abstract: A series of pyrite thin films were synthesized using a novel sequential evaporation technique to study the effects of substrate temperature on deposition rate and micro-structure of the deposited material. Pyrite was deposited in a monolayer-by-monolayer fashion using sequential evaporation of Fe under high vacuum, followed by sulfidation at high S pressures (typically > 1 mTorr to 1 Torr). Thin films were synthesized using two different growth processes; a one-step process in which a constant growth temperature is maintained throughout growth, and a three-step process in which an initial low temperature seed layer is deposited, followed by a high temperature layer, and then finished with a low temperature capping layer. Analysis methods to analyze the properties of the films included Glancing Angle X-Ray Diffraction (GAXRD), Rutherford Back-scattering Spectroscopy (RBS), Transmission Electron Microscopy (TEM), Secondary Ion Mass Spectroscopy (SIMS), 2-point IV measurements, and Hall effect measurements. Our results show that crystallinity of the pyrite thin film improves and grain size increases with increasing substrate temperature. The sticking coefficient of Fe was found to increase with increasing growth temperature, indicating that the Fe incorporation into the growing film is a thermally activated process. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2014

Arsenic is ubiquitous whether it is naturally occurring or produced by humans. It is found at sites on the National Priority List and at sites operated by DOE, where it is the second most commonly found contaminant. More wastes containing arsenic will be produced due to the lowering of the Maximum Contaminant Level (MCL) for arsenic in drinking water which will result in more treatment facilities for arsenic removal that will generate residuals. Furthermore, arsenic can be released from such wastes under the reduced conditions that are found in landfills. Pyrite (FeS2) is believed to be a compound that has a high affinity for arsenic and is stable under anoxic conditions. The first task of this research was to develop a method for making pyrite crystals of defined size with minimal reaction time and at high yield. Effects on the synthesis of pyrite particles of pH, the ratio of Fe/S, temperature and reaction time were investigated in batch reactor systems. Pyrite was synthesized within 24 hours at pH values ranging from pH 3.6 through pH 5.6, and at a ratio of Fe/S of 0.5. X-ray diffraction and scanning electron microscopy were used to size and characterize the pyrite particles. Experimental and analytical procedures developed for this work, included a hydride generation atomic absorption spectrometry method for measuring arsenic species (As(III), As(V)). The synthesized pyrite was applied to remove arsenic and its maximum capacity for arsenic removal was measured in batch adsorption experiments to be 3.23 μmol/g for As(III) and 113 μmol/g for As(V). Information obtained on the characteristics of chemical species before and after the reaction with arsenic showed that iron and sulfur were oxidized. Last, how strongly arsenic was bound to pyrite was investigated and it was determined that release of arsenic from As(III)-pyrite is not affected by pH, but release from As(V)-pyrite is affected by pH with minimum release in the range pH 5 to pH 8.

Reproduction de : Thèse de doctorat : Chimie inorganique : Reims : 2005. / Titre provenant de l'écran titre. Bibliogr. f. 213-228.

L'impacte des changements environnementaux marins (comme les conditions redox) sur l'explosion radiative du Cambrien inférieur des animaux à squelettes est une des questions majeures posées de longue date.Une étude intégrée sur l'enregistrement paléontologique, géochimique et des pyrites framboïdes, a été menée sur les Formations de Shuijingtuo et d'Hetang, qui affleurent respectivement dans les provinces de Hubei et de Zheijiang. Des fossiles de métazoaires et de petits coquillages fossiles (SSF) abondants et bien préservés sont ici rapportés au sein des roches sédimentaires noires de la Formation de Shuijingtuo. La présence de SSF, de microfossiles siliceux et d'acritarches est aussi documentée au sein des argilites et des jaspes noires de la Fm d'Hetang. Des proxys géochimiques indiquent que dans le bassin protégé de la région des Trois Gorges une diminution périodique de l'anoxie a eu lieu durant le Qiongzhusien, en relation avec des changements du niveau marin et d'upwelling. Des fluctuations fréquentes de l'oxygénation ont été détectées en utilisant appliquant des analyses en ICP-MS sur des lamines alternantes d'argilites noires et de carbonates lesquelles sont présentes sur la partie supérieure de la Fm de Shuijingtuo. L'augmentation de l'oxygénation de la plateforme de Yangtze a été également reconnue ici à travers l'étude de la Fm. Hetang et elle peut être corrélée avec des formations équivalentes qui affleurent dans d'autres endroits de Chine méridionale. Des différences bien distinctes de l'état d'oxygénation entre le Qiongzhusien inférieur et supérieur documentent la récupération progressive des environnements marins après les événements anoxiques du Cambrien inférieur, ainsi que l'étendu des masses d'eau oxygénées. Les indicateurs géochimiques de productivité et les données sur le COT indiquent que les océans du Cambrien inférieur ont du connaître une période longue de haute productivité. La bonne corrélation entre les indicateurs de productivité primaire et d'oxygénation des eaux du fond marin indiquent que la préservation de la matière organique a du être contrôlée principalement par le niveau des conditions oxygénée des masses d'eau du fond marin. / The impact of marine environmental changes (i.e. redox conditions) on the early Cambrian explosive radiation of skeletonized animals is a long-standing question. An integrated study of the paleontological, geochemical and framboidal pyrite record was carried out in the Shuijingtuo and Hetang Formations, which crop our respectively in the Hubei and Zhejiang Provinces. Abundant and well-preserved metazoan fossils and small shelly fossils (SFFs) are here reported from the black sedimentary rocks of the Shuijingtuo Fm. The occurrence of SSFs, siliceous microfossils and acritarchs is also documented from the black shales and cherts of the Hetang Fm. Geochemical proxies indicate that in the protected basin of the Three Gorges area periodical weakening of anoxia took place during the early to late Qiongzhusian, in relation with changes in sea level and upwelling. Frequent fluctuations in oxygenation were also detected by using high-resolution ICP-MS and geochemical indicators on black shale-carbonate laminations that occur in the upper part of the Shuijingtuo Fm. Increase in oxygenation of the Yangtze platform was also recognized here throughout the Hetang Fm. and it can be correlated with time equivalent formations that occur in other areas of South China. Distinct differences in the state of oxygenation between the early and late Qiongzhusian document the progressive recovery of marine environments after the lower Cambrian anoxic events, as well as the extent of oxygenated water masses. The geochemical proxies of productivity and TOC data indicate that the ocean experienced during the early Cambrian a long lasting interval of high primary productivity. The relatively highly correlated between primary productivity indicators and the bottom water condition implies that the preservation of organic carbon was mainly controlled by the level of oxygenated conditions of the bottom water.

The giant Detour Lake deposit is a Neoarchean orogenic Au ore body located in the northwestern Abitibi district within the Superior Province. The deposit is situated along the high strain Sunday Lake Deformation Zone (SLDZ) parallel to the broadly E-W trending Abitibi greenstone belt. The lower amphibolite facies assemblage (Act-Bt-Pl-Ep-Alm ±Cal ±Qz ±Ilm) suggests maximum temperatures reaching 550°C, exceeding conditions for pyrite plasticity, an important and ubiquitous Au-bearing phase that may ultimately represent the source for Au at orogenic style Au deposits. The metamorphic assemblage also obscures the relationship between Au and biotite, a visual indicator mineral within Au-rich ore zones. This work combines microstructural, geochemical and geochronological analyses to assess the influence of regional scale deformation and alteration on Au mineralization. EBSD and LA-ICP-MS analyses on pyrite reveal Au enrichment at microstructures supporting a syn- to post-peak metamorphic and deformation-assisted Au upgrading model. EMPA and 40Ar/39Ar analyses on biotite reveal one chemically homogeneous population, which yield variably reset ages that post-date regional metamorphism and Au mineralization.

Removing pyrite from coal and pyrrhotite from pentlandite play a critical role in coal and nickel production, respectively, to meet the stringent restriction on SO2 emission. The present project investigates first the mechanism of xanthate adsorption on gold using Atomic Force Microscope (AFM), then the depression of pyrite and pyrrhotite using the synthetic polymers developed by Cytec Industries. The results show that for xanthate/gold system, dixanthogen is the only species that renders the surface hydrophobic. Chemisorbed xanthate is observed on the gold surface but is hydrophilic. The synthetic polymers may adsorb on pyrite and pyrrhotite possibly through the hydrophobic interaction between the hydrophobic moiety of the polymer and the mineral surface that has been hydrophobized by collector adsorption. The hydrophilic moieties of the polymer are exposed to the aqueous phase and render the surface hydrophilic. / Master of Science

abstract: Pyrite is a 0.95 eV bandgap semiconductor which is purported to have great potential in widespread, low–cost photovoltaic cells. A thorough material selection process was used in the design of a pyrite sequential vapor deposition chamber aimed at reducing and possibly eliminating contamination during thin film growth. The design process focused on identifying materials that do not produce volatile components when exposed to high temperatures and high sulfur pressures. Once the materials were identified and design was completed, the ultra–high vacuum growth system was constructed and tested. Pyrite thin films were deposited using the upgraded sequential vapor deposition chamber by varying the substrate temperature from 250°C to 420°C during deposition, keeping sulfur pressure constant at 1 Torr. Secondary Ion Mass Spectrometry (SIMS) results showed that all contaminants in the films were reduced in concentration by orders of magnitude from those grown with the previous system. Characterization techniques of Rutherford Back–scattering Spectrometry (RBS), X–Ray Diffraction (XRD), Raman Spectroscopy, Optical Profilometry and UV/Vis/Near–IR Spectroscopy were performed on the deposited thin films. The results indicate that stoichiometric ratio of S:Fe, structural–quality (epitaxy), optical roughness and percentage of pyrite in the deposited thin films improve with increase in deposition temperature. A Tauc plot of the optical measurements indicates that the pyrite thin films have a bandgap of 0.94 eV. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2016

South Africa mines coal to supply in the growing energy demands of the country. A majority of these mines are opencast resulting in back filled pits and above ground disposal facilities. Leachate emanating from these disposal sites are saline and in most cases highly acidic. Currently the standard testing procedure to quantify expected leachate qualities include Acid Base Accounting (ABA), Net-acid Generating test (NAG), static-and kinetic leaching. The aim of this study is to model standard humidity cell leach tests performed using the PHREEQC code. This model can then be scaled up to field conditions to model 1D reactive transport. It is commonly accepted that the rate of pyrite oxidation in backfilled pits and waste storage facilities is governed by the rate of oxygen ingress and that no pyrite oxidation take place in the saturated zone. This is not the case for humidity cells, as sufficient oxygen is available for reaction. Pyrite reactions rates in humidity cells is expected to be governed by a combination of available reaction surface and ash layer resistance. This is modelled in PHREEQC (Parkhurst & Appelo, 2003) using the KINETIC block. Leachate composition is then modelled in the column by making use of the TRANSPORT block. The experimental data is fitted by using the reactive surface and ash layer diffusion coefficient as a fitting parameter. PHREEQC does not have a gas transport module to model oxygen diffusion through the column. Due to this shortfall of PHREEQC, the influence of oxygen ingress in the system can not be directly modelled under kinetic conditions. Davis and Ritchie (1986a) proposed an anlylitical solution in which the integrated sulphate production rate can be calculated for a waste heap dump. This rate takes into account the influence of oxygen ingress and the development of an ash layer resistance to the pyrite oxydation rate. This intergrated rate can then be defined in a RATES block in PHREEQC. The Aproximate Analytical Solution (AAS) model proposed by Davis and Ritchie (1986a) is used to scale up the model used for the humidity leach cell experiment. It was found from the modelling results and comparison with PYROX that the model under predicts the integrated sulphate production rate in the initial stages of the reacting waste heap dump. It does however show results that are in close agreement with the results obtained from PYROX in later stages of the lifespan of the waste heap dump. This highlight limitations to the AAS model’s applicability on geochemical problems. The model can only be applied to describe waste heap dumps where the particles at the top of the heap are fully oxidized. / MSc (Environmental Sciences), North-West University, Potchefstroom Campus, 2015

South Africa mines coal to supply in the growing energy demands of the country. A majority of these mines are opencast resulting in back filled pits and above ground disposal facilities. Leachate emanating from these disposal sites are saline and in most cases highly acidic. Currently the standard testing procedure to quantify expected leachate qualities include Acid Base Accounting (ABA), Net-acid Generating test (NAG), static-and kinetic leaching. The aim of this study is to model standard humidity cell leach tests performed using the PHREEQC code. This model can then be scaled up to field conditions to model 1D reactive transport. It is commonly accepted that the rate of pyrite oxidation in backfilled pits and waste storage facilities is governed by the rate of oxygen ingress and that no pyrite oxidation take place in the saturated zone. This is not the case for humidity cells, as sufficient oxygen is available for reaction. Pyrite reactions rates in humidity cells is expected to be governed by a combination of available reaction surface and ash layer resistance. This is modelled in PHREEQC (Parkhurst & Appelo, 2003) using the KINETIC block. Leachate composition is then modelled in the column by making use of the TRANSPORT block. The experimental data is fitted by using the reactive surface and ash layer diffusion coefficient as a fitting parameter. PHREEQC does not have a gas transport module to model oxygen diffusion through the column. Due to this shortfall of PHREEQC, the influence of oxygen ingress in the system can not be directly modelled under kinetic conditions. Davis and Ritchie (1986a) proposed an anlylitical solution in which the integrated sulphate production rate can be calculated for a waste heap dump. This rate takes into account the influence of oxygen ingress and the development of an ash layer resistance to the pyrite oxydation rate. This intergrated rate can then be defined in a RATES block in PHREEQC. The Aproximate Analytical Solution (AAS) model proposed by Davis and Ritchie (1986a) is used to scale up the model used for the humidity leach cell experiment. It was found from the modelling results and comparison with PYROX that the model under predicts the integrated sulphate production rate in the initial stages of the reacting waste heap dump. It does however show results that are in close agreement with the results obtained from PYROX in later stages of the lifespan of the waste heap dump. This highlight limitations to the AAS model’s applicability on geochemical problems. The model can only be applied to describe waste heap dumps where the particles at the top of the heap are fully oxidized. / MSc (Environmental Sciences), North-West University, Potchefstroom Campus, 2015

Initial published work suggested that Fe isotope fractionations recorded in sediments were a product of biological activity. Experiments and measurements of natural samples now indicate that Fe isotope fractionation can be the product of both biological and inorganic processes. Sedimentary iron sulphides provide unique information about the evolution of early life which developed under anoxic conditions. It is in these sedimentary Fe-S species and in particular in Archean and Proterozoic pyrites that the largest Fe isotope variations (up to a range of ~5‰ for δ56/54Fe) have been measured. Most research has focussed on potential processes responsible for the formation of a 56Fe depleted Fe(II) pool from which iron sulphides would precipitate without additional fractionation, recording the light Fe isotope composition of the pool. Much less attention has been given to the possibility that the iron sulphide forming mechanisms themselves could produce significant fractionations. The Fe-S system constitutes a diverse group of stable and metastable phases, the ultimate Fe sequestrating phase being pyrite. The aim of this study was to examine experimentally where Fe isotope fractionations occur during the abiotic formation of iron sulphides in order to assess whether or not the measured Fe isotope signatures in natural pyrite could be explained by chemical mechanisms only. Both analytical and experimental protocols were developed in order to determine the partition of Fe isotopes for each step towards diagenetic pyrite formation. 56/54Fe and 57/54Fe ratios were measured on an IsoProbe-P Micromass MC-ICP-MS, and all experiments were performed under oxygen-free N2 atmosphere. Supporting previously published data, the results indicate that the precipitation of the nanoparticulate iron(II) monosulphide mackinawite (FeSm) kinetically fractionates lighter isotopes with initial fractionations of Δ56FeFe(II)aq-FeS = 1.17 ± 0.16 ‰ at 25°C and Δ56FeFe(II)aq-FeS = 0.98 ± 0.16 ‰ at 2°C. The rate of isotopic exchange between Fe(II)aq and FeSm decreases as FeSm nanoparticles grow. Fe isotope exchange kinetics are consistent with i) FeSm nanoparticles that have a core–shell structure, in which case Fe isotope mobility is restricted to exchange between the surface shell and the solution and ii) a nanoparticle growth via an aggregation– growth mechanism. Because of the structure of FeSm nanoparticles, the approach to isotopic equilibrium is kinetically restricted at low temperatures. The equilibrium Fe isotope fractionation between Fe2+ aq and FeSm was determined using the three isotope method and is Δ56FeFe(II)-FeS = -0.33 ± 0.12 ‰ at 25°C and Δ56FeFe(II)-FeS = -0.52 ± 0.16 ‰ at 2°C. This suggests that at equilibrium, FeSm incorporates heavier isotopes with respect to Fe2+ aq, and the isotopic composition of most naturally occurring FeSm does not represent equilibrium. During pyrite formation, pyrite incorporates kinetically lighter isotopes with a fractionation Δ56FeFeS-pyrite ~ 2.2 ‰. Because pyrite is sparingly soluble in sedimentary environments, isotope exchange is prevented and pyrite does not equilibrate with its Fe(II) source. Combined fractionation factors between Fe2+ aq, mackinawite (FeSm) and pyrite permit the generation of pyrite with Fe isotope signatures that encapsulate the full range of sedimentary δ56Fepyrite recorded in both Archean and modern sediments. Archean Fe isotope excursions reflect various degrees of pyritisation, extent of Fe(II)aq utilisation, and variations in source composition rather than microbial dissimilatory Fe(III) reduction only. Our results show that sedimentary pyrite is not a passive recorder of the Fe isotope composition of the reactive Fe(II) reservoir forming pyrite. It is the formation process itself that influences pyrite Fe isotope signatures with consequent implications for the interpretation of sedimentary pyrite Fe isotope compositions throughout geological time.