Complexation by Organic Matter Controls Uranium Mobility in Anoxic Sediments.
Environmental science & technology
Uranium contamination threatens the availability of safe and clean drinking water globally. This toxic element occurs both naturally and as a result of mining and ore-processing in alluvial sediments, where it accumulates as tetravalent U [U(IV)], a form once considered largely immobile. Changing hydrologic and geochemical conditions cause U to be released into groundwater. Knowledge of the chemical form(s) of U(IV) is essential to understand the release mechanism, yet the relevant U(IV) species are poorly characterized. There is growing belief that natural organic matter (OM) binds U(IV) and mediates its fate in the subsurface. In this work, we combined nanoscale imaging (nano secondary ion mass spectrometry and scanning transmission X-ray microscopy) with a density-based fractionation approach to physically and microscopically isolate organic and mineral matter from alluvial sediments contaminated with uranium. We identified two populations of U (dominantly +IV) in anoxic sediments. Uranium was retained on OM and adsorbed to particulate organic carbon, comprising both microbial and plant material. Surprisingly, U was also adsorbed to clay minerals and OM-coated clay minerals. The dominance of OM-associated U provides a framework to understand U mobility in the shallow subsurface, and, in particular, emphasizes roles for desorption and colloid formation in its mobilization.
View details for DOI 10.1021/acs.est.9b04741
View details for PubMedID 31886668
Abiotic phosphorus recycling from adsorbed ribonucleotides on a ferrihydrite-type mineral: Probing solution and surface species.
Journal of colloid and interface science
2019; 547: 171–82
Iron (Fe) (oxyhydr)oxide minerals, which are amongst most reactive minerals in soils and sediments, are known to exhibit strong adsorption of inorganic phosphate (Pi) and organophosphate (Po) compounds. Beyond synthetic Po compounds, much still remains unknown about the reactivity of these minerals to transform naturally-occurring Po compounds to Pi, particularly with respect to solution versus surface speciation of Po hydrolysis. To investigate this reactivity with a ferrihydrite-type mineral and ribonucleotides, we employed high-resolution liquid chromatography-mass spectrometry (LC-MS), X-ray absorption near-edge structure (XANES), Fourier-transform infrared (FTIR) spectroscopy, and molecular modeling. Kinetic experiments were conducted with the mineral (1 g L-1) reacted with adenosine monophosphate, diphosphate, or triphosphate (respectively AMP, ADP, ATP; 50 M). Analysis of solution organic species by LC-MS implied that only adsorption occurred with AMP and ADP but both adsorption and dephosphorylation of ATP were evident. Maximum adsorption capacities per gram of mineral were 40.6 ± 0.8 mol AMP, 35.7 ± 1.6 mol ADP, and 10.9 ± 1.0 mol ATP; solution dephosphorylated by-products accounted for 15% of initial ATP. Subsequent XANES analysis of the surface species revealed that 16% of adsorbed AMP and 30% of adsorbed ATP were subjected to dephosphorylation, which was not fully quantifiable from the solution measurements. Molecular simulations predicted that ADP and ATP were complexed mainly via the phosphate groups whereas AMP binding also involved multiple hydrogen bonds with the adenosine moiety; our FTIR data confirmed these binding confirmations. Our findings thus imply that specific adsorption mechanisms dictate the recycling and subsequent trapping of Pi from ribonucleotide-like biomolecules reacted with Fe (oxyhydr)oxide minerals.
View details for DOI 10.1016/j.jcis.2019.03.086
View details for PubMedID 30954001
Advancing Chelation Chemistry for Actinium and Other +3 f-Elements, Am, Cm, and La.
Journal of the American Chemical Society
A major chemical challenge facing implementation of 225Ac in targeted alpha therapy-an emerging technology that has potential for treatment of disease-is identifying an 225Ac chelator that is compatible with in vivo applications. It is unclear how to tailor a chelator for Ac binding because Ac coordination chemistry is poorly defined. Most Ac chemistry is inferred from radiochemical experiments carried out on microscopic scales. Of the few Ac compounds that have been characterized spectroscopically, success has only been reported for simple inorganic ligands. Toward advancing understanding in Ac chelation chemistry, we have developed a method for characterizing Ac complexes that contain highly complex chelating agents using small quantities (μg) of 227Ac. We successfully characterized the chelation of Ac3+ by DOTP8- using EXAFS, NMR, and DFT techniques. To develop confidence and credibility in the Ac results, comparisons with +3 cations (Am, Cm, and La) that could be handled on the mg scale were carried out. We discovered that all M3+ cations (M = Ac, Am, Cm, La) were completely encapsulated within the binding pocket of the DOTP8- macrocycle. The computational results highlighted the stability of the M(DOTP)5- complexes.
View details for DOI 10.1021/jacs.9b10354
View details for PubMedID 31794205
The coordination chemistry of CmIII, AmIII, and AcIII in nitrate solutions: an actinide L3-edge EXAFS study.
2018; 9 (35): 7078-7090
Understanding actinide(iii) (AnIII = CmIII, AmIII, AcIII) solution-phase speciation is critical for controlling many actinide processing schemes, ranging from medical applications to reprocessing of spent nuclear fuel. Unfortunately, in comparison to most elements in the periodic table, AnIII speciation is often poorly defined in complexing aqueous solutions and in organic media. This neglect - in large part - is a direct result of the radioactive properties of these elements, which make them difficult to handle and acquire. Herein, we surmounted some of the handling challenges associated with these exotic 5f-elements and characterized CmIII, AmIII, and AcIII using AnIII L3-edge X-ray absorption spectroscopy (XAS) as a function of increasing nitric acid (HNO3) concentration. Our results revealed that actinide aquo ions, An(H2O) x 3+ (x = 9.6 ± 0.7, 8.9 ± 0.8, and 10.0 ± 0.9 for CmIII, AmIII, and AcIII), were the dominant species in dilute HNO3 (0.05 M). In concentrated HNO3 (16 M), shell-by-shell fitting of the extended X-ray fine structure (EXAFS) data showed the nitrate complexation increased, such that the average stoichiometries of Cm(NO3)4.1±0.7(H2O)5.7±1.3(1.1±0.2)-, Am(NO3)3.4±0.7(H2O)5.4±0.5(0.4±0.1)-, and Ac(NO3)2.3±1.7(H2O)8.3±5.2(0.7±0.5)+ were observed. Data obtained at the intermediate HNO3 concentration (4 M) were modeled as a linear combination of the 0.05 and 16 M spectra. For all three metals, the intermediate models showed larger contributions from the 0.05 M HNO3 spectra than from the 16 M HNO3 spectra. Additionally, these efforts enabled the Cm-NO3 and Ac-NO3 distances to be measured for the first time. Moreover, the AnIII L3-edge EXAFS results, contribute to the growing body of knowledge associated with CmIII, AmIII, and AcIII coordination chemistry, in particular toward advancing understanding of AnIII solution phase speciation.
View details for DOI 10.1039/c8sc02270d
View details for PubMedID 30310628
View details for PubMedCentralID PMC6137438
- The coordination chemistry of Cm-III, Am-III, and Ac-III in nitrate solutions: an actinide L-3-edge EXAFS study CHEMICAL SCIENCE 2018; 9 (35): 7078–90
Redox-Active vs Redox-Innocent: A Comparison of Uranium Complexes Containing Diamine Ligands
2018; 57 (11): 6530–39
Uranium complexes (MesDAE)2U(THF) (1-DAE) and Cp2U(MesDAE) (2-DAE) (MesDAE = [ArN-CH2CH2-NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), bearing redox-innocent diamide ligands, have been synthesized and characterized for a full comparison with previously published, redox-active diimine complexes, (MesDABMe)2U(THF) (1-DAB) and Cp2U(MesDABMe) (2-DAB) (MesDABMe = [ArN═C(Me)C(Me)═NAr]; Ar = Mes). These redox-innocent analogues maintain an analogous steric environment to their redox-active ligand counterparts to facilitate a study aimed at determining the differing electronic behavior around the uranium center. Structural analysis by X-ray crystallography showed 1-DAE and 2-DAE have a structural environment very similar to 1-DAB and 2-DAB, respectively. The main difference occurs with coordination of the ene-backbone to the uranium center in the latter species. Electronic absorption spectroscopy reveals these new DAE complexes are nearly identical to each other. X-ray absorption spectroscopy suggests all four species contain +4 uranium ions. The data also indicates that there is an electronic difference between the bis(diamide)-THF uranium complexes as opposed to those that only contain one diamide and two cyclopentadienyl rings. Finally, magnetic measurements reveal that all complexes display temperature-dependent behavior consistent with uranium(IV) ions that do not include ligand radicals. Overall, this study determines that there is no significant bonding difference between the redox-innocent and redox-active ligand frameworks on uranium. Furthermore, there are no data to suggest covalent bonding character using the latter ligand framework on uranium, despite what is known for transition metals.
View details for PubMedID 29749729
Oxidative uranium release from anoxic sediments under diffusion-limited conditions
AMER CHEMICAL SOC. 2018
View details for Web of Science ID 000435539901659
Microbially-mediated metal/radionuclide oxidation coupled to nitrate reduction in an alluvial aquifer
AMER CHEMICAL SOC. 2018
View details for Web of Science ID 000435539901629
Redox Controls over the Stability of U(IV) in Floodplains of the Upper Colorado River Basin
ENVIRONMENTAL SCIENCE & TECHNOLOGY
2017; 51 (19): 10954–64
Aquifers in the Upper Colorado River Basin (UCRB) exhibit persistent uranium (U) groundwater contamination plumes originating from former ore processing operations. Previous observations at Rifle, Colorado, have shown that fine grained, sulfidic, organic-enriched sediments accumulate U in its reduced form, U(IV), which is less mobile than oxidized U(VI). These reduced sediment bodies can subsequently act as secondary sources, releasing U back to the aquifer. There is a need to understand if U(IV) accumulation in reduced sediments is a common process at contaminated sites basin-wide, to constrain accumulated U(IV) speciation, and to define the biogeochemical factors controlling its reactivity. We have investigated U(IV) accumulation in organic-enriched reduced sediments at three UCRB floodplains. Noncrystalline U(IV) is the dominant form of accumulated U, but crystalline U(IV) comprises up to ca. 30% of total U at some locations. Differing susceptibilities of these species to oxidative remobilization can explain this variability. Particle size, organic carbon content, and pore saturation, control the exposure of U(IV) to oxidants, moderating its oxidative release. Further, our data suggest that U(IV) can be mobilized under deeply reducing conditions, which may contribute to maintenance and seasonal variability of U in groundwater plumes in the UCRB.
View details for PubMedID 28873299
Oxidative Uranium Release from Anoxic Sediments under Diffusion-Limited Conditions
ENVIRONMENTAL SCIENCE & TECHNOLOGY
2017; 51 (19): 11039–47
Uranium (U) contamination occurs as a result of mining and ore processing; often in alluvial aquifers that contain organic-rich, reduced sediments that accumulate tetravalent U, U(IV). Uranium(IV) is sparingly soluble, but may be mobilized upon exposure to nitrate (NO3-) and oxygen (O2), which become elevated in groundwater due to seasonal fluctuations in the water table. The extent to which oxidative U mobilization can occur depends upon the transport properties of the sediments, the rate of U(IV) oxidation, and the availability of inorganic reductants and organic electron donors that consume oxidants. We investigated the processes governing U release upon exposure of reduced sediments to artificial groundwater containing O2 or NO3- under diffusion-limited conditions. Little U was mobilized during the 85-day reaction, despite rapid diffusion of groundwater within the sediments and the presence of nonuraninite U(IV) species. The production of ferrous iron and sulfide in conjunction with rapid oxidant consumption suggested that the sediments harbored large concentrations of bioavailable organic carbon that fueled anaerobic microbial respiration and stabilized U(IV). Our results suggest that seasonal influxes of O2 and NO3- may cause only localized mobilization of U without leading to export of U from the reducing sediments when ample organic carbon is present.
View details for PubMedID 28876920
- Chemical changes in organic matter after fungal colonization in a nitrogen fertilized and unfertilized Norway spruce forest PLANT AND SOIL 2017; 419 (1-2): 113–26
- Thermodynamically controlled preservation of organic carbon in floodplains NATURE GEOSCIENCE 2017; 10 (6): 415-+
Molecular controls over uranium mobility in complex redox-active sediment systems
AMER CHEMICAL SOC. 2017
View details for Web of Science ID 000430569104798
Understanding controls on redox processes in floodplain sediments of the Upper Colorado River Basin.
The Science of the total environment
Floodplains, heavily used for water supplies, housing, agriculture, mining, and industry, are important repositories of organic carbon, nutrients, and metal contaminants. The accumulation and release of these species is often mediated by redox processes. Understanding the physicochemical, hydrological, and biogeochemical controls on the distribution and variability of sediment redox conditions is therefore critical to developing conceptual and numerical models of contaminant transport within floodplains. The Upper Colorado River Basin (UCRB) is impacted by former uranium and vanadium ore processing, resulting in contamination by V, Cr, Mn, As, Se, Mo and U. Previous authors have suggested that sediment redox activity occurring within organic carbon-enriched bodies located below the groundwater level may be regionally important to the maintenance and release of contaminant inventories, particularly uranium. To help assess this hypothesis, vertical distributions of Fe and S redox states and sulfide mineralogy were assessed in sediment cores from three floodplain sites spanning a 250km transect of the central UCRB. The results of this study support the hypothesis that organic-enriched reduced sediments are important zones of biogeochemical activity within UCRB floodplains. We found that the presence of organic carbon, together with pore saturation, are the key requirements for maintaining reducing conditions, which were dominated by sulfate-reduction products. Sediment texture was found to be of secondary importance and to moderate the response of the system to external forcing, such as oxidant diffusion. Consequently, fine-grain sediments are relatively resistant to oxidation in comparison to coarser-grained sediments. Exposure to oxidants consumes precipitated sulfides, with a disproportionate loss of mackinawite (FeS) as compared to the more stable pyrite. The accompanying loss of redox buffering capacity creates the potential for release of sequestered radionuclides and metals. Because of their redox reactivity and stores of metals, C, and N, organic-enriched sediments are likely to be important to nutrient and contaminant mobility within UCRB floodplain aquifers.
View details for DOI 10.1016/j.scitotenv.2017.01.109
View details for PubMedID 28359569
Uranium(IV) adsorption by natural organic matter in anoxic sediments
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2017; 114 (4): 711–16
Uranium is an important carbon-free fuel source and environmental contaminant that accumulates in the tetravalent state, U(IV), in anoxic sediments, such as ore deposits, marine basins, and contaminated aquifers. However, little is known about the speciation of U(IV) in low-temperature geochemical environments, inhibiting the development of a conceptual model of U behavior. Until recently, U(IV) was assumed to exist predominantly as the sparingly soluble mineral uraninite (UO2+x) in anoxic sediments; however, studies now show that this is not often the case. Yet a model of U(IV) speciation in the absence of mineral formation under field-relevant conditions has not yet been developed. Uranium(IV) speciation controls its reactivity, particularly its susceptibility to oxidative mobilization, impacting its distribution and toxicity. Here we show adsorption to organic carbon and organic carbon-coated clays dominate U(IV) speciation in an organic-rich natural substrate under field-relevant conditions. Whereas previous research assumed that U(IV) speciation is dictated by the mode of reduction (i.e., whether reduction is mediated by microbes or by inorganic reductants), our results demonstrate that mineral formation can be diminished in favor of adsorption, regardless of reduction pathway. Projections of U transport and bioavailability, and thus its threat to human and ecosystem health, must consider U(IV) adsorption to organic matter within the sediment environment.
View details for DOI 10.1073/pnas.1611918114
View details for Web of Science ID 000392597000045
View details for PubMedID 28069941
View details for PubMedCentralID PMC5278482
Regional importance of organic-rich sediments to uranium mobility in the upper Colorado River Basin
AMER CHEMICAL SOC. 2016
View details for Web of Science ID 000431905701331
Impact of redox conditions on interfacial uranium chemistry in complex natural sediments
AMER CHEMICAL SOC. 2016
View details for Web of Science ID 000431905701330
Persistent Secondary Contaminant Sources at a Former Uranium Mill Site, Riverton, Wyoming, USA
TU BERGAKADEMIE FREIBERG, INST MINING & SPECIAL CIVIL ENG. 2016: 398–404
View details for Web of Science ID 000402663400064
- Probing the sorption reactivity of the edge surfaces in birnessite nanoparticles using nickel(II) GEOCHIMICA ET COSMOCHIMICA ACTA 2015; 164: 191-204
Influence of natural organic matter on uranium mobility in the upper Colorado River Basin
AMER CHEMICAL SOC. 2015
View details for Web of Science ID 000411186500529
Mackinawite (FeS) Reduces Mercury(II) under Sulfidic Conditions
ENVIRONMENTAL SCIENCE & TECHNOLOGY
2014; 48 (18): 10681–89
Mercury (Hg) is a toxicant of global concern that accumulates in organisms as methyl Hg. The production of methyl Hg by anaerobic bacteria may be limited in anoxic sediments by the sequestration of divalent Hg [Hg(II)] into a solid phase or by the formation of elemental Hg [Hg(0)]. We tested the hypothesis that nanocrystalline mackinawite (tetragonal FeS), which is abundant in sediments where Hg is methylated, both sorbs and reduces Hg(II). Mackinawite suspensions were equilibrated with dissolved Hg(II) in batch reactors. Examination of the solid phase using Hg LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy showed that Hg(II) was indeed reduced in FeS suspensions. Measurement of purgeable Hg using cold vapor atomic fluorescence spectrometry (CVAFS) from FeS suspensions and control solutions corroborated the production of Hg(0) that was observed spectroscopically. However, a fraction of the Hg(II) initially added to the suspensions remained in the divalent state, likely in the form of β-HgS-like clusters associated with the FeS surface or as a mixture of β-HgS and surface-associated species. Complexation by dissolved S(-II) in anoxic sediments hinders Hg(0) formation, but, by contrast, Hg(II)-S(-II) species are reduced in the presence of mackinawite, producing Hg(0) after only 1 h of reaction time. The results of our work support the idea that Hg(0) accounts for a significant fraction of the total Hg in wetland and estuarine sediments.
View details for DOI 10.1021/es501514r
View details for Web of Science ID 000341801500020
View details for PubMedID 25180562
View details for PubMedCentralID PMC4167055
Hg reduction under sulfidic conditions by the nanocrystalline iron sulfide mackinawite
AMER CHEMICAL SOC. 2012
View details for Web of Science ID 000324475106027
Mechanisms of mercury sequestration by the iron sulfide mineral mackinawite
AMER CHEMICAL SOC. 2010
View details for Web of Science ID 000208189302662