Andreas Luttge

Reduction of graphene oxide via bacterial respiration.

Here we present that graphene oxide (GO) can act as a terminal electron acceptor for heterotrophic, metal-reducing, and environmental bacteria. The conductance and physical characteristics of bacterially converted graphene (BCG) are comparable to other forms of chemically converted graphene (CCG). Electron transfer to GO is mediated by cytochromes MtrA, MtrB, and MtrC/OmcA, while mutants lacking CymA, another cytochrome associated with extracellular electron transfer, retain the ability to reduce GO. Our results demonstrate that biodegradation of GO can occur under ambient conditions and at rapid time scales. The capacity of microbes to degrade GO, restoring it to the naturally occurring ubiquitous graphite mineral form, presents a positive prospect for its bioremediation. This capability also provides an opportunity for further investigation into the application of environmental bacteria in the area of green nanochemistries.

Variation in calcite dissolution rates:: A fundamental problem?

A comparison of published calcite dissolution rates measured far from equilibrium at a pH of ∼ 6 and above shows well over an order of magnitude in variation. Recently published AFM step velocities extend this range further still. In an effort to understand the source of this variation, and to provide additional constraint from a new analytical approach, we have measured dissolution rates by vertical scanning interferometry. In areas of the calcite cleavage surface dominated by etch pits, our measured dissolution rate is 10−10.95 mol/cm2/s (PCO2 10−3.41 atm, pH 8.82), 5 to ∼100 times slower than published rates derived from bulk powder experiments, although similar to rates derived from AFM step velocities. On cleavage surfaces free of local etch pit development, dissolution is limited by a slow, “global” rate (10−11.68 mol/cm2/s). Although these differences confirm the importance of etch pit (defect) distribution as a controlling mechanism in calcite dissolution, they also suggest that “bulk” calcite dissolution rates observed in powder experiments may derive substantial enhancement from grain boundaries having high step and kink density. We also observed significant rate inhibition by introduction of dissolved manganese. At 2.0 μM Mn, the rate diminished to 10−12.4 mol/cm2/s, and the well formed rhombic etch pits that characterized dissolution in pure solution were absent. These results are in good agreement with the pattern of manganese inhibition in published AFM step velocities, assuming a step density on smooth terraces of ∼9 μm−1.

A model for crystal dissolution

A comprehensive dissolution rate theory that integrates individual surface reactions into an overall rate is developed. The dissolution theory is based on the movement of dissolution stepwaves stemming from surface defects. The net bulk rate associated with dissolution stepwaves arises quite naturally from the equations describing the spreading of the train of steps from surface defects. The overall rate can be shown to approach a simple linear rate or transition-state theory-like equation far from equilibrium. However, one of the most important results is the strong nonlinear decrease in the rate as equilibrium conditions are approached, as is the case in most natural processes. The model is validated by extensive Monte Carlo simulations of crystal dissolution, which include a detailed treatment of surface defect energetics, adsorption, surface diffusion, transport of elements from solution, and the bonding dependence of detachment processes from the surface. Monte Carlo results show the generation of dissolution stepwaves and the nonlinear dependence of the overall rate on the saturation state. The final rate equations are consistent with both the far-from-equilibrium experimental work and several recent studies that approached equilibrium. The decrease in the rate as equilibrium is approached has far-reaching implications for both man-made problems ( e.g. , radioactive waste disposal, pollution, etc .) and natural processes from ground-water to metamorphic systems.

Etch pit coalescence, surface area, and overall mineral dissolution rates

Abstract A simple computer model for the dissolution kinetics of crystalline matter governed by etch-pit formation predicts different development, paths, and states for geometric, total (BET), and reactive surface area during the dissolution process. The model also explores the dynamics of the dissolution rate of a given model crystal surface as a function of the development of surface area. Because the surface area term is used in the normalization of bulk dissolution rates, results of this normalization reflect the large differences explored. Based on this evaluation, we discuss the application of the diversely defined surface area terms. In the light of this discussion, the likelihood of an unambiguous definition or application of reactive surface area is problematic. The model focuses on the relationship between the variation in total surface area and the global dissolution rate, and thus is independent of specific surface reaction mechanisms. The actual model calculations presented as an example in this paper utilize experimentally determined dissolution data of three dolomite [CaMg(CO3)2] cleavage surfaces obtained by vertical scanning interferometry (VSI). Similar data from minerals such as calcite, feldspars, and barite can be used and make this model applicable to a range of different crystalline phases.

Plasmonic Nature of the Terahertz Conductivity Peak in Single-Wall Carbon Nanotubes

Plasmon resonance is expected to occur in metallic and doped semiconducting carbon nanotubes in the terahertz frequency range, but its convincing identification has so far been elusive. The origin of the terahertz conductivity peak commonly observed for carbon nanotube ensembles remains controversial. Here we present results of optical, terahertz, and direct current (DC) transport measurements on highly enriched metallic and semiconducting nanotube films. A broad and strong terahertz conductivity peak appears in both types of films, whose behaviors are consistent with the plasmon resonance explanation, firmly ruling out other alternative explanations such as absorption due to curvature-induced gaps.

Direct Observation of Microbial Inhibition of Calcite Dissolution

ABSTRACT Vertical scanning interferometry (VSI) provides a method for quantification of surface topography at the angstrom to nanometer level. Time-dependent VSI measurements can be used to study the surface-normal retreat across crystal and other solid surfaces during dissolution or corrosion processes. Therefore, VSI can be used to directly and nondestructively measure mineral dissolution rates with high precision. We have used this method to compare the abiotic dissolution behavior of a representative calcite (CaCO3) cleavage face with that observed upon addition of an environmental microbe, Shewanella oneidensis MR-1, to the crystal surface. From our direct observations, we have concluded that the presence of the microbes results in a significant inhibition of the rate of calcite dissolution. This inhibition appears to be a 2nd-order effect that is related to the formation of etch pits. The opening of etch pits was greatly inhibited in the presence of added bacteria, suggesting that the bacterial cells exert their effect by inhibiting the formation of etch pits at high-energy sites at the crystal surface caused by lattice defects, e.g., screw or point dislocations. The experimental methodology thus provides a nondestructive, directly quantifiable, and easily visualized view of the interactions of microbes and minerals during weathering (or corrosion) processes or during mineral precipitation.

Mineralogical approaches to fundamental crystal dissolution kinetics

Abstract We introduce a general kinetic model for crystal dissolution that explicitly tracks all the various atoms in the crystal structure as part of the reaction mechanism. This model will be used in this and subsequent articles to develop a theory for the treatment of experimental and field water-rock kinetic data. The model is based on a many-body reaction mechanism. It is built from both elementary reactions, i.e., bond-breaking and bond-forming, and basic reactions, i.e., dissolution of surface units, adsorption and incorporation of solution units, and mobility of units at the crystal surface. The full crystal structure is used to calculate the interactions of neighboring atoms as well as possible defects of the crystal lattice in the model. This approach is different from models based on either molecular precursor complexes or adsorption. We analyze several fundamental concepts such as activation energy, surface free energy, the solubility product, inhibition/catalysis, and saturation-state dependence using our approach. In addition, surface features such as nucleation, steps, and defects are presented and put in a quantitative basis in this paper. The resulting kinetic framework can handle explicitly any crystal structure, treating the actual bonding and position of all atoms within a given surface orientation in the structure. Investigation of the properties of such a general kinetic model leads to new relations between the activation energy and the net energy changes in the hydrolyses reactions, between surface free energy and activation energies and between inhibition and the statistical mechanics of kink sites. The kinetic model can actually account for the emergence of a solubility product from a reaction mechanism involving independent kinetics for the different species using steady-state concepts on the behavior of surface sites. The possible ΔG dependence of the overall rate is studied with the general approach. Isotachs are used to exhibit the interplay of ΔG and inhibition within a simple AB mineral structure. The crystal-based reaction mechanism not only leads to a unified explanation of many observed water-rock features but also produce a series of modifications of kinetic results not fully understood before.

Single-crystal plagioclase feldspar dissolution rates measured by vertical scanning interferometry

Abstract Here we introduce a technique for simultaneous measurement of surface normal retreat rates of specific cleavage faces by vertical scanning interferometry and the bulk dissolution rate of a mineral powder. A hydrothermal reactor is used to contain both a well-characterized powder and oriented single crystals with a masked reference surface at elevated temperatures. We show examples using both anorthite and albite reacted at temperatures between 150 and 200 °C. In the case of albite, dissolution rates of fine-grained powders are substantially enhanced compared to those prevailing on large single-crystal cleavage surfaces. Rates developed on the (010) albite cleavage surface are also substantially faster than those on the (001) face, where etch-pit development was relatively modest and surface normal retreat was not detectable within the time frame of the experiment. The reasons for this difference are not immediately clear, but may be related to anisotropy in the distribution of Al-O-Si vs. Si-O-Si bonds in the albite structure, (010) twinning expressed on the (001) surface, and possible disruption of kink propagation across the twin plane.

Converged surface roughness parameters—A new tool to quantify rock surface morphology and reactivity alteration

The quantification of surface topography is essential in understanding the processes of dissolution and precipitation occurring at the rock-water interface. As a new approach to quantify rock surface alteration in the micrometer to nanometer scale we utilize the convergence of well-known surface roughness parameters. This approach allows the quantification of rock surface size and amplitude as well as its state of alteration during fluid-rock interaction. Vertical scanning interferometry (VSI) is our tool of choice for measuring rock surface topography because of its high vertical resolution and large field of view. Here, we present a case study to demonstrate the potential of surface roughness convergence for quantifying rock surface alteration during weathering. Black slates show different concentrations of organic matter (OM) due to different oxidative weathering ranks. Roughness and surface size data indicate that the original smooth slate surface with deviations of only some tens of nanometers was altered to a rough surface with pore diameters at a scale of some hundreds of nanometers up to several microns. Surface roughness data of a sample profile of three weathering stages (small, medium, large OM decrease) primarily indicate an increase of the OM’s surface area and roughness during weathering. However, during further weathering, surface area and roughness were decreased. From these data we conclude that those parts of the OM that do not directly adjoin to the slate’s clay minerals have a higher reactivity. This means that during ongoing OM weathering the rock surface reactivity and topography are controlled by the extent of OM degradation. Because both reactivity and topography of the observed surface will alter during reaction, it must be concluded that a constant term of “reactive” surface area must not be used to calculate the dissolution rates.

Retention of latex colloids on calcite as a function of surface roughness and topography.

Adhesion of colloidal particles to mineral and rock surfaces is important for environmental and technological processes. Surface topography variations of mineral and rock surfaces at the submicrometer scale may play a significant role in colloid retention in the environment. Here, we present colloid deposition data on calcite as a function of submicrometer surface roughness based on surface data over a field of view of several square millimeters, sufficient to trace the pattern of common inhomogeneities on mineral surfaces. A freshly cleaved calcite crystal was reacted to produce a well-defined etch pit density of approximately 3.4 +/- 1.2 to 8.3 +/- 1.6 [10(-3) microm(-2)] and etch pit depth ranging from approximately 4 to 50 nm. This surface was exposed at the point of zero charge (PZC) of calcite to a colloidal suspension. We used a bimodal particle size distribution of nonfunctionalized polystyrene latex spheres with average diameters of 499 and 903 nm. Vertical scanning interferometry (VSI) was applied to quantify calcite surface topography variations as well as the retention of latex colloids. For both particle sizes, the experiments showed a positive correlation between the surface roughness (Rq) and the number of adsorbed particles. Etch pits were preferred sites for colloidal deposition in contrast to surface steps. The majority of adsorbed particles were trapped at etch pit walls compared to etch pit bottoms. Increasing pit density (D) and depth (d) resulted in an increase of colloidal retention. Deposition of smaller particles exceeded that of the larger-sized fraction of the bimodal system investigated here. Our results show that colloidal deposition at rough mineral and rock surfaces is an important geochemical process. The results about surface roughness dependent particle adsorption will foster the understanding and predictability of colloidal retention for a multitude of natural and technical processes.