RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The solid/water interface reaction is investigated for the sorption of the Eu³⁺ion onto a well-characterized natural hematite at pH ≤6 in 0.1 mol/L NaClO₄. Sorption isotherms are determined at different pH. The sorption of Eu³⁺onto hematite at coverages <1% of the surface hydroxyl groups follows the ideal Nernst behavior and can be interpreted in terms of the surface complexation by the following equilibrium reaction: ≡XOH + Eu³⁺⇔ ≡XOEu²⁺+ H⁺(logK^int_x= 2.50 ± 0.19). Complexation constants are calculated by either graphically interpretating the sorption isotherms or fitting the data using the code FITEQL. At higher surface coverages a deviation from the Nernst behavior is observed which can be described satisfactorily by a two-site surface complexation model, taking into account the lateral electrostatic interaction of surface-bound Eu(III): ≡YOH + Eu³⁺⇔ ≡YOEu²⁺+ H⁺(logK^int_y= −0.82 ± 0.11). The experimental results are compared with data published in the literature, and the interpretation of sorption isotherms with regard to the sorption mechanisms is critically discussed.