Ola Karnland

Physico/chemical stability of smectite clays

Abstract There is convincing evidence from field data that smectite clay undergoes conversion primarily to illite and chlorite if it is fully water-saturated and heated. The conversion may take place through mixed-layer formation with increasing illite/smectite ratio at higher temperatures and pressures. This process requires dehydration of the interlamellar space, for which either an external pressure or drying are needed. An alternative mechanism that takes place without dehydration, is dissolution of smectite and neoformation of illite. Both processes imply reorganization of the smectite crystal lattice for which the activation energy is fairly high, meaning that the conversion is negligible at temperatures lower than about 60°C. At elevated temperatures the conversion rate is controlled by the access to potassium for either mechanism. An ongoing detailed investigation of this subject has led to a tentative model for the smectite-to-illite conversion in natural sediments and in canister-embedding clay in high-level radioactive waste (HLW) repositories.

Seeming Steady-State Uphill Diffusion of 22Na+ in Compacted Montmorillonite

Whereas the transport of solutes in nonreactive porous media can mostly be described by diffusion driven by the concentration gradients in the external bulk water phase, the situation for dense clays and clay rocks has been less clear for a long time. The presence of fixed negative surface charges complicates the application of Ficks laws in the case of ionic species. Here we report the seeming uphill diffusion of a (22)Na(+) tracer in compacted sodium montmorillonite, that is, transport directed from a low to a high tracer concentration reservoir. In contrast to the classical through-diffusion technique the present experiments were carried out under the conditions of a gradient in the background electrolyte and using equal initial (22)Na(+) tracer concentrations on both sides of the clay sample. We conclude that the dominant driving force for diffusion is the concentration gradient of exchangeable cations in the nanopores. Commonly used diffusion models, based on concentration gradients in the external bulk water phase, may thus predict incorrect fluxes both in terms of magnitude and direction.

INTERLABORATORY CEC AND EXCHANGEABLE CATION STUDY OF BENTONITE BUFFER MATERIALS: I. Cu(II)-TRIETHYLENETETRAMINE METHOD

Bentonites are candidate materials for encapsulation of radioactive waste. The cation exchange capacity (CEC) has proved to be one of the most sensitive parameters for detecting changes of mineral properties such as swelling capacity and illitization in alteration experiments. Whether measured differences in CEC values of bentonite buffer samples before and after an experiment are (1) actual differences caused by clay structural changes such as illitization or (2) simply data scatter due to the different methods used by international research teams is an open question. The aim of this study was to measure the CEC of clay samples in five different laboratories using the same method and to evaluate the precision of the values measured. The Cu-trien method and four reference materials of the Alternative Buffer Material (ABM) test project in Äspö, Sweden, were chosen for this interlaboratory study. The precision of the Cu-trien method, which uses visible spectroscopy, was very good with a standard deviation of ±0.7–2.1 meq/100 g for CECs that ranged from 11 to 87 meq/100 g. For the same CEC range, analysis of Cu-trien index cations using inductively coupled plasma (mass spectrometry) and atomic absorption spectroscopy were less precise with a standard deviation of ±2.8–3.9 meq/100 g. Based on the measured precision, greater measured differences in Cu-trien CEC and exchangeable cation values of bentonite buffer samples, before and after an experiment, might be actual differences. Great care must be taken when interpreting measured CEC differences, and analytical characterization of any structural changes may be needed. Compared with results from the ‘International Soil-Analytical Exchange’ (iSE) program for soils, most absolute concentrations were much larger for the clays studied; however, for the two parameters exchangeable Ca2+ and CEC the range was similar to the iSE ring test and, most importantly, the precision was comparable. Future studies should discuss the accuracy of CEC and exchangeable cation values and compare them to alternative CEC methods in which care is taken to prevent dissolution of soluble minerals, such as calcite and gypsum.

INTERLABORATORY CEC AND EXCHANGEABLE CATION STUDY OF BENTONITE BUFFER MATERIALS: II. ALTERNATIVE METHODS

Bentonites are candidate materials for encapsulation of radioactive waste. The cation exchange capacity (CEC) has proven to be one of the most sensitive parameters for detecting changes in mineral properties in bentonite-alteration experiments. An interlaboratory study of CECs and exchangeable cations for three reference bentonite buffer materials that were used in the Alternative Buffer Material test (ABM) project in Äspö, Sweden, was conducted to create a suitable database. The present study focused on CEC accuracy and compared CEC methods where care was taken to prevent dissolution of soluble minerals such as calcite and gypsum. The overall quality of the CEC and exchangeable cation values measured using non-Cu-trien CEC methods were good, with CECs of 74–91±0.5–3.3 meq/100 g and exchangeable cation values of 22–61±1.2–3.9 meq/100 g Na+, 7–23±0.8–1.5 meq/100 g Mg2+, and 19–39±0.8–1.6 meq/ 100 g Ca2+. The precision was comparable to the standard Cu-trien method even for exchangeable Ca2+, although the laboratories used different solution/solid ratios and reaction-time parameters for Cu-trien which affect carbonate dissolution. The MX80 and Dep.CAN bentonite exchangeable Ca2+ values were more accurate than standard-Cu-trien values. Using the optimized methods of this study, MX80 and Dep.CAN exchangeable Ca2+ values averaged 20.2±1.6 and 38.8±1.4 meq/100 g which amounts to ~70% of the inflated Cu-trien values. A more complex analysis of the CEC data using different methods, anion analyses, and mineralogical analysis is necessary to obtain plausible and accurate CEC values. Even with a more complicated analytical procedure, the CEC and exchangeable cation values were still not accurate enough because of excess anions. Chloride, sulfate, and dolomite might have increased the exchangeable Na+, Mg2+, and Ca2+ values.

A technique for modeling transport/conversion processes applied to smectite-to-illite conversion in HLW buffers

This paper describes an application of a technique developed for modeling chemical processes in buffer materials that are controlled by a reaction rate and by the transport of one component, which is essential for the process in question to occur. The application described here is the illitization of smectite by fixation of potassium ions in cation exchange positions, and with diffusion of dissolved potassium being the transport process. The technique is verified by comparison with analytical solutions. An overview, based on small models, is given which outlines under what constellations of assumptions the time scale for conversion of the buffer is controlled by reaction rate parameters and under which conditions transport controls this time scale. Examples are given of calculations performed for deposition holes, with potassium being supplied from the surroundings to the upper parts of the highly compacted bentonite buffer. It is concluded that restrictions in nearfield transport capacity have a very significant effect on the conversion time scale. Towards the end of the heating period about 98% of the smectite is found to remain, even for reaction rates and buffer transport conditions that would have left only 10% of the smectite unconverted without nearfield transport restrictions. It is also concluded that the modeling technique can be applied to other, similar, transport/conversion processes.

Redox properties of MX-80 and Montigel bentonite-water systems

The uptake of dissolved oxygen (O 2 ) has been studied in bentonite suspensions in 0.1 M NaCl media at (25±2)°C. MX-80 and Montigel bentonites were used in concentrations varying from 18 to 73 g/L. The experiments were performed in a magnetically stirred closed glass vessel, in an N 2 -glove box. Redox potentials where measured with Pt-wires, and dissolved O 2 was measured both with a membrane electrode and with an optode. The experiments with MX-80 show that dissolved O 2 disappears in ∼5 days under these conditions. Redox potentials decreased from ∼ +500 to ∼ +125 mVSHE ( versus Standard Hydrogen Electrode). The data for the Montigel bentonite show similar time scales for O 2 uptake but lower redox potentials at the end of the experiments ∼ −175 mVSHE. Pyrite oxidation is perhaps not the main process for O 2 uptake, as MX-80 contains 0.3% FeS 2 while Montigel bentonite only has a negligible amount.

Long-Term Test of Buffer Material at ASPO Hard Rock Laboratory, Sweden

Bentonite clay has been proposed as buffer material in several concepts for nuclear high level waste repositories. The “Long Term Test of Buffer Material” (LOT) series aims at validating models and hypotheses concerning physical properties in a bentonite buffer material and of related processes regarding mineralogy, microbiology, cation transport, copper corrosion and gas transport under conditions similar to those in a the Swedish KBS3 repository. The test series comprises 7 test parcels, which will be run for 1, 5 and 20 years. The testing principle is to place parcels containing heater, central copper tube, pre-compacted clay buffer, instruments, and parameter controlling equipment in vertical bore-holes in granitic rock. The parcels are equipped with heaters in order to simulate the decay power from spent nuclear fuel at standard KBS3 conditions (90°C) and adverse condition (130°C). Adverse conditions in this context refer also to high temperature gradients over the buffer, and additional accessory minerals leading to i.a. high pH and high potassium concentration in clay pore water. Temperature, total pressure, water pressure and water content are measured during the heating period. At test termination standard chemical and mineralogical analyses, and physical testing are made. The 2 pilot tests (1-year tests) have been completed and preliminary results concerning the bentonite analyses and tests are presented. The 5 test parcels in the main test series have been started during the fall, 1999.

The Corrosion Rate of Copper in a Test Parcel at the Äspö Hard Rock Laboratory

Cylindrical copper electrodes have been installed in a test parcel at the Aspo Hard Rock Laboratory and real-time corrosion monitoring was initiated in May 2001. The test parcel was emplaced on October 29, 1999, and will be retrieved in 2004. The three electrodes, each of about 100 cm 2 surface area, are installed in bentonite block 36, where the temperature is about 24°C. The corrosion monitoring is performed with linear polarization resistance (LPR), harmonic distortion analysis (HDA) and electrochemical noise techniques. A value on the Stern-Geary coefficient is required to calculate the corrosion rate from the measured LPR data. A default value of 10.3 mV has been used, but an actual value can in fact be obtained from the HDA. The corrosion rate will be overestimated if the frequency of the voltage perturbation for the LPR measurements is not low enough. Electrode impedance measurements have been performed to verify this. Two and a half year after emplacement the recorded corrosion rate of copper in the above bentonite block amounts to about 2.2 μm per year (using a default value of 10.3 mV for the Stern-Geary coefficient and a 0.01 Hz voltage perturbation frequency). The actual corrosion rate is estimated to less than 0.7 μm per year (considering a measured Stern-Geary coefficient of 6.5 mV and the findings from the electrode impedance measurements).

Soft X-ray Spectroscopic Characterization of Montmorillonite

Soft X-ray spectroscopy was applied to study a calcium bentonite from the Kutch area in India. We recorded the X-ray absorption spectra from the L-edge of calcium, silicon, and aluminum, and from K-edge of oxygen. The Ca absorption spectrum shows a quasi-atomic behavior, while the Si spectrum closely simulates the absorption spectrum of a pure silicon oxide. The O K spectrum shows a pre-peak, which is absent in the spectra of both the pure, bulk aluminum and silicon oxides. The Al L spectrum is complex and shows almost no resemblance to the absorption spectrum of aluminum oxides. The chemical state of the Al atoms (in octahedral coordination) must, thus, be quite different from what is common in the oxides. The obtained data show that soft X-ray spectroscopy is a promising technique for studying clay minerals. It is capable of supplying unique information that is complementary to information accessible using other techniques; especially, it can be used to determine the local electronic structure at various atomic sites in the complex samples.