Browsing by Subject "pyrite"
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Item Macroscopic and spectroscopic investigation of interactions of arsenic with synthesized pyrite(2009-05-15) Kim, Eun JungSulfide minerals have been suggested to play an important role in regulating dissolved metal concentrations in anoxic environments. Pyrite is the most common sulfide mineral and it has shown an affinity for arsenic, but little is known about the arsenic retention mechanisms of pyrite. In this study, interactions of arsenic with pyrite were investigated in an anoxic environment to understand geochemical cycling of arsenic better and to predict arsenic fate and transport in the environment better. A procedure using microwaves was studied to develop a fast and reliable method for synthesizing pyrite. Arsenic-pyrite interactions were investigated using macroscopic (solution phase experiments) and microscopic (X-ray photoelectron spectroscopic investigation) approaches. Pyrite was successfully synthesized within a few minutes via reaction of ferric iron and hydrogen sulfide under the influence of irradiation by a conventional microwave oven. The SEM-EDX study revealed that the nucleation and growth of pyrite occurred on the surface of elemental sulfur, where polysulfides are available. Compared to conventional heating, microwave energy results in rapid (< 1 minute) formation of smaller particulates of pyrite. Higher levels of microwave power can form pyrite even faster, but faster reaction can lead to the formation of pyrite with defects. Arsenic removal by pyrite was strongly dependent on pH and arsenic species. Both arsenite (As(III)) and arsenate (As(V)) had a strong affinity for the pyrite surface under acidic conditions, but As(III) was removed more effectively than As(V). Under acidic conditions, arsenic removal continued to occur almost linearly with time until complete removal was achieved. However, under neutral to alkaline conditions, fast removal was followed by slow removal and complete removal was not achieved in our experimental conditions. A BET isotherm equation provided the best fit to arsenic removal data, suggesting that surface precipitation occurred at high arsenic/pyrite ratio. The addition of competing ions did not substantially affect the ultimate distribution of arsenic between the pyrite surface and the solution, but changing pH affected arsenic stability on pyrite. X-ray photoelectron spectroscopy revealed that under acidic conditions, arsenic was removed and formed solid phases similar to As2S3 and As4S4 by reaction with pyrite. However, under neutral to alkaline conditions, arsenic was removed and formed As(III)-O and As(V)-O surface complexes, as well as As2S3/As4S4-like precipitates. As pH increases, the amount of arsenic that formed As2S3/As4S4-like precipitates decreased, while the amount that formed As(III)-O and As(V)-O surface complexes increased. Under alkaline conditions, a FeAsS-like phase was also detected.Item Mineralogy and Geochemistry of Pb, Zn and Ag Mine Tailings Originating From Carbonate-Rich Deposits(2012-11-29) McClure, Roberta 1981-Mining for silver, lead, zinc, and copper in Zimapan, Hidalgo State, Mexico has been ongoing since 1576. Unsecured tailings heaps and associated acid mine drainage have presented problems related to soil quality, water quality, and dust emission control in the Zimapan area. Objectives of the study of the mine tailings are (1) to determine mineralogy of the tailings in order to identify acid-producing minerals and heavy metals at risk for release in acidic conditions, and (2) to quantify carbonate minerals and (3) to determine heavy metal content that may be released by the products of sulfide mineral weathering. Representative mine tailings have been sampled from a site located north of Zimapan. Mineralogical characterization has been conducted with X-ray diffraction (XRD), and scanning and transmission electron microscopes (SEM and TEM). Total carbonates have been determined the Chittick procedure. X-Ray Fluorescence (XRF) has been utilized to determine total elemental composition. XRD and SEM analyses have confirmed the presence of pyrite and arsenopyrite indicating a potential for acid mine drainage. Calcite has been confirmed to have a significant presence in the unweathered samples by XRD and the Chittick procedure, with some samples containing an average of 19.4% calcite. NAA and XRF have revealed significant concentrations of toxic elements such as As, Pb and Zn in both the oxidized and unoxidized samples.