Dissolution of the barite (001) surface by the chelating agent DTPA as studied with non-contact atomic force microscopy

DTPA (diethylenetriaminepentaacetic acid) is a chelating agent widely used for removal of barium sulfate (barite) scale in the petroleum industry. In this paper we report ex-situ investigations of barite dissolution in deioni zed water and in 0.18 M DTPA aqueous solutions. Non-contact atomic force mi croscopy (NC-AFM) was used to observe dissolution on the BaSO4 (001) cleava ge surface. Dissolution was carried out at room temperature in a 10 mi reac tor. Each sample was first etched in solution and dried before examination by NC-AFM. Dissolution on the BaSO4 (001) surface took place via developmen t of etch pits. In deionized water, triangular etch pits were observed on t he (001) terraces at room temperature. And, zigzag shaped etch pits were fo und at the edges of steps. In DTPA solutions, etch pits on the (001) terrac es were observed and these became deeper and longer with increasing time. T he geometry of these etch pits was trapezoidal, and/or trapezohedral. To ex plain this characteristic morphology caused by dissolution we suggest that the active sites of one DTPA molecule bind to two or three Ba2+ cations exp osed on the (001) surface. (C) 1999 Elsevier Science B.V. All rights reserv ed.