Barite scale formation and dissolution at high ionic strength studied with atomic force microscopy

In-situ atomic force microscopy (AFM) experiments are used to demonstrate the influence of the ionic strength of the solution on growth and dissolution of barite with some comparative experiments on celestite. Growth and dissolution rates, as determined from monolayer step edge velocities, increase with increasing background electrolyte (NaCl) concentration. The electrolyte effect is interpreted as a consequence of decreased interfacial tension between barite and supersaturated aqueous solution at high ionic strength. The changes in the reaction rates do not only depend on the ionic strength but also on the crystallographic orientation of the monolayer step edges. We found that in solutions with high ionic strength, the relative stability of [010] steps is increased in comparison to the 〈120〉 direction, evident as growth islands and etch pits which are elongated in the [010] direction under growth and dissolution conditions, respectively. This indicates a specific interaction between the background electrolyte and certain sites on the mineral surface. The increased relative stability of steps parallel to [010] relative to those parallel to 〈120〉 can be explained by the formation of stabilized NaSO4NaSO4 or ClBaClBaCl chains along the step which are less polar than terraces bounded by either SO42− or Ba2+. The most likely explanation for the increased growth velocity is that Na+ ions in solution can attach to preexisting growth islands to start a new growth row, which is the rate limiting step for growth in solutions with a low salinity. For both minerals under investigation, barite and celestite, we found that surface features such as two-dimensional nuclei, growth spirals and etch pits which have formed in pure BaSO4/SrSO4 solution can be distinguished from surface features which have formed in solutions of high salinity.