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At least 433 records · Page 24Linked to original sources
USGS2009

Comprehensive isolation of natural organic matter from water for spectral characterizations and reactivity testing

A variety of approaches were tested to comprehensively isolate natural organic matter (NOM) from water . For waters with high NOM concentrations such as the Suwannee River, Georgia, approaches that used combinations of membrane concentrations, evaporative concentrations, and adsorption on nonionic XAD resins, ion exchange resins and iron oxide coated sand isolated over 90% of the NOM. However, for waters with low NOM concentrations, losses of half of the NOM were common and desalting of NOM isolates was a problem. A new comprehensive approach was devised and tested on the Seine River, France in which 100 L of filtered water was sodium softened by ion exchange and vacuum evaporated to 100 mL. Colloids (32% of the NOM) were isolated using a 3,500 Dalton membrane by dialysis against 0.1 M HCl and 0.2 M HF to remove salts and silica. On the membrane permeate, hydrophobic NOM (42%) was isolated using XAD-8 resin and hydrophilic NOM (26%) was isolated using a variety of selective desalting precipitations. The colloid fraction was characterized by IR and NMR spectroscopy as N-acetylamino sugars.

Georgia↗
USGS2011

Redox chemistry and natural organic matter (NOM): Geochemists' dream, analytical chemists' nightmare

Natural organic matter (NOM) is an inherently complex mixture of polyfunctional organic molecules. Because of their universality and chemical reversibility, oxidation/reductions (redox) reactions of NOM have an especially interesting and important role in geochemistry. Variabilities in NOM composition and chemistry make studies of its redox chemistry particularly challenging, and details of NOM-mediated redox reactions are only partially understood. This is in large part due to the analytical difficulties associated with NOM characterization and the wide range of reagents and experimental systems used to study NOM redox reactions. This chapter provides a summary of the ongoing efforts to provide a coherent comprehension of aqueous redox chemistry involving NOM and of techniques for chemical characterization of NOM. It also describes some attempts to confirm the roles of different structural moieties in redox reactions. In addition, we discuss some of the operational parameters used to describe NOM redox capacities and redox states, and describe nomenclature of NOM redox chemistry. Several relatively facile experimental methods applicable to predictions of the NOM redox activity and redox states of NOM samples are discussed, with special attention to the proposed use of fluorescence spectroscopy to predict relevant redox characteristics of NOM samples.

ACS Symposium Series↗
USGS2026

Comparing in vivo methylmercury detoxification in hunted duck: Implications for wildlife and human health

Great Salt Lake (GSL) waterfowl have elevated mercury (Hg) concentrations, and in vivo detoxification of neurotoxic and bioaccumulative methylmercury (MeHg) can occur through Hg-selenium (Se) complexation, influencing the suitability of Hg consumption advisories. Here, we present Hg chemical speciation and Hg stable isotope measurements of brain, breast muscle, liver, and kidney samples from two GSL duck species─northern shoveler ( Spatula clypeata ) and cinnamon teal (Spatula cyanoptera). Chemical and stable isotope measurements, along with high energy resolution fluorescence detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy, indicate the occurrence of in vivo MeHg demethylation in both bird species. The percentage of total Hg (THg) as MeHg (% MeHg) varied with tissue type (brain > breast muscle > kidney > liver), and differences between δ 202 THg and δ 202 MeHg were linearly correlated with % MeHg. Demethylated inorganic Hg was a mixture of Hg-dithiolate (Hg(SR) 2 ) and Hg-tetraselenolate (Hg(Sec) 4 ) in both bird species. Notably, liver THg concentrations were elevated in northern shoveler liver tissues relative to those of cinnamon teal (7.54 ± 4.69 mg/kg versus 2.19 ± 1.27 mg/kg, dry weight, respectively) and % MeHg liver values were significantly lower (40% ± 19% versus 68% ± 14%), indicating taxonomic differences in Hg detoxification and depuration. THg concentrations in waterfowl species from this study were comparable to previously reported levels, indicating that elevated Hg concentrations in northern shoveler and cinnamon teal at GSL have persisted over the past two decades. Due to in vivo demethylation, we show that THg is not an effective proxy for MeHg within duck species, carrying implications for current GSL waterfowl consumption advisories and assessments exposure risk. This study highlights important differences in MeHg detoxification between waterbird taxa and provides further insights into Hg toxicity risk to GSL waterbirds.

Utah↗
USGS2001

In situ spectroscopic and solution analyses of the reductive dissolution of Mn02 by Fe(II)

The reductive dissolution of MnO 2 by Fe(II) under conditions simulating acid mine drainage (pH 3, 100 mM SO 4 2 - ) was investigated by utilizing a flow-through reaction cell and synchrotron X-ray absorption spectroscopy. This configuration allows collection of in situ, real-time X-ray absorption near-edge structure (XANES) spectra and bulk solution samples. Analysis of the solution chemistry suggests that the reaction mechanism changed (decreased reaction rate) as MnO 2 was reduced and Fe(III) precipitated, primarily as ferrihydrite. Simultaneously, we observed an additional phase, with the local structure of jacobsite (MnFe 2 O 4 ), in the Mn XANES spectra of reactants and products. The X-ray absorbance of this intermediate phase increased during the experiment, implying an increase in concentration. The presence of this phase, which probably formed as a surface coating, helps to explain the reduced rate of dissolution of manganese(IV) oxide. In natural environments affected by acid mine drainage, the formation of complex intermediate solid phases on mineral surfaces undergoing reductive dissolution may likewise influence the rate of release of metals to solution.

Environmental Science & Technology↗
USGS2002

Limitations in the use of commercial humic acids in water and soil research

Seven samples of commercial "humic acids", purchased from five different suppliers, were studied, and their characteristics were compared with humic and fulvic acids isolated from streams, soils, peat, leonardite, and a dopplerite sample. Cross-polarization and magic-angle spinning 13C NMR spectroscopy clearly shows pronounced differences between the commercial materials and all other samples. Elemental and infrared spectroscopic data do not show such clear-cut differences but can be used as supportive evidence, with the 13C NMR data, to substantiate the above distinctions. As a result of these differences and due to the general lack of information relating to the source, method of isolation, or other pretreatment of the commercial materials, these commercial products are not considered to be appropriate for use as analogues of true soil and water humic substances, in experiments designed to evaluate the nature and reactivity of humic substances in natural waters and soils.

Environmental Science & Technology↗
USGS2001

Effects of sulfur impregnation temperature on the properties and mercury adsorption capacities of activated carbon fibers (ACFs)

Laboratory studies were conducted to determine the role of sulfur functional groups and micropore surface area of carbon-based adsorbents on the adsorption of Hg0 from simulated coal combustion flue gases. In this study, raw activated carbon fibers that are microporous (ACF-20) were impregnated with elemental sulfur between 250 and 650 ??C. The resulting samples were saturated with respect to sulfur content. Total sulfur content of the sulfur impregnated ACF samples decreased with increasing impregnation temperatures from 250 and 500 ??C and then remained constant to 650 ??C. Results from sulfur K-edge X-ray absorption near-edge structure (S-XANES) spectroscopy showed that sulfur impregnated on the ACF samples was in both elemental and organic forms. As sulfur impregnation temperature increased, however, the relative amounts of elemental sulfur decreased with a concomitant increase in the amount of organic sulfur. Thermal analyses and mass spectrometry revealed that sulfur functional groups formed at higher impregnation temperatures were more thermally stable. In general, sulfur impregnation decreased surface area and increased equilibrium Hg0 adsorption capacity when compared to the raw ACF sample. The ACF sample treated with sulfur at 400 ??C had a surface area of only 94 m2/g compared to the raw ACF sample's surface area of 1971 m2/g, but at least 86% of this sample's surface area existed as micropores and it had the largest equilibrium Hg0adsorption capacities (2211-11343 ??g/g). Such a result indicates that 400 ??C is potentially an optimal sulfur impregnation temperature for this ACF. Sulfur impregnated on the ACF that was treated at 400 ??C was in both elemental and organic forms. Thermal analyses and CS2extraction tests suggested that elemental sulfur was the main form of sulfur affecting the Hg0 adsorption capacity. These findings indicate that both the presence of elemental sulfur on the adsorbent and a microporous structure are important properties for improving the performance of carbon-based adsorbents for the removal of Hg0 from coal combustion flue gases.

Environmental Science & Technology↗
USGS2003

Binding of mercury(II) to aquatic humic substances: Influence of pH and source of humic substances

Conditional distribution coefficients ( K DOM ‘) for Hg(II) binding to seven dissolved organic matter (DOM) isolates were measured at environmentally relevant ratios of Hg(II) to DOM. The results show that K DOM ‘ values for different types of samples (humic acids, fulvic acids, hydrophobic acids) isolated from diverse aquatic environments were all within 1 order of magnitude (10 22.5 ± 1.0 −10 23.5 ± 1.0 L kg - 1 ), suggesting similar Hg(II) binding environments, presumably involving thiol groups, for the different isolates. K DOM ‘ values decreased at low pHs (4) compared to values at pH 7, indicating proton competition for the strong Hg(II) binding sites. Chemical modeling of Hg(II)−DOM binding at different pH values was consistent with bidentate binding of Hg(II) by one thiol group (p K a = 10.3) and one other group (p K a = 6.3) in the DOM, which is in agreement with recent results on the structure of Hg(II)−DOM bonds obtained by extended X-ray absorption fine structure spectroscopy (EXAFS).

Environmental Science & Technology↗
USGS2003

Arsenic speciation and reactivity in poultry litter

Recent U.S. government action to lower the maximum concentration levels (MCL) of total arsenic (As) (10 ppb) in drinking water has raised serious concerns about the agricultural use of As-containing biosolids such as poultry litter (PL). In this study, solid-state chemical speciation, desorbability, and total levels of As in PL and long-term amended soils were investigated using novel synchrotron-based probing techniques (microfocused (μ) synchrotron X-ray fluorescence (SXRF) and μ-X-ray absorption near-edge structure (XANES) spectroscopies) coupled with chemical digestion and batch experiments. The total As levels in the PL were as high as ≈50 mg kg - 1 , and As(II/III and V) was always concentrated in abundant needle-shaped microscopic particles (≈20 μm × 850 μm) associated with Ca, Cu, and Fe and to a lesser extent with S, Cl, and Zn. Post-edge XANES features of litter particles are dissimilar to those of the organo-As(V) compound in poultry feed (i.e., roxarsone), suggesting possible degradation/transformation of roxarsone in the litter and/or in poultry digestive tracts. The extent of As desorption from the litter increased with increasing time and pH from 4.5 to 7, but at most 15% of the total As was released after 5 d at pH 7, indicating the presence of insoluble phases and/or strongly retained soluble compounds. No significant As accumulation (<15 mg kg - 1 ) was found in long-term PL-amended agricultural surface soils. This suggests that As in the PL may have undergone surface and subsurface transport processes. Our research results raise concerns about long-term PL amendment effects on As contamination in surrounding soil−water environments.

Environmental Science & Technology↗
USGS2003

Molecular-scale characterization of uranium sorption by bone apatite materials for a permeable reactive barrier demonstration

Uranium binding to bone charcoal and bone meal apatite materials was investigated using U L III -edge EXAFS spectroscopy and synchrotron source XRD measurements of laboratory batch preparations in the absence and presence of dissolved carbonate. Pelletized bone char apatite recovered from a permeable reactive barrier (PRB) at Fry Canyon, UT, was also studied. EXAFS analyses indicate that U(VI) sorption in the absence of dissolved carbonate occurred by surface complexation of U(VI) for sorbed concentrations ≤ 5500 μg U(VI)/g for all materials with the exception of crushed bone char pellets. Either a split or a disordered equatorial oxygen shell was observed, consistent with complexation of uranyl by the apatite surface. A second shell of atoms at a distance of 2.9 Å was required to fit the spectra of samples prepared in the presence of dissolved carbonate (4.8 mM total) and is interpreted as formation of ternary carbonate complexes with sorbed U(VI). A U−P distance at 3.5−3.6 Å was found for most samples under conditions where uranyl phosphate phases did not form, which is consistent with monodentate coordination of uranyl by phosphate groups in the apatite surface. At sorbed concentrations ≥ 5500 μg U(VI)/g in the absence of dissolved carbonate, formation of the uranyl phosphate solid phase, chernikovite, was observed. The presence of dissolved carbonate (4.8 mM total) suppressed the formation of chernikovite, which was not detected even with sorbed U(VI) up to 12 300 μg U(VI)/g in batch samples of bone meal, bone charcoal, and reagent-grade hydroxyapatite. EXAFS spectra of bone char samples recovered from the Fry Canyon PRB were comparable to laboratory samples in the presence of dissolved carbonate where U(VI) sorption occurred by surface complexation. Our findings demonstrate that uranium uptake by bone apatite will probably occur by surface complexation instead of precipitation of uranyl phosphate phases under the groundwater conditions found at many U-contaminated sites.

Environmental Science & Technology↗
USGS2004

Macroscopic and microscopic observations of particle-facilitated mercury transport from New Idria and Sulphur Bank mercury mine tailings

Mercury (Hg) release from inoperative Hg mines in the California Coast Range has been documented, but little is known about the release and transport mechanisms. In this study, tailings from Hg mines located in different geologic settings-New Idria (NI), a Si-carbonate Hg deposit, and Sulphur Bank (SB), a hot-spring Hg deposit-were characterized, and particle release from these wastes was studied in column experiments to (1) investigate the mechanisms of Hg release from NI and SB mine wastes, (2) determine the speciation of particle-bound Hg released from the mine wastes, and (3) determine the effect of calcinations on Hg release processes. The physical and chemical properties of tailings and the colloids released from them were determined using chemical analyses, selective chemical extractions, XRD, SEM, TEM, and X-ray absorption spectroscopy techniques. The total Hg concentration in tailings increased with decreasing particle size in NI and SB calcines (roasted ore), but reached a maximum at an intermediate particle size in the SB waste rock (unroasted ore). Hg in the tailings exists predominantly as low-solubility HgS (cinnabar and metacinnabar), with NI calcines having >50% HgS, SB calcines having >89% HgS, and SB waste rock having ???100% HgS. Leaching experiments with a high-ionic-strength solution (0.1 M NaCl) resulted in a rapid but brief release of soluble and particulate Hg. Lowering the ionic strength of the leach solution (0.005 M NaCl) resulted in the release of colloidal Hg from two of the three mine wastes studied (NI calcines and SB waste rock). Colloid-associated Hg accounts for as much as 95% of the Hg released during episodic particle release. Colloids generated from the NI calcines are produced by a breakup and release mechanism and consist of hematite, jarosite/alunite, and Al-Si gel with particle sizes of 10-200 nm. ATEM and XAFS analyses indicate that the majority (???78%) of the mercury is present in the form of HgS. SB calcines also produced HgS colloids. The colloids generated from the SB waste rock were heterogeneous and varied in composition according to the column influent composition. ATEM and XAFS results indicate that Hg is entirely in the HgS form. Data from this study identify colloidal HgS as the dominant transported form of Hg from these mine waste materials.

Environmental Science & Technology↗
USGS2005

Freshwater mussel shells as environmental chronicles: Geochemical and taphonomic signatures of mercury-related extirpations in the North Fork Holston River, Virginia

This study utilized freshwater mussel shells to assess mercury (Hg) contamination in the North Fork Holston River that extirpated (caused local extinctions of) a diverse mussel fauna. Shells (n = 366) were collected from five sites situated upstream (two sites), just below (one site), and downstream (two sites) of the town of Saltville, Virginia, where Hg was used to produce chlorine and caustic soda from 1950 to 1972. Shell samples were used to test the (1) utility of geochemical signatures of shells for assessing the spatial variation in Hg levels in the river relative to the contamination source and (2) value of taphonomy (postmortem shell alteration) for distinguishing sites that differ in extirpation histories. Geochemical signatures of 40 shells, analyzed using atomic absorption spectroscopy, indicated a strong longitudinal pattern. All shells from the two upstream sites had low Hg concentrations (<5-31 ??g/kg), shells directly below Saltville had variable, but dramatically higher concentrations (23-4637 ??g/kg), and shells from the two downstream sites displayed intermediate Hg levels (<5-115 ??g/kg) that declined with distance from Saltville. Two pre-industrial shells, collected at Saltville in 1917, yielded very low Hg estimates (5-6 ??g/kg). Hg signatures were consistent among mussel species, suggesting that Hg concentrations were invariant to species type; most likely, highly variable Hg levels, both across sites and through time, overwhelmed any interspecific differences in Hg acquisition. Also, a notable postmortem incorporation of Hg in mussel shells seemed unlikely, as the Hg content was not correlated with shell taphonomy (r = 0.18; p = 0.28). The taphonomic analysis (n = 366) showed that the degree of shell alteration reliably distinguished sites with different extirpation histories. At Saltville, where live mussels have been absent for at least 30 years, shells were most heavily altered and fragmented. Conversely, fresh-looking shells abounded upstream, where reproducing mussel populations are still present. In summary, relic shells offered valuable spatiotemporal data on Hg concentrations in a polluted ecosystem, and shell taphonomic signatures discriminated sites with different extirpation histories. The shell-based strategies exemplified here do not require sampling live specimens and may augment more standard strategies applied to environmental monitoring. The approach should prove especially useful in areas with unknown extirpation and pollution histories. ?? 2005 American Chemical Society.

Environmental Science & Technology↗
USGS2005

Arsenate adsorption mechanisms at the allophane: Water interface

We investigated arsenate (As(V)) reactivity and surface speciation on amorphous aluminosilicate mineral (synthetic allophane) surfaces using batch adsorption experiments, powder X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS). The adsorption isotherm experiments indicated that As(V) uptake increased with increasing [As(V)] o from 50 to 1000 μM (i.e., Langmuir type adsorption isotherm) and that the total As adsorption slightly decreased with increasing NaCl concentrations from 0.01 to 0.1 M. Arsenate adsorption was initially (0−10 h) rapid followed by a slow continuum uptake, and the adsorption processes reached the steady state after 720 h. X-ray absorption spectroscopic analyses suggest that As(V) predominantly forms bidentate binuclear surface species on aluminum octahedral structures, and these species are stable up to 11 months. Solubility calculations and powder XRD analyses indicate no evidence of crystalline Al−As(V) precipitates in the experimental systems. Overall, macroscopic and spectroscopic evidence suggest that the As(V) adsorption mechanisms at the allophane−water interface are attributable to ligand exchange reactions between As(V) and surface-coordinated water molecules and hydroxyl and silicate ions. The research findings imply that dissolved tetrahedral oxyanions (e.g., H 2 PO 4 2 - and H 2 AsO 4 2 - ) are readily retained on amorphous aluminosilicate minerals in aquifer and soils at near neutral pH. The inner-sphere adsorption mechanisms might be important in controlling dissolved arsenate and phosphate in amorphous aluminosilicate-rich low-temperature geochemical environments.

Environmental Science & Technology↗
USGS2010

XANES evidence for rapid arsenic(III) oxidation at magnetite and ferrihydrite surfaces by dissolved O2 via Fe2+-mediated reactions

To reduce the adverse effects of arsenic on humans, various technologies are used to remove arsenic from groundwater, most relying on As adsorption on Fe-(oxyhydr)oxides and concomitant oxidation of As(III) by dissolved O 2 . This reaction can be catalyzed by microbial activity or by strongly oxidizing radical species known to form upon oxidation of Fe(II) by dissolved O 2 . Such catalyzed oxidation reactions have been invoked to explain the enhanced kinetics of As(III) oxidation in aerated water, in the presence of zerovalent iron or dissolved Fe(II). In the present study, we used arsenic K-edge X-ray absorption near edge structure (XANES) spectroscopy to investigate the role of Fe(II) in the oxidation of As(III) at the surface of magnetite and ferrihydrite under oxygenated conditions. Our results show rapid oxidation of As(III) to As(V) upon sorption onto magnetite under oxic conditions at neutral pH. Moreover, under similar oxic conditions, As(III) oxidized upon sorption onto ferrihydrite only after addition of Fe(II) aq within the investigated time frame of 24 h. These results confirm that Fe(II) is able to catalyze As(III) oxidation in the presence of dissolved O 2 and suggest that oxidation of As(III) upon sorption on magnetite under oxic conditions can be explained by an Fe 2+ -mediated Fenton-like reactions. Thus, the present study shows that magnetite might be an efficient alternative to the current use of oxidants and Fe(II) to remove As from aerated water. In addition, this study emphasizes that special care is needed to preserve arsenic oxidation state during laboratory sorption experiments as well as in collecting As-bearing samples from natural environments.

Environmental Science and Technology↗
USGS2011

Metallothionein-like multinuclear clusters of mercury(II) and sulfur in peat

Strong mercury(II)–sulfur (Hg-SR) bonds in natural organic matter, which influence mercury bioavailability, are difficult to characterize. We report evidence for two new Hg-SR structures using X-ray absorption spectroscopy in peats from the Florida Everglades with added Hg. The first, observed at a mole ratio of organic reduced S to Hg (S red /Hg) between 220 and 1140, is a Hg 4 S x type of cluster with each Hg atom bonded to two S atoms at 2.34 Å and one S at 2.53 Å, and all Hg atoms 4.12 Å apart. This model structure matches those of metal–thiolate clusters in metallothioneins, but not those of HgS minerals. The second, with one S atom at 2.34 Å and about six C atoms at 2.97 to 3.28 Å, occurred at S red /Hg between 0.80 and 4.3 and suggests Hg binding to a thiolated aromatic unit. The multinuclear Hg cluster indicates a strong binding environment to cysteinyl sulfur that might impede methylation. Along with a linear Hg(SR) 2 unit with Hg—S bond lengths of 2.34 Å at S red /Hg of about 10 to 20, the new structures support a continuum in Hg-SR binding strength in natural organic matter.

Environmental Science & Technology↗
USGS2011

Formation of nanocolloidal metacinnabar in mercury-DOM-sulfide systems

Direct determination of mercury (Hg) speciation in sulfide-containing environments is confounded by low mercury concentrations and poor analytical sensitivity. Here we report the results of experiments designed to assess mercury speciation at environmentally relevant ratios of mercury to dissolved organic matter (DOM) (i.e., <4 nmol Hg (mg DOM) −1 ) by combining solid phase extraction using C 18 resin with extended X-ray absorption fine structure (EXAFS) spectroscopy. Aqueous Hg(II) and a DOM isolate were equilibrated in the presence and absence of 100 μM total sulfide. In the absence of sulfide, mercury adsorption to the resin increased as the Hg:DOM ratio decreased and as the strength of Hg-DOM binding increased. EXAFS analysis indicated that in the absence of sulfide, mercury bonds with an average of 2.4 ± 0.2 sulfur atoms with a bond length typical of mercury-organic thiol ligands (2.35 Å). In the presence of sulfide, mercury showed greater affinity for the C 18 resin, and its chromatographic behavior was independent of Hg:DOM ratio. EXAFS analysis showed mercury–sulfur bonds with a longer interatomic distance (2.51–2.53 Å) similar to the mercury–sulfur bond distance in metacinnabar (2.53 Å) regardless of the Hg:DOM ratio. For all samples containing sulfide, the sulfur coordination number was below the ideal four-coordinate structure of metacinnabar. At a low Hg:DOM ratio where strong binding DOM sites may control mercury speciation (1.9 nmol mg –1 ) mercury was coordinated by 2.3 ± 0.2 sulfur atoms, and the coordination number rose with increasing Hg:DOM ratio. The less-than-ideal coordination numbers indicate metacinnabar-like species on the nanometer scale, and the positive correlation between Hg:DOM ratio and sulfur coordination number suggests progressively increasing particle size or crystalline order with increasing abundance of mercury with respect to DOM. In DOM-containing sulfidic systems nanocolloidal metacinnabar-like species may form, and these species need to be considered when addressing mercury biogeochemistry.

Environmental Science & Technology↗
USGS2013

Uranium(VI) interactions with mackinawite in the presence and absence of bicarbonate and oxygen

Mackinawite, Fe(II)S, samples loaded with uranium (10 -5 , 10 -4 , and 10 -3 mol U/g FeS) at pH 5, 7, and 9, were characterized using X-ray absorption spectroscopy and X-ray diffraction to determine the effects of pH, bicarbonate, and oxidation on uptake. Under anoxic conditions, a 5 g/L suspension of mackinawite lowered 5 × 10 -5 M uranium(VI) to below 30 ppb (1.26 × 10 -7 M) U. Between 82 and 88% of the uranium removed from solution by mackinawite was U(IV) and was nearly completely reduced to U(IV) when 0.012 M bicarbonate was added. Near-neighbor coordination consisting of uranium–oxygen and uranium-uranium distances indicates the formation of uraninite in the presence and absence of bicarbonate, suggesting reductive precipitation as the dominant removal mechanism. Following equilibration in air, mackinawite was oxidized to mainly goethite and sulfur and about 76% of U(IV) was reoxidized to U(VI) with coordination of uranium to axial and equatorial oxygen, similar to uranyl. Additionally, uranium-iron distances, typical of coprecipitation of uranium with iron oxides, and uranium-sulfur distances indicating bidentate coordination of U(VI) to sulfate were evident. The affinity of mackinawite and its oxidation products for U(VI) provides impetus for further study of mackinawite as a potential reactive medium for remediation of uranium-contaminated water.

Environmental Science & Technology↗
USGS2013

Evaluating chemical extraction techniques for the determination of uranium oxidation state in reduced aquifer sediments

Extraction techniques utilizing high pH and (bi)carbonate concentrations were evaluated for their efficacy in determining the oxidation state of uranium (U) in reduced sediments collected from Rifle, CO. Differences in dissolved concentrations between oxic and anoxic extractions have been proposed as a means to quantify the U(VI) and U(IV) content of sediments. An additional step was added to anoxic extractions using a strong anion exchange resin to separate dissolved U(IV) and U(VI). X-ray spectroscopy showed that U(IV) in the sediments was present as polymerized precipitates similar to uraninite and/or less ordered U(IV), referred to as non-uraninite U(IV) species associated with biomass (NUSAB). Extractions of sediment containing both uraninite and NUSAB displayed higher dissolved uranium concentrations under oxic than anoxic conditions while extractions of sediment dominated by NUSAB resulted in identical dissolved U concentrations. Dissolved U(IV) was rapidly oxidized under anoxic conditions in all experiments. Uraninite reacted minimally under anoxic conditions but thermodynamic calculations show that its propensity to oxidize is sensitive to solution chemistry and sediment mineralogy. A universal method for quantification of U(IV) and U(VI) in sediments has not yet been developed but the chemical extractions, when combined with solid-phase characterization, have a narrow range of applicability for sediments without U(VI).

Colorado↗
USGS2009

Arsenite and ferrous iron oxidation linked to chemolithotrophic denitrification for the immobilization of arsenic in anoxic environments

The objective of this study was to explore a bioremediation strategy based on injecting NO 3 − to support the anoxic oxidation of ferrous iron (Fe(II)) and arsenite (As(III)) in the subsurface as a means to immobilize As in the form of arsenate (As(V)) adsorbed onto biogenic ferric (Fe(III)) (hydr)oxides. Continuous flow sand filled columns were used to simulate a natural anaerobic groundwater and sediment system with co-occurring As(III) and Fe(II) in the presence (column SF1) or absence (column SF2) of nitrate, respectively. During operation for 250 days, the average influent arsenic concentration of 567 μg L −1 was reduced to 10.6 (±9.6) μg L −1 in the effluent of column SF1. The cumulative removal of Fe(II) and As(III) in SF1 was 6.5 to 10-fold higher than that in SF2. Extraction and measurement of the mass of iron and arsenic immobilized on the sand packing of the columns were close to the iron and arsenic removed from the aqueous phase during column operation. The dominant speciation of the immobilized iron and arsenic was Fe(III) and As(V) in SF1, compared with Fe(II) and As(III) in SF2. The speciation was confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results indicate that microbial oxidation of As(III) and Fe(II) linked to denitrification resulted in the enhanced immobilization of aqueous arsenic in anaerobic environments by forming Fe(III) (hydr)oxide coated sands with adsorbed As(V).

Environmental Science & Technology↗