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The Dissolution of Uranophane in CaCl2-SiO2(aq) Test Solutions

Published online by Cambridge University Press:  17 March 2011

James D. Prikryl  and
William M. Murphy
James D. Prikryl
Affiliation:
Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute®, San Antonio, TX 78228-0210, U.S.A., E-mail address:, [email protected]
William M. Murphy
Affiliation:
Department of Geological and Environmental Sciences, California State University, Chico, CA 95929-0205, U.S.A.
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Abstract

Uranophane [Ca(UO2)2(SiO3OH)2 · 5H2O] is a corrosion product of long-term leaching of spent fuel under oxidizing conditions and is a weathering product of uraninite in uranium ore deposits hosted by siliceous rocks. Incorporation of radionuclides into uranophane by coprecipitation may occur as a result of spent fuel alteration. Dissolution of uranophane leading to release of these radionuclides may therefore influence the longterm dissolved concentration and mobility of radionuclides at the proposed nuclear waste repository at Yucca Mountain, Nevada. In this study, the dissolution of uranophane in Ca- and Si-rich test solutions was investigated. Batch dissolution experiments were designed to approach uranophane equilibrium from undersaturated solutions at nearneutral pH (~6.0). Test solutions had initial U concentrations of 0.0 and 10-7 mol/L in matrices of ~10-2 mol/L CaCl2 and ~10-3 mol/L SiO2(aq). The test solutions were reacted with synthetic uranophane (confirmed by XRD and chemical analyses) and analyzed periodically over 10 weeks. Reaction quotients (Log Qs) derived from aqueous speciations of experimental solutions considered to be near equilibrium with uranophane ranged from 10.54 to 11.06 for the dissolution reaction: Ca(UO2)2(SiO3OH)2 · 5H2O + 6H+ ⇔ Ca2+ + 2UO22+ + 2SiO2(aq) + 9H2O.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 824: Symposium CC – Scientific Basis for Nuclear Waste Management XXVIII , 2004 , CC8.3
Copyright
Copyright © Materials Research Society 2004

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