UraniumVI sorption behavior on silicate mineral mixtures

doi:10.1016/S0169-7722(00)00153-4

Journal of Contaminant Hydrology

Volume 47, Issues 2–4, February 2001, Pages 241-253

https://doi.org/10.1016/S0169-7722(00)00153-4 Get rights and content

Abstract

Uranium^VI sorption experiments involving quartz and clinoptilolite, important mineral phases at the proposed US nuclear waste repository at Yucca Mountain, NV, were conducted to evaluate the ability of surface complexation models to predict U^VI sorption onto mineral mixtures based on parameters derived from single-mineral experiments. The experiments were conducted at an initial U^VI aqueous concentration of ∼2.0×10⁻⁷ mol·l⁻¹ (0.1 mol·l⁻¹ NaNO₃ matrix) and over the pH range ∼2.5 to ∼9.5. The U^VI solutions were reacted with either quartz or clinoptilolite only, or with mixtures of the two minerals. The experiments were carried out under atmospheric pCO₂(g) conditions (in loosely capped containers) or under limited pCO₂(g) (in capped containers or in a glove box).

Data from sorption experiments on quartz at atmospheric pCO₂ conditions were used to derive U^VI binding constants for a diffuse-layer surface complexation model (DLM). The DLM was then used with surface area as a scaling factor to predict sorption of U^VI onto clinoptilolite and clinoptilolite/quartz mixtures under both atmospheric and low pCO₂ conditions. The calculations reproduced many aspects of the pH-dependent sorption behavior. If this approach can be demonstrated for natural mineral assemblages, it may be useful as a relatively simple method for improving radionuclide transport models in performance-assessment calculations.

Introduction

The potential release of radionuclides as dissolved constituents in groundwater is a primary concern in performance assessments of proposed high-level nuclear waste (HLW) geologic repositories. Sorption onto mineral surfaces is an important mechanism for reducing radionuclide concentrations along groundwater flow paths and retarding radionuclide migration to the accessible environment. However, the dependence of sorption on aqueous solution properties (e.g., pH, radionuclide concentration, ionic strength, and complexing ligands) and sorptive phase characteristics (e.g., composition, surface area, and sorption site density) makes prediction of radionuclide retardation and transport difficult in natural geologic systems that are mineralogically and chemically heterogeneous. Recent sorption studies on a variety of mineral separates indicate that sorption of U^VI and Np^V through a surface complexation mechanism is important where the chemistry of the system favors the formation of hydroxy complexes in the aqueous phase, and that the magnitude of sorption is controlled primarily by the number of available sorption sites, which may be indirectly determined from surface area measurements Pabalan and Turner, 1997, Bertetti et al., 1998, Pabalan et al., 1998, Turner et al., 1996. Because rocks are multimineralic phases, an important uncertainty in predicting sorption in natural systems is the effect of the heterogeneity of the geologic materials.

In this study, experimental and surface complexation modeling studies were conducted to investigate the sorptive behavior of U^VI onto clinoptilolite, quartz, and binary mixtures of both minerals. Clinoptilolite and quartz are common sorptive mineral phases present in the rock matrix and in fractures at the proposed Yucca Mountain HLW repository (Bish and Chipera, 1989). The mineralogic and surface characteristics of clinoptilolite and quartz are distinct and could potentially influence U^VI sorption. The study was designed to determine whether model parameters derived from data on single minerals can be used successfully to predict U^VI sorption onto mineral mixtures.

Section snippets

Mineral preparation and characterization

Clinoptilolite was prepared from a clinoptilolite-rich tuff from Death Valley Junction, CA (Minerals Research, Clarkson, NY). The tuff was crushed and sieved to obtain material in the 100–200 mesh (0.150–0.074 mm) size range. Clinoptilolite was pretreated to remove mineral impurities such as soluble salts, carbonates, and iron oxides using a procedure described in Pabalan (1994). The purified clinoptilolite was converted to Na-form by ion exchange with 3 mol·l⁻¹ NaCl solutions at 90°C for about

Experimental results and discussion

Results of the U^VI sorption experiments are shown in Fig. 1 in terms of the distribution coefficient (K_d) vs. pH. The K_d was calculated from the equation: $K_{d} (ml · g^{−1})= equilibrium amount of U sorbed equilibrium amount of U in solution V M$ where V is the solution volume (ml) and M is the solid mass (g). The use of K_d provides a means of normalizing sorption results with respect to the M/V ratio and of taking into account the decrease in U^VI solution concentration that occurs during the course of the experiment.

Surface complexation modeling

The diffuse-layer surface complexation model (DLM) is based on the assumption of analogous behavior between aqueous complex formation in the bulk solution and formation of surface complexes at the mineral–water interface. Additional terms are included in the mass action expressions for the surface reactions to account for electrostatic interactions near the mineral surface. Details of this modeling approach are given elsewhere (Davis and Kent, 1990).

For U^VI, a generalized reaction can be

Conclusions

Similar pH-dependent trends in U^VI sorption behavior are observed for clinoptilolite, quartz, and mixtures of clinoptilolite and quartz. Uranium^VI sorption is enhanced under conditions that favor the formation of aqueous U^VI hydroxy complexes (e.g., near-neutral pH for atmospheric pCO₂ conditions and near-neutral and alkaline pH for low-pCO₂ conditions). Geochemical conditions that inhibit the formation of U^VI hydroxy complexes (e.g., low pH and formation of carbonate complexes) suppress U^VI

Acknowledgements

The reviews by F.P. Bertetti and B. Sagar are gratefully acknowledged. This work was funded by the Nuclear Regulatory Commission (NRC) under Contract No. NRC-02-97-009. The report is an independent product of the Center for Nuclear Waste Regulatory Analyses and does not necessarily reflect the views or regulatory position of the NRC.

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UraniumVI sorption behavior on silicate mineral mixtures

Abstract

Introduction

Section snippets

Mineral preparation and characterization

Experimental results and discussion

Surface complexation modeling

Conclusions

Acknowledgements

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

The sorption of actinides on igneous rocks

Nucl. Technol.

Studies of neptunium(V) sorption on quartz, clinoptilolite, montmorillonite, and ∀-alumina

Revised Mineralogic Summary of Yucca Mountain, Nevada

Surface complexation modeling in aqueous geochemistry

Rev. Mineral.

Surface Complexation Modeling: Hydrous Ferric Oxide

Groundwater

Thermodynamic properties of actinide species relevant to geochemical problems

Radiochim. Acta

Uranium^VI sorption behavior on silicate mineral mixtures