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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

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 O2and 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 O2during XAS measurements. With strict O2exclusion, however, both As(0) and arsenic sulfide were less prone to the photo-oxidation by Fe(III) (oxyhydr)oxides than NaAsO2and/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.

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.

Submarine groundwater discharge (SGD) is a widely recognized process that carries considerable amounts of groundwater and dissolved chemicals to the coastal ocean. Despite its importance, a lack of suitable tools to assess SGD's spatial and temporal variability has hampered a complete understanding of the process. Here we report, for the first time, use of an unmanned aerial vehicle (UAV or “drone”) to assess SGD variations. An octocopter UAV platform equipped with a thermal infrared (TIR) system was flown along a coastline on Jeju Island, Korea. The UAV clearly captured thermal signatures of SGD plumes and their dynamic temporal fluctuations modulated by tidal variations. Based on a plume area-SGD flux relation we developed by combining aerial and field data, we estimated that the SGD flux of the study site ranged from 33,000 to 54,000 m3 d−1. The drone approach enabled acquisition of time series plume imagery with easy control of spatial resolution, flexible field operations, and remote sensing of SGD at low cost compared to conventional aerial surveys. Combining the UAV-TIR images with on-site sampling enables one to determine fluxes of nutrients and other dissolved species. UAV-TIR mapping can thus serve as a powerful tool for study of SGD and other coastal processes.

Submarine groundwater discharge (SGD) is a global phenomenon that carries large volumes of groundwater and dissolved chemical species such as nutrient, metals, and organic compounds to coastal zones. We report the influence of SGD on the coastal waters of Jeju Island, Korea, using high-resolution aerial thermal infrared (TIR) mapping techniques and field investigations. An aircraft-based system was implemented using a cost-effective TIR camera for aerial TIR mapping. Ground-based calibrations and system integration with GPS/IMU (global positioning system/inertial measurement unit) were performed for the aerial systems. The aerial surveys showed distinct low-temperature signatures of SGD along the coasts of Jeju Island, revealing large groundwater inputs from the coastal aquifers to the ocean. Multiple aerial surveys over a range of seasons and tidal stages revealed that SGD rates dynamically affect the sea surface temperature (SST) of the coastal zone. The in-situ measurements supported that SGD has a substantial influence on the coastal water chemistry as well as SST. Our observations highlight the extent to which aerial-based TIR mapping can serve as a powerful tool for studying SGD and other coastal processes. Copyright © 2016 John Wiley & Sons, Ltd.

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.

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.

Cu(II) coordination in the interlayer of an expandable clay mineral montmorillonite is studied using X-ray absorption spectroscopy (XAS) along with electron paramagnetic resonance (EPR) and X-ray diffraction (XRD). Ab initio calculations are performed using FEFF code to reproduce the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) portion of the experimentally measured XAS spectra. Cu(II) coordination changes depending on the Cu(II) loading and hydration state of the interlayer. XRD shows that the Cu-saturated montmorillonite has d001-spacing values corresponding to the interlayer thickness of four and one water layer(s) for the hydrated and dehydrated interlayers, respectively. EPR shows that Cu(II) in the fully hydrated, unsaturated interlayer behaves similarly to free Cu(II) ion in a bulk aqueous solution, while Cu(II) forms a square planar complex in the dehydrated interlayer. Cu(II) in the fully hydrated, Cu-saturated montmorillonite has a characteristic singlet 1st derivative XANES spectrum. FEFF calculations show that this singlet feature originates from a quasi-regular octahedral coordination of water molecules around the interlayer Cu(II) atom. All other samples and model compounds including the dry Cu-saturated montmorillonite, wet and dry unsaturated montmorillonite, aqueous Cu(II), cupric nitrate salt (Cu(NO3)2·4.5H2O), and Cu(II) hydroxide precipitates have doublet 1st derivative XANES spectra. FEFF calculations suggest that the doublet features arise from an axially elongated octahedral coordination under the Jahn–Teller effect or square planar coordination. FEFF calculations of the EXAFS spectra as a function of the axial oxygen bond length demonstrate that a destructive interference between backscattering from equatorial oxygen (Oeq) and that from axial oxygen (Oax) atoms leads to an apparent coordination number (CN) less than six expected for the tetragonal coordination, with the farther, loosely bound axial oxygen atoms making a minor, yet negative contribution to the CN determined by the EXAFS analysis. This study shows that Cu(II) has interchangeable octahedral, tetragonal, and square planar coordinations in the interlayer of montmorillonite, depending on Cu(II) loading and degree of hydration. The quasi-regular octahedral coordination of the interlayer Cu(II) in montmorillonite is a new finding of this study.

Reactions with aqueous sulfide are important in determining uranium (U) geochemistry under sulfate reducing conditions. This paper reports on abiotic reduction of U(VI) by aqueous sulfide under a range of experimental conditions using batch reactors. Dissolved U concentration was measured as a function of time to study the effects of chemical variables including pH, U(VI), S(−II), total dissolved carbonate (CARB = H 2 CO 3 * + HCO 3 − + CO 3 2− ), and Ca 2+ concentration on the U(VI) reduction rate. Solid phase reaction products were characterized using X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy. The chemical variables had impacts on the solid phase U(VI) reaction products as well as the reduction rates by aqueous sulfide. The solid U reaction product at circumneutral pH was identified as uraninite (UO 2+ x (s) ). Under basic pH conditions, whether a precipitate occurred depended on Ca 2+ and CARB concentrations. U(VI) reduction was faster under higher S(−II) concentrations but was slowed by increased dissolved Ca 2+ or CARB concentration. In the absence of dissolved CARB and Ca 2+ , a rapid decrease in dissolved U concentration occurred at circumneutral pH, while virtually no decrease was observed at pH 10.7 within the experimental timeframe of two days. The U(VI) reduction rate was proportional to the total concentration of free uranyl plus its hydrolysis complexes even at minor to trace concentrations. Dissolved Ca 2+ and CARB slow abiotic U(VI) reduction by forming stable Ca–U(VI)–carbonato soluble complexes that are resistant to reaction with aqueous sulfide. U(VI) reduction was slow in a synthetic solution representative of groundwater at a uranium mill tailings site. This study illustrates that abiotic U reduction by aqueous sulfide can significantly vary under typical ranges of chemical conditions in groundwater and newly demonstrates the importance of dissolved Ca 2+ in the abiotic U(VI) reduction by aqueous sulfide. The results contribute to our understanding of the impact of sulfate reducing conditions on U speciation in groundwater systems undergoing bioreduction conversion of U(VI) to less mobile U(IV) solid phases.

The long-term success of in situ reductive immobilization of uranium (U) depends on the stability of U(IV) precipitates (e.g., uraninite) in the presence of natural oxidants, such as oxygen, Fe(III) hydroxides, and nitrite. Field and laboratory studies have implicated iron sulfide minerals as redox buffers or oxidant scavengers that may slow oxidation of reduced U(IV) solid phases. Yet, the inhibition mechanism(s) and reaction rates of uraninite (UO2) oxidative dissolution by oxic species such as oxygen in FeS-bearing systems remain largely unresolved. To address this knowledge gap, abiotic batch experiments were conducted with synthetic UO2 in the presence and absence of synthetic mackinawite (FeS) under simulated groundwater conditions of pH = 7, P O 2 = 0.02 atm, and P CO 2 = 0.05 atm. The kinetic profiles of dissolved uranium indicate that FeS inhibited UO2 dissolution for about 51 h by effectively scavenging oxygen and keeping dissolved oxygen (DO) low. During this time period, oxidation of structural Fe(II) and S(-II) of FeS were found to control the DO levels, leading to the formation of iron oxyhydroxides and elemental sulfur, respectively, as verified by X-ray diffraction (XRD), Mossbauer, and X-ray absorption spectroscopy (XAS). After FeS was depleted due to oxidation, DO levels increased and UO2 oxidative dissolution occurred at an initial rate of rm = 1.2 ± 0.4 × 10−8 mol g−1 s−1, higher than rm = 5.4 ± 0.3 × 10−9 mol g−1 s−1 in the control experiment where FeS was absent. XAS analysis confirmed that soluble U(VI)-carbonato complexes were adsorbed by iron oxyhydroxides (i.e., nanogoethite and lepidocrocite) formed from FeS oxidation, which provided a sink for U(VI) retention. This work reveals that both the oxygen scavenging by FeS and the adsorption of U(VI) to FeS oxidation products may be important in U reductive immobilization systems subject to redox cycling events.

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.

Iron sulfide minerals commonly found in natural anoxic Fe-S systems have been shown to reductively transform chlorinated hydrocarbons including trichloroethylene (TCE). In the present study, we tested the feasibility of applying an Fe(II) solution to a TCE-contaminated aquifer groundwater under simulated sulfide reducing conditions to enhance reductive transformation of TCE to nontoxic compounds. To achieve this goal, iron sulfide particles were precipitated under a range of pH and Fe:S molar ratios in aquifer groundwater samples from the Dugway Proving Grounds, Utah. Batch tests for abiotic reductive dechlorination of TCE were performed using the precipitates to establish the conditions for most favorable solids for dechlorination. Under all experimental conditions, the solids formed consisted mainly of mackinawite, a tetragonal reduced iron monosulfide FeS1-x . However, the precipitation conditions strongly affected the reactivity of the mackinawite particles formed. The results indicated that addition ...

Cu(II) sorbed on kaolinite (KGa-1b) was studied using electron paramagnetic resonance (EPR) spectroscopy over various pH, ionic strength, and Cu(II) concentration conditions. Two different absorption lines, namely, an isotropic ( g iso = 2.193) and an anisotropic signal ( g || = 2.365 and g ⊥ = 2.088) were observed at room temperature. The signals were assigned to an outer-sphere and an inner-sphere Cu(II) surface complex, respectively. The intensity of the two signals varied with the sorption conditions or post-sorptional treatments such as rinsing and dehydration/rehydration. The intensity of the isotropic signal was not directly proportional to the amount of sorbed Cu(II). Rinsing of the Cu-loaded surface had a different impact on the signal intensity depending on the sorption pH. The isotropic signal together with the anisotropic signal was simultaneously observed on the kaolinite surface air-dried at room temperature for as long as 13 months. On the other hand, only the anisotropic signal was observed on the surface dried at 105 °C overnight. The re-wetting of the once dried kaolinite with deionized water immediately restored the isotropic signal. These results show that an outer-sphere surface complex is the dominant form of the sorbed copper ions and that the drying and aging of the surface convert the outer-sphere complex into the inner-sphere complex or surface precipitate, at least a part of which was reversibly converted back into the outer-sphere complex by the rehydration of the surface. This work suggests that dehydration during aging as well as solution variables such as pH, concentrations of Cu(II) and competing cations can have an impact on the amount and long-term fate of the outer-sphere Cu(II) complex formed on the kaolinite surface.

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.

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 using2 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 and2 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.