Intact and treated biomass can remove heavy metals from water and wastewater. This study examined the ability of the activated, semi-intact and inactivated Azolla filiculoides (a small water fern) to remove Pb2+, Cd2+, Ni2+ and Zn2+ from the aqueous solution. The maximum uptake capacities of these metal ions using the activated Azolla filiculoides by NaOH at pH 10.5 +/- 0.2 and then CaCl2/MgCl2/NaCl with total concentration of 2 M (2: 1:1 mole ratio) in the separate batch reactors were obtained about 271, 111, 71 and 60 mg/g (dry Azolla), respectively. The obtained capacities of maximum adsorption for these kinds of the pre-treated Azolla in the fixed-bed reactors (N-o) were also very close to the values obtained for the batch reactors (Q(max)). On the other hand, it was shown that HCl, CH3OH, CH5OH, FeCl2, SrCl2, BaCl2) and AlCl3 in the pre-treatment processes decreased the ability of Azolla to remove the heavy metals in comparison to the semi-intact Azolla, considerably. The kinetic studies showed that the heavy metals uptake by the activated Azolla was done more rapid than those for the semi-intact Azolla. (c) 2005 Elsevier B.V. All rights reserved.