Modeling the complexation properties of mineral-bound organic polyelectrolyte: An attempt at comprehension using the model system alumina/polyacrylic acid/M (M = Eu, Cm, Gd)

International audienceThis paper contributes to the comprehension of kinetic and equilibrium phenomena governing metal ion sorption on organic-matter-coated mineral particles. Sorption and desorption experiments were carried out with Eu ion and polyacrylic acid (PAA)-coated alumina colloids at pH 5 in 0.1 M NaClO4 as a function of the metal ion loading. Under these conditions, M interaction with the solid is governed by sorbed PAA (PAAads). The results were compared with spectroscopic data obtained by time-resolved laser-induced fluorescence spectroscopy (TRLFS) with Cm and Gd. The interaction between M and PAAads was characterized by a kinetically controlled process: after rapid metal adsorption within less than 1 min, the speciation of complexed M changed at the particle surface till an equilibrium was reached after about 4 days. At equilibrium, one part of complexed M was shown to be not exchangeable. This process was strongly dependent on the ligand-to-metal ratio. Two models were tested to explain the data. In model 1, the kinetically controlled process was described through successive kinetically controlled reactions that follow the rapid metal ion adsorption. In model 2, the organic layer was considered as a porous medium: the kinetic process was explained by the diffusion of M from the surface into the organic layer. Model 1 allowed a very good description of equilibrium and kinetic experimental data. Model 2 could describe the data at equilibrium but could not explain the kinetic data accurately. In spite of this disagreement, model 2 appeared more realistic considering the results of the TRLFS measurements

Complexation studies of Eu(III) with polyacrylic acid either free in solution or adsorbed onto alumina

The complexation of Eu(III) with Polyacrylic acid (PAA) and alumina-bound polyacrylic acid (PAA-Al2O3) was studied at pH 5 in 0.1 M NaClO4. The experiments were carried out for two polyelectrolytes at 5000 Da and 50000 Da. The quantitative description of the interaction of Eu and PAA showed the formation of a 1:1 Eu:PAA complex and the existence of similar binding modes ("sites") of Eu with both polymers. Interaction studies of Eu(III) with alumina-bound PAA particles were carried out in conditions where their overall surface charge were equal to 0. The description of the data indicated the existence of a similar site in binary and ternary systems for high Eu loading. For low Eu loading, the existence of a new site on PAA-Al2O3 particles resulting from the PAA-Al2O3 interaction was suggested

Complexation Studies of Eu(III) with Alumina-Bound Polymaleic Acid: Effect of Organic Polymer Loading and Metal Ion Concentration

To contribute to the comprehension of the metal ion sorption properties in mixed mineral-organic matter systems, interaction studies between Eu(III) and polymaleic acid (PMA)-coated alumina colloids were carried out at pH 5 in 0.1 M NaClO4. The studied parameters were the metal ion concentration (1 × 10-10 to 1 × 10-4 M) and PMA loading on alumina ( = 10-70 mg/g). The data were described by a surface complexation model. The constant capacitance model was used to account for electrostatic interactions. The results showed that two sites were necessary to explain the sorption data. At high Eu loading, Eu is surrounded by one carboxylate group and one aluminol group. The existence of this ternary surface site was in agreement with time-resolved laser-induced fluorescence spectroscopy measurements. At low metal ion concentra tions, a surface site corresponding to a 1:1 Eu/COO- stoichiometry was deduced from modeling. Spectroscopic data did not corroborate the existence of this latter site. This discrepancy was explained by postadsorption kinetic phenomenon: a migration of the metal ion within the adsorbed organic layer