Consumption and diffusion of dissolved oxygen in sedimentary rocks

Area of Science:

• Geochemistry
• Environmental Science
• Materials Science

Background:

• Fe(II)-bearing minerals are crucial for consuming oxygen in geological repositories for high-level radioactive waste.
• Accurate estimation of dissolved oxygen diffusion and Fe(II) mineral oxidation rates is vital for repository safety assessments.

Purpose of the Study:

• To estimate effective diffusion coefficients (D_e) for dissolved oxygen (DO).
• To determine reaction rates for Fe(II)-bearing mineral oxidation in a repository environment.
• To compare results from diffusion-chemical reaction experiments with batch experiments.

Main Methods:

• Conducted diffusion-chemical reaction experiments using intact Mizunami sedimentary rock samples.
• Performed batch experiments on crushed sedimentary rock oxidation by DO in a closed system.
• Estimated effective diffusion coefficients for DO and second-order rate constants (k) for mineral oxidation.

Main Results:

• Effective diffusion coefficients for DO were estimated between 2.69×10^-11 and 6.30×10^-11 m²s^-1.
• Second-order rate constants (k) ranged from -3.66 to -2.83 Lmol^-1s^-1 (batch) and -3.87 to -2.22 Lmol^-1s^-1 (diffusion-chemical).
• Batch experiments yielded a higher concentration of reactive sites (10^-4molm^-2) compared to intact rock (10^-8molm^-2), indicating limited reaction sites in intact rock.

Conclusions:

• Diffusion-chemical reaction experiments enable simultaneous determination of kinetic parameters and diffusivity in intact rock.
• The reaction within intact rock is confined to accessible porosity, unlike powdered samples used in batch experiments.
• Results provide critical data for modeling oxygen consumption and ensuring the long-term safety of radioactive waste repositories.