Abstract

Characterising the geochemistry and speciation of major contaminant radionuclides is crucial in order to understand their behaviour and migration in complex environmental systems. Organic complexing agents used in nuclear decontamination have been found to enhance migration of radionuclides at contaminated sites; however, the mechanisms of the interactions in complex environments are poorly understood. In this work, radionuclide speciation and sorption behaviour were investigated in order to identify interactions between four key radionuclides with different oxidation states (Cs(I) and Sr(II) as important fission products; Th(IV) and U(VI) as representative actinides), three anthropogenic organic complexing agents with different denticities (EDTA, NTA and picolinic acid as common co-contaminants), and natural sand (as simple environmental solid phase). A UV spectrophotometric and an IC method were developed to monitor the behaviour of EDTA, NTA and picolinic acid in the later experiments. The optimised methods were simple, applied widely-available instrumentation and achieved the necessary analytical figures of merit to allow a compound specific determination over variable background levels of DOC and in the presence of natural cations, anions and radionuclides. The effect of the ligands on the solubility of the radionuclides was studied using a natural sand matrix and pure silica for comparison of anions, cations and organic carbon. In the silica system, the presence of EDTA, NTA and, to a lesser extent, picolinic acid, showed a clear net effect of increasing Th and U solubility. Conversely, in the sand system, the sorption of Th and U was kinetically controlled and radionuclide complexation by the ligands enhanced the rate of sorption, by a mechanism identified as metal exchange with matrix metals. Experiments in which excess EDTA, NTA and picolinic acid (40 – 100 fold excess) were pre-equilibrated with Th and U prior to contact with the sand, to allow a greater degree of radionuclide complex formation, resulted in enhanced rates of sorption. This confirmed that the radionuclide complexes interacted with the sand surface more readily than uncomplexed Th or U. Overall this shows that Th and U mobility would be lowered in this natural sand by the presence of organic co-contaminants. In contrast, the complexation of Sr with the complexing agents was rapid and the effect of the ligands was observed as a net increase on Sr solubility (EDTA, picolinic acid) or sorption (NTA). As expected, Cs did not interact with the ligands, and showed rapid sorption kinetics. Finally, ESI-MS was used to study competitive interactions in the aqueous Th-Mn-ligand ternary system. Quantification presented a challenge, however, the careful approach taken to determine the signal correction allowed the competitive interactions between Mn and Th for EDTA to be studied semi-quantitatively. In an EDTA limited system, Th displaced Mn from the EDTA complex, even in the presence of a higher Mn concentration, which was consistent with the higher stability constant of the Th-EDTA complex.

Keywords

Organic complexing agents, Sorption, Speciation, Migration, EDTA, NTA, Picolinic acid, Th(IV), U(VI), Sr(II), Cs(I), Natural sand, Radionuclide

Document Type

Thesis

Publication Date

2010

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