Smectite alteration and its influence on the barrier properties of smectite clay for a repository

Abstract

Hydrothermal tests were conducted to investigate smectite alteration and its influence on the barrier properties of a smectite clay for a repository. Examinations of the X-ray diffraction patterns of the starting material and reacted samples and the silica release rate in the solution revealed that the smectite was transformed into randomly interstratified illite–smectite by a smectite-to-illite conversion when it was hydrothermally treated under a potassium concentration of 0.5 M, maintaining a 1 g/20 ml of solid sample-to-solution ratio. Temperature was observed to be a key factor controlling the conversion reaction. The smectite alteration affected the barrier properties of smectite clay for a repository: when the temperature increased, the percentage of the expandable smectite layers in the randomly interstratified illite–smectite decreased, the layer charge was more negative, and the cation exchange capacity and the sorption capacity for the cesium and nickel ions were reduced.

Introduction

The major functions of the buffer in a high-level waste (HLW) repository are to inhibit the penetration of groundwater and to retard the release of radionuclides from the radioactive wastes to the surrounding environment. Smectite clay has been considered favorably as such a buffer material because of its high swelling and sorption capacities. However, when the smectite clay is exposed to an elevated temperature due to heat from radioactive decay and the particular geochemical conditions of a repository for a long time, it may be transformed into other minerals (e.g., illite, chlorite, etc.) which leads to a decrease in the swelling and sorption properties of the smectite clay and consequently increases its water penetration and radionuclide transport properties (Pusch and Carlsson, 1985, Bucher and Müller-Vonmoos, 1989). Therefore, an understanding of smectite alteration and its influence on the barrier properties of a smectite clay is essential to evaluate the long-term barrier performance of a buffer for a HLW repository.

Smectite alteration, especially smectite-to-illite conversion which may occur under commonly-prevailing pH conditions, has drawn considerable interest in the last few decades due to the implication of its reaction for clay diagenesis and migration of petroleum as well as the long-term barrier performance of a smectitic buffer for a repository. Very many investigators (Eberl, 1978, Roberson and Lahann, 1981, Inoue, 1983, Howard and Roy, 1985, Huang and Otten, 1987, Proust et al., 1990, Güven and Huang, 1991, Velde and Vasseur, 1992, Huang et al., 1993, Cuadros and Linares, 1996, Huertas et al., 2001, Bauer et al., 2005, Sato et al., 2005, Bauer et al., 2006) have studied the reaction mechanism and kinetics of smectite-to-illite conversion. On the basis of those studies, smectite-to-illite conversion has been suggested by a general consensus to proceed by two different mechanisms. One is a solid-state one-to-one transformation where T–O–T layers are conserved, and the reaction proceeds with a replacement of the tetrahedral Si⁴⁺by Al³⁺until the layer charge deficiency is sufficiently developed to dehydrate the interlayer cations, and a collapse occurs (Hower et al., 1976):

Smectite + Al³⁺ + K⁺ – > Illite + Si⁴⁺

The other is a mechanism in which a destruction of the T–O–T layers provides the source of aluminium for the transformation, which thus consumes more smectite than it produces illite (Boles and Franks, 1979):

Smectite + K⁺ – > Illite +Si⁴⁺

where the ratio of smectite consumed to illite produced is approximately 1.6:1, and no external source of aluminium is required. However, no consensus has been reached on which mechanism prevails because the reaction depends on different test conditions. The kinetics of a smectite-to-illite conversion has been studied to obtain a rate law for the overall conversion process (Pytte and Reynolds, 1989, Velde and Vasseur, 1992, Huang et al., 1993, Wei et al., 1993 Cuadros and Linares, 1996, Huertas et al., 2001). Most of these studies are based on field observation of clay diagenesis, while there are not many studies based on systematic laboratory work. From these studies, it is implied that, although the most important parameters for controlling smectite-to-illite conversion are the temperature, potassium concentration, and reaction time, the proposed rate laws have limitations in their application to evaluate the extent of smectite-to-illite conversion under different repository conditions, due to an uncertainty in the parameter values. On the other hand, insufficient data has been reported regarding the influence of smectite alteration on the barrier properties such as the percentage of expandable smectite (MacEwan and Wilson, 1980, Huang et al., 1993), layer charge (Howard and Roy, 1985, Güven and Huang, 1991), cation exchange capacity (CEC) (Oscarson, and Hume, 1993), and sorption capacity (Comans et al., 1991, Ohnuki et al., 1994) of a smectite clay for a buffer of a HLW repository.

The present study, in this connection, focuses on investigating to what extent the smectite may be altered when it is hydrothermally treated under a certain potassium concentration and as a result how the smectite alteration may affect the barrier properties of a smectite clay for a Korean HLW repository.