Electrocatalytic Reduction of Nitrate Using Magnéli Phase TiO₂ Reactive Electrochemical Membranes Doped with Pd-Based Catalysts

This research focused on synthesis, characterization, and application of point-of-use catalytic reactive electrochemical membranes (REMs) for electrocatalytic NO₃^– reduction. Deposition of Pd–Cu and Pd–In catalysts to the REMs produced catalytic REMs (i.e., Pd–Cu/REM and Pd–In/REM) that were active for NO₃^– reduction. Optimal performance was achieved with a Pd–Cu/REM and upstream counter electrode, which reduced NO₃^– from 1.0 mM to below the EPAs regulatory MCL (700 μM) in a single pass through the REM (residence time ∼2 s), obtaining product selectivity of <2% toward NO₂^–/NH₃. Nitrate reduction was not affected by dissolved oxygen and carbonate species and only slightly decreased in a surface water sample due to Ca²⁺ and Mg²⁺ scaling. Energy consumption to treat surface water was 1.1 to 1.3 kWh mol^–1 for 1 mM NO₃^– concentrations, and decreased to 0.19 and 0.12 kWh mol^–1 for 10 and 100 mM NaNO₃ solutions, respectively. Electrocatalytic reduction kinetics were shown to be an order of magnitude higher than catalytic NO₃^– reduction kinetics. Conversion of up to 67% of NO₃^–, with low NO₂^– (0.711 μM) and NH₃ formation (<10 μM), and low energy consumption obtained in this study suggest that Pd–Cu/REMs are a promising technology for distributed water treatment

Spectroscopic Investigation of Interfacial Interaction of Manganese Oxide with Triclosan, Aniline, and Phenol

We investigated the reaction of manganese oxide [MnO_<i>x</i>(s)] with phenol, aniline, and triclosan in batch experiments using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and aqueous chemistry measurements. Analyses of XPS high-resolution spectra suggest that the Mn­(III) content increased 8–10% and the content of Mn­(II) increased 12–15% in the surface of reacted MnO_<i>x</i>(s) compared to the control, indicating that the oxidation of organic compounds causes the reduction of MnO_<i>x</i>(s). Fitting of C 1s XPS spectra suggests an increase in the number of aromatic and aliphatic bonds for MnO_<i>x</i>(s) reacted with organic compounds. The presence of 2.7% Cl in the MnO_<i>x</i>(s) surface after reaction with triclosan was detected by XPS survey scans, while no Cl was detected in MnO_<i>x</i>-phenol, MnO_<i>x</i>-aniline, and MnO_<i>x</i>-control. Raman spectra confirm the increased intensity of carbon features in MnO_<i>x</i>(s) samples that reacted with organic compounds compared to unreacted MnO_<i>x</i>(s). These spectroscopy results indicate that phenol, aniline, triclosan, and related byproducts are associated with the surface of MnO_<i>x</i>(s)-reacted samples. The results from this research contribute to a better understanding of interactions between MnO_<i>x</i>(s) and organic compounds that are relevant to natural and engineered environments

Reactive Transport of U and V from Abandoned Uranium Mine Wastes

The reactive transport of uranium (U) and vanadium­(V) from abandoned mine wastes collected from the Blue Gap/Tachee Claim-28 mine site in Arizona was investigated by integrating flow-through column experiments with reactive transport modeling, and electron microscopy. The mine wastes were sequentially reacted in flow-through columns at pH 7.9 (10 mM HCO₃^–) and pH 3.4 (10 mM CH₃COOH) to evaluate the effect of environmentally relevant conditions encountered at Blue Gap/Tachee on the release of U and V. The reaction rate constants (<i>k</i>_<i>m</i>) for the dissolution of uranyl–vanadate (U–V) minerals predominant at Blue Gap/Tachee were obtained from simulations with the reactive transport software, PFLOTRAN. The estimated reaction rate constants were within 1 order of magnitude for pH 7.9 (<i>k</i>_<i>m</i> = 4.8 × 10^–13 mol cm^–2 s^–1) and pH 3.4 (<i>k</i>_<i>m</i> = 3.2 × 10^–13 mol cm^–2 s^–1). However, the estimated equilibrium constants (<i>K</i>_eq) for U–V bearing minerals were more than 6 orders of magnitude different for reaction at circumneutral pH (<i>K</i>_eq = 10^–38.65) compared to acidic pH (<i>K</i>_eq = 10^–44.81). These results coupled with electron microscopy data suggest that the release of U and V is affected by water pH and the crystalline structure of U–V bearing minerals. The findings from this investigation have important implications for risk exposure assessment, remediation, and resource recovery of U and V in locations where U–V-bearing minerals are abundant

Effect of Ca²⁺ and Zn²⁺ on UO₂ Dissolution Rates

The dissolution of UO₂ in a continuously stirred tank reactor (CSTR) in the presence of Ca²⁺ and Zn²⁺ was investigated under experimental conditions relevant to contaminated groundwater systems. Complementary experiments were performed to investigate the effect of adsorption and precipitation reactions on UO₂ dissolution. The experiments were performed under anoxic and oxic conditions. Zn²⁺ had a much greater inhibitory effect on UO₂ dissolution than did Ca²⁺. This inhibition was most substantial under oxic conditions, where the experimental rate of UO₂ dissolution was 7 times lower in the presence of Ca²⁺ and 1450 times lower in the presence of Zn²⁺ than in water free of divalent cations. EXAFS and solution chemistry analyses of UO₂ solids recovered from a Ca experiment suggest that a Ca–U­(VI) phase precipitated. The Zn carbonate hydrozincite [Zn₅(CO₃)₂(OH)₆] or a structurally similar phase precipitated on the UO₂ solids recovered from experiments performed in the presence of Zn. These precipitated Ca and Zn phases can coat the UO₂ surface, inhibiting the oxidative dissolution of UO₂. Interactions with divalent groundwater cations have implications for the longevity of UO₂ and the mobilization of U­(VI) from these solids in remediated subsurface environments, waste disposal sites, and natural uranium ores

Increased Sensitivity and Selectivity for As(III) Detection at the Au(111) Surface: Single Crystals and Ultraflat Thin Films Comparison

Electrochemical stripping voltammetry electroanalysis sensitivity and selectivity are oftentimes limited by wide variance in analyte electrode surface adsorption and desorption energies. The use of highly oriented Au(111) single crystal and thin film surfaces is shown to decrease this variance and improve detection for arsenic (As) in water. Cyclic voltammetry and linear stripping voltammetry (LSV) analysis on Au oriented and polyoriented electrode surfaces demonstrated that As deposition and oxidation is a complex surface-structure-dependent process. An electrochemical quartz microbalance indicated that As is deposited in multiple layers when in high concentrations and does not permanently reorganize the Au surface after stripping. LSV analysis of As(III) on the Au(111), Au(110), Au(100), and Au polyoriented single crystal, Au(Poly), model electrode surfaces showed that Au(111) had the highest peak to background ratio and narrowest peak width for As oxidative stripping. Furthermore, an ultraflat Au(111) thin film, Au(UTF), was then compared to the Au(111) and Au(Poly) single crystals and showed a bulk Au(111) single crystal-like response. The Au(UTF) was then used to perform a calibration curve to detect between 2.5 and 100 μg L–1 As(III) and resulted in a theoretical limit of detection of 0.0065 μg L–1 in 0.5 M H2SO4. The results from this study indicate that the Au(UTF) surface provides the sensitivity necessary for detection of trace concentrations of As in water at or below the maximum contaminant level (MCL) of 10 μg L–1

Metal Reactivity in Laboratory Burned Wood from a Watershed Affected by Wildfires

We investigated interfacial processes affecting metal mobility by wood ash under laboratory-controlled conditions using aqueous chemistry, microscopy, and spectroscopy. The Valles Caldera National Preserve in New Mexico experiences catastrophic wildfires of devastating effects. Wood samples of Ponderosa Pine, Colorado Blue Spruce, and Quaking Aspen collected from this site were exposed to temperatures of 60, 350, and 550 °C. The 350 °C Pine ash had the highest content of Cu (4997 ± 262 mg kg^–1), Cr (543 ± 124 mg kg^–1), and labile dissolved organic carbon (DOC, 11.3 ± 0.28 mg L^–1). Sorption experiments were conducted by reacting 350 °C Pine, Spruce, and Aspen ashes separately with 10 μM Cu­(II) and Cr­(VI) solutions. Up to a 94% decrease in Cu­(II) concentration was observed in solution while Cr­(VI) concentration showed a limited decrease (up to 13%) after 180 min of reaction. X-ray photoelectron spectroscopy (XPS) analyses detected increased association of Cu­(II) on the near surface region of the reacted 350 °C Pine ash from the sorption experiments compared to the unreacted ash. The results suggest that dissolution and sorption processes should be considered to better understand the potential effects of metals transported by wood ash on water quality that have important implications for postfire recovery and response strategies

Reducing Conditions Influence U(IV) Accumulation in Sediments during <i>In Situ</i> Bioremediation

This study presents field experiments conducted in a contaminated aquifer in Rifle, CO, to determine the speciation and accumulation of uranium in sediments during in situ bioreduction. We applied synchrotron-based X-ray spectroscopy and imaging techniques as well as aqueous chemistry measurements to identify changes in U speciation in water and sediment in the first days follwing electron donor amendment. Limited changes in U solid speciation were observed throughout the duration of this study, and non-crystalline U(IV) was identified in all samples obtained. However, U accumulation rates strongly increased during in situ bioreduction, when the dominant microbial regime transitioned from iron- to sulfate-reducing conditions. Results suggest that uranium is enzymatically reduced during Fe reduction, as expected. Mineral grain coatings newly formed during sulfate reduction act as reduction hotspots, where numerous reductants can act as electron donors [Fe(II), S(II), and microbial extracellular polymeric substances] that bind and reduce U. The results have implications for identifying how changes in the dominant reducing mechanism, such as Fe versus sulfate reduction, affect trace metal speciation and accumulation. The outcomes from this study provide additional insights into uranium accumulation mechanisms in sediments that could be useful for the refinement of quantitative models describing redox processes and contaminant dynamics in floodplain aquifers

Elevated Concentrations of U and Co-occurring Metals in Abandoned Mine Wastes in a Northeastern Arizona Native American Community

The chemical interactions of U and co-occurring metals in abandoned mine wastes in a Native American community in northeastern Arizona were investigated using spectroscopy, microscopy and aqueous chemistry. The concentrations of U (67–169 μg L^–1) in spring water samples exceed the EPA maximum contaminant limit of 30 μg L^–1. Elevated U (6,614 mg kg^–1), V (15,814 mg kg^–1), and As (40 mg kg^–1) concentrations were detected in mine waste solids. Spectroscopy (XPS and XANES) solid analyses identified U (VI), As (-I and III) and Fe (II, III). Linear correlations for the release of U vs V and As vs Fe were observed for batch experiments when reacting mine waste solids with 10 mM ascorbic acid (∼pH 3.8) after 264 h. The release of U, V, As, and Fe was at least 4-fold lower after reaction with 10 mM bicarbonate (∼pH 8.3). These results suggest that U–V mineral phases similar to carnotite [K₂(UO₂)₂V₂O₈] and As–Fe-bearing phases control the availability of U and As in these abandoned mine wastes. Elevated concentrations of metals are of concern due to human exposure pathways and exposure of livestock currently ingesting water in the area. This study contributes to understanding the occurrence and mobility of metals in communities located close to abandoned mine waste sites

Speciation and Reactivity of Uranium Products Formed during <i>in Situ</i> Bioremediation in a Shallow Alluvial Aquifer

In this study, we report the results of <i>in situ</i> U­(VI) bioreduction experiments at the Integrated Field Research Challenge site in Rifle, Colorado, USA. Columns filled with sediments were deployed into a groundwater well at the site and, after a period of conditioning with groundwater, were amended with a mixture of groundwater, soluble U­(VI), and acetate to stimulate the growth of indigenous micro­organisms. Individual reactors were collected as various redox regimes in the column sediments were achieved: (i) during iron reduction, (ii) just after the onset of sulfate reduction, and (iii) later into sulfate reduction. The speciation of U retained in the sediments was studied using X-ray absorption spectroscopy, electron microscopy, and chemical extractions. Circa 90% of the total uranium was reduced to U­(IV) in each reactor. Noncrystalline U­(IV) comprised about two-thirds of the U­(IV) pool, across large changes in microbial community structure, redox regime, total uranium accumulation, and reaction time. A significant body of recent research has demonstrated that noncrystalline U­(IV) species are more suceptible to remobilization and reoxidation than crystalline U­(IV) phases such as uraninite. Our results highlight the importance of considering noncrystalline U­(IV) formation across a wide range of aquifer parameters when designing <i>in situ</i> remediation plans