High-order multiple-scattering calculations of x-ray-absorption fine structure

Resolving a structural issue in cerium-nickel-based oxide: a single compound or a two-phase system?

CeNiO3 has been reported in the literature in the last few years as a novel LnNiO3 compound with promising applications in different catalytic fields, but its structure has not been correctly reported so far. In this research, CeNiO3 (RB1), CeO2 and NiO have been synthesized in a nanocrystalline form using a modified citrate aqueous sol-gel route. A direct comparison between the equimolar physical mixture (n(CeO2) : n(NiO) = 1 : 1) and compound RB1 was made. Their structural differences were investigated by laboratory powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) with an energy-dispersive X-ray spectroscopy (EDS) detector, and Raman spectroscopy. The surface of the compounds was analyzed by X-ray photoelectron spectroscopy (XPS), while the thermal behaviour was explored by thermogravimetric analysis (TGA). Their magnetic properties were also investigated with the aim of exploring the differences between these two compounds. There were clear differences between the physical mixture of CeO2 + NiO and RB1 presented by all of these employed methods. Synchrotron methods, such as atomic pair distribution function analysis (PDF), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were used to explore the structure of RB1 in more detail. Three different models for the structural solution of RB1 were proposed. One structural solution proposes that RB1 is a single-phase pyrochlore compound (Ce2Ni2O7) while the other two solutions suggest that RB1 is a two-phase system of either CeO2 + NiO or Ce1-xNixO2 and NiO.

Heavy metal removal potential of olivine

Industrial wastewater containing heavy metals, such as Cd and Pb, must be treated prior to discharge to meet increasingly stringent discharge guidelines and to limit the impact of toxic metals on ecosystems and human health. The application of olivine particles is a natural mineral-based solution to treat heavy metal-laden wastewaters, but little is known about the efficiency and mechanism of metal removal by this solid phase. In this work, we investigate the potential of olivine for heavy metal treatment by combining batch metal removal experiments with solid-phase characterization by synchrotron-based X-ray techniques and electron microscopy. We probed the removal behaviour of a variety of metal contaminants (Co, Ni, Cd, Zn, Cu, Pb; initial concentration = 1500 µg/L) and used Zn specifically to identify the metal removal pathway of olivine. We found that olivine in powdered (0.3 g/L) and granulated (0.5 g/L) forms was able to remove up to >90% of the initial metal, depending on the metal identity, with the efficiency increasing in order of Co ≤ Cd ≤ Ni Collapse

Key Words

• Industrial wastewater
• Mineral dissolution
• Secondary mineral precipitation
• Water treatment
• Zinc removal

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MESH Headings Collapse

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Affiliation(s)

C M van Genuchten Department of Geochemistry, Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, Copenhagen, Denmark.
H Hamaekers SIBELCO, De Zate 1, Dessel 2480, Belgium
D Fraiquin SIBELCO, Ankerpoort NV, Op de Bos 300, Maastricht 6223 EP, the Netherlands
S Hollanders SIBELCO, De Zate 1, Dessel 2480, Belgium
A Ahmad SIBELCO, Ankerpoort NV, Op de Bos 300, Maastricht 6223 EP, the Netherlands; Department of Sustainable Development, Environmental Science and Engineering, KTH Royal Institute of Technology, Stockholm, Sweden

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A local point of view of the Cu(100) → NiTPP charge transfer at the NiTPP/Cu(100) interface

A precise understanding, at the molecular level, of the massive substrate → adsorbate charge transfer at the NiTPP/Cu(100) interface has been gained through the application of elementary symmetry arguments to the structural determination of the NiTPP adsorption site by photoelectron diffraction (PED) measurements and Amsterdam density functional calculations of the free D4h NiTPP electronic structure. In particular, the PED analysis precisely determines that, among the diverse NiTPP chemisorption sites herein considered (fourfold hollow, atop, and bridge), the fourfold hollow one is the most favorable, with the Ni atom located at 1.93 Å from the surface and at an internuclear distance of 2.66 Å from the nearest-neighbors of the substrate. The use of elementary symmetry considerations enabled us to provide a convincing modeling of the NiTPP-Cu(100) anchoring configuration and an atomistic view of the previously revealed interfacial charge transfer through the unambiguous identification of the adsorbate π* and σ* low-lying virtual orbitals, of the substrate surface atoms, and of the linear combinations of the Cu 4s atomic orbitals involved in the substrate → adsorbate charge transfer. In addition, the same considerations revealed that the experimentally reported Ni(II) → Ni(I) reduction at the interface corresponds to the fingerprint of the chemisorption site of the NiTPP on Cu(100).

Atomic insights into the mechanisms of Al3+ or Cr3+ affecting ferrihydrite nucleation

Various exotic metal cations commonly coexist with ferrihydrite nanoparticles in natural environments. Lack of knowledge about the metal cations effects on ferrihydrite nucleation and growth greatly blights the deep understanding of ferrihydrite mineralogical properties and reactivities, and thus the fates of associated nutrients, heavy metals/metalloids, and organic pollutants. Here, the nucleation processes and mechanisms of ferrihydrite nanoparticles in the presence of Al3+ or Cr3+ were studied by combining visible spectroscopy, in situ quick X-ray absorption fine structure spectroscopy and quantum chemical calculations. The formation of ferrihydrite can be divided into three stages. In stage 1, Fe(H2O)63+ forms μ-oxo Fe dimers, with the gradual increase of Fe-O bond length (dFe-O) and disappearance of Fe-O multiple scattering. In stage 2, ferrihydrite particles begin to form and grow slowly, during which dFe-O continues to increase and edge- and corner-sharing Fe-Fe bonds appear. In stage 3, ferrihydrite growth rate increases significantly and continues to the end of the reaction, with the decreases of dFe-O. The presence of metal cations at a molar ratio of 0.1 to Fe hinders the formation of μ-oxo dimers by affecting the Fe3+ hydrolysis and polymerization at stage 1 and stage 2, but promotes the conversion of the μ-oxo dimer to the dihydroxo dimer with lower energy barrier and the creation of crystal growth sites and thus enhances the ferrihydrite nucleation and growth at stage 3.

Uranium oxides structural transformation in human body liquids

Uranium oxide microparticles ingestion is one of the potential sources of internal radiation doses to the humans at accidental or undesirable releases of radioactive materials. It is important to predict the obtained dose and possible biological effect of these microparticles by studying uranium oxides transformations in case of their ingestion or inhalation. Using a combination of methods, a complex examination of structural changes of uranium oxides in the range from UO₂ to U₄O₉, U₃O₈ and UO₃ as well as before and after exposure of uranium oxides in simulated biological fluids: gastro-intestinal and lung-was carried out. Oxides were thoroughly characterized by Raman and XAFS spectroscopy. It was determined that the duration of expose has more influence on all oxides transformations. The greatest changes occurred in U₄O₉, that transformed into U₄O_9-y. UO_2.05 and U₃O₈ structures became more ordered and UO₃ did not undergo significant transformation.

Investigating the local structure of Ti based MXene materials by temperature dependent X-ray absorption spectroscopy

The local structures of Ti based MXene-type electrode materials have been studied by Ti K-edge X-ray absorption fine structure measurements as a function of temperature to obtain direct information on the local bond lengths and their stiffness. In particular, the parent MAX phases Ti₂AlC and Ti₃AlC₂ and their etched MXene systems are characterized and their properties compared. We find that selective etching has a substantial effect on the local structural properties of the Ti based MXene materials. It leads to an increase in the interatomic distances, i.e. a decrease in the covalency, and corresponding bond stiffness, that is a likely cause of higher achievable performances. The obtained results underline the importance of the local atomic correlations as limiting factors in the diffusion capacity of ion batteries.

Saturation magnetisation as an indicator of the disintegration of barium hexaferrite nanoplatelets during the surface functionalisation

Barium hexaferrite nanoplatelets (BHF NPLs) are permanent nanomagnets with the magnetic easy axis aligned perpendicular to their basal plane. By combining this specific property with optimised surface chemistry, novel functional materials were developed, e.g., ferromagnetic ferrofluids and porous nanomagnets. We compared the interaction of chemically different phosphonic acids, hydrophobic and hydrophilic with 1-4 phosphonic groups, with BHF NPLs. A decrease in the saturation magnetisation after functionalising the BHF NPLs was correlated with the mass fraction of the nonmagnetic coating, whereas the saturation magnetisation of the NPLs coated with a tetraphosphonic acid at 80 °C was significantly lower than expected. We showed that such a substantial decrease in the saturation magnetisation originates from the disintegration of BHF NPLs, which was observed with atomic-resolution scanning transmission electron microscopy and confirmed by a computational study based on state-of-the-art first-principles calculations. Fe K-edge XANES (X-ray absorption near-edge structure) and EXAFS (Extended X-ray absorption fine structure) combined with Fourier-transformed infrared (FTIR) spectroscopy confirmed the formation of an Fe-phosphonate complex on the partly decomposed NPLs. Comparing our results with other functionalised magnetic nanoparticles confirmed that saturation magnetisation can be exploited to identify the disintegration of magnetic nanoparticles when insoluble disintegration products are formed.

Molecular Dynamics and Structural Studies of Zinc Chloroquine Complexes

Chloroquine (CQ) is a first-choice drug against malaria and autoimmune diseases. It has been co-administered with zinc against SARS-CoV-2 and soon dismissed because of safety issues. The structural features of Zn-CQ complexes and the effect of CQ on zinc distribution in cells are poorly known. In this study, state-of-the-art computations combined with experiments were leveraged to solve the structural determinants of zinc-CQ interactions in solution and the solid state. NMR, ESI-MS, and X-ray absorption and diffraction methods were combined with ab initio molecular dynamics calculations to address the kinetic lability of this complex. Within the physiological pH range, CQ binds Zn²⁺ through the quinoline ring nitrogen, forming [Zn(CQH)Cl_x(H₂O)_3-x]^(3+)-x (x = 0, 1, 2, and 3) tetrahedral complexes. The Zn(CQH)Cl₃ species is stable at neutral pH and at high chloride concentrations typical of the extracellular medium, but metal coordination is lost at a moderately low pH as in the lysosomal lumen. The pentacoordinate complex [Zn(CQH)(H₂O)₄]³⁺ may exist in the absence of chloride. This in vitro/in silico approach can be extended to other metal-targeting drugs and bioinorganic systems.

Effects of cobalt doping on the reactivity of hausmannite for As(III) oxidation and As(V) adsorption

Hausmannite is a common low valence Mn oxide mineral, with a distorted spinel structure, in surficial sediments. Although natural Mn oxides often contain various impurities of transitional metals (TMs), few studies have addressed the effect and related mechanism of TM doping on the reactivity of hausmannite with metal pollutants. Here, the reactivity of cobalt (Co) doped hausmannite with aqueous As(III) and As(V) was studied. Co doping decreased the point of zero charge of hausmannite and its adsorption capacity for As(V). Despite a reduction of the initial As(III) oxidation rate, Co-doped hausmannite could effectively oxidize As(III) to As(V), followed by the adsorption and fixation of a large amount of As(V) on the mineral surface. Arsenic K-edge EXAFS analysis of the samples after As(V) adsorption and As(III) oxidation revealed that only As(V) was adsorbed on the mineral surface, with an average As-Mn distance of 3.25-3.30 Å, indicating the formation of bidentate binuclear complexes. These results provide new insights into the interaction mechanism between TMs and low valence Mn oxides and their effect on the geochemical behaviors of metal pollutants.

CO2 Activation over Nanoshaped CeO2 Decorated with Nickel for Low-Temperature Methane Dry Reforming

Dry reforming of methane (DRM) is a promising way to convert methane and carbon dioxide into H₂ and CO (syngas). CeO₂ nanorods, nanocubes, and nanospheres were decorated with 1-4 wt % Ni. The materials were structurally characterized using TEM and in situ XANES/EXAFS. The CO₂ activation was analyzed by DFT and temperature-programmed techniques combined with MS-DRIFTS. Synthesized CeO₂ morphologies expose {111} and {100} terminating facets, varying the strength of the CO₂ interaction and redox properties, which influence the CO₂ activation. Temperature-programmed CO₂ DRIFTS analysis revealed that under hydrogen-lean conditions mono- and bidentate carbonates are hydrogenated to formate intermediates, which decompose to H₂O and CO. In excess hydrogen, methane is the preferred reaction product. The CeO₂ cubes favor the formation of a polydentate carbonate species, which is an inert spectator during DRM at 500 °C. Polydentate covers a considerable fraction of ceria's surface, resulting in less-abundant surface sites for CO₂ dissociation.

Correlation between the Cation Disorders of Fe3+ and Li+ in P3-Type Na0.67[Li0.1(Fe0.5Mn0.5)0.9]O2 for Sodium Ion Batteries

Various Fe-based layered oxide materials have received attention as promising cathode materials for sodium ion batteries because of their low cost and high specific capacity. Only a few P3-type Fe-based oxide materials, however, have been examined as cathodes because the synthesis of highly crystalline P3-type Fe-based oxides is not facile. For this reason, the structural merits of the P3 structure are not yet fully understood. Herein, highly crystalline P3-type Na0.67[Li0.1(Fe0.5Mn0.5)0.9]O2 heated at 900 °C is introduced to improve the electrochemical performance of Fe-based layered oxides. The structures, reaction mechanisms, and electrochemical performances of P3 Na0.67[Li0.1(Fe0.5Mn0.5)0.9]O2, P2 Na0.57[Li0.1(Fe0.5Mn0.5)0.9]O2, and P2 Na0.67[Fe0.5Mn0.5]O2 are compared to demonstrate the roles of Li+ doping in the improved electrochemical performance of P3 Na0.67[Li0.1(Fe0.5Mn0.5)0.9]O2, such as stable capacity retention over 100 cycles. P3 Na0.67[Li0.1(Fe0.5Mn0.5)0.9]O2 significantly suppresses the migration of Fe3+ ions to tetrahedral sites in the Na layer during cycling because the cation disorder of Li+ is more favorable than that of Fe3+. As a result, P3 Na0.67[Li0.1(Fe0.5Mn0.5)0.9]O2 shows better cycle performance than P2 Na0.67[Fe0.5Mn0.5]O2. P3 Na0.67[Li0.1(Fe0.5Mn0.5)0.9]O2 also exhibits an improved rate performance compared to P2 Na0.67[Fe0.5Mn0.5]O2. This finding provides fundamental insights to improve the electrochemical performance of layered oxide cathode materials for sodium ion batteries.

Insight into the interdependence of Ni and Al in bifunctional Ni/ZSM-5 catalysts at the nanoscale

Catalyst design is crucial for improving catalytic activity and product selectivity. In a bifunctional Ni/ZSM-5 zeolite type catalyst, catalytic properties are usually tuned via varying Al and Ni contents. While changes in acid properties associated with Al sites are usually closely investigated, Ni phases, however, receive inadequate attention. Herein, we present a systematic structural study of Ni in the Ni/ZSM-5 materials by using Ni K-edge XANES and EXAFS analyses, complemented by XRD and TEM, to resolve the changes in the local environment of Ni species induced by the different Al contents of the parent ZSM-5 prepared by a "green", template free technique. Ni species in Ni/ZSM-5 exist as NiO crystals (3-50 nm) and as charge compensating Ni²⁺ cations. The Ni K-edge XANES and EXAFS results enabled the quantification of Ni-containing species. At a low Al to Si ratio (n _Al/n _Si ≤ 0.04), the NiO nanoparticles predominate in the samples and account for over 65% of Ni phases. However, NiO is outnumbered by Ni²⁺ cations attached to the zeolite framework in ZSM-5 with a high Al to Si ratio (n _Al/n _Si = 0.05) due to a higher number of framework negative charges imparted by Al. The obtained results show that the number of highly reducible and active NiO crystals is strongly correlated with the framework Al sites present in ZSM-5 zeolites, which depend greatly on the synthesis conditions. Therefore, this kind of study is beneficial for any further investigation of the catalytic activities of Ni/ZSM-5 and other metal-modified bifunctional catalysts.

From EXAFS of reference compounds to U(VI) speciation in contaminated environments

Understanding the speciation of technogenic uranium in natural systems is crucial for estimating U migration and bioavailability and for developing remediation strategies for contaminated territories. Reference EXAFS data of model laboratory-prepared uranium compounds (`standards') are necessary to analyze U-contaminated samples from nuclear legacy sites. To minimize errors associated with measurements on different synchrotrons, it is important not only to compare data obtained on environmentally contaminated samples with the literature but also with `standards' collected at the same beamline. Before recording the EXAFS spectra, all reference compounds were thoroughly characterized by Raman spectroscopy and powder X-ray diffraction. The U(VI) local molecular environments in the reference compounds, i.e. uranyl oxyhydroxides, phosphates, carbonates and uranates, were examined using XAFS. Based on the EXAFS fitting results obtained, including the nature of the bonding, interatomic distances and coordination numbers, parameters that are typical for a particular U compound were differentiated. Using data for `standards', U speciation in the sample of radioactively contaminated soil was determined to be a mixture of U oxyhydroxide and carbonate phases.

Investigation of Cerium Reduction Efficiency by Grinding with Microwave Irradiation in Mechanochemical Processing

This study evaluated the efficiency of cerium reduction by grinding with microwave irradiation in mechanochemical processing. Grinding experiments with microwave irradiation were conducted using an agitating mixer. Since the structure of the ground samples was amorphous and the cerium concentration was much lower than those of other elements, the valence change and structural change of cerium after grinding with microwave irradiation were investigated using X-ray absorption fine structure (XAFS) analysis in the cerium K-edge. The X-ray absorption near-edge structure (XANES) analysis revealed that a portion of tetravalent cerium was reduced to trivalent cerium by grinding with microwave irradiation. In addition, it was confirmed by extended X-ray absorption fine structure (EXAFS) analysis that oxygen vacancies were produced as a result of the cerium reduction reaction. To evaluate the efficiency of cerium reduction efficiency, the percentage reduction by grinding with microwave irradiation was compared to that by planetary ball milling and microwave irradiation. As a result, it was revealed that the efficiency of cerium reduction via grinding with microwave irradiation was higher than that via microwave irradiation and the same as that via planetary ball milling. Moreover, a larger amount of tetravalent cerium could be reduced to trivalent cerium by grinding with microwave irradiation than when using planetary ball milling and microwave irradiation.

Structure and Population of Complex Ionic Species in FeCl2 Aqueous Solution by X-ray Absorption Spectroscopy

Technologies for mass production require cheap and abundant materials such as ferrous chloride (FeCl₂). The literature survey shows the lack of experimental studies to validate theoretical conclusions related to the population of ionic Fe-species in the aqueous FeCl₂ solution. Here, we present an in situ X-ray absorption study of the structure of the ionic species in the FeCl₂ aqueous solution at different concentrations (1–4 molL⁻¹) and temperatures (25–80 °C). We found that at low temperature and low FeCl₂ concentration, the octahedral first coordination sphere around Fe is occupied by one Cl ion at a distance of 2.33 (±0.02) Å and five water molecules at a distance of 2.095 (±0.005) Å. The structure of the ionic complex gradually changes with an increase in temperature and/or concentration. The apical water molecule is substituted by a chlorine ion to yield a neutral Fe[Cl₂(H₂O)₄]⁰. The observed substitutional mechanism is facilitated by the presence of the intramolecular hydrogen bonds as well as entropic reasons. The transition from the single charged Fe[Cl(H₂O)₅]⁺ to the neutral Fe[Cl₂(H₂O)₄]⁰ causes a significant drop in the solution conductivity, which well correlates with the existing conductivity models.

Probing Active Sites in CuxPdy Cluster Catalysts by Machine-Learning-Assisted X-ray Absorption Spectroscopy

Size-selected clusters are important model catalysts because of their narrow size and compositional distributions, as well as enhanced activity and selectivity in many reactions. Still, their structure-activity relationships are, in general, elusive. The main reason is the difficulty in identifying and quantitatively characterizing the catalytic active site in the clusters when it is confined within subnanometric dimensions and under the continuous structural changes the clusters can undergo in reaction conditions. Using machine learning approaches for analysis of the operando X-ray absorption near-edge structure spectra, we obtained accurate speciation of the Cu_xPd_y cluster types during the propane oxidation reaction and the structural information about each type. As a result, we elucidated the information about active species and relative roles of Cu and Pd in the clusters.

Resolving the Dilemma of Fe-N-C Catalysts by the Selective Synthesis of Tetrapyrrolic Active Sites via an Imprinting Strategy

Combining the abundance and inexpensiveness of their constituent elements with their atomic dispersion, atomically dispersed Fe-N-C catalysts represent the most promising alternative to precious-metal-based materials in proton exchange membrane (PEM) fuel cells. Due to the high temperatures involved in their synthesis and the sensitivity of Fe ions toward carbothermal reduction, current synthetic methods are intrinsically limited in type and amount of the desired, catalytically active Fe-N₄ sites, and high active site densities have been out of reach (dilemma of Fe-N-C catalysts). We herein identify a paradigm change in the synthesis of Fe-N-C catalysts arising from the developments of other M-N-C single-atom catalysts. Supported by DFT calculations we propose fundamental principles for the synthesis of M-N-C materials. We further exploit the proposed principles in a novel synthetic strategy to surpass the dilemma of Fe-N-C catalysts. The selective formation of tetrapyrrolic Zn-N₄ sites in a tailor-made Zn-N-C material is utilized as an active-site imprint for the preparation of a corresponding Fe-N-C catalyst. By successive low- and high-temperature ion exchange reactions, we obtain a phase-pure Fe-N-C catalyst, with a high loading of atomically dispersed Fe (>3 wt %). Moreover, the catalyst is entirely composed of tetrapyrrolic Fe-N₄ sites. The density of tetrapyrrolic Fe-N₄ sites is more than six times as high as for previously reported tetrapyrrolic single-site Fe-N-C fuel cell catalysts.

1,8-dihydroxy naphthalene (DHN) - melanin confers tolerance to cadmium in isolates of melanised dark septate endophytes

The contribution of 1,8-dihydroxy naphthalene (DHN) melanin to cadmium (Cd) tolerance in two dark septate endophytes (DSE) of the genus Cadophora with different melanin content was investigated in vitro. The DSE isolate Cad#148 with higher melanin content showed higher tolerance to Cd than the less melanised Cad#149. Melanin synthesis was significantly reduced by Cd in both isolates with uninhibited melanin synthesis, in a dose-dependent manner. Inhibition of melanin synthesis by tricyclazole reduced the relative growth of Cad#148 exposed to Cd and did not affect Cad#149. Cd accumulation was not altered by tricyclazole in the two isolates, but it increased catalase and reduced glutathione reductase activity in more melanised Cad#148, indicating higher stress levels. In contrast, in Cad#149 the enzyme activity was less affected by tricyclazole, indicating a more pronounced role of melanin-independent Cd tolerance mechanisms. Cd ligand environment in fungal mycelia was analysed by extended EXAFS (X-ray absorption fine structure). It revealed that Cd was mainly bound to O- and S-ligands, including hydroxyl, carboxyl, phosphate and thiol groups. A similar proportion of S- and O- ligands (~35% and ~65%) were found in both isolates with uninhibited melanin synthesis. Among O-ligands two types with Cd-O-C- and Cd-O-P- coordination were identified. Tricyclazole altered Cd-O- ligand environment in both fungal isolates by reducing the proportion of Cd-O-C- and increasing the proportion of Cd-O-P coordination. DHN-melanin, among other tolerance mechanisms, significantly contributes to Cd tolerance in more melanised DSE fungi by immobilising Cd to hydroxyl groups and maintaining the integrity of the fungal cell wall.

Cadmium Isotope Fractionation during Adsorption and Substitution with Iron (Oxyhydr)oxides

Cadmium (Cd) isotopes have great potential for understanding Cd geochemical cycling in soil and aquatic systems. Iron (oxyhydr)oxides can sequester Cd via adsorption and isomorphous substitution, but how these interactions affect Cd isotope fractionation remains unknown. Here, we show that adsorption preferentially enriches lighter Cd isotopes on iron (oxyhydr)oxide surfaces through equilibrium fractionation, with a similar fractionation magnitude (Δ^114/110Cd_{solid-solution}) for goethite (Goe) (-0.51 ± 0.04‰), hematite (Hem) (-0.54 ± 0.10‰), and ferrihydrite (Fh) (-0.55 ± 0.03‰). Neither the initial Cd²⁺ concentration or ionic strength nor the pH influence the fractionation magnitude. The enrichment of the light isotope is attributed to the adsorption of highly distorted [CdO₆] on solids, as indicated by Cd K-edge extended X-ray absorption fine-structure analysis. In contrast, Cd incorporation into Goe by substitution for lattice Fe at a Cd/Fe molar ratio of 0.05 preferentially sequesters heavy Cd isotopes, with a Δ^114/110Cd_{solid-solution} of 0.22 ± 0.01‰. The fractionation probably occurs during the transformation of Fh into Goe via dissolution and reprecipitation. These results improve the understanding of the Cd isotope fractionation behavior being affected by iron (oxyhydr)oxides in Earth's critical zone and demonstrate that interactions with minerals can obscure anthropogenic and natural Cd isotope characteristics, which should be carefully considered when applying Cd isotopes as environmental tracers.

Comparative analysis of XANES and EXAFS for local structural characterization of disordered metal oxides

In functional materials, the local environment around active species that may contain just a few nearest-neighboring atomic shells often changes in response to external conditions. Strong disorder in the local environment poses a challenge to commonly used extended X-ray absorption fine structure (EXAFS) analysis. Furthermore, the dilute concentrations of absorbing atoms, small sample size and the constraints of the experimental setup often limit the utility of EXAFS for structural analysis. X-ray absorption near-edge structure (XANES) has been established as a good alternative method to provide local electronic and geometric information of materials. The pre-edge region in the XANES spectra of metal compounds is a useful but relatively under-utilized resource of information of the chemical composition and structural disorder in nano-materials. This study explores two examples of materials in which the transition metal environment is either relatively symmetric or strongly asymmetric. In the former case, EXAFS results agree with those obtained from the pre-edge XANES analysis, whereas in the latter case they are in a seeming contradiction. The two observations are reconciled by revisiting the limitations of EXAFS in the case of a strong, asymmetric bond length disorder, expected for mixed-valence oxides, and emphasize the utility of the pre-edge XANES analysis for detecting local heterogeneities in structural and compositional motifs.

Firing Increases Arsenic Leaching from Ceramic Water Filters via Arsenic and Iron Phase Transformations

Ceramic water filters (CWFs) are produced globally using local clay sources and can effectively remove bacterial pathogens during point-of-use water treatment. The ceramic production process involves firing clay mixed with burnout material at temperatures of 800-1100 °C, which induces mineralogical changes leading to increased arsenic (As) leaching from CWF material compared to source clay. Unfired clay and fired CWFs from Cambodia, Canada, and Mexico, CWF from Laos, and test-fired clay from the United States were analyzed to determine the extent of As leaching from CWFs that range in As (<1 to 16 mg kg-1) and iron (Fe) (0.6 to 5%) content. Deionized water, NaOH, HCl, and oxalate extractions showed that firing increased As solubility and decreased Fe solubility compared to unfired clay, with up to 8 mg kg-1 of water-soluble As in Cambodian CWFs. X-ray absorption spectra of the Cambodian clay and CWF showed a decrease in the Fe-O distance from 2.01 to 1.91 Å and decreased Fe coordination number from 6.3 to 4.6 after firing, indicating a decrease in Fe-O coordination. Arsenic(V) was the dominant species in Cambodia clay and CWF, existing primarily as a surface complex with average As-Fe distance of 3.28 Å in clay while in CWF As was either an outer-sphere As(V) phase or a discrete arsenate phase with no significant As-Fe scattering contribution within the resolution of the data. Improved understanding of molecular-scale processes that cause increased As leaching from CWFs provides a basis for assessing As leaching potential prior to CWF factory capital investment as well as engineered solutions (e.g., modified firing temperature, material amendments, and leaching prior to distribution) to mitigate As exposure from CWFs.

First-principles design of a single-atom–alloy propane dehydrogenation catalyst

Nanoparticles of rhodium dispersed on metal oxides are generally poor catalysts for alkane dehydrogenation because the reactants bind too strongly to the metal. Hannagan et al. performed first-principle calculations indicating that single rhodium atoms in a copper surface should be stable and selective for conversion of propane to propene and hydrogen. Model studies of single rhodium atoms embedded in a copper (111) surface revealed a very high selectivity to propene and high resistance to the formation of surface carbon that would deactivate the catalyst.

Science , abg8389, this issue p. 1444

Local structural distortions in SnTe investigated by EXAFS

Extended x-ray absorption fine structure (EXAFS) has been measured at theKedge of Sn in SnTe in the temperature range from 5 to 480 K. EXAFS results are consistent with the presence of a local rhombohedral distortion in the full temperature range from 5 to 300 K, even well above the ferroelectric transition temperature, suggesting a partial order-disorder character of the transition. At and above 300 K, the anomalous behaviour of the third and fourth EXAFS cumulants reveals a modification of the anharmonicity of the effective pair potential, possibly connected with the softening of high frequency modes or to the presence of multiple phases.

Latent Representation Learning for Structural Characterization of Catalysts

Supervised machine learning-enabled mapping of the X-ray absorption near edge structure (XANES) spectra to local structural descriptors offers new methods for understanding the structure and function of working nanocatalysts. We briefly summarize a status of XANES analysis approaches by supervised machine learning methods. We present an example of an autoencoder-based, unsupervised machine learning approach for latent representation learning of XANES spectra. This new approach produces a lower-dimensional latent representation, which retains a spectrum-structure relationship that can be eventually mapped to physicochemical properties. The latent space of the autoencoder also provides a pathway to interpret the information content "hidden" in the X-ray absorption coefficient. Our approach (that we named latent space analysis of spectra, or LSAS) is demonstrated for the supported Pd nanoparticle catalyst studied during the formation of Pd hydride. By employing the low-dimensional representation of Pd K-edge XANES, the LSAS method was able to isolate the key factors responsible for the observed spectral changes.

Integrating biological As(III) oxidation with Fe(0) electrocoagulation for arsenic removal from groundwater

Arsenic (As) is a toxic element present in many (ground)water sources in the world. Most conventional As removal techniques require pre-oxidation of the neutral arsenite (As(III)) species to the negatively charged arsenate (As(V)) oxyanion to optimize As removal and minimize chemical use. In this work, a novel, continuous-flow As removal system was developed that combines biological As(III) oxidation by bacteria with Fe electrocoagulation (EC), an Fe(0)-based electrochemical technology that generates reactive Fe(III) precipitates to bind As. The bio-integrated FeEC system (bio-FeEC) showed effective oxidation and removal of 150 µg/L As(III), without the need of chemicals. To remove As to below the WHO guideline of 10 µg/L, 10 times lower charge dosage was required for the bio-FeEC system compared to conventional FeEC. This lower Fe dosage requirement reduced sludge production and energy consumption. The As(III) oxidizing biomass was found to consist of bacteria belonging to Comamonadaceae, Rhodobacteraceae and Acidovorax, which are capable of oxidizing As(III) and are common in drinking water biofilms. Characterization of the As-laden Fe solids by X-ray absorption spectroscopy indicated that both bio-FeEC and conventional FeEC produced solids consistent with a mixture of lepidocrocite and 2-line ferrihydrite. Arsenic bound to the solids was dominantly As(V), but a slightly higher fraction of As(V) was detected in the bio-FeEC solids compared to the conventional FeEC.

Spectroscopic study of volcanic ashes

Volcanic ashes particles are subjected to substantial modification during explosive eruptions. The mineralogical and compositional changes have important consequences on the environment and human health. Nevertheless, the relationship between the speciation of iron (Fe) and the mineralogical composition and particle granulometry of the ashes, along with their interaction with water, are largely unknown. In particular, the Fe oxidation state and the possible formation of new Fe-bearing phases in presence of S, Cl, and F in the plume are key points to assess the impact of the ashes. Fragmental material ejected during volcanic activity (tephra) in 2013, was collected on the Mt. Etna (Italy) and investigated using a multi-technique approach that included conventional Electron Paramagnetic Resonance (EPR), high field EPR (HFEPR), EchoEPR, and Fe K-edge X-ray Absorption Spectroscopy (XAS). These element-selective techniques allowed obtaining a detailed information on the oxidation state and coordination environment of Fe, and of its speciation in the ash samples as a function of the granulometry. A complex mineralogical assemblage, consisting of variable amounts of nanometric crystalline Fe inclusions in a glass matrix, and of Fe-oxides and Fe-sulfur phases was revealed. A risk assessment of the ashes is attempted.

Groundwater As Removal by As(III), Fe(II), and Mn(II) Co-Oxidation: Contrasting As Removal Pathways with O2, NaOCl, and KMnO4

Effective arsenic (As) removal from groundwater is a pressing need in view of increasingly stringent As drinking water limits in some US states and European countries. In this study, we compared the addition of weak (O₂), intermediate (NaOCl), and strong (KMnO₄) groundwater oxidants on the fate of As during As(III), Fe(II), and Mn(II) co-oxidation. Experiments were performed with 50 μg/L As(III), 5 mg/L Fe(II), and 0.5 mg/L Mn(II) in solutions containing relevant groundwater ions, with the reaction products characterized by As K-edge X-ray absorption spectroscopy (XAS). Adding O₂ by aeration was the least effective method, unable to decrease As to below 10 μg/L, which was attributed to inefficient As(III) oxidation. Dosing NaOCl (55 μM) consistently removed As to <10 μg/L (and often <5 μg/L). The As K-edge XAS data of the NaOCl samples indicated complete As(III) oxidation and As(V) sorption to coprecipitated hydrous ferric oxide (HFO) in the binuclear, bridging (²C) complex. The most effective As removal was observed with KMnO₄ (40 μM), which completely oxidized As(III) and yielded residual As concentrations that were less than (by as much as 50%) or equal to the NaOCl experiments. Furthermore, the average As-metal bond length of the KMnO₄ solids (R_As-Fe/Mn = 3.24 ± 0.02 Å) was systematically shorter than the NaOCl solids (R_As-Fe/Mn = 3.29 ± 0.02 Å), consistent with As(V) sorption to both MnO₂ and HFO. These findings can be used to optimize groundwater As treatment to meet relevant drinking water guidelines, while considering the As uptake mode and characteristics of the particle suspension (i.e., colloidal stability and filterability).

Graphite Nanoarrays-Confined Fe and Co Single-Atoms within Graphene Sponges as Bifunctional Oxygen Electrocatalyst for Ultralong Lasting Zinc-Air Battery

The inferior stability of bifunctional oxygen electrocatalysts in the air cathode is one of the main obstacles that impedes the commercialization of zinc-air batteries (ZABs). This work describes a self-assembly technique combined with subsequent calcination to prepare a bifunctional oxygen electrocatalyst of graphite nanoarrays-confined Fe and Co single-atoms within graphene sponges (FeCo-NGS). Specifically, graphene sponges overspread with graphite nanoarrays as a structure regulation, which can prevent the metal single-atoms from aggregating and accelerate the mass/electron transfer, provides a guarantee for the long-term operation. Furthermore, M-N₄ (M = Fe/Co) as the intrinsic activity regulation can effectively drive the heterogeneous oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic processes. Thanks to the rationally designed regulations, FeCo-NGS shows both extraordinary electrocatalytic activity for ORR and OER, even outperforming commercial Pt/C and IrO₂. Remarkably, ZABs with FeCo-NGS air cathode demonstrate a record-breaking cycle lifetime of more than 1500 h (over 9000 cycles) at 10 mA cm^-2 with a small charge-discharge gap.

The Active Center of Co-N-C Electrocatalysts for the Selective Reduction of CO2 to CO Using a Nafion-H Electrolyte in the Gas Phase

To contribute a solution for the global warming problem, the selective electrochemical reduction of CO₂ to CO was studied in the gas phase using a [CO₂(g), Co-N-C cathode | Nafion-H | Pt/C anode, H₂/water] system without using carbonate solutions. The Co-N-C electrocatalysts were synthesized by partial pyrolysis of precursors in inert gas, which were prepared from various N-bidentate ligands, Co(NO₃)₂, and Ketjenblack (KB). The most active electrocatalyst was Co-(4,4'-dimethyl-2,2'-bipyridine)/KB pyrolyzed at 673 K, denoted Co-4,4'-dmbpy/KB(673K). A high performance of CO formation (331 μmol h^-1 cm^-2, 217 TOF h^-1) at 0.020 A cm^-2 with 78% current efficiency was obtained at -0.75 V (SHE) and 273 K under strong acidic conditions of Nafion-H. Characterization studies using extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy-energy-dispersive X-ray (TEM-EDX), X-ray diffraction (XRD), and temperature-programmed desorption with mass spectrometry (TPD-MS) indicated the active site as Co coordinated with four N atoms bonding the surface of KB, abbreviated Co-N₄-C _x structure. A model of the reduction mechanism of CO₂ on the active site was proposed.

Characterization and environmental implications of selenate co-precipitation with barite

This study investigated the sequestration of dissolved selenate (SeO₄^2-) via co-precipitation in barite for a range of SeO₄^2- concentrations (0-~8650 mg/L), as well as its release at near neutral pH conditions (pH = ~5.5-6.5). Solid precipitates were characterized via X-ray diffraction and subsequent Rietveld refinements, Raman spectroscopy, Brunauer-Emmett-Teller surface area analyses, scanning electron microscopy, electron probe microanalyses (EPMA), inductively coupled plasma optical emission spectroscopy (ICP-OES), and X-ray absorption spectroscopy (XAS). ICP-OES results suggested barite efficiently removed >99% of SeO₄^2- from the test solutions during all co-precipitation experiments. EPMA results showed the SeO₄^2- was sequestered from the aqueous phase via co-precipitation with barite. XAS analyses indicated the SeO₄^2- tetrahedron is incorporated into the barite structure by substituting for sulfate (SO₄^2-) and bonding to Ba²⁺ atoms through bidentate mononuclear and bidentate binuclear complexes. Dissolution data showed the release of SeO₄^2- sequestered in barite to the aqueous phase is unlikely due to the low solubility and stability of the barite phase. As such, co-precipitation of SeO₄^2- with barite could be effective for removing SeO₄^2- from waters affected by mining and metallurgical operations.

Rapid and Efficient Arsenic Removal by Iron Electrocoagulation Enabled with in Situ Generation of Hydrogen Peroxide

Millions of people are exposed to toxic levels of dissolved arsenic in groundwater used for drinking. Iron electrocoagulation (FeEC) has been demonstrated as an effective technology to remove arsenic at an affordable price. However, FeEC requires long operating times (∼hours) to remove dissolved arsenic due to inherent kinetics limitations. Air cathode Assisted Iron Electrocoagulation (ACAIE) overcomes this limitation by cathodically generating H₂O₂ in situ. In ACAIE operation, rapid oxidation of Fe(II) and complete oxidation and removal of As(III) are achieved. We compare FeEC and ACAIE for removing As(III) from an initial concentration of 1464 μg/L, aiming for a final concentration of less than 4 μg/L. We demonstrate that at short electrolysis times (0.5 min), i.e., high charge dosage rates (1200 C/L/min), ACAIE consistently outperformed FeEC in bringing arsenic levels to less than WHO-MCL of 10 μg/L. Using XRD and XAS data, we conclusively show that poor arsenic removal in FeEC arises from incomplete As(III) oxidation, ineffective Fe(II) oxidation and the formation of Fe(II-III) (hydr)oxides at short electrolysis times (<20 min). Finally, we report successful ACAIE performance (retention time 19 s) in removing dissolved arsenic from contaminated groundwater in rural California.

SnO2-Containing Clinoptilolite as a Composite Photocatalyst for Dyes Removal from Wastewater under Solar Light

Due to their adsorbent, ion exchange and catalytic properties zeolites are suitable for a variety of applications. We report on the photocatalytic activity of a readily available and inexpensive natural zeolite clinoptilolite (Z) containing SnO2 (Sn-Z). The Sn-Z samples with 3–15 wt. % of Sn were prepared by using a precipitation–deposition method. Powder X-ray diffraction analysis showed that the zeolite structure was unaffected by the introduction of the Sn-phase. Diffuse reflectance UV/VIS spectra of the Sn-Z samples confirmed the presence of SnO2 and X-Ray absorption spectroscopy analyses suggested that the SnO2 particles mainly resided on the surface of the clinoptilolite, while ATR-FTIR analysis gave some clues that part of the SnO2 phase was incorporated in the pores of the zeolite. The presence of SnO2 in Sn-Z increased both adsorption capacity and photocatalytic performance which could be partially explained by higher surface area and partially with an increased negative potential of the surface. Adsorption and total degradation of methylene blue (MB) for the Sn-Z with the highest amount of Sn (15 wt.%) was about 30% and 45%, respectively, suggesting a synergetic effect between SnO2 and the clinoptilolite lattice. Reusability tests showed that these catalysts present a promising material for water purification.

Synthesis of orthogonally assembled 3D cross-stacked metal oxide semiconducting nanowires

Assemblies of metal oxide nanowires in 3D stacks can enable the realization of nanodevices with tailored conductivity, porous structure and a high surface area. Current fabrication methods require complicated multistep procedures that involve the initial preparation of nanowires followed by manual assembly or transfer printing, and thus lack synthesis flexibility and controllability. Here we report a general synthetic orthogonal assembly approach to controllably construct 3D multilayer-crossed metal oxide nanowire arrays. Taking tungsten oxide semiconducting nanowires as an example, we show the spontaneous orthogonal packing of composite nanorods of poly(ethylene oxide)-block-polystyrene and silicotungstic acid; the following calcination gives rise to 3D cross-stacked nanowire arrays of Si-doped metastable ε-phase WO₃. This nanowire stack framework was also tested as a gas detector for the selective sensing of acetone. By using other polyoxometallates, this fabrication method for woodpile-like 3D nanostructures can also be generalized to different doped metal oxide nanowires, which provides a way to manipulate their physical properties for various applications.

Disposition of Iridium on Ruthenium Nanoparticle Supported on Ketjenblack: Enhancement in Electrocatalytic Activity toward the Electrohydrogenation of Toluene to Methylcyclohexane

Electrohydrogenation of toluene (TL) to methylcyclohexane (MCH) has been recognized as a promising technology for the hydrogenation process in the organic hydride hydrogen storage system. Recently, we found that the Ketjenblack (KB)-supported Ru-Ir alloy electrocatalyst showed a high electrocatalytic activity for the electrohydrogenation of TL to MCH, and there was the synergy of Ir electrocatalysis for the formation of adsorbed hydrogen species (H_ad) (H⁺ + e^- → H_ad) and Ru catalysis for hydrogenation of TL (6H_ad + TL → MCH). In this paper, the Ir-modified Ru nanoparticle supported on KB (Ir/Ru/KB) electrocatalyst was synthesized by using a modified spontaneous deposition method. The method enables to control the surface structure of Ru-Ir nanoparticles. The Ir/Ru/KB cathode showed higher electrohydrogenation activity than the Ru-Ir alloy/KB cathodes even at lower loadings of precious Ir. Characterization studies using a scanning-transmission electron microscope with an energy-dispersive X-ray spectrometer and X-ray absorption fine structure proved selective and uniform modification of Ru nanoparticle with Ir. Cyclic voltammetry measurements in H₂SO₄ aqueous solution indicated higher electrochemical active surface areas of Ir at the Ir/Ru/KB electrocatalysts than that at the Ru-Ir alloy/KB electrocatalysts, which is the reason for the strong synergy of Ru and Ir for the electrohydrogenation of TL to MCH.

Achieving arsenic concentrations of <1 μg/L by Fe(0) electrolysis: The exceptional performance of magnetite

Consumption of drinking water containing arsenic at concentrations even below the World Health Organization provisional limit of 10 μg/L can still lead to unacceptable health risks. Consequently, the drinking water sector in the Netherlands has recently agreed to target 1 μg/L of arsenic in treated water. Unfortunately, in many poor, arsenic-affected countries, the costs and complexity of current methods that can achieve <1 μg/L are prohibitive, which highlights the need for innovative methods that can remove arsenic to <1 μg/L without costly support infrastructure and complicated supply chains. In this work, we used Fe(0) electrolysis, a low cost and scalable technology that is also known as Fe(0) electrocoagulation (EC), to achieve <1 μg/L residual dissolved arsenic. We compared the arsenic removal performance of green rust (GR), ferric (oxyhydr)oxides (Fe(III) oxides) and magnetite (Mag) generated by EC at different pH (7.5 and 9) in the presence of As(III) or As(V) (initial concentrations of 200-11,000 μg/L). Although GR and Fe(III) oxides removed up to 99% of initial arsenic, neither Fe phase could reliably meet the 1 μg/L target at both pH values. In contrast, EC-generated Mag consistently achieved <1 μg/L, regardless of the initial As(V) concentration and pH. Only solutions with initial As(III) concentrations ≥2200 μg/L resulted in residual arsenic >1 μg/L. As K-edge X-ray absorption spectroscopy showed that Mag also sorbed arsenic in a unique mode, consistent with partial arsenic incorporation near the particle surface. This sorption mode contrasts with the binuclear, corner sharing surface complex for GR and Fe(III) oxides, which could explain the difference in arsenic removal efficiency among the three Fe phases. Our results suggest that EC-generated Mag is an attractive method for achieving <1 μg/L particularly in decentralized water treatment.

Effects of Co(II) ion exchange, Ni(II)- and V(V)-doping on the transformation behaviors of Cr(III) on hexagonal turbostratic birnessite-water interfaces

Natural birnessite-like minerals are commonly enriched in various transitional metals (TMs), which greatly modify the mineral structure and properties. However few studies are yet conducted systematically on the effects of TM doping on birnessite reactivity towards Cr(III) oxidation. In the present study, the transformation behaviors of Cr(III) on Co-, Ni-, V-containing birnessites were investigated. Co and Ni doping generally decrease the mineral crystalline sizes and hydrodynamic sizes (D_H) while V-doping greatly decreases the crystalline sizes but not the D_H, owing to particle aggregation. Co and Ni firstly decrease and then increase the mineral zeta potentials (ζ) at pH4 while V decreases ζ. Electrochemical specific capacitances for Co-containing birnessites are gradually reduced, while those for Ni-doped birnessites are slightly reduced and for V-doped birnessites increased, which have a positively linear relationship with the amounts of Cr(III) oxidized by these samples. Cr(III) removal efficiencies from solution by these Co-, Ni- and V-containing birnessites are 26-51%, ∼62-72% and ∼96-100%, respectively, compared to ∼92% by pure birnessite. Cr(III) oxidation kinetics analysis demonstrates the gradual decrease of Mn(IV) and concurrent increase of Mn(III) and the adsorption of mainly Cr(III) on mineral surfaces. A negatively linear relationship exists between birnessite lateral sizes and the proportions of Mn(IV/III) consumed to oxidize Cr(III). Apparent initial Cr(III) oxidation rate (k_obs) for Co-containing birnessites are greatly reduced, while those for Ni-doped samples moderately decreased and for V-doped samples first increased and then decreased. A positively or negatively linear relationship exists between k_obs or the amount of Mn(II) released and the mineral Mn(IV) content respectively. Cr(III) oxidation probably initiates from layer edge sites of Ni-doped birnessites but the vacancies of Co- and V-containing birnessites. These results provide insights into the reaction mechanisms of Cr(III) with natural birnessite-like minerals.

Advancing Chelation Chemistry for Actinium and Other +3 f-Elements, Am, Cm, and La

A major chemical challenge facing implementation of ²²⁵Ac in targeted alpha therapy-an emerging technology that has potential for treatment of disease-is identifying an ²²⁵Ac chelator that is compatible with in vivo applications. It is unclear how to tailor a chelator for Ac binding because Ac coordination chemistry is poorly defined. Most Ac chemistry is inferred from radiochemical experiments carried out on microscopic scales. Of the few Ac compounds that have been characterized spectroscopically, success has only been reported for simple inorganic ligands. Toward advancing understanding in Ac chelation chemistry, we have developed a method for characterizing Ac complexes that contain highly complex chelating agents using small quantities (μg) of ²²⁷Ac. We successfully characterized the chelation of Ac³⁺ by DOTP^8- using EXAFS, NMR, and DFT techniques. To develop confidence and credibility in the Ac results, comparisons with +3 cations (Am, Cm, and La) that could be handled on the mg scale were carried out. We discovered that all M³⁺ cations (M = Ac, Am, Cm, La) were completely encapsulated within the binding pocket of the DOTP^8- macrocycle. The computational results highlighted the stability of the M(DOTP)^5- complexes.

Localization, ligand environment, bioavailability and toxicity of mercury in Boletus spp. and Scutiger pes-caprae mushrooms

This study provides information on mercury (Hg) localization, speciation and ligand environment in edible mushrooms: Boletus edulis, B. aereus and Scutiger pes-caprae collected at non-polluted and Hg polluted sites, by LA-ICP-MS, SR-μ-XRF and Hg L₃-edge XANES and EXAFS. Mushrooms (especially young ones) collected at Hg polluted sites can contain more than 100 μg Hg g^-1 of dry mass. Imaging of the element distribution shows that Hg accumulates mainly in the spore-forming part (hymenium) of the cap. Removal of hymenium before consumption can eliminate more than 50% of accumulated Hg. Mercury is mainly coordinated to di-thiols (43-82%), followed by di-selenols (13-35%) and tetra-thiols (12-20%). Mercury bioavailability, as determined by feeding the mushrooms to Spanish slugs (known metal bioindicators owing to accumulation of metals in their digestive gland), ranged from 4% (S. pes-caprae) to 30% (B. aereus), and decreased with increasing selenium (Se) levels in the mushrooms. Elevated Hg levels in mushrooms fed to the slugs induced toxic effects, but these effects were counteracted with increasing Se concentrations in the mushrooms, pointing to a protective role of Se against Hg toxicity through HgSe complexation. Nevertheless, consumption of the studied mushroom species from Hg polluted sites should be avoided.

Emerging investigator series: interdependency of green rust transformation and the partitioning and binding mode of arsenic

We investigated the impact of aging-induced structural modifications of carbonate green rust (GR), a mixed valent Fe(ii,iii) (hydr)oxide with a high oxyanion sorption affinity, on the partitioning and binding mode of arsenic (As). Suspensions of carbonate GR were produced in the presence of As(v) or As(iii) (i.e. co-precipitated with As(iii) or As(v)) and aged in anoxic and oxic conditions for up to a year. We tracked aqueous As over time and characterized the solid phase by X-ray absorption spectroscopy (XAS). In experiments with initial As(v) (4500 μg L-1, As/Fe = 2 mol%), the fresh GR suspension sorbed >99% of the initial As, resulting in approximately 14 ± 8 μg L-1 residual dissolved As. Anoxic aging of the As(v)-laden GR for a month increased aqueous As to >60 μg L-1, which was coupled to an increase in GR structural order revealed by Fe K-edge XAS. Further anoxic aging up to a year transformed As(v)-laden GR into magnetite and decreased significantly the aqueous As to <2 μg L-1. The As binding mode was also modified during GR transformation to magnetite from sorption to GR particle edges to As substitution for tetrahedral Fe in the magnetite structure. These GR structural modifications altered the ratio of As partitioning to the solid (μg As/mg Fe) and liquid (μg As per L) phase from 2.0 to 0.4 to 14 L mg-1 for the fresh, month, and year aged suspensions, respectively. Similar trends in GR transformation and As partitioning during anoxic aging were observed for As(iii)-laden suspensions, but occurred on more rapid timescales: As(iii)-laden GR transformed to magnetite after a day of anoxic aging. In oxic aging experiments, rapid GR oxidation by dissolved oxygen to Fe(iii) precipitates required only an hour for both As(v) and As(iii) experiments, with lepidocrocite favored in As(v) experiments and hydrous ferric oxide favored in As(iii) experiments. Aqueous As during GR oxidation decreased to <10 μg L-1 for both As(v) and As(iii) series. Knowledge of this interdependence between GR aging products and oxyanion fate improves biogeochemical models of contaminant and nutrient dynamics during Fe cycling and can be used to design more effective arsenic remediation strategies that rely on arsenic sorption to GR.

Characteristics of Fe and Mn bearing precipitates generated by Fe(II) and Mn(II) co-oxidation with O2, MnO4 and HOCl in the presence of groundwater ions

In this work, we combined macroscopic measurements of precipitate aggregation and chemical composition (Mn/Fe solids ratio) with Fe and Mn K-edge X-ray absorption spectroscopy to investigate the solids formed by co-oxidation of Fe(II) and Mn(II) with O₂, MnO₄, and HOCl in the presence of groundwater ions. In the absence of the strongly sorbing oxyanions, phosphate (P) and silicate (Si), and calcium (Ca), O₂ and HOCl produced suspensions that aggregated rapidly, whereas co-oxidation of Fe(II) and Mn(II) by MnO₄ generated colloidally stable suspensions. The aggregation of all suspensions decreased in P and Si solutions, but Ca counteracted these oxyanion effects. The speciation of oxidized Fe and Mn in the absence of P and Si also depended on the oxidant, with O₂ producing Mn(III)-incorporated lepidocrocite (Mn/Fe = 0.01-0.02 mol/mol), HOCl producing Mn(III)-incorporated hydrous ferric oxide (HFO) (Mn/Fe = 0.08 mol/mol), and MnO₄ producing poorly-ordered MnO₂ and HFO (Mn/Fe > 0.5 mol/mol). In general, the presence of P and Si decreased the crystallinity of the Fe(III) phase and increased the Mn/Fe solids ratio, which was found by Mn K-edge XAS analysis to be due to an increase in surface-bound Mn(II). By contrast, Ca decreased the Mn/Fe solids ratio and decreased the fraction of Mn(II) associated with the solids, suggesting that Ca and Mn(II) compete for sorption sites. Based on these results, we discuss strategies to optimize the design (i.e. filter bed operation and chemical dosing) of water treatment plants that aim to remove Fe(II) and Mn(II) by co-oxidation.