Synthesis, characterization and thermochemistry of synthetic Pb-As, Pb-Cu and Pb-Zn jarosites

The enthalpy of formation from the elements of well characterized Pb-As, Pb-Cu, and Pb-Zn synthetic jarosites, corresponding to chemical formulas (H3O)(0.68 +/- 0.03)Pb0.32 +/- 0.002Fe2.86 +/- 0.14(SO4)(1.69 +/- 0.08)(AsO4)(0.31 +/- 0.02)(OH)(5.59 +/- 0.28)(H2O)(0.41 +/- 0.02), (H3O)(0.67 +/- 0.03)Pb0.33 +/- 0.02Fe2.71 +/- 0.14Cu0.25 +/- 0.01(SO4)(2 +/- 0.00)(OH)(5.96 +/- 0.30)(H2O)(0.04 +/- 0.002) and (H3O)(0.57 +/- 0.03)Pb0.43 +/- 0.02Fe2.70 +/- 0.14 Zn-0.21 +/- 0.01(SO4)(2 +/- 0.00)(OH)(5.95 +/- 0.30)(H2O)(0.05 +/- 0.002), was measured by high temperature oxide melt solution calorimetry and gave Delta H degrees(f) = -3691.2 +/- 8.6 kJ/mol, Delta H degrees(f) = -3653.6 +/- 8.2 kJ/mol, and Delta H degrees(f) = -3669.4 +/- 8.4 kJ/mol, respectively. Using estimated entropies, the standard Gibbs free energy of formation from elements at 298 K Delta G degrees(f) of the three compounds were calculated to be -3164.8 +/- 9.1, -3131.4 +/- 8.7, and -3153.6 +/- 8.9 kJ/mol, respectively. Based on these free energies, their logKsp values are -13.94 +/- 1.89, -4.38 +/- 1.81 and -3.75 +/- 1.80, respectively. For this compounds, a log(10){Pb2+}-pH diagram is presented. The diagram shows that the formation of Pb-As jarosite may decrease aqueous arsenic and lead concentrations to meet drinking water standards. The new thermodynamic data confirm that transformation of Pb-As jarosite to plumbojarosite is thermodynamically possible. (C) 2013 Elsevier Ltd. All rights reserved.