Silicon isotope fractionation during the precipitation of quartz and the adsorption of H4SiO4(aq) on Fe(Ⅲ)-oxyhydroxide surfaces

Abstract:

Equilibrium Si isotope fractionation factors among orthosilicic acid (i.e.,H₄SiO_4(aq)), quartz and the adsorption complexes of H₄SiO_4(aq) on Fe (Ⅲ)-oxyhydroxide surface were calculated using the full-electron wave-function quantum chemistry methods (i.e., B3LYP/6-311G(2df,p)) with a new cluster-model-based treatment. Solvation effects were carefully included in our calculations via water-droplet method combined with implicit solvent models (e.g., PCM). The results revealed that, if it is under equilibrium conditions, heavy Si isotopes would be significantly enriched in quartz in comparison to H₄SiO_4(aq). However, most of the field observations suggested that quartz would have identical or even depleted d³⁰Si values compared to that of H₄SiO_4(aq). To explain this discrepancy between the equilibrium calculation results and the field observations, the kinetic isotope effect (KIE) associated with the formation of amorphous silica, which usually is the precursor of crystalline quartz, was investigated using quantum chemistry methods. The KIE results showed that amorphous silica would be significantly enriched in light Si isotopes during its formation. Our equilibrium fractionation results, however, matched a special type of quartz (i.e., Herkimer “diamond”) very well, due to its nearly equilibrated precipitation condition. Opposite to the case of precipitated quartz, a large equilibrium Si isotope fractionation (i.e., -3.0‰) was found between the absorbed bidentate Si surface complexes (i.e., ²C>Fe₂O₂Si(OH)₂) and H₄SiO_4(aq). This calculated equilibrium Si isotope fractionation factor largely differed from a previous experimental result (ca. -1.08‰). We found that the formation of transient or temporary surface complexes (e.g., ¹V>Fe₂OSi(OH)₃) may have accounted for the smaller net fractionation observed. With the equilibrium and kinetic Si isotope fractionation factors provided here, the distributions and changes of Si isotope compositions in the Earth’s surface systems can be better understood.