1,721,027 research outputs found
Deposition of Latex Colloids at Rough Mineral Surfaces: An Analogue Study Using Nanopatterned Surfaces
No full textDeposition of latex colloids on a structured silicon surface was investigated. The surface with well-defined roughness and topography pattern served as an analogue for rough mineral surfaces with half-pores in the submicrometer size. The silicon topography consists of a regular pit pattern (pit diameter = 400 nm, pit spacing = 400 nm, pit depth = 100 nm). Effects of hydrodynamics and colloidal interactions in transport and deposition dynamics of a colloidal suspension were investigated in a parallel plate flow chamber. The experiments were conducted at pH similar to 5.5 under both favorable and unfavorable adsorption conditions using carboxylate functionalized colloids to study the impact of surface topography on particle retention. Vertical scanning interferometry (VSI) was applied for both surface topography characterization and the quantification of colloidal retention over large fields of view. The influence of particle diameter variation (d = 0.3-2 mu m) on retention of monodisperse as well as polydisperse suspensions was studied as a function of flow velocity. Despite electrostatically unfavorable conditions, at all flow velocities, an increased retention of colloids was observed at the rough surface compared to a smooth surface without surface pattern. The impact of surface roughness on retention was found to be more significant for smaller colloids (d = 0.3, 0.43 vs. 1, 2 mu m). From smooth to rough surfaces, the deposition rate of 0.3 and 0.43 mu m colloids increased by a factor of similar to 2.7 compared to a factor of 1.2 or 1.8 for 1 and 2 mu m colloids, respectively. For a substrate herein, with constant surface topography, the ratio between substrate roughness and radius of colloid, Rq/r(c), determined the deposition efficiency. As Rq/r(c), increased, particle-substrate overall DLVO interaction energy decreased. Larger colloids (1 and 2 mu m) beyond a critical velocity (7 X 10(-5) and 3 X 10(-6) m/s) (when drag force exceeds adhesion force) tend to detach from the surface irrespective of the impact of roughness. For polydisperse solutions, an increase in the polydispersity and flow velocity resulted in a reduction of colloid deposition efficiency due to the resulting enhanced double-layer repulsion. Quantification of surface topography variations of two endmembers of natural grain surfaces showed that half-pore depths and roughness of sedimentary quartz grains are mainly in the micrometer range. Grains with diagenetically formed quartz overgrowths, however, show surface roughness mainly in the submicrometer range. Thus, surface topography features applied in the here presented analogue study and resulting variation in particle retention can serve as quantitative analogue for particle reactions in diagenetically altered quartz sands and sandstones. The reported impact of particle polydispersity can have an important application for quantitative prediction of retention of varying types of minerals, such as different clay minerals in the environment under prevailing unfavorable conditions
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Retention of Latex Colloids on Calcite as a Function of Surface Roughness and Topography
No full textAdhesion 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 similar to 3.4 +/- 1.2 to 8.3 +/- 1.6 [10(-3) mu m(-2)] and etch pit depth ranging from similar to 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 its 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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Kinetics and interfacial processes during the recrystallization of calcite and barite, and their influence on radionuclide incorporation
No full textInfluence of Carbonate on the Radium Uptake by Barite and Witherite
No full textThis Ph.D. thesis addresses the recrystallization of barite, BaSO4(s), in carbonate bearing aqueous solution into witherite, BaCO3(s), and the influence of carbonate on radium incorporation into both minerals. The uptake of Ra by barite has been investigated for many decades in the context of various environmental and industrial settings, such as Ra retention in nuclear waste repositories, Ra accumulation due to scaling processes in geothermal energy plants and in pipelines of petroleum production fields, removal of Ra from brackish groundwater in desalination plants, as well as other settings of Naturally Occurring Radioactive Materials.
Radium uptake by barite occurs when dissolved Ra2+(aq) cations react with barite, leading to Ra retention by formation of a (Ba,Ra)SO4 solid-solution. In studies under ambient geochemical conditions, it has been shown that barite crystals can recrystallize and spontaneously adapt their composition to the solution conditions through dissolution and re-precipitation processes, and incorporate Ra and other trace elements into the crystal structure of the barite host mineral. The database for describing (Ra,Ba)SO4(s) mixture thermodynamics has improved considerably in recent years. In addition, quantum chemical methods for determining the equilibrium position of Ra in such solid solutions have become established. However, experimental work also shows that equilibration times can vary between one year and extrapolated time spans of about ten thousands of years depending on the specific sample characteristics and pre-treatment of the initial barite mineral. This demonstrates that a reliable consideration of the Ra immobilization potential of a solid solution requires not only a sound thermodynamic description but also a fundamental quantitative understanding of the kinetics of the reactions involved.
In contrast to the knowledge on Ra uptake by barite via formation of a (Ra,Ba)SO4(s) solid solution, little is known about the uptake of Ra by witherite. The starting hypothesis for this work was that the reaction of barite with dissolved CO32-(aq) anions at elevated pH can lead to the recrystallization of barite into witherite via dissolution and consecutive precipitation processes, resulting in the formation of a (Ra,Ba)CO3(s) solid solution. Besides a quantitative description of the (Ra,Ba)CO3(s) mixture thermodynamics, the kinetics of the potential (Ra,Ba)SO4(s) to (Ra,Ba)CO3(s) recrystallization process is of interest. The presence of carbonate likely alters the chemical behavior of barite surfaces, via surface mixing or by witherite layer formation through dissolution-precipitation which in turn has an effect on the uptake process.
The final aim of this work is to explore the fate of Ra bearing barite during the recrystallization process of barite in the presence of carbonate. By integrating experimental, analytical and computational approaches, the influence of carbonate concentration in solution, thereby the influence of degree of oversaturation, and the influence of barite mineral properties on the recrystallization of BaSO4(s) to BaCO3(s) is intensively studied. Based on the achieved knowledge about the recrystallization of the barite to witherite, incorporation of radium into the two host minerals is investigated. Since strontium occurs as trace element in natural (Sr,Ba)SO4(s) and (Sr,Ba)CO3(s) solid solutions, the fate of Sr during the recrystallisation of a natural Sr-bearing barite to witherite is studied as an analogy for the fate of Ra to provide further insights into the transformation of (Ra,Ba)SO4(s) to (Ra,Ba)CO3(s) in the presence of carbonate.
The influence of carbonate on the Ra uptake by barite and witherite is studied in batch type recrystallisation experiments with large barite cubes and microcrystalline barite powders as well as in coprecipitation batch type coprecipitation experiments with Ba2+(aq) + SO42-(aq) + Ra2+(aq) and Ba2+(aq) + CO32-(aq) + Ra2+(aq) bearing solutions, respectively. In the recrystallisation experiments, coarse grained natural barite samples from Androvo (Bulgaria) and Iberg (Germany), freshly precipitated barite and commercial synthetic high purity barite powder (Sachtleben Chemie GmbH, Germany) are used as starting materials. Besides ultra-pure Sachtleben barite powder used in barite and witherite recrystallization experiments, powder samples of Sachtleben barite, which had been equilibrated with 226Ra2+(aq) bearing and carbonate-free solutions for seven years is used in (Ra,Ba)SO4(s) to (Ra,Ba)CO3(s) recrystallization experiments.
Different analytical and spectroscopic methods like scanning electron microscopy and energy / wavelength dispersive X-ray spectroscopy (SEM-EDS / WDS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD) as well as inductively coupled plasma-optical emission spectrometry (ICP-OES) and -spectroscopy are applied for the analysis of the composition of crystals and solutions studied in the batch experiments. A SEM equipped with a focused ion beam (FIB) cutting device is used for removing ultra-thin layers from surfaces of selected samples to reveal interior structures of altered barite. Using a combined FIB-SEM approach, precise imaging with nanometer resolution and simultaneous chemical analysis of the barite/witherite reaction fronts is achieved. In addition to the analytical and experimental methods, numerical simulations based on the Density Functional Theory (DFT) is used as a quantum-mechanical atomistic simulation tool to obtain electronic energies and atomic structures of the barite and witherite host minerals and the solid solutions (Ra,Ba)SO4(s) and (Ra,Ba)CO3(s). In case of the solid solution simulations, DFT-based electronic energies are employed in the Single Defect Method (SDM) approach to calculate the extent of solid solution non-ideality. Moreover geochemical modelling by means of the PHREEQC software package is used to calculate saturation levels, solid solutions mixing, precipitation of solid phases and diffusion processes at the barite / witherite reaction front.
In series of recrystallisation experiments under various temperatures, degrees of carbonate concentration and pH in solution it is observed that recrystallization of barite to witherite is a rather slow process in case of macroscopic single crystal cubes of Androvo and Iberg barite. It turned out that only under quite extreme conditions (60°C, 0.1 M Na2CO3, pH 11) the replacement (coupled recrystallization) of barite into witherite takes place to a measurable extent within the studied period of five weeks. Microscopic analyses of the initial and replaced barite cubes demonstrate that for the progress of the reaction, in particular the development of the porosity of the growing witherite layer is of importance. With increasing reaction time, the dissolution of barite and the formation of witherite slows down. The experimental results indicate that passivation of the surface by the growing witherite is to be expected over longer periods of time. Compared to results of PHREEQC calculations for full equilibration of the solid-solution system, the theoretical possible formation of approximately 40 wt.% witherite is not achieved or is only achieved over extremely long periods of time even in replacement experiments with barite cubes in 0.1 M Na2CO3 at pH 11 and 60°C. In crystal cubes of pure Androvo barite, a sharp interface between barite and witherite, associated with a sharp decrease in sulfur and an increase in carbon is observed. According to the thermodynamically expected distribution coefficients in (Sr,Ba)SO4(s) to (Sr,Ba)CO3(s) solid solutions, a significant change in the Sr/Ba ratio from the initial Sr-bearing Iberg barite to precipitated witherite would be expected. However, in experiments with Iberg barite the Sr/Ba ratio across the interface between barite and witherite does not change significantly. This indicates that at the barite-witherite interface where the replacement reaction proceeds, the cations released by barite dissolution are incorporated into witherite as they come, and that the thermodynamic affinities for incorporation into witherite under these transport-controlled conditions play no / or only a very minor role.
The initial reaction progress in recrystallization experiments with microcrystalline powder samples of natural Androvo barite, natural Iberg barite and of synthetic Sachtleben barite appears to be similar to that of the macroscopic Androvo and Iberg barite single crystal cubes. After a fast start, the transformation (uncoupled recrystallization) slows down and does not reach the theoretically achievable limit as calculated by means of PHREEQC for full equilibration of the solid-solution system. Still, the transformation of barite to witherite is considerably faster in the experiments with Androvo and Iberg barite powders compared to those with the respective single crystal cubes. Sachtleben and Iberg barite powders react at a closer rate. Similar to the kinetic trends in the replacement experiments with macroscopic single crystal cubes, the Androvo samples, which are ground from macroscopic single crystals, also shows in the powder transformation experiments a significantly lower reactivity compared to the reactivity of the Iberg barite powder. Experiments with Androvo and Iberg barite powder samples show that powders are by no means just small single crystals, but that an enormously increased complexity of the processes taking place can be observed here due to shifts in the reaction rates. SEM-EDX investigations demonstrate that in the powders, surface crystal rebuilding / transformation at a reactive barite-witherite interface plays a subordinate role. Instead, barite dissolves, while witherite crystals often form in idiomorphic shapes in various spatial arrangements with respect to the initial barite. The ratios between barite dissolution rate and witherite growth rate are decisive for this difference in the process sequence. If the barite dissolution is the slowest - rate-controlling process, a barite-witherite interface is formed, as in the single crystal experiments. If, on the other hand, witherite growth is slower than barite dissolution, and therefore rate-controlling, the witherite crystals grow increasingly independently of the initial barite.
In order to show what miscibility can be expected for radium incorporation in barite and witherite, both theoretical calculations using DFT and co-precipitation experiments are carried out. The DFT calculation results show that the incorporation of Ra in barite and witherite is almost ideal with rather small Guggenheim parameters ("non-ideality parameter") of 0.84 and 0.58, respectively. The value for Ra in barite agrees very well with calculated results of Vinograd et al., 2013. Using published solubility products of BaSO4(s) and RaSO4(s) at 60°C, a theoretical distribution coefficient of Dtheo = 0.42 is calculated. Coprecipitation experiments with Ba2+(aq) + SO42-(aq) + Ra2+(aq) at 60° result in a distribution coefficient of Dexp = 0.34 ± 0.14 which is also in excellent agreement with the thermodynamically predicted value for (Ra,Ba)SO4(s). Co-precipitation experiments for Ra-incorporation into witherite over a wide radium concentration range yield a distribution coefficient of Dexp = 0.15 ± 0.05 for (Ra,Ba)CO3(s). Only few literature data is available for the Ra-witherite system. An existing partition coefficient (D = 0.13 ± 0.07 of Yoshida et al., 2015) agrees exactly with the measured (Ra,Ba)CO3(s) composition. Together with the Guggenheim parameter of 0.58 for (Ra,Ba)CO3(s) calculated in this Ph.D. work, this results in partition coefficients of Dtheo = 0.06 , which closer to the lower limit of the value derived from the coprecipitation experiments with Ba2+(aq) + CO32-(aq) + Ra2+(aq), i.e. Dexp = 0.15 ± 0.05. The attempt to simulate the incorporation of sulfate into witherite by means of DFT calculation leads to a highly distorted structure indicating an extreme non-ideality of the solid solution. This corresponds to the experimental findings that sulfate could never be detected in witherite from the recrystallization experiments. Experiments on the recrystallization of Sachtleben barite powders, which had been equilibrated with 226Ra2+(aq) bearing and carbonate-free solutions for seven years, to Ra-bearing witherite shows that the various Ra-bearing barites investigated were quite inert. Interestingly, these long-time equilibrated (Ra,Ba)SO4(s) powders are less reactive compared to a Ra-free Sachtleben barite powder, which had been equilibrated in parallel with 133Ba2+(aq) bearing and carbonate-free solutions for seven years. Based on this observation, it is assumed that the Ra content in the barite host mineral actually has an inhibiting influence on the reactivity. Nevertheless, formation of microscopic (Ra,Ba)CO3(s) crystals is observed. But these do not contain enough Ra for a quantitative evaluation of Ra incorporation after recrystallization.
The comprehensive work undertaken within this Ph.D. represents an important step forward in the scientific understanding of the knowledge of radium geochemistry and contributes to a broader understanding of radium behavior in both natural and anthropogenic contexts. In addition to its contribution to the understanding of the fate of radium, the findings on barite recrystallization into witherite contributes to knowledge of the broader field of mineral dissolution and precipitation processes
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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