Influence of temperature and reducing conditions on the sorption of sulfate on magnetite.

The reactivity of aqueous sulfate toward magnetite was studied between 50 and 275 °C as a function of pH and the redox conditions. Under oxidizing conditions, we did not observe redox reactions and the sorption of aqueous sulfate is promoted under acidic conditions when the magnetite surface is positively charged. The effect of temperature on this retention is moderate but complex. From 50 to 125 °C the sorption edge is shifted toward low pH values, according to the variation of the point of zero charge. Above 125 °C, the effect of temperature is inverted, leading to a shift to basic pH values and an increase of the sorbed quantity. This inversion of the temperature effect is interpreted as related to changes in the nature of the complexes formed, correlated to the evolution of speciation of dissolved S(VI) species. Under reducing conditions (2bar hydrogen), sulfate is involved in redox reactions, likely as a consequence of the catalytic effect of the sorption that enhances the H(2)-sulfate reaction, producing sulfides in the gaseous, liquid, and solid phases. However, this effect is better evidenced at 125 °C than at 275 °C, illustrating the importance of surface speciation, assumed to change with temperature.

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.