Rarefied gas flow in functionalized microchannels.

The interaction of rarefied gases with functionalized surfaces is of great importance in technical applications such as gas separation membranes and catalysis. To investigate the influence of functionalization and rarefaction on gas flow rate in a defined geometry, pressure-driven gas flow experiments with helium and carbon dioxide through plain and alkyl-functionalized microchannels are performed. The experiments cover Knudsen numbers from 0.01 to 200 and therefore the slip flow regime up to free molecular flow. To minimize the experimental uncertainty which is prevalent in micro flow experiments, a methodology is developed to make optimal use of the measurement data. The results are compared to an analysis-based hydraulic closure model (ACM) predicting rarefied gas flow in straight channels and to numerical solutions of the linearized S-model and BGK kinetic equations. The experimental data shows that if there is a difference between plain and functionalized channels, it is likely obscured by experimental uncertainty. This stands in contrast to previous measurements in smaller geometries and demonstrates that the surface-to-volume ratio of 0.4 µ m - 1 seems to be too small for the functionalization to have a strong influence and highlights the importance of geometric scale for surface effects. These results also shed light on the molecular reflection characteristics described by the TMAC.

Discontinuous Dissolution Mechanism of Olivine Deduced from a Topography Observation Method.

Olivine dissolution plays an important role in environmental science and technology, from controlling global element circulation to carbon capture for climate change mitigation. Most studies have been focused on investigating its dissolution rates by monitoring chemical effluent changes under various conditions. However, only by observation of surface reactivity can we unravel the actual mechanism (s) of dissolution. Here, we studied the dissolution of an olivine (010) plane in a flow-through reaction cell with an acidic solution, a surface-controlled regime, and far-from-equilibrium conditions. Direct mineral surface topography measurements using vertical scanning interferometry and atomic force microscopy allowed for quantitative analyses of the spatial and temporal changes in the dissolution rate. The (010) plane dissolved discontinuously in time for different surface sites, resulting in a heterogeneously distributed rate map. Pits with different depths showed opposite dissolution rate distributions from the dislocation center to further out from the etch pit. Based on the step-wave model, we propose a mechanism of dissolution that is governed by the competition between Gibbs free energy of the dissolution process, ΔG, and the critical free energy of the opening of etch pits, i.e., ΔGcrit. The migration of step waves, the distribution of surface defects, the strain field of etch pits, and other dynamic elements, resulting in the instantaneous change of ΔGcrit on the surface, are important factors leading to the discontinuous dissolution of crystal materials. This discontinuous dissolution provides new insight into the guidance of crystalline mineral applications and the prediction of material properties regarding mineral dissolution variation.

A modeling approach for unveiling adsorption of toxic ions on iron oxide nanocrystals.

The era of advanced computer simulations in materials science enables a great potential to design in silico computational experiments for (nano-)material performance. The adsorption efficiency of nanoparticles in various environments can be unveiled by atomistic models and computer simulations. Arsenic (As) is one of the important globally distributed contaminants with a hazardous impact on human health and environment, and it can strongly bind with iron nanocrystals (e.g., hematite (Fe2O3)) depending on their shape and size. Here, we developed a novel Kinetic Monte Carlo (KMC) model capable of exploring and delineating shape-efficiency dependence for Fe2O3 nanocrystals in contact with arsenate-contaminated water. This newly designed model demonstrated the performance of nanocrystals for removal of toxic (As) ions on their surface. The current model opens new avenues for designing further advanced KMC models for nanoparticles-toxic ions interactions, under varying environmentally relevant situations, e.g., groundwater, wetlands, and water treatment systems. In addition to bidentate adsorption complexes, implemented in the model presented, monodentate and outer-sphere adsorption complexes should be incorporated into the KMC model. Detailed environmental controls can be addressed by implementation of pH and background ions.

The evaluation of arsenic contamination potential, speciation and hydrogeochemical behaviour in aquifers of Punjab, Pakistan.

In this study, we tested 123 groundwater wells from five different areas of Punjab, Pakistan for arsenic (As) contamination level and species, as well as delineated hydrogeochemical behaviour of As in aquifers. Results revealed that 75% and 41% of the groundwater wells exceeded the safe As limit of World Health Organisation (WHO, 10 µg L-1) and Pakistan-EPA (50 µg L-1), respectively. Arsenite (As(III)) and arsenate (As(V)) spanned 0-80% and 20-100% of total As (1.2-206 µg L-1), respectively. The mean As content (5.2 µg L-1) of shallow wells at 9-40 m depth did not exceed the WHO safe limit, representing a safe aquifer zone for pumping of groundwater compared to deeper wells at 41-90 m (51 µg L-1) and >90 m (23 µg L-1) depths. Piper-plot elucidated that the aqueous chemistry was dominated with Na-SO4, Na-Ca-SO4, Na-Mg-SO4 type saline water. Principal component analysis grouped As concentration with well depth, pH, salinity, Fe and CO3, exhibiting that these hydrogeochemical factors could have potential role in controlling As release/sequestration into the aquifers of study area. Geochemical modeling showed positive saturation indices only for iron (Fe) oxide-phases, indicating Fe oxides as the major carriers of As. Overall, this study provides insights to tackle emerging As threat to the communities in Punjab, Pakistan, as well as help develop suitable management/mitigation strategies - based on the baseline knowledge of As levels/species and factors governing As contamination in the study area.

Pulsating dissolution of crystalline matter.

Fluid-solid reactions result in material flux from or to the solid surface. The prediction of the flux, its variations, and changes with time are of interest to a wide array of disciplines, ranging from the material and earth sciences to pharmaceutical sciences. Reaction rate maps that are derived from sequences of topography maps illustrate the spatial distribution of reaction rates across the crystal surface. Here, we present highly spatially resolved rate maps that reveal the existence of rhythmic pulses of the material flux from the crystal surface. This observation leads to a change in our understanding of the way crystalline matter dissolves. Rhythmic fluctuations of the reactive surface site density and potentially concomitant oscillations in the fluid saturation imply spatial and temporal variability in surface reaction rates. Knowledge of such variability could aid attempts to upscale microscopic rates and predict reactive transport through changing porous media.

Arsenic removal by Japanese oak wood biochar in aqueous solutions and well water: Investigating arsenic fate using integrated spectroscopic and microscopic techniques.

In this study, we examined the sorption of arsenite (As(III)) and arsenate (As(V)) to Japanese oak wood-derived biochar (OW-BC) in aqueous solutions, and determined its efficiency to remove As from As-contaminated well water. Results revealed that, among the four sorption isotherm models, Langmuir model showed the best fit to describe As(III) and As(V) sorption on OW-BC, with slightly greater sorption affinity for As(V) compared to As(III) (QL=3.89 and 3.16mgg-1; R2=0.91 and 0.85, respectively). Sorption edge experiments indicated that the maximum As removal was 81% and 84% for As(III)- and As(V)-OW-BC systems at pH7 and 6, respectively, which decreased above these pH values (76-69% and 80-58%). Surface functional groups, notably OH, COOH, CO, CH3, were involved in As sequestration by OW-BC, suggesting the surface complexation/precipitation and/or electrostatic interaction of As on OW-BC surface. Arsenic K-edge X-ray absorption near edge structure (XANES) spectroscopy indicated that 36% of the added As(III) was partially oxidized to As(V) in the As(III) sorption experiment, and in As(V) sorption experiment, 48% of As(V) was, albeit incompletely, reduced to As(III) on OW-BC surface. Application of OW-BC to As-contaminated well water (As: 27-144µgL-1; n=10) displayed that 92 to 100% of As was depleted despite in the presence of co-occurring competing anions (e.g., SO42-, CO32-, PO43-). This study shows that OW-BC has a great potential to remove As from solution and drinking (well) water. Overall, the combination of macroscopic sorption data and integrated spectroscopic and microscopic techniques highlight that the fate of As on biochar involves complex redox transformation and association with surface functional moieties in aquatic systems, thereby providing crucial information required for implication of biochar in environmental remediation programs.

Arsenic removal by perilla leaf biochar in aqueous solutions and groundwater: An integrated spectroscopic and microscopic examination.

In this study, we examined the removal of arsenite (As(III)) and arsenate (As(V)) by perilla leaf-derived biochars produced at 300 and 700 °C (referred as BC300 and BC700) in aqueous environments. Results revealed that the Langmuir isotherm model provided the best fit for As(III) and As(V) sorption, with the sorption affinity following the order: BC700-As(III) > BC700-As(V) > BC300-As(III) > BC300-As(V) (QL = 3.85-11.01 mg g-1). In general, As removal decreased (76-60%) with increasing pH from 7 to 10 except for the BC700-As(III) system, where notably higher As removal (88-90%) occurred at pH from 7 to 9. Surface functional moieties contributed to As sequestration by the biochars examined here. However, significantly higher surface area and aromaticity of BC700 favored a greater As removal compared to BC300, suggesting that surface complexation/precipitation dominated As removal by BC700. Arsenic K-edge X-ray absorption near edge structure (XANES) spectroscopy demonstrated that up to 64% of the added As(V) was reduced to As(III) in BC700- and BC300-As(V) sorption experiments, and in As(III) sorption experiments, partial oxidation of As(III) to As(V) occurred (37-39%). However, XANES spectroscopy was limited to precisely quantify As binding with sulfur species as As2S3-like phase. Both biochars efficiently removed As from natural As-contaminated groundwater (As: 23-190 µg L-1; n = 12) despite in the presence of co-occurring anions (e.g., CO32-, PO43-, SO42-) with the highest levels of As removal observed for BC700 (97-100%). Overall, this study highlights that perilla leaf biochars, notably BC700, possessed the greatest ability to remove As from solution and groundwater (drinking water). Significantly, the integrated spectroscopic techniques advanced our understanding to examine complex redox transformation of As(III)/As(V) with biochar, which are crucial to determine fate of As on biochar in aquatic environments.

Variability of Zinc Oxide Dissolution Rates.

Zinc oxide (ZnO) is of widespread use for numerous applications, including many in the cosmetic industry. Thus, ZnO particles are quite likely to enter the environment. ZnO may be harmful because of the release of cytotoxic Zn2+ ions during dissolution reactions. Here, we analyze the dissolution kinetics of the polar zinc-terminated (000-1) and nonpolar (10-10) crystal surfaces in ultrapure water to examine the impact of the crystal defects on dissolution. By using a complementary approach of atomic force microscopy and vertical scanning interferometry, we quantify the difference in reaction rate between the crystal faces, the overall range of rate variability, and the rate components that combine to an overall rate. The mean dissolution rate of the (000-1) crystal surface is more than 4 times that of the (10-10) surface. By using the rate spectrum analysis, we observed an overall dissolution rate variability of more than 1 order of magnitude. The rate components and the range of dissolution rate are important input parameters in reactive transport models for the prediction of potential release of Zn2+ into the environment.

Direct Measurement of Surface Dissolution Rates in Potential Nuclear Waste Forms: The Example of Pyrochlore.

The long-term stability of ceramic materials that are considered as potential nuclear waste forms is governed by heterogeneous surface reactivity. Thus, instead of a mean rate, the identification of one or more dominant contributors to the overall dissolution rate is the key to predict the stability of waste forms quantitatively. Direct surface measurements by vertical scanning interferometry (VSI) and their analysis via material flux maps and resulting dissolution rate spectra provide data about dominant rate contributors and their variability over time. Using pyrochlore (Nd2Zr2O7) pellet dissolution under acidic conditions as an example, we demonstrate the identification and quantification of dissolution rate contributors, based on VSI data and rate spectrum analysis. Heterogeneous surface alteration of pyrochlore varies by a factor of about 5 and additional material loss by chemo-mechanical grain pull-out within the uppermost grain layer. We identified four different rate contributors that are responsible for the observed dissolution rate range of single grains. Our new concept offers the opportunity to increase our mechanistic understanding and to predict quantitatively the alteration of ceramic waste forms.

Lateral resolution enhancement of vertical scanning interferometry by sub-pixel sampling.

We apply common image enhancement principles and sub-pixel sample positioning to achieve a significant enhancement in the spatial resolution of a vertical scanning interferometer. We illustrate the potential of this new method using a standard atomic force microscope calibration grid and other materials having motifs of known lateral and vertical dimensions. This approach combines the high vertical resolution of vertical scanning interferometry and its native advantages (large field of view, rapid and nondestructive data acquisition) with important increases in lateral resolution. This combination offers the means to address a common challenge in microscopy: the integration of properties and processes that depend on, and vary as a function of observational length.

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.

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.

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.

Correlation between sub-micron surface roughness of iron oxide encrustations and trace element concentrations.

Iron oxide encrustations are formed on black slate surfaces during oxidative weathering of iron sulfide and phosphate bearing, organic matter-rich slates. Synchronously, trace elements are released during ongoing weathering. Laser ablation ICP-MS analyses of a weathered and encrusted slate showed that major portions of the V, Cu, As, Mo, Pb, Th, and U reside in the encrustation.Recently a potential relationship between several micrometer to 500 nm surface topography roughness of such encrustations and its uranium concentration was shown. Based on laser scanning microscopy measurements, the present study shows that this interrelation must be expanded to small submicron-sized half-pores with diameters between 100 nm and 500 nm. We demonstrate that the relationship is not limited to topography variations of a single encrustation in the hand-specimen scale. Surface topography and geochemical analyses of iron oxide encrustations from several locations but from the same geochemical environment and with similar weathering history showed that the concentrations of U, P, Cu, and Zn correlate inversely with the surface roughness parameter F. This parameter represents the total surface area and is - in this case - a proxy for the root-mean square surface roughness Rq.This study substantiates the environmental importance that micrometer- to submicrometer topography variations of fluid-rock interfaces govern the trapping of trace elements.

Relationship between micrometer to submicrometer surface roughness and topography variations of natural iron oxides and trace element concentrations.

The surface area and roughness of natural iron oxide precipitations were quantified by 3D optical microscopy in order to get information about fluid-rock interface topography in high-permeability zones. Converged surface roughness data of microscale to submicroscale topography show the predominance of macroporous half-pores (>500 nm) and the occurrence of smaller half-pores (<500 nm) that dominate the BET surface area of iron oxides. A relationship was found between the occurrence of macroporous surface structures (micrometer range) and the uranium content of iron oxide encrustations. Iron-normalized uranium concentrations of an X-ray amorphous iron oxide encrustation correlate linearly with maximum topography heights of 1 to 2 mum on hand specimen subsamples. Our study shows the potential importance of micrometer- to submicrometer-size surface features, whose environmental impact is often ignored.

Aluminosilicate dissolution kinetics: a general stochastic model.

We apply a kinetic model developed for understanding the behavior of crystal dissolution to aluminosilicate dissolution kinetics. Without making any assumptions about specific dissolution mechanisms, the model is a vigorous stochastic exploration of all of the elementary reactions and basic processes involved in dissolution: bond breakage, bond formation, surface diffusion, and departure and arrival of Si- and Al- units. In the stochastic model, the interdependence of these elementary reactions and basic processes is strictly determined by the complicated three-dimensional surface structure in which interconnected Si- and Al- atoms share oxygen atoms. The modeling results are consistent with experimental data in various aspects, such as saturation state dependence of the dissolution rate, aluminum inhibition effects, surface chemistry evolution, anisotropic dissolution, and alteration product. The stochastic model integrates all microscopic information at the atomic scale and elucidates the reasons for the observed kinetic results in experimental studies, improving our fundamental understanding of aluminosilicate dissolution.

Iron phosphide nanostructures produced from a single-source organometallic precursor: nanorods, bundles, crosses, and spherulites.

Single-source molecular precursors were found to produce iron phosphide materials. In a surfactant system of trioctylamine and oleic acid, H2Fe3(CO)9PtBu reacted to form Fe4(CO)12(PtBu)2, which decomposed to give Fe2P nanorods and "bundles." Control of the morphology obtained was possible by varying the surfactant system; addition of increasing amounts of oleic acid resulted in crystal splitting, while the addition of microliter amounts of an alkane enhanced the crystal splitting to give sheaflike structures. The different morphologies seen were attributed to imperfect crystal growth mechanisms.

Characterization of fibroblast morphology on bioactive surfaces using vertical scanning interferometry.

Tissue donor scarcity is a major hindrance to articular cartilage tissue engineering. Previous research shows that dermal fibroblasts express chondrocytic markers after seeding on aggrecan-coated surfaces. Since cell roundness appears to correlate with chondrocytic behavior of dermal fibroblasts, this study quantified roundness by measuring cell height and surface area-volume ratio. In addition to aggrecan as a surface coating, collagen type II and decorin, two other major extracellular matrix components of articular cartilage, were examined. Aggrecan, collagen type II, and decorin were coated onto a glass substrate using three application techniques: static drying, airbrush, and painting. Vertical scanning interferometry (VSI) is a novel technique that allows for the expedient morphological determination of single cells. Interferometry was used for the characterization of protein-coated surfaces in addition to characterizing the morphology of single dermal fibroblasts after 24 h of seeding. Fibroblast height was found to vary from 1.0 to 4.0 microm and protein coating, application technique, and seeding position were significant factors (p < 0.002). The largest cell heights were observed on aggrecan and collagen type II coated surfaces using the air brush and static applications. Additionally, variations were observed for surface area-volume ratio, ranging from 1.75 to 11.94 microm(-1) with decorin resulting in the lowest ratio, followed by collagen type II and aggrecan. This study identifies optimal coating conditions for stimulating morphology in dermal fibroblasts that is characteristic of the chondrocytic phenotype. These conditions can be employed to attempt articular cartilage regeneration and bypass difficulties due to a paucity of donor tissue.

Inhibitive properties and surface morphology of a group of heterocyclic diazoles as inhibitors for acidic iron corrosion.

The heterocyclic diazoles 3-amino-1H-isoindole, indazole, imidazole, 4-bromoimidazole, 4-methylimidazole, pyrazole, 4-nitropyrazole, and 4-sulfopyrazole were investigated as corrosion inhibitors of iron in 1 M HCl using ac and dc techniques. The polarization curves showed a decrease in corrosion current for the inhibitor-containing solution. Impedance spectra demonstrate that the charge-transfer resistance in the presence of these inhibitors was greater than in inhibitor-free solution, except for 4-nitropyrazole. The resistance increased with inhibitor concentration and with immersion time. The structural and electronic parameters of these diazoles were calculated using computational methodologies. The elemental composition and the speciation of the treated surfaces were investigated via XPS measurements, and morphological changes were monitored by vertical scanning interferometery.