Solubility of FeSe₂(cr) at 318 K in the presence of iron

The concentration of Se equilibrated with FeSe2(cr) was determined by solubility experiments under reducing conditions and 318 ± 1 K. Agreement between the solubility data obtained from the oversaturation and undersaturation directions suggested the attainment of equilibrium. X-ray diffraction analyses revealed the formation of the FeSe2(cr) phase only. The log10K0 values of −5.0 ± 1.4 and −14.9 ± 1.5 for the FeSe2(cr) solubility reactions of FeSe2(cr) ⇄ Fe2+ +0.5 Se42− + e− and FeSe2(cr) +2 H+ +2 e− ⇄ Fe2+ +2 HSe−, respectively were determined . Moreover, the selenium solubility in the presence of selenide and Fe under anoxic conditions was discussed with the results of our experiments and previous experimental work performed at 348 K. Under the experimental conditions corresponding to the typical pH and Eh of anoxic groundwater and equilibrium with stable FeSe2(cr), the Se solubilities were reduced and remained as constant values from 6.3 × 10−8 to 9.0 × 10−7 mol dm−3 over the pH range from 6.5 to 9.5, even in spite of the linear relationship between iron concentrations with pH. This indicated that Se was consumed before the formation of FeSe2(cr) and implied that consideration of reaction paths to equilibrium with FeSe2(cr) was necessary for an estimation of Se solubility for performance assessment in geological disposal system.