Complex formation of Eu(III) with humic acid and polyacrylic acid

With the intention of estimating the effect of the heterogeneous composition in the humic acid separately from its polyelectrolyte effect, comparative study on the interaction of Eu(III) with humic acid and polyacrylic acid has been carried out by the solvent extraction method with 10-3 M TTA and 10-3 M TBP in xylene at various total concentrations of Eu(III) (10-8 M to 10-5 M), pcH (5.0 to 5.5) and ionic strengths (0.02 to 1.0 M NaClO4). The interaction has been discussed in terms of the apparent complex formation constants, βα = [ML]/([M][R]), where [M] and [ML] are the concentrations of free and bound Eu3+ ion and [R] is the concentration of dissociated proton exchanging site. For both humate and polyacrylate, log βα increased with pcH or the degree of ionization and decreased with ionic strength. The Eu3+ ion concentration had no appreciable influence on the complex formation of Eu(III)-polyacrylate, and the values of log βα were obtained to be 7.57 ± 0.05 (I = 0.1, pcH = 5.53), 7.13 ± 0.04 (I = 0.1, pcH = 5.20), 6.68 ± 0.04 (I = 1.0, pcH = 5.54) and 5.61 ± 0.09 (I = 1.0, pcH = 4.73). On the other hand, log βα of humate remarkably decreased with the Eu3+ ion concentration; from 9.32 ([Eu3+] = 10-11.9 M) to 7.20 ([Eu3+] = 10-7.5 M) at pcH = 5.53 in 0.02 M NaClO4, from 8.63 ([Eu3+] = 10-12.5 M) to 6.35 ([Eu3+] = 10-7.2 M) at pcH = 5.53 in 0.1 M and from 7.59 ([Eu3+] = 10-10.7 M) to 5.20 ([Eu3+] = 10-7.0 M) at pcH = 5.52 in 1.0 M. The observed dependence of log βα of humate on the concentration of metal ion strongly suggests the heterogeneity of the complexing sites (the coexistence of weak and strong binding sites) in the humic acid. (orig.)