Surface behaviour of γ-FeOOH: Point of zero charge and specific ionic interactions

The acid-base dissociation behaviour of the surface of γ-FeOOH is investigated by potentiometric acid-base titrations of the oxide suspensions in aqueous solutions. The zero point of charge (p.z.c.) of the electrical double layer of the reversible oxide solution interface is determined: in KNO₃ titrations at various ionic strenghts cross over at pH = 7.6; by coincidence with the pH of the isoelectric point, determined by microelectrophoresis, the cross over of titrations in KNO₃ is identified as the point of zero charge of the surface. Specific effects regarding monovalent anions (Cl⁻, ClO₄⁻) are found to be absent, while SO₄⁻⁻ strongly adsorbs on the γ-FeOOH surface, thus modifying the balance of the positive and negative groups on the surface. The sequence of interaction of monovalent cations with the surface is reported: K⁺ < Na⁺ < Li⁺.

Surprising and of particular interest are the strong adsorption of Na⁺, which has always been reported to interact only electrostatically with oxide surfaces, and the absence of specific interactions with Cl⁻ ions.