The interaction between humic acids and europium shows significant var
iations 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 d
ecreasing molecular size of the humic material. However, because highe
r molecular weight humics are present in greater concentration, most o
f the metal is to be found associated with the higher molecular weight
fractions. Results show that >98% of europium is associated with humi
c species larger than 10.000 Daltons and over 50% of europium is assoc
iated with humics within the 10.000 - 30.000 dalton range. The effect
of humic acids on the mobility of metal ions in the environment is dis
cussed. Copyright (C) 1996 Elsevier Science Ltd