F. Marc Michel

The structure of ferrihydrite, a nanocrystalline material.

Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by traditional methods that rely on long-range order. We uncovered the atomic arrangement by real-space modeling of the pair distribution function (PDF) derived from direct Fourier transformation of the total x-ray scattering. The PDF for ferrihydrite synthesized with the use of different routes is consistent with a single phase (hexagonal space group P63mc; a = ∼5.95 angstroms, c = ∼9.06 angstroms). In its ideal form, this structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis δ-Keggin cluster. Real-space fitting indicates structural relaxation with decreasing particle size and also suggests that second-order effects such as internal strain, stacking faults, and particle shape contribute to the PDFs.

Crystallization by particle attachment in synthetic, biogenic, and geologic environments

Growing crystals by attaching particles Crystals grow in a number a ways, including pathways involving the assembly of other particles and multi-ion complexes. De Yoreo et al. review the mounting evidence for these nonclassical pathways from new observational and computational techniques, and the thermodynamic basis for these growth mechanisms. Developing predictive models for these crystal growth and nucleation pathways will improve materials synthesis strategies. These approaches will also improve fundamental understanding of natural processes such as biomineralization and trace element cycling in aquatic ecosystems. Science, this issue 10.1126/science.aaa6760 Materials nucleate and grow by the assembly of small particles and multi-ion complexes. BACKGROUND Numerous lines of evidence challenge the traditional interpretations of how crystals nucleate and grow in synthetic and natural systems. In contrast to the monomer-by-monomer addition described in classical models, crystallization by addition of particles, ranging from multi-ion complexes to fully formed nanocrystals, is now recognized as a common phenomenon. This diverse set of pathways results from the complexity of both the free-energy landscapes and the reaction dynamics that govern particle formation and interaction. Whereas experimental observations clearly demonstrate crystallization by particle attachment (CPA), many fundamental aspects remain unknown—particularly the interplay of solution structure, interfacial forces, and particle motion. Thus, a predictive description that connects molecular details to ensemble behavior is lacking. As that description develops, long-standing interpretations of crystal formation patterns in synthetic systems and natural environments must be revisited. Here, we describe the current understanding of CPA, examine some of the nonclassical thermodynamic and dynamic mechanisms known to give rise to experimentally observed pathways, and highlight the challenges to our understanding of these mechanisms. We also explore the factors determining when particle-attachment pathways dominate growth and discuss their implications for interpreting natural crystallization and controlling nanomaterials synthesis. ADVANCES CPA has been observed or inferred in a wide range of synthetic systems—including oxide, metallic, and semiconductor nanoparticles; and zeolites, organic systems, macromolecules, and common biomineral phases formed biomimetically. CPA in natural environments also occurs in geologic and biological minerals. The species identified as being responsible for growth vary widely and include multi-ion complexes, oligomeric clusters, crystalline or amorphous nanoparticles, and monomer-rich liquid droplets. Particle-based pathways exceed the scope of classical theories, which assume that a new phase appears via monomer-by-monomer addition to an isolated cluster. Theoretical studies have attempted to identify the forces that drive CPA, as well as the thermodynamic basis for appearance of the constituent particles. However, neither a qualitative consensus nor a comprehensive theory has emerged. Nonetheless, concepts from phase transition theory and colloidal physics provide many of the basic features needed for a qualitative framework. There is a free-energy landscape across which assembly takes place and that determines the thermodynamic preference for particle structure, shape, and size distribution. Dynamic processes, including particle diffusion and relaxation, determine whether the growth process follows this preference or another, kinetically controlled pathway. OUTLOOK Although observations of CPA in synthetic systems are reported for diverse mineral compositions, efforts to establish the scope of CPA in natural environments have only recently begun. Particle-based mineral formation may have particular importance for biogeochemical cycling of nutrients and metals in aquatic systems, as well as for environmental remediation. CPA is poised to provide a better understanding of biomineral formation with a physical basis for the origins of some compositions, isotopic signatures, and morphologies. It may also explain enigmatic textures and patterns found in carbonate mineral deposits that record Earth’s transition from an inorganic to a biological world. A predictive understanding of CPA, which is believed to dominate solution-based growth of important semiconductor, oxide, and metallic nanomaterials, promises advances in nanomaterials design and synthesis for diverse applications. With a mechanism-based understanding, CPA processes can be exploited to produce hierarchical structures that retain the size-dependent attributes of their nanoscale building blocks and create materials with enhanced or novel physical and chemical properties. Major gaps in our understanding of CPA. Particle attachment is influenced by the structure of solvent and ions at solid-solution interfaces and in confined regions of solution between solid surfaces. The details of solution and solid structure create the forces that drive particle motion. However, as the particles move, the local structure and corresponding forces change, taking the particles from a regime of long-range to short-range interactions and eventually leading to particle-attachment events. Field and laboratory observations show that crystals commonly form by the addition and attachment of particles that range from multi-ion complexes to fully formed nanoparticles. The particles involved in these nonclassical pathways to crystallization are diverse, in contrast to classical models that consider only the addition of monomeric chemical species. We review progress toward understanding crystal growth by particle-attachment processes and show that multiple pathways result from the interplay of free-energy landscapes and reaction dynamics. Much remains unknown about the fundamental aspects, particularly the relationships between solution structure, interfacial forces, and particle motion. Developing a predictive description that connects molecular details to ensemble behavior will require revisiting long-standing interpretations of crystal formation in synthetic systems, biominerals, and patterns of mineralization in natural environments.

Sulfidation Processes of PVP-Coated Silver Nanoparticles in Aqueous Solution: Impact on Dissolution Rate

Despite the increasing use of silver nanoparticles (Ag-NPs) in nanotechnology and their toxicity to invertebrates, the transformations and fate of Ag-NPs in the environment are poorly understood. This work focuses on the sulfidation processes of PVP-coated Ag-NPs, one of the most likely corrosion phenomena that may happen in the environment. The sulfur to Ag-NPs ratio was varied in order to control the extent of Ag-NPs transformation to silver sulfide (Ag₂S). A combination of synchrotron-based X-ray Diffraction (XRD) and Extended X-ray Absorption Fine Structure spectroscopy shows the increasing formation of Ag₂S with an increasing sulfur to Ag-NPs ratio. TEM observations show that Ag₂S forms nanobridges between the Ag-NPs leading to chain-like structures. In addition, sulfidation strongly affects surface properties of the Ag-NPs in terms of surface charge and dissolution rate. Both may affect the reactivity, transport, and toxicity of Ag-NPs in soils. In particular, the decrease of dissolution rate as a function of sulfide exposure may strongly limit Ag-NPs toxicity since released Ag⁺ ions are known to be a major factor in the toxicity of Ag-NPs.

Size-Controlled Dissolution of Organic-Coated Silver Nanoparticles

The solubility of Ag NPs can affect their toxicity and persistence in the environment. We measured the solubility of organic-coated silver nanoparticles (Ag NPs) having particle diameters ranging from 5 to 80 nm that were synthesized using various methods, and with different organic polymer coatings including poly(vinylpyrrolidone) and gum arabic. The size and morphology of Ag NPs were characterized by transmission electron microscopy (TEM). X-ray absorption fine structure (XAFS) spectroscopy and synchrotron-based total X-ray scattering and pair distribution function (PDF) analysis were used to determine the local structure around Ag and evaluate changes in crystal lattice parameters and structure as a function of NP size. Ag NP solubility dispersed in 1 mM NaHCO(3) at pH 8 was found to be well correlated with particle size based on the distribution of measured TEM sizes as predicted by the modified Kelvin equation. Solubility of Ag NPs was not affected by the synthesis method and coating as much as by their size. Based on the modified Kelvin equation, the surface tension of Ag NPs was found to be ∼1 J/m(2), which is expected for bulk fcc (face centered cubic) silver. Analysis of XAFS, X-ray scattering, and PDFs confirm that the lattice parameter, a, of the fcc crystal structure of Ag NPs did not change with particle size for Ag NPs as small as 6 nm, indicating the absence of lattice strain. These results are consistent with the finding that Ag NP solubility can be estimated based on TEM-derived particle size using the modified Kelvin equation for particles in the size range of 5-40 nm in diameter.

Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism

The natural nanomineral ferrihydrite is an important component of many environmental and soil systems and has been implicated as the inorganic core of ferritin in biological systems. Knowledge of its basic structure, composition, and extent of structural disorder is essential for understanding its reactivity, stability, and magnetic behavior, as well as changes in these properties during aging. Here we investigate compositional, structural, and magnetic changes that occur upon aging of “2-line” ferrihydrite in the presence of adsorbed citrate at elevated temperature. Whereas aging under these conditions ultimately results in the formation of hematite, analysis of the atomic pair distribution function and complementary physicochemical and magnetic data indicate formation of an intermediate ferrihydrite phase of larger particle size with few defects, more structural relaxation and electron spin ordering, and pronounced ferrimagnetism relative to its disordered ferrihydrite precursor. Our results represent an important conceptual advance in understanding the nature of structural disorder in ferrihydrite and its relation to the magnetic structure and also serve to validate a controversial, recently proposed structural model for this phase. In addition, the pathway we identify for forming ferrimagnetic ferrihydrite potentially explains the magnetic enhancement that typically precedes formation of hematite in aerobic soil and weathering environments. Such magnetic enhancement has been attributed to the formation of poorly understood, nano-sized ferrimagnets from a ferrihydrite precursor. Whereas elevated temperatures drive the transformation on timescales feasible for laboratory studies, our results also suggest that ferrimagnetic ferrihydrite could form naturally at ambient temperature given sufficient time.

Sulfidation Mechanism for Zinc Oxide Nanoparticles and the Effect of Sulfidation on Their Solubility

Environmental transformations of nanoparticles (NPs) affect their properties and toxicity potential. Sulfidation is an important transformation process affecting the fate of NPs containing metal cations with an affinity for sulfide. Here, the extent and mechanism of sulfidation of ZnO NPs were investigated, and the properties of resulting products were carefully characterized. Synchrotron X-ray absorption spectroscopy and X-ray diffraction analysis reveal that transformation of ZnO to ZnS occurs readily at ambient temperature in the presence of inorganic sulfide. The extent of sulfidation depends on sulfide concentration, and close to 100% conversion can be obtained in 5 days given sufficient addition of sulfide. X-ray diffraction and transmission electron microscopy showed formation of primarily ZnS NPs smaller than 5 nm, indicating that sulfidation of ZnO NPs occurs by a dissolution and reprecipitation mechanism. The solubility of partially sulfidized ZnO NPs is controlled by the remaining ZnO core and not quenched by a ZnS shell formed as was observed for partially sulfidized Ag NPs. Sulfidation also led to NP aggregation and a decrease of surface charge. These changes suggest that sulfidation of ZnO NPs alters the behavior, fate, and toxicity of ZnO NPs in the environment. The reactivity and fate of the resulting <5 nm ZnS particles remains to be determined.

Mineralogy of Iron Microbial Mats from Loihi Seamount

Extensive mats of Fe oxyhydroxides and associated Fe-oxidizing microbial organisms form in diverse geochemical settings – freshwater seeps to deep-sea vents – where ever opposing Fe(II)-oxygen gradients prevail. The mineralogy, reactivity, and structural transformations of Fe oxyhydroxides precipitated from submarine hydrothermal fluids within microbial mats remains elusive in active and fossil systems. In response, a study of Fe microbial mat formation at the Loihi Seamount was conducted to describe the physical and chemical characteristics of Fe-phases using extended X-ray absorption fine structure spectroscopy, powder X-ray diffraction, synchrotron radiation X-ray total scattering, low-temperature magnetic measurements, and Mössbauer spectroscopy. Particle sizes of 3.5–4.6 nm were estimated from magnetism data, and coherent scattering domain (CSD) sizes as small as 1.6 nm are indicated by pair distribution function (PDF) analysis. Disorder in the nanostructured Fe-bearing phases results in limited intermediate-range structural order: less than that of standard two-line ferrihydrite (Fh), except for the Pohaku site. The short-range ordered natural Fh (FhSRO) phases were stable at 4°C in the presence of oxygen for at least 1 year and during 400°C treatment. The observed stability of the FhSRO is consistent with magnetic observations that point to non-interacting nanoparticles. PDF analyses of total scattering data provide further evidence for FhSRO particles with a poorly ordered silica coating. The presence of coated particles explains the small CSD for the mat minerals, as well as the stability of the minerals over time and against heating. The mineral properties observed here provide a starting point from which progressively older and more extensively altered Fe deposits may be examined, with the ultimate goal of improved interpretation of past biogeochemical conditions and diagenetic processes.

Fe oxidation processes at Meridiani Planum and implications for secondary Fe mineralogy on Mars

Fe oxidation processes may have occurred during groundwater-mediated diagenesis in Meridiani Planum sediments. To address this question, melanterite oxidation experiments were conducted at epsomite saturation as a function of pH. Results show that schwertmannite is initially formed from acidic Fe oxidation and that its formation and aging to mixtures of jarosite and nanocrystalline goethite is strongly controlled by pH over the range b <2.0-4.0. The pH is controlled in turn by Fe oxidation and Fe3+ hydrolysis. In one 77-d oxidation experiment, nanocrystalline hematite was tentatively identified by MC6ssbauer spectroscopy. Accordingly, aging experiments with synthetic nanocrystalline goethite were conducted (1) to further resolve the formation mechanisms of Fe-phases identified from oxidation experiments and (2) to test whether low water activity (aw) controls the thermodynamically favored goethite to hematite transition at low temperature. MC6ssbauer spectroscopy and total X-ray scattering show no observable changes after 4 months of aging, and instead, these results point to a jarosite precursor for the tentatively identified hematite. On the basis of these results, we suggest that the oxidation and maturation of initially formed Fe2+-bearing saline minerals may account in large part for the distribution of secondary Fe minerals at the Martian surface, contributing to the association of Fe oxides and Mg/Ca sulfates observed from orbital surface analyses. We hypothesize that oxidation of Fe2+ sulfates at low temperature could account for sustained diagenetic acidity in addition to much of the observed Fe mineralogy in Meridiani Planum outcrop rocks. The origin of the gray crystalline hematite at Meridiani, however, is deserving of further experimental work to test this mechanism. Copyright 2008 by the American Geophysical Union.

Fate of Cd during Microbial Fe(III) Mineral Reduction by a Novel and Cd-Tolerant Geobacter Species

Fe(III) (oxyhydr)oxides affect the mobility of contaminants in the environment by providing reactive surfaces for sorption. This includes the toxic metal cadmium (Cd), which prevails in agricultural soils and is taken up by crops. Fe(III)-reducing bacteria can mobilize such contaminants by Fe(III) mineral dissolution or immobilize them by sorption to or coprecipitation with secondary Fe minerals. To date, not much is known about the fate of Fe(III) mineral-associated Cd during microbial Fe(III) reduction. Here, we describe the isolation of a new Geobacter sp. strain Cd1 from a Cd-contaminated field site, where the strain accounts for 10(4) cells g(-1) dry soil. Strain Cd1 reduces the poorly crystalline Fe(III) oxyhydroxide ferrihydrite in the presence of at least up to 112 mg Cd L(-1). During initial microbial reduction of Cd-loaded ferrihydrite, sorbed Cd was mobilized. However, during continuous microbial Fe(III) reduction, Cd was immobilized by sorption to and/or coprecipitation within newly formed secondary minerals that contained Ca, Fe, and carbonate, implying the formation of an otavite-siderite-calcite (CdCO3-FeCO3-CaCO3) mixed mineral phase. Our data shows that microbially mediated turnover of Fe minerals affects the mobility of Cd in soils, potentially altering the dynamics of Cd uptake into food or phyto-remediating plants.

Reactivity of ferritin and the structure of ferritin-derived ferrihydrite

BACKGROUND In nature or in the laboratory, the roughly spherical interior of the ferritin protein is well suited for the formation and storage of a variety of nanosized metal oxy-hydroxide compounds which hold promise for a range of applications. However, the linkages between ferritin reactivity and the structure and physicochemical properties of the nanoparticle core, either native or reconstituted, remain only partly understood. SCOPE OF REVIEW Here we review studies, including those from our laboratory, which have investigated the structure of ferritin-derived ferrihydrite and reactivity of ferritin, both native and reconstituted. Selected proposed structure models for ferrihydrite are discussed along with the structural and genetic relationships that exist among several different forms of ferrihydrite. With regard to reactivity, the review will emphasize studies that have investigated the (photo)reactivity of ferritin and ferritin-derived materials with environmentally relevant gaseous and aqueous species. MAJOR CONCLUSIONS The inorganic core formed from apoferritin reconstituted with varied amounts of Fe has the same structural topology as the inorganically derived ferrihydrite that is an important component of many environmental and soil systems. Reactivity of ferritin toward aqueous species resulting from the photoexcitation of the inorganic core of the protein shows promise for driving redox reactions relevant to environmental chemistry. GENERAL SIGNIFICANCE Ferritin-derived ferrihydrite is effectively maintained in a relatively unaggregated state, which improves reactivity and opens the possibility of future applications in environmental remediation. Advances in our understanding of the structure, composition, and disorder in synthetic, inorganically derived ferrihydrite are shedding new light on the reactivity and stability of ferrihydrite derived artificially from ferritin.