DISSOLUTION OF BARITE BY A CHELATING LIGAND: AN ATOMIC FORCE MICROSCOPY STUDY

Abstract

Atomic Force Microscopy (AFM) has been used to observe the dissolution of (001) cleavage surfaces of barite (BaSO4) in deionized water and in solutions of DTPA (diethylene triamine pentaacetic acid), a synthetic metal-ion chelating agent. Experiments were carried out in situ in a fluid cell as well as in air on samples previously etched and dried. In water the principal dissolution features are etch pits bound by {210} faces. In DTPA, there is a different dissolution process which on an (001) surface results in the removal of layers that are half-unit cell thick (3.5 a). Etch pits within each half-layer are bound by {110} and {100} faces and occur in two different orientations which are related by a 21 screw axis parallel to thec crystallographic axis. Only one pit orientation is found within each half-layer, and this orientation is determined by the position of the surface sulfate groups. The etch pit geometry suggests that the active ends of a DTPA molecule form bonds to Ba2+ ions along [110] directions.Two concentrations of DTPA solutions were studied, 0.05 and 0.5 M. These concentrations are used in the development of commercial solvents for barite scale deposits in oil wells. In these experiments the 0.05 M DTPA solution was found to be considerably more reactive as a solvent than the more concentrated solution, suggesting that surface passivation of the barite occurs at high DTPA concentrations.