Clay mineral weathering and contaminant dynamics in a caustic aqueous system: II. Mineral transformation and microscale partitioning

doi:10.1016/j.gca.2005.04.004

Geochimica et Cosmochimica Acta

Volume 69, Issue 18, 15 September 2005, Pages 4437-4451

https://doi.org/10.1016/j.gca.2005.04.004 Get rights and content

Abstract

Microscopic and spectroscopic studies were conducted to assess mineral transformation processes in aqueous suspensions of illite (Il), vermiculite (Vm) and montmorillonite (Mt) that were subjected to weathering in a simulated high-level radioactive tank waste leachate (0.05 m Al_T, 2 m Na⁺, 1 m NO₃⁻, pH ∼14, Cs⁺ and Sr²⁺ present as co-contaminants). Time series (0 to 369 d) experiments were conducted at 298 K, with initial [Cs]₀ and [Sr]₀ concentrations from 10⁻⁵ to 10⁻ mol kg⁻. Incongruent clay dissolution resulted in an accumulation of secondary aluminosilicate precipitates identified as nitrate-sodalite, nitrate-cancrinite and zeolite X, by molecular spectroscopy and electron microscopy (XRD, IR, NMR, SEM-EDS and TEM-EDS). Contaminant fate was dependent on competing uptake to parent clays and weathering products. TEM-EDS results indicated that high Il affinity for Cs was due to adsorption at frayed edge sites. The Il system also comprised Sr-rich aluminous precipitates after 369 d reaction time. In Mt systems, Cs and Sr were co-precipitated into increasingly recalcitrant spheroidal precipitates over the course of the experiment, whereas contaminant association with montmorillonite platelets was less prevalent. In contrast, Cs and Sr were found in association with weathered Vm particles despite the formation of spheroidal aluminosilicate precipitates that were comparable to those formed from Mt dissolution.

Introduction

∼373,400 m³ of high level radioactive waste have been stored in underground steel tanks at Department of Energy (DOE) sites across the U.S. including Hanford, WA; Savannah River, SC; Idaho Falls, ID; West Valley, NY and Oak Ridge, TN (DOE, 1995; Ahearne, 1997). Most of this waste (63%) is stored at the Hanford facility in 177 tanks ranging in size from 210 to 4100 m³ (Ahearne, 1997). Of these tanks, 67 have leaked, causing the release of over 3800 m³ of highly radioactive liquid waste containing ¹³⁷Cs and ⁹⁰Sr into the underlying sediment (Zachara et al., 2002). The leaking tank waste is characterized by high pH (> 13), high aluminate concentrations, and high ionic strength (dominantly NaNO₃) (Serne et al., 1998).

The sediments at the Hanford site comprise micaceous weathering products illite, vermiculite and montmorillonite (Serne et al., 2001) that are expected to contribute to Cs and Sr attenuation in the vadose zone. Under circumneutral conditions, these clays exhibit variable affinities for Sr, and particularly Cs, depending on the density and location of charge resulting from isomorphic substitution (Sawhney, 1972, Onodera et al., 1998, Sutton and Sposito, 2001, Zachara et al., 2002). However, they are also susceptible to high rates of dissolution at the caustic pH values encountered in tank waste leachate (Nagy, 1995, Chorover et al., 2003).

In a companion paper, we reported on the time-dependent coupling between Cs and Sr uptake and clay mineral (illite, vermiculite and montmorillonite) dissolution in a tank waste simulant (Choi et al., 2005). Dissolution rates were dependent on clay mineral type and co-contaminant concentrations, and reprecipitation of secondary solid phase products was evident in all systems. The objective of the present study was to employ microscopic and spectroscopic techniques to identify the nature of mineral transformations responsible for the element solubility patterns observed by Choi et al. (2005), and to elucidate the solid-phase fate of Cs and Sr in the weathering clay systems.

Section snippets

Materials and Methods

Montmorillonite (SWy-2) from Crook County, WY and illite (IMt-1) from Silver Hill, MT (both dioctahedral) were obtained from the Source Clays Repository of the Clay Minerals Society. Trioctahedral vermiculite from Phalaborwa in Transvall of South Africa was obtained from Ward's Scientific Inc., NY. Conventional mineralogical characterization (K and Mg saturation, ethylene glycol and glycerol solvation, and heat treatments) of this vermiculite sample by XRD (following cleaning) indicated that it

Scanning Electron Microscopy

In accordance with the Si and Al dissolution results (Choi et al., 2005), secondary phases were not detected in the illite systems by SEM before 190 d, regardless of Cs and Sr concentration. However, illite particles were found to decrease in size relative to unreacted samples (Fig. 1a) with increasing reaction time. At 190 d, precipitates with a ‘bundle of yarn’ (BY) shape were clearly evident, and subtle morphologic differences were observed between the treatments (Fig. 1b-c). The spheroidal

Discussion

Chemical reaction of 2:1 layer-type clays with a caustic high level radioactive waste simulant (STWL) affects the fate of contaminant radionuclides through concurrent adsorption and dissolution-precipitation reactions. Whereas mineral transformation is normally slow relative to adsorption-desorption, the former becomes relevant at the time-scale of the current experiments because reaction of STWL with clays promotes rapid Si dissolution. Our prior work indicated that initial Cs and Sr

Conclusions

Interacting effects of clay mineral (i) transformation rate and (ii) adsorption affinity combine to influence the localized siting of sorptive cesium and strontium ions in clay systems subjected to weathering in a STWL representative of that released into the Hanford site subsurface. Incongruent dissolution of 2:1 layer-type silicates results in the formation of nitrate-sodalite, nitrate-cancrinite and zeolite X with molar Si/Al ratios close to unity during precipitation of clay-derived Si with

Acknowledgments

We are grateful to B. Baeyens, an anonymous reviewer, and Associate Editor D. L. Sparks for constructive comments on an earlier draft. This research was supported by the Environmental Management Science Program of the Office of Science, U. S. Department of Energy, grants DE-FG07-99ER15012 and DE-FG07-02ER63504.

Associate editor: D. L. Sparks

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Clay mineral weathering and contaminant dynamics in a caustic aqueous system: II. Mineral transformation and microscale partitioning

Abstract

Introduction

Section snippets

Materials and Methods

Scanning Electron Microscopy

Discussion

Conclusions

Acknowledgments

Micropor. Mesopor. Mat.

Micropor. Mesopor. Mat.

Waste Man.

Micropor. Mesopor. Mat.

Zeol.

J. Cryst. Growth

Thermochim. Acta

Mat. Res. Bull.

J. Alloys Compd.

Solid State Nucl. Mag. Res.

J. Alloys Comp.

Thermochim. Acta

Micropor. Mesopor. Mater.

J. Cryst. Growth

J. Nucl. Mater.

Thermochim. Acta

Thermochim. Acta

Micropor. Mesopor. Mater.

Chem. Phys. Lett.

Waste Manag.

Zeol.

J. Contam. Hydrol.

J. Nucl. Mater.

Waste Man.

J. Colloid Interface Sci.

Zeol.

Catalys. Today

J. Nucl. Mater.

Geochim. Cosmochim. Acta