Sorption of Cs, Ba, Co, and Eu onto biotite: experiments and modelling

Sammanfattning: The sorption of 134Cs, 133Ba, 60Co and 152Eu, all at about [~10-8 M] onto biotite mineral was studied using the batch method with S: L = 1:50 at three different ionic strengths (0.001, 0.01 and 0.1M NaClO4) and five different pH (5,6,7,8,9) at 25°C for up to two months in an inert atmosphere glovebox. The results revealed that the sorption of all metals was dependent on both pH and ionic strength.   At 25°C, the measured sorption distribution coefficients (Rd -values) for 0.001 M were (pH 5-9): (Cs) 0.6-1.2, (Ba) 0.3-8.3, (Co) 0.01-1.9 and (Eu) 2.7-18 m3/kg, respectively. For I = 0.01 M, the corresponding results were (Cs) 0.1-0.7, (Ba) 0.01-4.4, (Co) 0.01-7.5 and (Eu) 0.2-4.3 m3/kg, respectively. For I= 0.1M, the corresponding results were (Cs) 0.01-0.2, (Ba) 0.03-0.4, (Co) 0.01-4.7 and (Eu) 2.1-6.7 m3/kg.   The surface acidity constants for the amphoteric surface site of biotite mineral were determined with the continuous potentiometric titration on a biotite suspension to pKa1= 0.30 and pKa2 = 0.20. These acidity constants were obtained by fitting titration data through the use of PHREEQC and PYTHON code optimization routines.   The biotite size fraction of 0.25-0.5 mm was characterized for acidic site density (ASD), cation exchange capacity (CEC) and the specific surface area (SSA) and were determined to be 6.7mmol/m2, 1.01  0.03 meq/100g and 0.47 m2/g by using tritium uptake, ammonium acetate (NH4Ac), and Kr-BET adsorption methods, respectively.     To model the experimental sorption data, PHREEQC geochemical modelling software coupled with PYTHON code optimization routines was used. The results shows that a two-step protolysis (2-pKa) non-electrostatic model (NEM) representing edge site with one additional ion-exchange site representing basal plane site is sufficient to reproduce the data for all four metals.