Micrometer-sized magnetite synthesis using Fe(OH)2(s) as a precursor for technetium sequestration from liquid nuclear waste streams

Abstract

• Transformation of Fe(OH) 2 (s) to micrometer-sized magnetite in waste stream at 75 °C. • Solution pH/temperature/oxidation kinetics play key roles in final mineral products. • An increase in pH or Fe(II) amendment improved tc removal from waste streams. • Tc sequestration as TcO 2 •2H 2 O or through Tc(IV)-magnetite incorporation. • More tc incorporation into magnetite at elevated temperatures and pH conditions. Systematic batch experiments under variable adjusted physicochemical conditions were conducted to explore optimization of micrometer-sized magnetite synthesis for Tc sequestration from radionuclide waste streams using Fe(OH) 2 (s) as the precursor. Extensive solid characterizations using X-ray diffraction, microscopy and spectroscopic methods were performed to identify mineral phases, assess changes in particle morphology and size distribution, as well as Tc speciation and incorporation, in the produced mineral phases. The results show that the solution pH, temperature, and oxidation kinetics play key roles in the final mineral products. Micrometer-sized magnetite crystals (0.62–0.96 µm, on average) with well-defined dodecahedral or octahedral structures were synthesized under near neutral (~pH 8) or alkaline (~pH 13) conditions at 75 °C, respectively; whereas goethite dominated the end products at room temperature. An increase in pH at 75 °C improved Tc removal from 27% (near neutral pH) to 42% (alkaline pH), but the removal process remained inhibited by redox-competitive Cr(VI) present in the waste streams. By adding additional Fe(II) to the system, Tc sequestration was dramatically improved up to 87% without observable changes in the solid product. The sequestrated Tc existed as TcO 2 •2H 2 O and/or Tc(IV) incorporated into magnetite, where extended X-ray absorption fine structure (EXAFS) spectroscopy showed that more Tc was incorporated into magnetite at elevated temperatures and pH conditions, with complete Tc(IV) incorporation into magnetite occurring under 75 °C-pH 13 conditions. Our results indicate that optimal micrometer-sized magnetite can be produced for Tc sequestration by reacting Fe(OH) 2 (s) with a waste stream simulant under elevated pH (~13) and temperature (75 °C) conditions. The incorporation of reduced Tc(IV) into stable micrometer-sized magnetite provides a viable supplemental immobilizing technology that may be used to improve nuclear waste treatment and disposal needs.