Scanning Force Microscopy Study of Etch Pits Formed during Dissolution of a Barite (001) Surface in CDTA and EDTA Solutions

Abstract

Dissolution of the barite (001) surface in aqueous solutions of 0.18 M CDTA (trans-1,2-cyclohexylenediaminetetraacetic acid) and 0.18 M EDTA (ethylenediaminetetraacetic acid) at pH 12 was investigated using ex situ scanning force microscopy. In both solutions, triangular and trapezoidal etch pits developed on the (001) surface and became deeper and longer with increasing dissolution time. The orientation of the etch pits in CDTA and EDTA solutions was elongated along the crystallographic b axis. Furthermore, dissolution of the (001) surface in a layer-by-layer fashion was observed. This resulted in the formation of “alternating” etch pits with heights of one half-unit cell (about 3.6 Å), 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 pits geometries as an assay for specific interactions between the crystal surface and organic molecules. Thus, we suggest that CDTA molecules bind to one Ba2+ cation along the 〈120〉 and/or 〈010〉 directions and EDTA molecules bind along the 〈110〉 directions to two Ba2+ cations exposed on the (001) surface.

Understanding barium sulfate precipitation onto stainless steel

“…First, NTA was added after scale had been deposited on the electrode to assess the impact of NTA on barium sulfate dissolution (chelators such as EDTA and NTA are shown to dissolve scales over time [35][36][37]). It was found that dissolution did not occur to an appreciable level on the timescale of the experiment.…”

The interaction of EDTA with barium sulfate

“…AFM studies of barite have tended to concentrate on its dissolution when EDTA is present [10,11]. These studies have shown that the etch pits in the presence of EDTA have a different morphology than those formed in pure water.…”

The MSXX Force Field for the Barium Sulfate−Water Interface

“…7,9,10 There have been many experiments to understand the morphology of barite crystals, 11 the nucleation and crystal growth process of barite, 12,13 and the morphology and growth rate of etch pits generated during the dissolution of barite. [14][15][16][17][18][19][20][21] These processes are known to be affected by organic and inorganic additives [22][23][24][25][26] or surfactants, 27 which should provide insight for the rational design of more efficient inhibitors and dissolvers.…”