AB-INITIO QUANTUM-MECHANICAL STUDIES OF THE KINETICS AND MECHANISMS OF SILICATE DISSOLUTION - H+(H3O+) CATALYSIS

It is generally believed that the hydrolysis of Si-O-Si or Si-O-Al bonds is the key step in the dissolution processes of most silicates and aluminosilicates. High level ab initio molecular orbital calculations have been carried out to investigate the atomic processes of mineral dissolution by studying the following reactions: =Si-O-Si=+H2O-->=Si-OH+=Si-OH, =Si-O-Al=+H2O-->=Si-OH+=Al-OH, =Si-O-Si=+H3O+-->=Si-OH+=Si-OH2+, =Si-O-Al=+H2O+-->=Si-OH+=Al-OH2+. Our ab initio results provide detailed molecular mechanisms, which can be used to explain the pH dependence of mineral dissolution rates, the formation of leached layers during feldspar dissolution, and recent experimental data on the kinetic isotope effect associated with quartz and feldspar dissolution. The hydrolysis reactions written above were studied by truncating the mineral surface so that a finite cluster of atoms could be used in the ab initio calculations. The potential energies involved in the adsorption of various aqueous species, as well as the reaction pathways leading to the formation of the transition state in each of the reactions, were then studied from first principles. The ab initio results indicate that: (1) The attack of H+(H3O+) onto the bridging oxygen site significantly weakens the Si-O-Si and Si-O-Al bridging bonds, (2) The calculated activation energies for the hydrolysis of Si-O-Si and Si-O-Al bonds by H2O are 29 (32) kcal/mol and 26 (27) kcal/mol, respectively, while under H3O+ catalysis, they are 24 (22) kcal/mol and 16 kcal/mol (at the MP2/6-31G* level). Therefore, our ab initio results provide direct evidence of both H3O+ catalysis and of the preferred hydrolysis of Si-O-Al linkages over Si-O-Si linkages, and (3) Calculations show that pure H2O hydrolysis leads to a rather big kinetic isotope effect(k(H2O)/k(D2O) = 2-4) while under H3O+ catalysis k(H3O+)/k(D3O+) = 0.7-1.1. The latter results are in good agreement with recent experimental data for quartz and feldspar dissolution at low PH (k(H3O+)/k(D3O+) = 1.1-1.3).