Leaching and transport of technetium from reducing cementitious waste forms in field lysimeters

Field lysimeters tests examined leaching of technetium-99 (⁹⁹Tc) from two types of cementitious waste forms and found that the presence of blast furnace slag reduced the overall leaching of ⁹⁹Tc from the waste form. The two cementitious waste forms were a slag-grout 45%/45%/10% mixture of fly ash, blast furnace slag, and cement, respectively, referred to as slag-grout or a 55%/45% mixture of cement and fly ash, respectively, referred to here simply as cement. Duplicate sources of each composition were buried in four lysimeters for approximately 10 months to evaluate leaching characteristics under natural meteorological conditions in South Carolina, USA. Effluent samples were collected four times during the experiment, and the distribution of ⁹⁹Tc in the sediment was determined by destructively segmenting the lysimeters at the end of the experiment. The transport of Tc within the lysimeter was simulated by assuming advection, dispersion, and sorption in partially saturated porous media, and by using a shrinking-core type approximation for the release of Tc from the source. The shrinking-core model predicted that the oxidation front created by the oxygenated infiltrating groundwater moved into the cementitious source at a rate of 14 μm/day. As this front moved through the source, Tc(IV) was oxidized to the highly mobile Tc(VII) (as TcO₄^-) species, which then was transported through the sediment primarily via advection due to a small partitioning coefficient (C_solid/C_aq; K_d = 0.14 mL/g). The simulations predicted a cycle of accumulation of Tc in sediment at the source between rainfall events, followed by downward advection due to infiltration during rainfall events. The anomalous upward movement of Tc peak was predicted to be due to upward flux caused by evaporation after the experiment was terminated by capping the lysimeter. These experiments demonstrate that Tc leaching from cementitious waste forms under simulated vadose zone oxidizing conditions can be reasonably approximated by the shrinking core model, and the migration of Tc through the sediment is profoundly influenced by the presence of slag in the grout formulation and hydraulic conditions due to the low sorption affinity of TcO₄^-.