Mechanistic Study of Radium Adsorption onto Goethite

Radium (Ra2+) is a radioactive element with a long half-life. It is used in industry and is often found in shallow aquifers due to natural or anthropogenic leakage or spill of brine from deep subsurface. A major factor influencing the transport and fate of Ra2+ in water is the adsorption/desorption process onto soil particles. Goethite, a ubiquitous natural mineral generally present as a coating of soil particles, contributes significantly to the adsorption of Ra2+ in the subsurface. However, the chemical reactions of adsorption of Ra2+ onto goethite are not well-established, yet. Previous studies reported that Ra2+ creates either tetradentate or monodentate complexes on goethite and no distinction between outer-sphere and inner-sphere types of complexes were made. Knowledge of the type and structure of Ra-goethite surface complexes is important to predict the behavior and the fate of Ra2+ in the subsurface, e.g., upon a spill and during the remediation of a contaminated site. Here, the adsorption of Ra2+ onto goethite is investigated by density functional theory (DFT) calculations. By using a combination of geometric structure, adsorption energy, and electronic state (i.e., density of states and magnetic moment) analyses, the outer-sphere adsorption was found to dominate Ra2+ complexation on goethite, suggesting a significant effect of salinity on Ra2+ transport in the subsurface. Inner-sphere adsorption configuration was not observed to be thermodynamically favorable, resulting from ionic rather than covalent interaction between Ra2+ and goethite. Based on these results, a surface complexation model was developed and validated successfully with literature data. This study provides insights into the mechanism of Ra2+ adsorption on soils containing goethite and provides chemical reactions of Ra-goethite surface interaction that can be coupled with a transport model to predict Ra2+ migration in the subsurface.