Effects of pH, ionic strength, and temperature on Cs, Ba, Co, and Eu sorption onto biotite: A combined experimental and modeling study

Abstract

The trace metal sorption onto the phyllosilicate mineral biotite, commonly found in granitic rock, has been studied with batch sorption experiments that were carried out for up to two months. The aim was to investigate the uptake mechanism of the radioisotopes ¹³⁴Cs, ¹³³Ba, ⁶⁰Co, and ¹⁵²Eu at trace concentrations (10⁻⁸ M) onto crushed biotite at a particle size 0.25–0.5 mm. under inert gas conditions ([O₂] < 1 ppm) and to provide data for a Surface Complexation Model. The experiments were conducted in triplicates with the pH adjusted to 5, 6, 7, 8, or 9 with pH-buffered solutions containing either 0.001, 0.01, or 0.1 M NaClO₄ at two temperatures: 40 and 60 °C. The results show that the sorption of Cs, Ba, Co and Eu onto biotite are strongly pH-dependent where each element shows a characteristic uptake behavior. The sorption of Cs and Ba appear to be particularly sensitive to increased ionic strength which decreases their sorption while Eu is less affected. In contrast, Co sorption seems to be largely non-affected by change of ionic strength. A temperature increase, on the other hand, has a positive effect on sorption for almost all elements. To estimate the surface acidity constants (pK_a) of biotite at 40 and 60 °C, titration experiments were performed with biotite suspended in solution over a pH range of approximately 3–11. Modeling of the titration data resulted in pK_a,1 = −5.0 ± 0.2 at 40 °C and 5.3 ± 0.2 at 60 °C. Similarly, pK_a,2 = −7.0 ± 0.3 at 40 °C and–6.9 ± 0.2 at 60 °C. The sorption data (R_d values) of all metals was successfully modelled with a combination of one amphoteric (2-pK_a) surface complexation site and one ion-exchange site. A non-electrostatic SCM, without any compensation for surface electrostatic effects from electrolyte solution, were used with the reaction constants and coefficients as fitting parameters. From these, the thermodynamic parameters $∆H$ and $∆S$for the sorption reactions were calculated. The results show that surface- complexation reactions are primarily entropy-driven, while ion-exchange reactions are enthalpy-driven.