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Quantum mechanical modeling of hydrolysis and H2O-exchange in Mg-, Ca-, and Nisilicate clusters: Implications for dissolution mechanisms of olivine minerals
-
Christin P. Morrow
,
Amanda A. Olsen
Published/Copyright:
November 12, 2014
Abstract
Barrier heights (BHs) for hydrolysis and H2O exchange reactions at M-O-Si (M = Ni2+, Mg2+, and Ca2+) linkages on olivine (M2SiO4) mineral surfaces were determined via DFT calculations. BHs for hydrolysis of protonated Ni-O-Si, Mg-O-Si, and Ca-O-Si sites are 76, 54, and 27 kJ/mol, respectively, and are 69 and 24 kJ/mol for H2O exchange reactions of protonated Mg-O-Si and Ca-O-Si sites, respectively. Rate constants were calculated via classical transition state theory (TST) using these BHs. For protonated Ni-O-Si, Mg-O-Si, and Ca-O-Si sites, these are 7.2 × 10-1, 4.7 × 104, and 1.5 × 109 s-1 [pseudo-first-order where (H2O) is assumed to be constant], respectively, and for H2O exchange at protonated Mg-O-Si and Ca-O-Si sites are 2.6 × 101 and 3.7 × 109 s-1 [pseudo-first-order where (H2O) is assumed to be constant], respectively. Approach of an H2O molecule from the second hydration sphere toward a protonated Ni-O-Si site leads to breakage of the Ni-O bond and subsequent release of Ni2+ to solution. For protonated Mg-O-Si sites, however, H2O exchange does not lead to rupture of the Mg-O bond and would not be a step toward dissolution of the mineral. Potential energy surface (PES) scans of H2O exchange indicated formation of a hepta-coordinated Ca2+, so neither H2O exchange nor hydrolysis of the Ca-O-Si linkage occurred in this case. Calculated rate constants are consistent with experimental data for end-member composition olivine minerals where observed rates of dissolution increase in the order Ni2+ < Mg2+ < Ca2+.
Keywords : Dissolution; silicate; reaction mechanism; density functional theory (DFT); quantum mechanical (QM); rate constant; H2O exchange; hydrolysis; olivine
Received: 2013-6-11
Accepted: 2014-5-13
Published Online: 2014-11-12
Published in Print: 2014-11-1
© 2014 by Walter de Gruyter Berlin/Boston
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Keywords for this article
Dissolution;
silicate;
reaction mechanism;
density functional theory (DFT);
quantum mechanical (QM);
rate constant;
H2O exchange;
hydrolysis;
olivine
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