U(VI) mono-hydroxo humate complexation

The complexation of the uranyl ion with humic acid is investigated. The humic acid ligand concentration is described as the concentration of reactive humic acid molecules based on the number of humic acid molecules, taking protonation of functional groups into account. Excess amounts of U(VI) are used and the concentration of the humic acid complex is determined by the solubility enhancement over the solid phase. pH is varied between 7.5 to 7.9 in 0.1M NaClO₄ under normal atmosphere and room temperature. The solubility of U(VI) in absence of humic acid is determined over amorphous solid phase between pH 4.45 and 8.62. With humic acid, only a limited range of data can be used for the determination of the complexation constant because of flocculation or sorption of the humic acid upon progressive complexation. Analysis of the complex formation dependency with pH shows that the dominant uranyl species in the concerned pH range are UO₂(OH)⁺ and (UO₂)₃(OH)₅ ⁺. The complexation constant is evaluated for the humate interaction with the to UO₂(OH)⁺ ion. The stability constant is found to be logβ = 6.94±0.3 l/mol. The humate complexation constant of the uranyl mono-hydroxo species thus is significantly higher than that of the nonhydrolyzed uranyl ion (6.2 l/mol). Published data on the Cm³⁺, CmOH²⁺ and Cm(OH)₂ ⁺ humate complexation are reevaluated by the present approach. The higher stability of the hydrolysis complex is also found for Cm(III) humate complexation.