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Title: The effect of calcium on aqueous uranium(VI) speciation and adsorption to ferrihydrite and quartz

Abstract

Recent studies of uranium(VI) geochemistry have focused on the potentially important role of the aqueous species, CaUO 2(CO 3) 3 2- and Ca 2UO 2(CO 3) 3 0(aq), on inhibition of microbial reduction and uranium(VI) aqueous speciation in contaminated groundwater. However, to our knowledge, there have been no direct studies of the effects of these species on U(VI) adsorption by mineral phases. The sorption of U(VI) on quartz and ferrihydrite was investigated in NaNO 3 solutions equilibrated with either ambient air (430 ppm CO 2) or 2% CO 2 in the presence of 0, 1.8, or 8.9 mM Ca 2+. Under conditions where the Ca 2UO 2(CO 3) 3 0(aq) species predominates U(VI) aqueous speciation, the presence of Ca in solution lowered U(VI) adsorption on quartz from 77% in the absence of Ca to 42% and 10% at Ca concentrations of 1.8 and 8.9 mM, respectively. U(VI) adsorption to ferrihydrite decreased from 83% in the absence of Ca to 57% in the presence of 1.8 mM Ca. Surface complexation model predictions that included the formation constant for aqueous Ca 2UO 2(CO 3) 3 0(aq) accurately simulated the effect of Ca 2+ on U(VI) sorption onto quartz and ferrihydrite within themore » thermodynamic uncertainty of the stability constant value. This study confirms that Ca 2+ can have a significant impact on the aqueous speciation of U(VI), and consequently, on the sorption and mobility of U(VI) in aquifers.« less

Authors:
; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
944778
Report Number(s):
PNNL-SA-44029
Journal ID: ISSN 0016-7037; GCACAK; KP1504010; TRN: US200902%%946
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geochimica et Cosmochimica Acta; Journal Volume: 70; Journal Issue: 6
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; ADSORPTION; AIR; AQUIFERS; CALCIUM; GEOCHEMISTRY; QUARTZ; SORPTION; STABILITY; THERMODYNAMICS

Citation Formats

Fox, Patricia M, Davis, James A, and Zachara, John M. The effect of calcium on aqueous uranium(VI) speciation and adsorption to ferrihydrite and quartz. United States: N. p., 2006. Web. doi:10.1016/j.gca.2005.11.027.
Fox, Patricia M, Davis, James A, & Zachara, John M. The effect of calcium on aqueous uranium(VI) speciation and adsorption to ferrihydrite and quartz. United States. doi:10.1016/j.gca.2005.11.027.
Fox, Patricia M, Davis, James A, and Zachara, John M. Mon . "The effect of calcium on aqueous uranium(VI) speciation and adsorption to ferrihydrite and quartz". United States. doi:10.1016/j.gca.2005.11.027.
@article{osti_944778,
title = {The effect of calcium on aqueous uranium(VI) speciation and adsorption to ferrihydrite and quartz},
author = {Fox, Patricia M and Davis, James A and Zachara, John M},
abstractNote = {Recent studies of uranium(VI) geochemistry have focused on the potentially important role of the aqueous species, CaUO2(CO3)32- and Ca2UO2(CO3)30(aq), on inhibition of microbial reduction and uranium(VI) aqueous speciation in contaminated groundwater. However, to our knowledge, there have been no direct studies of the effects of these species on U(VI) adsorption by mineral phases. The sorption of U(VI) on quartz and ferrihydrite was investigated in NaNO3 solutions equilibrated with either ambient air (430 ppm CO2) or 2% CO2 in the presence of 0, 1.8, or 8.9 mM Ca2+. Under conditions where the Ca2UO2(CO3)30(aq) species predominates U(VI) aqueous speciation, the presence of Ca in solution lowered U(VI) adsorption on quartz from 77% in the absence of Ca to 42% and 10% at Ca concentrations of 1.8 and 8.9 mM, respectively. U(VI) adsorption to ferrihydrite decreased from 83% in the absence of Ca to 57% in the presence of 1.8 mM Ca. Surface complexation model predictions that included the formation constant for aqueous Ca2UO2(CO3)30(aq) accurately simulated the effect of Ca2+ on U(VI) sorption onto quartz and ferrihydrite within the thermodynamic uncertainty of the stability constant value. This study confirms that Ca2+ can have a significant impact on the aqueous speciation of U(VI), and consequently, on the sorption and mobility of U(VI) in aquifers.},
doi = {10.1016/j.gca.2005.11.027},
journal = {Geochimica et Cosmochimica Acta},
number = 6,
volume = 70,
place = {United States},
year = {Mon Jan 30 00:00:00 EST 2006},
month = {Mon Jan 30 00:00:00 EST 2006}
}
  • Recent studies of uranium(VI) geochemistry have focused on the potentially important role of the aqueous species, CaUO2(CO3)32- and Ca2UO2(CO3)30(aq), on inhibition of microbial reduction (Brooks et al., 2003) and uranium(VI) aqueous speciation in contaminated groundwater (Davis et al., 2004; Wang et al., 2004). However, to our knowledge, there have been no direct studies of the effects of these species on U(VI) adsorption by mineral phases. The sorption of U(VI) on quartz and ferrihydrite was investigated in NaNO3 solutions equilibrated with either ambient air (430 ppm CO2) or 2% CO2 in the presence of 0, 1.8, or 8.9 mM Ca2+. Undermore » conditions where the Ca2UO2(CO3)30(aq) species predominates U(VI) aqueous speciation, the presence of Ca in solution lowered U(VI) adsorption on quartz from 77% in the absence of Ca to 42% and 10% at Ca concentrations of 1.8 and 8.9 mM, respectively. U(VI) adsorption to ferrihydrite decreased from 83% in the absence of Ca to 57% in the presence of 1.8mM Ca. Surface complexation model predictions that included the formation constant for aqueous Ca2UO2(CO3)30(aq) accurately simulated the effect of Ca2+ on U(VI) sorption onto quartz and ferrihydrite within the thermodynamic uncertainty of the stability constant value. This study confirms that Ca2+ can have a significant impact on the aqueous speciation of U(VI), and consequently, on the sorption and mobility of U(VI) in aquifers.« less
  • A study of U(VI) adsorption by ferrihydrite was conducted over a wide range of U(VI) concentrations, pH, and at two partial pressures of carbon dioxide. A two-site (strong- and weak-affinity sites, {triple_bond}Fe{sub s}OH and {triple_bond}Fe{sub w}OH, respectively) surface complexation model was able to describe the experimental data well over a wide range of conditions, with only one species formed with each site type: an inner-sphere, mononuclear, bidentate complex of the type ({triple_bond}FeO{sub 2})UO{sub 2}. The existence of such a surface species was supported by results of uranium EXAFS spectroscopy performed on two samples with U(VI) adsorption density in the uppermore » range observed in this study (10 and 18% occupancy of total surface sites). Adsorption data in the alkaline pH range suggested the existence of a second surface species, modeled as a ternary surface complex with UO{sub 2}CO{sub 3}{sup 0} binding to a bidentate surface site. Previous surface complexation models for U(VI) adsorption have proposed surface species that are identical to the predominant aqueous species, e.g., multinuclear hydrolysis complexes or several U(VI)-carbonate complexes. The results demonstrate that the speciation of adsorbed U(VI) may be constrained by the coordination environment at the surface, giving rise to surface speciation for U(VI) that is significantly less complex than aqueous speciation.« less
  • Adsorption on soil and sediment solids may decrease aqueous uranium concentrations and limit its propensity for migration in natural and contaminated settings. Uranium adsorption will be controlled in large part by its aqueous speciation, with a particular dependence on the presence of dissolved calcium and carbonate. Here we quantify the impact of uranyl speciation on adsorption to both goethite and sediments from the Hanford Clastic Dike and Oak Ridge Melton Branch Ridgetop formations. Hanford sediments were preconditioned with sodium acetate and acetic acid to remove carbonate grains, and Ca and carbonate were reintroduced at defined levels to provide a rangemore » of aqueous uranyl species. U(VI) adsorption is directly linked to UO{sub 2}{sup 2+} speciation, with the extent of retention decreasing with formation of ternary uranyl-calcium-carbonato species. Adsorption isotherms under the conditions studied are linear, and K{sub d} values decrease from 48 to 17 L kg{sup -1} for goethite, from 64 to 29 L kg{sup -1} for Hanford sediments, and from 95 to 51 L kg{sup -1} for Melton Branch sediments as the Ca concentration increases from 0 to 1 mM at pH 7. Our observations reveal that, in carbonate-bearing waters, neutral to slightly acidic pH values ({approx}5) and limited dissolved calcium are optimal for uranium adsorption.« less
  • The effect of pH and contact time on uranium extractability from quartz surfaces was investigated using either acidic or carbonate (CARB) extraction solutions, time-delayed spikes of different U isotopes (i.e., 238U and 233U), and liquid helium temperature time-resolved laser-induced fluorescence spectroscopy (LHeT TRLFS). Quartz powders were reacted with 238U(VI) bearing solutions that were equilibrated with atmospheric CO2 at pH 6, 7, and 8. After a 42 day equilibration period with 238U(VI), the suspensions were spiked with 233U(VI) and reacted for an additional 7 days. Sorbed U was then extracted with either dilute nitric acid or CARB. For the CARB extractionmore » there was a systematic decrease in extraction efficiency for both isotopes from pH 6 to 8. This was mimicked by less desorption of 238U, after the 233U spike, from pH 6 to 8. Further, the efficiency of 233U extraction was consistently greater than that of 238U, indicating a strong temporal component to the strength of U association with the surface that was accentuated with increasing pH. LHeT TRLFS revealed a strong correlation between carbonate extraction efficiency and differences in sorbed U speciation as a function of pH. In contrast, the acid extraction was consistently more efficient than the CARB extraction, with a smaller dependence on both pH and aging time. Collectively, the observations show that aging and pH are critical factors in determining the form and strength of uranium-silica interactions.« less