The adsorption of CaOH+ on (001) basal and (010) edge surface of Na-montmorillonite: a DFT study

Ca2+ cations were generally added to facilitate the coagulation of stable fine clay mineral dispersion due to the specific adsorption of their first hydrolysis CaOH+ species at pH near 10. The adsorption of CaOH+ on dry and hydrated (001) basal surface and (010) surface of Na-montmorillonite was investigated by using density functional theory method combined with the periodic slab model method. The adsorption energies and geometries, Mulliken charge, electron density difference, and density of state were presented and discussed. It was found that the adsorption energy of CaOH+ on (010) edge surface of Na-montmorillonite (-328.8kJ/mol) was much larger than that (-126.9kJ/mol) on (001) basal surface. The presence of waters could increase the adsorption energy of CaOH+ on (001) surface but affect that on (010) surface slightly. The protons in Si-OH and Al-OH2 groups as well as the OH2 ligands in Al-OH2 group on (010) edge surface were easily dissociated and coordinated to CaOH+ to form new waters. CaOH+ was the most steady adsorption species among CaOH+, Ca2+ cation, and H2O molecule on both (001) and (010) surfaces. Copyright (c) 2016 John Wiley & Sons, Ltd.