Solubility of KFe3(CrO4)(2)(OH)(6) at 4 to 35 degrees C

The solubility of KFe3(CrO4)(2)(OH)(6), the chromate analog of the sulfate mineral jarosite, was studied in a series of dissolution experiments. Experiments were conducted at 4 to 35 degrees C and pH values between 1.5 and 3.0 using synthetic KFe3(CrO4)(2)(OH)(6). The solids were kept in the reaction vessel for up to 6 months. Equilibrium was established in the experiments between 2 and 4 months. The log K-SP for the dissolution reaction of KFe3(CrO4)(2)(OH)(6) KFe3(CrO4)(2)(OH)(6) + 6H(+) reversible arrow K+ + 3Fe(3+) + 2CrO(4)(2+) + 6H(2)O at 25 degrees C is -18.3 +/- 0.6. Based on this measured solubility product, the free energy of formation, Delta G(f,298)degrees, is -3305.5 +/- 3.4 kJ mol(-1). The dissolution experiments at 25 degrees C indicate the formation of a FeCrO4+ complex. The dissolution experiments combined with previously published spectroscopic data between 0 and 25 degrees C yield a formation constant of the form 2.303log K-FeCrO4+ = - Delta H degrees/RT + Delta S degrees/R, where Delta H degrees = 19.1 +/- 2.2 kJ mol(-1) and Delta S degrees = 214 +/- 8 J K-1 mol(-1). At 25 degrees C, log K-FeCrO4+ is 7.8 +/- 0.5. The equilibrium ion activity products calculated from the experiments at 4, 15, 25, and 35 degrees C do not show a statistically significant trend indicating a weak temperature dependence of the solubility product over the temperature range of the experiments. The rate of the dissolution reaction can be described by a first-order model. The measured solubility indicates that the chromate analog of jarosite is stable over a wide range of conditions and could form in large parts of a Cr(VI)-contaminated aquifer.