This paper describes kinetic experiments to determine the rate at which Ba ions are mobilized from crystalline barium sulfate into aqueous solution by a solvent in which the active ingredient is a strong chelating agent for Ba ions (DTPA - diethylene triamine pentaacetic acid). The experiments define the temperature dependence and hence the activation energy of the dissolution process, the surface area dependence and most importantly the dependence on the concentration of the DTPA in solution.

In a 0.05M DTPA solution the dissolution rate of barium sulfate is highly temperature dependent with an activation energy of ~45kJ mol-1. This value suggests that the rate is controlled by the desorption of a Ba-DTPA surface complex. Surface complexation is further identified as the key to barium sulfate dissolution by the observation that over the concentration range 0.5M to 0.05M DTPA the initial dissolution rate is inversely related to the DTPA concentration. The activation energy remains unaltered but the absolute dissolution rate is increased. In other words, a 0.05M DTPA solution is more efficient as a solvent than a 0.5M solution. This unexpected result is interpreted in terms of a passivation of the barium sulfate surface by the formation of a surface complex layer at high DTPA concentrations.

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