Adsorption of barium(II) on montmorillonite: an EXAFS study

Migration of radioactive radium, ²²⁶Ra, in soil is an environmental concern, especially in areas adjacent to uranium processing facilities. Barium(II), as Ba²⁺, was used as a Ra analog and reacted with a Na-montmorillonite to obtain mechanistic insights into the interaction of Ra with soil matrices. The majority of sorbed Ba is associated with the permanently charged surface sites on the montmorillonite basal surface. This is indicated by the facts that (1) sorption of Ba(II) on montmorillonite is not highly sensitive to solution pH, although an increase of sorption was observed at higher pH values; and (2) displacement of sorbed Ba increased with increased NaNO₃ concentration. As demonstrated by EXAFS, a small fraction of Ba also adsorbed on the montmorillonite edge, forming an inner-sphere surface complex through sharing of oxygen atom(s) from deprotonated –OH group of the Al octahedral layer. The EXAFS measured distances between Ba and O at the first shell, and Ba and Al of the second shell are 2.7–2.8 and 3.7–3.9 Å, respectively, consistent with the results from geometry of a inner-sphere complex at the edge site. Results from bulk experiments and spectroscopic analysis suggest a co-existence of outer- and inner-sphere surface complexes for Ba sorbed to the montmorillonite surface.