Complexation of trivalent actinide and lanthanide ions by glycolic acid: a TRLFS study

Abstract

Complexation in the Cm(III) and Eu(III) glycolate systems has been studied by time-resolved laser fluorescence spectroscopy (TRLFS). Measurements have been performed at trace Cm(III) and Eu(III) concentrations (about 10⁻⁷ and 10⁻⁶ mol L⁻¹, respectively), at different concentrations of glycolic acid and at different pH using NaClO₄ as background electrolyte. Measurements at higher Eu(III) concentrations (10⁻³ mol L⁻¹) have also been performed in order to study the influence of metal ion concentration on the complexation reaction. By varying the glycolic acid concentration from 0.1 to 0.5 mol L⁻¹ at low pH ([H⁺] = 10⁻³ mol L⁻¹) the stepwise formation of glycolate complexes [Cm(HOCH₂CO₂⁻)_n(H₂O)_{9 − 2n}]^{3 − n} with n = 1–4 were confirmed spectroscopically. By varying the pH between 4.5 and 12.0 in 1 M glycolate three Cm(III) species were identified from the luminescence emission spectra (i) a hydrated tetraglycolate complex [Cm(HOCH₂CO₂⁻)₄(H₂O)]⁻ (Cm/complex 1) with a peak maximum at 602.3 nm and a luminescence emission lifetime of 206 ± 3 µs, (ii) a mixed hydroxide–glycolate complex [Cm(HOCH₂CO₂⁻)₄(OH)]²⁻ (Cm/complex 2) with a peak maximum at 605.6 nm and the same lifetime as Cm/complex 1 and (iii) a chelate complex [Cm(HOCH₂CO₂⁻)₃(⁻OCH₂CO₂⁻)(OH)]³⁻ or [Cm(HOCH₂CO₂⁻)₂(⁻OCH₂CO₂⁻)₂(H₂O)]³⁻ (Cm/complex 3) (peak maximum 611.3 nm) which is generated after deprotonation of one or two of the coordinated α-OH groups of the glycolate with a luminescence emission lifetime of 295 ± 15 µs. In the europium system there is evidence only for the corresponding Eu/complex 1 and 3. The corresponding europium mixed hydroxide–glycolate complex is not detectable spectroscopically. The luminescence decay is different in the Cm and Eu systems in the pH range from 7.8 up to 10.5; a bi-exponential decay behaviour was observed for the Cm system, while the Eu system shows mono-exponential decay. This indicates that the kinetics of the chelating process is much slower for Cm(III) than for Eu(III). The rate of protonation of the coordinated α-O⁻ group in the Eu/complex 3 is much faster than in the case of Cm/complex 3. Different spectra were observed for Eu(III)/glycolate complexes at europium concentrations of 3 × 10⁻⁶ and 1 × 10⁻³ mol L⁻¹ indicating the formation of poly-nuclear Eu(III)/glycolate complexes at high metal ion concentration.