RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The sorption behavior of Se(IV) on Grimsel granodiorite and its main minerals, plagioclase, K-feldspar, quartz and biotite, were investigated in Grimsel groundwater simulant in a large Se concentration range (from 1.66 × 10⁻¹⁰ M to 1 × 10⁻³ M). Experimental results show that the distribution coefficients (K_d values) of Se(IV) on the rock and mineral samples increased with the decreasing of Se(IV) concentration. The sorption of Se(IV) on biotite has the largest K_d value in low concentration area (<10⁻⁷ M) stabilizing between 0.0595 ± 0.0097 m³/Kg and 0.0713 ± 0.0164 m³/Kg. The K_d value of Se(IV) on K-feldspar was the second largest (0.0154 ± 0.0019 m³/Kg in 10⁻⁹ M) while the sorption on quartz was negligible. The sorption behavior of Se(IV) on Grimsel granodiorite followed the same trend as plagioclase, the most abundant mineral in Grimsel granodiorite, with K_d values of 0.0078 ± 0.0010 m³/Kg for Grimsel granodiorite and 0.0085 ± 0.0016 m³/Kg for plagioclase, when Se(IV) concentration was 10⁻⁹ M. HPLC-ICP-MS results show that all the Se(IV) remained in +IV oxidation state after more than 1 month experimental time and speciation modelling proved that the main species in Grimsel groundwater simulant were HSeO₃⁻ and SeO₃²⁻. Multi-site surface complexation modelling was performed by PHREEQC with the help of molecular modelling techniques which was performed with the CASTEP code implemented into Materials Studio. The modelling results predict that there are three kinds of sorption sites on the surface of biotite mineral, with sorption site densities differing in three magnitudes.