8 research outputs found
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THE ROLE OF 4-HYDROXYPHENYLPYRUVATE DIOXYGENASE IN ENHANCEMENT OF SOLID-PHASE ELECTRON TRANSFER BY SHEWANELLA ONEIDENSIS MR-1
While mechanistic details of dissimilatory metal reduction are far from being understood, it is postulated that the electron transfer to solid metal oxides is mediated by outer membrane-associated c-type cytochromes and redox active electron shuttling compounds. This study focuses on the production of homogensitate in Shewanella oneidensis MR-1, an intermediate of tyrosine degradation pathway, which is a precursor of a redox cycling metabolite, pyomelanin. In this study, we determined that two enzymes involved in this pathway, 4-hydroxyphenylpyruvate dioxygenase (4HPPD) and homogentisate 1,2-dioxygenase are responsible for homogentisate production and oxidation, respectively. Inhibition of 4-HPPD activity with the specific inhibitor sulcotrione (2-(2-chloro-4-methane sulfonylbenzoyl)-1,3-cyclohexanedione), and deletion of melA, a gene encoding 4-HPPD, resulted in no pyomelanin production by S. oneidensis MR-1. Conversely, deletion of hmgA which encodes the putative homogentisate 1,2-dioxygenase, resulted in pyomelanin overproduction. The efficiency and rates, with which MR-1 reduces hydrous ferric oxide, were directly linked to the ability of mutant strains to produce pyomelanin. Electrochemical studies with whole cells demonstrated that pyomelanin substantially increases the formal potential (E{sup o}{prime}) of S. oneidensis MR-1. Based on this work, environmental production of pyomelanin likely contributes to an increased solid-phase metal reduction capacity in Shewanella oneidensis
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Final Report: Bioelectrochemical Process Development
Work performed under the Independent Research and Development project ''Bioelectrochemical Process Development'' using Shewanella Oneidensis MR-1 henceforth MR-1, a model dissimilitory metal reducing (DMRB) bacteria successfully demonstrates the following: MR-1 transfers electrons by hydrogen oxidation to solid phase electrodes; MR-1 can act as an electron transfer agent when attached to a solid metal electrode surface - a modified electrode has been developed and tested; MR-1 transfers electrons from solution to a solid electrode. DMRB grow by transferring electrons to insoluble metals. When bacteria oxidize organic compounds or hydrogen this results in electron flow from the bacterial cell for the purpose of cellular energy production. Extracellular electron transfer to solid terminal electron acceptors permits coupling this oxidation process to electrodes. As a result, detection of organic compounds and hydrogen can be accomplished by bacterial contact to an electrode. The authors have designed a process that incorporates bacterial cells onto electrodes for the purpose of hydrogen detection. In addition, this type of bio-sensor also responds to the presence of FE III thereby providing potential utility as a Fe III sensor. Immediate uses of this technology include in-situ detection and quantification of organic compounds and hydrogen in the subsurface that provide energy for growth of indigenous bacteria
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HYDROGEN ISOTOPE RECOVERY USING PROTON EXCHANGE MEMBRANE ELECTROLYSIS OF WATER
A critical component of tritium glovebox operations is the recovery of high value tritium from the water vapor in the glove box atmosphere. One proposed method to improve existing tritium recovery systems is to replace the disposable hot magnesium beds used to separate the hydrogen and oxygen in water with continuous use Proton Exchange Membrane Electrolyzers (PEMEs). This study examines radiation exposure to the membrane of a PEME and examines the sizing difference that would be needed if the electrolyzer were operated with a cathode water vapor feed instead of an anode liquid water feed
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FISCAL YEAR 2006 REPORT ON ELECTROLYZER COMPONENT DEVELOPMENT FOR THE HYBRID SULFUR PROJECT
Thermochemical processes are being developed to provide global-scale quantities of hydrogen. A variant on sulfur-based thermochemical cycles is the Hybrid Sulfur (HyS) Process which uses a sulfur dioxide depolarized electrolyzer (SDE) to produce the hydrogen. In FY05, testing at the Savannah River National Laboratory (SRNL) explored a low temperature fuel cell design concept for the SDE. The advantages of this design concept include high electrochemical efficiency and small volumetric footprint that is crucial for successful implementation on a commercial scale. A key component of the SDE is the ion conductive membrane through which protons produced at anode migrate to the cathode and react to produce hydrogen. An ideal membrane for the SDE should have both low ionic resistivity and low sulfur dioxide transport. These features allow the electrolyzer to perform at high currents with low potentials, along with preventing contamination of both the hydrogen output and poisoning of the catalysts involved. Another key component is the electrocatalyst material used for the anode and cathode. Good electrocatalysts should be chemically stable and low overpotential for the desired electrochemical reactions. This report summarizes results from activities to evaluate different membrane and electrocatalyst materials for the SDE. Several different types of commercially-available membranes were analyzed for ionic resistance and sulfur dioxide transport including perfluorinated sulfonic acid, sulfonated poly-etherketone-ketone, and poly-benzimidazole membranes. Of these membrane types, the poly-benzimidazole (PBI) membrane, Celtec-L, exhibited the best combination of characteristics for use in an SDE. Testing examined the activity and stability of platinum and palladium as electrocatalyst for the SDE in sulfuric acid solutions. Cyclic and linear sweep voltammetry revealed that platinum provided better catalytic activity with much lower potentials and higher currents than palladium. Testing also showed that the catalyst activity is strongly influenced by concentration of the sulfuric acid. Various cell configurations were examined with respect to the deposition of electrocatalyst and use of conductive carbon materials such as carbon cloth and carbon paper. Findings from these evaluations and the results of the membrane and electrocatalyst testing, we prepared three different membrane electrode assemblies (MEA) for electrolyzer testing. The first MEA consisted of a Nafion{reg_sign} membrane with platinum electrocatalyst deposited on carbon cloths, which were heat pressed onto the membrane, an assembly identical to those used in proton exchange membrane fuel cells. The second MEA also used a Nafion membrane with the electrocatalysts deposited directly onto the membrane. The third MEA proved similar to the second but utilized a PBI membrane in place of the Nafion{reg_sign} membrane. Tailor of the membrane and catalysts properties for the SDE system was concluded as a required step for the technology to move forward. It was also recommended the evaluation of the tested and new developed materials at conditions closer to the SDE operating conditions and for longer period of time