The Effect of Ageing on the Clean-up of Dusts and Soils after a Nuclear Contamination Event

Cobalt is a waste product in many industrial processes and its most common radioactive isotope – 60Co – is a by-product of nuclear reactors. If released into the environment, cobalt would cause much harm as its stable isotope is toxic to plants and animals in large quantities and the radioactive isotope decays via gamma radiation. This thesis investigates the sorption of Co onto the surfaces of common soil constituents and composites of these, as well as several natural soils, to help aid decontamination procedures.
The key findings of this thesis were with regard to Co sorption behaviour to substances commonly found in soils. At high pH, Cobalt was found to form binuclear bidentate inner-sphere complexes with ferrihydrite, kaolinite, humic acid, as well as composites of the minerals with humic acid. At low pH, Co sorbed to kaolinite, humic acid, and the kaolinite-humic acid composite via outer-sphere complexes. STEM analysis showed that Co sorbed to one face of the kaolinite particles where high affinity AlOH sites are located. Above pH 5, humic acid behaves colloidally and Co bound to these colloidal particles are mobile. A surface complexation model was developed based on this information that was capable of modelling Co sorption to the end-member phases. However, Co sorption to the composite phases cannot be modelled assuming linear additivity of the end-member phases.
This model was further tested with Co sorption to soils with a range of constituents and carbon contents. Co sorption to peat was high across the entire pH range and the model produced a good fit to this. Sorption to the agricultural soil and sediment was low below pH 6, but increased to 100% by pH 8. Model fits to these replicated the overall behaviour but did not make accurate detailed predictions.
Co sorption to ferrihydrite and humic acid were not found to increase with ageing, though sorption to sorption to kaolinite increased up to 28 days. Co became incorporated into ferrihydrite with longer sorption periods, thus decreasing the amount of Co desorbed, and desorption by citric acid or HCl were the most effective due to dissolution of the ferrihydrite releasing incorporated Co. EDTA was the most effective at desorbing Co from kaolinite and humic acid as it desorbed Co via chelation. The most efficient method of decontamination based on these experiments would be to wash with EDTA within 7 days of the contamination event, as after this period of time EDTA removed from ferrihydrite similar amounts of Co as citric acid and HCl.
The findings of this thesis mark a significant advance in understanding the fundamental processes governing the sorption and desorption of Co in the environment. Particularly, it shows that while Co sorption to soils can be modelled assuming parameters based on sorption to pure phases, these models are missing the complexity required to fully model sorption to soils. Therefore it is essential for future work to further constrain the sorption processes controlling Co behaviour to soil phases. Furthermore, decontamination techniques are affected by the length of the sorption equilibrium period for iron oxides. This suggests that for soils with high Fe contents decontamination should be attempted as soon as possible.