Polymerization and complexation behavior of silicic acid: A review

Abstract

The role of dissolved silica (silicic acid) in the migration of radionuclides has been of interest for several decades. Studies have shown that the interaction of silicic acid with the metal cations is complicated due to a tendency to polymerize which is dependent on pH, ionic strength, temperature, etc. of the system. While polymerization of silicic acid has been studied in detail, uncertainty remains on the effect of these changes on cations in environmental systems. The nature and the role of silicate species in the environmental behavior of cationic radionuclides is the focus of this review. The interaction of metal cations such as UO22+, Eu 3+, Cm3+ and Cu2+ with polymeric silica is discussed. The effects of different experimental parameters (e.g., pH, ionic strength, temperature, acids, fluoride and hydroxyl anions, organic compounds, etc.) on the polymerization behavior of silicic acid are reviewed. Monomeric silicic acid is most stable at ca. pH 2 and most unstable at pH 7-8. Polymerization increases with increased ionic strength, temperature, and the aging time of the silicic acid solutions. Hydroxyl and fluoride ions catalyze the polymerization of silicic acid. The presence of metal ions, (e.g., aluminium, beryllium, thorium and iron) inhibits the polymerization process through complexation with hydroxyl and fluoride ions. However, the presence of mono- and divalent inorganic salts in silicic acid solutions accelerates its polymerization.