Abstract
Fluoride adsorption by a variety of oxides consisting of aluminum (Al), iron (Fe), titanium (Ti), and manganes (Mn), montmorillonite, kaolinite, and two allophanes were investigated in respect of solution pH, so as to grasp behavior of fluoride in environments and fixation mechanisms by the minerals. Almost all of the total fluoride species (T-F) are removed from the solution by anatase, amorphous TiO_2 and goethite in acidic condition and the removal percentages apparently decreased with increasing pH. Little fluoride adsorbed on amorphous MnO_2 and birnessite in the pH range (pH>4.0). In the minerals which contain Al as a major component, the fluoride ions (HF or F^-) removals were apparently higher than T-F removal in acidic condition. Especially, the removal percentage of HF or F^- on montmorillonite, natural allophane and synthetic allophane from the solutions showed nearly 100% in the pH range from 2.0 to 6.6. These phenomena meant that considerable amount of fluoride in the solutions in low pH range existed as soluble fluoro-aluminate complexes (AlF_n^<3-n>), leached from the minerals after sorption of fluoride onto surface Al-OH functional groups.
