Yu. L. Mikhlin

Spectroscopic and XRD studies of the air degradation of acid-reacted pyrrhotites

Abstract Monoclinic and hexagonal pyrrhotites leached in 1 mol/L HCl and exposed to the air at 100% and ∼10% relative humidity for up to 5 months were studied using X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy, Mossbauer spectroscopy, and electron paramagnetic resonance (EPR). The amorphous, nonequilibrium, iron-depleted layer (NL) produced by the leaching amounted to half of the residue mass and was composed of predominantly low-spin ferrous iron and polysulfide anions. Elemental sulfur and goethite were the only crystalline products of the NL decomposition. FTIR spectroscopy and XPS also revealed several sulfoxy species and, at low humidity, a small amount of ferric oxide. Neither alterations of the underlying pyrrhotite nor new iron sulfide phases (pyrite, pyrrhotite, etc.) crystallized from the amorphous NL were found. The NL decomposition was faster in the wet environment than in the dry one, and the oxidation of the NL was much more rapid than that of starting pyrrhotites. The intensity and quadruple split of the Mossbauer signal from the product (an isomer shift of 0.36 mm/s) were found to increase over the aging, indicating that the NL structure becomes more rigid and the singlet Fe(II) gradually converts to Fe(III). X-ray Fe Lα,β emission spectra showed the formation of intermediate, high-spin Fe(II) within the NL oxidized in the humid environment, but not in the dry air. No unpaired electron spins were detected by EPR; lines of paramagnetic Fe3+ appeared after the samples were aged in the dry air for 49 d and even later in the humid atmosphere. These phenomena are explained in terms of the formation of defects with negative correlation energy, similar to noncrystalline semiconductor chalcogenides, and of the fast electron exchange between the iron species, respectively. Mechanisms for reactions involved with the weathering of iron sulfides, which take into consideration the NL lattice elasticity, S-S and S-O bonding, oxygen incorporation, and oxidative and spin state of iron, are discussed. It is suggested in particular that the surface layer, strongly enriched in sulfur, as well as elemental sulfur and ferric oxyhydroxides, do not inhibit sulfide oxidation and acid production under weathering conditions, but the partially oxidized, disordered, nonstoichiometric layer may be passive.

Electronic structure of the non-equilibrium iron-deficient layer of hexagonal pyrrhotite

Abstract Natural hexagonal pyrrhotite before and after 1M HCl leaching has been studied, and X-ray and UV photoelectron, X-ray emission S K α, S K β, S L 2,3 , Fe L α and S K absorption spectra have been recorded and analyzed in conjunction with earlier reported Mossbauer spectroscopy data and available SCF-X α scattered wave MO calculations of the octahedral cluster FeS 6 10− and the molecular ion S 2 2− . The formation of a thick disordered non-equilibrium iron-deficient layer at the pyrrhotite surface after chemical treatment has been confirmed. Alterations of the X-ray spectra from the mineral as a result of acid etching indicated the sulphur-sulphur bonds and agreed well with the previous conclusion about the conversion of a ferrous ion from the quintet into the singlet state. The electronic structure of the metal-deficient layer has been described using a combination of structures of the S S bond containing group like S 2 2− , of the low-spin singlet cluster FeS 6 10− , and of a system of localized states.

Conditions for the Formation of Copper Nanoparticles by Reduction of Copper(II) Ions with Hydrazine Hydrate Solutions

Optimal conditions were found for the preparation of copper nanoparticles in aqueous solution via reduction of copper(II) ions with hydrazine hydrate. The effects of ligand environment of copper(II) in the initial solution (hydrate, ammonia, citrate, and glycine complexes), concentration, pH, surfactants, temperature, and mode of heating were examined. The obtained colloidal systems were studied by optical spectroscopy, X-ray photoelectron spectroscopy, X-ray powder diffraction, and atomic force microscopy. The examined colloids were found to contain generally spherical copper nanoparticles with a diameter of about 10 nm, which were coated with a copper(I) or copper(II) oxide and hydroxide film.

Pyrrhotite Electrooxidation in Acid Solutions

The electrooxidation of pyrrhotite, including that preliminarily etched in acid solutions, is studied by voltammetry, scanning electron microscopy, and X-ray electron spectroscopy. The oxidation mechanism includes the formation; “reversible” oxidation (which predominantly involves an increase in the amount of polysulfide ions) of a near-surface nonequilibrium metal-deficient layer (NL); and the layer destruction. The pyrrhotite passivation is caused not by a thick NL, but rather by a thin layer that contains oxidized iron, probably by a top, oxygen-containing zone of NL.

Conditions for the Formation of a Nonequilibrium Nonstoichiometric Layer on Pyrrhotite in Acid Solutions

Conditions for the formation of a nonequilibrium nonstoichiometric metal-deficient layer (NL) on natural pyrrhotites are studied, together with the NL electroreduction and the role NL plays in a nonoxidizing dissolution of pyrrhotite in solutions of sulfuric and hydrochloric acids. To estimate the NL weight, the charge connected with a cathodic peak at about –0.2 V (Ag/AgCl) is used. The peak reflects the irreversible reduction of NL with the formation of hydrogen sulfide, which is confirmed by SEM and XES data. Bulky NL forms in 0.5 M H2SO4at –0.1 to 0.0 V, where the nonoxidizing dissolution rate sharply alters, and at 0.5–1.1 V, where the pyrrhotite oxidation rate is high. The NL growth is controlled by solid-state diffusion, whereas nonoxidizing dissolution of iron is limited by diffusion at 20–30°C and a kinetic stage of dissolution of sulfur at higher temperatures.

Nanoparticles of noble metals in the supergene zone

Formation of noble metal nanoparticles is related to various geological processes in the supergene zone. Dispersed mineral phases appear during weathering of rocks with active participation of microorganisms, formation of soil, in aqueous medium and atmosphere. Invisible gold and other noble metals are incorporated into oxides, hydroxides, and sulfides, as well as in dispersed organic and inorganic carbonic matter. Sulfide minerals that occur in bedrocks and ores unaltered by exogenic processes and in cementation zone are among the main concentrators of noble metal nanoparticles.The ability of gold particles to disaggregate is well-known and creates problems in technological and analytical practice. When Au and PGE nanoparticles and clusters occur, these problems are augmented because of their unusual reactions and physicochemical properties. The studied gold, magnetite, titanomagnetite and pyrite microspherules from cementation zone and clay minerals of laterites in Republic of Guinea widen the knowledge of their abundance and inferred formation conditions, in particular, in the contemporary supergene zone. Morphology and composition of micrometer-sized Au mineral spherules were studied with SEM and laser microprobe. The newly formed segregations of secondary gold on the surface of its residual grains were also an object of investigation. The character of such overgrowths is the most indicative for nanoparticles. The newly formed Au particles provide evidence for redistribution of ultradispersed gold during weathering. There are serious prerequisites to state that microorganisms substantially control unusual nano-sized microspherical morphology of gold particles in the supergene zone. This is supported by experiments indicating active absorption of gold by microorganisms and direct evidence for participation of Ralstonia metallidurans bacteria in the formation of peculiar corroded bacteriomorphic surface of gold grains. In addition, the areas enriched in carbon and nitrogen have been detected with SEM on the surface of gold spherules from Guinea. Such organic compounds as serine, alanine, and glycine are identified on their surface with Raman spectroscopy. The experiments have been carried out and new data have been obtained indicating the role of micromycetes in concentration and distribution of noble metals in ferromanganese nodules of the World Ocean. Au and Pt were detected in the system with radioisotopes. It has been established that two forms of gold distribution develop within pseudomorphs of fungi colonies: (1) as pseudomorphic concentrates and (2) dispersed form unrelated to the colony structure. Inhomogeneities in distribution of dispersed platinum are manifested in the form of linear anomalies with elevated concentrations at the margins of the colonies.

Disordered surface layers of metal sulfides and their reactivity

The surface of metal sulfides during reactions of oxidation and corrosion in aqueous solutions is investigated using X-ray photoelectron spectroscopy, X-ray spectroscopy, Mössbauer spectroscopy, scanning electron microscopy, and scanning probe microscopy. Primary attention is focused on the state of the surface layer of the sulfide phase. It is demonstrated that the composition of this layer is changed in such a way that there arises a metal deficiency (as a rule, rather substantial) and the structure becomes disordered. Experimental evidence that the surface layer can be considered a chalcogenide glassy semiconductors is discussed. The role of the localization of electronic states and specific defects in the reactivity of sulfides, particularly in oxidation and acid dissolution, is analyzed.

Structural properties and state of the surface layer of zirconium dioxide modified by tungstate anions

A tetragonal metastable phase of zirconium dioxide formed after the addition of tungstate anions (>13 mol %) to the hydroxide precursor by different methods with heating (600–700°C), as revealed by X-ray diffraction analysis and X-ray photoelectron and IR spectroscopy. The W6+ and W5+ cations formed a solid solution with ZrO2. On the surface of the solid solution, the tungsten cations formed tungstate clusters (−WOx−)n. The formation of the WO3 phase was observed at concentrations of tungstate anions higher than 17.6 mol % or at temperatures of 850–870°C.

Influence of sulfide ions on the formation and properties of gold nanoparticles in aqueous solutions

The influence of sulfide ions on the formation of gold nanoparticles during reduction of HAuCl4 in an aqueous solution is studied using plasmon resonance spectroscopy and scanning tunneling microscopy. Upon the simultaneous addition of sodium citrate and sodium sulfide and for the molar ratios S/Au < 1, the nanoparticles have smaller sizes than those formed during the reduction only with sodium citrate under the same conditions; however, an increase in the sulfur content leads to an increase in the particle size. The gold nanoparticles are also obtained under microwave heating with a single-stage reaction of an AuCl4− aqueous solution with sodium sulfide. The particle size decreases with an increase in the S/Au ratio.

Investigation of gold nanoparticles immobilized on the surface of pyrite by scanning probe microscopy, scanning tunneling spectroscopy, and X-ray photoelectron spectroscopy

The characteristics of gold spontaneously deposited on the surface of pyrite from an HAuCl4 solution at room temperature are investigated by scanning probe microscopy, scanning tunneling spectroscopy, atomic-force microscopy, and X-ray photoelectron spectroscopy with synchrotron excitation. Within minutes after the onset of the deposition, gold is deposited in the form of metallic particles with a diameter ranging from 8 to 15 nm, which, in turn, subsequently form agglomerates with sizes up to several hundred nanometers. It is revealed that the Au 4f7/2 lines in the X-ray photoelectron spectra of the gold samples are shifted as compared to those for bulk gold and that tunneling in scanning tunneling spectra is suppressed. The effects caused, apparently, by the Coulomb blockage are unusually pronounced for such relatively large particles and decrease rather slowly upon the aggregation of particles.