Sorption Characteristics of Rare Earth Element Eu³⁺ onto Silica-Water Interfaces Studied by Fluorescence Spectroscopy

The sorption of Eu species onto nano-size silica-water interfaces was investigated using fluorescence spectroscopy for pH ranges of 1-8.5 and an initial Eu concentration (C_Eu) of 2×10^-4 M. The sorption rate of Eu was initially low, but significantly increased at pH > 4.The sorption density of Eu species on a silica surface was ~1.58×10^-7mol/m² when the dissolved Eu species were completely sorbed onto silica-water interfaces at pH＝~5.8.The sorbed Eu species at pH < 6 is aquo Eu³⁺, which is sorbed onto silica-water interfaces as an outer-sphere complex at pH < 5, but may be sorbed as an inner-sphere bidentate complex at 5 < pH < 6, due to the decrease of NH 2O to ~6 at pH＝6.At pH＝6 -8.5, Eu(OH)₂⁺, Eu(CO₃)⁺ and Eu(CO₃)₂^- formed in the solutions, and Eu(CO₃)⁺ is dominant at pH＝~7.5.These ions may be sorbed onto silica-water interfaces as inner-sphere bidentate complexes.