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Neptunium(V) Sorption Behavior on Clinoptilolite, Quartz and Montmorillonite

Published online by Cambridge University Press:  15 February 2011

F. P. Bertetti ,
R. T. Pabalan ,
D. R. Turner  and
M. G. Almendarez
F. P. Bertetti
Affiliation:
Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238–5166.
R. T. Pabalan
Affiliation:
Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238–5166.
D. R. Turner
Affiliation:
Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238–5166.
M. G. Almendarez
Affiliation:
Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238–5166.
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Abstract

Performance assessment models have identified 237Np as a radionuclide of concern in meeting release limits established for the geologic disposal of high-level nuclear waste at the proposed repository at Yucca Mountain, Nevada. In this study, quartz, clinoptilolite, and montmorillonite, which are minerals representative of phases that occur both in the rock matrix and as fracture coatings at Yucca Mountain, were reacted with 237Np-bearing solutions to characterize the sorption behavior of Np(V) on these minerals.

Batch experiments were conducted over a wide range of conditions in which pH of solution, surface loading, sorbent surface area, initial concentration of Np(V), and partial pressure of CO2 were varied. Initial Np(V) concentrations were between 1 × 10-7 and 1- 10-6 M in electrolyte solutions of 0.1 or 0.01 M NaNO3. The oxidation state of Np in solution was verified with NIR spectroscopy and by solvent extraction. Prior to the start of experiments, minerals were pretreated to eliminate impurities, and the clinoptilolite and montmorillonite were converted to Na-form by ion exchange with NaCI solutions.

Results indicate that, for all three minerals, Np(V) sorption begins at pH values coincident with the start of hydrolysis in solution (-7). For solutions undersaturated with respect to atmospheric CO2, sorption increases continuously with increasing pH. Under equilibrium with atmospheric CO2, Np(V) sorption is important in the pH range (7–9.5) where NpO2(OH)°(aq) is significant, whereas sorption is inhibited at higher pH where neptunyl carbonate complexes are the predominant species.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 412: Symposium V – Scientific Basis for Nuclear Waste Management XIX , 1995 , 631
Copyright
Copyright © Materials Research Society 1996

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