58 research outputs found

    Living in the World of Manmade Chemicals: Lessons Learned from the Migrations and Collapses of Civilizations

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    Many of the world’s natural surface and groundwater resources are getting contaminated with an increasing number of manmade chemicals, which include farm and household insecticides, industrial pollutants, and pharmaceuticals. As the sizes of potable water bodies are decreasing steeply, it is warranted that we find economic ways to preserve, and purify the available water resources. In this presentation, a discussion on two of the naturally occurring and most widespread contaminants in the world and their human exposure, how one of these contaminants contributed possibly to a collapse of a thriving ancient civilization and the lessons one can learn from these, and the development of novel nano technologies in the purification of water resources that contain these contaminants

    Role of Sulfhydryl Sites on Bacterial Cell Walls in the Biosorption, Mobility and Bioavailability of Mercury and Uranium

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    The goal of this exploratory study is to provide a quantitative and mechanistic understanding of the impact of bacterial sulfhydryl groups on the bacterial uptake, speciation, methylation and bioavailability of Hg and redox changes of uranium. The relative concentration and reactivity of different functional groups present on bacterial surfaces will be determined, enabling quantitative predictions of the role of biosorption of Hg under the physicochemical conditions found at contaminated DOE sites.The hypotheses we propose to test in this investigation are as follows- 1) Sulfhydryl groups on bacterial cell surfaces modify Hg speciation and solubility, and play an important role, specifically in the sub-micromolar concentration ranges of metals in the natural and contaminated systems. 2) Sulfhydryl binding of Hg on bacterial surfaces significantly influences Hg transport into the cell and the methylation rates by the bacteria. 3) Sulfhydryls on cell membranes can interact with hexavalent uranium and convert to insoluble tetravalent species. 4) Bacterial sulfhydryl surface groups are inducible by the presence of metals during cell growth. Our studies focused on the first hypothesis, and we examined the nature of sulfhydryl sites on three representative bacterial species: Bacillus subtilis, a common gram-positive aerobic soil species; Shewanella oneidensis, a facultative gram-negative surface water species; and Geobacter sulfurreducens, an anaerobic iron-reducing gram-negative species that is capable of Hg methylation; and at a range of Hg concentration (and Hg:bacterial concentration ratio) in which these sites become important. A summary of our findings is as follows- Hg adsorbs more extensively to bacteria than other metals. Hg adsorption also varies strongly with pH and chloride concentration, with maximum adsorption occurring under circumneutral pH conditions for both Cl-bearing and Cl-free systems. Under these conditions, all bacterial species tested exhibit almost complete removal of Hg from the experimental solutions at relatively low bacterial concentrations. Synchrotron based X-ray spectroscopic studies of these samples indicate that the structure and the coordination environment of Hg surface complexes on bacterial cell walls change dramatically- with sulfhydryls as the dominant Hg-binding groups in the micromolar and submicromolar range, and carboxyls and phosphoryls dominating at high micromolar concentrations. Hg interactions change from a trigonal or T-shaped HgS{sub 3} complex to HgS or HgS{sub 2} type complexes as the Hg concentration increases in the submicromolar range. Although all bacterial species studied exhibited the same types of coordination environments for Hg, the relative concentrations of the complexes change as a function of Hg concentration

    Potentially Massive and Global Non-Pyrogenic Production of Condensed Black Carbon through Biomass Oxidation

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    With the increased occurrences of wildfires worldwide, there has been an increase in scientific interest surrounding the chemistry of fire-derived black carbon (BC). Traditionally, wildfire research has assumed that condensed aromatic carbon (ConAC) is exclusively produced via combustion, and thus, ConAC is equated to BC. However, the lack of correlations between ConAC in soils or rivers and wildfire history suggests that ConAC may be produced non-pyrogenically. Here, we show quantitative evidence that this occurs during the oxidation of biomass with environmentally ubiquitous hydroxyl radicals. Pine wood boards exposed to iron nails and natural weather conditions for 12 years yielded a charcoal-like ConAC-rich material. ConAC was also produced during laboratory oxidations of pine, maple, and brown-rotted oak woods, as well as algae, corn root, and tree bark. Back-of-the-envelope calculations suggest that biomass oxidation could be producing massive non-pyrogenic ConAC fluxes to terrestrial and aquatic environments. These estimates (e.g., 163-182 Tg-ConAC/year to soils) are much higher than the estimated pyrogenic BC fluxes (e.g., 128 Tg-ConAC/year to soils) implying that environmental ConAC is primarily non-pyrogenic. This novel perspective suggests that wildfire research trajectories should shift to assessing non-pyrogenic ConAC sources and fluxes, developing new methods for quantifying true BC, and establishing a new view of ConAC as an intermediate species in the biogeochemical processing of biomass during soil humification, aquatic photochemistry, microbial degradation, or mineral-organic matter interactions. We also advise against using BC or pyrogenic carbon (pyC) terminologies for ConAC measured in environmental matrices, unless a pyrogenic source can be confidently assigned

    Role of S/Se ratio in chemical bonding of As-S-Se glasses investigated by Raman, x-ray photoelectron, and extended x-ray absorption fine structure spectroscopies

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    Chalcogenide glasses have attracted considerable attention and found various applications due to their infrared transparency and other optical properties. The As-S-Se chalcogenide glass, with its large glass-formation domain and favorable nonlinear property, is a promising candidate system for tailoring important optical properties through modification of glass composition. In this context, a systematic study on ternary As-S-Se glass, chalcogen-rich versus well-studied stochiometric compositions, has been carried out using three different techniques: Raman spectroscopy, x-ray photoelectron spectroscopy, and extended x-ray absorption fine structure spectroscopy. These complementary techniques lead to a consistent understanding of the role of S/Se ratio in chalcogen-rich As-S-Se glasses, as compared to stochiometric composition, and to provide insight into the structural units (such as the mixed pyramidal units) and evidence for the existence of homopolar bonds (such as Se-Se, S-S, and Se-S), which are the possible structural origin of the high nonlinearity in these glasses
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