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

Journal Article · Mon Jan 30 00:00:00 EST 2006 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2005.11.027· OSTI ID:944778

Fox, Patricia M; Davis, James A; Zachara, John M

Recent studies of uranium(VI) geochemistry have focused on the potentially important role of the aqueous species, CaUO₂(CO₃)₃^2- 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.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 944778
• Report Number(s): PNNL-SA-44029; GCACAK; KP1504010; TRN: US200902%%946
• Journal Information: Geochimica et Cosmochimica Acta, Vol. 70, Issue 6; ISSN 0016-7037
• Publisher: The Geochemical Society; The Meteoritical Society
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
ADSORPTION
AIR
AQUIFERS
CALCIUM
GEOCHEMISTRY
QUARTZ
SORPTION
STABILITY
THERMODYNAMICS