PNNL

Abstract

A remote spectroelectrochemical senosr and instrumentation package is being developed for the detection of technetium as aqueous pertechnetate (TcO4-) 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 TcO4- reduction at bare indium tin oxide (ITO) optically transparent electrodes (OTEs) show a poorly formed reduction wave for cyclic voltammetry and precipitation of technetium oxide (TcO2) on the electrode surface. Similar experiments at ITO OTEs coated with thin films containing cationic polymers show partitioning of TcO4- into the films. Three films were investigated: poly(dimethyldiallylammonium chloride) (PDMDAAC) and quaternized poly(4-vinylpyridine) (QPVP), both immodilized in porous glass by the sol-gel process, and poly(vinylbenzyltrimethylammonium chloride) (PVTAC) copolymerized with poly(vinylalcohol). The largest enhancement in the cyclic voltammetry reduction wave for TcO4- was for QPVP. The electrochemical mechanism changes to favor formation of a relatively long-lived soluble species that ultimately converts to TcO2. The electrodeposition of technetium oxide in these films was shown to be a method for the quantitative spectroelectrochemical determination of TcO4- and has been verified using radiochemistry dose measurements and scanning electron microscopy.