Sanders, Heather K.
(2018)
Interactions between anionic radionuclides (129I, 79Se and 99Tc) and soil geocolloids.
PhD thesis, University of Nottingham.
Abstract
The aim of this work was to investigate the interactions of anionic radionuclides 129I, 77Se (as a proxy for 79Se) and 99Tc with soil geocolloids under a range of conditions. These anionic fission products are of specific concern to policy makers regarding human and environmental risk assessments. Previous research has demonstrated strong links between soil organic matter (SOM) content and reduced mobility of these radionuclides. Negatively charged humic substances (HS), such as humic acid (HA) and fulvic acid (FA), may constitute 80% of organic matter and the mechanisms that allow anionic radionuclide to interaction with these HSs are not well understood. In the case of all three radionuclides, speciation plays a significant role in controlling their environmental mobility, therefore HPLC and SEC coupled to ICP-MS was used to monitor the speciation changes as the isotopes were progressively incorporated into HA. X-ray absorption spectroscopy was also employed in order to establish the solid phase speciation of Se after reaction with soil geocolloids.
Surface charge development of the HA significantly affected reaction with iodate (129IO3-) and iodide (129I-). Iodide added to HA systems demonstrated slow oxidation and formation of organically bound iodine (Org-129I) predominantly at higher pH (pH 6). Conversely IO3-, was rapidly transformed to form both I- and Org-I. As pH decreased, the rate of this reduction reaction increased. Increasing HA concentration also increased the rate of IO3- reduction and formation of Org-I. Previous research has suggested that the most likely mechanism is IO3- reduction to I2 or HOI which then binds with phenolic groups on OM forming Org-I species. However, IO3- was observed to rapidly bind to HA forming Org-I species with no initial evidence of I- formation; I- concentration then increased over time as Org-I decreased. Where Fe2+/Fe3+ was present increased reduction of IO3- to I- was observed, mediated by association with HA, resulting in less Org-I formation overall. Instantaneous reaction of I- with HA was observed in the presence of Fe2+/Fe3+, with bonding via cation bridging. Some I- was subsequently re-released as I- likely due to ongoing Fe hydrolysis. Modelling of the systems alone was successful and will assist the improvement of whole soil assemblage models.
Selenite (Se(IV)) reaction with HA was most rapid at low pH, with minimal/no reduction occurring at > pH 6. Reduction of selenate (Se(VI)) also occurred but this was less than for Se(IV), at low pH. No formation of Se(VI) from Se(IV) was observed, suggesting no oxidation took place, however some formation of Se(IV) from Se(VI ) was observed, also the formation of an unknown Se species suspected to be organic in nature. Humic acid concentration had no significant effect on the rate of Se(IV) or Se(VI) reduction, suggesting that HA itself was not responsible for the reduction. X-ray absorption spectroscopy (XAS) demonstrated the potential for significant reduction to Se(0) at pH 4 and bonding through a Se-O-C chain. The role of microbial communities on Se(IV) and Se(VI) reduction in the HA systems was demonstrated through the use of soil inoculum and glucose additions in sterile and non-sterile systems. No reduction of Se(IV) or Se(VI) and bonding to HA was observed in filter and -irradiation systems. Additions of inoculum and glucose increased the rate of reduction. Additions of Fe2+ did not increase reduction of Se(IV) or Se(VI) when compared to non-sterile HA systems, however XAS analysis demonstrated formation of HA-Fe cation bridges.
No reaction of pertechnetate (99Tc(VII)) with HA was observed in these aerobic systems. An unknown Tc species was occasionally observed (<0.005 µ L-1) and it is possible that this is an organic-Tc species. Significant incorporation of Tc into the solid phase was observed in aerobic soils, with most Tc(VII) being retained in soils with high OM contents and low pH.
The mechanisms considered here build upon the basic processes considered in current biosphere models for I and Se. Assemblage models must be used in order to reliably model the interactions of elements within soils due to the complexity of the systems. In order to understand the long-term radiological risks associated with geological repositories, the fine-scale mechanisms must be understood geochemically across a range of different soil types and conditions. The effect of I and Se speciation on bioavailability in soils determines both the potential transfer of radioactive isotopes to the food chain from GDF’s and from aerial sources of contamination. Alongside this, the work also has significant implications for advising on cost-effect fertiliser application methods for both I and Se, in order to tackle nutrient deficiencies worldwide.
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