferred from band assignments in all reduced glasses. The integrated area of Raman bands assigned to N 2 , CH 4 , NH 3 and H 2 vibrations in glasses increases with increasing molar volume of the melt, whereas that of CO decreases. Additionally, with increasing ƒ O2 , CO band areas increase while those of N 2 decrease, suggesting that the solubility of these neutral molecules is not solely determined by the melt molar volume under reduced conditions. Coexisting with these neutral molecules, other species as CN–, NH 2 – and OH– are chemically bonded within the silicate network. The observations indicate that, under reduced conditions, (1) H 2 , NH 2 –, NH 3 , CH 4 , CO, CN–, N 2 , and OH– species coexist in silicate glasses representative of silicate liquids in a magma ocean (2) their relative abundances dissolved in a magma ocean depend on melt composition, ƒ O2 and the availability of H and, (3) metal-silicate partitioning or degassing reactions of those magmatic volatile species must involve changes in melt and vapor speciation, which in turn may influence isotopic fractionation. [ABSTRACT FROM AUTHOR]

Speciation plays an important role in determining the fate and transport of metals in terrestrial surface and subsurface systems. Equilibrium speciation modeling in aqueous systems relies on thermodynamic constants (log K values) of complexes, which are subject to uncertainties. Here, using Monte Carlo (MC) simulations with Latin hypercube sampling (LHS) we systematically analyze the propagation of thermodynamic constant uncertainty through speciation modeling of an inorganic mercury-sulfide-chloride-water system. We find that seemingly small variances of the input log K normal distributions can lead to output species concentrations spanning multiple orders of magnitude, with highly skewed probability distributions. When equilibrium with mineral metacinnabar (β-HgS(s)) is neglected, the relative uncertainty of each output species is strongly positively correlated with the skewness of its concentration probability distribution, i.e., the lowest uncertainty occurs when the species concentration probability distribution is the most negatively skewed as its concentration approaches the total element concentration limit. The highest uncertainty in the identity of dominant species is located around species equivalence points. For cases where the mineral equilibrium is included, we derive analytical probability density functions for the log concentrations of all major species in the system. The mineral log K uncertainty is found to be an important contributor to all output concentration uncertainties. The analysis of combined effects of both pH and total sulfide concentration on output concentration uncertainties shows that high concentration uncertainties occur under highly sulfidic alkaline conditions and low uncertainties at low pH and sulfide concentrations. An analysis as presented here can distinguish between conditions that require a full uncertainty analysis and those for which the classical deterministic speciation modeling suffices. [ABSTRACT FROM AUTHOR]

Abstract Iron is among the most abundant elements in Earth's crust and is also a major aqueous solute in a variety of hydrothermal settings, yet major questions remain regarding the solubility and speciation of iron at hydrothermal conditions. Here, we conducted hydrothermal diamond-anvil cell experiments using synchrotron-radiation micro-XRF and XANES analyses as well as Raman spectroscopy, in situ at hydrothermal temperatures and pressures, to characterize the solubility and speciation of iron in fluids buffered by a variety of mineral assemblages. Our experiments included the assemblages hematite-magnetite (HM), fayalite-magnetite-quartz (FMQ), and magnetite-pyrite-pyrrhotite (MPP). Our results indicate highest solubilities of HM in HCl solutions. In sodium chloride solutions of similar molalities, FMQ shows higher Fe solubility than MPP. XANES data are interpreted as preponderance of ferrous iron in all experiments. Comparison of XANES spectra of these solutions with calculated XANES spectra from the literature indicates octahedral FeCl x (H 2 O) 6−x 2−x (x = 0–3) as predominant Fe(II) species at lower Cl-Fe ratios, and suggests additional tetrahedral FeCl 4 2− or FeCl 3 (H 2 O)− at high Cl-Fe ratios. Raman spectra to 600 °C show that the predominant species in ferric iron solutions is FeCl 2 (H 2 O) 4 + at temperatures less than 100 °C, which transitions to FeCl 4 − between 100 and 200 °C. An additional Raman band that occurred in some spectra of a H 2 O + HCl fluid equilibrated with hematite ± magnetite at temperatures greater than 300 °C may originate from an FeCl 3 (H 2 O) x (x = 0–3) species. All Raman spectra of ferrous iron solutions show a fairly broad band at about 280 cm−1, which can be interpreted to stem from Fe(II)–Cl vibrations of FeCl 3 (H 2 O) x − (x = 0–3) (point group D 3h) species or, at low Cl-Fe ratios of about two or less, from octahedral FeCl x (H 2 O) 6−x 2−x (x = 0–3) species. These results provide important constraints on the hydrothermal mobilization of iron and fluid-rock reactions involving iron- and chloride-bearing fluids. [ABSTRACT FROM AUTHOR]

Abstract The manganese content of aragonitic bivalve shells is a potential archival indicator of temporal Mn bioavailability in aquatic environments. The Mn speciation mechanism in biogenic aragonite minerals remains elusive because the analog is challenging to synthesize, and the metastable phase has yet to be fully resolved experimentally. In this study, we performed density functional theory (DFT) computations of hypothetical Mn-doped aragonite to examine its local coordination structure and thermodynamic and electronic properties. Our DFT calculations reproduced the experimental crystal structures and solubility product constants (K sp) of Mn-doped calcite. The magnetic moment of Mn was close to 5 μ B in both Mn-doped calcite and Mn-doped aragonite (Ca 1− x Mn x CO 3). The calculated K sp of Mn-doped aragonite was higher than that of Mn-doped calcite and increased with Mn content, indicative of the unfavorable coprecipitation of Mn with the aragonite phase. We found that the incorporation of a small mole fraction of Mn into aragonite created significant structural distortion around the Mn site, resulting in mixed coordination numbers of Mn (mainly five and seven). Valence-to-core X-ray emission spectroscopy (XES) measurements are useful in determining the coordination environment of Mn complexes. We calculated theoretical XES spectra, with a 1 s core hole in Mn, for Mn-doped calcite and four versions of Mn-doped aragonite. The Boltzmann-averaged spectrum for different coordination numbers in Mn-doped aragonite was akin to an experimental XES spectrum of aragonitic bivalve shells. The energy position of the K β 2,5 band was calculated to be insensitive to Mn speciation in CaCO 3 ; however, the band intensity was relatively sensitive to Mn speciation. The XES spectrum intensity decreased exponentially with increasing Mn–O distance. This quantitative XES relationship we report can reduce uncertainties in the spectral interpretation due to the absence of an Mn-doped aragonite reference spectrum. [ABSTRACT FROM AUTHOR]

Abstract Antimony is an environmental contaminant, whose mobility in soils, sediments and groundwater systems is strongly influenced by interactions with Fe(III) oxide minerals. When exposed to reducing conditions, these minerals can undergo reductive dissolution via the activity of Fe(III)-reducing microorganisms, thereby potentially liberating previously retained Sb. In addition, microbial Fe(III)-reduction and the consequent production of Fe(II) can induce the formation of secondary Fe(III)- and Fe(II)-bearing minerals, which may alter the speciation and partitioning of Sb. In this study, we examined Sb behaviour during (1) the microbially-mediated reduction and transformation of Sb(V)-bearing ferrihydrite by the dissimilatory Fe(III)-reducing bacterium, Shewanella putrefaciens (strain CN32), and (2) during the associated abiotic Fe(II)-catalyzed transformation of Sb(V)-bearing ferrihydrite. Antimony K-edge XANES spectroscopy showed negligible reduction of Sb(V) to Sb(III) in both experiments, reflecting the redox stability of Sb(V) under these conditions. X-ray diffraction and Fe K-edge EXAFS spectroscopy revealed that both microbially-mediated Fe(II) production as well as the experimental addition of aqueous Fe(II) under abiotic conditions triggered rapid transformation of the initial ferrihydrite to feroxyhyte (δ′-FeOOH) and goethite (α-FeOOH). Goethite has been widely observed as a product of the Fe(II)-catalyzed transformation of ferrihydrite. However, the present study is the first to document the formation of feroxyhyte via this pathway, with feroxyhyte formation appearing to be favored by the presence of Sb(V). The formation of these secondary Fe(III) oxides was associated with substantial decreases in aqueous Sb concentrations and in the amount of surface-bound Sb (as defined via extractions with 1 M PO 4 3−). This is consistent with the incorporation of Sb(V) into the newly formed feroxyhyte and goethite via substitution for Fe(III). The results of this study provide new perspectives on coupling between Sb geochemistry and Fe mineralogy by showing that microbial Fe(III)-reduction and associated secondary Fe(III)-oxide formation can help to immobilize Sb(V) in reducing environments. [ABSTRACT FROM AUTHOR]

Abstract Organic matter can be associated with mineralization in hydrothermal ore deposits. One hypothesis is that this organic matter represents remnants of organic fluids (crude oils) that were competing with aqueous fluids for metal transport and contributed to metal endowment. We investigated the transport of gold (Au) in model oil compounds (S-free n -dodecane, CH 3 (CH 2) 10 CH 3 , DD; and S-bearing 1-dodecanethiol, CH 3 (CH 2) 10 CH 2 SH; DDT) from 25 °C to 250 °C using in-situ synchrotron X-ray absorption spectroscopy (XAS) experiments to determine the speciation and the structural properties of gold complexes in the aqueous- and oil-based fluids. For most experiments, DD or DDT were in contact with Au-bearing acidified water, or acidified water plus 10 wt% NaCl (pH 25°C = 1.85 in both cases). Gold rapidly partitioned from the aqueous phase into DD and DDT. Below 125 °C, Au(III)Cl is dominant in the DD and the adjacent water with a refined coordination number (CN) of chloride of 4.0(3) and an Au Cl bond length of 2.28 Å, consistent with the tetrachloroaurate complex (AuCl 4 −) being stable in both the aqueous and organic phases. In contrast, Au(III) is rapidly reduced in the presence of DDT and an Au(I) complex dominates in both water and adjacent DDT with a CN of sulfur ∼2.0, suggesting a [RS-Au-SR]− (RS = DDT with deprotonated thiol group) complex with Au S bond lengths ranging from 2.29(1) Å to 2.31(3) Å. In an open system of DDT in contact with water, of which the water and DDT were analyzed separately, AuCl 4 − was dominant in the water phase, and Au(RS) 2 − dominant in DDT, possibly due to different equilibration kinetics in the beaker and glassy carbon tube. Since sulfur and organothiol compounds are ubiquitous and abundant components in natural oils, this study demonstrates the potential of natural oils to scavenge and enrich gold from co-existing gold-bearing brines. In particular, Au(I) organothiol complexes may contribute to transport in low-temperature (<125 °C) ore fluids such as those in basinal environments – in both hydrothermal fluids and oils. At temperatures ≥125 °C, gold was reduced to metallic gold in all experiments, suggesting that organo-stabilized nanoparticles may be the major form of gold to be scavenged, concentrated or transported in crude oils at these conditions. The results imply that brine-oil interactions may enrich Au in oils, and that oils may be an effective ore fluid in sedimentary environments. [ABSTRACT FROM AUTHOR]

Abstract The speciation, burial, and isotopic composition of vanadium (V) in seawater is predicted to be closely coupled to the redox state of the oceans. While the speciation and burial terms are reasonably constrained, the V isotopic composition of seawater has remained elusive owing to significant analytical challenges. To this end, for the first time we have developed and validated a new method to purify V from large volume (≥500 mL) seawater samples that we used to determine the V isotopic composition of seawater. Our method comprises four discrete V-purification steps that exploit ion-exchange chromatography including Nobias chelate and anion exchange resin(s) and measurement by multi-collector inductively-coupled plasma mass spectrometry. Results from several samples with addition of standard V solution with known isotope composition show no isotopic deviation in the chemical and/or analytical procedures and our reproducibility is within typical analytical error for vanadium isotopes measurements. Though V yields were non-quantitative (averaging ≈ 70%) for natural seawater samples, our approach was nonetheless validated with additional experiments. Therein, synthetic seawater solutions of known V isotopic composition with concentration similar to natural seawater were used to confirm that there is limited isotope fractionation during analytical procedures with similar yields. We further tested seawater samples using UV radiation, HNO 3 /HCl oxidation, and High-Pressure Asher treatments to ensure there was limited effects from potential non-dissolved phases or variable V speciation such as organic ligand binding of V. All tests except the High-Pressure Asher samples had similar recoveries (i.e. >70%) and all recorded similar isotopic values within error which suggest our method is robust and reliable for V isotopic measurement of seawater. Using our most optimal method, we report V isotope data for several seawater samples from surface and subsurface Atlantic Ocean and deep Pacific Ocean for the first time. Inter-laboratory sample comparison shows that the data was within analysis error (∼0.15‰, 2SD). Initial results imply that the deep ocean is isotopically homogeneous with respect to V, and the uptake of V in surface waters appear to cause very limited, if any, isotope fractionation as it is within analytical uncertainties. Thus, our results suggest a reference seawater V isotope composition of +0.20 ± 0.15‰ relative to V isotope standard AA solution. This work analyzes the first V isotope value of seawater which provides a key foundation for future work to constrain the modern marine V isotope cycle and budget and application for paleoceanographic research. [ABSTRACT FROM AUTHOR]

Highlights • Boiled basalt-hosted geothermal fluids have lower REE than submarine hydrothermal fluids. • REE are incorporated into well scale precipitates. • Mineral precipitation and chloride complexation control aqueous REE concentrations in boiled geothermal fluids. Abstract Hydrothermal processes that lead to REE fractionation and redistribution are important for understanding water-rock interactions in geothermal energy resources and mineral deposits, and for determining how submarine hydrothermal activity affects the composition of oceanic crust. Much previous work on REE transport and deposition has focused on submarine hydrothermal vents. We report REE concentrations in boiled fluids sampled from five subaerial, basalt-hosted geothermal fields, and explore controls on aqueous REE concentrations by ligand complexation and mineral supersaturation. Samples that boiled at pressures between 0.8 and 2.83 MPa were obtained from the Reykjanes, Svartsengi, Hellisheidi, and Nesjavellir geothermal systems in Iceland, and the Puna geothermal system in Hawaii. For comparison, we also report REE concentrations in hydrothermal fluids from the sediment hosted submarine Middle Valley hydrothermal system, which boiled at >250 MPa. The pH (25°C) values of the sampled subaerial geothermal fluids range from 3.94 to 6.77, and Cl concentrations range from near seawater (502 mmol/kg) to dilute (1.9 mmol/kg). La, Ce and Eu are the only REE present at levels above 5 picomole/kg (pmol/kg) in the boiled geothermal fluids; and there are notable CI chondrite normalized La and Eu anomalies in the saline fluids. REE concentrations in Middle Valley hydrothermal fluids fall within the typical range reported for submarine hydrothermal fluids and have around two orders of magnitude higher REE than the boiled subaerial geothermal fluids. Bulk samples of precipitates in pipes from the Reykjanes geothermal system have detectable REE, confirming that downhole fluids have lost REE during boiling and production of fluids for geothermal energy. Isenthalpic boiling models show that the proportions of La and Eu chloride complexes increase relative to other aqueous species as boiling progresses, attenuating the incorporation of La and Eu into precipitated well scale solids. Fluorapatite is calculated to precipitate on boiling of low pH and saline fluids and calcite is calculated to precipitate from dilute and near-neutral pH fluids, and these minerals likely sequester REE in boiled subaerial fluids. Submarine hydrothermal fluids are constrained to boiling at higher temperatures than subaerial geothermal fluids owing to pressure from overlying cold seawater, therefore secondary minerals and solids that incorporate REE are not extensively precipitated and REE concentrations in the fluids are higher. [ABSTRACT FROM AUTHOR]

To reliably utilize the information on growth and environment stored within the hard carbonate shells of long-lived bivalves, it is necessary to accurately count the annual growth lines from their shell cross-sections. In this study, we investigated the relationship between the distribution of minor elements and visual lines in the shell cross-sections of the long-lived mussel Margaritifera laevis collected from the Chitose River in Hokkaido, Japan. The distribution of minor elements, such as sulfur (S), phosphorus (P), strontium (Sr), magnesium (Mg), and sodium (Na), in a cross-section of the umbo was determined by X-ray fluorescence analysis using synchrotron soft X-ray radiation. The chemical properties of S and P were analyzed in-situ using soft X-ray photoabsorption spectroscopy, and the chemical form dependence of S distribution was determined by speciation mapping. In addition, the annual and disturbance lines were visualized by staining shell cross-sections. Our results showed that the distribution of reduced S ( i.e. , S C and S H) corresponded to the annual lines, whereas the distribution of oxidized S ( i.e. , SO 4 2− ) corresponded to the disturbance lines. Overall, this study revealed that speciation analysis of S in shell cross-sections can be used to determine the origin of visual lines in chronological studies. [ABSTRACT FROM AUTHOR]

As the most abundant transition metal in the Earth’s crust, iron is a key player in the planetary redox budget. Observations of iron minerals in the sedimentary record have been used to describe atmospheric and aqueous redox environments over the evolution of our planet; the most common method applied is iron speciation, a geochemical sequential extraction method in which proportions of different iron minerals are compared to calibrations from modern sediments to determine water-column redox state. Less is known about how this proxy records information through post-depositional processes, including diagenesis and metamorphism. To get insight into this, we examined how the iron mineral groups/pools (silicates, oxides, sulfides, etc.) and paleoredox proxy interpretations can be affected by known metamorphic processes. Well-known metamorphic reactions occurring in sub-chlorite to kyanite rocks are able to move iron between different iron pools along a range of proxy vectors, potentially affecting paleoredox results. To quantify the effect strength of these reactions, we examined mineralogical and geochemical data from two classic localities where Silurian-Devonian shales, sandstones, and carbonates deposited in a marine sedimentary basin with oxygenated seawater (based on global and local biological constraints) have been regionally metamorphosed from lower-greenschist facies to granulite facies: Waits River and Gile Mountain Formations, Vermont, USA and the Waterville and Sangerville-Vassalboro Formations, Maine, USA. Plotting iron speciation ratios determined for samples from these localities revealed apparent paleoredox conditions of the depositional water column spanning the entire range from oxic to ferruginous (anoxic) to euxinic (anoxic and sulfidic). Pyrrhotite formation in samples highlighted problems within the proxy as iron pool assignment required assumptions about metamorphic reactions and pyrrhotite’s identification depended on the extraction techniques utilized. The presence of diagenetic iron carbonates in many samples severely affected the proxy even at low grade, engendering an interpretation of ferruginous conditions in all lithologies, but particularly in carbonate-bearing rocks. Increasing metamorphic grades transformed iron in carbonates into iron in silicate minerals, which when combined with a slight increase in the amount of pyrrhotite, drove the proxy toward more oxic and more euxinic conditions. Broad-classes of metamorphic reactions (e.g. decarbonation, silicate formation) occurred at distinct temperatures-pressures in carbonates versus siliciclastics, and could be either abrupt between metamorphic facies or more gradual in nature. Notably, these analyses highlighted the importance of trace iron in phases like calcite, which otherwise might not be included in iron-focused research i.e. ore-system petrogenesis, metamorphic evolution, or normative calculations of mineral abundance. The observations show that iron is mobile and reactive during diagenesis and metamorphism, and these post-depositional processes can readily overprint primary redox information held by iron speciation. However, in principle, additional mineralogical and petrographic approaches can be combined with iron speciation data to help untangle many of these post-depositional processes and arrive at more accurate estimates of paleoenvironmental redox conditions and processes, even for metamorphosed samples. [ABSTRACT FROM AUTHOR]

Despite the pioneering efforts to explore the nature of carbon in carbon-bearing silicate melts under compression, experimental data for the speciation and the solubility of carbon in silicate melts above 4 GPa have not been reported. Here, we explore the speciation of carbon and pressure-induced changes in network structures of carbon-bearing silicate (Na 2 O-3SiO 2 , NS3) and sodium aluminosilicate (NaAlSi 3 O 8 , albite) glasses quenched from melts at high pressure up to 8 GPa using multi-nuclear solid-state NMR. The 27 Al triple quantum (3Q) MAS NMR spectra for carbon-bearing albite melts revealed the pressure-induced increase in the topological disorder around 4 coordinated Al ( [4] Al) without forming [5,6] Al. These structural changes are similar to those in volatile-free albite melts at high pressure, indicating that the addition of CO 2 in silicate melts may not induce any additional increase in the topological disorder around Al at high pressure. 13 C MAS NMR spectra for carbon-bearing albite melts show multiple carbonate species, including [4] Si(CO 3 ) [4] Si, [4] Si(CO 3 ) [4] Al, [4] Al(CO 3 ) [4] Al, and free CO 3 2– . The fraction of [4] Si(CO 3 ) [4] Al increases with increasing pressure, while those of other bridging carbonate species decrease, indicating that the addition of CO 2 may enhance mixing of Si and Al at high pressure. A noticeable change is not observed for 29 Si NMR spectra for the carbon-bearing albite glasses with varying pressure at 1.5–6 GPa. These NMR results confirm that the densification mechanisms established for fluid-free, polymerized aluminosilicate melts can be applied to the carbon-bearing albite melts at high pressure. In contrast, the 29 Si MAS NMR spectra for partially depolymerized, carbon-bearing NS3 glasses show that the fraction of [5,6] Si increases with increasing pressure at the expense of Q 3 species ( [4] Si species with one non-bridging oxygen as the nearest neighbor). The pressure-induced increase in topological disorder around Si is evident from an increase in peak width of [4] Si with pressure. 17 O NMR spectrum shows that the fraction of Na⋯O [5] Si in carbon-bearing NS3 glasses is less than that of carbon-free NS3 glasses at 6 GPa potentially due to the formation of bridging carbonate species. While its presence is not evident from the 17 O NMR spectrum primarily due to low carbon concentration, 13 C MAS NMR results imply the formation of bridging carbonates, [4] Si(CO 3 ) [4] Si, above 6 GPa. The spin-lattice relaxation time ( T 1 ) of CO 2 in albite melts increases with increasing pressure from 42 s (at 1.5 GPa) to 149 s (at 6 GPa). Taking the pressure-induced change in T 1 of carbon species into consideration, total carbon content in carbon-bearing albite melts increases with pressure from ∼1 wt% at 1.5 GPa to ∼4.1 wt% at 6 GPa. The results also reveal a noticeable drop in the peak intensity of free carbonates in carbon-bearing NS3 melts at 6 GPa, implying a potential non-linear change in the carbon solubility with pressure. The current results of carbon speciation in the silicate melts above 4 GPa provide an improved link among the atomic configurations around carbon species, their carbon contents, and isotope composition of carbon-bearing melts in the upper mantle. [ABSTRACT FROM AUTHOR]

Predicting the kinetics of heavy metal adsorption and desorption in soil requires consideration of multiple heterogeneous soil binding sites and variations of reaction chemistry conditions. Although chemical speciation models have been developed for predicting the equilibrium of metal adsorption on soil organic matter (SOM) and important mineral phases (e.g. Fe and Al (hydr)oxides), there is still a lack of modeling tools for predicting the kinetics of metal adsorption and desorption reactions in soil. In this study, we developed a unified model for the kinetics of heavy metal adsorption and desorption in soil based on the equilibrium models WHAM 7 and CD-MUSIC, which specifically consider metal kinetic reactions with multiple binding sites of SOM and soil minerals simultaneously. For each specific binding site, metal adsorption and desorption rate coefficients were constrained by the local equilibrium partition coefficients predicted by WHAM 7 or CD-MUSIC, and, for each metal, the desorption rate coefficients of various binding sites were constrained by their metal binding constants with those sites. The model had only one fitting parameter for each soil binding phase, and all other parameters were derived from WHAM 7 and CD-MUSIC. A stirred-flow method was used to study the kinetics of Cd, Cu, Ni, Pb, and Zn adsorption and desorption in multiple soils under various pH and metal concentrations, and the model successfully reproduced most of the kinetic data. We quantitatively elucidated the significance of different soil components and important soil binding sites during the adsorption and desorption kinetic processes. Our model has provided a theoretical framework to predict metal adsorption and desorption kinetics, which can be further used to predict the dynamic behavior of heavy metals in soil under various natural conditions by coupling other important soil processes. [ABSTRACT FROM AUTHOR]

The speciation of U in NaCl-bearing solutions at temperatures up to 250 °C and concentrations of NaCl up to 1.5 m has been investigated using an in situ spectroscopic technique. The recorded spectra permit us to identify the species present in the solutions as UO 2 2+ , UO 2 Cl + , and UO 2 Cl 2 °. UO 2 Cl 3 − is also likely present at high temperatures and NaCl concentrations, but concentrations of this species are insufficient for derivation of the formation constants. No evidence was found for species of higher ligand (Cl − ) number. Thermodynamic stability constants derived for these species show fair agreement with published data for 25 °C, but differ significantly from those predicted by an earlier high-temperature study (Dargent et al., 2013), which suggested that UO 2 Cl 4 2− and UO 2 Cl 5 3− contribute significantly to the mass balance of uranyl chloride complexes, especially at high temperature. In contrast, our data suggest that the main uranyl-chloride complex present in aqueous solutions at T > 150 °C and concentrations of NaCl relevant to natural hydrothermal systems is UO 2 Cl 2 °. The values of the logarithms of thermodynamic formation constants (β) for the reaction UO 2 2+  + Cl −  = UO 2 Cl + are 0.02, 0.25, 0.55, 1.09, 1.59, and 2.28 derived at 25, 50, 100, 150, 200, and 250 °C, respectively. For the reaction UO 2 2+  + 2Cl −  = UO 2 Cl 2 ° the values of log β derived at these temperatures are 0.4, 0.58, 0.74, 1.44, 2.18, and 3.42. Values of the formation constant estimated for uranyl-chloride species predict the high concentrations of U observed by Richard et al. (2011) in fluid inclusions of the giant McArthur River unconformity-type uranium deposit. [ABSTRACT FROM AUTHOR]

Despite the importance of phototrophic biofilms in metal cycling in freshwater systems, metal isotope fractionation linked to metal adsorption and uptake by biofilm remains very poorly constrained. Here, copper isotope fractionation by a mature phototrophic biofilm during Cu surface adsorption and incorporation was studied in batch reactor (BR) and open drip flow reactor (DFR) systems at ambient conditions. X-ray Absorption Spectroscopy (both Near Edge Structure, XANES, and Extended Fine Structure, EXAFS) at Cu K-edge of the biofilm after its interaction with Cu in BR experiments allowed characterizing the molecular structure of assimilated Cu and quantifying the degree of Cu II to Cu I reduction linked to Cu assimilation. For both BR and DFR experiments, Cu adsorption caused enrichment in heavy isotope at the surface of the biofilm relative to the aqueous solution, with an apparent enrichment factor for the adsorption process, ε 65 Cu ads , of +1.1 ± 0.3‰. In contrast, the isotope enrichment factor during copper incorporation into the biofilm (ε 65 Cu inc ) was highly variable, ranging from −0.6 to +0.8‰. This variability of the ε 65 Cu inc value was likely controlled by Cu cellular uptake via different transport pathways resulting in contrasting fractionation. Specifically, the Cu II storage induced enrichment in heavy isotope, whereas the toxicity response of the biofilm to Cu exposure resulted in reduction of Cu II to Cu I , thus yielding the biofilm enrichment in light isotope. EXAFS analyses suggested that a major part of the Cu assimilated by the biofilm is bound to 5.1 ± 0.3 oxygen or nitrogen atoms, with a small proportion of Cu linked to sulfur atoms (N S < 0.6) of sulfhydryl groups. XANES analyses showed that the proportion of Cu II vs Cu I , compared to the initial Cu II /Cu I ratio, decreased by 14% after the first hour of reaction and by 6% after 96 h of reaction. The value of ε 65 Cu inc of the biofilm exhibited a similar trend over time of exposure. Our study demonstrates the complexity of biological processes associated with live phototrophic biofilms, which produce large and contrasting isotope fractionations following rather small Cu redox and speciation changes during uptake, storage or release of the metal, i.e. , favoring heavy isotopes during complexation with carboxylate ligands and light isotopes during reduction of Cu II -O/N to Cu I -sulfhydryl moieties. [ABSTRACT FROM AUTHOR]