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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Mechanistic Investigation of Dimethylmercury Formation Mediated by a Sulfide Mineral Surface

Mercury (Hg) pollution is a global environmental problem, but many key transformations among 29 various Hg species are not well understood. Although anaerobic microorganisms are the primary 30 producers of monomethylmercury (MMeHg), the abiotic formation of dimethylmercury (DMeHg) 31 from MMeHg have been suggested to account for a large portion of DMeHg in the ocean. It has 32 been shown that abiotic formation of DMeHg from MMeHg can be facilitated by reduced sulfur 33 groups on mineral surfaces. However, a detailed mechanistic understanding of this reaction is 34 lacking. Here we perform density functional theory (DFT) calculations to explore mechanisms of 35 DMeHg formation on the 110 surface of a CdS(s) (hawleyite) nanoparticle. We show that 36 coordination of MMeHg substituents to adjacent reduced sulfur groups protruding from the surface 37 facilitate DMeHg formation and that the reaction proceeds through direct transmethylation from 38 one MMeHg substituent to another. Solvation effects were found to play an important role in the 39 surface reconstruction of the nanoparticle and in decreasing the energetic barrier for DMeHg 40 formation. We also investigate DMeHg formation from MMeHg mediated by organic dithiols and 41 compare the results to those mediated by the mineral surface. In contrast to the surface reaction, 42 the calculations suggest that dithiol-mediated formation of DMeHg instead proceeds through a 43 mechanism in which Hg–Hg interactions activate a methyl group from one MMeHg substituent 44 for intramolecular transfer to the adjacent MMeHg substituent. These findings refine previously 45 proposed mechanisms of abiotic DMeHg formation and fill gaps in our understanding of global 46 Hg cycling in the environment.

Lian, Peng↗
DOE OSTI2025

Modeling Pb(II) Adsorption on Mineral Surfaces: Bridging Density Functional Theory and Experiment with Thermodynamic Insights

Despite decades of work on aqueous lead (Pb) adsorption on a-Fe2O3 (hematite) and a-Al2O3 (alumina), gaps between measurements and modeling obscure molecular-level understanding. Achieving well-matched geometries between theory and experimental for mineral-water interfaces is a hurdle, as surface functional group type and distribution must be accounted for in determining mechanisms. Additionally, computational methods that can describe the substrate are often not appropriate to capture aqueous effects. Progress requires focusing on well-studied and relevant systems, such as key facets (001),(012), and (110) of hematite and alumina, and ubiquitous contaminants such as aqueous Pb. In the past, bulk-parametrized bond-valence principles were used to rationalize Pb(II) adsorption trends. These approaches can break down at surfaces, where flexible bonding environments and adsorption-induced surface relaxations play a critical role. Here, we adapt and apply a density functional theory (DFT) and thermodynamics framework, integrating DFT-calculated energies with experimental data and electrochemical principles, to predict Pb(II) adsorption. Our model results capture trends across the full set of surfaces and predict that inner-sphere Pb(II) sorption on (001) alumina varies from unfavorable to weakly favorable across a range of pH conditions. This aligns with experiment insights that Pb(II) interacts at that surface through outer-sphere interactions. Extending to Fe(II) adsorption, we demonstrate a coverage-dependent site preference, potentially explaining disorder in overlayers grown by the oxidative adsorption of Fe(II) on hematite (001).

lead contamination↗
DOE OSTI2017

Evaluation of accessible mineral surface areas for improved prediction of mineral reaction rates in porous media

The rates of mineral dissolution reactions in porous media are difficult to predict, in part because of a lack of understanding of mineral reactive surface area in natural porous media. Common estimates of mineral reactive surface area used in reactive transport models for porous media are typically ad hoc and often based on average grain size, increased to account for surface roughness or decreased by several orders of magnitude to account for reduced surface reactivity of field as opposed to laboratory samples. In this paper, accessible mineral surface areas are determined for a sample from the reservoir formation at the Nagaoka pilot CO 2 injection site (Japan) using a multi-scale image analysis based on synchrotron X-ray microCT, SEM QEMSCAN, XRD, SANS, and FIB-SEM. This analysis not only accounts for accessibility of mineral surfaces to macro-pores, but also accessibility through connected micro-pores in smectite, the most abundant clay mineral in this sample. While the imaging analysis reveals that most of the micro- and macro-pores are well connected, some pore regions are unconnected and thus inaccessible to fluid flow and diffusion. To evaluate whether mineral accessible surface area accurately reflects reactive surface area a flow-through core experiment is performed and modeled at the continuum scale. The core experiment is performed under conditions replicating the pilot site and the evolution of effluent solutes in the aqueous phase is tracked. Various reactive surface area models are evaluated for their ability to capture the observed effluent chemistry, beginning with parameter values determined as a best fit to a disaggregated sediment experiment (Beckingham et al., 2016) described previously. Simulations that assume that all mineral surfaces are accessible (as in the disaggregated sediment experiment) over-predict the observed mineral reaction rates, suggesting that a reduction of RSA by a factor of 10–20 is required to match the core flood experimental data. While the fit of the effluent chemistry (and inferred mineral dissolution rates) greatly improve when the pore-accessible mineral surface areas are used, it was also necessary to include highly reactive glass phases to match the experimental observations, in agreement with conclusions from the disaggregated sediment experiment. Finally, it is hypothesized here that the 10–20 reduction in reactive surface areas based on the limited pore accessibility of reactive phases in core flood experiment may be reasonable for poorly sorted and cemented sediments like those at the Nagaoka site, although this reflects pore rather than larger scale heterogeneity.

58 GEOSCIENCES↗
NASA NTRS1987

Chemical interactions between the present-day Martian atmosphere and surface minerals

Thermochemical and photochemical reactions between surface minerals and present-day atmospheric constituents are predicted to produce microscopic effects on the surfaces of mineral grains. Relevant reactions hypothesized in the literature include conversions of silicates and volcanic glasses to clay minerals, conversion of ferrous to ferric compounds, and formation of carbonates, nitrates, and sulfates. These types of surface-atmosphere interactions are important for addressing issues such as chemical weathering of minerals, biological potential of the surface environment, and atmospheric stability in both present and past Martian epochs. It is emphasized that the product of these reactions will be observable and interpretable on the microscopic surface layers of Martian surface rocks using modern techniques with obvious implications for sample return from Mars. Macroscopic products of chemical weathering reactions in past Martian epochs are also expected in Martian surface material. These products are expected not only as a result of reactions similar to those proceeding today but also due to aqueous reactions in past epochs in which liquid water was putatively present. It may prove very difficult or impossible however to determine definitively from the relic macroscopic product alone either the exact weathering process which led to its formation or the identity of its weathered parent mineral. The enormous advantages of studying Martian chemical weathering by investigating the microscopic products of present-day chemical reactions on sample surfaces are very apparent.

Prinn, Ronald↗
NASA NTRS1988

Chemical interactions between the present-day Martian atmosphere and surface minerals: Implications for sample return

Thermochemical and photochemical reactions between surface minerals and present-day atmospheric constituents are predicted to produce microscopic effects on the surface of mineral grains. Relevant reactions hypothesized in the literature include conversions of silicates and volcanic glasses to clay minerals, conversion of ferrous to ferric compounds, and formation of carbonates, nitrates, and sulfates. These types of surface-atmosphere weathering of minerals, biological potential of the surface environment, and atmospheric stability in both present and past Martian epochs. It is emphasized that the product of these reactions will be observable and interpretable on the microscopic surface layers of Martian surface rocks using modern techniques with obvious implications for sample return from Mars. Macroscopic products of chemical weathering reactions in past Martian epochs are also expected in Martian surface materials. These products are expected not only as a result of reactions similar to those proceeding today but also due to aqueous reactions in past epochs in which liquid water was putatively present. It may prove very difficult or impossible, however, to determine definitively from the relic macroscopic product alone either the exact weathering process which led to its formation of the identity of its weathering parent mineral. The enormous advantages of studying the Martian chemical weathering by investigating the microscopic products of present-day chemical reactions on sample surfaces are very apparent.

Prinn, Ronald↗
DOE OSTI2019

Mineral Surfaces as Agents of Environmental Proteolysis: Mechanisms and Controls

Here, we investigated the extent to which contact with mineral surfaces affected the molecular integrity of a model protein, with an emphasis on identifying the mechanisms (hydrolysis, oxidation) and conditions leading to protein alteration. To this end, we studied the ability of four mineral surface archetypes (negatively charged, positively charged, neutral, redox-active) to abiotically fragment a well-characterized protein (GB1) as a function of pH and contact time. GB1 was exposed to the soil minerals montmorillonite, goethite, kaolinite, and birnessite at pH 5 and pH 7 for 1, 8, 24, and 168 h and the supernatant was screened for peptide fragments using Tandem Mass Spectrometry. To distinguish between products of oxidative and hydrolytic cleavage, we combined results from the SEQUEST algorithm, which identifies protein fragments that were cleaved hydrolytically, with the output of a deconvolution algorithm (DECON-Routine) designed to identify oxidation fragments. All four minerals were able to induce protein cleavage. Manganese oxide was effective at both hydrolytic and oxidative cleavage. The fact that phyllosilicates—which are not redox active—induced oxidative cleavage indicates that surfaces acted as catalysts and not as reactants. Our results extend previous observations of proteolytic capabilities in soil minerals to the groups of phyllosilicates and Fe-oxides. We identified structural regions of the protein with particularly high susceptibility to cleavage (loops and β strands) as well as regions that were entirely unaffected (α helix).

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS2020

The Earth Surface Mineral Dust Source Investigation: An Earth Science Imaging Spectroscopy Mission

The Earth Surface Mineral Dust Source Investigation, EMIT, is planned to operate from the International Space Station starting no earlier than the fall of 2021. EMIT will use visible to short wavelength infrared imaging spectroscopy to determine the mineral composition of the arid land dust source regions of the Earth to advance our knowledge of the radiative forcing effect of these aerosols. Mineral dust emitted into the atmosphere under high wind conditions is an element of the Earth system with many impacts to the Earth’s energy balance, atmosphere, surface, and oceans. The Earth’s mineral dust cycle with source, transport, and deposition phases are studied with advanced Earth System Models. Because the chemical composition, optical and surface properties of soil particles vary strongly with the mineral composition of the source, these models require knowledge of surface soil mineral dust source composition knowledge to accurately understand dust impacts on the Earth system now and in the future. At present, compositional knowledge of the Earth’s mineral dust source regions from existing data sets is uncertain as a result of limited measurements. EMIT will use spectroscopically derived surface mineral composition to update the prescribe boundary conditions for state-of-the-art Earth System Models. The EMIT-initialized models will be used to investigate the impact of direct radiative forcing in the Earth system that depends strongly on the composition of the mineral dust aerosols emitted into the atmosphere. These new measurements and related products will be used to address the EMIT science objectives and made available to the science community for additional investigations. An overview of the EMIT science, development, and mission is presented in this paper.

Green, Robert O.↗
NASA NTRS1996

Synthesis of long Prebiotic Oligomers on Mineral Surfaces

Most theories of the origin of biological organization assume that polymers with lengths in the range of 30-60 monomers are needed to make a genetic system viable. But it has not proved possible to synthesize plausibly prebiotic polymers this long by condensation in aqueous solution, because hydrolysis competes with polymerization. The potential of mineral surfaces to facilitate prebiotic polymerization was pointed out long ago. Here we describe a system that models prebiotic polymerization by the oligomerization of activated monomers -both nucleotides and amino acids. We find that whereas the reactions in solution produce only short oligomers (the longest typically being a 10-mer), the presence of mineral surfaces (montmorillonite for nucleotides, illite and hydroxylapatite for amino adds) induces the formation of oligomers up to 55 monomers long. These are formed by successive "feedings" with the monomers; polymerization takes place on the mineral surfaces in a manner akin to solid-phase synthesis of biopolymers.

Ferris, James P.↗
NASA NTRS2020

Optical Design of the Earth Surface Mineral Dust Source Investigation (EMIT) Imaging Spectrometer

The Earth Surface Mineral Dust Source Investigation (EMIT) instrument is a high fidelity imaging spectrometer developed to characterize surface mineralogy of the Earth’s dust source regions over the spectral range of 380- 2500 nm and spectral sampling of 7.4 nm. EMIT will close the current knowledge gap in dust source mineral composition by collecting over 1 billion high signal-to-noise ratio spectra in this region of our planet. These new measurements will be used in conjunction with state-of-the-art Earth System Models to understand and reduce the uncertainty in the radiative forcing effect of mineral dust aerosols. EMIT will be deployed on the International Space Station that has an orbit that is well suited for measuring the arid land regions of the Earth. The optical design utilizes a Dyson spectrometer to reduce volume and mass for a fast (F/1.8) and wide swath (1240 samples) optical system. An overview of the EMIT optical design, development, and current status are discussed.

Green, Robert O.↗
DOE OSTI2022

Ab initio molecular dynamics free energy study of enhanced copper (II) dimerization on mineral surfaces

Understanding the adsorption of isolated metal cations from water on to mineral surfaces is critical for toxic waste retention and cleanup in the environment. Heterogeneous nucleation of metal oxyhydroxides and other minerals on material surfaces is key to crystal growth and dissolution. The link connecting these two areas, namely cation dimerization and polymerization, is far less understood. In this work we apply ab initio molecular dynamics calculations to examine the coordination structure of hydroxide-bridged Cu(II) dimers, and the free energy changes associated with Cu(II) dimerization on silica surfaces. The dimer dissociation pathway involves sequential breaking of two Cu 2+ -OH – bonds, yielding three local minima in the free energy profiles associated with 0-2 OH – bridges between the metal cations, and requires the design of a (to our knowledge) novel reaction coordinate for the simulations. Cu(II) adsorbed on silica surfaces are found to exhibit stronger tendency towards dimerization than when residing in water. Cluster-plus-implicit-solvent methods yield incorrect trends if OH – hydration is not correctly depicted. The predicted free energy landscapes are consistent with fast equilibrium times (seconds) among adsorbed structures, and favor Cu 2+ dimer formation on silica surfaces over monomer adsorption.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Prebiotic RNA Polymerisation: Energetics of Nucleotide Adsorption and Polymerisation on Clay Mineral Surfaces

We measured the binding energy and bonding parameters between model nucleotide functional groups and model clay mineral surfaces in solutions of acidic pH. We demonstrate that basal surfaces of clay minerals interact most strongly with nucleobases and show that the adsorption of the phosphate group to clay edges could facilitate polymerisation. Our results suggest that Al- and Fe-rich edge sites behave similarly in nucleotide polymerisation through change of the phosphodiester bond strength. We present an internally consistent set of thermodynamic parameters that represent the nucleotide–clay mineral system.

Jelavic, Stanislov↗
DOE OSTI2021

Characterizing the localization of organic C on mineral surfaces: a correlative microscopy/spectroscopy approach

It is estimated that organic carbon (OC) in soils exceeds the amount of OC in atmosphere and in vegetation pools combined.1 Given the current changes in climate and increasing temperatures in the environment, the fraction of OC sequestered in soils and sediments is more sensitive to decomposition and release as CO 2 in the atmosphere. Evidence implies that sorption and association of dissolved OC with inorganic soil components, i.e., soil minerals, is a key mechanism that contributes to stabilization and retention of OC in soil environment. Studies have shown a direct correlation between sorption of dissolved OC and soil mineral surface properties and their surface area. In addition, properties of organic compounds such as chemical structure and functional groups play a significant role in interaction of dissolved organic species with mineral surfaces. Association of OC with minerals via ligand exchange reactions, polyvalent tertiary bonding, or complexation with metal ions, is found to be preferential and dependent on the organic compounds’ functional groups.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1985

Microbial colonization and growth on metal sulfides and other mineral surfaces

To determine whether a bacterial film forms on sulfur minerals in situ, various sulfur containing and other minerals were incubated in Penitencia Creek. The rate of cell growth and attachment within the surface microenvironment of mineral surfaces was also determined. To determine whether surfaces enriched with soluble sulfur substrates (cysteine, glutathione, thioglycolate, sulfite, and thiosulfate) increased the rate of growth or attachment of natural communities, membrane enrichments were incubated. These rates were determined as described by Caldwell et al. (1981, 1983). The growth of Pseudomonas fluorescens, a heterotrophic sulfur oxidizer, was studied in batch cell suspensions and in continuous culture. In batch culture the cells were oxygen limited (growth rate 0.33 per hour under oxygen limitations and 0.52 per hour when vigorously aerated). Growth within the film was glucose limited. Several behavioral phenomena were observed for cells growing within the hydrodynamic boundary layer. Despite a flow of 10 cm per second in the environment, the bacteria were able to move freely in both directions within the hydrodynamic boundary layer.

Caldwell, D.↗
NASA NTRS2019

Chemical Ecosystem Selection on Mineral Surfaces Reveals Long-Term Dynamics Consistent with the Spontaneous Emergence of Mutual Catalysis

How did chemicals first become organized into systems capable of self-propagation and adaptive evolution? One possibility is that the first evolvers were chemical ecosystems localized on mineral surfaces and composed of sets of molecular species that could catalyze each other’s formation. We used a bottom-up experimental framework; chemical ecosystem selection (CES) to evaluate this perspective and search for surface-associated; and mutually catalytic chemical systems based on the changes in chemistry they are expected to induce. Here, we report the results of preliminary CES experiments conducted using a synthetic “prebiotic soup” and pyrite grains, which yield dynamical patterns that are suggestive of the emergence of mutual catalysis. While more research is needed to better understand the specific patterns observed here and determine whether they are reflective of self-propagation, these results illustrate the potential power of CES to test competing hypotheses for the emergence of protobiological chemical systems.

autocatalysis↗