RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The complexation of Pb(II) on a hydrous goethite surface has been investigated. Equilibrium measurements have been performed as potentiometric titrations at 298.2 K, within the range 2.7 < −log [H⁺] < 8.5. Three different ionic media were used: 0.1 mol dm⁻³ NaNO₃, 0.1 mol dm⁻³ NaCl, and a mixture (1:1) of both these media. The evaluation of experimental data was made, using the constant capacitance model, within which a previously determined model for the acid-base reactions was included. The formation of the following complexes within the system H⁺ — ≡FeOH − Pb²⁺, was found to describe the surface complexation: ≡FeOHPb²⁺ (logβ_0,1,1,0^s(int) = 8.20 ± 0.06); ≡FeOPb⁺(logβ_−1,1,1,0^s(int) = 0.17 ± 0.04); ≡FeOPbOH(logβ_−2,1,1,0^s(int) = −8.85 ± 0.08). Within the four component system H⁺-≡FeOH — Pb²⁺ — Cl⁻, the following complexes were added to the model: ≡FeOHPbCl⁺ (logβ_0,1,1,1^s(int) = 7.50 ± 0.08); ≡FeOPbCl(logβ_−1,1,1,1^s(int) = −0.35 ± 0.16); ≡FeOPbOHCl⁻(logβ_−2,1,1,1^s(int) = −8.00 ± 0.12).

In the presence of an excess of Pb(II) ions to surface binding sites, the formation of a polynuclear surface Pb(II) species is postulated. The proposed model has been verified by independent determination of total soluble Pb (II) concentrations. Model calculations, using concentrations typical for natural waters, demonstrate a strong adsorption of Pb(II) even at low concentrations of surface binding sites. The experimental data show fully reversible reactions.