Ab initio quantum mechanical studies of the kinetics and mechanisms of silicate dissolution: H{sup +}(H{sub 3}O{sup +}) catalysis

Journal Article · Thu Dec 01 04:00:00 EST 1994 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/0016-7037(94)90237-2· OSTI ID:28810

Xiao, Y; Lasaga, A C ^[1]

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1. Yale Univ., New Haven, CT (United States)

It is generally believed that the hydrolysis of Si-O-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: {triple_bond}Si-O-Si{triple_bond} + H{sub 2}O {yields} {triple_bond}Si-OH + {triple_bond}Si-OH, {triple_bond}Si-O-Al{triple_bond} + H{sub 2}O {yields} {triple_bond}Si-OH + {triple_bond}Al-OH, {triple_bond}Si-O-Si{triple_bond} + H{sub 3}O{sup +} {yields} {triple_bond}Si-OH + {triple_bond}Si-OH{sub 2}{sup +}, {triple_bond}Si-O-Al{triple_bond} + H{sub 3}O{sup +} {yields} {triple_bond}Si-OH + {triple_bond}Al-OH{sub 2}{sup +}. 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{sup +}(H{sub 3}O{sup +}) 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 H{sub 2}O are 29 (32) kcal/mol and 26(27) kcal/mol, respectively, while under H{sub 3}O{sup +} catalysis, they are 24(22) kcal/mol and 16 kcal/mol (at the MP2/6-31G* level).

• DOE Contract Number: FG02-90ER14153
• OSTI ID: 28810
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 24 Vol. 58; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
54 ENVIRONMENTAL SCIENCES
ADSORPTION
CATALYSIS
CHEMICAL REACTION KINETICS
DISSOLUTION
FELDSPARS
HYDROLYSIS
PH VALUE
QUARTZ
ROCK-FLUID INTERACTIONS
SILICATE MINERALS
SILICATES