Preparation of barium sulfate chelating agent DTPA-5Na and molecular dynamics simulation of chelating mechanism.

Abstract

Barium sulfate (BaSO4) scale is dense and hard, making it difficult to remove using conventional acid and alkali treatments. Diethylenetriaminepentaacetic acid (DTPA) and its complexes have been identified as important chelating agents for the removal of BaSO4 scale. However, DTPA has good solubility only under strong alkali conditions, which in turn exacerbate scaling. To improve the solubility and chelation effectiveness of DTPA, penta sodium diethylenetriamine-pentaacetate (DTPA-5Na) was synthesized using chloroacetic acid, diethylenetriamine, sodium carbonate, and sodium hydroxide as raw materials. The structure of DTPA-5Na was characterized by infrared spectroscopy and 1H-NMR, and its chelation effectiveness was evaluated. Experimentation demonstrated that under conditions of 50 °C and with a molar ratio of chloroacetic acid (ClCH2COOH), sodium carbonate (Na2CO3), sodium hydroxide (NaOH), and diethylenetriamine (DETA) of 5.00 : 2.50 : 5.25 : 1.00, the reaction for 6 hours resulted in the optimal chelation value of DTPA-5Na at 76.8 mg CaCO3·per g. Analysis of the chelation and dissolution of BaSO4 scale using DTPA-5Na and microstructural scanning electron microscopy of the BaSO4 crystal indicate that DTPA-5Na functions through solubilization, lattice distortion, and flaking dispersion to remove BaSO4. Molecular dynamics simulation software was used to simulate the chelation mechanism of DTPA-5Na, where the results indicated strong adsorption of DTPA-5Na to the surface of BaSO4. The adsorption energy follows the order of (120) surface > (001) surface > (100) surface > (210) surface. The adsorption is mainly a result of the interaction between the carboxylic "O" atom in DTPA-5Na and the (001), (100), and (120) surfaces of BaSO4 scale, while N atoms in DTPA-5Na structure primarily interact with the (210) surface. The adsorption of "O" atoms is stronger than that of "N" atoms in the DTPA-5Na structure.