Effect of pH and aging time on the kinetic dissociation of ²⁴³Am(III) from humic acid-coated γ-Al₂O ₃: A chelating resin exchange study

The chelating resin was studied to assess its influence on metal availability and mobility in the environment. The association of organic-inorganic colloid-borne trace elements was investigated in this work. The radionuclide ²⁴³Am(III) was chosen as the representative and chemical homologue for trivalent lanthanide and actinide ions present in radioactive nuclear waste. The kinetic dissociation behavior of ²⁴³Am(III) from humic acid-coated γ-Al₂O₃ was studied at pH values of 4.0 ± 0.1, 5.0 ± 0.2, and 6.0 ± 0.2 with a contact time of 2 days after the addition of a chelating cation exchanger resin. The concentrations of the components were: ²⁴³Am(III) 3.0 × 10^-7 mol/L, γ-Al ₂0₃ 0.5 g/L, HA 10 mg/L (pH 4.0 ± 0.1, 5.0 ± 0.2, and 6.0 ± 0.2) and 50 mg/L (pH 6.0 ± 0.2), respectively. The kinetics of dissociation of ²⁴³Am(III) after different equilibration time with humic acid-coated γ-Al₂O₃ was also investigated at pH 5.0 ± 0.2. The experiments were carried out in air and at ambient temperature. The results suggest that the fraction of irreversible bonding of radionuclides to HA-coated Al₂O₃ increases with increasing pH and is independent of aging time. The assumption of two different ²⁴³Am(III)-HA-Al₂O₃ species, with "fast" and "slow" dissociation kinetics, is required to explain the experimental results. ²⁴³Am(III) species present on HA-Al₂O₃ colloids moves from the "fast" to the "slow" dissociating sites with the increase of aging time.