文摘
The application of nanoparticles of predominantly zerovalent iron (nanoiron), either unsupported orsupported, to the separation and reduction of pertechnetate anions (TcO4-) from complex waste mixtureswas investigated as an alternative approach to current waste-processing schemes. Although applicable topertechnetate-containing waste streams in general, the research discussed here was directed at two specificpotential applications at the U.S. Department of Energy's Hanford Site: (1) the direct removal ofpertechnetate from highly alkaline solutions, typical of those found in Hanford tank waste, and (2) theremoval of dilute pertechnetate from near-neutral solutions, typical of the eluate streams from commercialorganic ion-exchange resins that may be used to remediate Hanford tank wastes. It was envisioned thatboth applications would involve the subsequent encapsulation of the loaded sorbent material into a separatewaste form. A high surface area (>200 m2/g) base-stable, nanocrystalline zirconia was used as a supportfor nanoiron for tests with highly alkaline solutions, while a silica gel support was used for tests withnear-neutral solutions. It was shown that after 24 h of contact time, the high surface area zirconia supportednanoiron sorbent removed about 50% (Kd = 370 L/kg) of the pertechnetate from a pH 14 tank wastesimulant containing 0.51 mM TcO4- and large concentrations of Na+, OH-, NO3-, and CO32- for aphase ratio of 360 L simulant per kg of sorbent. It was also shown that after 18 h of contact time, thesilica-supported nanoiron removed >95% pertechnetate from a neutral pH eluate simulant containing0.076 mM TcO4- for a phase ratio of 290 L/kg. It was determined that in all cases, nanoiron reduced theTc(VII) to Tc(IV), or possibly to Tc(V), through a redox reaction. Finally, it was demonstrated that amixture of 20 mass % of the solid reaction products obtained from contacting zirconia-supported nanoironwith an alkaline waste solution containing Re(VII), a surrogate for Tc(VII), with 80 mass % alkaliborosilicate based frit heat-treated at 700 C for 4 h sintered into an easily handled glass compositewaste form.
