PNNL

Abstract

Pertechnetate (TcO4-) is the most abundant chemical form of radioactive contaminant 99Tc present in the legacy nuclear waste streams and in the subsurface of the nuclear waste storage sites.One proposed remediation approach is reductive separation of TcO4-and sequestration in low-temperature waste forms. The development of the relevant technologies has been slow due to the lack of reductive materials that retain their functionalityand are otherwise suitable for application in multicomponent or aggressive media such as highly alkaline, brine-like solutions typifying nuclear tank wastes. This research prepared a tin-based reductive material and demonstrated its potential utility for the separation of TcO4- from the alkaline nuclear wastes. This material consists of Sn(II/IV) phosphate supported by the polycrystalline aluminophosphate matrix. The aluminophosphate matrix is inert to the reaction conditions and offers the benefits of high stability and low solubility in the concentrated alkaline solutions. This Sn(II/IV)-basedmaterial exhibitshigh loading capacity for Tc and selectively removed major fraction of TcO4-from the tank waste supernatant simulants, which contains 7.8 M total sodium and 2.43 M free hydroxide concentrations. The observed Kd values for Tc are about 13,000 and 2,200 mL g-1 for the simulant solutions containing no or 33 mM Cr(VI), respectively, positioning Sn(II/IV) aluminophosphate among the best performing reductive sorbents for TcO4- developed to date. This advanced behaviour is attributed to the synergistic combination of the Sn(II/IV) aluminophosphate functionalities. The presence of Sn(II/IV)-rich fibres facilitates the reduction of TcO4- to Tc(IV), which getsimbedded along the fibre branches. Importantly, the Sn(IV)-containing inert polycrystalline matrix alsoincorporates Tc(IV)which triggers its crystallization to rutile SnO2 phase andstabilizesTc(IV) in the polycrystalline matrix.