Abstract The sorption of uranium(VI) on two cationic resins containing different complexing groups, the imino- diacetic resin Chelex 100 and the weak carboxylic resin Amberlite CG-50, was investigated. The Gibbs–Donnan model was used to describe and to predict the sorption through the determination of the intrinsic complexation constants. These quantities, even though non-thermody- namic, characterize the sorption as being independent of experimental conditions. The sorption mechanism of the metal on the complex- ing resins was also studied by adding a competitive solu- ble ligand that shifts the sorption curves to higher pH val- ues. The ligand competes with the resin for the complex- ation with the metal ion. Uranium is also strongly sorbed on Chelex 100 at very acid pH, through formation of two complexes in the resin phase: ML with logβ 110i =–1.16, in more acidic solution, and ML 2 with logβ 120i =–5.72. Only the presence of the competitive ligand in solution makes the determination of the second complex possible. Also on Amberlite CG-50 the sorption is strong and involves the formation of the complex ML 2 , in more acidic solu- tion, with logβ 120i =–3.16. In the presence of the ligand EDTA, the complex ML 2 (OH) 2 was characterized with logβ 12–2i =–5.15. In all the experiments the hydrolysis re- action in the aqueous phase was quantitatively considered. Keywords Sorption of uranium(VI) on chelating resins · Gibbs–Donnan model · Competition of ligand for sorption · Chelex 100 · Amberlite CG-50 Introduction Uranium is one of the most seriously threatening heavy metals because of its high toxicity and some radioactivity (1 g of natural uranium produces a radioactivity of 2.523× 10 4 Bq). Excessive amounts of uranium have found their way into the environment through activities associated with the nuclear industry [1]. The determination of uranium in environmental sam- ples is generally carried out at trace levels; thus a concen- tration step, chemical separation, and final measurements are often required before determination [2]. Chemical separation commonly employs ion-exchange and chelating resins [3, 4, 5, 6]. The choice of the best op- erational conditions for sorption and elution of metal ions is a problem mostly dealt with on a purely experimental basis, while they could be usefully predicted if sorption equilibria were known. The protonation reactions and the sorption of different metal ions on many ion-exchange and chelating resins have previously investigated been on the basis of the Gibbs–Donnan based model [7, 8, 9, 10] for the ion-exchange resins. Satisfactory results in predic- tion of the sorption curves in different conditions were ob- tained showing that complexes with a different composi- tion can be formed in the resin phase, depending for in- stance on the pH of the aqueous solution in contact with the resin. The advantage in comparison with other models is that the concepts of the chemical equilibria in solution are ap- plied also to the resin phase. This has been extensively described in some previous publications [7, 8, 9, 10]. The objective of this research was to determine the sorption equilibria of uranium(VI) on two different chelating resins, the iminodiacetic chelating resin Chelex 100 and the weak cationic-exchange resin Amberlite CG-50. Thermodynamic model of the sorption of metal ions on complexing resins The model proposed is based on the Gibbs–Donnan de- scription of the ion-exchange resins [7, 8, 9, 10, 11]. Ac- cording to this approach, the resin is represented as a so- lution phase, usually concentrated, separated from the ex- ternal solution by an interface through which water, neu- Maria Pesavento · Raffaela Biesuz · Giancarla Alberti · Michela Sturini Characterization of the sorption of uranium(VI) on different complexing resins Anal Bioanal Chem (2003) 376 : 1023–1029 DOI 10.1007/s00216-003-1951-9 Received: 21 November 2002 / Revised: 17 March 2003 / Accepted: 27 March 2003 / Published online: 13 June 2003 SPECIAL ISSUE PAPER M. Pesavento · R. Biesuz (✉) · G. Alberti · M. Sturini Dipartimento di Chimica Generale dell’Università degli Studi di Pavia, Via Taramelli 12, 27100 Pavia, Italy e-mail: rbiesuz@unipv.it © Springer-Verlag 2003