QUANTUM-CHEMICAL STUDIES OF THE EFFECTS ON SILICATE MINERAL DISSOLUTION RATES BY ADSORPTION OF ALKALI-METALS

Quantum chemical calculations at the density functional level (B3LYP f unctional) with full geometry optimisation have been performed on the effect of protonation and of adsorption of alkali cations (Li+, Na+, K +, Rb+, and Cs+) on the siloxane bond strength in silicate minerals. T he influence of pH was modelled by assuming a fully protonated surface model, (OH)(3)Si-O-Si(OH)(3), at pH lower than the point of zero char ge (pzc), while for pH = pzc and pH > pzc, the cation was assumed to i nteract with a deprotonated surface O--(-) site. At low pH, addition o f cations is found to strengthen the siloxane bond in agreement with e xperiment for the alkali metals, but not for the interaction with H3O. At high pH, the siloxane bond is weakened by the addition of alkali, in agreement with experiment for feldspar dissolution. Inclusion of t he surface hydroxyl groups is found to be important particularly when solvation of the ions at the surface is considered; up to three water molecules have been included in the geometry optimisation. Solvation o f the ions interacting with the surface is found to give very importan t contributions to the computed reaction energies. Copyright (C) 1997 Elsevier Science Ltd.