Electroactive Materials for Anion Separation - Technetium from Nitrate

Many contaminants of interest to the U.S. Department of Energy (DOE) exist as anions. These include the high priority pollutants chromate, pertechnetate, and nitrate ions. In addition, there are also industrial and urban applications where the separation of anionic species from aqueous streams is critical. Examples include industrial water recycle and waste water treatment (e.g., chloride ion removal for the pulp and paper industry, borate ion in the chemical and nuclear industries) and drinking water and agricultural waste treatment (e.g., nitrate removal). In the proposed research, technetium is chosen as the target pollutant. Because of its half-life of 213,000 years, technetium (99Tc) presents a long-term hazard for waste disposal. Much of the 99Tc in the tank wastes is present as pertechnetate (TcO4-), accounting for its high solubility and mobility in aqueous systems. Several sorbents are available for removing TcO4- from alkaline waste brines, but each has important drawbacks. The use of commercial ion exchange (IX) resins to extract TcO4-, e.g., Reillex{trademark}-HPQ (Reilly Industries) and ABEC 5000 (Eichrom Industries), generates significant secondary waste. The elution of TcO4- from Reillex{trademark}-HPQ resins requires either concentrated nitric acid or a concentrated caustic solution of ethylene-diamine containing a small amount of tin chloride. This eluant has a short shelf life requiring frequent preparation, and the 99Tc is delivered in a complexed, reduced form. While TcO4- can be eluted from ABEC 5000 resins using de-ionized water, the much-reduced capacity of ABEC 5000 resins in comparison to the Reillex{trademark}-HPQ resins leads to a low column capacity. In general, unwanted secondary wastes are generated because (1) the only effective eluant happens to be hazardous and/or (2) the IX material has a low capacity or selectivity for the target ion, resulting in more frequent elution and column replacements. Alternative IX materials that have high capacities, can be regenerated easily, and are highly selective for TcO4- would avoid these problems. Electrochemically active IX media meet these criteria. Such an IX system uses electrically induced changes in the media to expel sorbed ions through a charge imbalance rather than requiring chemical eluants to ''strip'' them. Therefore, this medium eliminates the need to prepare, store, and dispose of many of the process chemicals normally required for IX operations. The focus of the project is to develop a fundamental understanding of how the physical and chemical properties of electroactive ion exchange (EaIX) materials control their efficiency when used as mass separation agents. Specifically, the desirable characteristics of EaIX materials for separation applications are (1) high reversibility, (2) high selectivity, (3) acceptable physical and chemical stability, (4) rapid intercalation and de-intercalation rates, and (5) high capacity. Because of these requirements, EaIX materials share many properties in common with conventional ion exchangers and electroactive polymers. For example, EaIX materials require the selectivity typically found in ion exchangers; they also require the redox reversibility of electroactive polymers. The results of this work will allow the rational design of new materials and processes tailored for the separation of specific anions