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.