Electrolyte-promoted demineralization of biogenic, vitreous, and crystalline silica: A density functional investigation

The dissolution of amorphous and crystalline varieties of SiO2 is an integral part of the global biogeochemical cycle of silicon. Nanoparticulate biogenic silica produced by marine phytoplankton and terrestrial plants are of particular interest because their enhanced reactivity and abundance make them important sources and sinks of dissolved silicon in natural environments. Recent experimental results on (100) surfaces of quartz show that the dominant dissolution mechanism in simple H2O solutions is by retreat of Q2 groups along step edges. In the presence of electrolytes, rates are accelerated by up to 100X in the presence by a crossover in the dominant dissolution mechanism to nucleation of vacancy islands at Q3 terminated species (Dove et al., PNAS, 2005). While the control of surface coordination in reactivity is clear, the molecular pathway by which electrolytes induce dissolution by a nucleated process remains poorly understood. The results of previous ab initio investigations of Si-O bond hydrolysis by water have demonstrated that the reaction proceeds through the dissociative adsorption of H2O at the silica surface, resulting in the formation of a pentacoordinated Si transition state, followed by the transfer of one of the water bound hydrogen atoms to a bridging oxygen in the SiO2 bonded network, and breakage of the Si-O bond. Assuming a similar reaction path, the specific effects of hydrated group II metal cations (Mg2+, Ca2+, Sr2+, Ba2+) on the energetics of Si-O bond hydrolysis have been investigated with density functional methods (B3LYP) and a relatively large neutral silica cluster (H8Si6O16). Reactant, product, and transition states for Q3 to Q2 hydrolysis in the presence and absence of the afore-mentioned cations have been determined with all electron (6-31G(d)) and effective core potential (SDDALL) Gaussian basis sets. The free energy of activation for Q3 to Q2 Si-O bond hydrolysis was determined to be approximately 5 kJ/mol lower for Ca2+ than Mg2+ at the 6-31G(d) level. Similar calculations for Ca2+, Sr2+ and Ba2+ using the SDDALL basis set yielded similar molecular geometries to the all-electron results, and free energies of activation for Sr2+ and Ba2+ that are ~10 kJ/mol and ~15 kJ/mol lower than for Ca2+ respectively. These results are consistent with experimental results, which show that these cations increase the maximum rate of silica dissolution in the order Mg2+ < Ca2+ < Sr2+ < Ba2+. Further investigation of other possible hydrolysis pathways including cationic species and or surface charge is also in progress.