No description available.

No description available.

This research project investigated the interaction between natural organics acids and selected metal ions. The aim of the project was to provide quantitative data on the speciation of metal ions when placed in systems containing natural organic acids. It was envisaged that such data will assist in the risk assessment of the Drigg low level waste site in Cumbria. The formation and complexing ability of these natural organic acids is discussed and the classing of these acids into high molecular weight organic acids and low molecular weight organic acids. Initial investigations used a potentiometric technique to study the interaction between nickel and europium and selected low molecular weight organic acids which were thought to occur in significant concentrations in soils and groundwaters. These experiments confirmed existing critically assessed literature values, and provided an experimental methodology for further 'in-house' measurement of such values. In addition, studies were also performed on systems containing two competing organic acids. Studies of such systems showed no synergistic effect and that they could be modelled using individual stability constants. A comprehensive investigation was performed on the interaction of nickel and europium with humic acid. High Performance Size Exclusion Chromatography (HPSEC) was assessed for its applicability to study such systems and was found to be suitable. Stability constants were determined for europium and nickel with humic acid. All data was modelled using MINTEQA2, a geochemical speciation code. Further work on these systems was carried out by a column ion exchange technique which confirmed the HPSEC data. A comparison was then carried out between batch and column exchange with the conclusion that column gave lower metal bound to the humic at high concentrations due to competition from the resin. HPSEC was used to investigate systems of low molecular weight organic acids with humic acids. In these systems no evidence for mixed complexes was found and that there was good agreement between experimental data and model predictions. The role of humic acid at alkaline pH was also investigated. It was found that humic acid showed enhanced complexation with europium and that this was attributable to phenolic groups on the humic molecule. In conclusion, results have been generated which provide data for a number of important reactions that occur in the environment.

No description available.

No description available.

It is government policy that the UK’s intermediate level radioactive wastes (ILW) will be disposed of in a deep geological disposal facility (GDF), where cementitious materials will be ubiquitous. After ILW disposal, groundwater ingress through the engineered facility is expected, forming a hyperalkaline plume from the cementitious materials into the surrounding host rock. This will form a persistent, high pH, “chemically disturbed zone” over timescales of 105 - 106 years, that will evolve from pH >13 to pH 10 over time. In the deep subsurface, microbial processes, particularly metal reduction may immobilise redox active radioactive contaminants in the waste, yet these reactions remain poorly characterised under these extreme conditions. In this project, microbiologically-mediated Fe(III) reduction was explored under alkaline conditions in sediment from a lime workings site in Buxton, UK, as an analogue for an ILW impacted subsurface environment. In addition, the impact of these processes on radionuclide (U, Tc and Np) behaviour was considered. Microcosms were set up using sediments taken from the site, adjusted to pH 10, augmented with electron donor (organic acids with yeast extract) and Fe(III), U(VI), Tc(VII) or Np(V) as electron acceptors. Biogeochemical processes were monitored using geochemistry, microbial ecology and X-ray absorption spectroscopy (XAS) techniques. A cascade of microbial reduction processes occurred at pH 10 – 10.5 in all microbially active systems. In Fe(III) enriched systems, the dominant post-reduction mineral phase was magnetite and the rate and extent of Fe(III) reduction was increased in the presence of extracellular (AQDS, Aldrich humic acid) and endogenous (riboflavin) electron shuttles. In U(VI) supplemented sediment systems, partial U(VI) reduction occurred to a non-uraninite phase, which was susceptible to reoxidation by air (O2) and nitrate. By contrast, in Fe(III)-augmented microcosms, more complete U removal to solids was noted, with uraninite identified as the end product, which was also reoxidised by air (O2) and nitrate. In these experiments there was, however, evidence to suggest that uranium was associated with the reoxidised Fe(III) mineral. In Tc supplemented microcosm experiments, complete Tc(VII) reduction occurred in systems with and without added Fe(III). In the microcosms with no added Fe(III) however, only partial Tc removal from solution occurred, despite evidence for complete reduction, suggesting that soluble or colloidal Tc(IV) may be present. Moderate Tc reoxidation occurred with air (O2) in both systems with and without added Fe(III) however no Tc remobilisation occurred during reoxidation with added nitrate. XAS on Fe(III) enriched sediments that had been microbially reduced and then re-oxidised by air, indicated that Tc may be associated with the reoxidised Fe mineral phase in these experiments. In the Np experiments, significant Np(V) sorption to sediments with and without added Fe(III) occurred initially, followed by Np(V) bioreduction to Np(IV). In all experiments, microbial (16S rRNA gene) profiling suggested a role for novel Gram-positive bacteria in Fe(III) and radionuclide reduction. These results highlight the significance of microorganisms on radionuclide biogeochemistry at high pH and have implications for the safe disposal of intermediate level nuclear wastes.