Pyrite (FeS2) is the most common sulfide mineral on Earth, forming through inorganic reactions in the crust and oceanic hydrothermal systems and via microbially driven processes in anaerobic sediments. The pyrite-water interface is the site of a wide range of adsorption and reaction processes in Earth systems including oxidation that dramatically affects the geochemistry of surface waters and influences global carbon and oxygen cycles. Mechanistic geochemical models of pyrite interfacial reactivity, however, are limited by the lack of experimentally derived atomistic structures of the reduced and reacting surfaces. Here we reveal the atomic-scale structure of the pyrite (001)-water interface that forms at very low oxygen partial pressures, relevant to suboxic environments in Earth. The interface structure and surface speciation were obtained using the crystal truncation rod method supported by ambient-pressure photoelectron spectroscopy and density functional theory calculations. The surface is dominantly composed of disulfide groups bound to a single oxygen atom, forming a sulfoxy group that has no known molecular or bulk mineral analog. This surface is interpreted as the first step in the oxidative dissolution of pyrite. The sulfoxy group is readily protonated through surface acid-base reactions that alter the structure of interfacial water and the free energy of interfacial reactions. Surface iron sites are not oxidized. Surprisingly, this interface can likely develop in equilibrium with bulk pyrite in some reducing and acidic solutions. This termination is therefore likely representative of pyrite surfaces under a vast range of experimental, industrial and Earth conditions.

Recovery of magnesium from brines can potentially be used to source MgO (periclase) as a CO2 sorbent or for Mg-based cements. However, it is not clear how common impurities in brines, such as lithium, affect the resulting MgO reactivity. To test the effect of lithium incorporation on MgO reactivity for hydration and carbonation, we combined computational simulations with experiments. Experimentally altered (Mg,Li)O with a low dopant concentration (0.012 +/- 0.002% w/w Li) was characterized using synchrotron-based X-ray scattering and high-resolution electron microscopy to measure reaction layer formation on (Mg,Li)O. Single-crystal X-ray diffraction analysis of (Mg,Li)O demonstrates that the incorporation of lithium leads to the formation of oxygen vacancies. The presence of vacancies is likely causing faster hydroxylation rates as predicted by ab initio molecular dynamics simulations. However, the faster hydroxylation rates likely lead to faster passivation of the surface because we observe thinner reaction layers on (Mg,Li)O samples both over short time periods (30 days) and over long time periods (28 years). After 28 years, the reaction layer on the (Mg,Li)O sample was less than one-third of the thickness of that of the pure MgO sample. In addition, over 30 days, reaction layers on (Mg,Li)O samples primarily formed at steps rather than on terraces, in contrast to our previous observations on MgO. Based on our results, naturally occurring impurities in MgO modify its reactivity even at very low concentrations and need to be considered for accurate reaction rate prediction for application of MgO as a CO2 sorbent or in cements.

Changes in the crystal structure of hematite (Fe2O3) during dissolution were analyzed using in situ, time-resolved X-ray diffraction (TRXRD) of synthetic hematite powders in 3 M HCl solutions. The refined Fe occupancy in hematite increased from 0.899(4) to 0.98(1) during dissolution, and lattice parameters (a, c) and unit-cell volume also increased. Because of the shared octahedral faces in the crystal structure of hematite, increasing Fe3+ occupancy enhanced repulsion between Fe3+ ions, promoting structural expansion. Previous TRXRD studies of nanohematite growth revealed that Fe occupancy also increases as crystals enlarge (Chen et al. 2023). The increase in Fe occupancy during both growth and dissolution suggests that structural changes induced by nanoscale sizing are not always reversible as a function of particle size. We attribute the increase in Fe occupancy as hematite dissolved to the sequential removal of defective surface layers. A higher concentration of Fe vacancies in the outer shells relative to the cores of the particles may account for the increase in Fe occupancy during both particle growth and dissolution.

Brucite [Mg(OH)2] is a promising sorbent for carbon dioxide removal (CDR) due to its availability and low calcination temperatures. However, natural and synthetic brucites tend to contain metal impurities, such as iron or manganese, and how these impurities affect the interfacial chemical reactivity is uncertain. Here, the impact of low concentrations of iron and manganese impurities on the carbonation efficiency of Mg(OH)2 was examined. Mg(OH)2 with small amounts (1-5 mol %) of Fe and Mn was synthesized. The increasing substitution of Fe into Mg(OH)2 was accompanied by the oxidation of Fe. The phase transformation sequence during the carbonation was found to be brucite [Mg(OH)2] -> amorphous magnesium carbonate (MgCO3nH2O) -> nesquehonite (MgCO33H2O), regardless of impurity concentration. Both the Fe- and Mn-doped Mg(OH)2 samples were more reactive than endmember Mg(OH)2, possibly due to their higher surface areas and lower stabilities. During carbonation, 3 mol % Fe- and Mn-doped Mg(OH)2 showed the highest reactivity. The variance in reactivity for Mn-doped Mg(OH)2 was less than that of Fe-doped Mg(OH)2. These results suggest that natural or industrial waste Mg(OH)2 with less than 5 mol % Fe and Mn impurities may be targeted as more effective CDR sorbents than endmember Mg(OH)2.

Upcycling of recycled LiNi0.6Mn0.2Co0.2O2 (NMC622) cathodes offers an economical route to produce cathode materials with increased energy density (i.e., LiNi0.8Mn0.1Co0.1O2, NMC811) that meet the performance needs of present-day electric vehicles. In this work, solid-state upcycling of NMC622 via calcination with Ni(OH)2 and LiOH was monitored using in situ synchrotron powder X-ray diffraction measurements. Sequential Rietveld refinements indicate that the calcination proceeds by initially converting Ni(OH)2 to a rocksalt NiO phase followed by lithiation of NiO to form LiNiO2 (LNO), with both NMC and LNO phases present in nearly equal proportions at the calcination endpoint. Variable-energy transmission X-ray microscopy tomograms of upcycled samples reveal that the NMC and LNO domains are intermixed at sub-micron length scales. Depth-dependent analysis of multi-elemental fitting maps matches the expected NMC811 composition at the secondary particle level and indicates that transition metal diffusion is not limited by the secondary particle size.

New sources of rare earth elements (REEs) are needed to support a green energy transition. REEs adsorbed to aluminum-rich clays in weathering deposits represent important resources, but the mechanisms responsible for their retention and ease of extraction are unresolved. Disordered coordination and co-occurrence of multiple species pose challenges to investigating REE adsorption processes via established spectroscopic methods. In this study, we applied element-specific surface crystallography methods to obtain a new perspective on the complexity of REE adsorption mechanisms and affinities. Alumina (001) and (012) crystal surfaces were utilized to evaluate surface-specific controls on Nd(III) and Yb(III) adsorption behavior. The REEs displayed similar total adsorption to alumina (001) as a mixture of inner- and outer-sphere complexes, but Nd displayed a greater proportion of inner-sphere binding. Adsorption of ordered inner- and outer-sphere REE species was substantially lower on alumina (012). These distinct behaviors reflect differences in the surface functional group charging and topography of the two surfaces. However, alumina (012) also hosted a substantial population of disordered adsorbed species, especially for Nd, potentially associated with Al vacancy surface defects. The accumulation of light versus heavy REEs via adsorption in weathering deposits likely results from multiple, competing reactions affected by clay particle morphology. Leaching procedures for resource recovery should account for differential rates of desorption by coexisting inner- and outer-sphere REE surface complexes.

Direct air capture (DAC) may be feasible to remove carbon dioxide (CO2) from the atmosphere at the gigaton scale, holding promise to become a major contributor to climate change mitigation. Mineral looping using magnesium oxide (MgO) is potentially an economical, efficient, and sustainable pathway to gigaton-scale DAC. The hydroxylation and carbonation of MgO determine the efficiency of the looping process, but their rates and mechanisms remain uncertain. In this work, MgO single crystals were reacted in air or CO2 at varying humidities and characterized by X-ray scattering, microscopy, and vibrational spectroscopy. Results show that the hydroxylation formed a brucite (Mg(OH)2)-like layer immediately after crystal cleaving. Concurrently, the carbonation formed hydrated magnesium carbonate phases, including barringtonite (MgCO3·2H2O) and nesquehonite (MgCO3·2H2O), in the layer. Rapid initial growth of the layer is also manifested in short-range bending/warping of nanocrystallites, resulting in multiple orientations of the same phases on the surface. The layer growth slowed down over time, indicating surface passivation. The formation of barringtonite and nesquehonite with 1:1 CO3/Mg ratio indicates an efficient carbonation when compared to other magnesium carbonate phases of lower ratio. Our results are essential for understanding surface passivation mechanisms and tackling the passivation issue of mineral looping DAC technology.

The time-resolved X-ray diffraction (TRXRD) technique enables in-situ studies of mineral transformations in aqueous solutions with a time resolution of ∼30 seconds. Rietveld analysis of sequential XRD data reveals the real-time evolution of phase fractions and crystal structure parameters of mineral transformations, as is especially important when the transformation occurs through metastable intermediate phases (i.e., by the Ostwald step rule). In contrast to traditional batch experiments, the high temporal resolution of TRXRD allows detailed observations of metastable intermediate phases, the calculation of high-quality kinetic data, and the determination of reaction mechanisms.Two examples of the successful application of TRXRD will be discussed: the hydration of periclase (MgO) to brucite (Mg(OH)2), and the transformation of calcite (CaCO3) to dolomite (CaMg(CO3)2). In order to determine whether the periclase transforms to brucite by solid-state diffusion or by dissolution-reprecipitation, synthetic periclase powders were placed in quartz glass capillaries in an acetic acid buffer solution with a pH of 4.96 and heated to 40 to 60 °C. TRXRD data revealed a continuous increase in the unit-cell c parameter for brucite. The microstrain along the c-direction initially was high but decreased as a function of reaction time. Our structure refinements suggest that stacking faults generated by the propagation of hydration on the (111) plane of periclase induced the transformation to brucite.The transformation of calcite to dolomite occurs via an intermediate phase: calcite (S.G.: R-3c) → disordered protodolomite (S.G: R-3c) → ordered dolomite (S.G: R-3). TRXRD of this reaction showed for the first time that protodolomite first crystallizes as a Mg-rich phase (∼60 mol% Mg) but evolves to a Ca-rich composition (∼60 mol%Ca). Moreover, the formation of ordered dolomite is inhibited by the presence of calcite. Both stages of dolomitization occur by dissolution and reprecipitation rather than solid-state cation exchange.

Despite extensive research on MgO's reactivity in the presence of CO2 under various conditions, little is known about whether impurities incorporated into the solid, such as iron, enhance or impede hydroxylation and carbonation reactions. The purity of the MgO required for the successful implementation of MgO looping as a direct air capture technology affects the deployment costs. With this motivation, we tested how incorporated iron impacts MgO (100) reactivity and passivation layer formation under ambient conditions by using atomic force microscopy, electron microscopy, and synchrotron-based X-ray scattering. Based on electron microprobe analysis, our MgO samples were 0.5 wt % iron, and Mössbauer spectroscopy results indicated that 70% of the iron is present as Fe(II). We find that even these low levels of iron dopants impeded both the hydroxylation at various relative humidities (10%, 33%, 75%, and >95%) and carbonation in CO2 (33%, 75%, and >95%) on the (100) surface. Crystalline reaction products were formed. Reaction layers on the sample were easily removed by exposing the sample to deionized water for 2 min. Overall, our findings demonstrate that the presence of iron dopants slows the reaction rate of MgO, indicating that MgO without incorporated iron is preferable for mineral looping applications.

Direct recycling of lithium-ion battery cathodes offers an alternative source of active cathode materials generated from end-of-life batteries with lower cost and reduced environmental impact compared to virgin battery manufacturing and conventional recycling methods. For batteries using NMC (LiNi x Mn y Co z O 2 ) cathodes, upcycling of low-nickel materials such as LiNi 0.33 Mn 0.33 Co 0.33 O 2 (NMC333) and LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) can be employed to produce cathode materials with higher capacity due to increased nickel content (i.e. LiNi 0.8 Mn 0.1 Co 0.1 O 2 , NMC811). As part of the ReCell Center, we are developing a solid state upcycling method which involves coating the low-nickel NMC material with a nickel-rich precursor phase and a lithium source (e.g. Ni(OH) 2 + LiOH) followed by calcination at 900°C to produce a layered NMC811 cathode. In this work, solid-state upcycling of NMC622 via calcination with Ni(OH) 2 and LiOH was monitored using in situ synchrotron powder X-ray diffraction and ex situ variable-energy transmission X-ray microscopy measurements. Sequential Rietveld refinements indicate that the calcination proceeds by initially converting Ni(OH) 2 to a rocksalt NiO phase followed by lithiation of NiO to form LiNiO 2 (LNO), with both NMC and LNO phases present in nearly equal proportions at the calcination endpoint. Variable-energy transmission X-ray microscopy tomograms of upcycled samples reveal that the NMC and LNO domains are intermixed at the primary particle level. Depth-dependent analysis of multi-elemental fitting maps matches the expected NMC811 composition at the secondary particle level and indicates that transition metal diffusion is not limited by the secondary particle size. Figure 1

Abstract One-dimensional (1D) olivine iron phosphate (FePO4) is widely proposed for electrochemical lithium (Li) extraction from dilute water sources, however, significant variations in Li selectivity were observed for particles with different physical attributes. Understanding how particle features influence Li and sodium (Na) co-intercalation is crucial for system design and enhancing Li selectivity. Here, we investigate a series of FePO4 particles with various features and revealed the importance of harnessing kinetic and chemo-mechanical barrier difference between lithiation and sodiation to promote selectivity. The thermodynamic preference of FePO4 provides baseline of selectivity while the particle features are critical to induce different kinetic pathways and barriers, resulting in different Li to Na selectivity from 6.2 × 102 to 2.3 × 104. Importantly, we categorize the FePO4 particles into two groups based on their distinctly paired phase evolutions upon lithiation and sodiation, and generate quantitative correlation maps among Li preference, morphological features, and electrochemical properties. By selecting FePO4 particles with specific features, we demonstrate fast (636 mA/g) Li extraction from a high Li source (1: 100 Li to Na) with (96.6 ± 0.2)% purity, and high selectivity (2.3 × 104) from a low Li source (1: 1000 Li to Na) with (95.8 ± 0.3)% purity in a single step.

Scaling minerals, such as barite, can cause detrimental consequences for oil/gas pipelines and water systems, but their formation can be inhibited by organic chelators such as ethylenediaminetetraacetic acid (EDTA). Here, we resolve how EDTA affects sorption and desorption of Pb at the barite (001) surface using a combination of X-ray scattering and microscopy measurements. In the presence of EDTA, Pb incorporated in the topmost part of the barite surface and adsorbed as inner-sphere complexes on the surface. In barite saturated solutions containing [Pb] >= 100 mu M, overgrowth films grew along step edges. These films were exclusively monolayer thick, indicating that their growth was a self-limiting process. Approximately half of the Pb was removed after 14.5 h reaction with a Pb-free EDTA solution where most of the desorption occurred to adsorbed Pb rather than incorporated Pb. Dissolution proceeded primarily via step retreat and etch pit formation in EDTA, but in deionized water, the secondary phase was quickly removed within 3 min. Together these results suggest EDTA binds to both the surface and Pb in solution, which limits Pb sorption. However, EDTA binding to the surface also inhibits removal of the secondary phase that formed at higher Pb concentrations.

Amorphous calcium carbonate (ACC) occurs as a precursor to geological and biogenic calcium carbonate (CaCO3), yet its transformation pathways and reaction mechanisms remain inconsistent and controversial. In this study, we investigated the transformation of ACC to calcite under both solution and dry conditions, in the presence and absence of impurity ions, utilizing operando time-resolved synchrotron X-ray diffraction (TRXRD) and reactive transport modeling. Results demonstrate that TRXRD techniques allow us to differentiate dissolution-reprecipitation versus solid-state transformation mechanisms for amorphous to crystalline phase transitions. Specifically, we observe that in environments with abundant water, ACC transforms to calcite through a dissolution-reprecipitation mechanism. This features an activation energy of 63 +/- 2 kJ/mol and unit cell volume contraction during calcite crystal growth. Conversely, under water-limited conditions, ACC to calcite transformation proceeds via a solid-state transformation mechanism, with an activation energy of 210 +/- 2 kJ/mol, three times greater than the dissolution-reprecipitation route, and a unit cell expansion during crystalline calcite growth. To illustrate the magnitude of these effects, the rates of calcite growth were similar during dissolution-reprecipitation at 3 degrees C [0.00207(35) s(-1)] and solid-state transformation at 280 degrees C [0.00134(11) s(-1)]. Moreover, the incorporation of an impurity, strontium, significantly retards the rate of calcite growth while expanding its unit cell but whose incorporation is history dependent. Reactive transport modeling of the dissolution-precipitation kinetics suggests that ACC must be dissolving as compact aggregates. These various transformation mechanisms drive diverse geological and biological carbonate formations, impacting their use as paleoenvironmental markers and functional materials synthesis.

Bastna''site ((Ce,La)-FCO3) is the primary mineral source of light rare earth elements, but its surface structure is not well understood. This presents a major challenge in improving beneficiation strategies. In this work, a synergistic combination of X-ray scattering and ab initio molecular dynamics (AIMD) was used to gain atomistic insight into the interfacial structure of bastna''site. Surface X-ray scattering was used to measure crystal truncation rods (CTRs) of the bastna''site (001) surface, a significant crystal face with a previously unknown termination. The best-fit atomic-scale model of the CTR data features a carbonate layer at the surface, which is stabilized by the relaxation of carbonate groups from their bulk structural positions. AIMD simulations predict similar surface relaxations, which are shown to be influenced by the protonation of oxygen atoms at the surface. Evidence of ordered water at the interface is also observed in the best-fit model and AIMD simulations. The presence of a carbonate layer at this dominant crystal surface is significant for improving separation technologies because most commonly used ligands utilize anionic functional groups to chelate metal cations at particle surfaces. Without modification, anionic ligands are expected to have poor affinity for the carbonate-terminated (001) surface.

Understanding the sorption behavior of ions at the mineral-water interface is important to determine the fate of elements in the environment. Here, we used barite to understand metal and organic molecule interactions with ionic crystals as organic molecules are used to remove scale that can form during oil recovery in sulfate-rich settings. The sorption behavior of strontium (Sr) on the (001) surface in the presence of ethylenediaminetetraacetic acid (EDTA) was measured using in situ specular X-ray reflectivity (XR) and resonant anomalous X-ray reflectivity (RAXR). Based on the XR results, the presence of EDTA disrupts the barite surface structure, particularly that of the top two monolayers. A maximum of 0.7 (+/- 0.2) EDTA molecules per surface unit cell are adsorbed, though the exact amount cannot be distinguished from that of other adsorbed species, such as Ba2+ and H2O. Based on the RAXR results, the presence of EDTA inhibits Sr incorporation into the barite surface and binds with Sr in solution to limit the amount of Sr2+ in solution. The Sr-EDTA2- complex then binds with barium ions at the barite surface, presumably in a bi-nuclear, bi-dentate state. Desorption of the Sr was also measured in the presence of EDTA. The desorption experiments show that similar to 75% of the sorbed strontium was removed following extended reactions with EDTA and deionized water. There were minimal changes in the surface structure following desorption measurements implying that changes following Sr sorption in the presence of EDTA may be irreversible.

To compare thermal expansion behaviors in isomorphic structures with different redox behaviors, we separately heated pyrolusite (Mn4+O2) and rutile (Ti4+O2) powders from 25 to-1000 degrees C at a rate of 0.05 degrees C/s at ambient pressure and analyzed them using synchrotron X-ray diffraction and Rietveld refinement. The pyrolusite exhibited two reduction-induced phase decompositions. Between 531 and 583 degrees C, pyrolusite decomposed to bixbyite (Mn3+2 O3), and between 972 and 998 degrees C, bixbyite decomposed to hausmannite (Mn2+Mn23+ O-4). The rutile experienced no phase decompositions or transitions over this same temperature interval. The refined bond lengths and angles for pyrolusite showed that the Mn4+ coordination octahedron became more distorted near the phase decomposition to bixbyite due to the approach of two coordinating oxygen atoms. The 3d electrons of each Mn4+ ion in pyrolusite make 7C bonds with the 2p electrons of the surrounding O-2-ions, whereas the Ti4+ of rutile has no 3d electrons. Thus, 7C bonding between octahedral Mn4+ and the surrounding O-2-anions increases the strength of the Mn-O bonds of pyrolusite relative to the Ti-O bonds of rutile. However, we observed a small decrease in O-O distances in pyrolusite before the decomposition, suggesting that at high temperature, increased 7C bonding between adjacent O anions precedes the release of O2 and the reduction of Mn4+ to Mn3+. Analogous behavior was observed for bixbyite before its reductive phase decomposition to hausmannite. In contrast, no anomalous changes in the O-O distances occurred for rutile. Our X-ray diffraction analyses provided accurate thermal expansion coefficients for these materials over a broader temperature range than reported in previous studies. The Mn-O bond lengths in pyrolusite were shorter and stronger than the Ti-O bonds of rutile; for example, at 60 degrees C, bond distances in pyrolusite refined to 1.8854(6) angstrom whereas the bond lengths of rutile were 1.9575(4) angstrom. Accordingly, the axial and volumetric thermal expansion coefficients refined for rutile were greater than those for pyrolusite. The axial coefficients of thermal expansion (CTE) for pyrolusite were 96% (a-axis) and 69% (c-axis) of the corresponding values for rutile, and the volumetric CTE for pyrolusite was 85% of that for rutile.

We have previously reported complex effects of cytokine-containing T cell supernatants on the interleukin (IL)4 plus phorbol 12-myristate 13-acetate (PMA)-induced proliferative response of murine thymocytes. Here we show that recombinant murine IL-2, IL-6, and IFN-γ each differentially regulate the IL-4/PMA-driven growth of thymocyte subpopulations. Thymocytes fractionated into four subpopulations on the basis of CD4 and CD8 expression were stimulated to proliferate by IL-4/PMA. Interferon-γ (IFN-γ) caused almost complete inhibition of the CD4+/CD8− response but had no measurable effect on the growth of CD4−/CD8+ or CD4−/ CD8− populations. This inhibitory effect was also observed on splenic CD4+/CD8− T cells. In contrast, IL-6 strongly enhanced the proliferative response of CD4+/CD8− thymocytes, but showed no effect on peripheral CD4+/CD8− T cells, suggesting that IL-6 may be an important regulator of growth in the thymus. IL-2 also enhanced the proliferation of both CD4−/CD8+ and CD4−/CD8− thymocytes to IL-4 and PMA. To test whether the IL-4/PMA stimulus provided all the signals required to initiate growth in each subpopulation, we titrated cell number and examined the relationship between cell dose and cell response. Growth of CD8+/CD4− cells was cell density independent, indicating that IL-4/PMA is sufficient stimulus to induce growth of these cells. In contrast, growth of CD4−/CD8− and CD4+/CD8− cells is cell density dependent, suggesting a requirement for another signal provided by the cells themselves. These observations suggest that more signals remain to be identified in this thymocyte growth system.

Mineralization by MgO is an attractive potential strategy for direct air capture (DAC) of CO2 due to its tendency to form carbonate phases upon exposure to water and CO2. Hydration of MgO during this process is typically assumed to not be rate limiting, even at ambient temperatures. However, surface passivation by hydrated phases likely reduces the CO2 capture capacity. Here, we examine the initial hydration reactions that occur on MgO(100) surfaces to determine whether they could potentially impact CO2 uptake. We first used atomic force microscopy (AFM) to explore changes in reaction layers in water (pH = 6 and 12) and MgO-saturated solution (pH = 11) and found the reaction layers on MgO are heterogeneous and nonuniform. To determine how relative humidity (R.H.) affects reactivity, we reacted samples at room temperature in nominally dry N-2 (similar to 11-12% R.H.) for up to 12 h, in humid (>95% R.H.) N-2 for 5, 10, and 15 min, and in air at 33 and 75% R.H. for 8 days. X-ray reflectivity and electron microscopy analysis of the samples reveal that hydrated phases form rapidly upon exposure to humid air, but the growth of the hydrated reaction layer slows after its initial formation. Reaction layer thickness is strongly correlated with R.H., with denser reaction layers forming in 75% R.H. compared with 33% R.H. or nominally dry N-2. The reaction layers are likely amorphous or poorly crystalline based on grazing incidence X-ray diffraction measurements. After exposure to 75% R.H. in air for 8 days, the reaction layer increases in density as compared to the sample reacted in humid N-2 for 5-15 min. This may represent an initial step toward the crystallization of the reaction layer. Overall, high R.H. favors the formation of a hydrated, disordered layer on MgO. Based on our results, DAC in a location with a higher R.H. will be favorable, but growth may slow significantly from initial rates even on short timescales, presumably due to surface passivation.