Quantum mechanical modeling of hydrolysis and H2O-exchange in Mg-, Ca-, and Ni- silicate clusters: Implications for dissolution mechanisms of olivine minerals

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 x 10(-1), 4.7 x 10(4), and 1.5 x 10(9) 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 x 10(1) and 3.7 x 10(9) 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+.