Spectroelectrochemical Sensor for Technetium - ACS Publications

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Chapter 13

Downloaded by MIT on April 20, 2013 | http://pubs.acs.org Publication Date: April 19, 2005 | doi: 10.1021/bk-2005-0904.ch013

Spectroelectrochemical Sensor for Technetium: Preconcentration and Quantification of Pertechnetate in Polymer-Modified Electrodes 1

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David J. Monk , Michael L. Stegemiller, Sean Conklin , Jean R. Paddock , William R. Heineman, Carl J. Seliskar , Thomas H. Ridgway, Samuel A. Bryan , and Timothy L. Hubler 1

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Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221 Radiochemical Processing Laboratory and Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352 2

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A remote spectroelectrochemical sensor and instrumentation package is being developed for the detection of technetium as aqueous pertechnetate (TcO ) in the vadose zone and associated groundwater. This sensor would be used to monitor the integrity of low-level and high-level nuclear waste containment at U.S. Department of Energy sites. Electrochemical studies of TcO reduction at bare indium doped tin oxide (ITO) optically transparent electrodes (OTEs) show a poorly formed reduction wave for cyclic voltammetry and precipitation of technetium oxide (TcO ) on the electrode surface. Similar experiments at ITO OTEs coated with thin films containing cationic polymers show partitioning of TcO into the films. Three films were investigated: poly(dimethyldiallylammonium chloride) (PDMDAAC) and quaternized poly(4-vinylpyridine) (QPVP), both immobilized in porous glass by the sol-gel process, and -

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© 2005 American Chemical Society In Subsurface Contamination Remediation; Berkey, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

307 poly(vinylbenzyltrimethylammonium chloride) (PVTAC) copolymerized with poly(vinylalcohol). The largest enhancement in cyclic voltammetry reduction wave for TcO was for QPVP. The electrochemical mechanism changes to favor the formation of a relatively long-lived soluble species that ultimately converts to TcO . The electrodeposition of technetium oxide in these films was shown to be a method for the quantitative spectroelectrochemical determination of TcO and has been verified using radiochemistry dose measurements and scanning electron microscopy. -

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Downloaded by MIT on April 20, 2013 | http://pubs.acs.org Publication Date: April 19, 2005 | doi: 10.1021/bk-2005-0904.ch013

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Introduction One of the goals of the U.S. Department of Energy (DOE) Environmental Management Science Program (EMSP) is to support basic research addressing fundamental issues that may be critical to advancing the remediation of Department of Energy (DOE) sites nationwide. One critical area of this remediation is the need to monitor radiochemical constituents in various areas, ranging from the containment of low- and high-level radioactive waste to monitoring of contaminant plumes in subsurface water. Present methods of analysis are hazardous and time consuming, usually requiring lengthy sampling, preparation, analysis, and data interpretation. A better approach would be to use sensors to perform rapid, sensitive, and economic in situ analyses for various constituents of interest. Technetium is one such constituent of radioactive waste where the need for a chemical means of detection exists, but a sensor does not. Technetium is not found in appreciable quantities in nature; however, the isotope "Tc is a byproduct of the thermal nuclear fission of U , U , and Pu at 6.1%, 4.8%, and 5.9% yields, respectively (/), and significant quantities of Tc exist at many DOE sites. "Tc exhibits rather weak β' decay ( E ^ = 0.292 keV), but it is of particular concern for two reasons: its long half life (2.13 χ 10 y) and the high solubility of its most common form in aqueous environmental media, pertechnetate (Tc0 ) (2). Pertechnetate does not readily adsorb to most minerals, and therefore in aqueous form and under suitable conditions, it may rapidly present itself to subsurface waters (3, 4). The goal of our current EMSP funded research has been to develop a sensor for Tc, capable of exceeding the U.S. Environmental Protection Agency (EPA) 900 pCi/L (-5 χ 10" M) standard for drinking water. Our previous research focused on a novel sensing technology combining three modes of selectivity 2 3 5

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Downloaded by MIT on April 20, 2013 | http://pubs.acs.org Publication Date: April 19, 2005 | doi: 10.1021/bk-2005-0904.ch013

308 (electrochemistry, spectrophotometry, and selective partitioning) into a single sensing technique, spectroelectrochemistry. We demonstrated that spectroelectrochemistry performed using ion-exchange-polymer-doped films coated on waveguides that also function as optically transparent electrodes (OTEs) provides a unique strategy to detect analytes in the presence of interferences (5). The ability of the ion-exchange-polymer-doped film to perform charge exclusion while increasing the concentration of die target analyte near the electrode surface combined with electrochemical modulation of an optical signal for quantification provides an effective strategy for the detection of many analytes. One such analyte that has been successfully detected and quantified by the spectroelectrochemical sensor is ferrocyanide. Ferrocyanide is a good example of an analyte that exhibits a large optical change when subjected to electrochemical modulation between ferrocyanide and ferricyanide. The concentration of ferrocyanide in some high-level radioactive waste storage tanks at the DOE Hanford Site is of importance. In the previous EMSP symposium and subsequent publication, we described the concept of sensing ferrocyanide using the spectroelectrochemistry approach (