Role of natural organic matter in binding uranium and incorporating radiocarbon in soils and sediments
Muir, Michael Robert
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The impact of long-lived radionuclides on human health depends on their behaviour in near-surface soils and sediments and the LORISE project aims to define the key controlling physical, chemical and biological processes at a number of ‘natural laboratories’ around the UK. Within this PhD project, the role of natural organic matter (NOM) is of especial importance because NOM can potentially facilitate the transport or attenuation of radiologically significant elements such as U and radiocarbon (14C). Peaty soils in the vicinity of natural uranium mineralisations are often highly enriched in U with concentrations of up to 3000 mg kg-1, 4000 mg kg-1 and 2500 mg kg-1 having been found in the US (Owen and Otten, 1995), Switzerland (Regenspurg et al., 2010), and the UK (Xu, 2013), respectively. The NOM within these soils has been implicated in U retention but the controlling processes and the nature of interactions are poorly characterised. The Needle’s Eye natural mineralisation, SW Scotland, provides a rare opportunity within the UK to investigate long-term U-NOM interactions. Similarly, the well characterised inputs of radiocarbon into the Irish Sea from the Sellafield nuclear reprocessing facility since the mid-1960s present an opportunity to investigate the transfer of anthropogenic 14C between environmental pools and trace its incorporation into coastal sediments. While the enrichment of sediment organic matter with anthropogenic 14C has been identified, (MacKenzie et al., 2004), the chemical and physical characteristics of NOM enriched with 14C have not been investigated. Currently, there is not a good understanding of which NOM components govern U binding or incorporate 14C, and therefore the implications of the influence of NOM on radionuclide mobilisation or immobilisation are hard to assess. In this study, the characteristics of organic matter from the Needle’s Eye natural analogue site have been examined and the relationship with the geochemical properties of the site and U binding have been investigated. Further to this, NOM extracted from the Needle’s Eye peat bog and from sediments of the Solway coast at the Southwick Merse has been fractionated and characterised using a range of spectroscopic techniques to identify the components of NOM which are responsible for binding U and those which incorporate anthropogenic 14C. The investigation of the properties of NOM at the Needle’s Eye natural analogue site showed a high degree of spatial heterogeneity over a relatively small area, with variation in NOM characteristics seen between samples collected from different zones of the site, particularly evident in the transition from the highly organic, reducing peat bog to the inner saltmarsh. This was signified by an increase in aromaticity and humic character due to the change in vegetation from deciduous tree cover to dense saltmarsh grasses. NOM analysed in depth profiles from the highly organic, U-rich bog showed characteristics consistent with ageing and microbially driven humification processes, with an increase in humic acid content, decrease in oxygen containing functional groups and increased aromaticity with depth. The analysis of U and other elements in the same samples showed that while the character of NOM is not the dominant factor in U accumulation, the characteristics of the NOM in the U-enriched zone, particularly the high amounts of aliphatic and oxygen containing NOM, are ideal for U binding. Analysis of Fe(II) and Fe(III) in porewaters suggest highly dynamic redox properties in the zone of U accumulation, first seen by Xu, (2013) and confirmed by this study. The variability in redox conditions is likely to lead to Fe-humic colloid precipitation and co-precipitation of U, acting as a mechanism of U removal to the solid phase. Analysis of samples from the Needle’s Eye bog by x-ray absorption spectroscopy (XAS) confirmed, for the first time at this site, that the U was present in the U(VI) oxidation state and was directly bound to the oxygen functional groups of organic matter. The application of multiple NOM fractionation and characterisation techniques has made it possible to draw robust conclusions about the properties of NOM samples. The results showed the differences in NOM qualities between physically and chemically separated samples and samples of different origin. Analysis of U in characterised NOM samples showed that it was preferentially bound in the large size fraction, which had more humic and aliphatic character, while smaller size fractions of more fulvic and aromatic character had a lesser association with U. These results showed that the type of NOM with U associated had relatively lower mobility and can therefore effectively retard U mobility for as long as the geochemical conditions remain favourable. Changes in pH or redox conditions could increase the mobility of NOM associated U. Characterisation of NOM from the Southwick Merse salt marsh sediment using spectroscopic techniques and δ13C analysis showed variation with depth due to the origin of NOM, with a greater proportion of marine NOM at depth which decreased towards the surface due to terrigenous inputs. NOM in the depth profile and in separated size fractions showed the increasingly aromatic character of terrigenous NOM compared to that of marine origin. 14C analysis of extracted NOM showed 14C concentrations enriched above background levels due to the anthropogenic inputs from Sellafield reprocessing facility. Analysis of extracted and fractionated NOM showed that the 0.1 M NaOH extractable NOM had increased 14C activity compared to the non-extractable sediment organic matter. Analysis of NOM separated into size fraction showed that the NOM which was more enriched in 14C was found in the larger size fraction, and was characterised by a more aliphatic character. The results of δ13C analysis showed that this fraction, relatively enriched in 14C, is of marine origin.