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A contribution to the surface characterization of alkali metal sulfates
No full textThe analytical characterization of surfaces of sulfur-bearing samples that present sulfides, polysulfides and/or elemental sulfur as reaction products can be difficult by simply relying on the binding energy of the S2p X-ray photoelectron signals, due to the small chemical shifts. In such cases the Auger parameter concept can be used to distinguish among different chemical states, but this requires a model to curve fit complex Auger SKLL signals in order to resolve the contributions arising from sulfur in different chemical states on the surface. With this scope a detailed X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES) surface analytical study of the group IA sulfates is presented in this paper. Sulfates were chosen as model compounds for curve fitting the X-ray induced SKLL spectra since in these compounds sulfur is present in a unique chemical state. For the first time the multicomponent SKLL spectra are fitted with model functions consisting of an intense 1D and a low intensity 1S contribution with constant energy difference of 8 eV. It was found that the kinetic energy of the SK2,3L2,3 (1D) line increases from 2105.1 ± 0.1 to 2107.5 ± 0.2 eV whereas the corresponding S2p 3/2 binding energy decreases from 169.5 ± 0.1 eV for Li 2SO4 to 167.8 ± 0.1 eV for Cs2SO 4. Shifts to lower binding energy values are observed also for S2p, S2s and O1s peaks. In the chemical state plot the alkali sulfates fall on a line with an Auger parameter of ca. 2275 eV whereas elemental sulfur is found at 2277 eV. A shift of 0.6 eV in the calculated Auger parameter α′ is observed from 2274.7 ± 0.1 eV for Li to 2275.3 ± 0.2 eV for Cs sulfate. These results are discussed with respect to the influence of the cation using fundamental data and concepts such as ion radius, oxidation state and ionization energy, and separating intra-atomic and inter-atomic effects. All the information show a relatively small interaction between the sulfate anion and the cations as a consequence of the strong group localization of the valence states over the sulfate tetrahedron. The sulfur SKLL spectrum of sulfates and elemental sulfur consisting of an intense 1D and a low intensity 1S contribution with constant energy difference of 8 eV can be considered a first step toward curve fitting of multicomponent sulfur SKLL spectra and the interpretation of complex mechanisms of oxidation and dissolution that involve sulfur
Exploiting XPS for the identification of sulfides and polysulfides
No full textThe identification of surface sulfide and polysulfide species based on the curve fitting of S2p photoelectron
spectra and, for the first time, of X-ray excited S KLL Auger spectra has been performed. The different sulfur
chemical states present on the surface (sulfide S2, central S and terminal S in polysulfide chains) could be
unambiguously assigned in the chemical state plot. Sulfur atoms in the central or terminal position,
respectively, are found on a line with slope ca. 3 irrespective of the cation indicating similar initial state
effects. On the other hand, for a given polysulfide, e.g. K2Sn, sulfur atoms both in central or terminal
positions are found on the same line with slope 1 indicating similar final state effects. This behavior can
be rationalized with the fact that the negative charge in polysulfide chains is located mainly on sulfur
atoms in the terminal position; indeed, sulfur present as central S shows a binding energy shift of 0.6
eV with respect to elemental sulfur (S8), and sulfur in terminal S a shift of 2.4 eV. An application of this
approach tested on commercial alkali polysulfides is provided for the curve fitting of SKLL signals and
sulfur speciation of three different sulfide minerals enargite (Cu3AsS4), chalcopyrite (CuFeS2) and
arsenopyrite (FeAsS). Also for the surface of mineral sulfides, terminal S atoms and central S atoms in the
polysulfide chains can successfully be identified
Recupero dei metalli nobili da schede elettroniche (PCB): un approccio multidisciplinare verso la sostenibilità
No full textNoble-metals recovery from printed circuit boards: a multidisciplinary approach towards sustainability
No full textA robust database of the noble metals (NM) content of different classes of Printed
Circuit Boards (PCB) was build through an accurate characterization of samples of different
origin, underwent mechanical comminution and representative sampling, by ICP-AES
quantitative chemical analysis, to work as reference for researcher and companies interested in
waste valorisation. The results identified RAM and mobile phone’s PCB as the “richest” classes
of PCB, while TV PCB as the “poorest” in term of NM content. On these basis, a sustainable
three-step NM recovery method, previously set-up on a finely shredded WEEE sample deprived
by non-metallic and ferrous materials, was applied on a coarse sample of shredded RAM
boards provided by companies, as a case study. Preliminary results highlighted that good NM
recovery yields (from 65% to quantitative) and limited by-products formation could be achieved,
despite a huge amount of composite materials was present in the mixture. Nevertheless, 10-
times increased leaching times found for copper dissolution suggested that improved
mechanical pretreatments might increase leaching effectiveness
From trash to resource: a green approach to noble-metals dissolution and recovery
No full textA process based on the lixiviant properties of organic mixtures of dihalogen/S,S-ligands, N,N′-dimethyl-perhydrodiazepine-2,3-dithione (Me2dazdt) and tetraalkylthiuramdisulphide (Et4TDS) in the presence of diiodine, for gold recovery from the non-ferrous metal fraction of real shredded waste electric and electronic equipment (WEEE), is presented here. Selective dissolution of metals is achieved through a sequence of three steps where the oxidation of different kinds of metals is achieved by using: (1) refluxing water solutions of HCl 1:5 under Ar atmosphere (Sn, Zn, etc.); (2) water solutions of NH3/(NH4)2SO4 mixtures in the presence of H2O2 on the resting sample(Cu, Ag); and (3) acetone solutions of Me2dazdt or Et4TDS/I2 mixtures on the final residue (Au). Each step is followed by a further treatment for: (1) metal recovery, in the case of Au, Cu, Ag; and (2) inertization, in the case of heavy metals. As a whole, the process is very promising for effective recovery of gold and other valuable noble-metals and for using non harmful reagents in mild conditions
Effect of the double doping mechanism on the phase diagram of Y1-xCaxBa2Cu3O6+y
No full textWe investigated the effect of hole doping and quenched disorder on the phase diagram of Y1-xCaxBa2Cu3O6+y, by exploiting the double doping mechanism of Y–Ca substitution and O intercalation in the undoped-to-heavily underdoped regime. We show that the insulator to metal transition, governed by the mobile hole concentration, reflects only the charges transferred by chain oxygen (the y fraction in the chemical formula). The transition is preceded by the suppression of antiferromagnetic order, which is replaced by a cluster spin glass ground state. We discuss the effect of doping and disorder on both magnetic states and on the appearance of superconductivit
Sulfur chemical state at mineral surfaces - An XPS and XAES investigation
No full textDuring the interaction of sulphide minerals with the environment many reactions may occur. The knowledge of sulphur chemical state is important in order to clarify the mechanisms of oxidation/dissolution and precipitation reactions also in biotic conditions on the surface of these minerals. To date different hypotheses have been proposed for identifying the sulphur compounds that may form but no consensus is reached so far because the identification has been carried out either only on the basis of the photoelectron signals or combining the photoelectron signal with the centroid of the X-ray induced SKLL Auger line.
The objectives of this thesis are: the development of an analytical strategy for the unambiguous identification of sulphur chemical state in nanometer thick layers that form on mineral surfaces after leaching. To this purpose the acquisition of detailed spectra of a series of metal sulphides and of model systems made of mixtures of sulphur and polysulphides prepared under controlled conditions is necessary. Second objective was the establishment of the curve-fitting procedure to allow the separation of the different components present on the surface (sulphide, polysulphides, sulphates) not only of the S2p XP-spectra but also of the SKLL spectra. Based on these results the Auger parameter values will be determined and will be used together with the chemical state plot to distinguish the chemical state of sulphur species formed on the mineral surfaces after different surface treatments. In this way, the surface reactivity of sulphide minerals and more in general, of sulphur –bearing particles will be determined
The present work is organized in six chapters: the first one presents a short overview on the environmental problems that may occur when sulphide minerals, especially, those bearing eco-toxic elements such as arsenic, are exposed to the acidic mine drainage in the presence of oxidants and/or of microorganisms which enhance oxidation kinetics. It then outlines the analytical strategy that was adopted in this work for the identification of the sulphur chemical state.
In the second chapter a literature survey is provided: it underlines the role of sulphur in sulphide bearing minerals: Section 2.1 reports information about the mineral enargite and the chemical state of sulphur on its surface as it is available to date. Section 2.2 deals with the literature on the possibility of using the KLL Auger transition. In chapter 3 a brief description of X-ray photoelectron and X-ray excited Auger electron spectroscopy is given while the materials and methods used in the present work are presented in chapter 4.
In chapter 5 – results and discussion - the X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES) analyses on alkaline and transition metal sulphide and sulphates, polysulphides and sulphide minerals are presented. In particular the spectra recorded on the S2p and S KLL lines of the sulphides and sulphates are shown: they were collected for getting the peak fitting parameters and apply them to the model systems obtained by mixing sulphur with sodium tetrasulphide and to investigate the composition of mineral surfaces. Minerals were analysed freshly cleaved, ground and after air exposure. The chemical state of sulphur in sulphates, sulphides (alkali- and transition metal compounds, minerals) and polysulphide is then discussed. For the first time the X-ray excited Auger SKLL lines were fitted with parameters based on standards. In this way, which can be considered as an extension of previous work, a separation of the different components and a more precise determination of their kinetic energy were achieved. The Auger parameter and the chemical state plot were obtained by combining the S2p photoelectron lines and the SKLL Auger lines. On this basis a clear distinction of the chemical state of sulphur in sulphates, in alkali sulphides and in transition metal sulphides was possible. The chemical state of sulphur in the bulk of minerals such as enargite or pyrite was determined. The chemical state of sulphur that formed on the surface of compounds after exposure to air or after grinding was found to vary according the coordinating metal and its concentration.
It can be assessed that the analytical approach developed in the presented thesis and based on the curve – fitting procedure applied to the X-ray induced Auger electron spectroscopy appears to be a very promising method for identifying the chemical state of sulphur and for providing a new insight in the surface chemistry of sulphide minerals
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
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