Novel Pathways to Treat Radioactively Contaminated Land: Bisphosphonate Ligands Enhance 99Tc Sequestration by Phosphate Minerals

Abstract Treatment of 99Tc in contaminated environments remains an important challenge in the remediation of legacy nuclear sites, including Hanford (USA) and Sellafield (UK). The highly mobile pertechnetate anion (Tc(VII)O4–) dominates in oxic shallow subsurface environments. Immobilization driven by biogeochemical reduction of Tc(VII)O4– to poorly soluble Tc(IV) species has been a key focus for remediation approaches. Here, we explore a novel 99Tc remediation strategy employing bisphosphonate ligands (e.g., medronic acid, MDP), which form soluble 99Tc(IV)-phosphate complexes that can subsequently be incorporated into calcium (Ca)-phosphate phases. We explored this MDP ligand-based approach in batch experiments containing 99Tc, hydroxyapatite, sediment, and synthetic groundwater representative of key nuclear sites. 99Tc concentration and speciation were tracked using a multitechnique approach including X-ray absorption spectroscopy. Additionally, we explored MDP-enhanced Tc scavenging in the column experiments. Here, in situ planar and tomographic gamma imaging employing 99mTc as a radiotracer showed enhanced removal of MDP-ligated 99mTc(IV) under dynamic flow conditions and field-relevant 99mTc concentrations. Finally, the feasibility of microbially mediated transformations to form MDP-ligated Tc(IV) was examined to explore the potential for this approach to support in situ bioremediation of technetium contamination. Here, 99Tc(VII) was bioreduced in the presence of MDP using the metal-reducing bacteria Shewanella oneidensis MR-1 to form the Tc(IV)-MDP precursor for enhanced removal by Ca phosphate. Overall, these findings demonstrate that 99Tc(VII) (bio)reduction and complexation with Tc(IV)-MDP enhanced 99Tc scavenging by Ca-phosphate phases.