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

Migration of radioactive radium, Ra-226, in soil is an environmental concer n, especially in areas adjacent to uranium processing facilities. Barium(II ), as Ba2+, was used as a Ra analog and reacted with a Na-montmorillonite t o obtain mechanistic insights into the interaction of Ra with soil matrices . The majority of sorbed Ba is associated with the permanently charged surf ace sites on the montmorillonite basal surface. This is indicated by the fa cts 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 co ncentration. As demonstrated by EXAFS, a small fraction of Ba also adsorbed on the montmorillonite edge, forming an in,ner-sphere surface complex thro ugh sharing of oxygen atom(s) from deprotonated -OH group of the Al octahed ral 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 A, respectively , consistent with the results from geometry of a inner-sphere complex at th e edge site. Results from bulk experiments and spectroscopic analysis sugge st a co-existence of outer- and inner-sphere surface complexes for Ba sorbe d to the montmorillonite surface. (C) 2001 Elsevier Science B.V. All rights reserved.