The Effect of Calcium on Aqueous Uranium (VI) Speciation and Adsorption to Ferrihydrite and Quartz
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+. 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 1.8mM 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.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 881093
- Report Number(s):
- PNNL-SA-48006; GCACAK; KP1504010; TRN: US200612%%756
- Journal Information:
- Geochimica et Cosmochimica Acta, 70(6):1379-1387, Vol. 70, Issue 6; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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