title: Incorporation and retention of 99-Tc(IV) in magnetite under high pH conditions.
abstract: Technetium incorporation into magnetite and its behavior during subsequent oxidation has been investigated at high pH to determine the technetium retention mechanism(s) on formation and oxidative perturbation of magnetite in systems relevant to radioactive waste disposal. Ferrihydrite was exposed to Tc(VII)(aq) containing cement leachates (pH 10.5-13.1), and crystallization of magnetite was induced via addition of Fe(II)aq. A combination of X-ray diffraction (XRD), chemical extraction, and X-ray absorption spectroscopy (XAS) techniques provided direct evidence that Tc(VII) was reduced and incorporated into the magnetite structure. Subsequent air oxidation of the magnetite particles for up to 152 days resulted in only limited remobilization of the incorporated Tc(IV). Analysis of both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data indicated that the Tc(IV) was predominantly incorporated into the magnetite octahedral site in all systems studied. On reoxidation in air, the incorporated Tc(IV) was recalcitrant to oxidative dissolution with less than 40% remobilization to solution despite significant oxidation of the magnetite to maghemite/goethite: All solid associated Tc remained as Tc(IV). The results of this study provide the first direct evidence for significant Tc(IV) incorporation into the magnetite structure and confirm that magnetite incorporated Tc(IV) is recalcitrant to oxidative dissolution. Immobilization of Tc(VII) by reduction and incorporation into magnetite at high pH and with significant stability upon reoxidation has clear and important implications for limiting technetium migration under conditions where magnetite is formed including in geological disposal of radioactive wastes.
authors.affiliation: Research Centre for Radwaste Disposal and Williamson Research Centre for Molecular Environmental Science, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester , Manchester, M13 9PL, United Kingdom.
authors.forename: Timothy A
authors.initials: TA
authors.lastname: Marshall
authors.forename: Katherine
authors.initials: K
authors.lastname: Morris
authors.forename: Gareth T W
authors.initials: GT
authors.lastname: Law
authors.forename: J Frederick W
authors.initials: JF
authors.lastname: Mosselmans
authors.forename: Pieter
authors.initials: P
authors.lastname: Bots
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Parry
authors.forename: Samuel
authors.initials: S
authors.lastname: Shaw
mesh_terms: D000327:Adsorption; D011232:Chemical Precipitation; D005290:Ferric Compounds / Q000737:chemistry; D052203:Ferrosoferric Oxide / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D010084:Oxidation-Reduction; D012996:Solutions; D013667:Technetium / Q000737:chemistry* / Q000302:isolation & purification; D056928:X-Ray Absorption Spectroscopy; D014961:X-Ray Diffraction
chemical_list: D005290:Ferric Compounds; D012996:Solutions; C000499:ferric oxide; D013667:Technetium; D052203:Ferrosoferric Oxide