Mineral Surface Structure Research Articles

The Surface Structure Change of Columbite-(Fe) Dissolution in H2SO4

The mineral surface structure and ions’ interaction were of significant interest to understanding mineral dissolution and reaction. In this study, X-ray photoemission spectroscopy combined with ICP emission spectrometer was used to investigate the influence of the leaching reaction conditions of 8 M dilute sulfuric acid and 12 M concentrated sulfuric acid on the surface chemical composition, chemical (valence) state and ion distribution of Columbite-(Fe) (FeNb2O6). The binding energy of the cations (Fe, Nb) bonding with different anions (O2−, SO42−) and the ratio of Fe3+/Fe2+ oxidation–reduction provided direct understanding of Fe and Nb releasing from the mineral surface during leaching. The results showed that the binding energy of the Nb5+-O bond was much smaller than that of Nb5+-SO4, and the binding energy decreased in sequence as Nb5+-O < Fe2+-O < Fe3+-O and increased in sequence as Fe3+-SO4 < Fe2+-SO4 < Nb5+-SO4. The mineral surface reaction during the leaching could be expressed with the formula: Fe-O + H2SO4 → Fe-SO4 + H2O, Nb-O + H2SO4 → Nb-SO4 + H2O. The results also revealed that Nb dissolution from Columbite-(Fe) occurred more easily compared to Fe. Nb dissolution from the mineral was owed to the content of H+ in solution, and increasing the H+ concentration could promote the dissolution. For Fe dissolution from the mineral, the oxidation potential could play an effective role in enhancement dissolution.

Molecular Insights on the Adsorption of Polycyclic Aromatic Hydrocarbons on Soil Clay Minerals

The role of soil clay minerals is often ignored in studying the polycyclic aromatic hydrocarbons (PAHs) adsorption in soil. In this work, Monte Carlo and molecular dynamics simulations are used to study the adsorption of PAHs on several typical soil clay mineral surfaces, such as (0 0 1) and (0 0 − 1) of kaolinite, pyrophyllite, mica, and montmorillonite. Results show that van der Waals and cation-π interactions dominate the interactions between PAHs and mineral surfaces. Bridging oxygen atom on the mineral surface structure is the main site of hydrophobic interaction with PAHs. The cation-π effect can significantly enhance the PAHs adsorption energy. In addition, the adsorption on clay minerals is enhanced with the increase in the number of PAHs aromatic rings. In the presence of water, different minerals exhibit different adsorption properties. A layer of water molecules is formed close to the kaolinite (0 0 1) surface, reducing PAHs adsorption. The self-diffusion coefficient (D) of PAHs in Ca2+-montmorillonite is more significant than that in K+-mica, due to the different hydration properties of mineral cations. These findings reveal the adsorption mechanism of PAHs on the surfaces of typical clay minerals at a molecular scale and provide a new perspective for soil adsorption of PAHs.

Interfacial X-Ray Scattering From Small Surfaces: Adapting Mineral-Fluid Structure Methods for Microcrystalline Materials

AbstractCrystal truncation rod (CTR) X-ray diffraction is an invaluable tool for measuring mineral surface and adsorbate structures, and has been applied to several environmentally and geochemically important systems. Traditionally, the method has been restricted to single crystals with lateral dimensions >3 mm. Minerals that meet this size criterion represent a minute fraction of those that are relevant to interfacial geochemistry questions, however. Crystal screening, data collection, and CTR measurement methods have been developed for crystals of <0.3 mm in lateral size using the manganese oxide mineral chalcophanite (ZnMn3O7·3H2O) as a case study. This work demonstrates the feasibility of applying the CTR technique to previously inaccessible surfaces, opening up a large suite of candidate substrates for future study.

Possible solubilization of various mineral elements in the rhizosphere of Lupinus albus L

ABSTRACT Lupinus albus L. (lupin) has a high tolerance for phosphorus deficient conditions as its roots can solubilize the unavailable phosphorus in the rhizosphere soil. The roots may also be able to solubilize other elements, but this requires further investigation. In this study, therefore, we conducted two experiments to comprehensively investigate the effects of lupin roots on the mineral dynamics of the rhizosphere soil. First, a mixed cropping experiment was conducted, in which lupin shared a rhizosphere with soybean (Glycine max (L.) Merr.) in a long-term experimental field with four fertilizer treatments: complete fertilization (+NPK), without nitrogen (−N), without phosphorus (−P), and without potassium (−K). The results of shoot dry weight of plants cultivated alone indicated that lupin is highly tolerant to all N, P, and K deficiencies, while soybean can adapt to N deficiency with the help of rhizobia but is less tolerant to P and K deficiencies than lupin. When mixed-cropped with lupin, the concentrations of many elements in the soybean leaf increased, particularly with the −N and −P treatments. Furthermore, soybean growth was significantly improved when cropped with lupin in the −N and −P treatments. Second, a comparison of the elemental profiles of hydroponically and field-soil-grown plants was conducted. Under hydroponic conditions, the rhizosphere effect is negligible when the culture medium is well circulated. The lupin/soybean ratios for leaf mineral concentrations were considerably larger in the field cultivated plants when compared with the hydroponic cultivations for elements such as sodium, potassium, cesium, phosphorus, iron, copper, and molybdenum, as there were lower concentrations of these elements in the soybean leaves in the field. These results indicate that lupin roots can solubilize a variety of insoluble elements in the soil, which may be the reason why lupin can adapt to various nutrient deficient soils. In the lupin rhizosphere, the solubilization of cesium, which is generally strongly fixed by soil minerals and not easily leached, was particularly pronounced. This implies that the surface structure of clay minerals might be altered in the lupin rhizosphere, resulting in the fixed forms of various elements becoming available.

Surface corrosion by microbial flora enhances the application potential of phosphate rock for cadmium remediation

Heavy metal pollution restricts the intensification of land and endangers human health through the food chain. Surface adsorption of heavy metals by soil natural minerals affects their mobility and biotoxicity, yet little is known about the fate of heavy metals after microorganisms affect the surface structure of minerals. Our study shows that ureolytic microbes corrode the surface of phosphate minerals through carbonate modification without changing their crystal structure but significantly enhancing their specific surface area. Compared with natural phosphate rock (PR), the concave surface of the flora-phosphate rock (FPR) exposes more Ca2+ exchange sites, more active functional groups, and more straightforward to release phosphates that co-precipitate with Cd2+, resulting in an approximately tenfold increase in Cd2+ adsorption. The adsorption of Cd2+ by FPR is a spontaneous endothermic single-layer adsorption, which can quickly remove Cd2+ in wastewater. FPR can also obviously change the pH of the soil by releasing phosphate to increase the stabilization effect of Cd2+. These results provide a sustainable approach to developing novel, cost-effective minerals for environmental remediation and contribute to understanding the function and role of microorganisms in immobilizing heavy metals at the mineral interface.

Biochars and Engineered Biochars for Water and Soil Remediation: A Review

Biochars (BCs) are considered as ecofriendly and multifunctional materials with significant potential for remediation of contaminated water and soils, while engineered biochars (E-BCs) with enlarged surface areas and abundant surface functional groups can perform even better in environmental remediation. This review systematically summarizes the key physical and chemical properties of BCs that affect their pollutant sorption capacities, major methods employed for modification of E-BCs, the performance of BCs/E-BCs in removing major types of organic (e.g., antibiotics and pesticides) and inorganic pollutants (e.g., heavy metals), and the corresponding removal mechanisms. The physical and chemical properties of BCs—such as ash or mineral contents, aromaticity, surface structures, pH, and surface functional groups (e.g., C=O, -COOH, -OH, and -NH2)—depend primarily on their feedstock sources (i.e., plant, sludge, or fecal) and the pyrolysis temperature. Ion exchange, precipitation, electrostatic attraction, and complexation are the main mechanisms involved in the adsorption of inorganic pollutants on BCs/E-BCs, whereas hydrogen bonding, pore filling, electrostatic attraction, hydrophobic interaction, and van der Waals forces are the major driving forces for the uptake of organic pollutants. Despite their significant promises, more pilot and field scale investigations are necessary to demonstrate the practical applicability and viability of BCs/E-BCs in water and soil remediation.

Influencing mechanisms of siderite and magnetite, on naphthalene biodegradation: Insights from degradability and mineral surface structure

Biodegradation is the most economical and efficient process for remediating polycyclic aromatic hydrocarbons (PAHs) such as naphthalene (Nap). Soil composition is pivotal in controlling PAH migration and transformation. Iron minerals such as siderite and magnetite are the primary components of soil and sediment and play key roles in organic pollutant biodegradation. However, it is unclear whether siderite and magnetite promote or inhibit Nap biodegradation. The effects of siderite and magnetite on Nap biodegradation were investigated through batch experiments in this study. The results indicated that siderite increased Nap biodegradation efficiency by 7.87%, whereas magnetite inhibited Nap biodegradation efficiency by 3.54%. In the presence of siderite, Nap-degrading bacteria with acid-producing effects promoted siderite dissolution via metabolic activity, resulting in an increased Fe (II) concentration in solution which accelerated the iron reduction process and promoted Nap biodegradation. In addition, the presence of iron minerals altered the genus-level community structure. Anaerobic sulfate-reducing bacteria such as Desulfosporosinus occurred in the presence of siderite, indicating that sulfate reduction occurred in advance under the influence of siderite. In the presence of magnetite, Fe (III) in iron minerals were converted to Fe (II), and under the mediation of microorganisms, Fe (II) combined with carbonate to form secondary minerals (e.g., siderite). Secondary minerals were attached to the surface of magnetite, which inhibited magnetite dissolution and reduced the efficiency of Fe (III) utilization by microorganisms. Furthermore, as the reaction proceeds, acid-producing microorganisms promoted magnetite further dissolution, resulting in a longer duration of the Fe (III) reduction process. Bacteria utilizing sulfuric acid as the terminal electron acceptor consumed organic matter more rapidly than those using iron as the terminal electron acceptor. Therefore, magnetite inhibited Nap degradation. These observations enhance our understanding of the interaction mechanisms of iron minerals, organic pollutants, and degrading bacteria during the biodegradation process.

Crystal face-dependent methylmercury adsorption onto mackinawite (FeS) nanocrystals: A DFT-D3 study

• Facet-dependent MeHg adsorption onto mackinawite (FeS) was calculated by DFT-D3. • The (0 1 1) surface is more favorable than (0 0 1) and (1 1 1) surfaces for MeHg adsorption. • MeHg adsorbs preferentially onto Fe sites of FeS surfaces relative to the S sites. Reactions occurring at mackinawite (FeS) surfaces play an important role in controlling methylmercury (MeHg) bioavailability and mobility. MeHg adsorption onto FeS is morphology dependent, and the mineral surface structure imparts distinct interfacial chemical properties to individual facets and can control the adsorption capacity at these interfaces. However, the underlying mechanisms for facet-specific adsorption of MeHg remain poorly understood at the molecular scale. A systematic first-principles study of MeHg adsorption onto the (0 0 1), (0 1 1), and (1 1 1) surfaces of FeS was conducted using density functional calculations, and the preferred adsorption sites and adsorption mechanisms were determined. MeHg binds preferentially with Fe sites relative to S sites on the FeS surface, with stronger adsorption onto the FeS (0 1 1) and FeS (1 1 1) surfaces compared to the FeS (0 0 1) surface. MeHg displays a variety of chemisorption geometries on each of the FeS surfaces. The most stable configuration on the FeS (0 0 1) surface is a monodentate S–Hg complex. In contrast, the most stable configuration on the FeS (0 1 1) surface is a monodentate Fe–Hg complex, and a bidentate Fe–Hg–Fe complex was found to be the most stable configuration on the FeS (1 1 1) surface. This work provides an important stepping stone in understanding facet effects and bonding mechanisms of MeHg adsorption on FeS surfaces at the molecular level and provides a framework to guide experimental studies that optimize the use of FeS as a scavenger material for MeHg in groundwater and soil systems.

Effects of divalent heavy metal cations on the synthesis and characteristics of magnetite

Despite decades-long research efforts, the remediation of groundwater contaminated by heavy metals has remained a significant challenge. Recent studies have demonstrated that in situ formation of magnetite might be an effective method to more permanently immobilize metal(loid)s. However, how the co-existence of additional heavy metals affects the synthesis of magnetite under environmentally relevant conditions and how it affects the associated heavy metal removal efficiency have remained unclear. Here, a common magnetite synthesis procedure was used to synthesize Cu-, Cd-, and Pb-substituted magnetites at circumneutral pH with variable heavy metal concentrations. The synthesized mineral products were then subjected to digestion, Mössbauer, XRD, SEM-EDS, magnetic susceptibility and surface area measurements, as well as isothermal adsorption experiments. The presence of additional divalent heavy metals during synthesis affected the characteristics of the Fe mineral products. Magnetite formed in the presence of heavy metals was poorly crystalline and impure. The mineralogy and morphology of the products was regulated by the surface affinity of the heavy metal. The co-existence of divalent heavy metal cations that have higher surface affinity than Fe(II) suppressed the mineral growth of more crystalline Fe oxides, including magnetite. Minerals synthesized in the absence of heavy metals had higher capacity and affinity for heavy metals than minerals synthesized in the presence of those metals because of the effects of altered mineral surface structure and surface area on adsorption. These results help us understand how different types and concentrations of heavy metals affect magnetite synthesis and its suitability for in situ groundwater remediation.

Monovalent – divalent cation competition at the muscovite mica surface: Experiment and theory

HypothesisIon adsorption on mineral surfaces depends on several factors, such as the mineral surface structure and the valency, size and hydration of the ion. In order to understand competitive adsorption at mineral surfaces, experimental techniques are required that can probe multiple ionic species at the same time. By comparing adsorption of two different cations, it should be possible to derive the factors governing ion adsorption. Divalent cations are expected to bind stronger to the negatively-charged muscovite surface than monovalent cations. ExperimentsHere, the competition between the monovalent Cs+ and the divalent Ca2+ cation for adsorption at the muscovite mica basal plane was investigated using surface X-ray diffraction. Using an extended surface complexation model, we simultaneously fit the measured cation coverages and net surface charges reported in literature. FindingsIn order to reproduce those complementary data sets, both cation adsorption and anion coadsorption were included in the surface complexation model. Moreover, the intrinsic muscovite surface charge and the maximum of available adsorption sites had to be reduced compared to existing literature values. Competition experiments revealed that the affinity of Cs+ for the muscovite surface is larger than the affinity of Ca2+, showing that hydration forces are more important than electrostatics.

Comparative Study on Surface Structure, Electronic Properties of Sulfide and Oxide Minerals: A First-Principles Perspective

First-principle calculations were used to investigate the surface structure and electronic properties of sulfide (pyrite, galena, and sphalerite) and oxide minerals (hematite, cerussite, and smithsonite). Surface relaxation and Femi energy, as well as projected DOS, are considered. Results show that the surface atoms of the sulfide minerals are more susceptible and more easily affected by the fracture bonds. The sulfide surfaces possess higher chemical potential than the corresponding oxide surfaces, and are more likely to be electron donors in reactions. The S 3p states are the mainly contributing states in the sulfide surface, while that in the oxide surface are O 2p states. The bonds of the sulfide surface have more covalent features and that of the oxide surface are ionic interactions. The O–M (M represents Fe, Pb or Zn) bonds are more stable, as the DOS of the oxide surfaces distribute in the lower energy range.

Changes in the functional chemical compositions of surface and structural defects of diamonds, due to the nonthermal influence of nanosecond high voltage pulses

Fourier transform infrared spectroscopy (FTIR), analytical electron and atomic force microscopy, and physicochemical examination of the structure and properties of mineral surfaces are used to study changes in the structural defects, functional and chemical compositions, and surface electrical properties and hydrophobicity of diamond surfaces exposed to nonthermal nanosecond high-voltage pulse treatment. The degradation of secondary mineral phase films, their peeling from the diamond surfaces, the growth of crystal hydrophobicity, and an increase in the number of B2 defects were observed after 50 s of treatment with electric pulses. Lengthening the period of irradiation (the dose) to 150 s resulted in oxidation of the diamond surfaces by products of water–air medium radiolytic decomposition, which led to the production of hydroxyl and/or carbonyl groups on the crystal surfaces, a further shift of the diamond electrokinetic and electrostatic potential into the region of negative values, and deterioration of the diamond’s hydrophobic properties.

Protein-Mineral Interactions: Molecular Dynamics Simulations Capture Importance of Variations in Mineral Surface Composition and Structure.

Molecular dynamics simulations, conventional and metadynamics, were performed to determine the interaction of model protein Gb1 over kaolinite (001), Na(+)-montmorillonite (001), Ca(2+)-montmorillonite (001), goethite (100), and Na(+)-birnessite (001) mineral surfaces. Gb1, a small (56 residue) protein with a well-characterized solution-state nuclear magnetic resonance (NMR) structure and having α-helix, 4-fold β-sheet, and hydrophobic core features, is used as a model protein to study protein soil mineral interactions and gain insights on structural changes and potential degradation of protein. From our simulations, we observe little change to the hydrated Gb1 structure over the kaolinite, montmorillonite, and goethite surfaces relative to its solvated structure without these mineral surfaces present. Over the Na(+)-birnessite basal surface, however, the Gb1 structure is highly disturbed as a result of interaction with this birnessite surface. Unraveling of the Gb1 β-sheet at specific turns and a partial unraveling of the α-helix is observed over birnessite, which suggests specific vulnerable residue sites for oxidation or hydrolysis possibly leading to fragmentation.

Characterisation of suspended particulate matter in the Rhone River: insights into analogue selection

Environmental contextThe fate and behaviour of pollutants such as pesticides, metals and nanoparticles in natural waters will influence their effects on the environment and human health. Owing to the complexity of natural waters and suspended particulate matter (SPM) that can interact with pollutants, as well as low pollutant concentrations, determination of pollutant fate and transport is non-trivial. Herein, we report a characterisation of the Rhone River chemistry to provide insight into selecting SPM analogues for experimental and modelling approaches. AbstractSelection of realistic suspended particulate matter (SPM) analogues remains vital for realising representative experimental and modelling approaches in predicting the environmental fate of pollutants. Here, we present the characterisation of dissolved-ion and SPM compositions for nine sampling sites over the length of the Rhone River. Dissolved-ion concentrations remained stable, but SPM concentrations varied among sampling sites. Size fractionation and mineralogical characterisation of the SPM revealed that the same minerals (e.g. quartz, calcite, muscovite) constituted every size class from 0.5 to >50µm, as is usually found with allochthonous and large-scale systems. To gain insight into SPM analogue selection, aggregation kinetics of silica, calcite, muscovite, feldspars and clays were monitored in the native filtrate and related to the respective zeta potentials (ζ). An SPM mixture of calcite (49%), muscovite (14%), feldspar (23%) and chlorite (14%) proved the best match for the Rhone SPM, demonstrating that mineral surface chemistry, structure and size are all important in selecting a realistic SPM analogue for a riverine system.

Nanoscale cross-correlated AFM, Kelvin probe, elastic modulus and quantum mechanics investigation of clay mineral surfaces: The case of chlorite

The physical and chemical surface properties of clay minerals have a significant impact on mineral–environment surface interactions. The study of the local surface properties of clay particles at the nanometre and sub-nm scale is of paramount importance to improve our knowledge on a variety of important interaction processes and interfacial phenomena, such as adsorption, coagulation, aggregation, sedimentation, filtration, catalysis, and ionic transport in porous media. In this work we investigated the physico-chemical properties of the (001) surface of chlorite by cross-correlating AFM, Kelvin probe force microscopy and ab initio quantum mechanics studies. Experimental measurements at the nanometre and sub-nm scale by atomic force microscopy and Kelvin probe force microscopy were performed to investigate the nanomorphology, elastic modulus and surface electrostatic potential of chlorite samples presenting Al substitutions in the tetrahedral sheets and in the hydroxylic interlayer. Quantum mechanics simulations were carried out to investigate the effect of the Al substitutions on both the mineral bulk and surface structures, and on the surface electrostatic potential. Experimental measurements and theoretical calculations were found in very good agreement, complementing each other. The approach here presented and the findings of this study can provide a valuable insight into clay mineral surface properties and a variety of interaction phenomena of clay minerals with important environmental, industrial and biotechnological applications.

Investigation of dodecylammonium adsorption on mica, albite and quartz surfaces by QM/MM simulation

The absorption mechanisms of collector and mineral surface structures play important roles in studies of lepidolite flotation. In this work, quantum mechanics (QM) and hybrid quantum mechanics/molecular mechanics (MM) methods were implemented to investigate the flotation mechanisms of lepidolite from muscovite, quartz and albite. The crystal structures, electron density distributions, bonds and the densities of states of lepidolite were calculated and compared with those of muscovite. The adsorption structures and energies of monomer dodecylammonium (DDA) on the three different minerals were also calculated. The headgroup of the DDA cation was found to adsorb on the surface of minerals, with its hydrophobic tail stretching into the vacuum slab, approximately perpendicular to the surface. Simulation results indicate that the purity of fine lepidolite is limited by the existence of muscovite, due to their similarities in surficial structure and properties. Other gangues were found to be removed efficiently with the use of acidic conditions. The results are in good agreement with other experiments. Compared with simple MM simulations, the use of the QM/MM methods to investigate the adsorption on minerals without specific forcefield parameters was concluded to be a more accurate method to attain monomer surfactant–mineral adsorption energies.

The role of chemistry and pH of solid surfaces for specific adsorption of biomolecules in solution—accurate computational models and experiment

Adsorption of biomolecules and polymers to inorganic nanostructures plays a major role in the design of novel materials and therapeutics. The behavior of flexible molecules on solid surfaces at a scale of 1–1000 nm remains difficult and expensive to monitor using current laboratory techniques, while playing a critical role in energy conversion and composite materials as well as in understanding the origin of diseases. Approaches to implement key surface features and pH in molecular models of solids are explained, and distinct mechanisms of peptide recognition on metal nanostructures, silica and apatite surfaces in solution are described as illustrative examples. The influence of surface energies, specific surface features and protonation states on the structure of aqueous interfaces and selective biomolecular adsorption is found to be critical, comparable to the well-known influence of the charge state and pH of proteins and surfactants on their conformations and assembly. The representation of such details in molecular models according to experimental data and available chemical knowledge enables accurate simulations of unknown complex interfaces in atomic resolution in quantitative agreement with independent experimental measurements. In this context, the benefits of a uniform force field for all material classes and of a mineral surface structure database are discussed.

Carbon Dioxide Binding at Dry FeOOH Mineral Surfaces: Evidence for Structure-Controlled Speciation

Interactions between CO2(g) and mineral surfaces are important to atmospheric and terrestrial settings. This study provides detailed evidence on how differences in mineral surface structure impact carbonate speciation resulting from CO2(g) adsorption reactions. It was achieved by resolving the identity of adsorption sites and geometries of (bi)carbonate species at surfaces of nanosized goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) particles. Fourier transform infrared spectroscopy was used to obtain this information on particles contacted with atmospheres of CO2(g). Vibrational modes of surface hydroxo groups covering these particles were first monitored. These showed that only one type of the surface groups that are singly coordinated to Fe atoms (-OH) are involved in the formation of (bi)carbonate species. Those of higher coordination numbers (μ-OH, μ3-OH) do not participate. Adsorption geometries were then resolved by investigating the C-O stretching region, assisted by density functional theoretical calculations. These efforts provided indications leaning toward a predominance of monodentate mononuclear species, -O-CO2Hx=[0,1]. In contrast, monodentate binuclear species of (-O)2-COHx=[0,1], are expected to form at particle terminations and surface defects. Finally, calculations suggested that bicarbonate is the dominant species on goethite, while a mixture of bicarbonate and carbonate species is present on lepidocrocite, a result stemming from different hydrogen bonding patterns at these mineral surfaces.

Adsorption of thymine and uracil on 1 : 1 clay mineral surfaces: comprehensive ab initio study on influence of sodium cation and water

This computational study performed using the density functional theory shows that hydrated and non-hydrated tetrahedral and octahedral kaolinite mineral surfaces in the presence of a cation adsorb the nucleic acid bases thymine and uracil well. Differences in the structure and chemistry of specific clay mineral surfaces led to a variety of DNA bases adsorption mechanisms. The energetically most predisposed positions for an adsorbate molecule on the mineral surface were revealed. The target molecule binding with the surface can be characterized as physisorption, which occurs mainly due to a cation-molecular oxygen interaction, with hydrogen bonds providing an additional stabilization. The adsorption strength is proportional to the number of intermolecular interactions formed between the target molecule and the surface. From the Atoms in Molecules analysis and comparison of binding energy values of studied systems it is concluded that the sorption activity of kaolinite minerals for thymine and uracil depends on various factors, among which are the structure and accessibility of the organic compounds. The adsorption is governed mostly by the surface type, its properties and presence of cation, which cause a selective binding of the nucleobase. Adsorbate stabilization on the mineral surface increases only slightly with explicit addition of water. Comparison of activity of different studied kaolinite mineral models reveals the following order for stabilization: octahedral-Na-water > octahedral-Na > tetrahedral-Na > tetrahedral-Na-water. Further investigation of the electrostatic potentials helps understanding of the adsorption process and confirmation of the active sites on the kaolinite mineral surfaces. Based on the conclusions that clay mineral affinity for DNA and RNA bases can vary due to different structural and chemical properties of the surface, a hypothesis on possible role of clays in the origin of life was made.

Adding reactivity to structure--reaction dynamics in a nanometer-size oxide ion in water

We examine oxygen-isotope exchanges in a nanometer-size oxide molecule in water and, separately, both its rates ot dissociation and molecular products. This molecule, the decaniobate ion ([H x Nb 10 O 28 ] (6-x) , is at the same size scale as geochemically interesting features on minerals, such as surface polymers and kink sites on growth steps, although it is structurally quite dissimilar. Unlike mineral surface structures, however, we have complete confidence in the aqueous structure of this molecule and it yields a clear spectroscopic signature as it reacts. We thus can follow proton-enhanced isotope exchanges and base-induced dissociation in unprecedented detail and clarity. The results are surprising and require new thinking about geochemical reactions at the molecular scale. For example, base-induced dissociation of the molecule, which is unprotonated, causes rates of oxygen-isotope exchanges of all structural oxygens to accelerate dramatically. Similarly, protonation of the molecule causes sets of oxygens to react, although protonation is limited. In general, all reactions are via concerted motions of many atoms and the reactivities vary as though the entire structure was responding to changes in solution composition. The site reactivities could not be inferred from the stable structure of the decaniobate molecule because so much of the structure is involved in each exchange event. Thus, computational models must be structurally faithful to an extraordinary degree, and inherently dynamic, or they will miss the essential chemistry.