Jillian F. Banfield

Morphology development and crystal growth in nanocrystalline aggregates under hydrothermal conditions: insights from titania

Abstract High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) peak broadening (Scherer) analysis were used to study coarsening and morphology development of nanocrystals. Experimental work used synthetic, equidimensional anatase (TiO2) particles about 5 nm in diameter. Under hydrothermal conditions (100–250°C, 15–40 bars), rapid growth occurs along [001], driven in part by the relatively high surface energy of (001) and in part by a kinetic effect involving a cyclic generation of highly reactive adsorption sites. Rapid growth along [001] depresses the growth rate until (001) surfaces shrink to a critical cluster size. A second major coarsening mechanism significantly reduces the surface energy, especially under acidic conditions. This mechanism involves topotactic attachment of primary particles at high energy surfaces (most commonly {112}, less commonly (001)) and can result in elongate single crystals. Oriented attachment may be an important coarsening mechanism under a wide range of conditions encountered in the laboratory and in natural environments.

Particle size effects on transformation kinetics and phase stability in nanocrystalline TiO 2

Abstract Kinetic studies conducted primarily between 465 and 525 ℃ demonstrate that the rate of the polymorphic anatase to rutile transformation increases dramatically when the reacting anatase is very finely crystalline. Coarsening of the reactant anatase and product rutile crystallites occurs simultaneously with the transformation. Kinetic behavior and quantification of transformation rate as a function of average crystallite size indicate that the increase in favorable nucleation sites is a likely cause of increase in transformation rate at small crystallite sizes. Additionally, experimental evidence supports the reversal of stabilities of anatase and rutile at small crystallite sizes. It is proposed that the reversal of stabilities is the result of rutile having a higher surface energy than the anatase phase. Data for coarsening kinetics of anatase and rutile supports the prediction that the surface energy of rutile is significantly larger than that of anatase. Thermodynamic data and theoretical estimates are used to show that a 15% greater surface energy for rutile causes the total free energy of rutile to be greater (less negative) than anatase at crystallite diameters in the few nanometer range. Given the fact that anatase and rutile structures have no polymerized octahedral fragments in common, this may be significant in determining the nature of the nucleated phase.

Phase transformation of nanocrystalline anatase-to-rutile via combined interface and surface nucleation

The kinetics of phase transformation of nanocrystalline anatase samples was studied using x-ray diffraction at temperatures ranging from 600 to 1150 °C. Kinetic data were analyzed with an interface nucleation model and a newly proposed kinetic model for combined interface and surface nucleation. Results revealed that the activation energy of nucleation is size dependent. In anatase samples with denser particle packing, rutile nucleates primarily at interfaces between contacting anatase particles. In anatase samples with less dense particle packing, rutile nucleates at both interfaces and free surfaces of anatase particles. The predominant nucleation mode may change from interface nucleation at low temperatures to surface nucleation at intermediate temperatures and to bulk nucleation at very high temperatures. Alumina particles dispersed among the anatase particles can effectively reduce the probability of interface nucleation at all temperatures.

New kinetic model for the nanocrystalline anatase-to-rutile transformation revealing rate dependence on number of particles

Abstract Existing kinetic models are unable to describe published experimental data for the anatase- to-rutile phase transformation in nanocrystalline samples. A new kinetic model is proposed that combines interface nucleation at certain contact areas between two anatase particles and formation and growth of rutile nuclei. Kinetic equations, incorporating massbalance considerations, derived for this ‘‘interface nucleation and constant growth’’ model fit the experimental data of Gribb and Banfield (1997) fairly well. Results confirm that the transformation is second order with respect to the number of particles of anatase. Over shorter reaction times, the net transformation rate is determined by the rate of nucleation, which is initiated from rutile-like structural elements in the contact area. The activation energy of 165.6 ± 1.1 kJ/mol for rutile nucleation within nanocrystalline anatase particles is much lower than values previously measured for rutile nucleation in coarse anatase samples (>330 kJ/mol). Nuclei growth proceeds at a constant rate with a very small activation barrier. Over longer reaction times, the net transformation rate is determined both by nucleation and nuclei growth. Results quantitatively explain the origin of the size dependence of phase transformation rates in this system.

EFFECT OF MICROORGANISMS AND MICROBIAL METABOLITES ON APATITE DISSOLUTION

The dissolution rate of apatite was determined in batch reactors in organic acid solutions and in microbial cultures. Inoculum for the cultures was from biotite plus apatite crystals from a granite weathering profile in South Eastern Australia. In both the biotic and the abiotic experiments, etching of the apatite surface leads to the formation of elongated spires parallel to the c axis. Apatite dissolution rates in the inorganic, acetate, and oxalate solutions increase as pH decreases from approximately 10 -11 mol/m -2 · s -1 at initial pH 5.5 to 10 -7 mol/m -2 · s -1 at initial pH 2. Under mildly acidic to near neutral pH conditions, both oxalate and acetate increased apatite dissolution by up to an order of magnitude compared to the inorganic conditions. Acetate catalyzed the reaction by forming complexes with Ca, either in solution or at the mineral surfaces. Oxalate forms complexes with Ca as well, and can also affect reaction rates and stoichiometry by forming Ca-oxalate precipitates, thus affecting solution saturation states. In all abiotic experiments, net phosphate release to solution approaches zero even when solutions are apparently undersaturated by several orders of magnitude with respect to the solubility of an ideal fluoroapatite mineral. In the microbial experiments, two enrichment cultures increased both apatite and biotite dissolution by producing organic acids, primarily pyruvate, fermentation products, and oxalate, and by lowering bulk solution pH to between 3 and 5. However, the microorganisms were also able to increase phosphate release from apatite (by two orders of magnitude) without lowering bulk solution pH by producing pyruvate and other compounds.

Microbial extracellular polysaccharides and plagioclase dissolution

Abstract Bytownite feldspar was dissolved in batch reactors in solutions of starch (glucose polymer), gum xanthan (glucose, mannose, glucuronic acid), pectin (poly-galacturonic acid), and four alginates (mannuronic and guluronic acid) with a range of molecular weights (low, medium, high and uncharacterized) to evaluate the effect of extracellular microbial polymers on mineral dissolution rates. Solutions were analyzed for dissolved Si and Al as an indicator of feldspar dissolution. At neutral pH, feldspar dissolution was inhibited by five of the acid polysaccharides, gum xanthan, pectin, alginate low, alginate medium, alginate high, compared to an organic-free control. An uncharacterized alginate substantially enhanced both Si and Al release from the feldspar. Starch, a neutral polysaccharide, had no apparent effect. Under mildly acidic conditions, initial pH ≈ 4, all of the polymers enhanced feldspar dissolution compared to the inorganic controls. Si release from feldspar in starch solution exceeded the control by a factor of three. Pectin and gum xanthan increased feldspar dissolution by a factor of 10, and the alginates enhanced feldspar dissolution by a factor of 50 to 100. Si and Al concentrations increased with time, even though solutions were supersaturated with respect to several possible secondary phases. Under acidic conditions, initial pH ≈ 3, below the pKa of the carboxylic acid groups, dissolution rates increased, but the relative increase due to the polysaccharides is lower, approximately a factor of two to ten. Microbial extracellular polymers play a complex role in mineral weathering. Polymers appear to inhibit dissolution under some conditions, possibly by irreversibly binding to the mineral surfaces. The extracellular polysaccharides can also enhance dissolution by providing protons and complexing with ions in solution.

Formation of rutile nuclei at anatase (112) twin interfaces and the phase transformation mechanism in nanocrystalline titania

Abstract Room-temperature volume measurements of the complete set of calcite-structure carbonates in the pressure range 0-8.1 GPa revealed systematic differences in compressibilities that depend on cation type, resulting in significant deviations from empirical relations of an inverse linear correlation between K0 and V0. The bulk modulus for MgCO3 lies approximately 18 GPa below the trend for the 3d transition metal carbonates, which show an expected inverse linear correlation of bulk modulus with ambient cell volume (and M-O bond length). The bulk modulus of CdCO3, whose M-O bond length and cell volume are only slightly smaller than those of CaCO3, lies up to 10 GPa above the trend of the 3d transition metal carbonates and about 30 GPa above that of calcite. These deviations in compressibility trends as a function of cell volume (or M-O bond length) are expressed as differences in a axis compressibility but not in c axis compressibility, which shows a nearly linear increase with M-O distance. Hence, systematic behavior is apparently limited to subsets of carbonates in which metal ions share the same valence electron character (i.e., s vs. 3d vs. 4d) and is primarily attributed to unexpected compressibility differences along the a axis. Crystal-field effects, beyond those reflected in the M-O distances, cannot account for the observed compressibilities. Nonbonded interactions also fail to explain the deviations from predicted trends. Variations of electronegativity and vibrational frequency with ionic radius for the relevant metal ions show differences that are qualitatively similar to the observed trends of bulk modulus, suggesting that differences in bonding character may contribute to the different behaviors among the subsets of calcite-structure carbonates. However, it is most likely that a combination of factors is necessary to account fully for the observed behavior.

Geomicrobiology of Pyrite (FeS2) Dissolution: Case Study at Iron Mountain, California

Geomicrobiology of pyrite weathering at Iron Mountain, CA, was investigated by molecular biological, surface chemical, surface structural, and solution chemical methods in both laboratory and field-based studies. Research focused at sites both within and peripheral to the ore-body. The acid-generating areas we have examined thus far at Iron Mountain (solution pH 35 C) were populated by species other than Thiobacillus ferrooxidans . 16S rDNA bacterial sequence analysis and domain- and specieslevel oligonucleotide probe-based investigations confirmed the presence of planktonic Leptospirillum ferrooxidans and indicated the existence of other species apparently related to other newly described acidophilic chemolithotrophs. T. ferrooxidans was confined to relatively moderate environments (pH 2-3, 20-30 C) that were peripheral to the orebody. Dissolution rate measurements indicated that, per cell, attached and planktonic species contributed comparably in acid release. Surface colonizati...

BIOLOGICALLY VERSUS INORGANICALLY MEDIATED WEATHERING REACTIONS : RELATIONSHIPS BETWEEN MINERALS AND EXTRACELLULAR MICROBIAL POLYMERS IN LITHOBIONTIC COMMUNITIES

Abstract Biophysical and biogeochemical weathering of amphibole syenite associated with the Stettin complex near Wausau, Wisconsin, has been examined by HRTEM, WDS, LM, and XRD. The rock consists of microperthitic feldspar, ferriannite, quartz, and ferrohastingsite. Crustose saxicolous lichens, Rhizocarpon grande and Porpidea albocaerulescens , penetrate the rock surface to a depth of 10 mm. Within the intact rock, amphibole surfaces along hyphae-filled cracks are highly corroded. Fungal hyphae exploit grain boundaries, cleavages, and cracks to gain access to mineral surfaces, resulting in accumulations of cleavage-bound mineral fragments as small as 5 μm within the lower thallus. Bacterial microcolonies are common and all mineral surfaces are completely coated in extracellular acidic mucopolysaccharides from fungal and bacterial sources. In the cases of amphibole, quartz, and feldspar, dissolution does not appear to involve pervasive leaching, for even the smallest mineral fragments retain their chemical and structural identity. Biotite directly in contact with the lichen thallus is intimately interpenetrated by fungal hyphae growing along (001) cleavages and is partially converted to vermiculite. No siliceous relics have been identified. Biologically mediated weathering involves a complex dissolution/ selective transport/recrystallization mechanism occurring within the acidic extracellular gels coating all mineral surfaces. A specialized weathering microenvironment around each mineral grain initially produces minute phyllosilicate crystallites. A rind of clay minerals forms around the dissolving parent phase, eventually culminating in abundant 5–10 μm diameter polymer-bound aggregates of face-to-face oriented clay minerals of homogeneous composition. Physiochemical weathering of ferrohastingsite produces topotactically oriented smectite and goethite. The cleavage-controlled reaction is neither isochemical nor isovolumetric.

Resistance to, and Accumulation of, Uranium by Bacteria from a Uranium-Contaminated Site

Heterotrophic bacteria were isolated from an acidic uranium-contaminated site. Five isolates and one reference strain, Deinococcus radiodurans, were studied for uranium accumulation and resistance to uranium. Cells were incubated in a pH 4 solution containing 80 ppm of U(VI) in the form of (UO2)2+ for 1 hr. Three isolates closely related to high G+C gram-positive bacteria were resistant to (UO2)2 +. One of the high G+C Gram-positive isolates, closely related to Arthrobacter ilicis, accumulated uranium intracellularly as precipitates closely associated with polyphosphate granules. It is interpreted that sequestration of uranium into polyphosphate is a detoxification mechanism. Deinococcus radiodurans, the most radiation resistant organism known, was vulnerable to chemical toxicity of (UO2)2+. D. radiodurans precipitated nanocrystals of a uranyl phosphate mineral extracellularly, probably as the result of phosphate release during cell lysis. It is possible that Arthrobacter spp. play important roles in natural attenuation and stimulated bioremediation of sites contaminated with radionuclides and toxic organic compounds.