Removal of Pertechnetate from Simulated Nuclear Waste Streams Using Supported Zerovalent Iron

Abstract

The application of zero-valent iron nanoparticles or "Ferragels," either unsupported or supported, to the separation and reduction of pertechnetate anions (TcO 4 -) from complex waste mixtures was investigated as an alternative approach to current waste processing schemes. Although applicable to pertechnetate-containing waste streams in general, the tests reviewed here were directed at two specific potential applications at the U.S. Department of Energy's Hanford Site: (1) the direct removal of pertechnetate from highly alkaline solutions, typical of those found in Hanford tank waste; and (2) the removal of dilute pertechnetate from near neutral solutions, typical of the eluate streams from commercial organic ion-exchange resins that may be used to remediate Hanford tank wastes. It was envisioned that both applications would involve the subsequent encapsulation of the loaded sorbent material into a separate waste form. A high surface area (>200 m 2 /g) base-stable, nano-crystalline zirconia was used as a support for Ferragel for tests with highly alkaline solutions, while a silica gel support was used for tests with near neutral solutions. It was shown that after 24 hours of contact time, the high surface area zirconia supported Ferragel sorbent removed about 50% (K d = 370 L/kg) of the pertechnetate from a pH 14 tank waste simulant containing 0.51 mM TcO 4 -and large concentrations of Na + , OH -, NO 3 -, and CO 3 2-for a phase ratio of 360 L simulant per kg of sorbent. It was also shown that after 18 hours of contact time, the silica supported Ferragel removed >95% pertechnetate from a neutral pH eluate simulant containing 0.076 mM TcO 4 -for a phase ratio of 290 L/kg. It was determined that in all cases, Ferragels reduced the Tc(VII) to Tc(IV), or possibly Tc(V), through a redox reaction. Finally, it was demonstrated that a mixture of 20 mass % of the solid reaction products obtained from contacting zirconia support Ferragel with an alkaline waste solution containing Re(VII) -a surrogate for Tc(VII) -with 80 mass % alkali borosilicate based frit heat treated at 700°C for 4 hours sintered into a handleable waste form. INTRODUCTIONA vast amount of solid and liquid radioactive waste had been generated during the approximately thirty years since the world's first full-scale nuclear reactors and processing plants needed for the production and isolation of plutonium-239 began operating in 1944 at the Hanford Site River in southeastern Washington,. One reference [1] indicates that these wastes consist of approximately 208,000 m 3 of mixed waste in 177 underground storage tanks. Current emphasis at U.S. Department of Energy (DOE) and commercial sites has been directed at remediation of such radioactive wastes.One plan for Hanford Site tank wastes, for example, has been to separate the waste into primarily solid, i.e., sludge containing insoluble high-activity waste (HAW) species as well as non-radioactive solids, and aqueous liquid portions that will contain tank supernatant and watersoluble species derived from HAW...

Current status and future direction for examining engineered nanoparticles in natural systems

“…[56][57][58] Other nanomaterials may form an oxide shell, altering the surface composition of the material and subsequently changing its physical and chemical properties. [59][60][61][62][63][64] In addition to chemical oxidation-reduction reactions, some materials may be susceptible to photooxidation and photoreduction, which can act to change the structure and properties of the ENM. Carbonaceous nanomaterials such as CNTs and fullerenes are prone to producing carboxy and hydroxyl groups on its surface as well as generating reactive oxygen species (ROS) in the presence of sunlight.…”

Tc(VII) reduction kinetics by titanomagnetite (Fe3−xTixO4) nanoparticles

“…Under oxidizing conditions, it exists predominantly as Tc(VII) in the form of the highly soluble pertechnetate anion [TcO 4 À ], a species that is generally resistant to adsorption on mineral surfaces over common pH values (Cui and Eriksen, 1996b). Under reducing conditions, it exists predominantly as Tc(IV) in the form of sparingly soluble solid phases, nominally TcO 2 ÁnH 2 O. Immobilizing subsurface TcO 4 À by reduction to its less soluble counterpart has therefore been widely regarded as a promising remediation strategy component (e.g., Fredrickson et al, 2004;Darab et al, 2007;Jaisi et al, 2009).…”

The application of zero-valent iron nanoparticles for the remediation of a uranium-contaminated waste effluent

“…The application of zero-valent INP for the remediation of radionuclides remains less widely researched than for the aforementioned heavy metals and organic contaminants. Studies are limited to the radioisotopes of Ba [27] and TcO 4 [24,28], and our group's U-sorption investigations [29,30]. It has long been known that scrap/bulk iron and iron-based minerals are highly effective scavengers of the uranium radionuclide [31,32].…”