Sorption of silicic acid from non-saturated aqueous solution by a complex of zinc ions with poly(vinylamine)

Silicifying organisms (diatom algae, sponges, etc.) build multifarious silica and composite nano-structures which attracts rapt attention of specialists in nanotechnology. These organisms can capture monomeric silicic acid from diluted solutions (<1 mM) and molecular mechanism of this process is still unknown. Some hypotheses include mediatorial role of zinc ions coordinated with a special protein transporter. We have studied several basic hydrogels and their complexes with double charged metal ions in sorption of silicic acid from non-saturated solutions. Cross-linked poly(vinylamine) coordinated with Zn²⁺ was able to capture silicic acid from 0.1 to 1.7 mM solutions. The other ions (Cu²⁺, Fe²⁺, Ni²⁺, Co²⁺, Cd²⁺ and Mn²⁺) and less basic poly(1-vinylimiazole) were not active in the sorption. A mechanism included oligomerization of silicic acid and formation of triple complex poly(vinylamine)-Zn²⁺-oligosiloxane was proposed. FTIR, NMR and EDS data confirm this conclusion. The found capture of silicic acid from diluted solutions with polyamine-zinc complex shows possibility in principle of the zinc-mediated transmembrane transport of Si(OH)₄ in silicifying organisms. On the other hand, the ability of zinc ions immobilized on the polyamine support to capture oligosilicates could be used in directed assembly of nanocomposite structures by the “bottom-up” approaches.