Silicate Binding and Precipitation on Iron Oxyhydroxides.

Silica-bearing waters in nature often alter the reactivity of mineral surfaces via deposition of Si complexes and solids. In this work, Fourier transform infrared (FTIR) spectroscopy was used to identify hydroxo groups at goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) surfaces that are targeted by ligand exchange reactions with monomeric silicate species. Measurements of samples first reacted in aqueous solutions then dried under N2(g) enabled resolution of the signature O-H stretching bands of singly (-OH), doubly (μ-OH), and triply coordinated (μ3-OH) groups. Samples reacted with Si for 3 and 30 d at pH 4 and 7 revealed that -OH groups were preferentially exchanged by silicate and that μ-OH and μ3-OH groups were not exchanged. Based on knowledge of the disposition of -OH groups on the major crystallographic faces of goethite and lepidocrocite, and the response of these groups to ligand exchange prior oligomerization, our work points to the predominance of rows of mononuclear monodentate silicate species, each separated by at least one -OH group. These species are the attachment sites from which oligomerization and polymerization reactions occur, starting at loadings exceeding ∼1 Si/nm2 and corresponding to soluble Si concentrations that can be as low as ∼0.7 mM after 30 d reaction time. Only above such loadings can reaction products grow away from rows of -OH groups and form hydrogen bonds with nonexchangeable μ-OH and μ3-OH groups. These findings have important repercussions for our understanding of the fate of waterborne silicate ions exposed to minerals.