The influence of surface structure on H4SiO4 sorption and oligomerization on goethite surfaces: An XPS study using goethites differing in morphology

Two important weathering products are solution phase silicic acid (H4SiO4) and the iron oxides. The chemistry of H4SiO4 on iron oxide surfaces is a complex mix of sorption and polymerization and this affects many iron oxide properties. For example the presence of H4SiO4 directs goethite growth to shorter and fatter needles which implies different H4SiO4 chemistry on different goethite faces and the purpose of this paper is to directly explore this and related phenomena. Isotherms and the Si 2s X-ray photoelectron spectra were measured for H4SiO4 on three goethite samples having different morphologies. Both the shape of the isotherms and the shifts in the Si 2s binding energies (BE) indicated different H4SiO4 chemistry on acicular crystalline goethite compared to previously reported behavior on ≈ 2 nm sized disordered particles of ferrihydrite. The H4SiO4 isotherm on acicular goethites had a plateau for silicic acid concentrations (Si(sol)) between 0.01 and 0.3 mM but the isotherm increases steeply for Si(sol) > 0.3 mM. The Si 2s BE for acicular goethites indicated monomeric sorption occurring at Si(sol) < 0.3 mM but then the BE increases for Si(sol) > 0.3 mM indicating the onset of H4SiO4 polymerization as also indicated by the increase in isotherm slope. The data are consistent with a model where monomers sorb on both the (110) and (021) faces (Pbnm space group) but at Si(sol) over ≈ 0.3 mM polymers form only on the (021) face. The arrangement of monomer sorption sites on the acicular goethite (021) face acts as a template for H4SiO4 polymerization. This would explain why the onset of polymerization occurs quite distinctly on the acicular goethite surface compared with that previously observed on ferrihydrite where there is a gradual increase in the significance of polymerization across the H4SiO4 isotherm.