Browsing by Subject "Arsenic"

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Archaea at the El Tatio Geyser Field : community composition, diversity, and distribution across hydrothermal features and geochemical gradients
(2012-05) Franks, Megan A.; Bennett, Philip C. (Philip Charles), 1959-; Bell, Christopher J.; Cardenas, Meinhard B.; Engel, Annette S.; Hawkes, Christine V.
Methanogenesis, a metabolic pathway unique to Archaea, is severely inhibited by the reduced form of arsenic (As). Despite this inhibition, methanogenic Archaea are present in some hydrothermal features at the El Tatio Geyser Field (ETGF), a high-arsenic site with 100+ hydrothermal features, including boiling pools, geyers, fumaroles, and springs. The ability of methanogenic Archaea and other microorganisms to withstand elevated arsenic concentrations, and a variety of other extreme environmental conditions at ETGF, may be due to unique adaptations or syntrophic relationships with other microorganisms. ETGF is situated in the Andes Mountains at an altitude of ~4300 meters. UV radiation is elevated in this region and air temperatures fluctuate widely. Most hydrothermal waters discharge at ~85˚C, the local boiling point, and rapidly evaporate due to the arid climate. This concentrates hydrothermal salts and metals, including arsenic (As) and antimony (Sb). Additionally, dissolved inorganic carbon (DIC) concentrations are extremely low in most features and may limit life. Water chemistry analyses done for this study show variability in dissolved constituents between features that are consistent over time. Variations may be due to the source or residence time of waters, and differences in chemistry could be responsible for the presence or absence of methanogenic Archaea at hydrothermal sites. The overlying control on microbial diversity and community composition may be water geochemistry, and potentially specific constituents. The goals of this study were to detect novel microbial taxa at ETGF, including novel methanogens, as well as to document microbial community composition at select hydrothermal features. The distribution and diversity of microorganisms at each feature was analyzed phylogenetically and within an ecological context in order to determine physicochemical and biological controls on community composition. Additionally, a model methanogen was used in laboratory analyses to determine how concentrations and oxidation states affected growth and methane production. This methanogen, Methanothermobacter thermautotrophicus, is found at ETGF, Yellowstone, and other hydrothermal fields, and thrives in high-temperature environments. MPN (most probable number) analyses show that culturable biomass from multiple sites contain metabolically active methanogens. These results support the biogenicity of dissolved methane detected in the field. 16S rRNA surveys of Archaea at four sites show that Archaea are diverse, and archaeal community composition varies across features. Phylogenetic tree construction indicates that Archaea from ETGF group together, suggesting that the isolation and broad environmental constrains on ETGF have some control on phylogenetic diversity. Laboratory analyses of As and Sb concentrations on M. thermautotrophicus suggest that Sb may decrease the inhibition of methanogenesis by As by preventing the formation of As(III) from As(V). Statistical analyses correlating microbial community composition and structure to physicochemical parameters show that archaeal and bacterial communities relate to different variables; with Bacteria correlating to water temperature, and Archaea correlating to dissolved constituents such as hydrogen gas and sulfate.
Dopant behavior in complex semiconductor systems
(2009-12) Kong, Ning; Kirichenko, Taras A.; Banerjee, Sanjay
As the size of modern transistors is continuously scaled down, challenges rise in almost every component of a silicon device. Formation of ultra shallow junction (USJ) with high activation level is particularly important for suppressing short channel effects. However, the formation of low resistance USJ is made difficult by dopant Transient Enhanced Diffusion (TED) and clustering-induced deactivation. In this work, we proposed a novel point defect engineering solution to address the arsenic TED challenge. By overlapping arsenic doped region with silicon interstitials and vacancies, we observed enhanced and retarded arsenic diffusion upon anneal, respectively. We explain this phenomenon by arsenic interstitial diffusion mechanism. In addition, we implemented this interstitial-based mechanism into a kinetic Monte Carlo (kMC) simulator. The key role of interstitials in arsenic TED is confirmed. And we demonstrated that the simulator has an improved prediction capability for arsenic TED and deactivation. As a long time unsolved process challenge, arsenic segregation at SiO₂/Si interface was investigated using density functional theory (DFT) calculation. The segregation-induced arsenic dose loss not only increases resistance but also may induce interface states. We identified three arsenic complex configurations, [chemical formula] , [chemical formula] and [chemical formula], which are highly stabilized at SiO₂/Si interface due to the unique local bonding environments. Therefore, they could contribute to arsenic segregation as both initial stage precursors and dopant trapping sites. Our calculation indicates that arsenic atoms trapped in such interface complexes are electrically inactive. Finally, the formation and evolution dynamics of these interface arsenic-defect complexes are discussed and kMC models are constructed to describe the segregation effects. A potential problem for the p-type USJ formation is the recently found transient fast boron diffusion during solid phase epitaxial regrowth process. Using DFT calculations and molecular dynamics simulation, we identified an interstitial-based mechanism of fast boron diffusion in amorphous silicon. The activation energy for this diffusion mechanism is in good agreement with experimental results. In addition, this mechanism is consistent with the experimentally reported transient and concentration-dependent features of boron diffusion in amorphous silicon.
Fate and transport of arsenic and antimony in the El Tatio Geyser Field, Chile
(2007-05) Landrum, Jeffrey Thomas, 1979-; Bennett, Philip C. (Philip Charles), 1959-
El Tatio Geyser Field (ETGF), northern Chile, hosts widespread geothermal activity, with very high aqueous concentrations of arsenic and antimony, higher than any other known geothermal system. Boiling springs (86°C) discharge circum-neutral pH, Na-Cl type waters with low organic carbon. Net discharge of a stream draining the ETGF basin is approximately 10 cfs. As(III), the dominant As species in discharge waters, rapidly oxidizes to As(V) at an estimated first order rate of 0.35 min-1, determined in the field by first arrival of a tracer. As and Sb concentrations and speciation in hydrothermal waters, deposits, and microbial biomass are evaluated as a product of microbial metabolism, sorption to metal-oxyhydroxides, and co-precipitation. Mechanisms controlling these reactions ( i.e. cooling, evaporation, changes in redox and pH) are evaluated and modeled. Sequential extractions reveal that As, sorbs to Fe and Mn oxy-hydroxide complexes. In contrast, Sb solid phase partitioning is influenced primarily by the co-precipitation of Sb-oxide minerals with siliceous sinter (up to 2% wt. Sb). Diurnal variations occur in spring water chemistry and may cause Sb-rich laminations in siliceous hydrothermal deposits. Microbial energetics calculations and enrichment experiments suggest that microbial activity influences the mobility of As, and probably Sb in the ETGF basin by altering redox speciation and sorption to microbial biomass.
First principles modeling of arsenic and fluorine behavior in crystalline silicon during ultrashallow junction formation
(2006) Harrison, Scott Anthony; Hwang, Gyeong S.; Edgar, Thomas F.
The 2005 International Technology Roadmap for Semiconductors predicts ultrashallow junctions (USJs) less than 7 nm deep with unprecedented dopant activation levels will be required for silicon transistors to be manufactured in 2010. To meet these requirements, it is necessary to have a better understanding of the dopant transient enhanced diffusion (TED) and clustering behaviors that undermine the achievement of these manufacturing specifications. Arsenic (As) is a commonly used n-type dopant in USJ formation and fluorine (F) is an impurity commonly co-implanted with dopants to reduce dopant diffusion and clustering during USJ formation. In this dissertation, density functional theory within the generalized gradient approximation is used to understand the behavior of As and F in crystalline silicon during USJ formation. In the first part of this dissertation, the influence of silicon interstitials on As behavior during thermal annealing that follows dopant implantation is investigated. As a result of dopant implantation, a net excess of silicon interstitial defects exist in the silicon. First, it is shown that silicon interstitials can easily annihilate existing Asvacancy complexes in silicon with negligible recombination energy barriers. Second, experimentally observed As TED mediated by interstitials is explained by the formation of a highly mobile As-silicon interstitial pair that can exist in positive, neutral, and negative charge states. Finally, it is shown that large As-silicon interstitial complexes may form when excess interstitials are present and provide a kinetic route to As clustering that leads to As deactivation. In the second part of this dissertation, the interaction of F impurities with silicon interstitials and B dopants is investigated. First, the formation and diffusion of a highly mobile fluorine-silicon interstitial pair which has been suggested by experiment is detailed. Second, an immobile B-Sii-F structure is identified in which B has a deactivated configuration. This structure may play a role in deactivating and immobilizing B when implanted B and F profiles coincide. This research provides fundamental insight into the behavior of As dopants and F impurities during USJ formation. As the future of silicon-based devices relies on the ability to perform precise doping, these findings should be of great importance to device manufacturers.
The microbial ecology and biogeochemistry of cyanobacteria in the arsenic-rich and inorganic carbon-limited geothermal waters of El Tatio Geyser Field, Chile
(2015-05) Myers, Kimberly Dawn; Bennett, Philip C. (Philip Charles), 1959-; Bell, Christopher J; Omelon, Christopher R; Shanahan, Timothy M; Hawkes, Christine V
Geothermal settings are some of the best-known analogs for early earth environments and among the best places to investigate the impact of extreme conditions on microbial life. El Tatio Geyser Field (ETGF) is a geothermal setting located at 4,300m in the Atacama Desert region of Chile. Its high-elevation desert position leads to high UV-flux, rapid evaporation, and mineral precipitation. El Tatio geothermal waters also possess extremely limited concentrations of life-essential nutrients, such as dissolved inorganic carbon (DIC as CO2(aq) + HCO3-), contain among the highest naturally occurring concentrations of the toxic element arsenic (As as H3AsO30 + HXAsO43-X), and are buffered to circumneutral pH by arsenate (H2AsO4-/HAsO42-; pKa ~ 6.9 at 25°C). Cyanobacteria were found to be the most important primary producers supporting microbial communities in El Tatio geothermal waters. The objective of this dissertation work was to characterize the role of cyanobacteria in the ETGF microbial ecosystem, and determine the response of cyanobacteria to the high-As and low-DIC conditions present at ETGF. Field observations, geochemical analyses, and next-generation 16S rRNA gene sequencing approaches were used to determine the geochemical controls on cyanobacterial distribution, the phylogenetic diversity of El Tatio cyanobacteria, and the corresponding microbial community structure at sites with and without cyanobacteria. Four cultured cyanobacterial strains were isolated from ETGF mat material, and experiments were performed to assess the growth and carbon-uptake response of these strains to low DIC, AsIII, and AsV. AsIII and temperature negatively controlled the abundance and distribution of cyanobacteria in geothermal outflows throughout ETGF, whereas AsV positively influenced these factors. In the laboratory, AsIII inhibited the growth of cultured strains, while AsV stimulated growth. Closed-system experiments showed significantly increased carbon uptake and growth in the presence of AsV, due to the ability of arsenate to offset the rapid upward pH shift that often occurs in mats during photosynthesis, thereby maintaining DIC in the preferred forms for cyanobacterial uptake. These results showed that AsV plays a positive role in the ETGF microbial ecosystem by increasing the productivity of cyanobacterial mats under low DIC and arsenate-buffered conditions.
Removal of inorganic contaminants and natural organic matter by enhanced alum coagulation : defluoridation at the pilot scale and application to arsenic
(2016-12) Gee, Isabella Marie; Lawler, Desmond F.; Katz, Lynn Ellen
The removal of inorganic contaminants is a primary concern in drinking water treatment. Fluoride and arsenic both naturally occur in ground and surface waters throughout the world. Recent health concerns regarding extended exposure to high levels of fluoride in drinking water have prompted the United States Environmental Protection Agency (USEPA) to review the fluoride maximum contaminant level (MCL). Arsenic is one of the most well-known and toxic inorganic contaminants regulated by the USEPA. While typically associated with groundwater, arsenic could be an increasing concern for surface water sources in the future. Small water systems may not have the resources to adjust their treatment scheme to accommodate a lower fluoride MCL or target arsenic removal. In this study, enhanced alum coagulation was investigated as a treatment strategy for both fluoride and arsenic. Facilities with surface water sources face a particular challenge, as the presence of natural organic matter (NOM) may interfere with the removal of fluoride and/or arsenic during coagulation. This work builds upon previous investigation of the interactions between fluoride, NOM, and aluminum during coagulation by comparing two pilot studies in Texas and Colorado. Each pilot study confirmed that enhanced alum coagulation was able to remove fluoride during continuous flow experiments using natural source water; the comparison of the studies revealed that source water composition impacts the maximum efficacy of alum coagulation for fluoride removal. A higher influent organic concentration appears to reduce the maximum efficacy of fluoride removal. However, the use of pH control may also be a contributing factor to the discrepancy in fluoride removal between pilot studies. The pilot study with a lower maximum fluoride removal had a higher influent DOC concentration, but was run without pH control. Synthetic water jar testing confirmed the ability of alum coagulation to remove arsenic (V). A maximum arsenic removal of 99% was observed for As(V) at alum doses of 100 mg/L and above, and an alum dose of 20 mg/L achieved an As(V) removal of 97%. Aluminum residuals suggest that the presence of As(V) lowers the point of zero charge for aluminum hydroxide solid.