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


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

Stumpf, T.; Fanghänel, T.; Grenthe, I.

The complexation in the Cm(III) and Eu(III) glycolate systems have been studied by time-resolved laser fluorescence spectroscopy (TRLFS). The measurements have been performed at trace Cm(III) and Eu(III) concentrations (about 10-7 and 10-6 mol/L, 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) 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 at low pH ([H+]=10-3) the stepwise formation of glycolate complexes [Cm(HOCH2COO-)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 the following three Cm(III) species were identified from the fluorescence emission spectra

  • a hydrated tetraglycolate complex [Cm(HOCH2COO-)4(H2O)]- (Cm/complex 1) with a peak maximum at 602.3 nm and a fluorescence emission lifetime of 206 ± 3 µs,
  • a mixed hydroxide-glycolate complex [Cm(HOCH2COO-)4(OH)]2- (Cm/complex 2) with a peak maximum at 605.6 nm and the same lifetime as Cm/complex 1 and
  • a chelate complex [Cm(HOCH2COO-)3(-OCH2COO-)(OH)]3- or [Cm(HOCH2COO-)2(-OCH2COO-)2(H2O)]3- (Cm/complex 3 ) (peak maximum 611.3 nm) which is generated after deprotonation of one or two of the coordinated a-OH groups of the glycolate with a fluorescence 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 fluorescence 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 a-O- group in the Eu/complex 3 is much faster than in case of Cm/complex 3. Different spectra were observed for Eu(III)/glycolate complexes at europium concentrations of 3x10-6 mol/L and 1x10-3 mol/L indicating the formation of poly-nuclear Eu(III)/glycolate complexes at high metal ion concentration.
  • Journal of the Chemical Society - Dalton Transactions, (20): 3799-3804, 2002

Permalink: https://www.hzdr.de/publications/Publ-4488
Publ.-Id: 4488