Inhibition of Strontium Adsorption and Desorption by Ethylenediaminetetraacetic Acid at the Barite (001)–Water Interface

Abstract

Understanding the sorption behavior of ions at the mineral–water interface is important to determine the fate of elements in the environment. Here, we used barite to understand metal and organic molecule interactions with ionic crystals as organic molecules are used to remove scale that can form during oil recovery in sulfate-rich settings. The sorption behavior of strontium (Sr) on the (001) surface in the presence of ethylenediaminetetraacetic acid (EDTA) was measured using in situ specular X-ray reflectivity (XR) and resonant anomalous X-ray reflectivity (RAXR). Based on the XR results, the presence of EDTA disrupts the barite surface structure, particularly that of the top two monolayers. A maximum of 0.7 (±0.2) EDTA molecules per surface unit cell are adsorbed, though the exact amount cannot be distinguished from that of other adsorbed species, such as Ba2+ and H2O. Based on the RAXR results, the presence of EDTA inhibits Sr incorporation into the barite surface and binds with Sr in solution to limit the amount of Sr2+ in solution. The Sr-EDTA2– complex then binds with barium ions at the barite surface, presumably in a bi-nuclear, bi-dentate state. Desorption of the Sr was also measured in the presence of EDTA. The desorption experiments show that ∼75% of the sorbed strontium was removed following extended reactions with EDTA and deionized water. There were minimal changes in the surface structure following desorption measurements implying that changes following Sr sorption in the presence of EDTA may be irreversible.