The Influence of pH, Ionic Strength and Temperature on Cs, Ba, Co, and Eu Sorption on Biotite - Experiments and Modelling

The sorption behavior of ¹³⁴Cs, ¹³³Ba, ⁶⁰Co and ¹⁵²Eu, in a mixture, at tracer concentration [10^-8 M] on crushed Na-converted biotite (particle size 0.25-0.5 mm) was investigated by the batch sorption experiment with S:L ratio of 1:50 under the inert gas conditions ([O₂] <1 ppm) for up to two months. The experiment was carried out in triplicate with pH adjusted to 5, 6, 7, 8 or 9 with buffered solutions of 0.001, 0.01 and 0.1 M NaClO_4 at 40 and 60 C. Titration experiments on the biotite surface were conducted over a pH range of around 3 to 11 at 40 and 60 C respectively. The results were modelled to obtain pKa,1 = -7.10 ±0.05 at 40 C and -6.93 ±0.32 at 60 C, pKa,2 = -4.90 ±0.32 at 40 C, and -5.00 ±0.13 at 60 C using PHREEQC chemical speciation calculations coupled with an error minimization routine. Sorption results for the pH-range 5-9 show that for Cs, Ba, Co, and Eu, at 40 C and I= 0.001 M the Rd values were 0.6-2.3, 0.2-3.8, 0.01-4.0 and 2.4-11.3 m³/kg respectively. Corresponding results for 40 C and I= 0.01 M were Rd = 0.2-0.8, 0.03-5.1, 0.01-10.9 and 0.2-22.3 m³/kg and for 40 C and I= 0.1 M, were Rd = 0.05-0.2, 0.02-0.4, 0.01-8.61 and 2.7-13.63 m³/kg. For 60 C and I= 0.001 M the Rd values for Cs, Ba, Co, and Eu were 0.7-7.8, 0.3-4.4, 0.03-5.77 and 3.1-18.3 m³/kg respectively. Corresponding results for 60 C and I= 0.01 M were Rd = 0.2-3.2, 0.02-8.1, 0.01-19.3 and 0.2-22.9 m³/kg and for 60 C and I= 0.1 M Rd = 0.1-0.9, 0.1-0.8, 0.01-18.0 and 5.3-30.0 m³/kg. The sorption of all four elements is dependent on pH, ionic strength, and temperature. A computer code was developed that coupled PHREEQC geochemical modeling software with Python programming language to fit the sorption experimental data with the compensation for biotite dissolution. Sorption of all metals was successfully modelled with a combination of one amphoteric (i.e. 2-pKa) surface complexation site and one ion exchange site. The model was fitted to the data without any compensation for electrostatic effects. The modeling result shows that the Cs sorption is primarily governed by two surface species: SOCs, a surface complex, and CsX, an ion exchange species. However, in the case of Ba, two surface complexes: SOBa⁺ and SOBaOH and one ion-exchange species, BaX₂, mainly contribute to sorption. The Co sorption data was fitted by considering three-surface complexes: SOCo⁺, SOCoOH, and SOCo(OH)₂^- but no ion exchange is involved. The model predicts that Eu is sorbed as three different surface complexes: SOEu²⁺, SOEuOH⁺, and SOEu(OH)₂, and in addition, also one ion-exchange species EuX₃