The aqueous dissolution kinetics of the barium/lead sulfate solid solution series at 25 and 60C

The dissolution behavior of the Ba{sub x}Pb({sub 1-x})SO{sub 4} solid solution series and pure endmembers was investigated. The solids were participated from sulfuric acid solutions, then dissolved in slightly acidified water at 25 and 60C for 24 h. Endmembers equilibrated within several h at 25C and several min at 60C. At 25C the solid solutions dissolved congruently up to 30 min, then the Ba{sup 2+} concentration decreased and Pb{sup 2+} concentration increased for duration of the experiments. At 60C 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 Ba{sup 2+}, while maintaining steady states of undersaturation with respect to the stoichiometric saturation. The {sup 25}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 60C results could not be modeled due to a complete lack of data in the congruent dissolution region. The model was able to generate close fits to the authors Ba{sup 2+} data, but not their Pb{sup 2+} 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.