This award supports Professor Lynn C. Francesconi at CUNY Hunter College to determine the chemical bonding, redox chemistry, and speciation of technetium in metal-oxide materials. Polyoxometalates (POMs) will serve as models for solid-state materials to garner critical information concerning the chemical and redox stability and speciation of 99Tc in metal oxide hosts. The specific objectives are: 1) An examination of the impact of the steric and electronic differences of the 1- and 2- Wells-Dawson POM isomers on the binding strength and redox properties of technetium. 2) A determination of the stability of a technetium-POM materials as a function of the steric and electronic features of the polyoxometalate combined with specific low valent technetium cores. 3) Development of a new approach for the reduction of pertechnetate and stabilization of the generated low valent Tc. This strategy employs tunable reduced POMs to transfer electrons to pertechnetate and stabilize the reduced Tc.
A broader impact of this project will be the training of postdocs and graduate students in radiochemistry and in the handling of radioactive materials. The research will also draw on the large pool of minority students in the sciences at Hunter and CUNY to bring diversity into the pool of radiochemists. The training component of this project is enhanced through collaborations with the University of Nevada-Las Vegas and Lawrence Berkeley National Laboratory.
The element Technetium (symbol: Tc) possesses only radioactive isotopes and is an artificial element; however Tc is not a rare element. One isotope of technetium, Tc-99, is a major by-product of Uranium-235 fission in nuclear power plants. It is estimated that 9.5 kg of Tc-99 are produced every day from nuclear power plants worldwide. Technetium-99 is mildly radiotoxic and possesses a half-life of 200,000 years rendering it a problem if released into the environment. Technetium-99 can be separated from spent nuclear fuel using the UREX (Uranium Extraction) process. The UREX process results in Tc-99 in the form of pertechnetate, the highest oxidation state of Tc (+7), i.e. where no electrons are associated with the outermost shell of Tc. Presently, there are no viable strategies to process Tc from separated pertechnetate into a suitable form for storage. Complicating the issue is that Tc tends to accept multiple electrons and lose multiple electrons depending on its chemical environment. The number of electrons surrounding the Tc nucleus is referred to as the oxidation state of technetium. For example, no electrons surrounding the Tc nucleus, the oxidation state of Tc is referred to as Tc +7; one electron surrounding the Tc nucleus: Tc +6, two electrons: Tc +5, three electrons: Tc +4, and so on. This NSF funded project examines the chemistry of technetium-99 in metal oxide molecules, called polyoxometalates (POMs) with the overall goal to identify specific oxygen environments of the POMs that form stable complexes with specific oxidation states of Tc. Intellectual Merit: 1. This project demonstrated that molecular coordination environments of POMs containing four oxygen atom donors can stabilize multiple oxidation states of technetium (Tc +4, Tc+5, Tc+6). Increasing the basicity (i.e. electron donation) of the four oxygen atoms of the POMs increases the ability to stabilize these oxidation states. 2. This project identified that POMs can serve as "electron transfer agents" that add electrons to pertechnetate to form the Tc +4 oxidation state and moreover, stabilize the Tc +4 either on the surface of the POM or by removing an electron and imbedding Tc +5 into the POM. The mechanism for this process is by photocatalysis using Ultraviolet light. Along with this, this project identified that a common inexpensive material, titanium dioxide, that is used in sunscreens, cosmetics, self-cleaning windows, and photocells, can perform the same function and may form a Tc +4 /titanium dioxide complex where the Tc +4 coats the titanium dioxide and cannot be removed. This Tc +4 /titanium dioxide complex is being considered as a precursor that can be further processed into a stable Tc +4 /titanium dioxide wasteform for storage of technetium. Broader Impacts: 1. This study identified potential oxide environments that stabilize the fission product technetium-99 . This information is important to nuclear scientists (chemists and engineers) who are designing wasteforms for technetium and fission products. The oxide environments of the molecular POMs that stabilize Tc-99 found in this study can be reproduced as solid-state oxide materials for storage of Tc-99. 2. This study identified a process that can seamlessly take pertechnetate from the waste stream (once separated from the spent nuclear fuel rods) to a stable wasteform. This will be investigated by the PI and collaborators at National Laboratory sites. 3. This project also provided exceptional training for postdoctoral fellows, graduate students and undergraduates in the issues and challenges associated with radioactive materials with applications in nuclear fuel cycle, environment, and in medical imaging and therapy. Moreover, the students were trained in handling radioactive materials and in the appropriate techniques and methods to study radioactive materials.
