Toward a mechanistic understanding of cesium adsorption to todorokite: A molecular dynamics simulation study.

A mechanistic understanding of cesium (Cs) adsorption to soil mineral phases is essential for effective mitigation of Cs mobility in the subsurface environment. Todorokite, a common tunnel-structured manganese oxide in soil, exhibits sorption capacity for Cs comparable to the capacities of clay minerals. However, the adsorption sites and molecular species of Cs+ adsorbed to todorokite remain uncertain in comparison with those of clay minerals. In this study, we explored adsorption of Cs+ to hydrated todorokite surfaces via atomistic molecular dynamics (MD) simulations. We performed the first MD simulations based on atomic pair potentials for Mn-oxide edge surfaces interfaced with an aqueous solution. MD simulations predicted that Cs+ forms only inner-sphere (IS) complexes within todorokite tunnels; however, Cs+ forms both IS and outer-sphere (OS) complexes at the external (010) and (100)/(001) external surfaces. On the (010) surface, the positions between IS and OS complexes of Cs+ were interchangeable during MD simulations. Detailed molecular structures of IS and OS Cs+ surface complexes are compared to those of Cs+ in an aqueous solution. The current MD simulation results can be used as an atomistic structural proxy for spectroscopic analysis of adsorbed metal speciation and surface complexation modeling of metal adsorption to Mn oxides.

Thermal effect on the leachability of extraframework Co2+ in zeolite X.

A partially Co2+-exchanged zeolite X was thermally treated to simulate the effect of decay heat on the leachability of extraframework Co2+. To have a mechanistic insight into thermal effect, X-ray diffraction, scanning electron microscopy, 27Al magic angle spinning nuclear magnetic resonance spectroscopy, and Co K-edge X-ray absorption spectroscopy were employed with leaching tests. Although thermal treatment at ≤ 600 °C did not lead to the collapse of zeolite framework, it removed H2O molecules from the coordination shell of extraframework Co2+, which in turn changed its coordination structure in a way to strengthen the interaction between Co2+ and the lattice oxygens. In leaching tests, the sample treated at higher temperature for a longer period showed less remobilized Co2+ by forming a Co(OH)2-like surface precipitate and a Co hydrotalcite-like phase. Notably, the formation of the latter phase indicated the abstraction of the framework Al, the extent of which increased with the treatment temperature and duration. Two mechanisms, the concurrent extraction of Al with Co2+ remobilization and the hydrolysis-promoted Al abstraction, were proposed to account for thermally promoted dealumination. This study suggests that the exposure of Co2+-exchanged zeolite X to decay heat lessen the risk of extraframework Co2+ to be reintroduced into groundwater.

A multidisciplinary assessment of the impact of spilled acids on geoecosystems: an overview.

We developed and applied a multidisciplinary approach to the impact of an accidentally spilled acid on the underlying geomedia and subsurface environment, based on the concept of geoecosystem. We used mineralogical, geochemical, microbiological, and ecotoxicological techniques to identify and assess the multiple aspects involved. First, we constructed a conceptual model for the acid interactions with the underlying subsurface environment by introducing the concept of a geoecosystem-a multicomponent system composed of inorganic, organic, and biological components to describe the subsurface environment. Second, we designed and manufactured a two dimensional cell to visualize acid transport through geomedia. Third, we hypothesized that the acids are neutralized through dissolution of minerals and protonation of functional groups on the surfaces of minerals and organic matter. We tested this hypothesis by conducting batch-type geomedia-acid reaction and surface titration experiments. Fourth, we observed changes in soil microbial communities before and after the acid exposure and neutralization treatment. Fifth, we performed flow-through experiments using columns packed with soil samples pre-contaminated with arsenic to investigate potential longer term, secondary effects of remnant acids on geoecosystems. Finally, we conducted ecotoxicological investigations using various geomedia and observed that suitability of the geoecosystem as a habitat deteriorated to different degrees depending on the respective systems' acid neutralizing power. We conclude that a holistic understanding of the interactions among the multiple components of geoecosystems and subsequent estimation of the influenced area requires a multidisciplinary approach such as those used in this study. Based on the findings of this study, we propose geoecosystems' vulnerability defined as the reciprocal of their acid-neutralizing capacity against the moving acid fronts and present this concept as central to a quantitative assessment of the impact of acid spills on geoecosystems. We also inventoried the essential components, factors, and parameters necessary in developing geoecosystems' acid vulnerability assessment system.

Beam-induced redox transformation of arsenic during As K-edge XAS measurements: availability of reducing or oxidizing agents and As speciation.

During X-ray absorption spectroscopy (XAS) measurements of arsenic (As), beam-induced redox transformation is often observed. In this study, the As species immobilized by poorly crystallized mackinawite (FeS) was assessed for the susceptibility to beam-induced redox reactions as a function of sample properties including the redox state of FeS and the solid-phase As speciation. The beam-induced oxidation of reduced As species was found to be mediated by the atmospheric O2 and the oxidation products of FeS [e.g. Fe(III) (oxyhydr)oxides and intermediate sulfurs]. Regardless of the redox state of FeS, both arsenic sulfide and surface-complexed As(III) readily underwent the photo-oxidation upon exposure to the atmospheric O2 during XAS measurements. With strict O2 exclusion, however, both As(0) and arsenic sulfide were less prone to the photo-oxidation by Fe(III) (oxyhydr)oxides than NaAsO2 and/or surface-complexed As(III). In case of unaerated As(V)-reacted FeS samples, surface-complexed As(V) was photocatalytically reduced during XAS measurements, but arsenic sulfide did not undergo the photo-reduction.

Soil microbial community responses to acid exposure and neutralization treatment.

Changes in microbial community induced by acid shock were studied in the context of potential release of acids to the environment due to chemical accidents. The responses of microbial communities in three different soils to the exposure to sulfuric or hydrofluoric acid and to the subsequent neutralization treatment were investigated as functions of acid concentration and exposure time by using 16S-rRNA gene based pyrosequencing and DGGE (Denaturing Gradient Gel Electrophoresis). Measurements of soil pH and dissolved ion concentrations revealed that the added acids were neutralized to different degrees, depending on the mineral composition and soil texture. Hydrofluoric acid was more effectively neutralized by the soils, compared with sulfuric acid at the same normality. Gram-negative ß-Proteobacteria were shown to be the most acid-sensitive bacterial strains, while spore-forming Gram-positive Bacilli were the most acid-tolerant. The results of this study suggest that the Gram-positive to Gram-negative bacterial ratio may serve as an effective bio-indicator in assessing the impact of the acid shock on the microbial community. Neutralization treatments helped recover the ratio closer to their original values. The findings of this study show that microbial community changes as well as geochemical changes such as pH and dissolved ion concentrations need to be considered in estimating the impact of an acid spill, in selecting an optimal remediation strategy, and in deciding when to end remedial actions at the acid spill impacted site.

Effect of the redox dynamics on microbial-mediated As transformation coupled with Fe and S in flow-through sediment columns.

Arsenic (As) biogeochemistry coupled with iron (Fe) and sulfur (S) was studied using columns packed with As(V)-contaminated sediments under two phases: a reduction phase followed by an oxidation phase. During the reduction phase, four identical columns inoculated with G. sulfurreducens were stimulated with 3mM acetate for 60days. The As(III) in the effluent rapidly increased then gradually decreased. The Fe(II) and sulfate concentration indicated ferrous sulfide precipitation inside the column after day 14 and X-ray absorption near edge structure spectra showed that As(III) was enriched at the column outlet. The genera Desulfosporosinus and Anaeromyxobacter as well as the Geobacter inoculum played a primary role in As reduction. During the oxidation phase, dissolved oxygen was consumed by heterotrophic aerobes belonging to the phylum Cloroflexi in the column with acetate, resulting in more As in the effluent. When only nitrate was injected, sulfur-oxidizing bacteria such as Thiobacillus thioparus instantly oxidized the sulfide formed during the first phase, resulting in less As(V) in the aqueous phase compared to the column with dissolved oxygen alone. This study showed that redox gradients and dynamics linked to Fe and S biogeochemistry have an important role in controlling As mobility in subsurface environments.

Hydrologic Controls on Nitrogen Cycling Processes and Functional Gene Abundance in Sediments of a Groundwater Flow-Through Lake.

The fate and transport of inorganic nitrogen (N) is a critically important issue for human and aquatic ecosystem health because discharging N-contaminated groundwater can foul drinking water and cause algal blooms. Factors controlling N-processing were examined in sediments at three sites with contrasting hydrologic regimes at a lake on Cape Cod, MA. These factors included water chemistry, seepage rates and direction of groundwater flow, and the abundance and potential rates of activity of N-cycling microbial communities. Genes coding for denitrification, anaerobic ammonium oxidation (anammox), and nitrification were identified at all sites regardless of flow direction or groundwater dissolved oxygen concentrations. Flow direction was, however, a controlling factor in the potential for N-attenuation via denitrification in the sediments. Potential rates of denitrification varied from 6 to 4500 pmol N/g/h from the inflow to the outflow side of the lake, owing to fundamental differences in the supply of labile organic matter. The results of laboratory incubations suggested that when anoxia and limiting labile organic matter prevailed, the potential existed for concomitant anammox and denitrification. Where oxic lake water was downwelling, potential rates of nitrification at shallow depths were substantial (1640 pmol N/g/h). Rates of anammox, denitrification, and nitrification may be linked to rates of organic N-mineralization, serving to increase N-mobility and transport downgradient.

Abiotic reductive dechlorination of cis-DCE by ferrous monosulfide mackinawite.

Cis-1,2,-dichloroethylene (cis-DCE) is a toxic, persistent contaminant occurring mainly as a daughter product of incomplete degradation of perchloroethylene (PCE) and trichloroethylene (TCE). This paper reports on abiotic reductive dechlorination of cis-DCE by mackinawite (FeS1-x), a ferrous monosulfide, under variable geochemical conditions. To assess in situ abiotic cis-DCE dechlorination by mackinawite in the field, mackinawite suspensions prepared in a field groundwater sample collected from a cis-DCE contaminated field site were used for dechlorination experiments. The effects of geochemical variables on the dechlorination rates were monitored. A set of dechlorination experiments were also carried out in the presence of aquifer sediment from the site over a range of pH conditions to better simulate the actual field situations. The results showed that the suspensions of freshly prepared mackinawite reductively transformed cis-DCE to acetylene, whereas the conventionally prepared powder form of mackinawite had practically no reactivity with cis-DCE under the same experimental conditions. Significant cis-DCE degradation by mackinawite has not been reported prior to this study, although mackinawite has been shown to reductively transform PCE and TCE. This study suggests feasibility of using mackinawite for in situ remediation of cis-DCE-contaminated sites with high S levels such as estuaries under naturally achieved or stimulated sulfate-reducing conditions.

Growth of Desulfovibrio vulgaris when respiring U(VI) and characterization of biogenic uraninite.

The capacity of Desulfovibrio vulgaris to reduce U(VI) was studied previously with nongrowth conditions involving a high biomass concentration; thus, bacterial growth through respiration of U(VI) was not proven. In this study, we conducted a series of batch tests on U(VI) reduction by D. vulgaris at a low initial biomass (10 to 20 mg/L of protein) that could reveal biomass growth. D. vulgaris grew with U(VI) respiration alone, as well as with simultaneous sulfate reduction. Patterns of growth kinetics and solids production were affected by sulfate and Fe(2+). Biogenic sulfide nonenzymatically reduced 76% of the U(VI) and greatly enhanced the overall reduction rate in the absence of Fe(2+) but was rapidly scavenged by Fe(2+) to form FeS in the presence of Fe(2+). Biogenic U solids were uraninite (UO2) nanocrystallites associated with 20 mg/g biomass as protein. The crystallite thickness of UO2 was 4 to 5 nm without Fe(2+) but was <1.4 nm in the presence of Fe(2+), indicating poor crystallization inhibited by adsorbed Fe(2+) and other amorphous Fe solids, such as FeS or FeCO3. This work fills critical gaps in understanding the metabolic utilization of U by microorganisms and formation of UO2 solids in bioremediation sites.

Kinetic study of cis-dichloroethylene (cis-DCE) and vinyl chloride (VC) dechlorination using green rusts formed under varying conditions.

Abiotic degradation of cis-dichloroethylene (cis-DCE) and vinyl chloride (VC) was investigated using Fe hydroxides obtained by hydrolyzing Fe(II) salts over a pH range of 7.7-8.0. Within this narrow pH range, a green rust (GR) precipitated. The dechlorination reactivity of the resulting GR precipitates increased with the dissolved Fe(II) concentration remaining in solution after precipitation. Controls run using only the dissolved Fe(II) supernatant were not reactive, suggesting the relative amount of Fe(II) on the surface of precipitated GRs was the causative agent in the relative reactivity. To test this, a series of GR batches with varying dissolved Fe(II) concentrations were prepared by acid-base titration and examined for cis-DCE and VC dechlorination kinetics under reducing conditions. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses of these batches were performed to characterize the bulk mineralogy and the excess surface Fe(II), respectively. Cis-DCE and VC dechlorination results along with solid phase characterization show that different surface Fe(II)/Fe(III) compositions are responsible for the different reactivity of GRs formed within the GR precipitation zone.

Uranium(VI) reduction by iron(II) monosulfide mackinawite.

Reaction of aqueous uranium(VI) with iron(II) monosulfide mackinawite in an O(2) and CO(2) free model system was studied by batch uptake measurements, equilibrium modeling, and L(III) edge U X-ray absorption spectroscopy (XAS). Batch uptake measurements showed that U(VI) removal was almost complete over the wide pH range between 5 and 11 at the initial U(VI) concentration of 5 × 10(-5) M. Extraction by a carbonate/bicarbonate solution indicated that most of the U(VI) removed from solution was reduced to nonextractable U(IV). Equilibrium modeling using Visual MINTEQ suggested that U was in equilibrium with uraninite under the experimental conditions. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy showed that the U(IV) phase associated with mackinawite was uraninite. Oxidation experiments with dissolved O(2) were performed by injecting air into the sealed reaction bottles containing mackinawite samples reacted with U(VI). Dissolved U measurement and XAS confirmed that the uraninite formed from the U(VI) reduction by mackinawite did not oxidize or dissolve under the experimental conditions. This study shows that redox reactions between U(VI) and mackinawite may occur to a significant extent, implying an important role of the ferrous sulfide mineral in the redox cycling of U under sulfate reducing conditions. This study also shows that the presence of mackinawite protects uraninite from oxidation by dissolved O(2). The findings of this study suggest that uraninite formation by abiotic reduction by the iron sulfide mineral under low temperature conditions is an important process in the redistribution and sequestration of U in the subsurface environments at U contaminated sites.

Simultaneous removal of nitrate and arsenic from drinking water sources utilizing a fixed-bed bioreactor system.

A novel bioreactor system, consisting of two biologically active carbon (BAC) reactors in series, was developed for the simultaneous removal of nitrate and arsenic from a synthetic groundwater supplemented with acetic acid. A mixed biofilm microbial community that developed on the BAC was capable of utilizing dissolved oxygen, nitrate, arsenate, and sulfate as the electron acceptors. Nitrate was removed from a concentration of approximately 50 mg/L in the influent to below the detection limit of 0.2 mg/L. Biologically generated sulfides resulted in the precipitation of the iron sulfides mackinawite and greigite, which concomitantly removed arsenic from an influent concentration of approximately 200 ug/L to below 20 ug/L through arsenic sulfide precipitation and surface precipitation on iron sulfides. This study showed for the first time that arsenic and nitrate can be simultaneously removed from drinking water sources utilizing a bioreactor system.

Surface complexation modeling of U(VI) adsorption by aquifer sediments from a former mill tailings site at Rifle, Colorado.

A study of U(VI) adsorption by aquifer sediment samples from a former uranium mill tailings site at Rifle, Colorado, was conducted under oxic conditions as a function of pH, U(VI), Ca, and dissolved carbonate concentration. Batch adsorption experiments were performed using <2 mm size sediment fractions, a sand-sized fraction, and artificial groundwater solutions prepared to simulate the field groundwater composition. To encompass the geochemical conditions of the alluvial aquifer at the site, the experimental conditions ranged from 6.8 x 10(-8) to 10(-5) M in [U(VI)](tot), 7.2 to 8.0 in pH, 3.0 x 10(-3) to 6.0 x 10(-3) M in [Ca(2+)], and 0.05 to 2.6% in partial pressure of carbon dioxide. Surface area normalized U(VI) adsorption K(d) values for the sand and <2 mm sediment fraction were similar, suggesting a similar reactive surface coating on both fractions. A two-site two-reaction, nonelectrostatic generalized composite surface complexation model was developed and successfully simulated the U(VI) adsorption data. The model successfully predicted U(VI) adsorption observed from a multilevel sampling well installed at the site. A comparison of the model with the one developed previously for a uranium mill tailings site at Naturita, Colorado, indicated that possible calcite nonequilibrium of dissolved calcium concentration should be evaluated. The modeling results also illustrate the importance of the range of data used in deriving the best fit model parameters.

Spectroscopic investigation of the uptake of arsenite from solution by synthetic mackinawite.

As(III) uptake from solution by synthetic mackinawite is examined as a function of pH and initial As(III) concentration using X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). XAS data indicate that when mackinawite is reacted at pH 5, 7, and 9 with 5 x 10(-4) M As(III), arsenic is reduced from its original +3 valence state and is primarily coordinated as As-S (approximately 2.26 angstroms) and As-As (approximately 2.54 angstroms), which is consistent with the formation of a realgar-like phase in agreement with XRD data. At 5 x 10(-5) M As(III), samples are markedly different from those collected at an order of magnitude higher concentration and differ at each pH value. The XAS analysis of mackinawite samples reacted with 5 x 10(-5) M As(III) shows a transition from As-O coordination to As-S coordination as pH decreases, with the sample reacted at pH 5 resembling realgar. Under alkaline conditions, arsenic retains its original valence state of +3 and is primarily coordinated to oxygen at a distance of 1.75 angstroms. This may be attributed to uptake by adsorption as an As(III) oxyanion. These results provide the basis for selecting the reactions needed for modeling and are beneficial in understanding the mechanisms of arsenite uptake by mackinawite under anoxic sulfidic conditions.

An electron paramagnetic resonance study of Cu(II) sorbed on quartz.

The nature of the interaction among Cu(II), adsorbed water, and quartz surface was studied using electron paramagnetic resonance (EPR) spectroscopy. The EPR lineshape gave information concerning the motional status of sorbed Cu(II) that revealed its binding strength at the surface. Two distinct absorption lines of sorbed Cu(II), namely, the liquid-type and the solid-type signal, were simultaneously observed at the fully hydrated surface at room temperature. The absorption lines and the variation of their intensity with experimental and measurement conditions such as degree of hydration, pH, ionic strength, and surface coverage indicated that there exist three kinds of Cu(II) entities, the inner-sphere surface complex, the outer-sphere surface complex, and the surface precipitate on the quartz surface, and that their concentrations change with experimental conditions. The reversible conversion of the liquid-type signal to the solid-type one during the drying-wetting or freezing-melting of the surface suggested the development of multiple layers of adsorbed water molecules on the quartz surface. It is assumed that the innermost layer of the water layers contains the inner-sphere Cu(II) surface complexes, while the outer layers contain the outer-sphere complexes whose binding strength decreases outward with increasing distance from the surface. The result of this work suggests that the sorption mechanism of a metal cation on a given mineral surface; hence its mobility in the environment may change significantly with the solution pH, the ionic strength, and the surface coverage.