Inorganic tin aluminophosphate nanocomposite for reductive separation of pertechnetate

Abstract

Pertechnetate (TcO₄⁻) is the most abundant chemical form of the radioactive contaminant ⁹⁹Tc present in legacy nuclear waste streams and in the subsurface of nuclear waste storage sites. One proposed remediation approach is reductive separation of TcO₄⁻ and sequestration in low-temperature waste forms. The development of relevant technologies has been slow due to the lack of reductive materials that retain their functionality and are otherwise suitable for application in multicomponent and aggressive media such as highly alkaline, brine-like solutions typifying nuclear tank wastes. In this work, a tin-based reductive material was prepared, and its potential utility for the separation of TcO₄⁻ from alkaline nuclear wastes was demonstrated. This material consists of Sn(II/IV) phosphate supported by a polycrystalline aluminophosphate matrix. The aluminophosphate matrix is inert to the reaction conditions and offers the benefits of high stability and low solubility in concentrated alkaline solutions. This Sn(II/IV)-based material exhibits a high loading capacity for Tc and selectively removes a major fraction of TcO₄⁻ from the tank waste supernatant simulant, which contains 7.8 M total sodium and 2.43 M free hydroxide concentrations. Observed K_d values for Tc are about 13 000 and 2200 mL g⁻¹ for 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 is embedded along the fibre branches. Importantly, the Sn(IV)-containing inert polycrystalline matrix also incorporates Tc(IV) which triggers its crystallization to cassiterite SnO₂ phase and stabilizes Tc(IV) in the polycrystalline matrix.