99Tc will be present in significant quantities in radioactive
wastes including intermediate-level waste (ILW). The internationally
favored concept for disposing of higher activity radioactive wastes
including ILW is via deep geological disposal in an underground engineered
facility located ∼200–1000 m deep. Typically, in the
deep geological disposal environment, the subsurface will be saturated,
cement will be used extensively as an engineering material, and iron
will be ubiquitous. This means that understanding Tc biogeochemistry
in high pH, cementitious environments is important to underpin safety
case development. Here, alkaline sediment microcosms (pH 10) were
incubated under anoxic conditions under “no added Fe(III)”
and “with added Fe(III)” conditions (added as ferrihydrite)
at three Tc concentrations (10–11, 10–6, and 10–4 mol L–1). In the 10–6 mol L–1 Tc experiments with no
added Fe(III), ∼35% Tc(VII) removal occurred during bioreduction.
Solvent extraction of the residual solution phase indicated that ∼75%
of Tc was present as Tc(IV), potentially as colloids. In both biologically
active and sterile control experiments with added Fe(III), Fe(II)
formed during bioreduction and >90% Tc was removed from the solution,
most likely due to abiotic reduction mediated by Fe(II). X-ray absorption
spectroscopy (XAS) showed that in bioreduced sediments, Tc was present
as hydrous TcO2-like phases, with some evidence for an
Fe association. When reduced sediments with added Fe(III) were air
oxidized, there was a significant loss of Fe(II) over 1 month (∼50%),
yet this was coupled to only modest Tc remobilization (∼25%).
Here, XAS analysis suggested that with air oxidation, partial incorporation
of Tc(IV) into newly forming Fe oxyhydr(oxide) minerals may be occurring.
These data suggest that in Fe-rich, alkaline environments, biologically
mediated processes may limit Tc mobility.