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

The sorption of ¹³⁴Cs, ¹³³Ba, ⁶⁰Co and ¹⁵²Eu onto crushed biotite mineral of size fraction 0.25-0.5 mm at pH 5, 6, 7, 8 and 9 with pH-buffered solutions of 0.001, 0.01 and 0.1 M NaClO₄ were investigated. Experimental data was collected by both batch sorption and potentiometric titrations, together with measurements of the acidic site density and cation exchange capacity of the biotite. Titrations gave biotite surface pK_a1 = -4.9 ± 0.1 and pK_a2 = -7.1 ± 0.2. Batch sorption results show that sorption of all radionuclides is highly dependent on pH. In general sorption increase with pH, but for Co its aqueous hydroxide complexes compete with sorption at pH values > 8 while for Eu, this competition starts at pH > 7. Sorption of both Cs and Ba is strongly dependent on ionic strength, Eu shows an intermediate response while Co was found to be insensitive to this parameter. The measured apparent R_d values were found to increase with time, presumably due to in-diffusion. Therefore, equilibrium R_d was evaluated with a new model for diffusion into porous particles. A non-electrostatic Surface Complexation Model (SCM) was applied on the sorption data and the reaction constants were evaluated in a computation process which couples the PHREEQC chemical speciation program with an iterative error minimization routine. The sorption onto biotite of all four tracer elements was successfully modelled with a combination of one amphoteric 2-pK_a surface complexation site and one ion exchange site. The results show that the R_d-values may vary up to two orders of magnitude, depending on tracer element and water conditions. Such large changes should be considered in the safety evaluation of a repository in granitic rock, more specifically for scenarios where groundwater composition is expected to change.