Scanning force microscopy study of etch pits formed during dissolution of a barite (001) surface in CDTA and EDTA solutions

Dissolution of the barite (001) surface in aqueous solutions of 0.18 M CDTA (trans-1,2-cyclohexylene-diaminetetraacetic acid) and 0.18 M EDTA (ethylen ediaminetetraacetic acid) at pH 12 was investigated using ex situ scanning force microscopy. In both solutions, triangular and trapezoidal etch pits d eveloped on the (001) surface and became deeper and longer with increasing dissolution time. The orientation of the etch pits in CDTA and EDTA solutio ns was elongated along the crystallographie b axis. Furthermore, dissolutio n of the (001) surface in a layer-by-layer fashion was observed. This resul ted in the formation of "alternating" etch pits with heights of one half-un it cell (about 3.6 Angstrom), with the orientations of any two consecutive etch pits pointing oppositely to each other. In CDTA, etch pits within the half-unit cell were frequently bounded along the (120) and (010) directions . However, in EDTA, etch pits within the half-unit cell were bounded along the (110) and (010) directions. The dissolution behavior of barite in these two solutions is different based on the observed differences in the etch p its geometries as an assay for specific interactions between the crystal su rface and organic molecules, Thus, we suggest that CDTA molecules bind to o ne Ba2+ cation along the (120) and/or (010) directions and EDTA molecules b ind along the (110) directions to two Ba2+ cations exposed on the (001) sur face.