In situ AFM study on barite (001) surface dissolution in NaCl solutions at 30°C

This paper reports in situ observations on barite (001) surface dissolution behavior in 0.1-0.001M NaCl solutions at 30°C using atomic force microscopy (AFM). The step retreating on barite (001) surfaces changed with increasing NaCl solution concentrations. In solutions with a higher NaCl concentration (≥0.01M), many steps showed curved or irregular fronts during the later experimental stage, while almost all steps in solutions with a lower NaCl concentration exhibited straight or angular fronts, even during the late stage. The splitting phenomenon of the initial 〈hk0〉 one-layer steps (7.2Å) into two half-layer steps (3.6Å) occurred in all NaCl solutions, while that of the initial [010] one-layer steps observed only in the 0.1M NaCl solution. The step retreat rates increased with an increasing NaCl solution concentration. We observed triangular etch pit and deep etch pit formation in all NaCl solutions, which tended to form late in solutions with lower NaCl concentrations. The deep etch pit morphology changed with increasing NaCl solution concentrations. A hexagonal form elongated in the [010] direction was bounded by the {100}, {310}, and (001) faces in a 0.001M NaCl solution, and a rhombic form was bounded by the {510} and (001) faces in 0.01M and 0.1M NaCl solutions. An intermediate form was observed in a 0.005M NaCl solution, which was defined by {100}, a curved face tangent to the [010] direction, {310}, and (001) faces: the intermediate form appeared between the hexagonal and rhombic forms in solutions with lower and higher NaCl concentrations, respectively. The triangular etch pit and deep etch pit growth rates also increased with the NaCl solution concentration. Combining the step and face retreat rates in NaCl solutions estimated in this AFM study as well as the data on the effect of water temperature on the retreat rates reported in our earlier study, we produced two new findings. One finding is that the retreat rates increase by approximately two-fold when the NaCl solution concentration increases by one order of magnitude, and the other finding is that the retreat rate increase due to a one order of magnitude increase in the NaCl concentration corresponds to an increase of approximately 8°C in water temperature. This correlation may help to understand and evaluate increasing dissolution kinetics induced by the different mechanisms where barite dissolution is promoted by the catalytic effect of Na⁺ and Cl^- ions (through an increase in the NaCl solution concentration) or by an increase in the hydration of Ba²⁺ and SO₄^2- (through an increase in water temperature).