THE AQUEOUS DISSOLUTION KINETICS OF THE BARIUM LEAD SULFATE SOLID-SOLUTION SERIES AT 25-DEGREES-C AND 60-DEGREES-C

The dissolution behavior of the BaxPb(1-x)SO4 solid solution series and pure endmembers was investigated. The solids were precipitated from sulfuric acid solutions, then dissolved in slightly acidified water at 25 and 60-degrees-C for 24 h. Endmembers equilibrated within several h at 25-degrees-C and several min at 60-degrees-C. At 25-degrees-C the solid solutions dissolved congruently up to 30 min, then the Ba2+ concentration decreased and Pb2+ concentration increased for the duration of the experiments. At 60-degrees-C the results were similar except the region of congruent dissolution was over before the first samples were taken at 3 min. Solid solution-aqueous solution (SSAS) reaction paths were followed on Lippmann diagrams. During congruent dissolution the reaction paths dissolved toward stoichiometric saturation. During incongruent dissolution the systems moved toward two-component thermodynamic equilibrium at lower aqueous mole fractions of Ba2+, while maintaining steady states of undersaturation with respect to the stoichiometric saturation. At 60-degrees-C the solutions maintained steady states closer to stoichiometric saturation than at 25-degrees-C. The 25-degrees-C results were fit to the two parameter kinetic model of DENIS and MICHARD (1983) with an additional parameter to account for the precipitation rate dependence of the distribution coefficient. The 60-degrees-C results could not be modelled due to a complete lack of data in the congruent dissolution region. The model was able to generate close fits to our Ba2+ data, but not our Pb2+ data. The model was unable to generate steady states significantly below the stoichiometric saturation given the observed rates of congruent dissolution. This indicates a process occurs which acts to decrease the congruent dissolution rate after the formation of a secondary phase.