RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Acid/base reactions and Al(III) complexation at the goethite-solution interface have been investigated at 298.2 K in NaNO₃ solutions at a constant ionic strength of 0.1 M. Equilibrium measurements were performed as potentiometric titrations using a glass electrode. The experimental data were evaluated on the basis of the constant capacitance model. The acid/base properties are described by the equilibria and the intrinsic equilibrium constants: ≡FeOH + H⁺ ⇆ ≡FeOH⁺₂; log β_1,1,0(int)^S = 7.47±0.02, and ≡FeOH ⇆ ≡FeO⁻ + H⁺; log β_{−1,1,0(int)}^S = −9.51±0.04. The specific capacitance was determined to be 1.28 F m⁻². Aluminium (III) complexation within the range 3.0 < −log [H ⁺] < 8.5 results in the formation of the monodentate species ≡Fe-O-Al-OH⁺ (log β_{−2,1,1(int)}^S = −1.49±0.04) and ≡Fe-O-Al-(OH)₂⁰ (log β_{−3,1,1(int)}^S = −9.10±0.08). Whereas the acid base reactions were fully reversible, the aluminium desorption was found to be extremely slow and showed poor reversibility.