1,721,005 research outputs found
Mineralogical-geochemical study of the anionic competition effect on the octacalcium phosphate reaction into fluorapatite
Get PDFThe unstable compound octacalcium phosphate (OCP) is one of the crystalline precursors of the apatite mineral series composed by hydroxyapatite, fluorapatite and chlorapatite. The feature of OCP to react into apatite, depending on the media conditions, has been mainly exploited for biomedical applications as bone and tooth substitute material. Recently, some important applications of OCP have been documented: e.g. as electrode material for supercapacitors and as fluoride remover reagent for environmental purposes. With the aim of deepening the property of OCP to be the crystalline precursor of apatite and assessing if and how the anionic competition can influence the formation of the different apatite end-members, the OCP → apatite reaction has been here investigated placing 0.223 mmol of OCP in 50 mL aqueous solution with 0.368 mmol of dissolved fluoride, chloride, hydroxyl and carbonate anions (fluoride alone, fluoride with each of the other anions, and all the anions together) at room temperature. The post-experiment analyses of solid and liquid phases, conducted by using XRD, ESEM and ICP-OES, show that fluoride is always the main anion removed from solution during the OCP transformation reaction. The precise mineralogical characterization of solid phases formed, performed using the Rietveld algorithm, shows that fluorapatite is always the main resulting apatitic phase, followed by hydroxyapatite. Taking into account the different application fields of OCP, these results could be significant in better defining the OCP → apatite reaction in aqueous solutions where different competing anions are involved
Defluoridation of water through the transformation of octacalcium phosphate into fluorapatite
Get PDFThe consumption of water with fluoride concentration higher than 1.5 mg/L (WHO recommended limit) is
recognized to cause serious diseases, and fluoride removal from natural contaminated waters is a health priority
for more than 260 million people worldwide. The octacalcium phosphate (OCP), a mineralogical precursor of bioapatite, is here tested as a fluoride remover. A new two-step method for the synthesis of OCP is proposed: 1)
synthesis of brushite from calcium carbonate and phosphoric acid; 2) subsequent hydrolysis of brushite. Fluoride
removal experiments are performed in batch-mode using different initial concentrations of fluoride (from 40 to
140 mg/L) and reaction times. Most of fluoride is removed within the first 2 h of all experiments, and the
drinkable limit of 1.5 mg/L is reached within a minimum of 3 h for an initial fluoride concentration of 40 mg/L.
The experimental fluoride removal capacity of OCP is 25.7 mg/g, and 4 g of OCP can effectively treat 1 L of water
with fluoride concentration up to 50 times higher than the drinking limit of 1.5 mg/L. XRD and chemical
characterization of the solid phases, before and after the removal experiments, indicate that OCP transforms into
fluorapatite (FAP) uptaking fluoride from solution
Unravelling the luminescence spectrum of garnet grossular var. tsavorite: The role of chromium (III), manganese (II) and misattribution of vanadium (II)
Get PDFGem quality garnet var. tsavorite from Tanzania is investigated by means of SEM-EDS, XPS and fluorescence spectroscopy to decipher its luminescence spectrum. The EDS analysis confirms that the main constituents of the tsavorite samples from Tanzania are those of grossularia (i.e. Ca, Al, Si, O), with V, Mn, Cr, Ti and Mg as minor but characteristic elements (particularly V, Mn and Cr) of the tsavorite variety. The XPS analysis shows that the oxidation state of Mn is +2 and that of V is +3. The luminescence of tsavorite in the yellow region (591 nm) is originated by Mn 2+ in dodecahedral coordination (D 2 point symmetry) while the luminescence in the red region
(690–750 nm) is originated by Cr 3+ in octahedral coordination (O h point symmetry) in a strong crystal field. Contrary to what is often reported in the literature, this study demonstrates that V 2+ (or other 3d transition metal ions as well) is not responsible for the red emissions of tsavorite
Willingness to pay for fluoride-free water in Tanzania: disentangling the importance of behavioural factors
Get PDFApproximately 200 million people, mainly concentrated in rural areas of the Great East African Rift Valley, suffer from fluorosis caused by excess of fluoride naturally contained in water. This study employs the RANAS (Risk, Attitude, Norm, Ability, Self-regulation) model to understand how behavioural factors influence Tanzanian rural communities' willingness to pay for fluoride-free water obtained from a new defluoridator device. Results show that perceived risk, knowledge, attitudes and descriptive norms significantly influence the adoption of the proposed healthy behaviour. Policy implications are discussed taking into account how rural communities could achieve equitable and affordable access to safe water
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
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