Reactivity of Tc at the Groundwater-Surface Water Interface

Technetium-99 (t^1/2 =211,000y) is environmentally mobile as the pertechnetate oxyanion [⁹⁹Tc(VII)O₄^-_(aq)]. Tc(VII) may react to less soluble Tc(IV) at intermediate redox potentials (E^o = -0.36 V) through heterogeneous reduction with solid-phase biogenic reaction products. ⁹⁹Tc is forecast to migrate through groundwater to the Columbia River at the U.S. DOE Hanford site in Washington State. Discharge to surface water will occur through a groundwater-surface water interaction zone with complex hydrogeology and biogeochemistry that is stimulated by the overlapping nutrient regimes of groundwater and surface water. The reactivity of pertechnetate in reduced sediments from this zone was investigated to determine effects of biogenic ferrous-Fe and sulfide-S on Tc(VII) reduction rate; and the resulting speciation, mineral association, and physical location of Tc(IV).

⁹⁹Tc(VII) was reduced to near detection (<10^-9 Mol/L) over periods of days to months. Tc(VII) reduction rate was first order in [Tc(VII)]_aq and sediment mass, but correlations with specific biogenic reductant concentrations [(Fe(II), ferrous mono-sulfide] were not found. Tc(IV) was isolated to fine-grained aggregates (0.1 to 0.5 mm) of “mud”, consisting of primary mineral material embedded within a phyllosilicate or clay matrix. EXAFS revealed that product Tc(IV) existed as combinations of a Tc(IV)O₂-like phase ,Tc(IV)-Fe surface clusters, and/or TcS_x. Ferrous mono-sulfide was implicated as a more selective reductant. Migration of Tc(VII) through the interaction zone will be controlled by water residence time and the density and spatial distribution of fine-grained aggregates that host reductive biogeochemical processes in otherwise coarse-textured, partially oxygenated sediments.