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Radium and barium uptake onto ferrihydrite and goethite have been studied in the concentration range 1 nM to 5 mM and from pH 4 -10, to develop a model to predict radium behaviour in legacy uranium mining wastes. For ferrihydrite, uptake of Ra 2+ at nM concentrations was strong at pH > 7. At higher concentrations, Ba 2+ sorption to ferrihydrite was slightly weaker than that of Ra 2+ . Experiments with goethite showed weaker binding for both metal ions in all systems. The interactions of radium with both ferrihydrite and goethite are fully reversible. The behaviour of radium during transformation of ferrihydrite to goethite has been studied, and no evidence for irreversible incorporation within the goethite lattice was found; radium uptake to goethite was the same, whether or not it was present during its formation. Calcium competed with radium for ferrihydrite sorption only at high calcium concentrations (>10 mM). Barium is a more effective competitor, and a concentration of 1 mM reduced radium sorption. Sediment samples from a legacy uranium mining site have been analysed, and the in situ R d values are consistent with radium uptake by surface coatings of ferrihydrite or goethite like phases. Surface complexation models have been developed for radium sorption to ferrihydrite and goethite which simulate the experimental data successfully. In both cases, approaches based on a single surface functional group and tetradentate binding sites simulated the data successfully. These data could be used in underpinning the safety case for legacy mining sites.

Wetlands often act as sinks for uranium and other trace elements. Our previous work at a mining-impacted wetland in France showed that a labile noncrystalline U(IV) species consisting of U(IV) bound to Al−P−Fe−Si aggregates was predominant in the soil at locations exhibiting a Ucontaining clay-rich layer within the top 30 cm. Additionally, in the porewater, the association of U(IV) with Fe(II) and organic matter colloids significantly increased U(IV) mobility in the wetland. In the present study, within the same wetland, we further demonstrate that the speciation of U at a location not impacted by the clay-rich layer is a different noncrystalline U(IV) species, consisting of U(IV) bound to organic matter in soil. We also show that the clay-poor location includes an abundant sulfate supply and active microbial sulfate reduction that induce substantial pyrite (FeS 2 ) precipitation. As a result, Fe(II) concentrations in the porewater are much lower than those at clay-impacted zones. U porewater concentrations (0.02−0.26 μM) are also considerably lower than those at the clay-impacted locations (0.21−3.4 μM) resulting in minimal U mobility. In both cases, soil-associated U represents more than 99% of U in the wetland. We conclude that the low U mobility reported at clay-poor locations is due to the limited association of Fe(II) with organic matter colloids in porewater and/or higher stability of the noncrystalline U(IV) species in soil at those locations.

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Qualitative chemical information is used as a guideline for correlations between equilibrium constants or between equilibrium constants and atomic charges (deduced from quantum mechanics calculations). Pa(V) and Nb(V) hydrolysis constants are also recalculated from experimental data. log K3 Þ; and 10:0 5 AE 0:8 8 ðHPO 2À 4 Þ are estimated based on the trend of affinity for An cations in the series CO 2À 3 > HPO 2À 4 zSO 2À 3 > SO 2À 4 zS 2 O 2À 3 : These ideas and values are used to discuss U(IV) chemistry in S-containing ground-waters.