Oxidation Of Sulfur Species Research Articles

SIMS studies of oxidation mechanisms and polysulfide formation in reacted sulfide surfaces

The surface oxidation of metal sulfides in air and aqueous solution is of central importance in mineral separation and environmental control of acid mine drainage. Mechanisms of oxidation, dissolution and surface restructuring have been extensively studied using XPS. High binding energy components in S 2p XPS spectra have been attributed to metal-deficiency, formation of polysulfide S n 2−, elemental sulfur and electronic defect structures (ie Cu(I)/ZnS). The assignment of these components in S 2p XPS spectra has, however, left significant uncertainties particularly in the formation of SS bonding in polysulfide species requiring confirmation from other surface analytical techniques. The use of static ToF-SIMS has provided a new avenue for identification of these species and their development in oxidation of the sulfide surfaces. For the iron sulfides, there is a systematic change in the FeS 2/FeS fragment ratio from troilite (FeS) through pyrrhotite (Fe 1−xS) to pyrite (FeS 2) with ratios varying from 0.59, 1.2 to 32 respectively. Similarly, high ratios for FeS n/FeS are found for pyrite compared with pyrrhotite and troilite mirrored in the S n/S fragment ratios. Changes in surface oxidation, represented in atomic concentrations and S 2p XPS spectra, are seen in the ToF-SIMS signals for S n/SO n ratios in the same iron sulfide sequence. These mass markers, reflecting increased SS bonding, increase in surfaces after oxidation giving further confidence in XPS assignment to polysulfide species. Freshly cleaved galena PbS surfaces reacted in pH8 aqueous solution for increasing periods of time have also shown a systematic increase in S n/S ratios with increasing at.% of oxidised S n 2− species from XPS spectra. Statistical analysis of oxidised galena has shown that the ratios 206PbO +/ 206Pb + and 208PbOH +/ 208Pb + directly reflect the degree of oxidation of the surface lead species whilst the O −/S −, S −/total — ion yield and SO 3 −/S − are the best measures for following the oxidation of sulfur species. Results from these ratios suggest that initial air oxidation takes place predominantly on the S sites rather than Pb sites but, in solution at pH9, both sites are oxidised. The ToF-SIMS results appear to directly reflect the surface chemistry of the metal and sulfur species and are not consistent with recombination or fragmentation of secondary neutral or ionic species. The results strongly suggest increasing polymerisation of SS species with increasing oxidation in accord with the XPS assignment to polysulfide of increasing chain length.

Evidence of atmospheric sulphur in the martian regolith from sulphur isotopes in meteorites.

Sulphur is abundant at the martian surface, yet its origin and evolution over time remain poorly constrained. This sulphur is likely to have originated in atmospheric chemical reactions, and so should provide records of the evolution of the martian atmosphere, the cycling of sulphur between the atmosphere and crust, and the mobility of sulphur in the martian regolith. Moreover, the atmospheric deposition of oxidized sulphur species could establish chemical potential gradients in the martian near-surface environment, and so provide a potential energy source for chemolithoautotrophic organisms. Here we present measurements of sulphur isotopes in oxidized and reduced phases from the SNC meteorites--the group of related achondrite meteorites believed to have originated on Mars--together with the results of laboratory photolysis studies of two important martian atmospheric sulphur species (SO2 and H2S). The photolysis experiments can account for the observed sulphur-isotope compositions in the SNC meteorites, and so identify a mechanism for producing large abiogenic 34S fractionations in the surface sulphur reservoirs. We conclude that the sulphur data from the SNC meteorites reflects deposition of oxidized sulphur species produced by atmospheric chemical reactions, followed by incorporation, reaction and mobilization of the sulphur within the regolith.

Reduction of surface sulphur upon microbial devulcanization of rubber materials

Thiobacillus ferrooxidans, T. thioparus, Acidianus brierleyi and TH2 Lund, an archaeal isolate, were tested for sulphur oxidising capacity on ground rubber from spent car tyres. T. ferrooxidans converted 8 % (w/w) of the sulphur in cryoground spent rubber to sulphate in 20 days. Desulphurization of fresh made natural rubber was studied with scanning electron microscope with energy-dispersive x-ray spectrometer and electron spectroscopy for chemical analysis. The average depletion of sulphur on the treated surface was 24 % compared to the bulk region. Oxidised sulphur species were detected on the surface after microbial treatment. © Rapid Science Ltd. 1998

Surface Modification of Electroactive Polymer Films by Ozone Treatment

Surface modification of polypyrrole (PPY), polyaniline (PAN) and poly(3-alkylthiophene) films by ozone was studied by angle-resolved x-ray photoelectron spectroscopy (XPS). In the case of the N-containing polymers, oxidation occurs readily and mainly at the carbon atoms, resulting in the formation of C-O, C=O, COOH and even O-COOH (especially for PAN) species. The carbon atoms of doped PPY and PAN films are significantly more resistant to ozone oxidation, but the samples suffer some loss of the dopant in the surface region. Ozone treatment does not involve the direct oxidation of the nitrogen heteroatoms to form the - NO x species, even at high extent of carbon oxidation. In addition, a substantial decrease in the intrinsic oxidation state ([=N-]/[-NH-] ratio) was observed in the 25% deprotonated PPY (DP-PPY) base, the 50% intrinsically oxidized emeraldine (EM) base and the 75% intrinsically oxidized nigraniline (NA) base after ozone treatment and subsequent atmospheric exposure. In the case of the S-containing polymers, ozone treatment results predominantly in the oxidation of sulphur heteroatoms to give rise to the sulphone and peroxide species. The oxidized sulphur species, however, are readily reduced or consumed in a photochemical reaction.

Surface Modification of Electroactive Polymer Films by Ozone Treatment

Surface modification of polypyrrole (PPY), polyaniline (PAN) and poly(3-alkylthiophene) films by ozone was studied by angle-resolved x-ray photoelectron spectroscopy (XPS). In the case of the N-containing polymers, oxidation occurs readily and mainly at the carbon atoms, resulting in the formation of CνO, CξO, COOH and even OνCOOH (especially for PAN) species. The carbon atoms of doped PPY and PAN films are significantly more resistant to ozone oxidation, but the samples suffer some loss of the dopant in the surface region. Ozone treatment does not involve the direct oxidation of the nitrogen heteroatoms to form theνNOx species, even at high extent of carbon oxidation. In addition, a substantial decrease in the intrinsic oxidation state ([ξNν]/[νNHν] ratio) was observed in the 25% deprotonated PPY (DP-PPY) base, the 50% intrinsically oxidized emeraldine (EM) base and the 75% intrinsically oxidized nigraniline (NA) base after ozone treatment and subsequent atmospheric exposure. In the case of the S-containing polymers, ozone treatment results predominantly in the oxidation of sulphur heteroatoms to give rise to the sulphone and peroxide species. The oxidized sulphur species, however, are readily reduced or consumed in a photochemical reaction.

The age and origin of the barite-fluorite (PbZn) veins of the Sierra del Guadarrama (Spanish Central System, Spain): a radiogenic (Nd, Sr) and stable isotope study

Fluorite and barite-rich veins (+ sulphides) of the Sierra del Guadarrama (Spanish Central System) are usually hosted by Hercynian granitoids. Zoned fluorites from one of these veins have a SmNd age of 145 ± 18 Ma. This age is similar to the KAr ages obtained in other hydrothermal rocks of the Spanish Central System (156–152 Ma) and confirms the existence of an important hydrothermal event during the Late Jurassic, probably related to extensional fracturing accompanying the opening of the North Atlantic Ocean. The 87 Sr 86 Sr ratio found in fluorite and barite (0.7154–0.7207) is in most cases lower than that found in the host granitoids (0.7194 and 0.7209), suggesting more than one source of strontium. This, in turn, supports previous models that involve mixing of two fluids during mineralization, an ascending hot (> 300°C) and low-salinity fluid and a shallow, cooler (< 100°C) and more saline brine. The ascending fluid probably reacted to a variable extent with the host granitoids, and the most likely source for the shallow fluid derived from a marine/evaporitic environment, with a low 87 Sr 86 Sr ratio (∼0.7069 in the Late Jurassic). The calculated ϵ Nd composition of the fluorites (−8.8 to −7.6) as well as the 147 Sm 144 Nd variation found in this mineral (0.153–0.405) do not conflict with the mixing model although they can be largely explained by interaction of the ascending fluid with the host granitoids. Sulphur isotopic compositions of sulphides range between −3.4 and −0.8%. and those of barites from +15.5 to +15.8‰. The near 0‰ δ 34S-value of the sulphides suggests that their sulphur was derived from the hydrothermal leaching of the granitoids by the ascending fluid. The δ 34S-value of barite is consistent with Upper Jurassic marine sulphate as the source of sulphate in the shallow fluid. The absence of equilibrium between reduced and oxidized sulphur species can be easily interpreted as a consequence of the short residence time of the mixed fluid in the open fractures at temperatures between 250° and 100°C. Although no pre-Cretaceous sediments are found on this part of the Iberian Hercynian basement, the data presented here support the existence of a shallow platform joining central Spain with the Iberian or the Subbetic realm in Late Jurassic times.

XPS study of the oxidation of cuprous sulphide in aerated aqueous solutions

The oxidation of cuprous sulphide (chalcocite) in aerated water and at various pH values at room temperature has been studied by X-ray photoelectron spectroscopy (XPS). In acidic solution, an almost monolayer coverage of adsorbed oxygen, water and hydroxide groups was found. In neutral and basic solutions, the surface consists mainly of copper (I) and copper (II) hydroxides and of some copper (II) carbonate. The amount of these products increases with the pH of the solution. No oxidized sulphur species were observed in the surface layer.

The behaviour of sulphur in the oxidative leaching of sulphidic minerals

The acidic oxidative leaching of (mono) -sulphidic minerals has been investigated, with a paticular view to identifying the chemical reactions leading to sulphur and sulphate ions. A reaction scheme, based on the “Wackenroder” reaction, is proposed to account quantitatively for the observed yields of elemental sulphur and sulphate. The first step in this mechanism is the oxidation of sulphidic S 2− to either S 2+ or S +, depending on the nature of the oxidation agent used. These oxidation reactions are followed by hydrolysis and decomposition of the H 2S 2O 2 and H 2SO 2 formed, according to the overall reactions 2H 2S 2O 2→3S+H 2SO 3+H 2O and 2H 2SO 2→S+H 2SO 3+H 2O leading ultimately to 75 and 50% elemental sulphur yields, respectively. Higher sulphur yields can be achieved at pH levels below a certain value, which is specific for the particular mineral, by direct formation of H 2S, which in turn reacts with oxidized sulphur species, for example according to: H 2S+H 2S 2O 2→S+2H 2O