ELSEVIER Talanta 44 (1997) 781-786 Talanta Europium complexation by an aquatic fulvic acid - effects of competing ions Maria Nord6n, James H. Ephraim, Bert Allard Department o[ Water and Environmental Studies, Link6ping University, S-581 83 Linkdping, Sweden Received 15 December 1995; received in revised form 10 September 1996; accepted 16 September 1996 Abstract Effects of competing ions, Fe2+/Fe 3+ and Al3+, on Eu 3 + complexation with an aquatic fulvic acid (FA), have been investigated using an ion exchange technique. The influence of different concentrations (10-6 10--4 M) of the competing ions on the distribution coefficient for Eu was measured, and the overall complex formation function, ~ov, was resolved for the Eu systems with Fe and A1. All systems showed pH-dependent/~o~-functions. The presence of 10 -4 M concentration of competing ion reduced the resolved complex formation function (log flo,) for Eu complexation with fulvic acid by 0.6 and 0.4 log units at pH 5 for Fe and A1, respectively. This indicates that Fe has a more perturbing effect on Eu-FA complexation than AI. In similar competition studies Sr and Eu were found not to perturb each others complexation with fulvic acid, suggesting therefore that the two metals probably bind to different sites on the fulvic acid molecule. © 1997 Elsevier Science B.V. Keywords: Competing ions; Europium; Fulvic acid; Ion exchange technique I. Introduction Humic substances are ubiquitous in natural wa- ters and have significant effects on metal specia- tion and mobility in aquatic environments [1]. The metal complexes formed with humic substances are influenced by pH, ionic strength, concentra- tions of metal and humic matter and presence of competing ions. Few studies that investigate the effects of competing metals have been carried out [2 5]. The influence of Ca 2 + [2-4], Mg 2+ [2,3] and Cd 2 + [5] on Cu 2 + binding by naturally oc- curring dissolved organic matter are some of the few investigations where competition effects are considered. Usually in these studies only minor effects are observed. In a more recent work A13 + was found to compete with Cu 2 + for binding sites in a fulvic acid [6]. The overall objective of this study has been to quantify the perturbing effects of Fe, AI and Sr on the Eu-FA complexation. Europium was cho- sen as a model element for trivalent radionuclides that might be released into the environment from, e.g. radioactive waste in geologic deposits [7]. Aluminium and Fe are common metals in the environment, and therefore likely competitors. Aluminium can be found in fresh waters in the concentration range of 10 -6 10 .4 M and Fe in the concentration range of 10 7--10 -4 M [1]. Strontium is present in radioactive waste and its 0039-9140/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0039-91 40(96)02102-9