B32A-01 INVITED
Nano-sized Minerals of Elemental Selenium and Tellurium Formed by Bacterial Dissimilatory Reduction of Se- and Te-Oxyanions.
Selenium and tellurium are both Group 16 elements that have curious opto-electrical properties making them of potential interest for photovoltaic applications. The process of dissimilatory reduction of selenate and selenite by 3 diverse species of anaerobes, Bacillus selenitireducens, Sulfurospirillum barnesii, and Selenihalanaerobacter shriftii resulted in the accumulation of many uniformly-sized nanospheres (diameter = approx. 300 nm) that aggregated on the outside of their cell envelopes (Oremland et al., 2004). Despite their uniformity of shape, purified Se-nanospheres from the 3 different species displayed significantly different spectral properties (UV- visible light and Raman) indicating differing internal arrangements of their Se atoms. Se-nanospheres from all 3 species also had lower bandgap energies than that of elemental selenium formed by chemical means. We subsequently determined that S. barnesii and B. selenitireducens could grow by dissimilatory reduction of Te- oxyanions, although progress was hampered by the fact that Te concentrations above 0.6 mM proved toxic to cells (Baesman et al., 2007). Unlike the case for Se-nanospheres, the Te-nanoparticles formed by the two microbes were entirely different. S. barnesii formed small, irregularly shaped spheroids (smaller than 50 nm diameter) that coalesced into larger aggregates. In contrast, B. selenitreducens formed nano-rods (10 nm diameter x 200 nm length) that coalesced into larger shards which formed even larger rosette-shaped aggregates once they sloughed off the cells. Spectroscopy of purified Te-rosettes indicated an internal trigonally-shaped array of Te atoms. Future research on Te(0) nano-materials formed by anaerobic bacteria would be aided by isolation of novel species adapted to growth at high batch culture concentrations of Te-oxyanions (approx. 10 mM). Furthermore, the ability of microbes like B. selenitreducens to form selenide by reduction of Se(0) suggests an application in the biological formation of CdSe or perhaps CdTe type-nanomaterials that bypasses the need for highly reactive compounds required for chemical syntheses. Oremland, R.S. et al. (2004). Appl. Environ. Microbiol. 70: 52 – 60. Baesman, S.M., et al. (2007). Appl. Environ. Microbiol. 73: 2135 – 2143.
B32A-02
Bacterial Interactions with CdSe Quantum Dots
Cadmium selenide quantum dots (QDs) are semiconductor nanoparticles that are manufactured for biomedical imaging, photovoltaics, and other applications. While metallic nanoparticles can be made biotically by bacteria and fungi, and thus occur in nature, the fate of either natural or engineered QDs and relationships to nanoparticle size, conjugate and biotic conditions are mostly unknown. Working with several different bacterial strains and QDs of different sizes and conjugate chemistries, including QDs synthesized by a Fusarium fungal strain, we show that QDs can enter cells through specfic receptor-mediated processes, that QDs are broken down by bacteria during cell association, and that toxicity to cells is much like that imposed by Cd(II) ions. The mechanisms of entry and toxicity are not fully understood, but preliminary evidence suggests that electron transfer between cells and QDs occurs. Also, cell membranes are compromised, indicating oxidative stress is occurring. Results with planktonic and biofilm bacteria are similar, but differently, biofilms tend to accumulate Cd(II) associated with QD treatments.
B32A-03
Biomineralization of Metallic Copper and Copper Sulfide Nanomaterials in a Flooded Soil: Impact on Contaminant Mobility
Colloidal nanomaterials may enhance the mobility of strongly sorbing contaminants that are otherwise immobile in soils and sediments. We investigated the formation of biogenic nanomaterials in a contaminated wetland soil upon flooding and microbially-mediated soil reduction using microcosm experiments. Combining electron microscopy and X-ray absorption spectroscopy, we characterized the newly formed nanomaterials and evaluated their effect on the mobility of selected contaminants (Cu, Cd, Pb). In the pore water of the flooded soil, we observed the formation of <20 nm metallic Cu(0) nanocrystals associated with bacterial cells. The Cu(0) nanocrystals grew in size until the onset of microbial sulfate respiration, when Cu(0) started to transform into poorly crystalline hollow Cu sulfide structures on bacteria. Concurrently, we observed the precipitation of <40 nm Cu sulfide nanoparticles dispersed in the pore water. Both Cu sulfide hollow spheres and nanoparticles were apparently mobile and contained substantial quantities of Cd and Pb in addition to Cu, dominating their respective pore water speciation. Over extended periods of flooding, the colloids were slowly removed by deposition following apparent first-order kinetics. We have conducted additional experiments to elucidate the formation mechanism of the bacteria-associated nanomaterials observed. First evidence suggests that the biomineralization of the Cu(0) nanocrystals occurs when Cu(I) released by bacteria, likely exported by homeostasis proteins, disproportionates on the outer cell membrane. Their transformation into hollow structures upon reaction with sulfide is interpreted to result from inward diffusion of vacancies balancing outward diffusing Cu in a Kirkendall-like process. Our results demonstrate that the formation of mobile Cu sulfide colloids enhances Cu, Cd, and Pb mobility in the flooded soil. The findings thus point to a novel pathway for the translocation of chalcogenic contaminants from wetland soils to adjacent surface and groundwater bodies.
B32A-04 INVITED
A preliminary study on the transport of biogenic nanoparticles in aquifer environments
Immobilizing metals and radionuclides as minerals via microbial reduction and precipitation has been proposed as a cost-effective strategy for remediating contaminated aquifers. Biogenic minerals often occur as aggregates of nanoparticles with sizes as small as several to hundreds of nanometers. Because of relatively large chemical reactivity of the nanoparticles, the mobility of biogenic nanoparticles and aggregates are important in evaluating the efficacy and long-term impact of in situ bioremediation. To investigate the mobility of biogenic nanoparticles in aquifer environments, we take iron sulfide produced by pure cultures of sulfate reducing bacteria as an example and characterized the nanoparticles using Fourier transform inferred spectroscopy (FTIR). The FTIR spectra showed that extracellular polymeric substances on the surfaces of the nanoparticles were dominantly proteins and fatty acids. To explore how the surface organic coatings affect the interactions among nanoparticles, we characterized the growth kinetics of biogenic iron sulfide aggregates under various ionic strengths and microbial activities using chemostat reactors and dynamic light scattering. Contrary to the predictions of classical colloid theories, sizes of biogenic aggregates were controlled dominantly by the rates of sulfide production, but not the surface potentials of the nanoparticles. We further quantified the impact of the surface organic coatings on the interactions between the nanoparticle aggregates and aquifer sediments using flow-through column experiments. The experiments under various ionic strengths and aggregate sizes demonstrated that the ionic strength was not significant in controlling the mobility of biogenic aggregates. These preliminary results demonstrated that, because of the organic surface coatings, the production and mobility of biogenic nanoparticle aggregates in aquifers differ significantly from the predictions based on synthesized nanoparticles and colloids. Future experiments are required to elucidate the impact of aquifer hydrogeochemistry on the nature of the surface organic coatings and the mobility of biogenic nanoparticles in aquifers.
B32A-05
A Comparison of Naturally-Occurring and Artificially Stimulated Uranium(VI) Bioreduction in Sediment from a Field-scale Experiment in Rifle, CO
Long-term remediation of uranium (U)-contaminated groundwater poses one of the greatest challenges in the clean-up of impacted sites. One solution is to reduce dissolved U(VI) to insoluble U(IV) precipitates by stimulating indigenous metal reducing bacterial populations in situ. Contamination from a former U mine tailings repository (Rifle, CO) provides a research site to study the efficacy of biostimulated U(VI) reduction at the field scale. Several cores were drilled in June 2007 across a region of naturally-occurring U(VI) bioreduction. The cores represent a cross section of sediment that ranges from minimally reducing to highly reducing. Anaerobic sediment samples from the cores were analyzed for labile U(VI) content by carbonate extraction in anoxic conditions (pH 9.4, 14 mM NaHCO3, 2.8 mM Na2CO¬3). A subset of the same core sections were dried and oxidized by exposure to air for 2 weeks. The carbonate extraction was repeated, and the amount of U(IV) present in the anaerobic sample was calculated by difference between the anoxic and oxidized extractions. An acid extraction was also performed on the oxidized sediments to compare the carbonate extractable and the acid extractable U fractions. The highest U concentrations were found in the highly bioreduced sediment, with the majority of U present as U(IV) (66-92%). The regions of highest bioreduction also correspond to elevated concentrations of solid phase organic carbon, suggesting that natural bioreduction is stimulated by zones of increased organic carbon content. The same field site was then used for an artificially stimulated bioreduction experiment, where the indigenous bacterial community was stimulated by injecting acetate upgradient of the core collection location. Carbonate and acid extractions were performed on core samples taken after the completion of the acetate injection. This work evaluates the composition of the sediment before and after biostimulation as a way of directly comparing the extent of natural U(VI) bioreduction to acetate-stimulated bioreduction in order to facilitate the design of a more effective bioremediation strategy for this site.
B32A-06
Detecting Naturally-Produced Sulfide Nanoparticles by Adsorptive, Cathodic Stripping Voltammetry.
Growing evidence implies that metal sulfide nanoparticles of natural origin exist in some aquatic environments. These nanoparticles could play important roles as mediators of trace metal nutrition and toxicity. Thermodynamics suggests that in sulfidic environments (total transition metal<total sulfide) the most abundant transition metal (usually Fe) will form the predominant sulfide nanoparticle. In oxic/suboxic environments (total transition metal>total sulfide) the most insoluble metal sulfide (usually Hg or Cu) will form the predominant sulfide nanoparticle. New experimental methods for detecting and distinguishing between such nanoparticles are needed. We report that mercury electrodes effectively preconcentrate a number of different metal sulfide nanoparticles, enabling their detection by adsorptive cathodic stripping voltammetry. Voltammetrically, nanoparticulate analytes differ fundamentally from dissolved analytes; e.g. analyte accumulation is very sensitive to electrolyte composition and concentration in accord with the Schulze-Hardy Rule. EDTA or acid treatment of samples is useful for distinguishing highly insoluble nanoparticles (HgS, CuS) from FeS. Nanoparticulate sulfur potentially interferes. Supersaturated solutions can generate artifactual analyte on Hg electrode surfaces. Despite such potential pitfalls, progress is encouraging. Preliminary, qualitative results from natural waters will be reported.
B32A-07
Utilizing Societal Concerns in Nanomaterials R and D Decision-Making
Nanotechnology has demonstrated the potential to transform many aspects of human existence. From extraordinary ways to deliver pharmaceuticals to the tiniest of electronic circuits to the finest of filters, technologies made with nanomaterials can shrink the computer chip or remove contaminants from water. Nanotechnology systems can deliver drugs to targets inside individual cells or serve as components in sensors that detect chemical and biological agents. As nanomaterials are commercialized, there are potential consequences, both positive and negative. At each stage of development of nanotechnology, it is important to assess both the potential benefits and the potential costs. Scientists need more tools to simultaneously develop new technologies and consider whether their methods or materials might contribute to health or environmental hazards down the line. Early information on potential risks guides the development of these materials as they move toward industrial production. Some indications of problems have turned-up. If thorough assessments are not done, the potential exists to extend projects (and their related expense) long past the point where "fatal flaws" could be identified. As nanotechnology graduates from infancy and it is possible to predict the course and impacts of the technology beyond the next several years. There are well-founded concerns with nanotechnology, especially relating to human health, the environment, and public perception in general. This paper will include a matrix of potential effects, means of ranking or scoring them and examples of how the technique can be applied to current research and development.