RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The amount of adsorption of Co²⁺ ions on spherical magnetite particles was measured as a function of the pH (5.5–8.0), the concentration of Co²⁺ ions (1.7 × 10⁻⁸−1.7 × 10⁻⁴ mole dm⁻³), the ionic strength (I = 10⁻⁴−10⁻¹ mole dm⁻³), and the temperature (25–75°C). The amount of adsorption was found to increase with increasing pH, equilibrium concentration of Co²⁺ ions, and temperature, and with decreasing ionic strength. Acid-base titrations of magnetite dispersions were carried out to estimate the number of surface sites of −FeOH, −FeOH₂⁺, and −FeO⁻ types as a function of pH. The adsorption behavior can be explained quantitatively by considering a surface complex to form at the neutral site (−FeOH), combined with the release of one or two protons: CO²⁺ + −FeOH ⇌ −FeOCo⁺ + H⁺, K₁^o; Co²⁺ + 2−FeOH ⇌ −(FeO)₂Co + 2H⁺, β₂^o. The values of the equilibrium constants K₁^o and β₂^o, which pertain to a surface on which the amount of cobalt is negligible, were found to be 3.6 × 10⁻³ and 3.4 × 10⁻² dm⁻¹ at 25°C, respectively.