Pl~rgamon PIh S0045-6535(96)00218-4 Chemosphere, Vol. 33, No. 4, pp. 659-670, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0045-6535/96 $15.00+0.00 INTERACTION BETWEEN EUROPIUM IONS AND SELECTED SIZE FRACTIONS OF HUMIC ACIDS Lars Carlsen*l, Pia Lassen 1, Peter Warwick 2 and Amanda Randall 2 1National Environmental Research Institute, Department of Environmental Chemistry, DK-4000 Roskilde, Denmark, 2Department of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire LE11 3TU, UK (Received in Germany 25 March 1996; accepted 10 May 1996) ABSTRACT The interaction between humic acids and europium shows significant variations with the molecular size of the humic acids. Results presented in this paper show that the complexing capacity, defined as the number of millimoles of metal per gramme of humic material, increases with decreasing molecular size of the humic material. However, because higher molecular weight humics are present in greater concentration, most of the metal is to be found associated with the higher molecular weight fractions. Results show that >98% of europium is associated with humic species larger than 10.000 Daltons and over 50% of europium is associated with hu- mics within the 10.000 - 30.000 dalton range. The effect of humic acids on the mobility of metal ions in the environment is discussed. Copyright © 1996 Elsevier Science Ltd INTRODUCTION Humic materials, i.e. humic and fulvic acids, are omnipresent compounds in the terre- strial environment and constitute the more important fraction of organic carbon in natural waters, surface as well as ground waters [Stumm and Morgan, 1981; Thurman, 1985; Aiken et al., 1985]. It is widely accepted that these naturally occurring organic substances exhibit polyelec- trolytic properties as observed for hydrophilic and negatively charged colloids [Stumm and Morgan, 1981; Choppin and Allard, 1985]. Furthermore, due to their strong complexing ability towards polyvalent metal ions [Choppin and Allard, 1985; Kim, 1985], the role of these substances in natural aquifers appears to be of major importance, owing to their possible influence on the migration behaviour of metal ions in the geosphere [Carlsen, 1989; Carlsen et al., 1989]. Humic substances, considered as negatively charged colloids can a priori be expected to ex- hibit properties generally associated with this type of colloids, e.g. the tendency to flocculate in 659