Europium binding by fulvic acids J.R. Lead 1,a , J. Hamilton-Taylor a,* , A. Peters a , S. Reiner a , E. Tipping b a Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, LA1 4YQ, UK b Institute of Freshwater Ecology, Ambleside, Cumbria, LA22 0LP, UK Received 13 October 1997; received in revised form 20 March 1998; accepted 23 March 1998 Abstract The binding of Eu to fulvic acid (FA) as a function of pH was determined by Schubert's method and by equilibrium dialysis. Data obtained by both methods showed a strong pH dependence, which was reproduced well by the model used to interpret the data (humic ion-binding model VI). The model was also able to account for ®ve other data sets from the literature, and there was reasonable agreement among the values of the characteristic binding parameter for Eu±FA interactions. Data sets investigating the effects of aluminum competition were also explained well by Model VI. The model was used to calculate the speciation of Eu in natural waters as a function of the most-likely competitive solution species (protons, aluminium and calcium). The calculations indicate that under environmentally realistic concentrations both, aluminium and calcium can be signi®cant competitors at low and high pH, respectively, although FA binding appears to dominate Eu speciation under most conditions. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Binding; Equilibrium dialysis; Eu (europium); Fulvic acid; Model; Schubert's method; Speciation 1. Introduction The behaviour of trace metals in the environment depends to a substantial extent on their reactions with natural ligands and surfaces. Humic substances are a particularly important category of ligands because of their high concentration, strong af®nity for metals and ubiquity in soils, sediments and waters [1]. Interest in Eu binding to humic substances is due primarily to the use of the metal as an analogue for the actinide metals, in particular americium, which constitute a radiolo- gical hazard. The assumption that americium and Eu behave in comparable ways is supported by a number of studies [2±4]. A previous modelling study [3] of Eu and actinide binding to fulvic acid (FA) raised questions about our understanding of the environmental behaviour of Eu, because two data sets [5,6] gave binding constants which differed by nearly an order of magnitude. In order to attempt to resolve this inconsistency we carried out experimental studies of Eu±FA binding, using the ion-exchange method of Schubert [7,8] and equilibrium dialysis. We also discovered additional data in the literature [9±11]. To try to rationalize all the available data, modelling was performed using humic ion-binding model VI [12]. Analytica Chimica Acta 369 (1998) 171±180 *Corresponding author. Tel.: 0044 1524 593893; fax: 0044 1524 593985; e-mail: J.Hamilton-Taylor@lancaster.ac.uk 1 Present address: De Âpartement de Chimie Mine Ârale, Analytique at Applique Âe, Sciences II, 30 Quai E. Ansermet, CH-1211, Gene Áve 4, Switzerland. 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00230-X