Investigating the Role of Iron Sulfide on the Long-Term Stability of Reduced Uranium under Oxic Groundwater Conditions.

The historical accumulation and improper disposal of radioactive waste from extensive extraction and processing activities has caused widespread uranium contamination of groundwater and soils in the United States. Because uranium is a toxic heavy metal and radiological hazard, its migration poses serious human health and ecological risk. While successful remediation has been practiced at selected uranium contaminated sites, recent concerns are raised over maintaining the long-term immobilization of reduced uranium solids in the subsurface when oxidants re-enters the reducing zone. Previous studies reported that iron sulfide minerals formed during bioreduction may retard UO2 reoxidation by consuming dissolved oxygen, yet limited mechanistic information is available detailing the thermodynamic and kinetic constraints that control UO2 oxidative dissolution in the presence of iron sulfide in groundwater. This research aims at understanding the role of iron sulfide in affecting the stability of uraninite (UO2(s)) under oxic groundwater conditions. Synthetic nano-particulate mackinawite (FeS) and uraninite solids were prepared to simulate the reduced precipitates in groundwater systems dominated by sulfate reducing conditions. Completely mixed batch and flow-through reactor experiments were conducted to investigate UO2 oxidative dissolution rate in artificial groundwater as a function of pH, FeS content, and carbonate and oxygen concentrations. FeS and UO2 oxidation products were characterized by various analytical techniques to examine reaction pathways and rate-controlling mechanisms during oxidation. This research demonstrates that FeS serves as an effective oxygen scavenger and inhibits UO2 oxidative dissolution. The dissolution rate of UO2 in the presence of FeS is over one order of magnitude lower than those in the absence of FeS. The preferential reaction of FeS with oxygen leads to surface-oxidation limited dissolution of UO2, which is facilitated by a fast detachment of ternary Ca-U(VI)-CO3 complexes. When FeS concentration significantly diminishes, increasing oxygen concentration may passivate UO2 surface by forming a less reactive U(VI) layer. However, dissolved calcium and carbonate can reduce the formation of passivation layer by promoting soluble U(VI) release rate from UO2 surface. The results contribute to the understanding of uranium fate and transport in the presence of iron sulfide during periods of persistent oxygen intrusion in heterogeneous groundwater systems.PHDEnvironmental EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/108922/1/ybi_1.pd

Removal of Radium from Synthetic Shale Gas Brines by Ion Exchange Resin

Rapid development of hydraulic fracturing for natural gas production from shale reservoirs presents a significant challenge related to the management of the high-salinity wastewaters that return to the surface. In addition to high total dissolved solids (TDS), shale gas-produced brines typically contain elevated concentrations of radium (Ra), which must be treated properly to prevent contamination of surface waters and allow for safe disposal or reuse of produced water. Treatment strategies that isolate radium in the lowest volume waste streams would be desirable to reduce disposal cost and generate useful treatment by-products. The present study evaluates the potential of a commercial strong acid cation exchange resin for removing Ra2+ from high-TDS brines using fixed-bed column reactors. Column reactors were operated with varying brine chemistries and salinities in an effort to find optimal conditions for Ra2+ removal through ion exchange. To overcome competing divalent cations present in the brine for exchange sites, the chelating agent, EDTA, was used to form stable complexes predominantly with the higher concentration Ca2+, Mg2+, and Sr2+ divalent cations, while isolating the much lower concentration Ra2+ species. Results showed that Ra2+ removal by the resin strongly depended on the TDS concentration and could be improved with careful selection of EDTA concentration. This strategy of metal chelation coupled with ion exchange resins may be effective in enhancing Ra2+ removal and reducing the generation and disposal cost if volume reduction of low-level radioactive solid waste can be achieved.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140367/1/ees.2016.0002.pd

Rapid mobilization of noncrystalline U(IV) coupled with FeS oxidation

The reactivity of disordered, noncrystalline U(IV) species remains poorly characterized despite their prevalence in biostimulated sediments. Because of the lack of crystalline structure, noncrystalline U(IV) may be susceptible to oxidative mobilization under oxic conditions. The present study investigated the mechanism and rate of oxidation of biogenic noncrystalline U(IV) by dissolved oxygen (DO) in the presence of mackinawite (FeS). Previously recognized as an effective reductant and oxygen scavenger, nanoparticulate FeS was evaluated for its role in influencing U release in a flow through system as a function of pH and carbonate concentration. The results demonstrated that noncrystalline U(IV) was more susceptible to oxidation than uraninite (UO2) in the presence of dissolved carbonate. A rapid release of U occurred immediately after FeS addition without exhibiting a temporary inhibition stage, as was observed during the oxidation of UO2, although FeS still kept DO levels low. X-ray photoelectron spectroscopy (XPS) characterized a transient surface Fe(III) species during the initial FeS oxidation, which was likely responsible for oxidizing noncrystalline U(IV) in addition to oxygen. In the absence of carbonate, however, the release of dissolved U was significantly hindered as a result of U adsorption by FeS oxidation products. This study illustrates the strong interactions between iron sulfide and U(IV) species during redox transformation and implies the lability of biogenic noncrystalline U(IV) species in the subsurface environment when subjected to redox cycling events

Comparison of multiple PM2.5 exposure products for estimating health benefits of emission controls over New York State, USA

Ambient exposure to fine particulate matter (PM2.5) is one of the top global health concerns. We estimate the PM2.5-related health benefits of emission reduction over New York State (NYS) from 2002 to 2012 using seven publicly available PM2.5 products that include information from ground-based observations, remote sensing and chemical transport models. While these PM2.5 products differ in spatial patterns, they show consistent decreases in PM2.5 by 28%–37% from 2002 to 2012. We evaluate these products using two sets of independent ground-based observations from the New York City Community Air Quality Survey (NYCCAS) Program for an urban area, and the Saint Regis Mohawk Tribe Air Quality Program for a remote area. Inclusion of satellite remote sensing improves the representativeness of surface PM2.5 in the remote area. Of the satellite-based products, only the statistical land use regression approach captures some of the spatial variability across New York City measured by NYCCAS. We estimate the PM2.5-related mortality burden by applying an integrated exposure-response function to the different PM2.5 products. The multi-product mean PM2.5-related mortality burden over NYS decreased by 5660 deaths (67%) from 8410 (95% confidence interval (CI): 4570–12 400) deaths in 2002 to 2750 (CI: 700–5790) deaths in 2012. We estimate a 28% uncertainty in the state-level PM2.5 mortality burden due to the choice of PM2.5 products, but such uncertainty is much smaller than the uncertainty (130%) associated with the exposure-response function

Nano-FeS Inhibits UO₂ Reoxidation under Varied Oxic Conditions

Bioreductive <i>in situ</i> treatment of U-contaminated groundwater can convert soluble U­(VI) species to immobile reduced U­(IV) solid phases such as UO₂(s) to contain U movement. Once active bioremediation is halted, UO₂ may be subsequently reoxidized if oxidants such as oxygen enter the reducing zone. However, iron sulfide minerals that form during bioreduction may serve as electron sources or oxygen scavengers and inhibit UO₂ reoxidation upon oxygen intrusion. In this study, flow-through reactor experiments examined the abiotic kinetics of UO₂ oxidative dissolution in the presence of oxygen and nanoparticulate FeS as a function of pH, dissolved oxygen (DO) concentration, and FeS content. The UO₂ dissolution rates in the presence of FeS were over 1 order of magnitude lower than those in the absence of FeS under otherwise comparable oxic conditions. FeS effectively scavenged DO and preferentially reacted with oxygen, contributing to a largely unreacted UO₂ solid phase during an “inhibition period” as determined by X-ray absorption spectroscopy (XAS). The removal of DO by FeS was significant but incomplete during the inhibition period, resulting in surface-oxidation-limited dissolution and greater UO₂ dissolution rate with increasing influent DO concentration and decreasing FeS content. Although the rate was independent of solution pH in the range of 6.1–8.1, the length of the inhibition period was shortened by substantial FeS dissolution at slightly acidic pH. The reducing capacity of FeS was greatest at basic pH where surface-mediated FeS oxidation dominated

Surface Passivation Limited UO₂ Oxidative Dissolution in the Presence of FeS

Iron sulfide minerals produced during in situ bioremediation of U can serve as an oxygen scavenger to retard uraninite (UO₂) oxidation upon oxygen intrusion. Under persistent oxygen supply, however, iron sulfides become oxidized and depleted, giving rise to elevated dissolved oxygen (DO) levels and remobilization of U­(IV). The present study investigated the mechanism that regulates UO₂ oxidative dissolution rate in a flow-through system when oxygen breakthrough occurred as a function of mackinawite (FeS) and carbonate concentrations. The formation and evolution of surface layers on UO₂ were characterized using XAS and XPS. During FeS inhibition period, the continuous supply of carbonate and calcium in the influent effectively complexed and removed oxidized U­(VI) to preserve an intermediate U₄O₉ surface. When the FeS became depleted by oxidization, a transient, rapid dissolution of UO₂ was observed along with DO breakthrough in the reactor. This rate was greater than during the preceding FeS inhibition period and control experiments in the absence of FeS. With increasing DO, the rate slowed and the rate-limiting step shifted from surface oxidation to U­(VI) detachment as U­(VI) passivation layers developed. In contrast, increasing the carbonate concentrations facilitated detachment of surface-associated U­(VI) complexes and impeded the formation of U­(VI) passivation layer. This study demonstrates the critical role of U­(VI) surface layer formation versus U­(VI) detachment in controlling UO₂ oxidative dissolution rate during periods of variable oxygen presence under simulated groundwater conditions

One-step solid-state-chemistry synthesized layered bismuth oxyiodide crystal for efficient solar-driven CO2 photoreduction

We develop a facile one-step solid-state-chemistry strategy for synthesizing BiOI crystals with layered microstructure. The BiOI photocatalyst shows typical mesoporous feature with available surface area, considerable wide-spectrum light absorption and carriers lifetime properties, and remarkable solar to thermal effect for efficient solar-driven photocatalytic CO2 reduction to CH4. The engineered BiOI displays a superior CH4 formation performance with the space-time yield of 13.1 μmol g−1 h−1 and the selectivity of 82.3% as well as excellent photostability for CO2 photoreduction under solar light irradiation. Furthermore, the versatile BiOI can boost the photo-stimulated efficiency on clean H2 generation and pollutant degradation

Yttrium Residues in MWCNT Enable Assessment of MWCNT Removal during Wastewater Treatment

Many analytical techniques have limited sensitivity to quantify multi-walled carbon nanotubes (MWCNTs) at environmentally relevant exposure concentrations in wastewaters. We found that trace metals (e.g., Y, Co, Fe) used in MWCNT synthesis correlated with MWCNT concentrations. Because of low background yttrium (Y) concentrations in wastewater, Y was used to track MWCNT removal by wastewater biomass. Transmission electron microscopy (TEM) imaging and dissolution studies indicated that the residual trace metals were strongly embedded within the MWCNTs. For our specific MWCNT, Y concentration in MWCNTs was 76 &#181;g g&#8722;1, and single particle mode inductively coupled plasma mass spectrometry (spICP-MS) was shown viable to detect Y-associated MWCNTs. The detection limit of the specific MWCNTs was 0.82 &#181;g L&#8722;1 using Y as a surrogate, compared with &gt;100 &#181;g L&#8722;1 for other techniques applied for MWCNT quantification in wastewater biomass. MWCNT removal at wastewater treatment plants (WWTPs) was assessed by dosing MWCNTs (100 &#181;g L&#8722;1) in water containing a range of biomass concentrations obtained from wastewater return activated sludge (RAS) collected from a local WWTP. Using high volume to surface area reactors (to limit artifacts of MWCNT loss due to adsorption to vessel walls) and adding 5 g L&#8722;1 of total suspended solids (TSS) of RAS (3-h mixing) reduced the MWCNT concentrations from 100 &#181;g L&#8722;1 to 2 &#181;g L&#8722;1. The results provide an environmentally relevant insight into the fate of MWCNTs across their end of life cycle and aid in regulatory permits that require estimates of engineered nanomaterial removal at WWTPs upon accidental release into sewers from manufacturing facilities

Influence of Iron Sulfides on Abiotic Oxidation of UO₂ by Nitrite and Dissolved Oxygen in Natural Sediments

Iron sulfide precipitates formed under sulfate reducing conditions may buffer U­(IV) insoluble solid phases from reoxidation after oxidants re-enter the reducing zone. In this study, sediment column experiments were performed to quantify the effect of biogenic mackinawite on U­(IV) stability in the presence of nitrite or dissolved oxygen (DO). Two columns, packed with sediment from an abandoned U contaminated mill tailings site near Rifle, CO, were biostimulated for 62 days with an electron donor (3 mM acetate) in the presence (BRS+) and absence (BRS−) of 7 mM sulfate. The bioreduced sediment was supplemented with synthetic uraninite (UO₂(<i>s</i>)), sterilized by gamma-irradiation, and then subjected to a sequential oxidation by nitrite and DO. Biogenic iron sulfides produced in the BRS+ column, mostly as mackinawite, inhibited U­(IV) reoxidation and mobilization by both nitrite and oxygen. Most of the influent nitrite (0.53 mM) exited the columns without oxidizing UO₂, while a small amount of nitrite was consumed by iron sulfides precipitates. An additional 10-day supply of 0.25 mM DO influent resulted in the release of about 10% and 49% of total U in BRS+ and BRS– columns, respectively. Influent DO was effectively consumed by biogenic iron sulfides in the BRS+ column, while DO and a large U spike were detected after only a brief period in the effluent in the BRS– column