Environmental stability of a uranium-plutonium-carbide phase.

A plutonium-rich carbide, (U,Pu)(Al,Fe)3C3, was discovered in a hot particle from the Maralinga nuclear testing site in South Australia. The particle was produced between 1960 and 1963 and has been exposed to ambient conditions since then. The new phase belongs to a group of ternary carbides known as 'derivative-MAX phases'. It formed at high temperature within an explosion cloud via rapid eutectic crystallisation from a complex Al-Fe-U-Pu-C-O melt, and is the major Pu host in this particle. Despite signs of volume expansion due to radiation damage, (U,Pu)(Al,Fe)3C3 remains highly X-ray crystalline 60 years after its formation, with no evidence of Pu leaching from the crystals. Our results highlight that the high-energy conditions of (sub-)critical explosions can create unexpected species. Even micro-particles of a derivative-MAX phase can effectively retain low-valence (metallic-like character) Pu under environmental conditions; the slow physical and chemical weathering of these particles may contribute to the slow release of radionuclides over decades, explaining constant low-levels of radionuclides observed in fauna. This study further suggests that rapidly quenched eutectic melts may be engineered to stabilise actinides in nuclear waste products, removing the need for hydrometallurgical processing.

Microbial adaptations and biogeochemical cycling of uranium in polymetallic tailings.

Microorganisms inhabiting uranium (U)-rich environments have specific physiological and biochemical coping mechanisms to deal with U toxicity, and thereby play a crucial role in the U biogeochemical cycling as well as associated heavy metals. We investigated the diversity and functional capabilities of indigenous bacterial communities inhabiting historic U- and Rare-Earth-Elements-rich polymetallic tailings from the Mount Painter Inlier, Northern Flinders Ranges, South Australia. Bacterial diversity profiling identified Actinobacteria as the predominant phylum in all samples. GeoChip analyses revealed the presence of diverse functional genes associated with biogenic element cycling, metal homeostasis/resistance, stress response, and secondary metabolism. The high abundance of metal-resistance and stress-tolerance genes indicates the adaptation of bacterial communities to the "harsh" environmental (metal-rich and semi-arid) conditions of the Northern Flinders Ranges. Additionally, a viable bacterial consortium was enriched from polymetallic tailings. Laboratory experiments demonstrated that the consortium scrubbed uranyl from solution by precipitating a uranyl phosphate biomineral (chernikovite), thus contributing to U biogeochemical cycling. These specialised microbial communities reflect the high specificity of the mineralogy/geochemistry, and biogeography of these U-rich settings. This study provides the fundamental knowledge to develop future applications in securing long-term stability of polymetallic mine waste, and for reprocessing this "waste" to further extract critical minerals.

From biomolecules to biogeochemistry: Exploring the interaction of an indigenous bacterium with gold.

Specialised microbial communities colonise the surface of gold particles in soils/sediments, and catalyse gold dissolution and re-precipitation, thereby contributing to the environmental mobility and toxicity of this 'inert' precious metal. We assessed the proteomic and physiological response of Serratia proteamaculans, the first metabolically active bacterium enriched and isolated directly from natural gold particles, when exposed to toxic levels of soluble Au3+ (10 µM). The results were compared to a metal-free blank, and to cultures exposed to similarly toxic levels of soluble Cu2+ (0.1 mM); Cu was chosen for comparison because it is closely associated with Au in nature due to similar geochemical properties. A total of 273 proteins were detected from the cells that experienced the oxidative effects of soluble Au, of which 139 (51%) were upregulated with either sole expression (31%) or had synthesis levels greater than the Au-free control (20%). The majority (54%) of upregulated proteins were functionally different from up-regulated proteins in the bacteria-copper treatment. These proteins were related to broad functions involving metabolism and biogenesis, followed by cellular process and signalling, indicating significant specificity for Au. This proteomic study revealed that the bacterium upregulates the synthesis of various proteins related to oxidative stress response (e.g., Monothiol-Glutaredoxin, Thiol Peroxidase, etc.) and cellular damage repair, which leads to the formation of metallic gold nanoparticles less toxic than ionic gold. Therefore, indigenous bacteria may mediate the toxicity of Au through two different yet simultaneous processes: i) repairing cellular components by replenishing damaged proteins and ii) neutralising reactive oxygen species (ROS) by up-regulating the synthesis of antioxidants. By connecting the fields of molecular bacteriology and environmental biogeochemistry, this study is the first step towards the development of biotechnologies based on indigenous bacteria applied to gold bio-recovery and bioremediation of contaminated environments.

Nature and coordination geometry of geologically relevant aqueous Uranium(VI) complexes up to 400 ºC: A review and new data.

The structure of the uranyl aqua ion (UO22+) and a number of its inorganic complexes (specifically, UO2Cl+, UO2Cl20, UO2SO40, [Formula: see text] , [Formula: see text] and UO2OH42-) have been characterised using X-Ray absorption spectroscopy/extended X-Ray absorption fine structure (XAS/EXAFS) at temperatures ranging from 25 to 326 ºC. Results of ab initio molecular dynamics (MD) calculations are also reported for uranyl in chloride and sulfate-bearing fluids from 25 to 400 ºC and 600 bar to 20 kilobar (kb). These results are reported alongside a comprehensive review of prior structural characterisation work with particular focus given to EXAFS works to provide a consistent and up-to-date view of the structure of these complexes under conditions relevant to U mobility in ore-forming systems and around high-grade nuclear waste repositories. Regarding reported EXAFS results, average equatorial coordination was found to decrease in uranyl and its sulfate and chloride complexes as temperature rose - the extent of this decrease differed between species and solution compositions but typically resulted in an equatorial coordination number of ∼3-4 at temperatures above 200 ºC. The [Formula: see text] complex was observed at temperatures from 25 to 247 ºC and exhibited no major structural changes over this temperature range. UO2OH42- exhibited only minor structural changes over a temperature range from 88 to 326 ºC and was suggested to manifest fivefold coordination with four hydroxyl molecules and one water molecule around its equator. Average coordination values derived from fits of the reported EXAFS data were compared to average coordination values calculated using the experimentally derived thermodynamic data for chloride complexes reported by Dargent et al. (2013) and Migdisov et al. (2018b), and for sulfate complexes reported by Alcorn et al. (2019) and Kalintsev et al. (2019). Sulfate EXAFS data were well described by available thermodynamic data, and chloride EXAFS data were described well by the thermodynamic data of Migdisov et al. (2018b), but not by the data of Dargent et al. (2013). The ab initio molecular dynamics calculations confirmed the trends in equatorial coordination observed with EXAFS and were also able to provide an insight into the effect of pressure in equatorial water coordination - for a given temperature, higher pressures appear to lead to a greater number of equatorially bound waters counteracting the temperature effect.

Silica-assisted pyro-hydrolysis of CaCl2 waste for the recovery of hydrochloric acid (HCl): Reaction pathways with the evolution of Ca(OH)Cl intermediate by experimental investigation and DFT modelling.

The chlorine evolution mechanism remains unclear during the thermal treatment of CaCl2/Ca(OH)Cl-containing solid waste. In this paper, we have conducted both experimental investigation and density functional theory (DFT) calculation to elucidate the mechanism of pyro-hydrolysis of CaCl2 with and without SiO2 in the temperature ranges of 400-900 °C. It was determined that pyro-hydrolysis of CaCl2 alone generated a maximum of 12% HCl by decomposition into Ca(OH)Cl, which is a stable intermediate that can be reverted to CaCl2 at 800 °C. Upon the addition of SiO2 at an equimolar ratio to CaCl2, the HCl release extent was accelerated to 50% at 900 °C. Both experiments and DFT calculations prove that the added SiO2 can promote the dissociation of water molecules which provides hydroxyl ions that enable the conversion of CaCl2 into Ca(OH)Cl at low temperatures. The resulting Ca(OH)Cl can also quickly react with SiO2 to form Cl-bearing silicates such as Ca2SiO3Cl2 and Ca3SiO4Cl2 with weakened CaCl bond that are relatively easy to cleave into Cl-free CaSiO3 and HCl(g) from 800 °C.

Carbonate complexation enhances hydrothermal transport of rare earth elements in alkaline fluids.

Rare earth elements (REE), essential metals for the transition to a zero-emission economy, are mostly extracted from REE-fluorcarbonate minerals in deposits associated with carbonatitic and/or peralkaline magmatism. While the role of high-temperature fluids (100 < T < 500 °C) in the development of economic concentrations of REE is well-established, the mechanisms of element transport, ore precipitation, and light (L)REE/heavy (H)REE fractionation remain a matter of debate. Here, we provide direct evidence from in-situ X-ray Absorption Spectroscopy (XAS) that the formation of hydroxyl-carbonate complexes in alkaline fluids enhances hydrothermal mobilization of LREE at T ≥ 400 °C and HREE at T ≤ 200 °C, even in the presence of fluorine. These results not only reveal that the modes of REE transport in alkaline fluids differ fundamentally from those in acidic fluids, but further underline that alkaline fluids may be key to the mineralization of hydrothermal REE-fluorcarbonates by promoting the simultaneous transport of (L)REE, fluoride and carbonate, especially in carbonatitic systems.

Kinetically driven successive sodic and potassic alteration of feldspar.

The dynamic evolutions of fluid-mineral systems driving large-scale geochemical transformations in the Earth's crust remain poorly understood. We observed experimentally that successive sodic and potassic alterations of feldspar can occur via a single self-evolved, originally Na-only, hydrothermal fluid. At 600 °C, 2 kbar, sanidine ((K,Na)AlSi3O8) reacted rapidly with a NaCl fluid to form albite (NaAlSi3O8); over time, some of this albite was replaced by K-feldspar (KAlSi3O8), in contrast to predictions from equilibrium reaction modelling. Fluorine accelerated the process, resulting in near-complete back-replacement of albite within 1 day. These findings reveal that potassic alteration can be triggered by Na-rich fluids, indicating that pervasive sequential sodic and potassic alterations associated with mineralization in some of the world's largest ore deposits may not necessarily reflect externally-driven changes in fluid alkali contents. Here, we show that these reactions are promoted at the micro-scale by a self-evolving, kinetically-driven process; such positive feedbacks between equilibrium and kinetic factors may be essential in driving pervasive mineral transformations.

The nature of Pu-bearing particles from the Maralinga nuclear testing site, Australia.

The high-energy release of plutonium (Pu) and uranium (U) during the Maralinga nuclear trials (1955-1963) in Australia, designed to simulate high temperature, non-critical nuclear accidents, resulted in wide dispersion µm-sized, radioactive, Pu-U-bearing 'hot' particles that persist in soils. By combining non-destructive, multi-technique synchrotron-based micro-characterization with the first nano-scale imagining of the composition and textures of six Maralinga particles, we find that all particles display intricate physical and chemical make-ups consistent with formation via condensation and cooling of polymetallic melts (immiscible Fe-Al-Pu-U; and Pb ± Pu-U) within the detonation plumes. Plutonium and U are present predominantly in micro- to nano-particulate forms, and most hot particles contain low valence Pu-U-C compounds; these chemically reactive phases are protected by their inclusion in metallic alloys. Plutonium reworking was observed within an oxidised rim in a Pb-rich particle; however overall Pu remained immobile in the studied particles, while small-scale oxidation and mobility of U is widespread. It is notoriously difficult to predict the long-term environmental behaviour of hot particles. Nano-scale characterization of the hot particles suggests that long-term, slow release of Pu from the hot particles may take place via a range of chemical and physical processes, likely contributing to on-going Pu uptake by wildlife at Maralinga.

Trace element catalyses mineral replacement reactions and facilitates ore formation.

Reaction-induced porosity is a key factor enabling protracted fluid-rock interactions in the Earth's crust, promoting large-scale mineralogical changes during diagenesis, metamorphism, and ore formation. Here, we show experimentally that the presence of trace amounts of dissolved cerium increases the porosity of hematite (Fe2O3) formed via fluid-induced, redox-independent replacement of magnetite (Fe3O4), thereby increasing the efficiency of coupled magnetite replacement, fluid flow, and element mass transfer. Cerium acts as a catalyst affecting the nucleation and growth of hematite by modifying the Fe2+(aq)/Fe3+(aq) ratio at the reaction interface. Our results demonstrate that trace elements can enhance fluid-mediated mineral replacement reactions, ultimately controlling the kinetics, texture, and composition of fluid-mineral systems. Applied to some of the world's most valuable orebodies, these results provide new insights into how early formation of extensive magnetite alteration may have preconditioned these ore systems for later enhanced metal accumulation, contributing to their sizes and metal endowment.

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.

Synchrotron X-ray absorption spectroscopy study of the evolution of chlorine during the pyro-hydrolysis of calcium and magnesium chloride waste.

Effective liberation of chlorine (Cl) as a recoverable product such as hydrochloric acid is crucial for the clean disposal of massive Cl-bearing industrial solid waste. This study aims to clarify the evolution of Cl upon the pyrohydrolysis of CaCl2 waste. Particularly, the use of silica and MgCl2 to promote the breakage of Ca-Cl bonds to release HCl gas has been investigated, via synchrotron X-ray absorption spectroscopy (XAS). As confirmed, in the presence of silica, the pyrohydrolysis of CaCl2 commences from 800 °C, lower than the minimum temperature predicted based on the existing thermodynamic database. The attraction of Ca2+ by SiO44- breaks the Ca-Cl bond successfully. The addition of Mg2+ can also improve the HCl regeneration extent to nearly 100%. Upon the addition of Mg2+, a structure of Ca-O-Mg-Cl is very likely to form, in which the second coordination shell of Ca2+ is occupied by both Cl- and Mg2+. Consequently, the incorporated Mg2+ bonds with Cl-, "pushing" the Ca2+ in the third shell further away, leading to a distorted and less crystalline silicate matrix from which the liberation of Cl- is easier. The Cl K-edge XANES shows that the reaction residues feature a unique, long-range multi-scattering phenomenon; this differs from the fully molten Cl-bearing glasses that bear a high similarity with CaCl2.

Metal resistant bacteria on gold particles: Implications of how anthropogenic contaminants could affect natural gold biogeochemical cycling.

In Earth's near-surface environments, gold biogeochemical cycling involves gold dissolution and precipitation processes, which are partly attributed to bacteria. These biogeochemical processes as well as abrasion (via physical transport) are known to act upon gold particles, thereby resulting in particle transformation including the development of pure secondary gold and altered morphology, respectively. While previous studies have inferred gold biogeochemical cycling from gold particles obtained from natural environments, little is known about how metal contamination in an environment could impact this cycle. Therefore, this study aims to infer how potentially toxic metal contaminants could affect the structure and chemistry of gold particles and therefore the biogeochemical cycling of gold. In doing so, river sediments and gold particles from the De Kaap Valley, South Africa, were analysed using both microanalytical and molecular techniques. Of the metal contaminants detected in the sediment, mercury can chemically interact with gold particles thereby directly altering particle morphology and "erasing" textural evidence indicative of particle transformation. Other metal contaminants (including mercury) indirectly affect gold cycling by exerting a selective pressure on bacteria living on the surface of gold particles. Particles harbouring gold-tolerant bacteria with diverse metal resistant genes, such as Arthrobacter sp. and Pseudomonas sp., contained nearly two times more secondary gold relative to particles harbouring bacteria with less gold-tolerance. In conclusion, metal contaminants can have a direct or indirect effect on gold biogeochemical cycling in natural environments impacted by anthropogenic activity.

Synthesis of in-situ Al3+-defected iron oxide nanoflakes from coal ash: A detailed study on the structure, evolution mechanism and application to water remediation.

The recovery of value-added materials from coal ash waste is of highly economic value and sustainable significance. However, researches on the synthesis of defect-engineering nanomaterials from coal ash are still blank. Herein, iron oxide (Fe1.72Al0.28O3, simplified as FAO) nanoflakes were successfully synthesized from a brown coal fly ash (BCFA) waste. The obtained FAO nanoflakes possess a round-shape morphology with a diameter of around 300 nm and 50 nm in thickness. With the progress of hydrothermal treatment, the impure Al3+ gradually replaced part of the Fe3+ in the α-Fe2O3 crystal. Specifically, Al3+ was preferentially adsorbed on the (001) facet, hindering the growth of Fe3+ on the [001] direction and thus causing the flattening of the resultant FAO. The introduced Al3+ also serves as the disordered defects on the hematite surface, leading to decreased crystal parameters for hematite, the formation of a compact first shell and a reduced periodical symmetry for the central cation Fe3+. The defects were also found to significantly improve the adsorption capacity of the resultant FAO for Cr(VI), As(V), As(III) and Congo red in waste water, with the maximum adsorption capacity of 68.3, 80.6, 61.1 and 213.8 mg g-1, respectively. Cyclic tests also confirmed a relatively strong stability for the as-synthesised adsorbents.

The aqueous chemistry of polonium (Po) in environmental and anthropogenic processes.

The longest-lived naturally occurring isotope of polonium is polonium-210, one of the daughters of uranium-238 (138 days half-life). As a daughter radionuclide of radon-222, polonium-210 can become enriched in pore fluids in U-bearing rocks, leading to contents in excess of 100 Bq.g-1 in some products from the mineral, coal, oil and gas industries (e.g., anode slimes in copper refinement; sludge from the oil and gas industry). Since 2006, IAEA recommendation limits require polonium and other radionuclides from the U- and Th-series decay to be regulated for products and wastes that contain >1 Bq.g-1, which results in the classification of large amounts of industrial products and waste as radioactive. To develop effective methods for polonium removal and/or control, it is necessary to acquire an understanding of its aqueous chemistry. Based on a review of available experimental data, we developed a self-consistent thermochemical model for polonium in inorganic aqueous solutions. Polonium exists mainly in two oxidation states in aqueous solutions: Po(IV) and Po(II). The importance of Po(II) is unique, as Te(II) or Se(II) complexes do not appear to play a significant role in aqueous solution at room temperature. The model is used to discuss polonium speciation in some environmental and process waters.

Garnet peridotites reveal spatial and temporal changes in the oxidation potential of subduction.

Changes in the oxygen fugacity (fO2) of the Earth's mantle have been proposed to control the spatial and temporal distribution of arc-related ore deposits, and possibly reflect the evolution of the atmosphere over billions of years. Thermodynamic calculations and natural evidence indicate that fluids released from subducting slabs can oxidise the mantle, but whether their oxidation potential varied in space and time remains controversial. Here, we use garnet peridotites from western Norway to show that there is a linear decrease in maximum fO2 with increasing depth in the mantle wedge. We ascribe this relation to changes in the speciation of sulfur released in slab fluids, with sulfate, controlling maximum oxidation, preferentially released at shallow depths. Even though the amount of sulfate in the Precambrian oceans, and thus in subducted lithologies, is thought to have been dramatically lower than during the Phanerozoic, garnet peridotites metasomatised during these two periods have a comparable fO2 range. This opens to the possibility that an oxidised mantle with fO2 similar to modern-day values has existed since the Proterozoic and possibly earlier. Consequently, early magmas derived from partial melting of metasomatised mantle may have had suitable fO2 to generate porphyry Cu-Au and iron-oxide Cu-Au deposits.

Temporal Evolution of Copper Distribution and Speciation in Roots of Triticum aestivum Exposed to CuO, Cu(OH)2, and CuS Nanoparticles.

Utilization of nanoparticles (NP) in agriculture as fertilizers or pesticides requires an understanding of the NP properties influencing their interactions with plant roots. To evaluate the influence of the solubility of Cu-based NP on Cu uptake and NP association with plant roots, wheat seedlings were hydroponically exposed to 1 mg/L of Cu NPs with different solubilities [CuO, CuS, and Cu(OH)2] for 1 h then transferred to a Cu-free medium for 48 h. Fresh, hydrated roots were analyzed using micro X-ray fluorescence (µ-XRF) and imaging fluorescence X-ray absorption near edge spectroscopy (XANES imaging) to provide laterally resolved distribution and speciation of Cu in roots. Higher solubility Cu(OH)2 NPs provided more uptake of Cu after 1 h of exposure, but the lower solubility materials (CuO and CuS) were more persistent on the roots and continued to deliver Cu to plant leaves over the 48 h depuration period. These results demonstrate that NPs, by associating to the roots, have the potential to play a role in slowly providing micronutrients to plants. Thus, tuning the solubility of NPs may provide a long-term slow delivery of micronutrients to plants and provide important information for understanding mechanisms responsible for plant uptake, transformation, and translocation of NPs.

Recrystallization of Manganite (γ-MnOOH) and Implications for Trace Element Cycling.

The recrystallization of Mn(III,IV) oxides is catalyzed by aqueous Mn(II) (Mn(II)aq) during (bio)geochemical Mn redox cycling. It is poorly understood how trace metals associated with Mn oxides (e.g., Ni) are cycled during such recrystallization. Here, we use X-ray absorption spectroscopy (XAS) to examine the speciation of Ni associated with Manganite (γ-Mn(III)OOH) suspensions in the presence or absence of Mn(II)aq under variable pH conditions (pH 5.5 and 7.5). In a second set of experiments, we used a 62Ni isotope tracer to quantify the amount of dissolved Ni that exchanges with Ni incorporated in the Manganite crystal structure during reactions in 1 mM Mn(II)aq and in Mn(II)-free solutions. XAS spectra show that Ni is initially sorbed on the Manganite mineral surface and is progressively incorporated into the mineral structure over time (13% after 51 days) even in the absence of dissolved Mn(II). The amount of Ni incorporation significantly increases to about 40% over a period of 51 days when Mn(II)aq is present in solution. Similarly, Mn(II)aq promotes Ni exchange between Ni-substituted Manganite and dissolved Ni(II), with around 30% of Ni exchanged at pH 7.5 over the duration of the experiment. No new mineral phases are detected following recrystallization as determined by X-ray diffraction and XAS. Our results reveal that Mn(II)-catalyzed mineral recrystallization partitions Ni between Mn oxides and aqueous fluids and can therefore affect Ni speciation and mobility in the environment.

Characterization of uranium redox state in organic-rich Eocene sediments.

The presence of organic matter (OM) has a profound impact on uranium (U) redox cycling, either limiting or promoting the mobility of U via binding, reduction, or complexation. To understand the interactions between OM and U, we characterised U oxidation state and speciation in nine OM-rich sediment cores (18 samples), plus a lignite sample from the Mulga Rock polymetallic deposit in Western Australia. Uranium was unevenly dispersed within the analysed samples with 84% of the total U occurring in samples containing >21 wt % OM. Analyses of U speciation, including x-ray absorption spectroscopy and bicarbonate extractions, revealed that U existed predominately (∼71%) as U(VI), despite the low pH (4.5) and nominally reducing conditions within the sediments. Furthermore, low extractability by water, but high extractability by a bi-carbonate solution, indicated a strong association of U with particulate OM. The unexpectedly high proportion of U(VI) relative to U(IV) within the OM-rich sediments implies that OM itself does not readily reduce U, and the reduction of U is not a requirement for immobilizing uranium in OM-rich deposits. The fact that OM can play a significant role in limiting the mobility and reduction of U(VI) in sediments is important for both U-mining and remediation.