11 research outputs found
Quantifying Differences in the Impact of Variable Chemistry on Equilibrium Uranium(VI) Adsorption Properties of Aquifer Sediments
Uranium adsorption ā desorption on sediment samples collected from the Hanford 300-Area, Richland, WA varied extensively over a range of field-relevant chemical conditions, complicating assessment of possible differences in equilibrium adsorption properties. Adsorption equilibrium was achieved in 500ā1000 h although dissolved uranium concentrations increased over thousands of hours owing to changes in aqueous chemical composition driven by sediment-water reactions. A nonelectrostatic surface complexation reaction, \u3eSOH + UO22+ + 2CO32- = \u3eSOUO2(CO3HCO3)2- , provided the best fit to experimental data for each sediment sample resulting in a range of conditional equilibrium constants (logKc) from 21.49 to 21.76. Potential differences in uranium adsorption properties could be assessed in plots based on the generalized massaction expressions yielding linear trends displaced vertically by differences in logKc values. Using this approach, logKc values for seven sediment samples were not significantly different. However, a significant difference in adsorption properties between one sediment sample and the fines (Kc uncertainty were improved by capturing all data points within experimental errors. The massaction expression plots demonstrate that applying models outside the range of conditions used in model calibration greatly increases potential errors
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Transient groundwater chemistry near a river: Effects on U(VI) transport in laboratory column experiments
In the 300 Area of a U(VI)-contaminated aquifer at Hanford, Washington, USA,
inorganic carbon and major cations, which have large impacts on U(VI) transport, change on
an hourly and seasonal basis near the Columbia River. Batch and column experiments were
conducted to investigate the factors controlling U(VI) adsorption/desorption by changing
chemical conditions over time. Low alkalinity and low Ca concentrations (Columbia River
water) enhanced adsorption and reduced aqueous concentrations. Conversely, high alkalinity
and high Ca concentrations (Hanford groundwater) reduced adsorption and increased
aqueous concentrations of U(VI). An equilibrium surface complexation model calibrated
using laboratory batch experiments accounted for the decrease in U(VI) adsorption observed
with increasing (bi)carbonate concentrations and other aqueous chemical conditions. In the
column experiment, alternating pulses of river and groundwater caused swings in aqueous
U(VI) concentration. A multispecies multirate surface complexation reactive transport model
simulated most of the major U(VI) changes in two column experiments. The modeling
results also indicated that U(VI) transport in the studied sediment could be simulated by
using a single kinetic rate without loss of accuracy in the simulations. Moreover, the
capability of the model to predict U(VI) transport in Hanford groundwater under transient
chemical conditions depends significantly on the knowledge of real-time change of local
groundwater chemistry
Characterization of the intragranular water regime within subsurface sediments: Pore volume, surface area, and mass transfer limitations
Although āāintragranularāā pore space within grain aggregates, grain fractures, and mineral surface coatings may contain a relatively small fraction of the total porosity within a porous medium, it often contains a significant fraction of the reactive surface area, and can thus strongly affect the transport of sorbing solutes. In this work, we demonstrate a batch experiment procedure using tritiated water as a high-resolution diffusive tracer to characterize the intragranular pore space. The method was tested using uranium contaminated sediments from the vadose and capillary fringe zones beneath the former 300A process ponds at the Hanford site (Washington). Sediments were contacted with tracers in artificial groundwater, followed by a replacement of bulk solution with tracer-free groundwater and the monitoring of tracer release. From these data, intragranular pore volumes were calculated and mass transfer rates were quantified using a multirate first-order mass transfer model. Tritium-hydrogen exchange on surface hydroxyls was accounted for by conducting additional tracer experiments on sediment that was vacuum dried after reaction. The complementary (āāwetāā and āādryāā) techniques allowed for the simultaneous determination of intragranular porosity and surface area using tritium. The Hanford 300A samples exhibited intragranular pore volumes of ~1% of the solid volume and intragranular surface areas of ~20%ā35% of the total surface area. Analogous experiments using bromide ion as a tracer yielded very different results, suggesting very little penetration of bromide into the intragranular porosity
Transient groundwater chemistry near a river: Effects on U(VI) transport in laboratory column experiments
In the 300 Area of a U(VI)-contaminated aquifer at Hanford, Washington, USA, inorganic carbon and major cations, which have large impacts on U(VI) transport, change on an hourly and seasonal basis near the Columbia River. Batch and column experiments were conducted to investigate the factors controlling U(VI) adsorption/desorption by changing chemical conditions over time. Low alkalinity and low Ca concentrations (Columbia River water) enhanced adsorption and reduced aqueous concentrations. Conversely, high alkalinity and high Ca concentrations (Hanford groundwater) reduced adsorption and increased aqueous concentrations of U(VI). An equilibrium surface complexation model calibrated using laboratory batch experiments accounted for the decrease in U(VI) adsorption observed with increasing (bi)carbonate concentrations and other aqueous chemical conditions. In the column experiment, alternating pulses of river and groundwater caused swings in aqueous U(VI) concentration. A multispecies multirate surface complexation reactive transport model simulated most of the major U(VI) changes in two column experiments. The modeling results also indicated that U(VI) transport in the studied sediment could be simulated by using a single kinetic rate without loss of accuracy in the simulations. Moreover, the capability of the model to predict U(VI) transport in Hanford groundwater under transient chemical conditions depends significantly on the knowledge of real-time change of local groundwater chemistry
Quantifying Differences in the Impact of Variable Chemistry on Equilibrium Uranium(VI) Adsorption Properties of Aquifer Sediments
Uranium adsorptionādesorption on sediment samples collected from the Hanford 300-Area, Richland, WA varied extensively over a range of field-relevant chemical conditions, complicating assessment of possible differences in equilibrium adsorption properties. Adsorption equilibrium was achieved in 500ā1000 h although dissolved uranium concentrations increased over thousands of hours owing to changes in aqueous chemical composition driven by sediment-water reactions. A nonelectrostatic surface complexation reaction, >SOH + UO<sub>2</sub><sup>2+</sup> + 2CO<sub>3</sub><sup>2-</sup> = >SOUO<sub>2</sub>(CO<sub>3</sub>HCO<sub>3</sub>)<sup>2ā</sup><sub>,</sub> provided the best fit to experimental data for each sediment sample resulting in a range of conditional equilibrium constants (log<i>K</i><sup>c</sup>) from 21.49 to 21.76. Potential differences in uranium adsorption properties could be assessed in plots based on the generalized mass-action expressions yielding linear trends displaced vertically by differences in log<i>K</i><sup>c</sup> values. Using this approach, log<i>K</i><sup>c</sup> values for seven sediment samples were not significantly different. However, a significant difference in adsorption properties between one sediment sample and the fines (<0.063 mm) of another could be demonstrated despite the fines requiring a different reaction stoichiometry. Estimates of log<i>K</i><sup>c</sup> uncertainty were improved by capturing all data points within experimental errors. The mass-action expression plots demonstrate that applying models outside the range of conditions used in model calibration greatly increases potential errors
Hydrologic Controls on Nitrogen Cycling Processes and Functional Gene Abundance in Sediments of a Groundwater Flow-Through Lake
Hydrologic Controls on Nitrogen Cycling Processes and Functional Gene Abundance in Sediments of a Groundwater Flow-Through Lake
The fate and transport of inorganic
nitrogen (N) is a critically
important issue for human and aquatic ecosystem health because discharging
N-contaminated groundwater can foul drinking water and cause algal
blooms. Factors controlling N-processing were examined in sediments
at three sites with contrasting hydrologic regimes at a lake on Cape
Cod, MA. These factors included water chemistry, seepage rates and
direction of groundwater flow, and the abundance and potential rates
of activity of N-cycling microbial communities. Genes coding for denitrification,
anaerobic ammonium oxidation (anammox), and nitrification were identified
at all sites regardless of flow direction or groundwater dissolved
oxygen concentrations. Flow direction was, however, a controlling
factor in the potential for N-attenuation via denitrification in the
sediments. Potential rates of denitrification varied from 6 to 4500
pmol N/g/h from the inflow to the outflow side of the lake, owing
to fundamental differences in the supply of labile organic matter.
The results of laboratory incubations suggested that when anoxia and
limiting labile organic matter prevailed, the potential existed for
concomitant anammox and denitrification. Where oxic lake water was
downwelling, potential rates of nitrification at shallow depths were
substantial (1640 pmol N/g/h). Rates of anammox, denitrification,
and nitrification may be linked to rates of organic N-mineralization,
serving to increase N-mobility and transport downgradient
Evaluating Chemical Extraction Techniques for the Determination of Uranium Oxidation State in Reduced Aquifer Sediments
Extraction
techniques utilizing high pH and (bi)Ācarbonate concentrations
were evaluated for their efficacy in determining the oxidation state
of uranium (U) in reduced sediments collected from Rifle, CO. Differences
in dissolved concentrations between oxic and anoxic extractions have
been proposed as a means to quantify the UĀ(VI) and UĀ(IV) content of
sediments. An additional step was added to anoxic extractions using
a strong anion exchange resin to separate dissolved UĀ(IV) and UĀ(VI).
X-ray spectroscopy showed that UĀ(IV) in the sediments was present
as polymerized precipitates similar to uraninite and/or less ordered
UĀ(IV), referred to as non-uraninite UĀ(IV) species associated with
biomass (NUSAB). Extractions of sediment containing both uraninite
and NUSAB displayed higher dissolved uranium concentrations under
oxic than anoxic conditions while extractions of sediment dominated
by NUSAB resulted in identical dissolved U concentrations. Dissolved
UĀ(IV) was rapidly oxidized under anoxic conditions in all experiments.
Uraninite reacted minimally under anoxic conditions but thermodynamic
calculations show that its propensity to oxidize is sensitive to solution
chemistry and sediment mineralogy. A universal method for quantification
of UĀ(IV) and UĀ(VI) in sediments has not yet been developed but the
chemical extractions, when combined with solid-phase characterization,
have a narrow range of applicability for sediments without UĀ(VI)