Separating selenium species by diffusion in Brazilian bentonite: a mathematical modeling approach

Area of Science:

• Environmental Science
• Geochemistry
• Materials Science

Background:

• Selenium is an emerging contaminant of environmental concern.
• Bentonite is a clay mineral often used in geological barrier applications.
• Understanding selenium mobility in bentonite is crucial for environmental risk assessment.

Purpose of the Study:

• To investigate the diffusion of selenium species in bentonite.
• To determine the apparent and effective diffusion coefficients of selenite and selenate.
• To assess the mobility of different selenium species within bentonite.

Main Methods:

• Planar source method for diffusion studies.
• Application of a double Gaussian function to model diffusion data.
• Sequential extraction and distribution coefficient measurements to evaluate mobility.

Main Results:

• Two distinct diffusion coefficients were observed, corresponding to selenite and selenate.
• Selenate exhibited higher apparent diffusion coefficients (approx. 10^-10 m² s^-1) than selenite (approx. 10^-12 m² s^-1).
• Sequential extraction and distribution coefficients confirmed greater mobility for selenate compared to selenite.

Conclusions:

• Bentonite diffusion characteristics highlight differential mobility of selenium species.
• Interconversion between selenite and selenate can mobilize selenium, increasing contamination risk.
• These findings necessitate careful consideration of selenium speciation and mobility in geological barrier designs.