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Layered, poorly crystalline Mn(IV)O(2) phases are abundant in the environment. These mineral phases may rapidly oxidize Cr(III) to more mobile and toxic Cr(VI) in soils. There is still, however, little knowledge of how Cr(III) oxidation by Mn(IV)O(2) proceeds at the microscopic and molecular levels. Therefore, the sorption mechanisms of Cr(III) and Cr(VI) on Random Stacked Birnessite (RSB), δ-MnO(2), and Acid Birnessite (AB) were determined by Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). These three synthetic Mn(IV)O(2), which are poorly crystalline phases and have layered structures, were reacted with 50 mM Cr(III) at pH 2.5, 3, and 3.5 before being analyzed by EXAFS. The results indicated that Cr(VI) was loosely sorbed as an outer-sphere complex on Mn(IV)O(2), while Cr(III) was tightly sorbed as an inner-sphere complex. Further research is needed to understand why Cr(III) stopped being significantly oxidized by Mn(IV)O(2) after 30 min. This study, however, demonstrated that the formation of a Cr surface precipitate is not necessarily responsible for the cessation in Cr(III) oxidation. Indeed, no Cr surface precipitate was detected at the microscopic and molecular levels on Mn(IV)O(2) surfaces reacted with Cr(III) for 1 h, although the Cr(III) oxidation ceased before 1 h of reaction at most employed experimental conditions.

Manganese (Mn) oxides are among the most reactive natural minerals and play an important role in elemental cycling in oceanic and terrestrial environments. A large portion of naturally-occurring Mn oxides tend to be poorly-crystalline and/or nanocrystalline, with not fully resolved crystal structures. In this study, the crystal structures of their synthetic analogs including acid birnessite (AcidBir), δ-MnO2, polymeric MnO2 (PolyMnO2) and a bacteriogenic Mn oxide (BioMnOx), have been revealed using atomic pair distribution function (PDF) analysis. Results unambiguously verify that these Mn oxides are layered materials. The best models that accurately allow simulation of pair distribution functions (PDFs) belong to the monoclinic C12/m1 space group with a disk-like shape. The single MnO6 layers in the average structures deviate significantly from hexagonal symmetry, in contrast to the results of previous studies based on X-ray diffraction analysis in reciprocal space. Manganese occupancies in MnO6 layers are estimated to be 0.936, 0.847, 0.930 and 0.935, for AcidBir, BioMnOx, δ-MnO2 and PolyMnO2, respectively; however, occupancies of interlayer cations and water molecules cannot be accurately determined using the models in this study. The coherent scattering domains (CSDs) of PolyMnO2, δ-MnO2 and BioMnOx are at the nanometer scale, comprising one to three MnO6 layers stacked with a high disorder in the crystallographic c-axis direction. Overall, the results of this study advance our understanding of the mineralogy of Mn oxide minerals in the environment.

Trypanothione reductase (TR) is found in the trypanosomatid parasites, where it catalyses the NADPH-dependent reduction of the glutathione analogue, trypanothione, and is a key player in the parasite's defenses against oxidative stress. TR is a promising target for the development of antitrypanosomal drugs; here, we report our synthesis and evaluation of compounds 3-5 as low micromolar Trypanosoma cruzi TR inhibitors. Although 4 and 5 were designed as potential irreversible inhibitors, these compounds, as well as 3, displayed reversible competitive inhibition. Compound 3 proved to be the most potent inhibitor, with a K(i) = 2 µM.

Arsenite (As(III)) oxidation by manganese oxides (Mn-oxides) serves to detoxify and, under many conditions, immobilize arsenic (As) by forming arsenate (As(V)). As(III) oxidation by Mn(IV)-oxides can be quite complex, involving many simultaneous forward reactions and subsequent back reactions. During As(III) oxidation by Mn-oxides, a reduction in oxidation rate is often observed, which is attributed to Mn-oxide surface passivation. X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) data show that Mn(II) sorption on a poorly crystalline hexagonal birnessite (δ-MnO₂) is important in passivation early during reaction with As(III). Also, it appears that Mn(III) in the δ-MnO₂ structure is formed by conproportionation of sorbed Mn(II) and Mn(IV) in the mineral structure. The content of Mn(III) within the δ-MnO₂ structure appears to increase as the reaction proceeds. Binding of As(V) to δ-MnO₂ also changes as Mn(III) becomes more prominent in the δ-MnO ₂ structure. The data presented indicate that As(III) oxidation and As(V) sorption by poorly crystalline δ-MnO₂ is greatly affected by Mn oxidation state in the δ-MnO₂ structure.

Massively parallel sequencing has provided a more affordable and high-throughput method to study microbial communities, although it has mostly been used in an exploratory fashion. We combined pyrosequencing with a strict indicator species statistical analysis to test if bacteria specifically responded to ethanol injection that successfully promoted dissimilatory uranium(VI) reduction in the subsurface of a uranium contamination plume at the Oak Ridge Field Research Center in Tennessee. Remediation was achieved with a hydraulic flow control consisting of an inner loop, where ethanol was injected, and an outer loop for flow-field protection. This strategy reduced uranium concentrations in groundwater to levels below 0.126 μM and created geochemical gradients in electron donors from the inner-loop injection well toward the outer loop and downgradient flow path. Our analysis with 15 sediment samples from the entire test area found significant indicator species that showed a high degree of adaptation to the three different hydrochemical-created conditions. Castellaniella and Rhodanobacter characterized areas with low pH, heavy metals, and low bioactivity, while sulfate-, Fe(III)-, and U(VI)-reducing bacteria ( Desulfovibrio , Anaeromyxobacter , and Desulfosporosinus ) were indicators of areas where U(VI) reduction occurred. The abundance of these bacteria, as well as the Fe(III) and U(VI) reducer Geobacter , correlated with the hydraulic connectivity to the substrate injection site, suggesting that the selected populations were a direct response to electron donor addition by the groundwater flow path. A false-discovery-rate approach was implemented to discard false-positive results by chance, given the large amount of data compared.

Arsenic (As) contamination of drinking water is a threat to global health. Manganese(III/IV) (Mn) oxides control As in groundwater by oxidizing more mobile AsIII to less mobile AsV. Both As species sorb to the Mn oxide. The rates and mechanisms of this process are the subject of extensive research; however, as a group, study results are inconclusive and often contradictory. Here, the existing body of literature describing AsIII oxidation by Mn oxides is examined, and several potential reasons for inconsistent kinetic data are discussed. The oxidation of AsIII by Mn(III/IV) oxides is generally biphasic, with reported first order rate constants ranging seven orders of magnitude. Reanalysis of existing datasets from batch reactions of AsIII with δ-MnO2 reveal that the first order rate constants reported for As depletion are time-dependent, and are not well described by pure kinetic rate models. This finding emphasizes the importance of mechanistic modeling that accounts for differences in reactivity between MnIII and MnIV, and the sorption and desorption of AsIII, AsV, and MnII. A thorough understanding of the reaction is crucial to predicting As fate in groundwater and removing As via water treatment with Mn oxides, thus ensuring worldwide access to safe drinking water.

Todorokite, as one of three main Mn oxide phases present in oceanic Mn nodules and an active MnO{sub 6} octahedral molecular sieve (OMS), has garnered much interest; however, its formation pathway in natural systems is not fully understood. Todorokite is widely considered to form from layer structured Mn oxides with hexagonal symmetry, such as vernadite ({delta}-MnO{sub 2}), which are generally of biogenic origin. However, this geochemical process has not been documented in the environment or demonstrated in the laboratory, except for precursor phases with triclinic symmetry. Here we report on the formation of a nanoscale, todorokite-like phase from biogenic Mn oxides produced by the freshwater bacterium Pseudomonas putida strain GB-1. At long- and short-range structural scales biogenic Mn oxides were transformed to a todorokite-like phase at atmospheric pressure through refluxing. Topotactic transformation was observed during the transformation. Furthermore, the todorokite-like phases formed via refluxing had thin layers along the c* axis and a lack of c* periodicity, making the basal plane undetectable with X-ray diffraction reflection. The proposed pathway of the todorokite-like phase formation is proposed as: hexagonal biogenic Mn oxide {yields} 10-{angstrom} triclinic phyllomanganate {yields} todorokite. These observations provide evidence supporting the possible bio-related origin of natural todorokites andmore » provide important clues for understanding the transformation of biogenic Mn oxides to other Mn oxides in the environment. Additionally this method may be a viable biosynthesis route for porous, nano-crystalline OMS materials for use in practical applications.« less

Biogenic Mn-oxides (BioMnO(x)), produced by microorganisms, possess an extraordinary ability to sequester metals. BioMnO(x) are generally layered structures containing varying amounts of Mn(III) and vacant sites in the Mn layers. However the relationship between the varying structure of BioMnO(x) and metal sorption properties remains unclear. In this study, BioMnO(x) produced by Pseudomonas putida strain GB-1 was synthesized at either pH 6, 7, or 8 in CaCl(2) solution, and Ni(II) sorption mechanisms were determined at pH 7 and at different Ni(II) loadings, using isotherm and extended X-ray absorption fine structure (EXAFS) spectroscopic analyses. Our data demonstrate that Ni(II) sorbs at vacant sites in the interlayer of the BioMnO(x) and the maximum Ni(II) sorption capacity increases as the formation pH of BioMnO(x) decreases. This relation indicates that the quantity of BioMnO(x) vacant sites increases as formation conditions become more acidic, which is in good agreement with our companion study. Contents of the vacant sites were quantitatively estimated based on maximum Ni(II) sorption capacity. Additionally, this study reveals that imidazole groups are involved in Ni(II) binding to biomaterials, and have a higher Ni(II) sorption affinity, but a lower site density compared to carboxyl groups.

A pilot-scale field test system with an inner loop nested within an outer loop was constructed for in situ U(VI) bioremediation at a US Department of Energy site, Oak Ridge, TN. The outer loop was used for hydrological protection of the inner loop where ethanol was injected for biostimulation of microorganisms for U(VI) reduction/immobilization. After 2 years of biostimulation with ethanol, U(VI) levels were reduced to below drinking water standard (30 microg l(-1)) in the inner loop monitoring wells. To elucidate the microbial community structure and functions under in situ uranium bioremediation conditions, we used a comprehensive functional gene array (GeoChip) to examine the microbial functional gene composition of the sediment samples collected from both inner and outer loop wells. Our study results showed that distinct microbial communities were established in the inner loop wells. Also, higher microbial functional gene number, diversity and abundance were observed in the inner loop wells than the outer loop wells. In addition, metal-reducing bacteria, such as Desulfovibrio, Geobacter, Anaeromyxobacter and Shewanella, and other bacteria, for example, Rhodopseudomonas and Pseudomonas, are highly abundant in the inner loop wells. Finally, the richness and abundance of microbial functional genes were highly correlated with the mean travel time of groundwater from the inner loop injection well, pH and sulfate concentration in groundwater. These results suggest that the indigenous microbial communities can be successfully stimulated for U bioremediation in the groundwater ecosystem, and their structure and performance can be manipulated or optimized by adjusting geochemical and hydrological conditions.

Methylated forms of arsenic (As), monomethylarsenate (MMA) and dimethylarsenate (DMA), have historically been used as herbicides and pesticides. Because of their large application to agriculture fields and the toxicity of MMA and DMA, the sorption of methylated As to soil constituents requires investigation. MMA and DMA sorption on amorphous aluminum oxide (AAO) was investigated using both macroscopic batch sorption kinetics and molecular scale extended X-ray absorption fine structure (EXAFS) and Fourier transform infrared (FTIR) spectroscopic techniques. Sorption isotherm studies revealed sorption maxima of 0.183, 0.145, and 0.056 mmol As/mmol Al for arsenate (As{sup V}), MMA, and DMA, respectively. In the sorption kinetics studies, 100% of added As{sup V} was sorbed within 5 min, while 78% and 15% of added MMA and DMA were sorbed, respectively. Desorption experiments, using phosphate as a desorbing agent, resulted in 30% release of absorbed As{sup V}, while 48% and 62% of absorbed MMA and DMA, respectively, were released. FTIR and EXAFS studies revealed that MMA and DMA formed mainly bidentate binuclear complexes with AAO. On the basis of these results, it is proposed that increasing methyl group substitution results in decreased As sorption and increased As desorption on AAO.

The oxidation state of uranium plays a major role in determining uranium mobility in the environment. Under anaerobic conditions, common metal respiring bacteria enzymatically reduce soluble U(VI) to U(IV), resulting in the formation of sparingly soluble UO(2(bio)) (biogenic uraninite). The stability of biologically precipitated uraninite is critical for determining the long-term fate of uranium and is not well characterized within soils and sediments. Here, we demonstrate that biogenic uraninite oxidation by ferrihydrite, an environmentally ubiquitous, disordered Fe(III) (hydr)oxide, appears to proceed through a soluble U(IV) intermediate and results in the concomitant production of Fe(II) and dissolved U(VI). Uraninite oxidation rates are accelerated under conditions that increase its solubility and decrease uraninite surface passivation, which include high bicarbonate concentration and pH values deviating from neutrality. Thus, our results demonstrate that UO(2(bio)) oxidation by Fe(III) (hydr)oxides is controlled by the rate of uraninite dissolution and that this process may limit uranium(IV) sequestration in the presence of Fe(III) (hydr)oxides.

In-situ stabilization using phosphate (P) amendments, such as P-based fertilizers and rock, are a potentially cost-effective and minimally disruptive alternative for stabilizing Pb in soils. We examined the effect of time (0-365 d), in vitro extraction pH (1.5 vs. 2.3), and dosage of three P-based amendments on the bioaccessibility (as a surrogate for oral bioavailability) of Pb in 10 soils from U.S. Department of Defense facilities. Initial untreated soil bioaccessibility consistently exceeded the U.S. Environmental Protection Agency default value of 60% relative bioavailability, with higher bioaccessibility consistently observed at an in vitro extraction pH of 1.5 vs. 2.3. Although P-based amendments statistically (P < 0.05) reduced bioaccessibility in many instances, with reductions dependent on the amendment and dosage, large amendment dosages (approximately 20-25% by mass to yield 5% P by mass) were required to reduce average bioaccessibility by approximately 25%. For most amendment combinations, reductions continued to occur for periods up to 1 yr, indicating that the observed reductions were not merely experimental artifacts of the in vitro extraction procedure. Although our results indicated that reductions in Pb bioaccessibility with P amendments are technically feasible, relatively large amendment masses were required to achieve relatively modest reductions in bioaccessibility. The cost and potential environmental implications of adding such large amounts of P may limit the practicality of in situ immobilization for some Pb-contaminated soils, industrial and firing range soils in particular.

Batch U(VI) sorption/reduction experiments were completed on sterilized and non-sterilized sediment samples to elucidate biological and geochemical reduction mechanisms. Results from X-ray absorption near-edge structure (XANES) spectroscopy revealed that γ-sterilized sediments were actually better sorbents of U(VI), despite the absence of any measurable biological activity. These results indicate that γ-irradiation induced significant physico-chemical changes in the sediment which is contrary to numerous other studies identifying γ-sterilization as an effective and minimally invasive technique. To identify the extent and method of alteration of the soil as a result of γ-sterilization, untreated soil samples, physically separated size fractions, and chemically extracted fractions of the soil were analyzed pre- and post-sterilization. The effects of sterilization on mineralogy, pH, natural organic matter (NOM), cation exchange capacity (CEC), and iron oxidation state were determined. Results indicated that major mineralogy of the clay and whole sediment samples was unchanged. Sediment pH decreased only slightly with γ-irradiation; however, irradiation produced a significant decrease in CEC of the untreated sediments and affected both the organic and inorganic fractions. Mossbauer spectra of non-sterile and γ-sterilized sediments measured more reduced iron present in γ-sterilized sediments compared to non-sterile samples. Our results suggest that sterilization by γ-irradiation induced iron reduction that may have increased the sorption and/or reduction of U(VI) onto these sediments. However, Mossbauer and batch sorption data are somewhat contradictory, the former indicates that the iron oxide or iron hydroxide minerals are more significantly reduced while the later indicates that reduced clay minerals account for greater sorption of U(VI).

Oscillations between reducing and oxidizing conditions are observed at the interface of anaerobic/oxic and anaerobic/anoxic environments, and are often stimulated by an alternating flux of electron donors (e.g., organic carbon) and electron acceptors (e.g., O2 and NO3(-)). In iron (Fe) rich soils and sediments, these oscillations may stimulate the growth of both Fe-reducing bacteria (FeRB) and Fe-oxidizing bacteria (FeOB), and their metabolism may induce cycling between Fe(II) and Fe(III), promoting the transformation of Fe (hydr)oxide minerals. Here, we examine the mineralogical evolution of lepidocrocite and ferrihydrite, and the adaptation of a natural microbial community to alternating Fe-reducing (anaerobic with addition of glucose) and Fe-oxidizing (with addition of nitrate or air) conditions. The growth of FeRB (e.g., Geobacter) is stimulated under anaerobic conditions in the presence of glucose. However, the abundance of these organisms depends on the availability of Fe(III) (hydr)oxides. Redox cycling with nitrate results in decreased Fe(II) oxidation thereby decreasing the availability of Fe(III) for FeRB. Additionally, magnetite is detected as the main product of both lepidocrocite and ferrihydrite reduction. In contrast, introduction of air results in increased Fe(II) oxidation, increasing the availability of Fe(III) and the abundance of Geobacter. In the lepidocrocite reactors, Fe(II) oxidation by dissolved O2 promotes the formation of ferrihydrite and lepidocrocite, whereas in the ferrihydrite reactors we observe a decrease in magnetite stoichiometry (e.g., oxidation). Understanding Fe (hydr)oxide transformation under environmentally relevant redox cycling conditions provides insight into nutrient availability and transport, contaminant mobility, and microbial metabolism in soils and sediments.

Sterilized and non-sterilized soil columns were amended with three different carbon sources to elucidate the potential for geochemical and biological Cr6+ reduction. Cr6+ was reduced to Cr3+ in the non-sterilized lactate, ethanol, and acetate-amended soils; however, soils amended with lactate reduced significantly more chromium. Soils sterilized by γ-irradiation reduced almost no Cr6+, indicating that Cr6+ reduction was at least indirectly biological in nature. Analyses of small subunit (ssu) rRNA genes amplified from the column sediments showed significantly different bacterial populations within the amended soils that may be due to carbon source or to aerobic micropockets within the sediment columns.

The bioreduction of soluble uranium [U(VI)] to sparingly soluble U(IV) species is an attractive remedial technology for contaminated soil and groundwater due to the potential for immobilizing uranium and impeding its migration in subsurface environments. This manuscript describes a column study designed to simulate a three-step strategy proposed for the remediation of a heavily contaminated site at the U.S. Department of Energy's NABIR Field Research Center in Oak Ridge, TN. The soil is contaminated with high concentrations of uranium, aluminum, and nitrate and has a low, highly buffered pH (approximately 3.5). Steps proposed for remediation are (i) flushing to remove nitrate and aluminum, (ii) neutralization to establish pH conditions favorable for biostimulation, and (iii) biostimulation for U(VI) reduction. We simulated this sequence using a packed soil column containing undisturbed aggregates of U(VI)-contaminated saprolite that was flushed with an acidified salt solution (pH 4.0), neutralized with bicarbonate (60 mM), and then biostimulated by adding ethanol. The column was operated anaerobically in a closed-loop recirculation setup. However, during the initial month of biostimulation, ethanol was not utilized, and U(VI) was not reduced. A bacterial culture enriched from the site groundwaterwas subsequently added, and the consumption of ethanol coupled with sulfate reduction immediately ensued. The aqueous concentration of U(VI) initially increased, evidently because of the biological production of carbonate, a ligand known to solubilize uranyl. After approximately 50 days, aqueous U(VI) concentrations rapidly decreased from approximately 17 to1 mg/L. At the conclusion of the experiment,the presence of reduced solid phase U(IV) was confirmed using X-ray absorption near edge structure spectroscopy. The results indicate that bioreduction to immobilize uranium is potentially feasible at this site; however, the stability of the reduced U(IV) and its potential reoxidation will require further investigation, as do the effects of groundwater chemistry and competitive microbial processes, such as methanogenesis.

Naturally occurring manganese (Mn(iii/iv)) oxides are ubiquitous in a wide range of environmental settings and play a key role in numerous biogeochemical cycles. In addition, Mn(iii/iv) oxides are powerful oxidants that are capable of oxidizing a wide range of compounds. This review critically assesses the reactivity of Mn oxides with organic contaminants. Initial work with organic reductants employed high concentrations of model compounds (e.g., substituted phenols and anilines) and emphasized the reductive dissolution of the Mn oxides. Studies with lower concentrations of organic contaminants demonstrate that Mn oxides are capable of oxidizing a wide range of compounds (e.g., antibacterial agents, endocrine disruptors, and pesticides). Both model compounds and organic contaminants undergo similar reaction mechanisms on the oxide surface. The oxidation rates of organic compounds by manganese oxides are dependent upon solution conditions, such as pH and the presence of cations, anions, or dissolved organic matter. Similarly, physicochemical properties of the minerals used affect the rates of organic compound oxidation, which increase with the average oxidation state, redox potential, and specific surface area of the Mn oxides. Due to their reactivity with contaminants under environmentally relevant conditions, Mn oxides may oxidize contaminants in soils and/or be applied in water treatment applications.

International audience; The influence of arsenite [As(III)] on natural microbial communities and the capacity of exposed communities to oxidize As(III) has not been well explored. In this study, we conducted soil column experiments with a natural microbial community exposed to different carbon conditions and a continuous flow of As(III). We measured the oxidation rates of As(III) to As(V), and the composition of the bacterial community was monitored by 454 pyrosequencing of 16S rRNA genes. The diversity of As(III)-oxidizing bacteria was examined with the aox gene, which encodes the enzyme involved in As(III) oxidation. Arsenite oxidation was high in the live soil regardless of the carbon source and below detection in sterilized soil. In columns amended with 200 lmol kg À1 of As (III), As(V) concentrations reached 158 lmol kg À1 in the column effluent, while As(III) decreased to unmeasurable levels. Although the number of bacterial taxa decreased by as much as twofold in treatments amended with As(III), some As(III)-oxidizing bacterial groups increased up to 20-fold. Collectively, the data show the large effect of As(III) on bacterial diversity, and the capacity of natural communities from a soil with low initial As contamination to oxidize large inputs of As(III).

The Cr(III)-oxidizing capacity of three layered poorly crystalline Mn(IV)O(2) phases, i.e. δ-MnO(2), Random Stacked Birnessite (RSB), and Acid Birnessite (AB), was determined in real-time and in situ, using Quick X-ray Absorption Fine Structure Spectroscopy (Q-XAFS). The results obtained with this technique, which allows the measurement of the total amount of Cr(VI) produced in the system, indicated that the Cr(III) oxidation reaction had ceased between 30 min and 1 h under most experimental conditions. However, this cessation was not observed with a traditional batch technique, which only allows the measurement of Cr(VI) present in solution and thus neglects the amount of Cr(VI) that may be sorbed to Mn(IV)O(2). This study also demonstrated that the Mn(IV)O(2) phase oxidizing the highest amount of Cr(III), which is positively charged in solution, was the mineral featuring the most negatively charged surface. Also, the results indicated that the presence of Mn(II) and/or Mn(III) impurities inside the Mn(IV)O(2) structure could enhance the mineral's capacity to oxidize Cr(III). The information provided in this study will be useful in predicting the capabilities of naturally occurring Mn oxide minerals, which are similar to the three synthetic Mn(IV)O(2) investigated, to oxidize Cr(III) to toxic and mobile Cr(VI) in the soil of contaminated sites.

Arsenic (As) mobility in the environment is greatly affected by its oxidation state and the degree to which it is sorbed on metal oxide surfaces. Manganese oxides (Mn oxides) have the ability to decrease overall As mobility both by oxidizing toxic arsenite (As(III)) to less toxic arsenate (As(V)), and by sorbing As. However, the effect of competing ions on the mobility of As sorbed on Mn-oxide surfaces is not well understood. In this study, desorption of As(V) and As(III) from a poorly crystalline phyllomanganate (δ-MnO(2)) by two environmentally significant ions is investigated using a stirred-flow technique and X-ray absorption spectroscopy (XAS). As(III) is not observed in solution after desorption under any conditions used in this study, agreeing with previous studies showing As sorbed on Mn-oxides exists only as As(V). However, some As(V) is desorbed from the δ-MnO(2) surface under all conditions studied, while neither desorptive used in this study completely removes As(V) from the δ-MnO(2) surface.

This study determined Cd speciation and release kinetics in a Cd-Zn cocontaminated alkaline paddy soil, under various flooding periods and draining conditions, by employing synchrotron-based techniques, and a stirred-flow kinetic method. Results revealed that varying flooding periods and draining conditions affected Cd speciation and its release kinetics. Linear least-squares fitting (LLSF) of bulk X-ray absorption fine structure (XAFS) spectra of the air-dried, and the 1 day-flooded soil samples, showed that at least 50% of Cd was bound to humic acid. Cadmium carbonates were found as the major species at most flooding periods, while a small amount of cadmium sulfide was found after the soils were flooded for longer periods. Under all flooding and draining conditions, at least 14 mg/kg Cd was desorbed from the soil after a 2-hour desorption experiment. The results obtained by micro X-ray fluorescence (μ-XRF) spectroscopy showed that Cd was less associated with Zn than Ca, in most soil samples. Therefore, it is more likely that Cd and Ca will be present in the same mineral phases rather than Cd and Zn, although the source of these two latter elements may originate from the same surrounding Zn mines in the Mae Sot district.

Manganese-oxides (Mn-oxides) are quite reactive, with respect to arsenite (As(III)) oxidation. However, studies regarding the pathways of As(III) oxidation, over a range of time scales, by poorly crystalline Mn-oxides, are lacking. In stirred-flow experiments, As(III) oxidation by δ-MnO₂ (a poorly crystalline form of hexagonal birnessite) is initially rapid but slows appreciably after several hours of reaction. Mn(II) is the only reduced product of δ-MnO₂ formed by As(III) oxidation during the initial, most rapid phase of the reaction. There seems to be evidence that the formation of Mn(III) observed in previous studies is a result of conproportionation of Mn(II) sorbed onto Mn(IV) reaction sites rather than from direct reduction of Mn(IV) by As(III).The only evidence of arsenic (As) sorption during As(III) oxidation by δ-MnO₂ is during the first 10 h of reaction, and As sorption is greater when As(V) and Mn(II) occur simultaneously in solution. Our findings indicate that As(III) oxidation by poorly crystalline δ-MnO₂ involves several simultaneous reactions and reinforces the importance of studying reaction mechanisms over time.

Biologically catalyzed Mn(II) oxidation produces biogenic Mn-oxides (BioMnO(x)) and may serve as one of the major formation pathways for layered Mn-oxides in soils and sediments. The structure of Mn octahedral layers in layered Mn-oxides controls its metal sequestration properties, photochemistry, oxidizing ability, and topotactic transformation to tunneled structures. This study investigates the impacts of cations (H(+), Ni(II), Na(+), and Ca(2+)) during biotic Mn(II) oxidation on the structure of Mn octahedral layers of BioMnO(x) using solution chemistry and synchrotron X-ray techniques. Results demonstrate that Mn octahedral layer symmetry and composition are sensitive to previous cations during BioMnO(x) formation. Specifically, H(+) and Ni(II) enhance vacant site formation, whereas Na(+) and Ca(2+) favor formation of Mn(III) and its ordered distribution in Mn octahedral layers. This study emphasizes the importance of the abiotic reaction between Mn(II) and BioMnO(x) and dependence of the crystal structure of BioMnO(x) on solution chemistry.

In order to learn about in situ structural changes in materials at subseconds time scale, we have further refined the techniques of quick extended x-ray absorption fine structure (QEXAFS) and quick x-ray absorption near edge structure (XANES) spectroscopies at beamline X18B at the National Synchrotron Light Source. The channel cut Si (111) monochromator oscillation is driven through a tangential arm at 5 Hz, using a cam, dc motor, pulley, and belt system. The rubber belt between the motor and the cam damps the mechanical noise. EXAFS scan taken in 100 ms is comparable to standard data. The angle and the angular range of the monochromator can be changed to collect a full EXAFS or XANES spectrum in the energy range 4.7-40.0 KeV. The data are recorded in ascending and descending order of energy, on the fly, without any loss of beam time. The QEXAFS mechanical system is outside the vacuum system, and therefore changing the mode of operation from conventional to QEXAFS takes only a few minutes. This instrument allows the acquisition of time resolved data in a variety of systems relevant to electrochemical, photochemical, catalytic, materials, and environmental sciences.

During the last two decades, X-ray absorption spectroscopy (XAS) has developed into a mature technique for obtaining the speciation (e.g., oxidation state) and short-range structure of elements present in soils and sediments. XAS encompasses both X-ray absorption near-edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. XAS has a number of advantageous qualities for studying soils and sediments, which include elemental specificity, sensitivity to the local chemical and structural state of an element, and the ability to analyze materials in situ. This information allows accurate determination of oxidation state, type of nearest neighbors, coordination number, bond distance, and orbital symmetries of the X-ray absorbing element. In this review, we examine the application of a wide variety of synchrotron X-ray techniques to fundamental issues in environmental soil chemistry. Additionally, we examine the application of microfocused and time-resolved XAS to determine speciation (e.g., oxidation state and/or local coordination environment) and transformation kinetics of contaminants in heterogeneous environmental systems. During the last three decades, XAS has a played a critical role in furthering our understanding of a myriad of environmental systems and will continue to do so into the foreseeable future.

Life and element cycling on Earth is directly related to electron transfer (or redox) reactions. An understanding of biogeochemical redox processes is crucial for predicting and protecting environmental health and can provide new opportunities for engineered remediation strategies. Energy can be released and stored by means of redox reactions via the oxidation of labile organic carbon or inorganic compounds (electron donors) by microorganisms coupled to the reduction of electron acceptors including humic substances, iron-bearing minerals, transition metals, metalloids, and actinides. Environmental redox processes play key roles in the formation and dissolution of mineral phases. Redox cycling of naturally occurring trace elements and their host minerals often controls the release or sequestration of inorganic contaminants. Redox processes control the chemical speciation, bioavailability, toxicity, and mobility of many major and trace elements including Fe, Mn, C, P, N, S, Cr, Cu, Co, As, Sb, Se, Hg, Tc, and U. Redox-active humic substances and mineral surfaces can catalyze the redox transformation and degradation of organic contaminants. In this review article, we highlight recent advances in our understanding of biogeochemical redox processes and their impact on contaminant fate and transport, including future research needs.

The initial kinetics of Cr(III) oxidation on mineral surfaces is poorly understood, yet a significant portion of the oxidation process occurs during the first seconds of reaction. In this study, the initial rates of Cr(III) oxidation on hydrous manganese oxide (HMO) were measured at three different pH values (pH 2.5, 3, and 3.5), using a quick X-ray absorption fine structure spectroscopy (Q-XAFS) batch method. The calculated rate constants were 0.201, 0.242, and 0.322 s(-1) at pH 2.5, 3, and 3.5, respectively. These values were independent of both [Cr(III)] and [Mn(II)] and mixing speed, suggesting that the reaction was "chemically" controlled and not dependent upon diffusion at the time period the rate parameters were measured. A second-order overall rate was found at three pH values. This represents the first study to determine the chemical kinetics of Cr(III) oxidation on Mn-oxides. The results have important implications for the determination of rapid, environmentally important reactions that cannot be measured with traditional batch and flow techniques. An understanding of these reactions is critical to predicting the fate of contaminants in aquatic and terrestrial environments.

Microbial enumeration, 16S rRNA gene clone libraries, and chemical analysis were used to evaluate the in situ biological reduction and immobilization of uranium(VI) in a long-term experiment (more than 2 years) conducted at a highly uranium-contaminated site (up to 60 mg/liter and 800 mg/kg solids) of the U.S. Department of Energy in Oak Ridge, TN. Bioreduction was achieved by conditioning groundwater above ground and then stimulating growth of denitrifying, Fe(III)-reducing, and sulfate-reducing bacteria in situ through weekly injection of ethanol into the subsurface. After nearly 2 years of intermittent injection of ethanol, aqueous U levels fell below the U.S. Environmental Protection Agency maximum contaminant level for drinking water and groundwater (Desulfovibrio, Geobacter, Anaeromyxobacter, Desulfosporosinus , and Acidovorax spp. The predominant sulfate-reducing bacterial species were Desulfovibrio spp., while the iron reducers were represented by Ferribacterium spp. and Geothrix spp. Diversity-based clustering revealed differences between treated and untreated zones and also within samples of the treated area. Spatial differences in community structure within the treatment zone were likely related to the hydraulic pathway and to electron donor metabolism during biostimulation.

Aluminum sulfate (alum; Al2(SO4)3{center_dot}14H2O) is used as a chemical treatment of poultry litter to reduce the solubility and release of phosphate, thereby minimizing the impacts on adjacent aquatic ecosystems when poultry litter is land applied as a crop fertilizer. The objective of this study was to determine, through the use of X-ray absorption near edge structure (XANES) spectroscopy and sequential extraction, how alum amendments alter P distribution and solid-state speciation within the poultry litter system. Our results indicate that traditional sequential fractionation procedures may not account for variability in P speciation in heterogeneous animal manures. Analysis shows that NaOH-extracted P in alum amended litters is predominantly organic ({approx}80%), whereas in the control samples, >60% of NaOH-extracted P was inorganic P. Linear least squares fitting (LLSF) analysis of spectra collected of sequentially extracted litters showed that the P is present in inorganic (P sorbed on Al oxides, calcium phosphates) and organic forms (phytic acid, polyphosphates, and monoesters) in alum- and non-alum-amended poultry litter. When determining land application rates of poultry litter, all of these compounds must be considered, especially organic P. Results of the sequential extractions in conjunction with LLSF suggest that no P species is completely removed by a singlemore » extractant. Rather, there is a continuum of removal as extractant strength increases. Overall, alum-amended litters exhibited higher proportions of Al-bound P species and phytic acid, whereas untreated samples contained Ca-P minerals and organic P compounds. This study provides in situ information about P speciation in the poultry litter solid and about P availability in alum- and non-alum-treated poultry litter that will dictate P losses to ground and surface water systems.« less

Groundwater within Area 3 of the U.S. Department of Energy (DOE) Environmental Remediation Sciences Program (ERSP) Field Research Center at Oak Ridge, TN (ORFRC) contains up to 135 microM uranium as U(VI). Through a series of experiments at a pilot scale test facility, we explored the lower limits of groundwater U(VI) that can be achieved by in-situ biostimulation and the effects of dissolved oxygen on immobilized uranium. Weekly 2 day additions of ethanol over a 2-year period stimulated growth of denitrifying, Fe(III)-reducing, and sulfate-reducing bacteria, and immobilization of uranium as U(IV), with dissolved uranium concentrations decreasing to low levels. Following sulfite addition to remove dissolved oxygen, aqueous U(VI) concentrations fell below the U.S. Environmental Protection Agengy maximum contaminant limit (MCL) for drinking water (30/microg L(-1) or 0.126 microM). Under anaerobic conditions, these low concentrations were stable, even in the absence of added ethanol. However, when sulfite additions stopped, and dissolved oxygen (4.0-5.5 mg L(-1)) entered the injection well, spatially variable changes in aqueous U(VI) occurred over a 60 day period, with concentrations increasing rapidly from0.13 to 2.0 microM at a multilevel sampling (MLS) well located close to the injection well, but changing little at an MLS well located further away. Resumption of ethanol addition restored reduction of Fe(III), sulfate, and U(VI) within 36 h. After 2 years of ethanol addition, X-ray absorption near-edge structure spectroscopy (XANES) analyses indicated that U(IV) comprised 60-80% of the total uranium in sediment samples. Atthe completion of the project (day 1260), U concentrations in MLS wells were less than 0.1 microM. The microbial community at MLS wells with low U(VI) contained bacteria that are known to reduce uranium, including Desulfovibrio spp. and Geobacter spp., in both sediment and groundwater. The dominant Fe(III)-reducing species were Geothrix spp.

The nickel (Ni) hyperaccumulator Alyssum murale has been developed as a commercial crop for phytoremediation/phytomining Ni from metal-enriched soils. Here, metal co-tolerance, accumulation and localization were investigated for A. murale exposed to metal co-contaminants. A. murale was irrigated with Ni-enriched nutrient solutions containing basal or elevated concentrations of cobalt (Co) or zinc (Zn). Metal localization and elemental associations were investigated in situ with synchrotron X-ray microfluorescence (SXRF) and computed-microtomography (CMT). A. murale hyperaccumulated Ni and Co (> 1000 microg g(-1) dry weight) from mixed-metal systems. Zinc was not hyperaccumulated. Elevated Co or Zn concentrations did not alter Ni accumulation or localization. SXRF images showed uniform Ni distribution in leaves and preferential localization of Co near leaf tips/margins. CMT images revealed that leaf epidermal tissue was enriched with Ni but devoid of Co, that Co was localized in the apoplasm of leaf ground tissue and that Co was sequestered on leaf surfaces near the tips/margins. Cobalt-rich mineral precipitate(s) form on leaves of Co-treated A. murale. Specialized biochemical processes linked with Ni (hyper)tolerance in A. murale do not confer (hyper)tolerance to Co. A. murale relies on a different metal storage mechanism for Co (exocellular sequestration) than for Ni (vacuolar sequestration).

One method for the in situ remediation of uranium contaminated subsurface environments is the removal of highly soluble U(VI) from groundwater by microbial reduction to the sparingly soluble U(IV) mineral uraninite. Success of this remediation strategy will, in part, be determined by the extent and products of microbial reduction. In heterogeneous subsurface environments, microbial processes will likely yield a combination of U(IV) and U(VI) phases distributed throughout the soil matrix. Here, we use a combination of bulk X‐ray absorption spectroscopy (XAS) and micro‐focused XAS and X‐ray diffraction to determine uranium speciation and distribution with sediment from a pilot‐scale uranium remediation project located in Oak Ridge, TN.

In aerobic environments, uranium is generally found in the hexavalent oxidation state, is quite soluble, and readily forms complexes with calcium and carbonate. However, under anaerobic conditions, common metal respiring bacteria can enzymatically reduce U(VI) to U(IV), resulting in the formation of sparingly soluble UO 2 (uraninite). Uranium(VI) reduction, therefore, has a prominent role in natural attenuation of uranium and is being explored as a potential remediation option for this hazardous element. The stability of biologically precipitated uraninite is critical for determining the long-term fate of uranium and is not well characterized within soils and sediments. Given their environmental predominance, Fe(III) (hydr)oxides, which act as both U(VI) reductants and U(IV) oxidants, exert a prominent role in determining uranium chemical fate and associated mobility. Here, we examine several factors controlling the extent of uraninite oxidation by Fe(III) (hydr)oxides, including iron oxide and bicarbonate concentration, in addition to iron oxide type. Our analysis reveals that the extent of uraninite oxidation increases under conditions that increase the thermodynamic probability of the reaction; however, recrystallization of poorly crystalline Fe(III) (hydr)oxides to more crystalline forms may ultimately limit the uraninite oxidation reaction. Nevertheless, uraninite oxidation by Fe(III) (hydr)oxides may be a limiting factor on uraninite stability in the environment.

In situ microbial reduction of soluble U(VI) to sparingly soluble U(IV) was evaluated at the site of the former S-3 Ponds in Area 3 of the U.S. Department of Energy Natural and Accelerated Bioremediation Research Field Research Center, Oak Ridge, TN. After establishing conditions favorable for bioremediation (Wu, et al. Environ. Sci. Technol. 2006, 40, 3988-3995), intermittent additions of ethanol were initiated within the conditioned inner loop of a nested well recirculation system. These additions initially stimulated denitrification of matrix-entrapped nitrate, but after 2 months, aqueous U levels fell from 5 to approximately 1 microM and sulfate reduction ensued. Continued additions sustained U(VI) reduction over 13 months. X-ray near-edge absorption spectroscopy (XANES) confirmed U(VI) reduction to U(IV) within the inner loop wells, with up to 51%, 35%, and 28% solid-phase U(IV) in sediment samples from the injection well, a monitoring well, and the extraction well, respectively. Microbial analyses confirmed the presence of denitrifying, sulfate-reducing, and iron-reducing bacteria in groundwater and sediments. System pH was generally maintained at less than 6.2 with low bicarbonate level (0.75-1.5 mM) and residual sulfate to suppress methanogenesis and minimize uranium mobilization. The bioavailability of sorbed U(VI) was manipulated by addition of low-level carbonate (5 mM) followed by ethanol (1-1.5 mM). Addition of low levels of carbonate increased the concentration of aqueous U, indicating an increased rate of U desorption due to formation of uranyl carbonate complexes. Upon ethanol addition, aqueous U(VI) levels fell, indicating that the rate of microbial reduction exceeded the rate of desorption. Sulfate levels simultaneously decreased, with a corresponding increase in sulfide. When ethanol addition ended but carbonate addition continued, soluble U levels increased, indicating faster desorption than reduction. When bicarbonate addition stopped, aqueous U levels decreased, indicating adsorption to sediments. Changes in the sequence of carbonate and ethanol addition confirmed that carbonate-controlled desorption increased bioavailability of U(VI) for reduction.

Uranium mobility in the environment is partially controlled by its oxidation state, where it exists as either U(VI) or U(IV). In aerobic environments, uranium is generally found in the hexavalent form, is quite soluble, and readily forms complexes with carbonate and calcium. Under anaerobic conditions, common metal respiring bacteria can reduce soluble U(VI) species to sparingly soluble UO2 (uraninite); stimulation of these bacteria, in fact, is being explored as an in situ uranium remediation technique. However, the stability of biologically precipitated uraninite within soils and sediments is not well characterized. Here we demonstrate that uraninite oxidation by Fe(III) (hydr)oxides is thermodynamically favorable under limited geochemical conditions. Our analysis reveals that goethite and hematite have a limited capacity to oxidize UO2(biogenic) while ferrihydrite can lead to UO2(biogenic) oxidation. The extent of UO2(biogenic) oxidation by ferrihydrite increases with increasing bicarbonate and calcium concentration, but decreases with elevated Fe(II)(aq) and U(VI)(aq) concentrations. Thus, our results demonstrate that the oxidation of UO2(biogenic) by Fe(III) (hydr)oxides may transpire under mildly reducing conditions when ferrihydrite is present.

Chromate is a widespread contaminantthat has deleterious impacts on human health, the mobility and toxicity of which are diminished by reduction to Cr(III). While biological and chemical reduction reactions of Cr(VI) are well resolved, reduction within natural sediments, particularly of arid environments, remains poorly described. Here, we examine chromate reduction within arid sediments from the Hanford, WA site, where Fe(III) (hydr)oxide and carbonate coatings limit mineral reactivity. Chromium(VI) reduction by Hanford sediments is negligible unless pretreated with acid; acidic pretreatment of packed mineral beds having a Cr(VI) feed solution results in Cr(III) associating with the minerals antigorite and lizardite in addition to magnetite and Fe(II)-bearing clay minerals. Highly alkaline conditions (pH14), representative of conditions near high-level nuclearwaste tanks, result in Fe(II) dissolution and concurrent Cr(VI) reduction. Additionally, Cr(III) and Cr(VI) are found associated with portlandite, suggesting a secondary mechanism for chromium retention at high pH. Thus, mineral reactivity is limited within this arid environment and appreciable reduction of Cr(VI) is restricted to highly alkaline conditions resulting near leaking radioactive waste disposal tanks.

A field-scale experiment to assess biostimulation of uranium reduction is underway at the Natural and Accelerated Bioremediation Research Field Research Center (FRC) in Oak Ridge, Tennessee. To simulate the field experiment, we established replicate batch microcosms containing well-mixed contaminated sediment from a well within the FRC treatment zone, and we added an inoculum from a pilot-scale fluidized bed reactor representing the inoculum in the field experiment. After reduction of nitrate, both sulfate and soluble U(VI) concentration decreased. X-ray absorption near edge structure (XANES) spectroscopy confirmed formation of U(IV) in sediment from biostimulated microcosms, but did not detect reduction of solid-phase Fe(III). Two to three fragments dominated terminal restriction fragment length polymorphism (T-RFLP) profiles of the 16S rDNA gene. Comparison to a clone library indicated these fragments represented denitrifying organisms related to Acidovorax, and Acidovorax isolates from the inoculum were subsequently shown to reduce U(VI). Investigation using the T-RFLP Analysis Program (TAP T-RFLP) and chemical analyses detected the presence and activity of fermenting and sulfate-reducing bacteria after 2 weeks. These organisms likely contributed to uranium reduction. In some microcosms, soluble U(VI) concentration leveled off or rebounded, indicating microbial and/or mineralogical heterogeneity among samples. Sulfate, acetate, and ethanol were depleted only in those microcosms exhibiting a rebound in soluble U(VI). This suggests that rates of U(VI) desorption can exceed rates of U(VI) reduction when sulfate-reducing bacteria become substrate-limited. These observations underscore the importance of effective chemical delivery and the role of serial and parallel processes in uranium reduction.