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.