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

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

Arsenic Waste Management: A Critical Review of Testing and Disposal of Arsenic-Bearing Solid Wastes Generated during Arsenic Removal from Drinking Water

Water treatment technologies for arsenic removal from groundwater have been extensively studied due to widespread arsenic contamination of drinking water sources. Central to the successful application of arsenic water treatment systems is the consideration of appropriate disposal methods for arsenic-bearing wastes generated during treatment. However, specific recommendations for arsenic waste disposal are often lacking or mentioned as an area for future research and the proper disposal and stabilization of arsenic-bearing waste remains a barrier to the successful implementation of arsenic removal technologies. This review summarizes current disposal options for arsenic-bearing wastes, including landfilling, stabilization, cow dung mixing, passive aeration, pond disposal, and soil disposal. The findings from studies that simulate these disposal conditions are included and compared to results from shorter, regulatory tests. In many instances, short-term leaching tests do not adequately address the range of conditions encountered in disposal environments. Future research directions are highlighted and include establishing regulatory test conditions that align with actual disposal conditions and evaluating nonlandfill disposal options for developing countries

Uranium(VI) Reduction by Iron(II) Monosulfide Mackinawite

Reaction of aqueous uranium­(VI) with iron­(II) monosulfide mackinawite in an O₂ and CO₂ free model system was studied by batch uptake measurements, equilibrium modeling, and <i>L</i>_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 O₂ 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 O₂. 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

Optimization of Arsenic Removal Water Treatment System through Characterization of Terminal Electron Accepting Processes

Terminal electron accepting process (TEAP) zones developed when a simulated groundwater containing dissolved oxygen (DO), nitrate, arsenate, and sulfate was treated in a fixed-bed bioreactor system consisting of two reactors (reactors A and B) in series. When the reactors were operated with an empty bed contact time (EBCT) of 20 min each, DO-, nitrate-, sulfate-, and arsenate-reducing TEAP zones were located within reactor A. As a consequence, sulfate reduction and subsequent arsenic removal through arsenic sulfide precipitation and/or arsenic adsorption on or coprecipitation with iron sulfides occurred in reactor A. This resulted in the removal of arsenic-laden solids during backwashing of reactor A. To minimize this by shifting the sulfate-reducing zone to reactor B, the EBCT of reactor A was sequentially lowered from 20 min to 15, 10, and 7 min. While 50 mg/L (0.81 mM) nitrate was completely removed at all EBCTs, more than 90% of 300 μg/L (4 μM) arsenic was removed with the total EBCT as low as 27 min. Sulfate- and arsenate-reducing bacteria were identified throughout the system through clone libraries and quantitative PCR targeting the 16S rRNA, dissimilatory (bi)­sulfite reductase (<i>dsrAB</i>), and dissimilatory arsenate reductase (<i>arrA</i>) genes. Results of reverse transcriptase (RT) qPCR of partial <i>dsrAB</i> (i.e., <i>dsrA</i>) and <i>arrA</i> transcripts corresponded with system performance. The RT qPCR results indicated colocation of sulfate- and arsenate-reducing activities, in the presence of iron­(II), suggesting their importance in arsenic removal

Growth of <i>Desulfovibrio vulgaris</i> When Respiring U(VI) and Characterization of Biogenic Uraninite

The capacity of <i>Desulfovibrio vulgaris</i> 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 <i>D. vulgaris</i> at a low initial biomass (10 to 20 mg/L of protein) that could reveal biomass growth. <i>D. vulgaris</i> 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 Fe²⁺. Biogenic sulfide nonenzymatically reduced 76% of the U­(VI) and greatly enhanced the overall reduction rate in the absence of Fe²⁺ but was rapidly scavenged by Fe²⁺ to form FeS in the presence of Fe²⁺. 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 Fe²⁺ but was <1.4 nm in the presence of Fe²⁺, indicating poor crystallization inhibited by adsorbed Fe²⁺ 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

Anaerobic Disposal of Arsenic-Bearing Wastes Results in Low Microbially Mediated Arsenic Volatilization

The removal of arsenic from drinking water sources produces arsenic-bearing wastes, which are disposed of in a variety of ways. Several disposal options involve anaerobic environments, including mixing arsenic waste with cow dung, landfills, anaerobic digesters, and pond sediments. Though poorly understood, the production of gaseous arsenic species in these environments can be a primary goal (cow dung mixing) or an unintended consequence (anaerobic digesters). Once formed, these gaseous arsenic species are readily diluted in the atmosphere. Arsenic volatilization can be mediated by the enzyme arsenite S-adenosylmethionine methyltransferase (ArsM) or through the enzymes involved in methanogenesis. In this study, methanogenic mesocosms with arsenic-bearing ferric iron waste from an electrocoagulation drinking water treatment system were used to evaluate the role of methanogenesis in arsenic volatilization using methanogen inhibitors. Arsenic volatilization was highest in methanogenic mesocosms, but represented <0.02% of the total arsenic added. 16S rRNA cDNA sequencing, qPCR of <i>mcrA</i> transcripts, and functional gene array-based analysis of <i>arsM</i> expression, revealed that arsenic volatilization correlated with methanogenic activity. Aqueous arsenic concentrations increased in all mesocosms, indicating that unintended contamination may result from disposal in anaerobic environments. This highlights that more research is needed before recommending anaerobic disposal intended to promote arsenic volatilization