8 research outputs found
Nano-FeS Inhibits UO<sub>2</sub> Reoxidation under Varied Oxic Conditions
Bioreductive <i>in situ</i> treatment of U-contaminated
groundwater can convert soluble UĀ(VI) species to immobile reduced
UĀ(IV) solid phases such as UO<sub>2</sub>(s) to contain U movement.
Once active bioremediation is halted, UO<sub>2</sub> 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 UO<sub>2</sub> reoxidation
upon oxygen intrusion. In this study, flow-through reactor experiments
examined the abiotic kinetics of UO<sub>2</sub> oxidative dissolution
in the presence of oxygen and nanoparticulate FeS as a function of
pH, dissolved oxygen (DO) concentration, and FeS content. The UO<sub>2</sub> 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 UO<sub>2</sub> solid phase during an āinhibition periodā 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 UO<sub>2</sub> dissolution rate with increasing influent DO concentration and decreasing
FeS content. Although the rate was independent of solution pH in the
range of 6.1ā8.1, 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 UO<sub>2</sub> 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 (UO<sub>2</sub>) 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 UĀ(IV).
The present study investigated the mechanism that regulates UO<sub>2</sub> 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 UO<sub>2</sub> 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 UĀ(VI) to
preserve an intermediate U<sub>4</sub>O<sub>9</sub> surface. When
the FeS became depleted by oxidization, a transient, rapid dissolution
of UO<sub>2</sub> 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 UĀ(VI) detachment as UĀ(VI) passivation layers developed. In contrast,
increasing the carbonate concentrations facilitated detachment of
surface-associated UĀ(VI) complexes and impeded the formation of UĀ(VI)
passivation layer. This study demonstrates the critical role of UĀ(VI)
surface layer formation versus UĀ(VI) detachment in controlling UO<sub>2</sub> oxidative dissolution rate during periods of variable oxygen
presence under simulated groundwater conditions
Influence of Iron Sulfides on Abiotic Oxidation of UO<sub>2</sub> by Nitrite and Dissolved Oxygen in Natural Sediments
Iron
sulfide precipitates formed under sulfate reducing conditions
may buffer UĀ(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 UĀ(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
(BRSā) of 7 mM sulfate. The bioreduced sediment was supplemented
with synthetic uraninite (UO<sub>2</sub>(<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 UĀ(IV) reoxidation and mobilization
by both nitrite and oxygen. Most of the influent nitrite (0.53 mM)
exited the columns without oxidizing UO<sub>2</sub>, 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 BRSā 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 BRSā 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 uraniumĀ(VI) with ironĀ(II) monosulfide
mackinawite
in an O<sub>2</sub> and CO<sub>2</sub> free model system was studied
by batch uptake measurements, equilibrium modeling, and <i>L</i><sub>III</sub> edge U X-ray absorption spectroscopy (XAS). Batch
uptake measurements showed that UĀ(VI) removal was almost complete
over the wide pH range between 5 and 11 at the initial UĀ(VI) concentration
of 5 Ć 10<sup>ā5</sup> M. Extraction by a carbonate/bicarbonate
solution indicated that most of the UĀ(VI) removed from solution was
reduced to nonextractable UĀ(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 UĀ(IV) phase associated with mackinawite was uraninite.
Oxidation experiments with dissolved O<sub>2</sub> were performed
by injecting air into the sealed reaction bottles containing mackinawite
samples reacted with UĀ(VI). Dissolved U measurement and XAS confirmed
that the uraninite formed from the UĀ(VI) reduction by mackinawite
did not oxidize or dissolve under the experimental conditions. This
study shows that redox reactions between UĀ(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<sub>2</sub>. 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 Ī¼g/L (4 Ī¼M) 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)Āsulfite 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 ironĀ(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
UĀ(VI) was studied previously with nongrowth conditions involving a
high biomass concentration; thus, bacterial growth through respiration
of UĀ(VI) was not proven. In this study, we conducted a series of batch
tests on UĀ(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 UĀ(VI) respiration alone,
as well as with simultaneous sulfate reduction. Patterns of growth
kinetics and solids production were affected by sulfate and Fe<sup>2+</sup>. Biogenic sulfide nonenzymatically reduced 76% of the UĀ(VI)
and greatly enhanced the overall reduction rate in the absence of
Fe<sup>2+</sup> but was rapidly scavenged by Fe<sup>2+</sup> to form
FeS in the presence of Fe<sup>2+</sup>. Biogenic U solids were uraninite
(UO<sub>2</sub>) nanocrystallites associated with 20 mg/g biomass
as protein. The crystallite thickness of UO<sub>2</sub> was 4 to 5
nm without Fe<sup>2+</sup> but was <1.4 nm in the presence of Fe<sup>2+</sup>, indicating poor crystallization inhibited by adsorbed Fe<sup>2+</sup> and other amorphous Fe solids, such as FeS or FeCO<sub>3</sub>. This work fills critical gaps in understanding the metabolic utilization
of U by microorganisms and formation of UO<sub>2</sub> solids in bioremediation
sites
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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