1,720,959 research outputs found
Mineral dissolution and groundwater arsenic (As) and uranium (U) contamination in Newfoundland
No full textThe studies presented in this thesis provide critical insights in understanding: (i) arsenic and uranium hosting minerals in natural sediments, and (ii) mechanisms that control arsenic and uranium release from natural sediments to groundwater.
Two distinct sediment samples were collected, characterized, and examined with batch leaching experiments. Scanning electronic microscopy equipped with energy dispersive spectroscopy (SEM-EDX) and sequential extraction results showed that silicate minerals are the main arsenic hosting mineral, containing 75% of the total arsenic. Carbonate, Fe-Mn oxyhydroxides and silicate minerals are the major uranium hosting mineral. Batch leaching experiments showed that, besides desorption, dissolution of Fe oxyhydroxides and silicate minerals is an important mechanism controlling arsenic release. Uranium release increased with increasing pH, Eh, citrate, bicarbonate and natural organic matter (NOM) concentrations. Uranium desorption is the dominant uranium release mechanism under most of the water chemistry conditions tested in our leaching experiments.Includes bibliographical references (pages 135-158)
THERMODYNAMIC MODELING OF METAL IMMOBILIZATION BY BIOCHAR
Get PDFBiochar is a cost-effective sorbent owing to its low production cost and its ability to efficiently remove metals and organics from water. Developing mechanistic and predictive geochemical models of metals immobilization by biochar that are predictive across wide ranges of environmental conditions is important to understand its behavior toward metals in nature and to enhance its use for remedial application. Such models, requires detailed information about biochar surface chemistry and reactivity. The studies in this thesis provide critical insights into metal-biochar surface reactions at molecular-scale by answering questions including: (1) Can the adsorption of divalent cations and radionuclides at the biochar surface be predicted by the surface complexation modeling (SCM) approach across a wide range of water chemistry conditions? (2) Can the SCM approach predict metal adsorption in multi-sorbent systems involving biochar? (3) To what extent can biochar, whether by itself or as part of a composite material, reduce redox-sensitive metals such as Cr(VI)?
Following thorough characterization of biochar, I employ two thermodynamic approaches, (i) surface complexation modeling (SCM) and (ii) isothermal titration calorimetry (ITC), supported by synchrotron based X-ray absorption spectroscopy (XAS), to develop predictive and mechanistic models of the binding of divalent metals (Ni(II) and Zn(II)) and a radionuclide (U(VI)) to biochar. The results show that Ni (II), Zn(II) and U(VI) adsorbed through proton-active -COOH and -OH functional groups on the biochar surface, as confirmed by FT-IR studies coupled with EXAFS. The SCM approach is able to accurately predict the adsorption of Ni(II), Zn(II) and U(VI) across a wide range of water chemistries and at varying adsorbent to adsorbate concentrations. The thermodynamic driving forces of protonation adsorption reactions on the biochar surface suggest that the biochar surface contains anionic oxygen ligands, and Ni(II), Zn(II) and U(VI)-biochar surface adsorption reactions have slightly exothermic to slightly endothermic enthalpies. Ni(II), Zn(II) and U(VI) form both inner- and outer-surface complexes with the biochar surface. The combined SCM-ITC approaches, supported by EXAFS, enhance our understanding of the molecular scale mechanisms of divalent cations and radionuclides adsorption to the surface of biochar. I also test a non-electrostatic SCM approach to predict the adsorption of Cd and/or Se in mixtures of biochar to two agricultural soils. The results show that the metal adsorption to biochar amended soils is successfully predicted by SCM the approach. However, it may be necessary to invoke ternary complexes to accurately predict metal removal from solution if organic ligands are present in solution. Generally, this approach could be useful to develop predictive models of metal distribution in biochar containing multi-sorbent systems. Moreover, I test the redox properties of biochar to reduce chromate from solution, and the efficiency of biochar as a composite material combined with magnetite nanoparticles to enhance Cr(VI) removal from aqueous solution. The results show that both adsorption and reduction of Cr(VI) is enhanced by magnetite nanoparticle – biochar composites compared to biochar and magnetite nanoparticles alone, suggesting a synergetic interaction between magnetite nanoparticle and biochar. Cr(VI) immobilization by magnetite nanoparticle – biochar composites occurs through adsorption and intraparticle diffusion followed by reduction. Synchrotron-based XAS shows that all of the Cr(VI) is reduced to Cr(III) and that the Cr(III)-bearing precipitates Cr(OH)3 and chromite (Cr2FeO4) are formed. This result has important implications for the use of magnetite nanoparticle – biochar composites as a low-cost and efficient material for treating chromate-containing solutions
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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