Plutonium (IV) complexation with citric and alginic acids at low Pu_T concentrations

Summary

The conditional stability constants for trace-level concentrations of Pu(IV) complexing with citric and alginic acids were determined at pH 4 and an ionic strength of 0.1 M NaCl using an ion-exchange technique. The results of these experiments were analyzed with: 1) the graphical Schubert's approach, 2) a non-linear simulation of ‘Schubert’-type data, and 3) an affinity distribution approach to describe the alginic acid. Analysis of the results of ion-exchange data for Pu(IV)-citric acid systems using Schubert's method indicates the formation of a 1 : 1 Pu-citrate complex with a stability constant (log β_1,1) of 16.536 ± 0.015 at I = 0.1 M. Alginic acid exhibits a strong affinity for plutonium and, unlike citric acid, the experimental results were interpreted as a mixture of 1 : 2 and 1 : 3 Pu : alginic acid complexes using non-linear Schubert's analysis of ion-exchange data; average conditional stability constants (log β) of 13.479 ± 0.057 and 14.755 ± 0.305 at I = 0.1 M NaCl, respectively, were determined. The results from the ion-exchange experiments were also modeled based on a non-electrostatic, discrete ligand approach in which alginic acid is conceptualized as being composed of a suite of monoprotic acids, HL_i, of arbitrarily-assigned pK_a(i) values (e.g., 2, 4, 6 and 8). The experimental ion-exchange data were best described by postulating complexation reactions between Pu(IV) and the model ligands L₂ (pK_a= 4) and L₃ (pK_a= 6), forming the species Pu(L₂)₂²⁺ and Pu(L₃)₃⁺, indicating that the Pu(IV) binding by alginic acid occurs mainly through carboxylic groups under the experimental conditions studied.