RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The interfacial behavior of two hydrous oxides of iron, namely an amorphous variety (amor. FeOOH) and a synthetic akaganeite (β-FeOOH), in different electrolyte media has been studied by potentiometric titration (both rapid and batch procedures) and particle electrophoresis methods. The background electrolyte medium was varied from simple KNO₃ and NaCl solutions to a complex electrolyte solution resembling seawater. In KNO₃ solution the pH_pzc and the pH_iep are almost the same for both samples. In NaCl solution, the pH_iep shifts by about a pH unit toward the acid side for amor. FeOOH due to specific sorption of anions; for β-FeOOH the pH_iep shifts slightly toward the alkaline side due to the small specific sorption of cations. Consequently, the pH_pzc of amor. FeOOH in 0.5 M NaCl, although it shifts slightly to the acid side in the presence of MgCl₂, has no corresponding shift to the alkaline side observable in the presence of Na₂SO₄ solution. Calculations for surface species in different electrolyte media suggest that while surface ionization is stronger than complexation for amor. FeOOH, surface complexation is the stronger phenomenon contributing to the total surface charge on β-FeOOH. All these observations are in conformity with the ΔpK^int_a value of amor. FeOOH being smaller than that of β-FeOOH.