Sajih, and 7 co-authors, reported in 2014 that radium and barium uptake onto ferrihydrite and goethite have been studied in the concentration range 1 nM to 5 mM and from pH 4 to 10, to develop a model to predict radium behavior in legacy uranium mining wastes. For ferrihydrite, uptake of Ra2+ at nM concentrations was strong at pH >7. At higher concentrations, Ba2+ sorption to ferrihydrite was slightly weaker than that of Ra2+. 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 behavior 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 Rd 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.
The authors concluded that, as expected, radium sorption to both ferrihydrite and goethite is linear up to radium concentrations of 12 nM (2.71 ppb). For ferrihydrite and radium, a typical sorption step is observed, centred at pH ≈ 6.25. The barium step is slightly displaced by approximately 0.3 pH units, indicating slightly weaker sorption. The radium interaction with ferrihydrite becomes stronger with increasing pH, with Rd increasing from 45 to 470,000 l/kg across the pH range 6– 9. Radium uptake by goethite is slightly weaker; Rd increases from 32 to 220,000 l/kg across the same pH range, and the sorption step is at pH ≈ 7. For both ferrihydrite and goethite in a 0.1 M NaClO4 background electrolyte system, the interaction with Ra2+ is completely reversible, so that thermodynamic modelling approaches (such as surface complexation modelling) are suitable for predicting the effect of sorption to these phases and Ra2+ mobility in the environment. Further, there is no evidence for incorporation of radium within goethite during phase transformation from ferrihydrite to goethite. Hence, a coupled chemical transport model that contains routines to account for sorption to ferrihydrite and goethite should be able to simulate radium behavior, even where mineral phase transformations are taking place due to, for example, bioreduction processes (e.g., Thorpe et al., 2012). Calcium has been found to compete with radium for sorption to ferrihydrite, but the effect is only significant at high concentrations ([Ca] > 0.01 M at pH 7). Although barium is able to suppress radium sorption at lower concentrations (1 mM), it will have lower concentrations in the environment, and so
in most cases, competition will not be significant.
It has been established previously that sorption of radium in the environment is expected to be dominated by iron phases (e.g., Ames et al., 1983; Gonneea et al., 2008) and here analysis of the samples from a legacy uranium mining site has shown that uptake onto the in situ sediments is consistent with this model: The Rd values determined here for ferrihydrite and goethite do predict the solid: solution partition for the mine site samples. This is despite the fact that the mine site samples are predominantly calcite, quartz and silicate phases, and so it seems that radium behavior at the site may be controlled by small amounts of ferrihydrite-like and goethite-like phases that are coating the bulk minerals.
Surface complexation models have been developed to simulate the interactions of Ra2+ and Ba2+ with ferrihydrite and goethite. For both phases, two equations have been
defined for each ion with each surface type. All of the surface species are tetradentate in terms of a single surface functional group. For each surface, surface complexes with the same stoichiometry are able to simulate both radium and barium behaviour. However, different complexes are required for each phase: [(≡HfoOH)3 (≡HfoO)M]+ and [(≡HfoOH2)2(≡HfoOH)2M]4+for ferrihydrite; [(≡GoeO) (≡GoeOH)3M]+ and [(≡GoeOH)4M]2+ for goethite. Higher equilibrium constants are required for radium binding for both phases. This may be due to the large differences in concentration between the radium and barium data, or it could be due to a small intrinsic difference between the Ra2+ and Ba2+ interactions. Given that the interaction of radium with both phases is reversible and sorption is unaffected by phase transformations, a surface complexation approach should be able to predict radium solubility and mobility in the environment where these two phases dominate the iron mineralogy.
The authors concluded that the tetradentate model of Sverjensky provided the best fit to the experimental data. However, the ferrihydrite (hydrous ferric oxide) surface complexation model of Dzombak and Morel (1990) is very widely applied and complexation constants exist for a wide range of metal ions. Therefore, Appendix E contains the best possible fit to the ferrihydrite data using this approach, so that constants are available for this model too.
Managed & Built by I2M Consulting, LLC | © 2010-
