Leaching of technetium from engineered waste forms and subsequent transport through far-field environments is governed by coupled hydrological and chemical processes, primarily the oxidation of sparsely soluble, immobile Tc(IV) to mobile Tc(VII) species by infiltrating dissolved oxygen in pore waters. The objective of this study was to characterize changes in speciation of Tc during sequestration and leaching from cementitious reducing grout waste forms and integrate the conceptual model developed from these studies into a reactive transport simulation. X-ray absorption analysis of Tc, Fe, and S speciation in cementitious waste forms confirm that variable S species contribute to the long-term stability of partially sufidized or polymerized Tc(IV) which did not leach from the waste form after 485 days. All leached technetium was determined to be pertechnetate in batch leaching tests under oxidizing and reducing conditions, indicating infiltration of dissolved oxygen as the primary means of Tc(IV) oxidation. Technetium transport in field lysimeters experiments 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 moved into the 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 Kd = 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 generalized conceptual chemical model could not full explain the observed lysimeter transport behavior, demonstrating the importance of coupling hydrologic and chemical models into integrated reactive transport simulations. The results of this study suggest a strong redox coupling between Tc and S within reducing grouts that limits Tc(IV) oxidation and the high mobility of leached pertechnetate under environmental conditions at the Savannah River Site.

Long-term field and laboratory studies of technetium sequestration in reducing grout waste forms

ACS Fall 2021

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