Pentavalent Uranium Incorporated in the Structure of Proterozoic Hematite.

Characterizing the chemical state and physical disposition of uranium that has persisted over geologic time scales is key for modeling the long-term geologic sequestration of nuclear waste, accurate uranium-lead dating, and the use of uranium isotopes as paleo redox proxies. X-ray absorption spectroscopy coupled with molecular dynamics modeling demonstrated that pentavalent uranium is incorporated in the structure of 1.6 billion year old hematite (α-Fe2O3), attesting to the robustness of Fe oxides as waste forms and revealing the reason for the great success in using hematite for petrogenic dating. The extreme antiquity of this specimen suggests that the pentavalent state of uranium, considered a transient, is stable when incorporated into hematite, a ubiquitous phase that spans the crustal continuum. Thus, it would appear overly simplistic to assume that only the tetravalent and hexavalent states are relevant when interpreting the uranium isotopic record from ancient crust and contained ore systems.

Intermobility of barium, strontium, and lead in chloride and sulfate leach solutions.

Production of radionuclide-free copper concentrates is dependent on understanding and controlling the deportment of daughter radionuclides (RNs) produced from 238U decay, specifically 226Ra, 210Pb, and 210Po. Sulfuric acid leaching is currently employed in the Olympic Dam processing plant (South Australia) to remove U and fluorine from copper concentrates prior to smelting but does not adequately remove the aforementioned RN. Due to chemical similarities between lead and alkaline earth metals (including Ra), two sets of experiments were designed to understand solution interactions between Sr, Ba, and Pb at various conditions. Nanoscale secondary ion mass spectrometry (NanoSIMS) isotopic spatial distribution maps and laser ablation inductively coupled-plasma mass spectrometry transects were performed on laboratory-grown crystals of baryte, celestite, and anglesite which had been exposed to different solutions under different pH and reaction time conditions. Analysis of experimental products reveals three uptake mechanisms: overgrowth of nearly pure SrSO4 and PbSO4 on baryte; incorporation of minor of Pb and Ba into celestite due to diffusion; and extensive replacement of Pb by Sr (and less extensive replacement of Pb by Ba) in anglesite via coupled dissolution-reprecipitation reactions. The presence of H2SO4 either enhanced or inhibited these reactions. Kinetic modelling supports the experimental results, showing potential for extrapolating the (Sr, Ba, Pb)SO4 system to encompass RaSO4. Direct observation of grain-scale element distributions by nanoSIMS aids understanding of the controlling conditions and mechanisms of replacement that may be critical steps for Pb and Ra removal from concentrates by allowing construction of a cationic replacement scenario targeting Pb or Ra, or ideally all insoluble sulfates. Experimental results provide a foundation for further investigation of RN uptake during minerals processing, especially during acid leaching. The new evidence enhances understanding of micro- to nanoscale chemical interactions and not only aids determination of where radionuclides reside during each processing stage but also guides development of flowsheets targeting their removal.

Ab initio crystal structures and relative phase stabilities for the aleksite series, PbnBi4Te4Sn+2.

Density functional theory methods are applied to crystal structures and stabilities of phases from the aleksite homologous series, PbnBi4Te4Sn+2 (n = homologue number). The seven phases investigated correspond to n = 0 (tetradymite), 2 (aleksite-21R and -42R), 4 (saddlebackite-9H and -18H), 6 (unnamed Pb6Bi4Te4S8), 8 (unnamed Pb8Bi4Te4S10), 10 (hitachiite) and 12 (unnamed Pb12Bi4Te4S14). These seven phases correspond to nine single- or double-module structures, each comprising an odd number of atom layers, 5, 7, (5.9), 9, (7.11), 11, 13, 15 and 17, expressed by the formula: S(MpXp+1)·L(Mp+1Xp+2), where M = Pb, Bi and X = Te, S, p ≥ 2, and S and L = number of short and long modules, respectively. Relaxed structures show a and c values within 1.5% of experimental data; a and the interlayer distance dsub decrease with increasing PbS content. Variable Pb-S bond lengths contrast with constant Pb-S bond lengths in galena. All phases are n-fold superstructures of a rhombohedral subcell with c/3 = dsub*. Electron diffraction patterns show two brightest reflections at the centre of dsub*, described by the modulation vector qF = (i/N) ·â€…dsub*, i = S + L. A second modulation vector, q = γ ·â€…csub*, shows a decrease in γ, from 1.8 to 1.588, across the n = 0 to n = 12 interval. The linear relationship between γ and dsub allows the prediction of any theoretical phases beyond the studied compositional range. The upper PbS-rich limit of the series is postulated as n = 398 (Pb398Bi4Te4S400), a phase with dsub (1.726 Å) identical to that of trigonal PbS within experimental error. The aleksite series is a prime example of mixed layer compounds built with accretional homology principles.

Selective radionuclide co-sorption onto natural minerals in environmental and anthropogenic conditions.

Anthropogenic activities can redistribute the constituents of naturally occurring radioactive materials (NORM), posing potential hazards to populations and ecosystems. In the present study, the co-sorption of several RN from the U decay chain- 238U, 230Th, 226Ra, 210Pb and 210Po, onto common minerals associated with mining activities (chalcopyrite, bornite, pyrite and barite) was investigated, in order to identify the various factors that control long-term NORM mobility and retentivity in environmental acid-mine drainage systems and hydrometallurgical processing. The results show selective RN co-sorption to the various natural minerals, suggesting that mineral-specific mechanisms govern the variability in NORM mobility and retentivity. Both 226Ra and 210Po underwent significant sorption onto the natural minerals investigated in this study. The order of co-sorption in sulfate media for chalcopyrite and bornite was 210Po>226Ra>206Pb>210Pb>238U/230Th. Conversely, both pyrite and barite showed increased affinity for 226Ra; the order of co-sorption in sulfate media was 226Ra>210Po>206Pb/210Pb>238U/230Th for pyrite and 226Ra>206Pb/210Pb>230Th/238U/210Po for barite. Similar orders of co-sorption were observed in the nitrate media: for chalcopyrite and bornite the order was 210Po>226Ra/206Pb/210Pb/238U/230Th compared to 226Ra>210Po/206Pb/210Pb/238U/230Th for pyrite and barite. The behavior of 210Po was found to the anomalous: in both sulfate and nitrate solutions, 210Po had little affinity for barite compared to the sulfides. Thermodynamic modeling indicated the formation of a reduced PoS(s) phase at the surface of sulfide minerals, leading to the suggestion that 210Po likely undergoes reductive precipitation on the surface of sulfide minerals. The high sorption of both 206Pb and 210Pb observed in the sulfate systems were likely as a result of co-precipitation as insoluble anglesite compared to nitrate where they mainly remained in solution. Overall, barite showed the highest affinity for 226Ra, given its propensity to sorb 226Ra (similar ionic size). Both 238U and 230Th were highly mobile in acidic sulfate and nitrate solutions. The results highlighted here identify the various constraints on the natural variability and fractionation of NORM in the environment, as well as the mineral-specific mechanisms that control co-sorption of RN. This information provides a framework for predicting RN transport within soils and ground waters with variable geochemical conditions and in metallurgical extraction processes, in order to develop effective strategies towards NORM mitigation.

Localised solution environments drive radionuclide fractionation in uraninite.

We explore the role of various solution environments - chloride brines, acid mine drainage (sulfate) and groundwater (carbonate), as well as pore pressure in producing secular disequilibrium among the various radionuclides (RN) in the U-decay series upon leaching of uraninite - the most abundant U-ore and a widespread accessory mineral in U-rich rocks. We observed that the end products of the U-decay chain, 206Pb and 207Pb, exist primarily at the surface/edges of grains or within large pores in the uraninite. In contrast, the intermediate daughters 226Ra, 210Pb, 210Po, and 234/230Th, exist primarily within the bulk of uraninite, requiring breakdown by leaching for subsequent mobility to occur. Overall, pore pressure had little effect on RN mobility, with solution environment being the primary factor in creating significant mobility and disequilibrium among the RN, as it drives the initial breakdown of uraninite and influences the subsequent differential solubility of individual RNs. This was particularly the case for carbonate-bearing fluids, leading to significant fractionation of the various daughter RN arising from variable complexation and sorption phenomena. Understanding the geochemical behaviour of the RN in the U-decay series is important for predicting and managing the risks associated with RN in both environmental (acid-mine drainage) and engineered (metallurgical extraction) processes. Effective modelling of long-term RN behaviour should incorporate this strong relative fractionation caused by contrasting geochemical behaviour of individual RN during and after their release into the water from uraninite and subsequent interaction with the surrounding aquifer host rocks.

Understanding the mobility and retention of uranium and its daughter products.

Knowledge of the behavior of technologically enhanced naturally occurring radioactive materials derived through the decay of U and its daughter products, and their subsequent fractionation, mobilization and retention, is essential to develop effective mitigation strategies and long-term radiological risk prediction. In the present study, multiple state-of-the-art, spatially resolved micro-analytical characterization techniques were combined to systematically track the liberation and migration of radionuclides (RN) from U-bearing phases in an Olympic Dam Cu flotation concentrate following sulfuric-acid-leach processing. The results highlighted the progressive dissolution of U-bearing minerals (mainly uraninite) leading to the release, disequilibrium and ultimately upgrade of daughter RN from the parent U. This occurred in conjunction with primary Cu-Fe-sulfide minerals undergoing coupled-dissolution reprecipitation to the porous secondary Cu-mineral, covellite. The budget of RN remaining in the leached concentrate was split between RN still hosted in the original U-bearing minerals, and RN that were mobilized and subsequently sorbed/precipitated onto porous covellite and auxiliary gangue mineral phases (e.g. barite). Further grinding of the flotation concentrate prior to sulfuric-acid-leach led to dissolution of U-bearing minerals previously encapsulated within Cu-Fe-sulfide minerals, resulting in increased release and disequilibrium of daughter RN, and causing further RN upgrade. The various processes that affect RN (mobility, sorption, precipitation) and sulfide minerals (coupled-dissolution reprecipitation and associated porosity generation) occur continuously within the hydrometallurgical circuit, and their interplay controls the rapid and highly localized enrichment of RN. The innovative combination of tools developed here reveal the heterogeneous distribution and fractionation of the RN in the ores following hydrometallurgical treatment at nm to cm-scales in exquisite detail. This approach provides an effective blueprint for understanding of the mobility and retention of U and its daughter products in complex anthropogenic and natural processes in the mining and energy industries.