Applicability of the second-order rate law for modeling dissolved O2 consumption by Fe(II)-bearing geologic media

Abstract

Fe(II)-bearing geologic media are promising agents for the consumption of dissolved oxygen (DO) remaining in geologic formations after the backfilling of radioactive waste repositories, which helps prevent redox-sensitive radionuclides from becoming soluble and mobile in groundwater. A second-order rate law has been proposed to describe DO consumption due to the oxidation of structural Fe(II) at the mineral surface; however, it has only been tested against a few Fe(II)-bearing geologic media via DO consumption experiments. Thus, this study investigates the applicability of the second-order rate law to Fe(II)-bearing geologic media and minerals by reanalyzing data from previous batch experiments of DO consumption. Additional DO consumption experiments were also performed on diatomaceous and siliceous mudstones to examine the influence of clay components and coexisting Fe(II)-bearing minerals on the DO consumption rate constant. The reanalysis results showed that the second-order rate law can effectively describe DO consumption by both natural geologic media and purified minerals. The derived consumption rate constants are relatively similar for natural geologic media and purified minerals, but not for media with a high clay content. In particular, the rate constants for sedimentary rocks with smectite components exhibit large variations and tend to be smaller than the theoretical lower limit for structural Fe(II) on silicate minerals. Conversely, the rate constants for mudstones containing non-swelling chlorite exhibit small variations. This implies that, in batch experiments on powdered samples containing smectite, DO is consumed not only by structural Fe(II) at the external surface but also by Fe(II) in the interlayers of the smectite, which is less accessible to DO in bulk solution. This may explain the prolonged reaction time and relatively small rate constants of sedimentary rocks with smectite components. This study suggests that the second-order rate law can be applied to model DO consumption in a variety of Fe(II)-bearing rocks. However, the rate constants may be underestimated with large variations when testing powdered rocks containing smectite.