CO₂ effect on the pH of compacted bentonite buffer on the laboratory scale

Abstract

Disposal of Finnish spent nuclear fuel is planned to be based on the KBS-3 repository concept. The role of the bentonite buffer in this concept is essential, and thus the behaviour of the bentonite has to be known. The experiments in this paper concentrated on providing information about the effects of carbon dioxide CO2(g) partial pressure on compacted sodium bentonite, giving an insight into the buffering capacity. The experimental setup consisted of a hermetic box which had a CO2-adjusted atmosphere, and the bentonite was in contact with this atmosphere through water reservoirs. The results indicated that it is possible to measure online the changing pH in the porewater inside compacted bentonite using IrOx electrodes. It was found that the pH fell if the CO2 partial pressure increased above atmospheric conditions. The experimental results indicated a greater fall in pH than in our model in the test cases where CO2 was present. The pH in the experiment with 0 PCO2 remained nearly constant throughout the 5 month period. On the other hand, the pH dropped to near 6 with 0.3 PCO2 and to 5.5 with 1 PCO2.

Methodology for studying the composition of non-interlamellar pore water in compacted bentonite

“…After completion of the squeezing test, the syringe was decoupled from the device and the exact amount of squeezed pore water in the syringe was determined gravimetrically. The squeezed pore-water samples were analysed by ICP-OES to determine Ca, Na, K, Mg and Fe concentrations, by IC to determine sulfate and chloride concentrations, and by titration to determine bicarbonate according to the Gran method (Gran, 1950), and the pH value by an IrOx electrode (Muurinen, 2009;Itälä et al, 2013). The pore-water samples were kept in air-tight syringes before the bicarbonate and pH determinations.…”