Binary and Ternary Uranium(VI) Humate Complexes Studied by Attenuated Total Reflection Fourier-transform Infrared Spectroscopy

The complexation of U(VI) with humic acid (HA) in aqueous solution has been investigated at an ionic strength of 0.1 M (NaCl) in the pH range between pH 2 and 10 at different carbonate concentrations by attenuated total reflection Fourier-transform infrared (ATR FT IR) spectroscopy. For the first time, the formation of binary and ternary U(VI) humate complexes was directly verified by in-situ spectroscopic measurements. The complex formation constants for the binary U(VI) humate complex (UO₂HA_(II)) and the for ternary U(VI) mono hydroxo humate complex (UO₂(OH)HA_(I)) as well as the ternary U(VI) dicarbonato humate complex (UO₂(CO₃)₂HA_(II) ⁴⁻) determined from the spectroscopic data amount to log β_0.1 M = 6.70 ± 0.25, log β_0.1 M = 15.14 ± 0.25 and log β_0.1 M = 24.47 ± 0.70, respectively and verify literature data.