Sorption of sulfate ions onto magnetite

In order to acquire data necessary to understand and predict the behavior of oxide particles in the secondary circuits of pressurized water reactors (PWR) and study the role of redox and interface reactions, the acid-base properties of magnetite and sorption of sulfate ions were studied at 25 degrees C. Redox reactions with magnetite predicted from thermodynamic data were not observed and sulfur species always remain as sulfate. From zetametric measurements, mass titrations and acid-base titrations an IEP at 6.7 and a PZC at 6.2 were found. Acid-base experimental data were modeled in the 2pK surface complexation model in CCM or BSM. Sulfate sorption increase with decreasing pH is typical for anionic species on oxides. For the modeling of sorption data, the choice of surface species was guided by the slope of the sorption curves and by the evolution of zeta potential. In the proposed model, sulfate ions are sorbed essentially as outer-sphere complexes, with a partial sorption as a neutral bidentate inner-sphere complex below pH 5.