Probing subtle alterations of smectite layer charge by the spectroscopic OD method in bentonite from the Alternative Buffer Materials tests in Äspö, Sweden

Abstract

The recent and efficient spectroscopic, “O-D (using D₂O water) method”, for layer charge (LC) determination of materials containing smectitic, wettable surfaces was used to identify subtle LC changes of thermally reacted samples (between ∼80 and 250 °C) from the Alternative Buffer Material (ABM) tests 2 and 5 conducted at the Äspö Hardrock Laboratory (HRL), Sweden. The tests were in situ intermediate-scale field experiments designed to simulate an engineered barrier around canisters containing radioactive waste within a deep geological repository. The tests consisted of several different bentonites compacted into 31 (ABM2) and 30 (ABM5) discs, stacked vertically around a central heater, and emplaced in a crystalline rock borehole in the HRL. Previous investigations of the ABM2 and 5 bentonites did not reveal specific mineralogical reactions of smectites, which may be observed at temperatures above 100 °C. The data presented in the present study, however, showed LC changes, in the ABM5 test, within the individual bentonite discs along a thermal gradient from the outside edge of the discs toward the heater contact, at which the temperature reached a maximum of 250 °C. Considerably less LC changes were observed for the bentonites in ABM2, in which heating reached a maximum of only ∼140 °C. Moreover, the LC data presented in the present study were found to be consistent with previously obtained cation exchange capacity (CEC) results and explained the observed changes in CEC along the thermal gradients across the discs. Both CEC and LC displayed a strong decreasing trend toward the heater contact in ABM5, and very weak increases toward the heater contact in ABM2. Of the several bentonites used in the tests, those with an initially higher LC, generally showed the greatest decrease of apparent LC after being reacted at high temperatures in the ABM5 package. The bentonites with an initially low to medium smectitic LC, appeared to undergo the least changes at the high temperatures achieved in the ABM5 package. The observed decrease of apparent LC is interpreted as resulting from the adsorption of interlayer cations at the siloxane surface, possibly in the ditrigonal cavity of the tetrahedral sheet, causing partial neutralization of the negative charge of the 2:1 layer. The minute increase in LC in the ABM2 package also showed a correlation with CEC but was within the measurement uncertainty.