Extent of pyrite oxidation by dissolved oxygen in alkaline solutions including purified sodium bentonite

Abstract

Ferrous iron-bearing minerals such as pyrite are promising reducing agents for the consumption of atmospheric oxygen in repositories used for the geological disposal of radioactive waste. Thus, the consumption of dioxygen (O2) by pyrite around repositories is essential for assessing atmospheric O2 which may be dissolved into underground water during installation of the repository. However, quantitative evaluation of the long-term oxidation of pyrite by dissolved oxygen (DO) has rarely been attempted. In particular, few observations of the particle morphology and internal inhomogeneity during the oxidation of pyrite have been recorded. Our study presents a quantitative investigation of the internal morphology of pyrite after approximately five years of oxidation by DO in carbonate-buffered solutions at varying ionic strengths. The effects of ionic strength (by applying various concentrations of sodium chloride (NaCl)) and of purified sodium bentonite (SB) on pyrite oxidation were studied. We observed that the surface of oxidized pyrite was covered with Fe(III) oxyhydroxides, which did not passivate the pyrite against further oxidation, and the interior of pyrite generated numerous flow-path-like voids in random directions during long-term oxidation. The oxidation of pyrite by DO was accelerated with increasing pH (9.3 ≤ pH ≤ 10.9) but was not associated with the increase in ionic strength by the addition of NaCl. The presence of SB increased pyrite oxidation when the pH was 10 or higher, while there was no effect when pH was 9.6 or lower. These observations suggest that pyrite can effectively consume O2 trapped underground in enclosed radioactive waste repositories.