BARITE IN PURE WATER AT 25C

In research, barite (BaSO4) has served as a model substance in many laboratory investigations of crystal growth and dissolution using atomic force microscopy (AFM). Barite also creates a significant commercial concern due to its tendency to form scales during oil production. Most recently, the role of barite in biogeochemical cycles has been explored (e.g., Dickens et al. 2002). This is motivation enough to study the dissolution kinetics of barite under various laboratory conditions. A number of experimental studies have focused on the dissolution/growth rates in the presence of a chelating agent. EDTA, for example, is a commonly used chelating “scrubber” that is used to remove barite scale down the water-injection well in oil production. Here, we are focusing on the study of barite dissolution kinetics in pure water (18 MegaOhm) at 25C using vertical scanning interferometry (VSI). This analytical technique, in combination with standard flow-through cells, provides the possibility to measure slow mineral dissolution (or growth) rates, i.e., surface normal retreat (or advance) rates (e.g., Luttge et al. 1999, 2002).

Our (steady state) dissolution rate for the (001) barite surface measured by VSI is 10-6.9 moles m-2 s-1. This is in good agreement with rates reported from bulk dissolution studies that are slightly slower, 10-7.1 moles m-2 s-1 (Dove and Czank, 1995) but rates measured by AFM are about a factor 5 slower than our rates, i.e., 10-7.4 moles m-2 s-1 (Dove and Platt, 1995). We observed an initial phase during which no large etch pits are opening, and reaction rates are significantly slower than the reported rate. After about 6 hours the opening of etch pits was observed; consequently, dissolution rates increased by almost an order of magnitude. We assume that steady state is reached after the opening of the etch pits. These results are in good agreement with theoretical consideration by Lasaga and Luttge (2001).