RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Thermodynamic data for all fate-determining processes are needed in order to predict the fate and transport of metals in natural systems. The surface complexation properties of a synthetic MnO₂, δ-MnO₂, have accordingly been investigated using glass electrode potentiometry. Experimental data were interpreted according to the surface complexation model in conjunction with the diffuse double layer model of the solid/solution interface. Adsorption constants were determined using the non-linear optimisation program FITEQL. Surface complexation parameters determined in this way were validated against results obtained from the literature. Best fits of alkalimetric titration data were obtained with a 2-site, 3 surface-species model of the δ-MnO₂ surface. Site concentrations of 2.23×10⁻³ mol g⁻¹ and 7.66×10⁻⁴ mol g⁻¹ were obtained. Corresponding logarithms of formation constants for the postulated surface species are −1.27 (≡XO⁻), −5.99 (≡YO⁻) and 3.52 (≡YOH₂⁺) at I=0.1 M. The surface speciation of δ-MnO₂ is dominated by ≡XO⁻ over the pH range investigated. Metal adsorption was modelled with surface species of the type ≡XOM⁺, ≡XOMOH, ≡YOM⁺, ≡YOMOH (M=Cu, Ni, Zn, Cd and Pb) and ≡XOM₂OH²⁺ (M=Pb). For Cu, Ni and Zn, titration data could be modelled with ≡XOM⁺, ≡XOMOH, ≡YOM⁺ and ≡YOMOH, whereas for Cd, ≡XOM⁺ and ≡YOM⁺ were sufficient. Lead data were best modelled by assuming the dinuclear species ≡XOM₂OH²⁺ to be the only surface species to form. Adsorption constants determined for Ni, Cu and Zn follow the Irving-Williams sequence. The model suggests an adsorption order of (Pb, Cu) > (Ni, Zn) > Cd. The discrepancy between model predictions and published adsorption results is similar to the variability observed in experimental results from different laboratories.