Montmorillonite interlayer surface chemistry: effect of magnesium ion substitution on cation adsorption

Abstract

Montmorillonite is a clay mineral and the main component in bentonite clay, which is utilized in various applications including its planned use as a buffer material for long-term nuclear waste disposal. In the present paper, a quantum chemical study is presented providing an insight into montmorillonite structure and its surface chemistry, which plays a key role in understanding montmorillonite behavior at the molecular level. A model is first designed by calculating the positions of Mg-substitutions in the octahedral sheet of the layer structure. This model is then used to study (1) charge distribution in the system and (2) the energetics of Na+/Ca2+ cation adsorption on the interlayer surfaces. The results show and verify that the Mg-substitutions are positive charge deficits and the only significant charge defects in the structure. Therefore, the energetics of cation adsorption is found to correlate linearly with the inverse distances between cations and Mg-substitutions in a dry, fully periodic montmorillonite lattice.