Interaction of Uranium(VI) and Humic Acid under Neutral pH Conditions

Due to their strong ability for complex and colloid formation, humic acids influence the speciation and consequently the migration behavior of actinide ions in the environment. In order to perform a trustworthy risk assessment for the conception of future nuclear waste repositories as well as for the remediation of former uranium mining and milling areas, reliable thermodynamic data on the interaction of uranium with humic acids under geogenic conditions are necessary.
The U(VI) complexation by humic acids was already the subject of a number of previous investigations. However, these studies were predominately performed in the acidic pH range, were the hydrolysis of the uranyl ion can be neglected. Only few studies exist on the interaction of U(VI) with humic acids under neutral pH conditions where ternary U(VI) humate complexes are formed.
In the present study we investigate the complexation of U(VI) by purified Aldrich humic acid at pH 7 under exclusion of CO2 ([HA]: 2 mg/L; [U]: 1·10-6-1·10-5 M, I: 0.1 M NaClO4). Under the studied experimental conditions and in absence of humic acid, U(VI) occurs in form of the following species: UO2OH+, UO2(OH)2(aq), (UO2)3(OH)5+, (UO2)4(OH)7+. The speciation is dominated by 62-73 % of (UO2)3(OH)5+. Based on this speciation, the formation of ternary uranyl hydroxy humate complexes is assumed. For determination of the U(VI) and humic acid speciation in solution we apply the conventional time-resolved laser-induced fluorescence spectroscopy (TRLFS) and TRLFS with ultrafast pulses, respectively. For identification and quantification of the U(VI) species in solution, fluorescence spectra were measured for the relevant U(VI) hydrolysis species. The experimental data are evaluated applying the metal ion charge neutralization model [1], which describes the metal ion complexation by humic acids.

[1] Kim, J.I., Czerwinski, K.R.: Complexation of Metal Ions with Humic Acids: Metal Ion Charge Neutralization Model. Radiochim. Acta 73, 5 (1996).