Unusual varieties of Early Cretaceous (120 ± 3 Ma) rhyolites in the Nyalga basin (Central Mongolia) bear exceptionally high percentages of fluorite reaching 30–36 wt%. The origin of fluorite in the ...rhyolites does not fit the existing models of magmatic crystallization in haplogranitic melts or subsolidus postmagmatic and hydrothermal fluoritization. We suggest that the fluorite-enriched rhyolites formed after eruption of magma that contained a mixture (emulsion) of immiscible rhyolitic and F-Ca melts. The F-Ca melt separated from the rhyolitic magma as a result of fluoride-silicate liquid immiscibility when the concentration of F in the Ca-bearing subaluminous–peraluminous rhyolitic melt locally increased to 1–2 wt%. Trace and minor elements, mainly REE, Y, Sr, P, Zr, Hf, Ta and Nb partitioned between the two immiscible melts. The F-Ca melt existed in the course of rhyolitic magma evolution from the crystallization of phenocrysts in the magma source till eruption and quenching on the Earth's surface. The effective viscosity of the magma (mixed rhyolitic and F-Ca melts with fluid bubbles) may have been of the same order of magnitude as that of viscous liquid during the formation of the rhyolite matrix at 750–650°С. The quenching of the F-Ca melt formed an F-Ca phase consisting of submicrometer fluorite particles, while crystallization of rhyolitic glass produced quartz-sanidine symplectites. According to SEM EDS and LA-ICP-MS data, the F-Ca phase typically shows large ranges of O, Y, La, Ce, Nd, Sr, Sc, P, Si (wt%) and trace element (ppm) concentrations varying for orders of magnitude. The variations result from recrystallization of the F-Ca phase under the effect of a low-density (0.05–0.1 g/cm3) aqueous fluid released from the degassing rhyolitic melt. Upon interaction with the fluid, micrometer fluorite particles of the F-Ca phase liberated from impurities and transformed into larger crystalline segregations. The process occurred within the 570–780°С temperature range at a high oxygen fugacity of ΔlgfO2 Ni-NiO (0.9–1.7). The oxidized environment maintained crystallization of ferrian ilmenite, monazite-group As-bearing minerals, and cerianite, as well as replacement of titanomagnetite by hematite. The O2− → F− substitution in the fluorite structure led to the formation of O-vacancy centers responsible for luminescence of the F-Ca phase in the rhyolite matrix upon laser excitation. The compositions close to the primary F-Ca phase have preserved in relict globules existing as inclusions in the matrix and in enclosed minerals. The F-Ca melt could exist in a metastable liquid state during the eruption of rhyolitic magma at an oxygen fugacity remaining high till the subsolidus temperature of the rhyolitic melt. The fluoride-silicate liquid immiscibility with participation of an oxygenated F-Ca melt may be a common feature of F-rich silicate magmas at high oxygen fugacity. Fluorite and related mineralization in many igneous rocks and magmatic ores possibly resulted from transformation of the F-Ca melt enriched in REE, Y and other minor elements.
•Fluorite-rich rhyolite formed after quenching of immiscible rhyolitic and F-Ca melts.•Minor and trace elements partitioned between the two immiscible melts.•F-Ca melt remained liquid till the subsolidus temperatures of the rhyolitic melt.•F-Ca melt may have kept its supercooled metastable state at high oxygen fugacity.•Fluorite and related ore mineralization may result from F-Ca melt transformation.
Spectral variable selection is an important step in spectroscopic data analysis, as it tends to parsimonious data representation and can result in multivariate models with greater predictive ability. ...In this study, we used VIS-NIR (visible to near-infrared) diffuse reflectance and DRIFT (diffuse reflectance infrared Fourier transform in the mid-infrared range, MIR) spectroscopy to determine a series of chemical and biological soil properties. Multivariate calibrations were performed with partial least squares regression (PLSR) using the full absorbance spectra (VIS-NIR: 400–2500nm with 5-nm intervals; MIR: 4000–800cm−1 with 4-cm−1 intervals) and with a combination of PLSR and CARS (competitive adaptive reweighted sampling) to integrate only the most informative key variables. The CARS procedure has as yet not been applied in the field of soil spectroscopy. As set heterogeneity is crucial for an optimal calibration, we tested these approaches to a sample set of 60 agricultural samples covering a broad range of different parent materials, soil textures, organic matter contents and soil pH values. Soil samples were taken from the Ap horizon (0–10cm depth), air-dried and pulverised before the lab spectroscopic measurements were performed. In a cross-validation approach, the CARS–PLSR method was markedly more accurate than full spectrum-PLSR for all investigated soil variables and both spectral regions. With MIR data and CARS–PLSR, excellent results (indicated by a residual prediction deviation (RPD) greater than 3.0) were obtained for organic carbon (OC), nitrogen (N), microbial biomass-C (Cmic) and pH values; for hot water extractable C (Chwe), RPD was 2.60. The accuracies obtained with VIS-NIR data were considerably lower than those with the MIR spectra; best results were retrieved for pH and Cmic (approximately quantitative as indicated by RPD values between 2.0 and 2.5). The information content of the MIR data was substantially different from the VIS-NIR information, as indicated by 2D correlation analysis. We found an overall blurred 2D correlation pattern between both spectral regions with moderate to low correlation coefficients, which suggested that the heterogeneity of the studied soil sample population had led to a very complex blurring of overtones and combination bands in the NIR region.
Statistical CARS selections were physically reasonable. MIR key wavenumbers for the studied C fractions were inter alia identified at the bands at 2920cm−1 and 2850cm−1 (both aliphatic CH-groups) and the region between 1740 and 1600cm−1 (CO-groups) and represent hydrophobic and hydrophilic compounds of soil organic matter. Important VIS-NIR wavelengths for assessing C fractions and N were located nearby the prominent water absorption band at 1915nm and the hydroxyl band at 2200nm.
The simplicity of the approach, parsimony of the multivariate models, accuracy levels in the cross-validation and physically reasonable selections indicated a successful operation of the CARS procedure. It should be further examined with a larger number of samples using separate calibration and validation sets.
•Informative key spectral variables were effectively selected with the CARS method.•CARS–PLSR improved cross-validated accuracies compared to full spectrum-PLSR.•Statistically selected MIR wavenumbers matched with bands of relevant functional groups.•2D-correlations did not reflect physical couplings between MIR and NIR regions.
Converting biomass waste into biochar by slow pyrolysis with subsequent soil amendment is a prospective approach with multiple environmental benefits including soil contamination remediation, soil ...amelioration and carbon sequestration. This study selected cow manure as precursor to produce biochar under 300 °C, 400 °C, 500 °C and 600 °C, and a remarkable promotion of carbon (C) retention in biochar by incorporation of exogenous Ca was achieved at all investigated pyrolysis temperatures. The C retention was elevated from 49.2 to 68.3% of pristine biochars to 66.1–79.7% of Ca-composite biochars. It was interesting that extent of this improvement increased gradually with rising of pyrolysis temperature, i.e., doping Ca in biomass promoted pyrolytic C retention in biochar by 16.6%, 23.4%, 29.1% and 31.1% for 300 °C, 400 °C, 500 °C and 600 °C, respectively. Thermogravimetric-mass spectrometer (TG-MS) and X-ray photoelectron spectroscopy (XPS) showed that Ca catalyzed thermal-chemical reactions and simultaneously suppressed the release of small organic molecular substances (C2–C7) via physical blocking (CaO, CaCO3, and CaClOH) and chemical bonding (CO and OC–O). The catalyzation mainly occurred at 200–400 °C, while the suppression was more prominent at higher temperatures. Raman spectra and 2D FTIR analysis on biochar microstructure showed that presence of Ca had negative influence on carbon aromatization and thus weakened biochar's stability, while increasing pyrolysis temperature enhanced the stability of carbon structure. Finally, with integrating “C retention” during pyrolysis and “C stability” in biochar, the maximum C sequestration (56.3%) was achieved at 600 °C with the participation of Ca. The study highlights the importance of both Ca and pyrolysis temperature in enhancing biochar's capacity of sequestrating C.
Display omitted
•Exogenous mineral Ca in pyrolysis could promote more carbon retained in biochar.•Extent of this promotion increased gradually with rising of pyrolysis temperature.•Biochar stability was determined by interaction of Ca and pyrolysis temperature.•Ca suppressed release of small molecules via physical blocking/chemical bonding.•Optimal carbon sequestration (56.3%) was achieved at 600 °C with Ca participation.
In this study, polymetal(iod)s-contaminated mining soil from the Huainan coalfield, Anhui, China, was used to investigate the synergistic effects of biochar (BC), raw coal (RC), and hydrothermally ...treated coal (HTC) on the immobilization, speciation, transformation, and accumulation of Cd, Cr, and Pb in a soil–plant system via geochemical speciation and advanced spectroscopic approaches. The results revealed that the BC-2% and BC–HTC amendments were more effective than the individual RC, and/or HTC amendments to reduce ethylene-diamine-tetraacetic acid (EDTA)-extractable Cd, Cr, and Pb concentrations by elevating soil pH and soil organic carbon content. Soil pH increased by 1.5 and 2.5 units after BC-2% and BC–HTC amendments, respectively, which reduced EDTA-extractable Cd, Cr, and Pb to more stabilized forms. Metal speciation and X-ray photoelectron spectroscopy analyses suggested that the BC–HTC amendment stimulated the transformation of reactive Cd, Cr, and Pb (exchangeable and carbonate-bound) states to less reachable (oxide and residual) states to decrease the toxicity of these heavy metals. Fourier transform infrared spectroscopy and X-ray diffraction analyses suggested that reduction and adsorption by soil colloids may be involved in the mechanism of Cd(II), Cr(VI), and Pb(II) immobilization via hydroxyl, carbonyl, carboxyl, and amide groups in the BC and HTC. Additionally, the BC-2% and BC–HTC amendments reduced Cd and Pb accumulation in maize shoots, which could mainly be ascribed to the reduction of EDTA-extractable heavy metals in the soil and more functional groups in the roots, thus inhibiting metal ion translocation by providing the electrons necessary for immobilization, compared to those in roots grown in the unamended soil. Therefore, the combined application of BC and HTC was more effective than the individual application of these amendments to minimize the leaching, availability, and exchangeable states of Cd, Cr, and Pb in polymetal(iod)s-contaminated mining soil and accumulation in maize.
Display omitted
•Cd, Cr, and Pb adsorption capacity enhanced by combined application of biochar and hydrothermally treated coal.•Biochar and hydrothermally treated coal increased surface functionality, SOC, and pH in the soil.•Combined application of both reduced the exchangeable fractions of heavy metals.•Application of biochar and hydrothermally treated coal reduced plants metal uptake.•Hydrothermally treated coal with biochar could be served as a novel cost-effective soil amendment.
The ubiquity of Au-bearing arsenian pyrite in hydrothermal ore deposits suggests that the coupled geochemical behaviour of Au and As in this sulfide occurs under a wide range of physico-chemical ...conditions. Despite significant advances in the last 20years, fundamental factors controlling Au and As ratios in pyrite from ore deposits remain poorly known. Here we explore these constraints using new and previously published EMPA, LA-ICP-MS, SIMS, and μ-PIXE analyses of As and Au in pyrite from Carlin-type Au, epithermal Au, porphyry Cu, Cu–Au, and orogenic Au deposits, volcanogenic massive sulfide (VHMS), Witwatersrand Au, iron oxide copper gold (IOCG), and coal deposits. Pyrite included in the data compilation formed under temperatures from ∼30 to ∼600°C and in a wide variety of geological environments. The pyrite Au-As data form a wedge-shaped zone in compositional space, and the fact that most data points plot below the solid solubility limit defined by Reich et al. (2005) indicate that Au1+ is the dominant form of Au in arsenian pyrite and that Au-bearing ore fluids that deposit this sulfide are mostly undersaturated with respect to native Au. The analytical data also show that the solid solubility limit of Au in arsenian pyrite defined by an Au/As ratio of 0.02 is independent of the geochemical environment of pyrite formation and rather depends on the crystal-chemical properties of pyrite and post-depositional alteration. Compilation of Au–As concentrations and formation temperatures for pyrite indicates that Au and As solubility in pyrite is retrograde; Au and As contents decrease as a function of increasing temperature from ∼200 to ∼500°C. Based on these results, two major Au–As trends for Au-bearing arsenian pyrite from ore deposits are defined. One trend is formed by pyrites from Carlin-type and orogenic Au deposits where compositions are largely controlled by fluid-rock interactions and/or can be highly perturbed by changes in temperature and alteration by hydrothermal fluids. The second trend consists of pyrites from porphyry Cu and epithermal Au deposits, which are characterised by compositions that preserve the Au/As signature of mineralizing magmatic-hydrothermal fluids, confirming the role of this sulfide in controlling metal ratios in ore systems.
The capacity of biochar to take up heavy metals from contaminated soil and water is influenced by the pyrolysis temperature. We have prepared three biochar samples from Jerusalem artichoke stalks ...(JAS) by pyrolysis at 300, 500 and 700 °C, denoted as JAS300, JAS500, and JAS700, respectively. A variety of synchrotron-based techniques were used to assess the effect of pyrolysis temperature on the molecular properties and copper (Cu) sorption capacity of the samples. The content of oxygen-containing functional groups in the biochar samples decreased, while that of aromatic structures and alkaline mineral components increased, with a rise in pyrolysis temperature. Scanning transmission X-ray microscopy indicated that sorbed Cu(II) was partially reduced to Cu(I), but this process was more evident with JAS300 and JAS700 than with JAS500. Carbon K-edge X-ray absorption near edge structure spectroscopy indicated that Cu(II) cations were sorbed to biochar via complexation and Cu-π bonding. With rising pyrolysis temperature, Cu(II)-complexation weakened while Cu-π bonding was enhanced. In addition, the relatively high ash content and pH of JAS500 and JAS700 facilitated Cu precipitation and the formation of langite on the surface of biochar. The results of this investigation will aid the conversion of halophyte waste to useable biochar for the effective remediation of Cu-contaminated soil and water.
Display omitted
•Features of JAS biochar were closely related to pyrolysis temperature.•STXM and XANES were used to clarify mechanisms of Cu sorption by JAS biochar.•Reduction of Cu(II) to Cu(I) on the surface of biochar was imaged by STXM.•High pyrolysis temperature favored Cu-π binding with aromatic components.
Advanced synchrotron-based techniques were helpful in probing the effect of pyrolysis temperature on the molecular properties and copper-sorption capacity of biochar.
Microbial carbonate precipitation is known as an efficient process for the remediation of heavy metals from contaminated soils. In the present study, a urease positive bacterial isolate, identified ...as Bacillus cereus NS4 through 16S rDNA sequencing, was utilized on a large scale to remove nickel from industrial soil contaminated by the battery industry. The soil was highly contaminated with an initial total nickel concentration of approximately 900 mg kg−1. The soluble-exchangeable fraction was reduced to 38 mg kg−1 after treatment. The primary objective of metal stabilization was achieved by reducing the bioavailability through immobilizing the nickel in the urease-driven carbonate precipitation. The nickel removal in the soils contributed to the transformation of nickel from mobile species into stable biominerals identified as calcite, vaterite, aragonite and nickelous carbonate when analyzed under XRD. It was proven that during precipitation of calcite, Ni2+ with an ion radius close to Ca2+ was incorporated into the CaCO3 crystal. The biominerals were also characterized by using SEM-EDS to observe the crystal shape and Raman-FTIR spectroscopy to predict responsible bonding during bioremediation with respect to Ni immobilization. The electronic structure and chemical-state information of the detected elements during MICP bioremediation process was studied by XPS. This is the first study in which microbial carbonate precipitation was used for the large-scale remediation of metal-contaminated industrial soil.
Display omitted
•Microbial carbonate precipitation for large-scale remediation of metal-contaminated soil.•Soluble-exchangeable Ni fraction was reduced to 38 mg kg−1 after treatment from 898 mg kg−1.•Ni transformation from mobile species into stable biominerals, mainly calcite and NiCO3.•Characterization of biominerals by SEM-EDS, Raman-FTIR spectroscopy and XRD.
Microbial carbonate precipitation for nickel remediation was studied. Soluble-exchangeable Ni fraction from contaminated industrial soil was reduced from initial 898 mg kg−1 to 38 mg kg−1.
The Samail ophiolite in Oman is undergoing modern hydration and carbonation of peridotite and may host a deep subsurface biosphere. Previous investigations of hyperalkaline fluids in Oman have ...focused on fluids released at surface seeps, which quickly lose their reducing character and precipitate carbonates upon contact with the O2/CO2-rich atmosphere. In this work, geochemical analysis of rocks and fluids from the subsurface provides new insights into the operative reactions in serpentinizing aquifers. Serpentinite rock and hyperalkaline fluids (pH>10), which exhibit millimolar concentrations of Ca2+, H2 and CH4, as well as variable sulfate and nitrate, were accessed from wells situated in mantle peridotite near Ibra and studied to investigate their aqueous geochemistry, gas concentrations, isotopic signatures, mineralogy, Fe speciation and microbial community composition.
The bulk mineralogy of drill cuttings is dominated by olivine, pyroxene, brucite, serpentine and magnetite. At depth, Fe-bearing brucite is commonly intermixed with serpentine, whereas near the surface, olivine and brucite are lost and increased magnetite and serpentine is detected. Micro-Raman spectroscopy reveals at least two distinct generations of serpentine present in drill cuttings recovered from several depths from two wells. Fe K-edge X-ray absorption near-edge spectroscopy (XANES) analysis of the lizardite shows a strong tetrahedral Fe coordination, suggesting a mixture of both Fe(II) and Fe(III) in the serpentine. Magnetite veins are also closely associated with this second generation serpentine, and 2–10μm magnetite grains overprint all minerals in the drill cuttings. Thus we propose that the dissolved H2 that accumulates in the subsurface hyperalkaline fluids was evolved through low temperature oxidation and hydration of relict olivine, as well as destabilization of pre-existing brucite present in the partially serpentinized dunites and harzburgites. In particular, we hypothesize that Fe-bearing brucite is currently reacting with dissolved silica in the aquifer fluids to generate late-stage magnetite, additional serpentine and dissolved H2.
Dissolved CH4 in the fluids exhibits the most isotopically heavy carbon in CH4 reported in the literature thus far. The CH4 may have formed through abiotic reduction of dissolved CO2 or through biogenic pathways under extreme carbon limitation. The methane isotopic composition may have also been modified by significant methane oxidation. 16S rRNA sequencing of DNA recovered from filtered hyperalkaline well fluids reveals an abundance of Meiothermus, Thermodesulfovibrionaceae (sulfate-reducers) and Clostridia (fermenters). The fluids also contain candidate phyla OP1 and OD1, as well as Methanobacterium (methanogen) and Methylococcus sp. (methanotroph). The composition of these microbial communities suggests that low-temperature hydrogen and methane generation, coupled with the presence of electron acceptors such as nitrate and sulfate, sustains subsurface microbial life within the Oman ophiolite.
Stibnite (Sb2S3) dissolution and transformation on mineral surfaces are the fundamental steps controlling the fate of antimony (Sb) in the environment. The molecular-level understanding of ...Sb2S3-mineral-water interfacial reactions is of great importance. Herein, Sb2S3 oxidative dissolution and sequestration on pyrite (FeS2) were explored. The results show that FeS2 accelerated the rate of Sb2S3 oxidative dissolution by a factor of 11.4-fold under sunlight due to heterogeneous electron transfer. The electron transfer from Sb2S3 to FeS2 separated photogenerated hole (h+) and electron (e−) pairs, facilitating the generation of hydroxyl radicals (OH) on Sb2S3 and FeS2, and superoxide radicals (O2-) on FeS2. Surface O2- was the dominant oxidant for Sb(III) oxidation with 91% contribution, as evidenced by radical trapping experiments. OH was preferentially adsorbed on Sb2S3, but was released with Sb2S3 dissolution, and subsequently contributable to Sb(III) oxidation in solution. The Sb(III) oxidation and sequestration on FeS2 surface coupled Fe2+/Fe3+ cycling and inhibited FeS2 dissolution, as evidenced by X-ray absorption near edge structure and X-ray photoelectron spectroscopy. The insights gained from this study further our understanding of Sb2S3 transformation and transport at the environmental mineral-water interfaces.
Display omitted
•FeS2 accelerated Sb2S3 oxidative dissolution via a heterogeneous electron transfer process.•Surface O2- radicals dominated Sb(III) oxidation on FeS2 and contributed to Sb2S3 dissolution.•Sb2S3 assisted OH radical’s release from FeS2 surface to solution to oxidize soluble Sb(III).•Sb(III) oxidation and sequestration coupled Fe2+/Fe3+ cycling and inhibited FeS2 dissolution.
PDF
