Cs, Ba, Co, and Eu sorption on biotite - experiments at ambient temperature and modelling

Area of Science:

• Geochemistry and Environmental Science
• Radiochemistry and Nuclear Science
• Materials Science and Mineralogy

Background:

• Understanding radionuclide sorption onto geological materials is critical for nuclear waste disposal safety.
• Biotite, a common mica mineral in granitic rocks, plays a significant role in radionuclide retention.
• The influence of solution chemistry (pH, ionic strength) on sorption processes requires detailed investigation.

Purpose of the Study:

• To investigate the sorption behavior of ¹³⁴Cs, ¹³³Ba, ⁶⁰Co, and ¹⁵²Eu onto biotite.
• To determine the effect of pH and ionic strength on radionuclide sorption.
• To model and predict radionuclide sorption using a Surface Complexation Model (SCM).

Main Methods:

• Batch sorption experiments were conducted using crushed biotite (0.25-0.5 mm) at varying pH (5-9) and ionic strengths (0.001-0.1 M NaClO₄).
• Potentiometric titrations were used to determine biotite surface acidity (pK_a1, pK_a2) and cation exchange capacity.
• A non-electrostatic Surface Complexation Model (SCM) coupled with PHREEQC was applied to evaluate sorption data and reaction constants.

Main Results:

• Radionuclide sorption onto biotite was strongly dependent on pH, generally increasing with pH.
• ⁶⁰Co and ¹⁵²Eu sorption were affected by aqueous hydroxide complexation at higher pH values.
• Sorption of ¹³⁴Cs and ¹³³Ba was highly sensitive to ionic strength, while ⁶⁰Co was insensitive.
• Apparent R_d values increased with time due to in-diffusion, necessitating equilibrium R_d evaluation using a diffusion model.
• The SCM successfully modeled the sorption of all four tracer elements using a combination of amphoteric surface complexation and ion exchange sites.

Conclusions:

• Radionuclide sorption onto biotite is complex and influenced by multiple factors including pH, ionic strength, and tracer element properties.
• The developed SCM provides a robust framework for predicting radionuclide retention in granitic environments.
• Significant variations in R_d values (up to two orders of magnitude) highlight the importance of considering dynamic groundwater conditions in repository safety assessments.