5 research outputs found
Flux of Total Mercury and Methylmercury to the Northern Gulf of Mexico from U.S. Estuaries
To
better understand the source of elevated methylmercury (MeHg)
concentrations in Gulf of Mexico (GOM) fish, we quantified fluxes
of total Hg and MeHg from 11 rivers in the southeastern United States,
including the 10 largest rivers discharging to the GOM. Filtered water
and suspended particles were collected across estuarine salinity gradients
in Spring and Fall 2012 to estimate fluxes from rivers to estuaries
and from estuaries to coastal waters. Fluxes of total Hg and MeHg
from rivers to estuaries varied as much as 100-fold among rivers.
The Mississippi River accounted for 59% of the total Hg flux and 49%
of the fluvial MeHg flux into GOM estuaries. While some estuaries
were sources of Hg, the combined estimated fluxes of total Hg (∼5200
mol y<sup>–1</sup>) and MeHg (∼120 mol y<sup>–1</sup>) from the estuaries to the GOM were less than those from rivers
to estuaries, suggesting an overall estuarine sink. Fluxes of total
Hg from the estuaries to coastal waters of the northern GOM are approximately
an order of magnitude less than from atmospheric deposition. However,
fluxes from rivers are significant sources of MeHg to estuaries and
coastal regions of the northern GOM
Nanostructured Metal Oxide Sorbents for the Collection and Recovery of Uranium from Seawater
The ability to collect uranium from
seawater offers the potential
for a long-term green fuel supply for nuclear energy. However, extraction
of uranium, and other trace minerals, is challenging because of the
high ionic strength and low mineral concentrations in seawater. Herein
we evaluate the use of nanostructured metal oxide sorbents for the
collection and recovery of uranium from seawater. Chemical affinity,
chemical adsorption capacity, and uptake kinetics of sorbent materials
were evaluated. Materials with higher surface area clearly produced
better sorbent performance. Uptake kinetics showed that the materials
could rapidly equilibrate in a few hours with effective solution contact.
Manganese, iron oxide, and especially Mn–Fe nanostructured
composites provided the best performance for uranium collection from
seawater. The preferred materials were demonstrated to extract uranium
from natural seawater with up to 3 mg U/g-sorbent in 4 h of contact
time. Inexpensive nontoxic carbonate solutions were demonstrated to
be an effective and environmentally benign method of stripping the
uranium from the metal oxide sorbents. Various formats for the utilization
of the nanostructured metals oxide sorbent materials are discussed,
including traditional methods and nontraditional methods such as magnetic
separation
Pyrogenic Inputs of Anthropogenic Pb and Hg to Sediments of the Hood Canal, Washington, in the 20th Century: Source Evidence from Stable Pb Isotopes and PAH Signatures
Combustion-derived PAHs and stable Pb isotopic signatures
(<sup>206</sup>Pb/<sup>207</sup>Pb) in sedimentary records assisted
in
reconstructing the sources of atmospheric inputs of anthropogenic
Pb and Hg to the Hood Canal, Washington. The sediment-focusing corrected
peak fluxes of total Pb and Hg (1960–70s) demonstrate that
the watershed of Hood Canal has received greater atmospheric inputs
of these metals than its mostly rural land use would predict. The
tight relationships between the Pb, Hg, and organic markers in the
cores indicate that these metals are derived from industrial combustion
emissions. Multiple lines of evidence point to the Asarco smelter,
located in the Main Basin of Puget Sound, as the major emission source
of these metals to the watershed of the Hood Canal. The evidence includes
(1) similar PAH isomer ratios in sediment cores from the two basins,
(2) the correlations between Pb, Hg, and Cu in sediments and previously
studied environmental samples including particulate matter emitted
from the Asarco smelter’s main stack at the peak of production,
and (3) Pb isotope ratios. The natural rate of recovery in Hood Canal
since the 1970s, back to preindustrial metal concentrations, was linear
and contrasts with recovery rates reported for the Main Basin which
slowed post late 1980s
Elution of Uranium and Transition Metals from Amidoxime-Based Polymer Adsorbents for Sequestering Uranium from Seawater
High-surface-area amidoxime and carboxylic
acid grafted polymer
adsorbents developed at Oak Ridge National Laboratory were tested
for sequestering uranium in a flowing seawater flume system at the
PNNL-Marine Sciences Laboratory. FTIR spectra indicate that a KOH
conditioning process is necessary to remove the proton from the carboxylic
acid and make the sorbent effective for sequestering uranium from
seawater. The alkaline conditioning process also converts the amidoxime
groups to carboxylate groups in the adsorbent. Both Na<sub>2</sub>CO<sub>3</sub>–H<sub>2</sub>O<sub>2</sub> and hydrochloric
acid elution methods can remove ∼95% of the uranium sequestered
by the adsorbent after 42 days of exposure in real seawater. The Na<sub>2</sub>CO<sub>3</sub>–H<sub>2</sub>O<sub>2</sub> elution method
is more selective for uranium than conventional acid elution. Iron
and vanadium are the two major transition metals competing with uranium
for adsorption to the amidoxime-based adsorbents in real seawater.
Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt, 1 M)
can remove iron from the adsorbent very effectively at pH around 7.
The coordination between vanadiumÂ(V) and amidoxime is also discussed
based on our <sup>51</sup>V NMR data
Investigations into the Reusability of Amidoxime-Based Polymeric Adsorbents for Seawater Uranium Extraction
The ability to reuse amidoxime-based
polymeric adsorbents is a
critical component in reducing the overall cost of the technology
to extract uranium from seawater. This report describes an evaluation
of adsorbent reusability in multiple reuse (adsorption/stripping)
cycles in real seawater exposures with potassium bicarbonate (KHCO<sub>3</sub>) elution using several amidoxime-based polymeric adsorbents.
The KHCO<sub>3</sub> elution technique achieved ∼100% recovery
of uranium adsorption capacity in the first reuse. Subsequent reuses
showed significant drops in adsorption capacity. After the fourth
reuse with the ORNL AI8 adsorbent, the 56-day adsorption capacity
dropped to 28% of its original capacity. FTIR spectra revealed that
there was a conversion of the amidoxime ligands to carboxylate groups
during extended seawater exposure, becoming more significant with
longer exposure times. Ca and Mg adsorption capacities also increased
with each reuse cycle supporting the hypothesis that long-term exposure
resulted in converting amidoxime to carboxylate, enhancing the adsorption
of Ca and Mg. Shorter seawater exposure (adsorption/stripping) cycles
(28 vs 42 days) had higher adsorption capacities after reuse, but
the shorter exposure cycle time did not produce an overall better
performance in terms of cumulative exposure time. Recovery of uranium
capacity in reuses may also vary across different adsorbent formulations.
Through multiple reuses, the AI8 adsorbent can harvest 10 g uranium/kg
adsorbent in ∼140 days, using a 28-day adsorption/stripping
cycle, a performance much better than would be achieved with a single
use of the adsorbent through a very long-term exposure (saturation
capacity of 7.4 g U/kg adsorbent). A time dependent seawater exposure
model to evaluate the cost associated with reusing amidoxime-based
adsorbents in real seawater exposures was developed. The predicted
cost to extract uranium from seawater ranged from 830/kg
U. Model simulation suggests that a short seawater exposure cycle
(<15 days) is the optimal deployment period for lower uranium production
cost in seawater uranium mining