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Title: Irreversible sorption of radionuclides resulting from 2-line ferrihydrite transformation
Author: Kenyon, Graham
ISNI:       0000 0004 7971 0928
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2017
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A major challenge for the remediation of contaminated land, and in the design of safe long-term geological high-level waste repositories, is an accurate understanding of radionuclide migration in the environment. This requires knowledge about the key processes that can affect radionuclide mobility. Fe(III) (oxyhydr)oxides (e.g. goethite, α-FeOOH) are reactive mineral phases common in subsurface terrestrial and marine environments and can strongly influence the mobility of radionuclides in contaminated environments through various sorption processes. In particular, irreversible sorption of radionuclides to Fe(III) (oxyhydr)oxides may act both to impede and facilitate radionuclide transport, depending on the mobility of the mineral phase. Further, a significant cause of irreversible sorption of radionuclides is incorporation, whereby metal ions become bound within the mineral structure. Indeed, there is a growing body of evidence that radionuclides, as well as other trace metal ions, can become incorporated during the transformation of poorly-ordered nanoparticulate ferrihydrite into the more thermodynamically stable and crystalline Fe(III) (oxyhydr)oxide phases of hematite and goethite. However, the studies so far have predominantly concerned incorporation at elevated radionuclide concentrations. As a consequence, there is need to study incorporation in these systems at “ultra-dilute” radionuclide concentrations relevant to the natural environment. The following study investigated the effects of ferrihydrite transformation to hematite and goethite on radionuclide sorption reversibility at ultra-dilute radionuclide concentrations. This was achieved through a combination of desorption kinetics measurements, to explore sorption reversibility, and acid extraction techniques, to probe any potentially incorporated radionuclides. A range of radionuclides were studied in the investigation, Am(III), Eu(III), U(VI), Th(IV), and Ra(II). The desorption kinetics results showed that up to 42 % Am(III), 30 % Eu(III), and 65 % U(VI) were irreversibly sorbed to the hematite/goethite phase. Indeed, the acid extractions suggest that this was likely due to radionuclide incorporation within the hematite/goethite phase. Acid extractions also suggest that ~70 % Th(IV) and ~20 % Ra(II) was likely incorporated into the hematite/goethite phase during ferrihydrite transformation. Interestingly, the study also highlights that the fraction of radionuclide incorporated during ferrihydrite transformation is independent of the radionuclide loadings on the ferrihydrite solids prior to transformation. This suggests that the incorporation mechanism is independent of the radionuclide loadings on the ferrihydrite solids and may indicate that incorporation is controlled by particle mediated crystal growth processes such as oriented attachment. Overall, these findings provide greater insight into the incorporation behaviour of radionuclides during ferrihydrite transformation, and thus allow the risk of radionuclide migration in the environment to be better understood and characterised.
Supervisor: Shaw, Samuel ; Morris, Katherine Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: colloid ; oriented aggregation. ; oriented attachment ; surface exchange ; radium ; iron oxide ; uranium ; europium ; americium ; Desorption ; thorium