Determination of Sulfide Consumption by Fe-bearing Components of Bentonites

Abstract

Geologic repositories for spent nuclear fuel use bentonite as a buffer to protect the metallic containers confining the radioactive material. Sulfate-reducing bacteria, which may be present in groundwater, at the bentonite–host rock interface or eventually within the bentonite may produce sulfide, representing a potential threat for the metallic canisters, particularly copper. Bentonites can act as potential sulfide scavengers. Little is yet known, however, regarding the underlying mechanisms, the maximum extent of sulfide consumption, and the potential impacts on bentonite structure under repository conditions. In the current study, concentrated (4–150 mM) sulfide solutions were reacted in batch experiments with six natural Fe-bearing bentonites, with various purified Fe-bearing components of bentonite (a series of purified montmorillonites and three iron (oxyhydr)oxides), and with one synthetic mixture, for up to 1.5 months at pH values ranging from 7 to 13. The solutions were analyzed by colorimetry to determine sulfide and polysulfide concentrations and the solids were analyzed by 57Fe Mössbauer spectrometry to determine iron speciation. Important sulfide consumption coupled with a reduction of structural Fe in the clay samples was observed. Not all clay structural Fe was reactive toward sulfide; the proportion of active structural Fe depended on the clay structure and pH. In the presence of excess sulfide in solution regarding Fe in the solid sample, the clay structural Fe was found to be the main reactant while the reaction with iron (oxyhydr)oxides was largely inhibited. Three bentonite groups were distinguished, based on the sulfide oxidation capacity of their main clayey component.