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  1. Competitive and cooperative effects of chloride on palladium(II) adsorption to iron (oxyhydr)oxides: Implications for mobility during weathering

    Wright, Emily G. ; He, Xicheng ; Flynn, Elaine D. ; ... - Geochimica et Cosmochimica Acta
    In surface and near-surface weathering environments, the mobilization and partial loss of palladium (Pd) under oxidizing and weakly acidic conditions has been attributed to aqueous chloride complexation. However, prior work has also observed that a portion of Pd is retained by iron (oxyhydr)oxides in the weathering zone. The effect chloride has on the relative amount of Pd mobilization versus retention by iron (oxyhydr)oxides is currently unclear. We studied the effect of chloride complexation on Pd(II) adsorption to two iron (oxyhydr)oxides, hematite and 2-line ferrihydrite, at pH 4. Increasing chloride concentration suppresses Pd adsorption for both hematite and ferrihydrite, which displaymore » similar binding affinities under the conditions studied. Thermodynamic modeling of aqueous Pd speciation indicates that greater suppression of binding to iron (oxyhydr)oxides should occur than is observed because of the strength of Pd-Cl complexation, implying that additional interactions at the mineral surface are counteracting this effect. While increasing dissolved chloride concentration does not measurably impact mineral surface charging, extended X-ray absorption fine structure (EXAFS) spectra indicate that ternary Pd-Cl surface complexes form on both hematite and ferrihydrite. The number of Cl ligands in the surface species increase at greater chloride concentration. A mixture of bidentate and monodentate surface species are indicated by the EXAFS spectra, although the fitting uncertainties precludes determining whether these vary in relative abundance with chloride concentration. In order to offset the effect of strong aqueous Pd-Cl complexation and align with our EXAFS results, a surface complexation model developed for Pd adsorption to hematite involves a mixture of three ternary surface complexes containing 1, 2, and 3 chloride ligands. Our results show that Pd is mobilized as a chloride complex in platinum group element-rich weathering zones. As a result, porewater chloride concentrations are thus a dominant control on Pd retention by iron (oxyhydr)oxides in these weakly acidic environments.« less

  2. Adsorption of Neodymium, Dysprosium, and Ytterbium to Goethite under Varying Aqueous Chemistry Conditions

    He, Xicheng ; Sharma, Neha ; Stagg, Olwen ; ... - ACS Earth and Space Chemistry
    The adsorption of rare earth elements (REEs) to iron oxides can regulate the mobility of REEs in the environment and is heavily influenced by water chemistry. This study utilized batch experiments to examine the adsorption of Nd, Dy, and Yb to goethite under varying pH, electrolyte (type and concentration), and concentrations of dissolved inorganic carbon and citrate. REE adsorption was strongly influenced by pH, with an increase from essentially no adsorption at pH 3.0 to nearly complete adsorption at pH 6.5 and higher. Citrate enhanced the adsorption of REEs at low pH (<5.0), likely by forming goethite-REE-citrate ternary surface complexes.more » However, citrate inhibited the adsorption of REEs at higher pH (>5.0) by forming aqueous REE-citrate complexes. Ionic strength had a small influence on REE adsorption, and the presence of dissolved inorganic carbon had no discernible effect. Equilibrium adsorption was interpreted with a triple-layer surface complexation model (SCM). The selection of surface complexation reactions was guided by extended X-ray absorption fine structure spectra. An SCM with a single bidentate inner-sphere surface complexation reaction for Nd and two inner-sphere surface complexation reactions (one monodentate and one bidentate reaction) for Dy and Yb effectively simulated adsorption across a broad range of conditions in the absence of citrate. Accounting for the effects of citrate on REE adsorption required the addition of up to two ternary REE-citrate-goethite surface complexes. The SCM can enable predictions of REE transport in subsurface environments that have goethite as an important adsorbent mineral. Furthermore, this predictive capability could contribute to identifying potential REE sources and facilitating efficient extraction of REEs.« less

  3. Uranium and Nickel Partitioning in a Contaminated Riparian Wetland

    Lin, Peng ; Boyanov, Maxim I. ; O’Loughlin, Edward J. ; ... - Water (Basel)
    Uranium (U) and nickel (Ni) released 50 years ago have been immobilized in the Tims Branch wetlands located on the Savannah River Site in the United States. Sediments were collected from seven locations to identify the factors responsible for this attenuation. Ni and U contents in the solids were significantly correlated, suggesting that depositional as opposed to chemical processes contributed to their spatial distribution. Based on sequential extractions, 63 ± 16% of the U was partitioned into the organic fraction, whereas Ni was distributed between several sediment fractions. An inverse pH-organic matter (OM) correlation and positive correlations of OM withmore » total U and organic-bound U/Ni suggest that increased OM preservation and binding to the mineral surfaces were likely responsible for Ni- and especially U-sediment retention (Tims Branch pH = 4.84 ± 0.68). EXAFS analysis indicated the predominance of U(VI) coordinated with clay minerals (~65%), together with ~35% coordinated to either OM (in areas with elevated OM levels) or iron oxides. The desorption-Kd coefficients of U (3972 ± 1370 L/kg) and Ni (30 ± 8 L/kg) indicate that dissolved Ni poses a greater long-term risk than dissolved U for migrating downstream. This study suggests that a delicate balance of geochemical properties controls whether wetlands behave as sinks or sources of contaminants.« less

  4. Unlocking Syngas Synthesis from the Catalytic Gasification of Lignocellulose Pinewood: Catalytic and Pressure Insights

    Tewari, Kshitij ; Balyan, Sonit ; Jiang, Changle ; ... - ACS Sustainable Chemistry & Engineering
    Modern technologies transform biomass into commodity chemicals, biofuels, and solid charcoal, making it appear as a renewable resource rather than organic waste. The effectiveness of Mo, Fe, Co, and Ni metal catalysts was investigated during the gasification of lignocellulosic pinewood. The primary goal was to compare the performance of iron and nickel catalysts in the lowand high-pressure production of syngas from pinewood. This is the first study that has reported high-pressure gasification of pinewood without the use of an external gasifying agent, producing syngas containing hydrogen, carbon monoxide, and carbon dioxide along with considerable amounts of methane with or withoutmore » a catalyst. Also, the same gasification at low pressures was compared. In this study, the iron catalyst produces syngas more efficiently at higher pressure and 800 °C, and contains 43 mol % H2, 22 mol % CO2, 26 mol % CH4, and 8 mol % CO in comparison to the nickel catalyst. High pressure produces a large amount of methane too. The nickel catalyst produces higher syngas at low pressure and 850 °C, and contains 55 mol % H2, 9 mol % CO2, 5 mol % CH4, and 30 mol % CO. Low-pressure gasification produces less amounts of CH4 and CO2. Also, the H2/CO ratio is ~1.81 using the nickel catalyst at low pressures, which is good for utilizing syngas as a feedstock. These results highlight the importance of catalyst selection, reactor configuration, and operating circumstances in adjusting gasification product composition. The study’s findings provide information about optimizing syngas production from pinewood, which is critical for the development of sustainable and efficient energy conversion technologies.« less

  5. Incorporation of actinides into iron (oxyhydr)oxides: A long-term environmental barrier to radionuclide migration

    Stagg, Olwen ; Morris, Katherine ; Townsend, Luke T. ; ... - Applied Geochemistry
    Iron (oxyhydr)oxide minerals are ubiquitous in the sub-surface environment (e.g. soils and sediments) and have the potential to act as a long-term barrier to the migration of radionuclides. The long-lived actinides (e.g. 237Np, 2.14 million year half-life) are particularly challenging, and pose multigenerational technical and policy issues for the remediation of contaminated land and the disposal of radioactive wastes. Over the last decade, increasing evidence indicates that actinides may substitute for Fe in the structure of iron (oxyhydr)oxides. Given the thermodynamic stability of iron (oxyhydr)oxides over the crustal continuum, incorporation may provide long-term immobilization pathways for anthropogenic actinide contaminants. Here,more » the speciation and coordination adopted by key actinides (i.e. U, Np, Pu and Am) on incorporation into iron (oxyhydr)oxides is reviewed. The stability of these phases under fluctuating environmental conditions is also discussed, to provide constraints on the long-term fate of actinides incorporated into iron (oxyhydr)oxides.« less

  6. Toward Accelerated Activation of Fe-ZSM-5 in Methane Dehydroaromatization

    Deng, Yifan ; Veser, Götz - Energy and Fuels
    Fe-ZSM-5 is a promising catalyst for methane dehydroaromatization (MDA) but currently suffers from long activation periods that exacerbate a comparatively low activity vs the state-of-the-art Mo-ZSM-5 catalysts. Iron can exist in ZSM-5 in many forms, and while it is well-known that metal speciation impacts the activation period, the underlying process is poorly understood to date. The present study aimed to fill this gap by elucidating the effect of Fe speciation on catalyst activation as a guide for rational catalyst design. Iron speciation was carefully controlled via different synthesis pathways (wet impregnation, ion exchange, and isomorphous substitution), and its effect onmore » the activation period in MDA was examined. A higher degree of aggregation of iron oxide was found to be beneficial for the acceleration of the activation period, which could be explained by the involvement of reduction and carburization steps in the formation of the active phase. Aggregated iron oxide is more readily carburized than atomically dispersed iron in the framework, which cannot be carburized directly and needs to be transformed into aggregated iron oxide before being reduced and carburized into the active phase. This suggests a fine balance required for synthesizing an optimal Fe-ZSM-5 catalyst for MDA, which balances fast activation enabled by high Fe agglomeration against a large active surface area for high reactivity.« less

  7. The structure of molten calcium ferrite under various redox conditions

    Shi, Caijuan ; Alderman, Oliver G. ; Tamalonis, Anthony ; ... - Philosophical Transactions of the Royal Society. A, Mathematical, Physical and Engineering Sciences
    Laser-heated melts based on the 43CaO–57Fe2O3-x eutectic, close to the calcium ferrite (CF) composition, were measured with high-energy X-ray diffraction using aerodynamic levitation over a range of redox states controlled by CO/CO2 gas atmospheres. The iron–oxygen coordination number was found to rise from 4.4 ± 0.3 at 15% Fe3+ to 5.3 ± 0.3 at 87% Fe3+. Empirical potential structure refinement modelling was used to obtain the ferric and ferrous partial pair distribution functions. It was found that the Fe2+ iron–oxygen coordination number is consistently approximately 10% higher in CF than in pure iron oxide, while Fe3+ is essentially identical inmore » all but the most oxygen-rich environments (where it is higher in CF compared with FeOx). The model also shows calcium octahedra to be the dominant species across all redox environments, although the population of CaO7 increases with the availability of oxygen at the expense of CaO4 and CaO5.« less

  8. Machine learning assisted phase and size-controlled synthesis of iron oxide particles

    Liu, Juejing ; Zhang, Zimeng ; Li, Xiaoxu ; ... - Chemical Engineering Journal
    Synthesis of iron oxides with specific phases and particle sizes is a crucial challenge in various fields, including materials science, energy storage, biomedical applications, environmental science, and earth science. However, despite significant advances in this area, much of the current palette of particle outcomes has been based on time-consuming trial-and-error exploration of synthesis conditions. The present study was designed to explore a very different approach to 1) predict the outcome of synthesis from specified reaction parameters based on using machine learning (ML) techniques, and 2) correlate sets of parameters to obtain products with desired outcomes by a newly designed recommendationmore » algorithm. To achieve this, four ML algorithms were tested, namely random forest, logistic regression, support vector machine, and k-nearest neighbor. Among the models, random forest outperformed the others, attaining 96% and 81% accuracy when predicting the phase and size of iron oxide particles in the test dataset. Surprisingly, the permutation feature importance analysis revealed that volume, which may strongly relate to pressure, was one of the important features, along with precursor concentration, pH, temperature, and time, influencing the phase and size of iron oxide particles during synthesis. To verify the robustness of the random forest models, prediction and experimental results were compared based on 24 randomly generated methods in additive and non-additive systems not included in the datasets. The predictions of product phase and particle size from the models agreed well with the experimental results. Furthermore, a searching and ranking algorithm was developed to recommend potential synthesis parameters for obtaining iron oxide products with the desired phase and particle size from previous studies in the dataset. Furthermore, this study lays the foundation for a closed-loop approach in materials synthesis and preparation, beginning with suggesting potential reaction parameters from the dataset and predicting potential outcomes, followed by conducting experiments and analyses, and ultimately enriching the dataset.« less

  9. Transition Metal Dissolution in Lithium-Ion Cells: A Piece of the Puzzle

    Son, Seoung-Bum ; Zhang, Zhengcheng ; Gim, Jihyeon ; ... - Journal of Physical Chemistry. C
    Static leaching tests were performed using the chemically delithiated positive electrode materials, LiFePO4, LiCoO2, LiMn2O4, LiNiO2, and LiNi0.8Mn0.1Co0.1O2. Instead of the common electrolyte, which contains LiPF6, the solvent consisted of only ethylene carbonate (EC) and ethyl methyl carbonate (EMC), limiting the possible reactions to only those that depend on the solvent. The product liquids from these experiments showed that there were lithium-bearing species in common, such as Li(EC)+ and Li(EMC)+. Interestingly, we found evidence of electrolyte degradation products in both the positive-and negative-ion mass spectral results. The positive-ion results showed that the products tended to coordinate to lithium. In conclusion,more » the negative-ion results showed that most of the products tended to complex with transition metals. It was difficult to discern which positive ion was associated with which negative ion.« less

  10. Removal of As(V) from wastewaters using magnetic iron oxides formed by zero-valent iron electrocoagulation

    Wang, Yi ; Liu, Lihu ; Yang, Xiong ; ... - Journal of Environmental Management
    Electrocoagulation of zero-valent iron has been widely applied to the removal of dissolved arsenic, but the solid-liquid separation of arsenic-containing precipitates remains technically challenging. In this work, zero-valent iron was electrochemically oxidized to magnetic iron oxides for the removal of As(V) from simulated and actual mining wastewaters. The results indicated that lepidocrocite was formed when zero-valent iron was oxidized by dissolved oxygen, but ferrihydrite and green rust were first formed and then transformed to magnetic iron oxides (mainly magnetite and maghemite) in the electrochemical oxidation from 0 to 0.9 V (vs. SCE), which facilitates the adsorption of As(V) and subsequentmore » solid-liquid separation under a magnetic field. In simulated As(V)-containing solution with initial pH 7.0, zero-valent iron was electrochemically oxidized to magnetite and maghemite at 0.6 V (vs. SCE) for 2 h. The As(V) concentration first decreased from 5127.5 to 26.8 μg L-1 with a removal ratio of 99.5%. In actual mining wastewaters, zero-valent iron was electrochemically oxidized to maghemite at 0.6 V (vs. SCE) for 24 h, and the As(V) concentration decreased from 5486.4 to 3.6 μg L-1 with a removal ratio of 99.9%. The removal ratio of As(V) increased slightly with increasing potential, and increased first and then decreased with increasing initial pH. Compared with that of SO42- and NO3-, the presence of Cl- significantly enhanced the removal of As(V). This work provides a highly efficient, facile and low-cost technique for the treatment of arsenic-containing wastewaters.« less
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