Synergetic Effects of Soil Organic Matter Components During Interactions with Minerals.

Mineral-associated soil organic matter (SOM) is critical for stabilizing organic carbon and mitigating climate change. However, mineral-SOM interactions at the molecular scale, particularly synergetic adsorption through organic-organic interaction on the mineral surface known as organic multilayering, remain poorly understood. This study investigates the impact of organic multilayering on mineral-SOM interactions, by integrating macroscale experiments and molecular-scale simulations that assess the individual and sequential adsorption of major SOM compounds-lauric acid (lipid), pentaglycine (amino acid), trehalose (carbohydrate), and lignin onto soil minerals.

Chemical information from XPS: Theory and experiment for Ni(OH)2.

The features and the electronic character of the states for the Ni 2p x-ray photoelectron spectroscopy (XPS) of Ni(OH)2 were analyzed. This detailed analysis is based on ab initio molecular orbital wavefunctions for a cluster model of Ni(OH)2. The theory is validated by comparison with experiment.

Detecting underscreening and generalized Kirkwood transitions in aqueous electrolytes.

We establish the connection between the measured small angle x-ray scattering signal and the charge-charge correlations underlying Kirkwood transitions (KTs) in 1:1, 2:1, and 3:1 aqueous electrolytes. These measurements allow us to obtain underscreening lengths for bulk electrolytes independently verified by theory and simulations. Furthermore, we generalize the concept of KTs beyond those theoretically predicted for 1:1 electrolytes, which involves the inverse screening length, a0, and the inverse periodicity length, Q0.

SAXS of murine amelogenin identifies a persistent dimeric species from pH 5.0 to 8.0.

Amelogenin is an intrinsically disordered protein essential to tooth enamel formation in mammals. Using advanced small angle X-ray scattering (SAXS) capabilities at synchrotrons and computational models, we revisited measuring the quaternary structure of murine amelogenin as a function of pH and phosphorylation at serine-16. The SAXS data shows that at the pH extremes, amelogenin exists as an extended monomer at pH 3.

Cerium Nanophases from Cerium Ammonium Nitrate.

Ceria nanomaterials with facile Ce redox behavior are used in sensing, catalytic, and therapeutic applications, where inclusion of Ce has been correlated with reactivity. Understanding assembly pathways of CeO nanoparticles (NC-CeO) in water has been challenged by "blind" synthesis, including rapid assembly/precipitation promoted by heat or strong base. Here, we identify a layered phase denoted with a proposed formula Ce(OH)(NO)·HO ( ≈ 2.

Facet-dependent dispersion and aggregation of aqueous hematite nanoparticles.

Nanoparticle aggregates in solution controls surface reactivity and function. Complete dispersion often requires additive sorbents to impart a net repulsive interaction between particles. Facet engineering of nanocrystals offers an alternative approach to produce monodisperse suspensions simply based on facet-specific interaction with solvent molecules.

Cation coordination polyhedra lead to multiple lengthscale organization in aqueous electrolytes.

Understanding multiple lengthscale correlations in the pair distribution functions (PDFs) of aq. electrolytes is a persistent challenge. Here, the coordination chemistry of polyoxoanions supports an ion-network of cation-coordination polyhedra in NaNO and NaNO that induce long-range solution structure.

Local density changes and carbonate rotation enable Ba incorporation in amorphous calcium carbonate.

Incorporation of a Ba impurity in amorphous calcium carbonate (ACC) is shown with molecular dynamics simulations to have a long-range effect on its atomic-level structure and to be energetically favoured relative to incorporation in crystalline calcium carbonate polymorphs. The ability of carbonate ions to rotate and of ACC to undergo local density changes explain ACC's propensity for incorporating divalent metal impurities with a wide range of ionic radii. These findings provide an atomic-level basis for understanding the significant effects of low concentrations of impurities on the structure of ACC.

Structural water in amorphous carbonate minerals: molecular dynamics simulations of X-ray pair distribution experiments.

Water is known to play a controlling role in directing mineralization pathways and stabilizing metastable amorphous intermediates in hydrous carbonate mineral MCO·HO systems, where M is a divalent metal cation. Despite this recognition, the nature of the controls on crystallization is poorly understood, largely owing to the difficulty in characterizing the dynamically disordered structures of amorphous intermediates at the atomic scale. Here, we present a series of atomistic models, derived from molecular dynamics simulation, across a range of experimentally relevant cations (M = Ca, Mg, Sr) and hydration levels (0 ≤ ≤ 2).

Residue-Specific Insights into the Intermolecular Protein-Protein Interfaces Driving Amelogenin Self-Assembly in Solution.

Amelogenin, the dominant organic component (>90%) in the early stages of amelogenesis, orchestrates the mineralization of apatite crystals into enamel. The self-association properties of amelogenin as a function of pH and protein concentration have been suggested to play a central role in this process; however, the large molecular weight of the self-assembled quaternary structures has largely prevented structural studies of the protein in solution under physiological conditions using conventional approaches. Here, using perdeuterated murine amelogenin (0.

Heteroatom Manipulation of Zeolite Crystallization: Stabilizing Zn-FAU against Interzeolite Transformation.

The preparation of metastable zeolites is often restricted to a limited range of synthesis conditions, which is exemplified in commercial syntheses lacking organics to stabilize the crystal structure. In the absence of an organic structure-directing agent, interzeolite transformation is a common phenomenon that can lead to undesirable products or impurities. Many studies have investigated the substitution of Si and Al in zeolite frameworks with alternative elements (heteroatoms) as a means of tailoring the properties of zeolites; however, relatively few studies have systematically explored the impact of heteroatoms on interzeolite transformations and their concomitant effects on zeolite crystallization.

Pentavalent Uranium Incorporated in the Structure of Proterozoic Hematite.

Characterizing the chemical state and physical disposition of uranium that has persisted over geologic time scales is key for modeling the long-term geologic sequestration of nuclear waste, accurate uranium-lead dating, and the use of uranium isotopes as paleo redox proxies. X-ray absorption spectroscopy coupled with molecular dynamics modeling demonstrated that pentavalent uranium is incorporated in the structure of 1.6 billion year old hematite (α-FeO), attesting to the robustness of Fe oxides as waste forms and revealing the reason for the great success in using hematite for petrogenic dating.

In situ imaging of amorphous intermediates during brucite carbonation in supercritical CO.

Progress in understanding crystallization pathways depends on the ability to unravel relationships between intermediates and final crystalline products at the nanoscale, which is a particular challenge at elevated pressure and temperature. Here we exploit a high-pressure atomic force microscope to directly visualize brucite carbonation in water-bearing supercritical carbon dioxide (scCO) at 90 bar and 50 °C. On introduction of water-saturated scCO, in situ visualization revealed initial dissolution followed by nanoparticle nucleation consistent with amorphous magnesium carbonate (AMC) on the surface.

Cluster defects in gibbsite nanoplates grown at acidic to neutral pH.

Gibbsite [α-Al(OH)] is the solubility limiting phase for aluminum across a wide pH range, and it is a common mineral phase with many industrial applications. The growth mechanism of this layered-structure material, however, remains incompletely understood. Synthesis of gibbsite at low to circumneutral pH yields nanoplates with substantial interlayer disorder.

Thin Water Films Enable Low-Temperature Magnesite Growth Under Conditions Relevant to Geologic Carbon Sequestration.

Injecting supercritical CO (scCO) into basalt formations for long-term storage is a promising strategy for mitigating CO emissions. Mineral carbonation can result in permanent entrapment of CO; however, carbonation kinetics in thin HO films in humidified scCO is not well understood. We investigated forsterite (MgSiO) carbonation to magnesite (MgCO) via amorphous magnesium carbonate (AMC; MgCO·HO, 0.

Resolving Configurational Disorder for Impurities in a Low-Entropy Phase.

Hematite (α-FeO) exerts a strong control over the transport of minor but critical metals in the environment and is used in multiple industrial applications; the photocatalysis community has explored the properties of hematite nanoparticles over a wide range of transition metal dopants. Nonetheless, simplistic assumptions are used to rationalize the local coordination environment of impurities in hematite. Here, we use ab initio molecular dynamics (AIMD)-guided structural analysis to model the extended X-ray absorption fine structure (EXAFS) of Cu- and Zn-doped hematite nanoparticles.

Competitive TcO, IO, and CrO Incorporation into Ettringite.

Ettringite is a naturally occurring mineral found in cementitious matrices that is known for its ability to incorporate environmentally mobile oxyanion contaminants. To better assess this immobilization mechanism for contaminants within cementitious waste forms intended for nuclear waste storage, this work explores how mixed oxyanion contaminants compete for ettringite incorporation and influence the evolving mineralogy. Ettringite was precipitated in the presence of TcO, IO, and/or CrO, known contaminants of concern to nuclear waste treatment, over pre-determined precipitation periods.

Using Atom Dynamics to Map the Defect Structure Around an Impurity in Nano-Hematite.

The bulk behavior of materials is often controlled by minor impurities that create nonperiodic localized defect structures due to ionic size, symmetry, and charge balance mismatches. Here, we used transmission electron microscopy (TEM) of atom-resolved dynamics to directly map the topology of Fe vacancy clusters surrounding structurally incorporated U in nanohematite (α-FeO). Ab initio molecular dynamic simulations provided additional independent constraints on coupled U, Fe, and vacancy mobility in the solid.

Immobilizing Pertechnetate in Ettringite via Sulfate Substitution.

Technetium-99 immobilization in low-temperature nuclear waste forms often relies on additives that reduce environmentally mobile pertechnetate (TcO) to insoluble Tc(IV) species. However, this is a short-lived solution unless reducing conditions are maintained over the hazardous life cycle of radioactive wastes (some ∼10,000 years). Considering recent experimental observations, this work explores how rapid formation of ettringite [CaAl(SO)(OH)·26(HO)], a common mineral formed in cementitious waste forms, may be used to directly immobilize TcO.

Low temperature and limited water activity reveal a pathway to magnesite amorphous magnesium carbonate.

Forsterite carbonated in thin H2O films to magnesite via amorphous magnesium carbonate during reaction with H2O-bearing liquid CO2 at 25 °C. This novel reaction pathway contrasts with previous studies that were carried out at higher H2O activity and temperature, where more highly hydrated nesquehonite was the metastable intermediate.

Association of Defects and Zinc in Hematite.

Zn is an essential micronutrient that is often limited in tropical, lateritic soils in part because it is sequestered in nominally refractory iron oxide phases. Stable phases such as goethite and hematite, however, can undergo reductive recrystallization without a phase change under circumneutral pH conditions and release metal impurities such as Zn into aqueous solutions. Further, the process appears to be driven by Fe vacancies.

A new method for in situ structural investigations of nano-sized amorphous and crystalline materials using mixed-flow reactors.

Structural investigations of amorphous and nanocrystalline phases forming in solution are historically challenging. Few methods are capable of in situ atomic structural analysis and rigorous control of the system. A mixed-flow reactor (MFR) is used for total X-ray scattering experiments to examine the short- and long-range structure of phases in situ with pair distribution function (PDF) analysis.