Installation of a pilot experimental trench at the Little Forest legacy site

During 2017, a pilot experimental trench was constructed at the Little Forest Legacy Site (LFLS). The objective of installing this trench was to facilitate experimental field-work aimed at further characterising the site, in particular the hydrology of the excavated trenches and of the near-surface layers in which the trenches are located. The test trench is of similar depth to the waste disposal trenches at the legacy site (3 metres) and extends 6 m in length. However, unlike the disposal trenches, the experimental trench contains no waste materials of any kind. Instead, the trench contains a number of sampling points and other instrumentation, and is back filled with river gravel to provide a uniform composition and maintain structural stability. It is intended that the pilot trench will be followed by other trenches with specific experimental objectives. The purposes of this report are to discuss the background, rationale for, and implementation of the facility; to provide a detailed description of the pilot trench; and to compile information and photographs documenting the excavation process. Although some preliminary hydrological data and comparisons with the legacy trenches are presented, the scientific data will be fully discussed and interpreted in future scientific reports

Kinetics of chlorite dissolution.

A model for the dissolution of chlorite has been developed based on fast ligand assisted proton attack of the alumina tetrahedra within the alumina-silica lattice followed by slower dissolution of the remnant silica lattice. While the rate determining step is within the silica dissolution regime, the rate is a function of the H+ and Al3+ concentrations and the dominant aqueous Al species. Individual rates may be described by a generic rate equation applicable across the spectrum of Al species: where rn is the rate subscripted for the nth Al species, k is the rate constant of the rate controlling step, K is the surface exchange constant, β is the solution stability constant subscripted for the Al species, a is the species activity subscripted for species and raised to the power of the stoichiometry, p and q are stoichiometric coefficients, z is the ligand charge and τ is the fractional coefficient for the precursor of the rate defining step. The observed rate is the sum of the individual rates. When the observed rate is in a domain of dominance for a single aluminium species and in the absence of strong complexing agents such as oxalate, the observed rate is proportional to (a3H+/aAl3+)τn. The model is supported by experimental data for the dissolution of chlorite over a pH range of 3–10 and temperature range 25–95°C. The results have hydrometallurgical application. © 2007, Elsevier Ltd

Environmental mobility of cobalt as influenced by solid phase characteristics and groundwater chemistry.

The adsorption of cobalt on samples from a potential waste repository site in an arid region was investigated in batch experiments, as a function of various solution phase parameters including the pH and ionic strength. The samples were characterized using a range of techniques, including BET surface area measurements, total clay content and quantitative X-ray diffraction. The statistical relationships between the measured cobalt distribution coefficients (Kd values) and the solid and liquid phase characteristics were assessed. The sorption of cobalt increased with the pH of the aqueous phase. In experiments with a fixed pH value, the measured Kd values were strongly correlated to the BET surface area, but not to the amount of individual clay minerals (illite, kaolinite or smectite). A further set of sorption experiments was undertaken with two samples of distinctive mineralogy and surface area, and consequently different sorption properties. A simple surface complexation model (SCM) that conceptualized the surface sites as having equivalent sorption properties to amorphous Fe-oxide was moderately successful in explaining the pH dependence of the sorption data on these samples. Two different methods of quantifying the input parameters for the SCM were assessed. While a full SCM for cobalt sorption on these complex environmental substrates is not yet possible, the basic applicability and predictive capability of this type of modeling is demonstrated. A principal requirement to further develop the modeling approach is adequate models for cobalt sorption on component mineral phases of complex environmental sorbents. © 2009, Elsevier Ltd.Ministry of Science and Technology of Korean Government; Korea Radioisotope Associatio

Applicability of surface area normalised distribution coefficients (Ka) for uranium sorption

Adsorption of radionuclides on soils and sediments is commonly quantified by distribution coefficients (Kd values). This paper examines the relationship between Kd values for uranium(VI) adsorption and the specific surface area (SSA) of geologic materials. We then investigate the potential applicability of normalising uranium (U) Kd measurements using the SSA, to produce ‘Ka values’ as a generic expression of the affinity of U for the surface. The data for U provide a reasonably coherent set of Ka values on various solid phases, both with and without ligands. The Ka representation provides a way of harmonising datasets obtained for materials having different specific surface areas, and accounting for the effects of ligands in different systems. In addition, this representation may assist in developing U sorption models for complex materials. However, a significant limitation of the Ka concept is that sorption of radionuclides at trace levels can be dominated by interactions with specific surface sites, whose abundances are not reflected by the SSA. Therefore, calculated Ka values should be interpreted cautiously. © 2011, Elsevie

Inhibition of Uranium (VI) Sorption on Titanium Dioxide by Surface Iron (III) Species in Ferric Oxide/Titanium Dioxide Systems.

Uranium (U(VI)) sorption in systems containing titanium dioxide (TiO2) and various Fe(M)-oxide phases was investigated in the acidic pH range (pH 2.5-6). Studies were conducted with physical mixtures of TiO2 and ferrihydrite, TiO2 with coprecipitated ferrihydrite, and with systems where Fe(III) was mostly present as crystalline Fe(III) oxides. The presence of ferrihydrite resulted in decreased U(VI) sorption relative to the pure TiO2 systems, particularly below pH 4, an unexpected result given that the presence of another sorbent would be expected to increase U(VI) uptake. In mixtures of TiO2 and crystalline Fe(III) oxide phases, U(VI) sorption was higher than for the analogous mixtures of TiO2 with ferrihydrite, and was similar to U(VI) sorption on TiO2 alone. X-ray absorption spectroscopy of the TiO2 system with freshly precipitated Fe(III) oxides indicated the presence Fe(III) surface phase that inhibits U(VI) sorption-a reaction whereby Fe(III) precipitates as lepidocrocite and/or ferrihydrite effectively blocking surface sorption sites on the underlying TiO2. Competition between dissolved Fe(III) and U(VI) for sorption sites may also contribute to the observed decrease in U(VI) sorption. The present study demonstrates the complexity of sorption in mixed systems, where the oxide phases do not necessarily behave in an additive manner, and has implications for U(VI) mobility in natural and impacted environments where Fe(III) (oxyhydr)oxides are usually assumed to increase the retention of U(VI). © 2012, American Chemical Society

Uranium sorption in iron oxide/titanium oxide systems

Not availabl

Uranium sorption on various forms of Ti02 - influence of surface area, surface charge and impurities.

Titanium dioxide has properties that make it an excellent substrate for experimental study and theoretical development of adsorption models, including negligible solubility and a near neutral point of zero charge 1. A number of different forms of Ti-oxide have been used in experimental studies, including hydrous Ti-oxide, anatase, rutile and various commercially available samples that contain a mixture of anatase and rutile. The aim of our work is to investigate uranium sorption phenomena and the influence of surface area, surface charge and impurities for a range of thoroughly characterised Ti-oxide surfaces. We have undertaken uranium(VI) sorption studies on a number of commercially available Ti oxides, some of which were aggressively pre-treated to remove inherent impurities. Characterisations performed on the various Ti-oxides comprised a range of chemical and physical methods including XRD, XRF, ATR FT-IR, chemical assays, BET determinations, and electroacoustic measurements. The sorption of U on these Ti oxides was studied by a baTch sorption method and the effect of pH, ionic strength, mass loading, and U concentration on uranium sorption was also investigated for several of these Tioxides. We found that the sorption of uranium (VI) on these Tioxides was extremely strong and much greater than many other common environmental sorbents on a surface area basis. Aggressive pre-treatment of one Ti-oxide significantly altered its isoelectric point, but did not appear to significantly impact its sorption behaviour. Differences in sorption behaviour between the various Ti-oxides were related to the surface area of these materials. The data provide insights into the effect of different source materials and surface properties on radionuclide sorption, and will be useful in assessing data obtained in diverse experimental studies involving Ti oxides

Sorption of U(VI) at the TiO2–water interface: An in situ vibrational spectroscopic study

Molecular-scale knowledge of sorption reactions at the water-mineral interface is important for predicting U(VI) transport processes in the environment. In this work, in situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy was used in a comprehensive investigation of the sorption processes of U(VI) onto TiO2. The high sensitivity of the in situ ATR FT-IR technique allows the study of U(VI) concentrations down to the low micromolar range, which is relevant to most environmental scenarios. A set of highly purified and well characterized TiO2 phases differing in their origin, the ratio of the most stable polymorphs (anatase and rutile), in specific surface area, isoelectric points and in particle size distribution was investigated. Irrespective of the composition of the mineral phase, it was shown that U(VI) forms similar surface complexes, which was derived from the antisymmetric stretching mode nu(3)(UO2) showing a characteristic shift to lower wavenumbers compared to the respective aqueous species. The availability of a fast scanning IR device makes it feasible to perform time-resolved experiments of the sorption processes with a time resolution in the sub-minute range. It is shown that during the early stages of the U(VI) uptake, a surface species on the mineral phase is formed, characterized by a significantly red-shifted absorption maximum which is interpreted as a bidendate inner-sphere complex. After prolonged sorption, the IR spectra indicate the formation of a second surface species showing a smaller shift compared to the aqueous species. These findings were verified by a series of spectroscopic experiments performed on a U(VI)-saturated surface, at different U(VI) concentrations, pH values and in the absence of atmospheric-derived carbonate. This work provides new molecular insights into the sorption processes of U(VI) on TiO2. Basic thermodynamic ideas of surface complexation are substantiated by in situ infrared spectroscopy. © 2012, Elsevier Ltd

Uranium sorption on various forms fo titanium dioxide - influence of surface area, surface charge and impurities

Titanium dioxide (TiO(2)) has often served as a model substrate for experimental sorption studies of environmental contaminants. However, various forms of Ti-oxide have been used, and the different sorption properties of these materials have not been thoroughly studied. We investigated uranium sorption on some thoroughly characterized TiO(2) surfaces with particular attention to the influence of surface area, surface charge, and impurities. The sorption of U(VI) differed significantly between samples. Aggressive pretreatment of one material to remove impurities significantly altered the isoelectric point, determined by an electroacoustic method, but did not significantly impact U sorption. Differences in sorption properties between the various TiO(2) materials were related to the crystallographic form, morphology, surface area, and grain size, rather than to surface impurities or surface charge. In-situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopic studies showed that the spectra of the surface species of the TiO(2) samples are not significantly different, suggesting the formation of similar surface complexes. The data provide insights into the effect of different source materials and surface properties on radionuclide sorption. © 2011, American Chemical Societ