Separation and processing of waste at Department of Defense (DoD) and Department of Energy (DOE) sites has resulted in a variety of high-level waste streams. In the past, separations have produced waste salts such as strontium fluoride and cesium chloride, and, currently, separations schemes are being considered for the removal of cesium (Cs), strontium (Sr), iodine, plutonium, and technetium from liquid waste streams at several DOE sites. In addition, waste streams that contain toxic heavy metals such as lead, mercury, chromium, and cadmium are present at many DoD sites. The need for an efficient separations and waste conversion process is common to all of these sites.
The project was designed to determine the feasibility of alternative glass or ceramic waste forms for the containment of titanium dioxide (TiO2)-rich and phosphate-rich wastes, which, unless they are greatly diluted, are either insoluble or incompatible with the baseline borosilicate glass currently proposed for the containment of radioactive wastes.
The project examined ion exchangers with the potential for removing hazardous cationic and anionic species from tank wastes in the context of conversion into final waste forms. The overall program included the following two components: (1) synthesis and/or evaluation of two exchangers thought to be effective at removal of species such as Cs+, Sr2+, and technetium oxide (TcO4). Exchangers were evaluated for chemical stability, selectivity and ion exchange capacity, and compatibility with existing ion exchange processes and (2) evaluation and development of methods for converting potential feed stocks produced via ion exchange into solid waste forms. A baseline activity included determining whether the potential feed is compatable with borosilicate waste glass. Alternate waste form options also were explored.
In place of using borosilicate glasses to contain TiO2-rich and phosphate-rich wastes, previous work had demonstrated that many cesium silicotitanate compositions could be melted directly to form durable glasses containing as much as 22 percent TiO2 by weight and form pollucite-like crystalline phases with leach resistances better than borosilicate glass.
Two new cesium silicotitanate zeolites with unique crystal structures that encapsulate cesium in covalently bonded molecular cages were discovered. These zeolites can be used as an ion exchanger to separate aqueous Cs ions and then can be further thermally processed to a stable waste form. Both of these zeolites as well as several other glass and ceramic compositions are highly durable and would serve as viable waste forms for containment of Cs sorbed onto silicotitanate ion exchangers. Iron phosphate glass was identified as an alternative host for phosphate and iron-rich tank waste and for cesium chloride and strontium fluoride capsule waste. Iron phosphate glasses also were shown to dissolve 30 percent (by weight) cesium chloride or strontium fluoride while maintaining exceptional durabilities (20 to 50 times better than borosilicate glass). This technology is targeted for remediation of waste at DOE sites. Potential applications also exist in the areas of ion exchange and catalysis.
Less dilution of the wastes of concern would reduce the volume of stabilized, immobilized, radioactive waste glass that must be stored, resulting in substantial cost and storage capacity savings. The silicotitanates and iron phosphate glasses developed in this project together could provide suitable waste forms to handle more than 90 percent of the radioactive wastes at the DOE Hanford, WA, site.