Characterisation of thorium–ethylenediaminetetraacetic acid and thorium–nitrilotriacetic acid species by electrospray ionisation-mass spectrometry

Electrospray ionisation-mass spectrometry (ESI-MS) has been used to investigate the speciation of Th in the presence of two common organic complexing agents found in radioactive wastes, EDTA and NTA. These ligands may enhance radionuclide migration from nuclear wastes and contaminated land, and characterisation of the complexes formed will improve our understanding of their environmental mobility. When acetic acid and ammonia were used to adjust the pH, the dominant Th–EDTA species changed from the mixed ligand [ThEDTAac] − complex to [ThEDTAOH] − and [ThEDTA(OH) 2ac] 3− as the pH increased, with all species co-existing, to some extent, over the pH range (2.5–10.8). A previously suggested Th–NTA species, [ThNTA 2] 2−, dominates from pH 6.0–8.5 but at high pH (10.0–10.8) NTA was not able to solubilise Th to a measurable extent. In the presence of both EDTA and NTA, Th formed a mixed ligand complex, [ThEDTANTA] 3− which has also been characterised in independent experiments. The identification of the importance of [ThNTA 2] 2−, [ThEDTANTA] 3− and [ThEDTA(OH) 2] 2− (with an additional acetate ligand) highlights that these species are not included in current speciation databases and thus may impact on predictions of the environmental mobility of Th. ESI-MS has therefore been used to identify and confirm fundamentally important Th complexes. It has proved to be a useful tool for elucidating radionuclide speciation, adding to our knowledge of Th geochemistry and providing a means of critically examining existing stability constant data.