Use of the ion exchange method for the determination of stability constants of trivalent metal complexes with humic and fulvic acids II. Tb3+ Yb3+ and Gd3+ complexes in weakly alkaline conditions.
The conditional stability constants for tracer concentrations of Tb(III), Yb(II), and Gd(III) with three soil humic acids, three soil fulvic acids and a fulvic acid from weathered coal were determined at pH 9.0-9.1 (these values are similar to those in calcareous soils) in the presence of NaHCO3 by using the anion exchange method. It was found that 1:1 and 1:2 complexes were simultaneously formed in the weakly alkaline conditions. The conditional stability constants of these 1:1 and 1:2 complexes were calculated from the distribution coefficients of rare earth elements at various concentrations of humate or fulvate. The stability constants indicate the very high stability of trivalent Tb3+, Yb3+ and Gd3+ complexes with humic substances in weakly alkaline conditions. The key parameters necessary for the experimental determination of the conditional stability constants of metal ions with humic substances in the presence of NaHCO3 by using an anion exchange method were discussed. The conditional stability constants of these 1:1 and 1:2 complexes were compared in this paper. It was found that stabilities of Tb3+ 1:1 and 1:2 complexes with humic acid are greater than the corresponding ones with fulvic acid from the same soil. In addition, the effect of the presence of Ca2+ as a competitor on the stabilities of 1:1 and 1:2 complexes of Yb was examined and no pronounced change of stabilities of 1:1 complex was found, even though Ca2+ is in a 10(3) excess to Yb3+.
Wenming
Dong
D
Radiochemistry Laboratory, College of Chemistry and Chemical Engineering, Lanzhou University, People's Republic of China. dongwm@lzu.edu.cn
Weijuan
Li
L
Zuyi
Tao
T